Search

Adding ionic species can critically affect the structure of weak polyelectrolyte (PE) chains, whose charge density in aqueous solution can be greatly regulated by bathing solution conditions such as pH and added ions. Distinct from simple ions that can be treated as point charges, multivalent macroions of finite size, including many charged nanoparticles and biopolymers, could show strong electrostatic coupling with PEs and effectively modify the conformation and assembly of PEs in aqueous solution. In this work, we have compared the effects of hydrophilic multivalent macroion of finite size and simple divalent ion on the conformational transition of a model weak polybase, poly(2-vinylpyridine) (P2VP), in dilute aqueous solution. By using fluorescence correlation spectroscopy combined with photon counting histogram analysis, we have examined the swollen-to-collapsed conformational transition and local electric potential of a P2VP chain in ionic aqueous solution at a single-molecule level. Adding inorganic polytungstate ([W12]) macroion bearing eight negative charges per [W12] of ~0.8 nm in diameter at increased concentration from 10-9 to 10-5 mM can lead to a shift of the critical conformational transition pH, pHcr, of P2VP to lower pH values, in an opposite trend to the previously reported effect of adding simple monovalent anion. Conversely, adding simple divalent sulfate anion can lead to a nonmonotonic change of pHcr when increasing its concentration from 0.03 to 15 mM. Additionally, at pH < pHcr where P2VP is highly protonated and adopts a swollen conformation, a monotonic decrease of P2VP size is observed with increased sulfate ionic concentration, exhibiting the typical ionic screening effect. In contrast, the size of the P2VP chain shows little change with increasing [W12] concentration before the precipitation of P2VP from water. To investigate the distinct effects of multivalent ion and macroion on the conformational transition of P2VP in aqueous solution, we have also measured the local proton concentration in the vicinity to a P2VP chain by an attached pH-sensitive fluorescence probe. In both cases, we have observed the monotonic reduction of the local electric potential of a swollen P2VP chain with increased ionic concentration, despite the increased protonation degree of P2VP. The results suggest that counterion condensation of multivalent ion and macroion can modify the effective net charge density of P2VP chains in dilute aqueous solution. However, possibly due to its high multivalency and finite size, multivalent [W12] macroion is much more effective in modifying the local electric environment and structure of P2VP chains at 3-7 orders of magnitude lower concentrations than simple sulfate counterion. [ABSTRACT FROM AUTHOR]

Aggregation behavior of a series of neat 1-ethyl 3-methylimidazolium alkyl sulfate (alkyl = ethyl, butyl, hexyl, and octyl) ionic liquids has been investigated through combined time-resolved fluorescence spectroscopy, 1-D and 2-D NMR spectroscopy, and fluorescence correlation spectroscopy (FCS). Interestingly, experimentally measured rotational relaxation times (τr) for ethyl, butyl, hexyl and octyl systems are measured to be 2.25, 1.64, 1.36, and 1.32 times higher than the estimated (from Stokes-Einstein-Debye theory) values for the same respective systems. This indicates that the emitting species is not the monomeric imidazolium moiety rather an associated species, and volume of the rotating fluorescing species decreases even though the length of the alkyl moiety on the anions is increased. The shift in the ¹H proton signal as well as a change in the width of the same signal upon dilution of the neat ionic liquids indicates that ionic liquids exist in the aggregated form. Further investigation through the 2D-ROESY experiment shows that interaction between imidazolium and sulfate is relatively stronger in the ethyl system than that of the longer octyl system. FCS measurements independently show that the hydrodynamic volume decreases with an increase in the anion chain length. The NMR and FCS results are consistent with the findings of the fluorescence anisotropy study. [ABSTRACT FROM AUTHOR]

A polarizableforce field for the sulfate anion SO42–has been developed and extended from nonpolarizableforce fields in order to reproduce its structural and thermodynamicsproperties in aqueous solution. Two force fields with different atomicpartial charges on S and O have been tested and used with moleculardynamics with explicit polarization. The results obtained with ourdeveloped force field are in good agreement with the experimentalhydration properties of the sulfate anion. In addition to moleculardynamics simulations of the sulfate anion in aqueous solution, potentialsof mean force of sulfate electrolytes have been calculated via umbrella-samplingmolecular dynamics simulations, i.e., MgSO4, EuSO4+, and UO2SO4. These potentials allow for calculating pairassociation constants directly comparable to the experimental ones.In the case of monoatomic cations such as Mg2+and Eu3+, the association constants calculated are in very good agreementwith the experimental values, i.e., pKcalc= 2.21 (vs 2.21 experimentally) and 3.86 (vs 3.56–3.78 experimentally)for MgSO4and EuSO4+, respectively. In the case of purely molecularelectrolyte (UO2SO4), the association constantcalculated (pKcalc= 1.58–2.07)is in agreement with the range of values available in the literature(pKexp= 1.17–3.14). [ABSTRACT FROM AUTHOR]

Ionicliquids have been shown to have application in several areasof importance in the context of sustainable industrial activity. Oneapplication of particular interest is the ability of certain ionicliquids to dissolve biomass. This clearly marks them as useful materialswith application within biorefineries. In this contribution, we presentnew coarse-grained soft-SAFT models and experimental density dataof chloride (Cl–), methylsulfate ([MeSO4]−), and dimethylphosphate ([Me2PO4]−) based ionic liquids which are relevantfor biomass deconstruction processes. Model parameters were obtainedby fitting to pure component temperature density data, and the modelswere subsequently tested by assessing their ability to accuratelycalculate viscosity and interfacial surface tension. We also developedmodels of mixtures of the ionic liquids with water and short-chainlinear alcohols. We decomposed the contributions to the excess Gibbsenergy of mixing to chemical and structural contributions, and usedthis to provide some insight into the driving forces for solubilityof molecular species in these ionic liquids. [ABSTRACT FROM AUTHOR]

Aqueous alkyltrimethylammonium bromides,or dialkyldimethylammonium ones, were mixed with sodium alkyl sulfatesand dialkanesulfonates. Depending on the overall surfactant concentration,charge and/or mole ratios, cat-anionic vesicles were formed by mixingnonstoichiometric amounts of oppositely charged species. The resultingvesicles are thermodynamically and kinetically stable. ζ-potentialand dynamic light scattering characterized the systems. As a rule,cat-anionic vesicles have sizes in the 102–103nm range and bear significant amounts of surface charges.At fixed surfactant concentration, the vesicle surface charge densityscales with mole ratios and tends to zero as the latter approach unity.Conversely, the hydrodynamic radius diverges when the cationic/anionicmole ratio is close to 1. The double-layer thickness and surface chargedensity are controlled by mole ratios and addition of NaBr, whichplays a role in vesicle stability. The salt screens the surface chargedensity and modulates both vesicle size and double-layer thickness.Slightly higher concentrations of NaBr induce the transition fromvesicles toward lamellar phases. The electrokinetic properties ofcat-anionic dispersions were analyzed by dielectric relaxation experiments.The measured properties are sensitive to vesicle size distributions.In fact, the relaxation frequency shifts in proportion to vesiclepolydispersity. Model calculations proposed on that purpose supportedthe experimental findings. [ABSTRACT FROM AUTHOR]

3-Methyl-1-octylimidazoliumdodecylsulfate, [C8mim][C12OSO3],a vesicle forming biamphiphilic ionic liquid(BAIL) (J. Phys. Chem. B 2012, 116, 14363–14374), has beenfound to induce significant folding alterations in thestructure of bovine serum albumin (BSA) in the aqueous medium at pH7.0. Such alterations have been investigated in detail using variousphysicochemical and spectroscopic techniques. Different concentrationregimes (monomeric, shared aggregation, and post-vesicular) of [C8mim][C12OSO3]–BSA interactionshave been defined through adsorption and binding isotherms using tensiometryand isothermal titration calorimetry (ITC). Fluorimetry, circulardichroism (CD), and dynamic light scattering (DLS) measurements haveshown that [C8mim][C12OSO3] inducesa small unfolding of BSA in the monomeric regime at low concentration(designated as Cf), which is followedby a refolding up to critical aggregation concentration (CAC) (designatedas C1). Above C1, i.e., in the shared aggregation concentration regime, again a smallunfolding of BSA was observed up to critical vesicular concentration(CVC) (designated as C2). In the vesicularand post-vesicular regimes, the BSA remained stable against foldingalterations. The kinetic stability of BSA in the vesicular concentrationregimes was studied for a month using turbidimetry. It has been foundthat [C8mim][C12OSO3] stabilizesBSA against the aggregation which is the major cause of protein destabilization.The present study gives insights for the design of surface activeILs for protein stabilization as a potential replacement for the mixedmicelles of conventional surfactants used in detergent industriesfor enzyme stabilization and as an artificial chaperone. [ABSTRACT FROM AUTHOR]

Solvation and mesoscale ordering of sulfuric acid and other strong acid solutions leads to suppressed freezing points and strong rheological changes with varying concentration. While the solid-state structures are well-understood, studies focused on the evolving solvation structure in the solution phase have probed a limited concentration range (∼1-6 M). This study applies a total scattering approach in both the wide-angle X-ray scattering (WAXS) and pair distribution function (PDF) regimes to elucidate the evolving solvation structure over its full range of acid concentration (0-18 M). The emergence of a prepeak in the WAXS regime at intermediate concentrations indicates a transition from noninteracting sulfate molecules in the dilute limit to sterically limited sulfates at concentrations near its deep eutectic point. Fits to the PDF data quantify this trend, showing a transition from octahedrally hydrated sulfates up to 6-7 M concentrations, followed by gradual dehydration, and eventually reaching a solution structure similar to that of water-in-salt electrolyte systems at high acid concentrations.

Using molecular dynamics simulationswe have studied the structureof alkylsulfate-based ionic liquids: 1-ethyl-3-methylimidazolium n-alkylsulfate [C2C1im][CnSO4] (n= 2, 4, 6 and8). The structure of the different ionic liquids have been interpretedtaking into account radial and spatial distribution functions, andstructure factors, that allowed us to characterize the morphologyof the polar and nonpolar domains present in this family of liquids.The size of the nonpolar regions depends linearly on the anion alkylchain length. Furthermore, properties of the surface of ionic liquids,such as surface tension, ordering, and charge and density profiles,were studied using molecular simulation. We were able to reproducethe experimental values of the surface tension with a maximum deviationof 10%, and it was possible to relate the values of the surface tensionwith the structure of the liquid–vacuum interfaces. Microscopicstructural analysis of orientational ordering at the interface anddensity profiles along the direction normal to the interface suggestthat the alkyl chains of the anions tend to protrude toward the vacuum,and the presence of the interface leads to a strong organization ofthe liquid phase in the region close to the interface, stronger whenthe side-chain length of the anions increases. [ABSTRACT FROM AUTHOR]

The influence of a humid or dry atmosphere on acousticallylevitatedionic liquid droplets was studied by volumetric analysis and vibrationalspectroscopy. Imidazolium-based ionic liquids with two types of anions,fluorinated (BF4and PF6) and alkylsulfate anions,were investigated. The amount of absorbed water was correlated withstructural differences of the ionic liquids and analyzed in termsof equilibrium mole fraction as well as absorption rate. The typeof anion had the most significant influence on the amount of adsorbedwater from the atmosphere. Furthermore, spectral changes in the insitu Raman spectra due to absorbed water were studied for all investigatedionic liquids. For 1-ethyl-3-methylimidazolium ethylsulfate, an exemplarydetailed analysis of the intermolecular interactions between cations,anions and water was carried out based on the spectroscopic data.The observed band shifts were explained with a hydrogen bond betweenthe anion and water. [ABSTRACT FROM AUTHOR]

We report a joined X-ray diffraction and molecular dynamicsstudyon the series of 1-ethyl-3-methylimidazolium alkyl sulfates (alkyl= ethyl, butyl, hexyl, octyl) ionic liquids. A general good agreementbetween experimental and theoretical structure functions has beenfound for each term of the series in all ranges of qvalues. Once the quality of the employed force field in reproducingstructural data was tested, we used dynamics simulations to accessinformation on morphology and properties of these systems. The seriesof ionic liquids presents nanoscale structural heterogeneity, whosesize depends on the anion alkyl chain size. Analyzing our simulationdata on the basis of alkyl chain length, we propose a structural modelconsistent with the presence of low qpeaks. [ABSTRACT FROM AUTHOR]

The hydrated bisulfate ion clusters (HSO4–(H2O)n, n= 1–10) were optimized at the M06/6-311++G(d,p)level. Thefactors affecting ν2-SOH of the clusters involvedvibration coupling between ν2-SOH and the water wagginglibration mode (W-H2O) and hydrogen bonding effect. Inorder to understand the vibration coupling between W-H2O and ν2-SOH for the bisulfate clusters, D2O instead of H2O and Se instead of S were used to estimatethe uncoupling frequency of ν2-SOH and W-H2O, respectively. For HSO4–·H2O-I, the uncoupling frequencies of ν2-SOHand W-H2O were obtained at 752.0 and 753.4 cm–1. After coupling, the frequencies appeared at 782.2 and 732.6 cm–1. H2S and NH4+insteadof D2O in HSO4–·D2O-II were compared to analyze the effect of hydrogen bond.The sequence of hydrogen bond strength was found to be HSO4–·H2S-II < HSO4–·D2O-II < HSO4–·NH4+-II with the respective ν2-SOH at 736.7, 740.5, and 802.2 cm–1increasingin the same order. In HSO4–·(H2O)n, coupling appeared when nwas from 1 to 8. For HSO4–·(D2O)n, no couplingbetween ν2-SOH and D2O librations madeit possible to understand the hydrogen bonding effect on the ν2-SOH. The frequencies of ν2-SOH for clustersHSO4–(D2O)nalmost linearly decreased from 752.0 to 854.6 cm–1with nfrom 1 to 10. [ABSTRACT FROM AUTHOR]

The fluorescence resonance energy transfer (FRET) usingCoumarin-153(C-153) as the donor and Rhodamine 6G (R6G) as the acceptor is studiedin an aqueous solution of N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate (SB-16) micellesby steady-state and picosecond time-resolved fluorescence spectroscopy.We have determined the rate of FRET (kFRET) from the rise of the acceptor (R6G) emission. In the absence ofdonor (C-153), the acceptor (R6G) displays a single-exponential decaywith average lifetime of 4.77 ns, whereas in presence of donor (C-153),the acceptor (R6G) exhibits a biexponential fluorescence transienthaving a distinct rise component of 0.94 ns and decay component of5.16 ns. We have carried out a comparative study of changes in FRETparameters upon addition of three different ionic liquids (ILs), 1-ethyl-3-methylimidazoliumethylsulfate [C2mim][C2SO4], 1-ethyl-3-methylimidazolium n-butylsulfate [C2mim][C4SO4], and 1-ethyl-3-methylimidazolium n-hexylsulfate[C2mim][C6SO4], where each ionicliquid bears the same cationic part and the anionic parts differ inthe alkyl chain length only. It has been observed that with gradualaddition of the ILs [C2mim][C2SO4], [C2mim][C4SO4], and [C2mim][C6SO4], the rise component gradually decreasesand the rate of FRET (kFRET) graduallyincreases. The kFRETwas found to be 1.06× 109s–1in 28 mM aqueous SB-16micelles. With the addition of 100 mM [C2mim][C2SO4], the kFRETincreasesby a factor of 1.33 (1.41 × 109s–1), whereas with the addition of 100 mM [C2mim][C6SO4] it increases by a factor of 3.25 (3.45 × 109s–1). This rapid increase in kFRETin the case of [C2mim][C6SO4] can be explained by our earlier observation (Rao, V. G.; Ghatak, C.; Ghosh, S.; Mandal, S.; Sarkar, N. J. Phys. Chem. B2012, 116, 3690–3698), where we have shown that with the additionof [C2mim][C6SO4], C-153 moves towardthe outer surface of the micelle. This movement of C-153 causes reductionin donor–acceptor distance and enhancement in FRET rate (kFRET). This is well-supported by the reduceddonor–acceptor distance (RDA) observedwith the addition of [C2mim][C6SO4]. The RDAwas found to be 29.1 Åin 28 mM aqueous SB-16 micelles. With the addition of 100 mM [C2mim][C6SO4], the RDAdecreases to 24.8 Å. With further increase in the concentrationof [C2mim][C6SO4], the RDAdecreases, but the time constant for the rise of acceptoremission decreases to such an extent that we are unable to observeit by our instrumental setup. [ABSTRACT FROM AUTHOR]

Sulfate–water clusters play an important rolein environmentaland industrial processes, yet open questions remain on their physicaland chemical properties. We investigated the smallest hydrated sulfateanion clusters believed to have a full solvation shell, with 12 or13 water molecules. We used ab initiomolecular dynamicsand electronic structure calculations based on density functionaltheory, with semilocal and hybrid functionals. At both levels of theorywe found that configurations with the anion at the surface of thecluster are energetically favored compared to fully solvated ones,which are instead metastable. We show that infrared spectra of theanion with different solvation shells have similar vibrational signatures,indicating that a mixture of surface and internally solvated geometriesare likely to be present in the experimental samples at low temperature.In addition, the computed electronic density of states of surfaceand internally solvated clusters are hardly distinguishable at finitetemperature, with the highest occupied molecular orbital belongingto the anion in all cases. The equilibrium structure determined forSO42–·(H2O)13differs from that previously reported; we find that the additionof one water molecule to a 12-water cluster modifies its hydrationshell and that water–water bonds are preferred over water–anionbonds. [ABSTRACT FROM AUTHOR]

The standard partial molar volumes, V∞(i,T), of 12 univalent ions (alkalimetal, ammonium,halide, nitrate, and perchlorate) and five divalent ions (alkalineearth and sulfate) in water at 125, 150, 175, and 200 °C andat 2 MPa were derived from the data of Ellis. Similar data for NH4+and NO3–at 0–100°C, not included in Part 4, were added, derived from his datatoo. The (negative) electrostrictive volumes, ΔVelstr(i,T), of these ions at infinitedilution were obtained from the shell-by-shell calculation of theelectrostriction according to Marcus and Hefter that takes into accountthe mutual dependence of the relative permittivity of the water aroundthe ion and the electrical field strength at it. The expanded volumesof the ions, defined as VExpan(i,T) = V∞(i,T) – ΔVelstr(i,T), were then derived and compared with their intrinsic volumes, calculatedaccording to Glueckauf. The calculation yields also the spatial extensionof the dielectrically saturated region around the ions. The numbersof water molecules, the molar volume of which is affected by the ionsat infinite dilution, were estimated from the ratio of ΔVelstr(i,T) and VWel(T), the latter being the averagemolar electrostriction of the water in the vicinity of the ions. [ABSTRACT FROM AUTHOR]

We examined the correlation between the dynamically sampledanharmonicfrequency of the OH stretching motion and the corresponding instantaneousgeometric parameters associated with the structure of crystallinesodium hydrogen bis(sulfate), which is a benchmark system with anextremely short hydrogen bond. We analyzed the trajectory obtainedby a conventional Car–Parrinello molecular dynamics simulation,followed by an a posteriori quantization of the proton motion. Forstatistical analysis we applied the established methodologies of multiplelinear regression, principal component analysis, principal componentregression, and Kohonen neural networks. No simple correlation schemebetween the OH stretching frequency and any particular geometry parameter(or their combination) was found. In comparison to the establishedcorrelation schemes (e.g., Mikenda and Novak) that consider a series of systems, our study provides a complementary insightinto the nature of hydrogen bonding of a single system, in the sense that it considers the important aspects of fluctuationsof the environment and the resulting broadening of the OH stretchingband, which cannot be adequately assessed by experiment. The absenceof appreciable correlations gives strong evidence of the extreme complexityof short hydrogen bonding. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations have been used to studythe micellizationbehavior of atomistic models for sodium alkyl sulfates in explicitwater. A major finding of the present work is the observation of astrong dependence of free surfactant concentration on overall surfactantconcentration, that has not been reported previously and that is keyto comparing simulation results for the critical micelle concentration(CMC) to experimental data. The CMC and aggregate size distributionswere obtained for alkyl tail lengths from six to nine at temperaturesfrom 268 to 363 K, from 400 ns simulations covering a number of surfactantand water model combinations. The free surfactant concentration ismuch lower than the critical micelle concentration for strongly micellizingsystems at the relatively high concentrations accessible by simulations.Thus, counterion association must be accounted for in determiningthe CMC from the raw simulation data. Simulation results are in qualitativeagreement with experimental trends for aggregate size and CMC as functionsof alkyl tail length and temperature. [ABSTRACT FROM AUTHOR]

The effect of ethylene glycol (EG) and its derivatives,ethyleneglycol monomethyl ether (EGMME), or ethylene glycol dimethyl ether(EGDME), on the aggregation behavior of a surfactant-like ionic liquid(IL), 1-butyl-3-methyl imidazolium octylsulfate, [C4mim][C8OSO3], in aqueous solutions is investigated usingconductivity, surface tension, fluorescence, 1H NMR, anddynamic light scattering (DLS) measurements. Thermodynamic parameterssuch as Gibbs free energy (ΔGm°), standard enthalpy (ΔHm°), and standard entropy (ΔSm°) of aggregation are determinedfrom the temperature dependence of conductivity. The interfacial propertiesof IL at the air/water interface in various mixed solvents are evaluatedfrom surface tension measurements. Information about the local microenvironmentand size of the aggregates is obtained from steady-state fluorescenceusing pyrene as a polarity probe and DLS measurements, respectively. 1H NMR data has been employed to get detailed insight intothe effect of organic additives on the IL aggregate structure andaggregation number. It has been observed that the addition of organicadditives to water decreases the spontaneity of aggregation of IL. [ABSTRACT FROM AUTHOR]

Halogen-free, low-cost alkyl sulfate-based surface active ionic liquids (SAILs), 1-butyl-3-methylimidazolium dodecyl sulfate ([C4mim][C12SO4]), and N-butyl-N-methylpyrrolidinium dodecyl sulfate ([C4MP][C12SO4]) were easily synthesized through ion exchange reaction. The aggregation behaviors of [C4mim][C12SO4] and [C4MP][C12SO4] in aqueous solution were investigated by surface tension, electric conductivity, and static fluorescence quenching. Both [C4mim][C12SO4] and [C4MP][C12SO4] have rather lower cmc, γcmcvalues and higher pC20, πcmcvalues than those reported for the traditional ionic surfactant, sodium dodecyl sulfate (SDS), and imidazolium-based SAIL, 1-dodecyl-3-methylimidazolium bromide ([C12mim]Br), with the same hydrocarbon chain length. The thermodynamic parameters evaluated from electric conductivity measurements show that the micelle formation of [C4mim][C12SO4] and [C4MP][C12SO4] is entropy-driven in the temperature range investigated. Lower average aggregation number indicates that the micelles of two SAILs present much looser structure. It is found that both the nature and the ring type of counterions can affect the aggregation behavior in aqueous solution. 1H NMR results of [C4mim][C12SO4] were used to further verify the mechanism of micelle formation. Hydration ability and steric hindrance of the imidazolium or pyrrolidinium counterion as well as the cooperative hydrophobic interaction of longer alkyl chain of [C12SO4] anion and comparatively shorter alkyl chain of [C4mim] or [C4MP] cation are proposed to play critical roles in the aggregation of [C4mim][C12SO4] and [C4MP][C12SO4]. [ABSTRACT FROM AUTHOR]

Rotational dynamics of two neutral organic solutes, coumarin-153 (C-153) and 4-aminophthalimide (AP), with only the latter having hydrogen-bond-donating ability, has been investigated in a series of 1-ethyl-3-methylimidazolium alkyl sulfate ionic liquids as a function of temperature. The ionic liquids differ only in the length of the linear alkyl side chain (alkyl = ethyl, butyl, hexyl, and octyl) on the anionic moiety. The present study has been undertaken to examine the role of alkyl side chains on the rotational dynamics of the two solutes in these ionic liquids. Analysis of the results using Stokes–Einstein–Debye hydrodynamic theory indicates that the rotational dynamics of C-153 lies between the stick and slip boundary condition in the ethyl analogue and finally reaches subslip condition as in case of the octyl substituent. The observed rotational behavior of C-153 has been explained on the basis of an increase in the size of the solvent, which offers lower friction for solute rotation. On the other hand, AP shows superstick behavior in the ethyl system and exceeds the stick limit in the octyl derivative. Superstick behavior of AP has been attributed to the specific hydrogen-bonding interaction between AP and the sulfate moiety. Proton NMR investigation confirms the hydrogen-bonding interaction between the N–H hydrogen of AP and the ionic liquid. The decrease in rotational coupling constant values for AP with increasing length of alkyl side chains has been attributed to the decrease in the solute–solvent-specific interaction with an increase in the alkyl side chain length on the sulfate moiety. [ABSTRACT FROM AUTHOR]

The properties of large molecules confined in nanopores are expected to be different from those of the bulk. This study reports changes in the phase behavior and vibrational frequencies of an imidazolium-based ionic liquid (IL), namely, 1-butyl-3-methyl imidazolium octyl sulfate ([BMIM][OcSO4]) in a nanoporous silica gel matrix. Nanoporous silica gel matrixes have been synthesized by a one-step sol–gel technique using tetraethylorthosilicate (TEOS) as a starting precursor. The synthesized gel has been characterized by differential scanning calorimetry (DSC), BET, TEM and FTIR. The FTIR spectra show shifts in many vibrational bands; particularly, the vibrational bands related to the imidazolium ring, aliphatic chain, and SO3of the IL are found to shift significantly upon confinement. The DSC results show significant changes in the melting point (ΔTm≈ 52 °C), crystallization temperature (ΔTC≈ 14 °C), and glass transition temperature (ΔTg≈ 2 °C). The IL used in the present study has a large anion ([OcSO4]), and ΔTmfor this is much larger than those reported earlier for many other ILs with relatively smaller anions. A new approach, based on the liquid-drop model, has been suggested to explain this. [ABSTRACT FROM AUTHOR]

Competitive adsorption of a neutral amphiphilic polymer, namely poly(ethylene oxide) (PEO) and an ionic surfactant, i.e., sodium dodecyl sulfate (SDS), is investigated at the free water surface by computer simulation methods at 298 K. The sampled equilibrium configurations are analyzed in terms of the novel identification of the truly interfacial molecules (ITIM) method, by which the intrinsic surface of the aqueous phase (i.e., its real surface corrugated by the capillary waves) instead of an ideally flat surface approximating its macroscopic surface plane, can be taken into account. In the simulations, the surface density of SDS is gradually increased from zero up to saturation, and the structural, dynamical, and energetic aspects of the gradual squeezing out of the PEO chains from the surface are analyzed in detail. The obtained results reveal that this squeezing out occurs in a rather intricate way. Thus, in the presence of a moderate amount of SDS the majority of the PEO monomer units, forming long bulk phase loops in the absence of SDS, are attracted to the surface of the solution. This synergistic effect of SDS of moderate surface density on the adsorption of PEO is explained by two factors, namely by the electrostatic attraction between the ionic groups of the surfactant and the moderately polar monomer units of the polymer, and by the increase of the conformational entropy of the polymer chain in the presence of the surfactant. This latter effect, thought to be the dominant one among the above two factors, also implies the formation of similar polymer/surfactant complexes at the interface than what are known to exist in the bulk phase of the solution. Finally, in the presence of a large amount of SDS the more surface active surfactant molecules gradually replace the PEO monomer units at the interfacial positions, and squeezing out the PEO molecules from the surface in a monomer unit by monomer unit manner. [ABSTRACT FROM AUTHOR]

An extensive dynamic and structural characterization of the supramolecular complexes that can be formed by mixing α-, β-, and γ-cyclodextrin (CD) with sodium dodecyl sulfate (SDS) in water at 283, 298, and 323 K was performed by means of computational molecular dynamics simulations. For each CD at the three temperatures, seven different initial conformations were used, generating a total of 63 trajectories. The observed stoichiometries, intermolecular distances, and relative orientation of the individual molecules in the complexes, as well as the most important interactions which contribute to their stability and the role of the solvent water molecules were studied in detail, revealing clear differences and similarities between the three CDs. Earlier reported findings in the inclusion complexes field are also discussed in the context of the present results. For any of the three native cyclodextrins, the CD2SDS1species in the head-to-head conformation appears to be a promising building block for nanotubular aggregates both in the bulk and at the solution/air interface, as earlier suggested for the case of α-CD. Moreover, the observed noninclusion arrangements involving β-CD are proposed as the seed for the premicellar (β-CD)-induced aggregation of SDS described in the literature. [ABSTRACT FROM AUTHOR]

A kinetic study for the electrosynthesis of polypyrrole (Ppy) doped with SO42−ions is presented. Ppy films were electrochemically polymerized onto a graphite-epoxy resin electrode. Experimental current density transients (j−t) were obtained for three different potentiometric behaviors: anionic, cationic, and a combination. Theoretical models were used to fit the experimental j−tdata to determine the nucleation and growth processes controlling the polymer synthesis. It was encountered that, in all cases, pyrrole electropolimerization involves two concomitant processes, namely, a Ppy diffusion limited multiple 3D nucleation and growth and pyrrole electro-oxidation on the growing surface of the Ppy nuclei. SEM analysis of the electrodes surfaces reveals that Ppy deposition occurred over most of the electrode surface by multiple nucleation of hemispheres, as the theoretical model used for the analysis of the current transients required. Hemispherical particles formed the polymeric film displaying different sizes. The order for the particle size was as follows: anionic > anionic−cationic > cationic. These results are congruent with those obtained by theoretical analysis of the corresponding current transients. Analysis of the impedance measurements recorded on the anionic Ppy film, immersed in an aqueous solution with different sulfate ion concentrations evidenced that SO42−ions diffuse through the Ppy film provoking a decrease of its electrical resistance and an increase of its dielectric constant. From the Warburg impedance coefficient, the sulfate coefficient of diffusion in the Ppy film was 1.38 × 10−9cm2s−1. [ABSTRACT FROM AUTHOR]

Sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB) dispersed in aqueous solution form catanionic vesicles. Depending on composition, such vesicles show different net charge, stability, and interaction capability, indicative of the strong impact that catanionic systems may have in gene therapy and drug delivery technologies. To reveal the interplay among composition, net charge, sensitivity to temperature changes, vesicle size, and inner structure, a series of experiments on catanionic vesicles prepared at different SDS/CTAB mole ratios was performed. Dynamic light scattering, small-angle X-ray scattering, and ζ-potential experiments allow one to characterize an unexpected critical phenomenon at the nanoscale level. On heating, vesicles increase in size, but at a critical temperature an abrupt vesicle size reduction has been observed, together with a transition from multi- to a unilamellar state. The critical temperature regularly depends on the SDS/CTAB mole ratio. The unilamellar state obtained upon heating is retained for weeks. These phenomena suggest a new way to produce stable unilamellar vesicles with tunable size and charge. [ABSTRACT FROM AUTHOR]

Addition of small amounts of lauric acid (LA) to a micellar solution of sodium dodecyl sulfate (SDS, 11.5 wt %) and cocamidopropyl betaine (CAPB, 3 wt %) has a dramatic effect on the rheological properties and phase behavior of the system. The viscosity increases by more than 1 order of magnitude up to a weight ratio LA/SDS = 0.17 and decreases for further LA loading. The decrease in viscosity is associated with the formation of a birefringent liquid crystalline phase. The evolution of the system from isotropic micelles in the absence of LA to lyotropic liquid crystals up to a weight ratio LA/SDS = 0.30 was probed by a combination of 23Na NMR quadrupolar splitting, measurements of water and surfactant self-diffusion coefficients via 1H-PGSE-NMR, and rheology. The evolution of the water self-diffusion coefficients indicates that LA induced a dramatic increase in the anisotropy of disk-shaped micelles. Birefringent samples always showed a well developed 23Na quadrupolar splitting with a line shape typical of monodomain samples. This suggests that the whole sample is easily oriented within the spectrometer electromagnet, as usually observed for nematic liquid crystals. Sample spinning first destroys the alignment (only a single peak is discernible in the 23Na NMR spectrum). Then, upon prolonged spinning, the alignment develops again. This indicates that the system is composed by disklike micelles aligning themselves with their normal perpendicular to the magnetic field. On the other hand, the linear viscoelastic response close to the nematic transition shows features usually observed in wormlike micellar systems (e.g., nearly Maxwellian behavior). To reconciliate the rheological data and the NMR evidence of disklike micelles, the formation of columnar stacks of disklike micelles is proposed. The rheology of the isotropic phase can therefore be interpreted in terms of entanglements of “living columnar stacks” of disklike micelles, and the nematic phase observed at high LA content could be attributed to a nematic columnar phase NColformed by the alignment of such stacks. [ABSTRACT FROM AUTHOR]

The calculation of association constants from computer simulations has historically been complicated because of difficulties in validating metal ion force fields for solution simulations. Here we develop a method that produces a force field for divalent metal ions in metal sulfate solutions (i.e., Mg2+SO42−, Ca2+SO42−, Mn2+SO42−, Fe2+SO42−, Co2+SO42−, Ni2+SO42−, Cu2+SO42−, and Zn2+SO42−). Using free energy of perturbation calculations, we are able to calibrate the potential of mean force W(r) for these metal sulfate solutions. The calibrated free energy profiles then allow us to produce association constants for contact, solvent-shared, and solvent-separated ion pairs that are in excellent agreement with available ultrasonic and dielectric spectroscopic data. This metal solution force field is accurate for the calculation of relative free energies from physical and biophysical computer simulations. [ABSTRACT FROM AUTHOR]

The temperature−composition phase diagrams of 40 binary mixtures composed of a haloalkane dissolved in either 1-ethyl-3-methylimidazolium ethylsulfate or 1-ethyl-3-methylimidazolium ethylsulfonate were measured from ambient temperature to the boiling point temperature of the solute. The coexistence curves corresponding to liquid−liquid equilibria (LLE) boundaries were visually determined and the experimental results have been correlated using either the nonrandom two-liquid (NRTL) model or a set of empirical equations capable of describing the corresponding upper critical solution temperatures (UCSTs) loci. The different types of LLE behavior were discussed in terms of the type of ionic liquid solvent, the alkyl-chain length of the solute, and the type and pattern of halogen substitution present in the haloalkane. Auxiliary simulation data (obtained by ab initio or by molecular dynamics methods) were used to corroborate some of the experimental findings. Also, they correlate in a semiquantitative way the observed LLE behavior with the dipole moments of the different solutes. [ABSTRACT FROM AUTHOR]

In this work, we perform a series of molecular dynamics (MD) simulations on the category of sodium alkyl sulfate (SDS-type) surfactant monolayers at the water/trichloroethylene (TCE) interface. Three separate tail-length SDS-type molecules are used. We investigate the conformation of surfactant chain (i.e., packing, orientation, and order), interfacial properties (i.e., interfacial thickness, interfacial tension, area compressibility, and bending modulus), their dependence on the chain length, and the average area per surfactant chain. We also examine the behavior of the surfactant monolayer in the metastable regime of negative surface tension with reference to collapse. The simulation has clearly shown that the very dilute monolayer is well described as a two-dimensional gas. With the increase of interfacial surfactant coverage, the monolayer is in the liquid-expanded (LE) phase. The surfactant tails at the interface become straighter, more ordered, and thicker at higher surfactant coverage. At the same time, interfacial tension of long-tail systems is always lower than that of short-tail systems. In the LE phase, the area compressibility modulus and the bending modulus increase with an increase in tail length. With a further decrease in molecular areas, the monolayer with large negative surface tension becomes unstable. Our simulations show that buckling of the monolayers is of dynamic nature as a response to mechanical instability. The further transformation pathway from buckling to bud can be controlled by the bending modulus, which depends crucially on the tail length and interfacial surfactant coverage. At a given area per molecule, the short tail chain makes the monolayer softer, and the budding process becomes more probable. For the supersaturated softer SDS monolayer, the collapse transition is initiated by the buckling of monolayers, followed primarily by budding and detachment of the nanoscale swollen micelle from the monolayer. Despite a number of extensive studies of monolayer collapse at the air/water interface, to our knowledge the conversion of surfactants from the liquid−liquid interface to swollen micellar aggregates as described here has not been reported in the literature. [ABSTRACT FROM AUTHOR]

Conjugates of 5-(4-carboxyphenyl)-10,15,20-tris(4-methylpyridinium-4-yl)porphyrin (P−H) are promising photoactive agents for medical applications. As their ultimate efficacy will depend on the behavior of initial excited states, photophysical parameters have been determined with conventional steady-state absorption and fluorescence as well as time-resolved femto- and nanosecond spectroscopies. The fluorescence quantum yield of P−H and P−H conjugated to uncharged groups increases from ∼0.03 in pH 7 buffer to ∼0.05 in Triton X100 micelles (TX100) and in ethanol and to 0.12 in sodium dodecyl sulfate (SDS) micelles. Corresponding 1S1lifetimes are ∼5−10 ns. In buffer, an equilibrium between P−H monomers and small-size aggregates is observed. Conjugation with poly-S-lysine (P−(Lys)n) results in fluorescence quenching in all solvents. Structural reorganization of conjugates bearing a Di-O-isopropylidene-α-d-galactopyranosyl or a α/β-d-galactopyranosyl group occurs in ethanol (k∼0.15 ps−1) after 1S1state solvation (∼700 fs). Relaxation of bulky P−(Lys)npolypeptide chains takes place on a longer time scale in all solvents (k≤ 0.01 ps−1) with enhanced internal conversion. Triplet state (3T1) transient spectra of all derivatives in PBS, SDS, TX100, and ethanol exhibit a strong absorbance with a broad maximum in the 460−475 nm region and minor maxima at ∼540, 630, and 690 nm. In ethanol, energy transfer from the P−H 3T1state to β-carotene provides an estimate of ε ∼40 000 M−1cm−1at 460 nm for the P−H 3T1state. Using triplet meso-tetraphenylporphyrin as an actinometer, the P−H triplet quantum yield (ΦT) is estimated to be ∼0.50 in all solvents. This high ΦTleads to effective singlet oxygen production in buffered solutions. [ABSTRACT FROM AUTHOR]

Solvent dynamics effects on electroreduction of peroxodisulphate anion on mercury electrode (a typical bond breaking electron transfer reaction) are explored in the framework of the Sumi−Marcus model. The reaction three-dimensional free energy surface is constructed using the Anderson model Hamiltonian. A new interpretation of short- and long-time survival times is presented as well. Since the reduction is assumed to proceed from aqueous sucrose and glucose solutions of different concentrations (which are used to vary the solution viscosity), unavoidable changes in the Pekar factor (static effect) are also taken into account. The results of model calculations are employed to interpret challenging experimental data on nonmonotonous constant rate vs solution viscosity dependence reported earlier (in part, appearance of an ascent plot). The influence of mixed solvent composition on the reaction rate and transfer coefficient is explained in terms of the saddle point avoidance in the vicinity of activationless discharge. Splitting of the reaction coordinates into slow (solvent) and fast (intramolecular) ones is argued to be crucial, as the most important reaction features cannot be described by means of more simplified models, even if both static and dynamic effects are addressed. [ABSTRACT FROM AUTHOR]

Self-assembled semiconducting, paramagnetic polyaniline nanotubes have been synthesized by the oxidative polymerization of aniline with ammonium peroxydisulfate in aqueous medium in the presence of colloidal silica particles of an average diameter ∼12 nm, without added acid. The electrical conductivity of polyaniline nanotubes/silica nanocomposites is in the range (3.3−4.0) × 10−3S cm−1. The presence of paramagnetic polaronic emeraldine salt form of polyaniline and phenazine units in nanocomposites was proved by FTIR, Raman, and EPR spectroscopies. The influence of the initial silica/aniline weight ratio on the morphology of polyaniline/silica nanocomposites was studied by scanning and transmission electron microscopies. Nanocomposites synthesized by using the initial weight ratio silica/aniline ≤0.2 contain polyaniline nanotubes which have a typical outer diameter of 100−250 nm and an inner diameter of 10−80 nm, and nanorods with a diameter of 60−100 nm, accompanied with polyaniline/silica nanogranules, while the nanocomposite synthesized at weight ratio silica/aniline ∼2 contains polyaniline/silica nanogranules with an average diameter of 35−70 nm. The evolution of molecular and supramolecular structure of polyaniline in the presence of colloidal silica is discussed. [ABSTRACT FROM AUTHOR]

The considerable interest aroused in recent years by the unique properties and industrial applications of ionic liquids has given rise to the need for a detailed statement of the linkage between their molecular features and the observed macroscopic behavior. A combined experimental/computational approach to the study of ionic liquids is submitted here and applied to the relevant, nonhalogenated ionic liquids 1,3-dimethylimidazolium methylsulfate and 1-ethyl-3-methylimidazolium tosylate. To establish a reliable equation of state pertinent to these fluids, density data over wide pressure (0.1−60 MPa) and temperature (318.15−428.15 K) ranges, along with high pressure (1−70 MPa) viscosities and other selected ambient pressure properties were measured to assemble sufficient experimental information for the seek of predictive models for process design. A computational method based on ab initio and classical molecular dynamics yielded a deal of structural information, borne out by the experimental readings. Likewise, the predictive ability of the force field applied in molecular dynamics simulations was faced with the measured properties. The pictorial description of the selected ionic liquids reached this way may become widespread to other relevant examples in order to infer valuable structure/property relationships. [ABSTRACT FROM AUTHOR]

Photon counting histogram (PCH) analysis is a statistical method that provides independent information about the relative brightness and the number of molecular entities in the system. This suggests that it could be very advantageous in the analysis of intermolecular interactions in multicomponent systems. In this paper we employ PCH to study the interaction of Rhodamine 6G (Rh-6G) with sodium dodecyl sulfate (SDS) micelles and show how the method can unveil the mechanism of quenching of Rh-6G by methyl viologen (MV). [ABSTRACT FROM AUTHOR]

We have performed a 50 ns of molecular dynamics study of poly(diallyldimethylammonium chloride) (PDADMAC)/sodium dodecyl sulfate (SDS)/decanol/water systems. The influence of the cationic polyelectrolyte on the anionic SDS-based lamellar liquid crystalline system was investigated. The main structural parameters have been calculated and compared with experimental data. We obtain two types of PDADMAC conformation, a more folded structure A and a structure B where the PDADMAC molecule is adsorbed at the anionic head groups of the surfactant molecules. The polyelectrolyte-induced coexistence of two lamellar phases at a concentration of 2−3% of PDADMAC is observed, which is in agreement with experimental findings. [ABSTRACT FROM AUTHOR]

The effect of different mixing protocols on the bulk and surface properties of the aqueous mixtures of linear poly(vinylamine) (PVAm) and sodium dodecyl sulfate (SDS) has been investigated using pH, electrophoretic mobility, dynamic light scattering, coagulation kinetics, and surface tension measurements. For the preparation of the solutions, two kinds of mixing protocols were applied. The so-called “stop flow mixing” enables a very rapid mixing whereas in the case of “gentle mixing” the mixing of the components is less efficient. At high surfactant concentrations a kinetically stable colloid dispersion of the PVAm/SDS particles is formed via the application of the stop flow mixing method. The mixing protocols have a significant effect on the bulk properties of the PVAm/SDS system, in particular, at the low pH range and at large PVAm concentrations. The effect of mixing can be qualitatively understood in terms of the enhanced local rate of coagulation of the PVAm/SDS complexes as well as of the appearance of polyelectrolyte/surfactant aggregates via the application of a less efficient mixing. The study also reveals that the applied methods of solution preparation do not have a major impact on the bound amount of the surfactant as well as on the surface tension isotherms of the system. This latter finding is attributed to the hindered adsorption of the large polyelectrolyte/surfactant aggregates at the air/water interface. [ABSTRACT FROM AUTHOR]

Cationic conjugated polymers (CCPs) have been widely utilized as signal amplifiers in biosensors to improve the detection sensitivity through fluorescence resonance energy transfer (FRET) from CCPs to dye-labeled probes or targets. This paper investigates the effect of sodium dodecyl sulfate (SDS) on energy transfer between a cationic polyfluoreneethynylene copolymer ( P1) and Texas Red labeled single-stranded DNA (ssDNA-TR). The presence of SDS in solution affects both the optical properties of P1and TR emission within P1/ssDNA-TR complexes, which provides basic information on the role of SDS in FRET between P1and ssDNA-TR. Although the quantum yield of P1decreases in the presence of low concentrations of SDS, the presence of SDS reduces TR fluorescence quenching within P1/ssDNA-TR complexes and increases the number of optically active polymer repeat units within the proximity of TR, which are beneficial to P1-sensitized TR emission. In the absence of SDS, FRET from P1to ssDNA-TR provides a 2.6-fold enhancement in TR emission intensity as compared to that upon direct excitation of TR at 595 nm. At the optimum SDS concentration (5 μM), P1-sensitized TR signal output increases to 11.3-fold relative to direct excitation of TR. This study highlights the importance of modulation of the CCP/ssDNA-dye interaction in improving the signal output of dye-labeled DNA by CCP through FRET. [ABSTRACT FROM AUTHOR]

The effects of the anionic surfactant sodium dodecyl sulfate (SDS) on the spatio-temporal and temporal dynamics of the ferroin-catalyzed Belousov−Zhabotinsky (BZ) reaction have been studied over a wide surfactant concentration range. For the first time, investigations were performed also for unstirred systems. The presence of SDS in the reaction mixture influences the oscillatory parameters to an extent that significantly depends on the surfactant concentration. The trend of the wave speed vupon the increasing amount of SDS was found to have a maximum at [SDS] = 0.075 mol dm −3( v= 0.071 mm s −1), after which the speed decreased to 0.043 mm s −1at [SDS] = 0.5 mol dm −3, which is below the value found in the absence of the surfactant ( v= 0.055 mm s −1). The response of the oscillatory BZ system to the addition of SDS has been ascribed to two different causes: (a) the peculiar capability of the organized surfactant assemblies to affect the reactivity by selectively sequestering some key reacting species and (b) the modifications induced by SDS on the physical properties of the medium. These hypotheses have been corroborated by performing spectrophotometric investigations on the stirred BZ system. Complementary viscosity measurements gave useful hints for the clarification of the surfactant role. [ABSTRACT FROM AUTHOR]

The photophysical behavior of 3-pyrazolyl-2-pyrazoline derivative (PZ), a newly synthesized biologically active compound has been studied in micellar solutions of anionic sodium dodecyl sulfate (SDS), cationic cetyl trimethylammonium bromide (CTAB) and nonionic p- tert-octylphenoxy polyoxyethanol (Triton X-100, TX-100) micelle using steady state and time-resolved fluorescence spectroscopy technique. Influence of the micelles on the photophysics of PZ has also been investigated using different approaches. The location of the fluorophore PZ in the micelle has been identified by cetyl pyridinium chloride (CpCl) induced fluorescence quenching and micropolarity surrounding that fluorophore in micellar solution. The effect of urea on the steady state fluorescence and relaxation dynamics of the micelle bound probe has also been observed. The results have been interpreted in terms of the model that urea displaces water molecules from the micellar interface and the consequent destabilization leads to the expulsion of the probe molecules from the interfacial region. An attempt has been made to determine probe sensing microviscosities for these micellar microenvironments in the light of average reorientation times of the probe PZ. [ABSTRACT FROM AUTHOR]

Aggregation behavior of a novel anionic amphiphilic molecule, sodium N-(2-(n-dodecylamino)ethanoyl)-l-alaninate (C12Ala), was studied in the presence of sodium dodecyl sulfate (SDS) surfactant at different C12Ala/SDS molar ratios and concentrations. The viscosity of aqueous SDS solution increased in the presence of C12Ala surfactant. The bulk viscosity of water was found to increase upon increase of both molar ratio and total surfactant concentration. The microenvironments of the self-assemblies were investigated using the fluorescence probe technique. Fluorescence anisotropy studies indicated formation of rodlike micelles. Both dynamic light scattering and small-angle neutron scattering measurements were performed to obtain the size and shape of the microstructures. The concentration and composition dependence of the hydrodynamic diameter of the aggregates were investigated. Transmission electron micrographs revealed the presence of a hexagonal liquid crystal phase in dilute solutions of the C12Ala−SDS mixture. The micrographs of moderately concentrated solution, however, showed cholesteric liquid-crystal structures with fingerprint-like texture. Temperature-dependent phase behavior of the self-assemblies was studied by use of the fluorescence probe technique. [ABSTRACT FROM AUTHOR]

This article deals with the solution properties of poly(vinylpyrrolidone) (PVP) in salt and surfactant environment. The cloud point (CP) of PVP has been found to be induced by the salts NaCl, KCl, KBr, Na2SO4, MgSO4, and Na3PO4. On the basis of CP values for a salt at different PVP, the energetics of the clouding process have been estimated. The effect of the surfactant, sodium dodecyl sulfate (SDS), on the salt-induced CP has also been studied, and reduction in CP at low SDS and increase in CP at high SDS have been observed. The water vapor adsorption of PVP has been determined by isopiestic method. The results display a BET Type III isotherm whose analysis has helped to obtain the monolayer capacity of PVP and formation of multilayer on it. The solvation of PVP in a solution of water and a water-isopropanol mixture has been determined by conductometry from which contribution of the individual components were estimated. The interaction of PVP with SDS in solution led to formation of a complex entity, which has been studied also by conductometry adopting a binding-equilibrium scheme. SDS has been found to undergo two types of binding as monomers in the pre- critical aggregation concentration (CAC) range and as small clusters in the post CAC region. The stoichiometries of binding and binding constant were evaluated. [ABSTRACT FROM AUTHOR]

Specific ion effects on micelles have been addressed by molecular dynamics simulations of Li+, Na+, and NH4+ cations with dodecyl sulfate (DS) micelles in aqueous solution. Of the three systems, the most compact structure for a dodecyl sulfate micelle is observed in the case of Li+ ions, which penetrate considerably deeper into the micelle than either Na+ or NH4+. Comparison of molecular trajectories of micellar solutions and those of methyl sulfate solutions with the same counterion demonstrates that the micelle surface is effective at disrupting the ionic hydration shells to allow direct cation-sulfate interaction. The structure of the second coordination shell of the sulfate groups on the micelle surface is, in turn, affected by the cations involved in contact ion pairs. The 4.9 Å peak in the sulfur-sulfur radial distribution function (rdf) in the LiDS micelle was attributed to direct coordination of two or more headgroups to a lithium ion. A second peak in the rdf is 1.2 Å farther, corresponding to a water molecule intervening between Li+ and one of the sulfates. The sulfur-sulfur rdfs for both NaDS and NH4DS feature single peaks at distances that suggest a lesser role of these cations in headgroup structuring. The direct coordination of the ions to the headgroups requires their dehydration and shows that hydrated-ion radii are inadequate for models of the micelle interfacial region. [ABSTRACT FROM AUTHOR]

Images by cryo-transmission electron microscopy show two types of structure in dilute aqueous mixtures of amylose (AM) and sodium dodecyl sulfate (SDS): Loose fibrous networks and unilamellar vesicles. Neither of the structures was observed in reference solutions of AM or SDS alone. It is proposed that both types of structure consist of inclusion complexes where SDS molecules are enclosed in V-type single helices of amylose. In the vesicles, the complexes are packed, by folding of the AM helices, into lamellae. [ABSTRACT FROM AUTHOR]