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Achieving a controlled preparation of nanoparticle superstructures with spatially periodic arrangement, also called superlattices, is one of the most intriguing and open questions in soft matter science. The interest in such regular superlattices originates from the potentialities in tailoring the physicochemical properties of the individual constituent nanoparticles, eventually leading to emerging behaviors and/or functionalities that are not exhibited by the initial building blocks. Despite progress, it is currently difficult to obtain such ordered structures; the influence of parameters, such as size, softness, interaction potentials, and entropy, are neither fully understood yet and not sufficiently studied for 3D systems. In this work, we describe the synthesis and characterization of spatially ordered hierarchical structures of coated cerium oxide nanoparticles in water suspension prepared by a bottom-up approach. Covering the CeO 2 surface with amphiphilic molecules having chains of appropriate length makes it possible to form ordered structures in which the particles occupy well-defined positions. In the present case superlattice arrangement is accompanied by an improvement in photoluminescence (PL) efficiency, as an increase in PL intensity of the superlattice structure of up to 400 % compared with that of randomly dispersed nanoparticles was observed. To the best of our knowledge, this is one of the first works in the literature in which the coexistence of 3D structures in solution, such as face-centered cubic (FCC) and Frank-Kasper (FK) phases, of semiconductor nanoparticles have been related to their optical properties., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Hypothesis: Implementation of tissue adhesives from natural sources endowed with good mechanical properties and underwater resistance still represents a challenging research goal. Inspired by the extraordinary wet adhesion properties of mussel byssus proteins resulting from interaction of catechol and amino residues, hydrogels from soy protein isolate (SPI) and selected polyphenols i.e. caffeic acid (CA), chlorogenic acid (CGA) and gallic acid (GA) under mild aerial oxidative conditions were prepared., Experiments: The hydrogels were subjected to chemical assays, ATR FT-IR and EPR spectroscopy, rheological and morphological SEM analysis. Mechanical tests were carried out on hydrogels prepared by inclusion of agarose. Biological tests included evaluation of the antibacterial and wound healing activity, and hemocompatibility., Findings: The decrease of free NH 2 and SH groups of SPI, the EPR features, the good cohesive strength and excellent underwater resistance (15 days for SPI/GA) under conditions relevant to their use as surgical glues indicated an efficient interaction of the polyphenols with the protein in the hydrogels. The polyphenols greatly also improved the mechanical properties of the SPI/ agarose/polyphenols hydrogels. These latter proved biocompatible, hemocompatible, not harmful to skin, displayed durable adhesiveness and good water-vapour permeability. Excellent antibacterial properties and in some cases (SPI/CGA) a favourable wound healing activity on dermal fibroblasts was obtained., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Halloysite nanotubes (HNTs) represent a versatile core structure for the design of functional nanosystems of biomedical interest. However, the development of selective methodologies for the site-controlled functionalization of the nanotubes at specific sites is not an easy task. This study aims to accomplish a procedure for the site-selective/specific, "pin-point", functionalization of HNTs with polydopamine (HNTs@PDA). This goal was achieved, at pH 6.5, by exploiting the basicity of ZnO nanoparticles anchored on the HNTs external surface (HNTs@ZnO) to induce a punctual polydopamine polymerization and coating. The morphology and the chemical composition of the nanomaterial was demonstrated by several techniques. Turbidimetric analysis showed that PDA coating affected the aqueous stability of HNTs@PDA compared to both HNTs@ZnO and HNTs. Notably, hyperthermia studies revealed that the nanomaterial induced a local thermic rise, up to 50 °C, under near-infrared (NIR) irradiation. Furthermore, secondary functionalization of HNTs@PDA by selective grafting of biotin onto the PDA coating followed by avidin binding was also accomplished., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Lipopolysaccharides (LPS) are complex amphiphilic macromolecules forming the external leaflet of the outer membrane of Gram-negative bacteria. The LPS glycolipid portion, named Lipid A, is characterized by a disaccharide backbone carrying multiple acyl chains. Some Lipid A bear very-long-chain-fatty-acids (VLCFA), biosynthesized to span the entire lipid membrane profile. The synbiontic Bradyrhizobium BTAi1 strain carries an unique Lipid A specie, named HoLA, in which VLCFA terminus is covalently-bonded to hopanoid, a triterpenoid displaying structural similarity with eukaryotic sterols. Here, we investigate the role of HoLA in regulating self-assembly, microstructure and thermotropicity of lipid membranes composed by 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-1'-rac-glycerol. DLS and SANS indicated the formation of multilamellar aggregates whose size increases when the hopanoid is present in the bilayer. EPR and DSC showed that HoLA induced a more rigid and ordered organization among the lipids in the bilayer, evocative of a mechanical strengthening. Notably, the presence of calcium ions promote an increase of the bilayer thickness and vesicles size, leading to low curvature aggregates. These results highlight the key role of the hopanoid covalently-linked to VLCFA in defining the physico-chemical properties of bacterial envelope, thus offering a robust scientific basis for the interpretation of the biological activity of the considered strain., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Hypothesis: The rational design of branched-tail surfactants is a suitable strategy to obtain low-viscosity surfactant-rich isotropic aqueous mixtures with negligible effects on biodegradability. This opens a way to the design of concentrated ("water-free") surfactant formulations, highly attractive for their ecological and economic benefits., Experiments: The aggregation behaviour of N,N-dimethyl-2-propylheptan-1-amine oxide (C 10 DAO-branched) in aqueous mixtures is investigated across the entire composition range by polarized optical microscopy, small angle X-ray and neutron scattering, electron paramagnetic resonance, and pulse-gradient stimulated echo nuclear magnetic resonance. The humidity scanning quartz crystal microbalance with dissipation monitoring technique is validated as a tool for the fast screening of surfactants phase behaviour. Furthermore, the shear viscosities and viscoelastic moduli of the systems are determined by rheological measurements., Findings: With respect to the linear isomer, C 10 DAO-branched presents a much lower tendency to form lyotropic liquid crystalline phases. Except for a narrow composition and temperature range in which a lamellar structure is observed, C 10 DAO-branched aqueous mixtures are isotropic liquids whose microstructure changes, with increasing concentration, from micellar solutions to unstructured dispersions of hydrated surfactant molecules. Low-viscosity was found for all these mixtures, including the most concentrated ones. Thus, the introduction of a single short side-chain in the tail is demonstrated to be an effective approach to increase the active concentration in surfactant formulations., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Hypothesis: Catanionic vesicles based on large-scale produced surfactants represent a promising platform for the design of innovative, effective and relatively inexpensive nano-vehicles for a variety of actives. Structural, dynamic and functional behavior of these aggregates is finely tuned by the molecular features of their components and can be opportunely tailored for their applications as drug carriers., Experiments: Here we investigate the aggregates formed by CTAC and SDS, two of the most diffused surfactants, by means of Dynamic Light Scattering, Small Angle Neutron Scattering and Electron Paramagnetic Resonance spectroscopy (EPR). The exploitation of these aggregates as nano-vehicles is explored using the poorly water-soluble antioxidant trans-resveratrol (t-RESV), testing t-RESV solubility and antioxidant activity by means of UV, fluorescence spectroscopy and EPR., Findings: The presence of a large stability region of catanionic vesicles on the CTAC-rich side of the phase diagram is highlighted and interpreted in terms of the mismatch between the lengths of the surfactant tails and of first reported effects of the chloride counterions. CTAC-SDS vesicles massively solubilize t-RESV, which in catanionic vesicles exerts a potent antioxidant and radical-scavenging activity. This behavior arises from the positioning of the active at the surface of the vesicular aggregates thus being sufficiently exposed to the external medium., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Nanostructured hydrogels composed by biocompatible molecules are formulated and characterized. They are based on a polymer network formed by hydrophobically modified chitosans (HMCHIT or CnCHIT) in which vesicles of monoolein (MO) and oleic acid or sodium oleate (NaO), depending on pH, are embedded. The best conditions for gel formation, in terms of pH, length of the hydrophobic moieties of chitosan, and weight proportion among the three components were estimated by visual inspection of a large number of samples. Among all possible combinations, the system C12CHIT-MO-NaO in the weight proportion (1:1:1) is optimal for the formation of a well-structured gel-like system, which is also confirmed by rheological experiments. Electron paramagnetic resonance (EPR) measurements unambiguously show the presence of lipid bilayers in this mixture, indicating that MO-NaO vesicles are stabilized by C12CHIT even at acid pH. A wide small angle neutron scattering investigation performed on several ternary systems of general formula CnCHIT-MO-NaO shows that the length of the hydrophobic tail Cn is a crucial parameter in stabilizing the polymer network in which lipid vesicles are embedded. Structural parameters for the vesicles are determined by using a multilamellar model that admits the possibility of displacement of the center of each shell. The number of shells tends to be reduced by increasing the polymer content. The thickness and the distance between consecutive lamellae are not influenced by either the polymer or MO-NaO concentration. The hydrogel presented in this work, being fully biocompatible and nanostructured, is well-suited for possible application in drug delivery., (Copyright © 2015 Elsevier Inc. All rights reserved.)

The micro- and mesoscopic structure of reverse Pluronic 25R4 in aqueous mixtures has been studied by SANS, SAXS and shear rheology. These techniques have been able to give a deep insight into the complex structure of the system phase diagram, that includes an isotropic water-rich liquid phase L(1), and liquid crystalline phases with hexagonal, E, or lamellar order, D. Particular attention has been paid to the isotropic water-rich phase L(1), which has a large stability region in the temperature-composition phase diagram. This region is crossed by a large "cloudy zone". Below it, namely at low temperature and composition, SANS data show the presence of polymer unimers in a gaussian coil conformation. Above the "cloudy zone", at higher temperature and composition, the L(1) phase is structured as a network of interconnected multimeric micelles. Rheology adds information about the structuring of the L(1) phase showing its incipient hexagonal pre-structuring. This technique is also able to highlight the defective structure of the E phase itself. In the temperature and concentration ranges in which a lamellar phase D is present, SANS and SAXS results are in complete agreement, showing how interlamellar distance is influenced by both polymer composition and temperature according to an "ideal deswelling" or a "not ideal swelling" mechanism. In addition, in the D phase rheology suggests the presence of densely packed vesicles., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Ionophores are an important class of synthetic molecules which mimic natural ion channels or carriers. Here we report the aggregation behavior in pseudo-physiological environment of three Cyclic Phosphate-Linked Oligosaccharides (CyPLOS) derivatives, synthetic ion transporters based on cyclic, phosphate-linked disaccharide skeleton differing for the nature of the tails (tetraethylene-TEG glycol and/or n-undecyl chains) attached to the C-2 and C-3 of the constitutive monosaccharides. Their aggregation behavior has been studied by a combined use of dynamic light scattering (DLS), electron paramagnetic resonance spectroscopy (EPR) and Small Angle Neutron Scattering (SANS). DLS measurements were performed to reveal the formation and size distribution of the CyPLOS aggregates. EPR measurements, by using 5-doxyl stearic acid (5-DSA) as spin-probe, showed that the aggregates are mainly due to the formation of double layers and allowed to analyze the local fluidity. Finally, SANS measurements allowed estimating the layer thickness of the double layers. Our results indicate that the three CyPLOS analogs show self-aggregation properties that depend on the different nature of the inserted tails., (Copyright © 2010 Elsevier Inc. All rights reserved.)

The present work deals with an interpretation of the differences in micellar structure of two surfactants, sodium decyl sulfonate and sodium decyl sulfate, whose only difference in molecular structure is the presence, for sulfate, of an oxygen atom bridging alkyl chains from a common SO(3)(-) charged head group. Using a simple geometrical approach, a different exposure of the surfactant head groups on the micellar surface is proposed. It is also shown that the different interphase microstructure could be the reason that decyl sulfate forms ellipsoidal aggregates, in contrast to decyl sulfonate, which forms nearly spherical micelles., (Copyright 2010 Elsevier Inc. All rights reserved.)

Differential scanning calorimetry (DSC), dynamic light scattering (DLS), and turbidity measurements have been used to study the effect of sodium decylsulfate, C10OS, on 1,2-dipalmitoylphosphatidylcholine, DPPC, vesicles. DSC measurements, performed soon after addition of C10OS solutions to DPPC vesicular dispersion, at several C10OS/DPPC molar ratios, showed the contemporary presence of maxima ascribable both to transitions of an unmodified DPPC membrane and to transitions related to membrane domains in which C10OS is also present. In all the considered samples only unilamellar vesicles are present as shown by DLS measurements carried out on aqueous DPPC and C10OS-DPPC systems. Also the aging of the samples confirmed the presence of SUV in the time scale of DSC measurements. On the basis of DSC results the pseudo-phase diagram of the system was drawn. This diagram does not represent a system in equilibrium, from a thermodynamic point of view, but only kinetically probable states. In fact the prolonged aging of the solutions used both for DSC and DLS scans highlight the disappearance of the unmodified DPPC membrane and its conversion into a mixed membrane. Turbidity measurements at 25 degrees C, performed as a function of surfactant composition, showed that C10OS initially contributes to increase the vesicles dimensions and allowed to calculate the saturation ratio C10OS/DPPC in the membrane. In contrast, at 45 degrees C the presence of surfactant gives origin immediately to smaller structures in comparison to vesicles, probably gel-state micelles., (2009 Elsevier Inc. All rights reserved.)

Lysophosphatidylcholines are widely used as biomembrane mimics. In order to furnish a structural basis for this application, in this work the self-aggregation behaviour of n-acyl-lysophosphatidylcholines (C(n)lysoPC, n=6,8,10,12), in aqueous solution has been investigated by the PGSTE-NMR and spin probing EPR techniques at 25 degrees C. The experimental data show that C(n)lysoPCs behave as zwitterionic surfactants, and permit evaluation of the influence of the acyl chain length on the phospholipid micellization. For all the C(n)lysoPCs considered, the phospholipid intradiffusion coefficient trend shows a slope change corresponding to the critical micelle concentration (CMC). In the micellar composition range, solubilized tetramethylsilane (TMS) molecules were used to determine the micelle intradiffusion coefficient, from which the aggregate radii and the aggregation numbers were obtained. The solvent intradiffusion coefficient in the C(n)lysoPC aqueous mixtures has been also measured. The results show that the C(n)lysoPC micelles present a thick external layer constituted by strongly hydrated glycerophosphocholine groups. The ability of this layer to embed either anionic or cationic guest molecules has been studied by EPR spectroscopy, employing 3-carboxy-PROXYL in its deprotonated form (CP(-)) or TEMPO-choline (TC) as spin probes. In all the considered systems, the nitrogen isotropic hyperfine coupling constant of the spin probe, A(N), decreases and the correlation time, tau(C), increases with increasing phospholipid molality. The results show that C(n)lysoPC micelles can establish a variety of interaction with different guests. In fact, CP(-) anions interact with the C(n)lysoPC choline groups adsorbing on the micelle surface, while TC cations interacting with the C(n)lysoPC phosphate groups are embedded in thick external layer of the micelles.

The effect of the polymer molecular weight on the interaction between pentaethylene glycol n-octyl ether (C(8)E(5)) and poly(acrylic acid) (PAA) has been investigated by a combined experimental strategy including tensiometry, potentiometry, calorimetry, fluorescence quenching and intradiffusion (pulsed gradient spin echo-NMR) measurements. PAA samples with an average molecular weight varying in a wide range (M (w)=2000, 100,000, 250,000, and 450,000) have been considered. The measurements have been performed at constant polymer concentration (0.1% w/w) with varying surfactant molality. In all the considered systems, at low surfactant concentration, adsorption of surfactant monomers onto the polymer chain has been detected. At a C(8)E(5) molality (T(1)) independent of the PAA M (w), surfactant molecules start to aggregate, forming clusters to which the polymer co-participates. Above this concentration, the behavior of the system depends on M (w). In fact, if polymer samples with high molecular weight (M (w)100,000) are employed, all the added surfactant aggregates onto the polymer leading to the polymer saturation and, subsequently, to free micelles formation. Both saturation and free micellization occur at surfactant concentrations which are independent of the polymer molecular weight. C(8)E(5) aqueous mixtures containing PAA with low molecular weight (M (w)=2000) behaves differently, in that, above T(1), only a fraction ( approximately 20%) of the added surfactant molecules interact with the polymer, forming aggregates to which more than one PAA chain participate. In this case, C(8)E(5) free micellization occurs before polymer saturation. The experimental evidences have been interpreted in terms of the subtle balance between the various molecular interactions driving the surfactant-polymer aggregation.

The interaction in aqueous solution between either the normal block copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide): Pluronic PE6200 [(EO)(11)-(PO)(28)-(EO)(11)], or the reverse block copolymer poly(propylene oxide)-poly(ethylene oxide)-poly(propylene oxide): Pluronic 25R4 [(PO)(19)-(EO)(33)-(PO)(19)] and the surfactants sodium decylsulfate, C(10)OS, decyltrimethyl ammonium bromide, C(10)TAB, and pentaethylene glycol monodecyl ether, C(10)E(5), was investigated and the aggregation behavior of these surfactants with Pluronics was compared. Surface tension measurements show that Pluronics in their non-aggregated state better interact with the anionic surfactant C(10)OS than with cationic and non-ionic ones. The presence of the two Pluronics induces the same lowering of the aggregation number of C(10)OS as shown by fluorescence quenching measurements. The number of polymer chains necessary to bind each C(10)OS aggregate has been estimated to be approximately 6 for PE6200 and approximately 2 for 25R4. Furthermore, this surfactant also induces the same increment in the gyration radius of the polymers as revealed by viscosimetry. Calorimetric results have been reasonably reproduced by applying a simple equilibrium model to the aggregation processes.

The effect of glycerol on the micellization of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and of the ethoxylated nonionic surfactant Brij 58 has been investigated by various experimental techniques. For both surfactants the critical micellar concentration (cmc), determined by surface tension measurements, is almost unaffected by the presence of glycerol in the mixture; only at high glycerol concentrations (>/=20% w/w) does the cmc significantly increase. The area per surfactant molecule at the air-solution interface, A, increases with increasing glycerol weight percentage, w(g). Fluorescence quenching measurements indicate that the presence of glycerol induces a lowering of the aggregation number of both surfactants. The glycerol intradiffusion coefficient has been measured by the pulsed-gradient spin-echo NMR technique as a function of glycerol content at constant surfactant concentration. It is almost unaffected by the presence of the surfactants, indicating that no direct glycerol-surfactant interaction occurs in the mixture. The surfactant intradiffusion coefficient has been also measured. In the case of CTAB, it increases with increasing glycerol concentration, a reflection of the decreased aggregation number. For Brij 58, in spite of the lowering of the aggregation number, the surfactant intradiffusion coefficient decreases with increasing glycerol concentration, suggesting an increase of the intermicellar interaction. The experimental evidence shows that for both surfactants the micellization is affected by the presence of glycerol through an indirect, solvent-mediated mechanism. In the case of CTAB, the main effect of glycerol is a lowering of the medium dielectric constant, which enhances the electrostatic interactions in solution. In the case of Brij 58, the results can be interpreted in terms of a salting-out effect according to which glycerol competes with the surfactant for water molecules, causing a dehydration of the surfactant ethoxylic headgroup.

The phase behavior (temperature vs composition) and microstructure for the two binary systems Pluronic 25R4 [(PO)19(EO)33(PO)19]-water and Pluronic 25R2 [(PO)21(EO)14(PO)21]-water have been studied by a combined experimental approach in the whole concentration range and from 5 to 80 degrees C. The general phase behavior has been identified by inspection under polarized light. Precise phase boundaries have been determined by analyzing 2H NMR line shape. The identification and microstructural characterization of the liquid crystalline phases have been achieved using small-angle X-ray scattering (SAXS). The isotropic liquid solution phases have been investigated by self-diffusion measurements (PGSE-NMR method). 25R2 does not form liquid crystals and is miscible with water in the whole concentration range; with increasing temperature, the mixtures split into water-rich and a copolymer-rich solutions in equilibrium. 25R4 shows rich phase behavior, passing, with increasing copolymer concentration, from a water-rich solution to a lamellar and copolymer-rich solution. A small hexagonal phase, completely encircled in the stability region of the water-rich solution, is also present. In water-rich solutions, at low temperatures and low copolymer concentrations, the copolymers are dissolved as independent macromolecules. With increasing copolymer concentrations an interconnected network of micelles is formed in which micellar cores of hydrophobic poly(propylene oxide) are interconnected by poly(ethylene oxide) strands. In copolymer-rich solutions water is molecularly dissolved in the copolymer. The factors influencing the self-aggregation of Pluronic R copolymers (PPO-PEO-PPO sequence) are discussed, and their behavior in water is compared to that of Pluronic copolymers (PEO-PPO-PEO sequence).

The interaction between pentaethylene glycol n-octyl ether (C8E5) and low-molecular-weight poly(acrylic acid) (PAA, M(w)=2000) in aqueous solution has been investigated by various experimental techniques at constant polymer concentration (0.1% w/w) with varying surfactant molality. Spectrofluorimetry, using pyrene as molecular probe, shows (i) the formation of surfactant-polymer aggregates at a surfactant molality (T(1)) lower than the critical micelle concentration (cmc) of C8E5 in water and (ii) the formation of free micelles at a surfactant molality (T(2)) slightly higher than the cmc. Fluorescence quenching measurements indicate that the presence of PAA induces a lowering of the C8E5 aggregation number. Calorimetry confirms spectrofluorimetric evidence; in addition, it shows the presence of weak interactions below T(1) between monomeric surfactant molecules and the polymer chains. Tensiometry shows that, above T(1), only a low fraction of surfactant molecules interact with the polymer and that free micelle formation occurs before polymer saturation. The peculiarities of the interaction between surfactants and low-molecular-weight polymers have been discussed.

Apparent molar relative enthalpies were measured for the nonionic ethoxylated surfactant CH(3)-(CH(2))(5)-(OCH(2)-CH(2))(5)OH (C(6)E(5)) in aqueous solution at constant molality of the ionic surfactant CH(3)-(CH(2))(5)-SO(-)(3)Na(+)(C(6)SNa) at 25 degrees C. The experimental data obtained by a stepwise dilution process allowed evaluation of the C(6)E(5) first interaction parameter at several constant molalities of C(6)SNa. The C(6)E(5) critical micelle composition as a function of the C(6)SNa molality was also estimated. The experimental calorimetric data, together with the mixed micelles composition computed in the past by some of us [Ciccarelli et al., Langmuir 14, 7130 (1998)], allowed computation of the Deltah(Mic) of micellization. The experimental data are compared to those predicted by the ideal solution model and regular solution model of mixed micellization. From a calorimetric study performed on the water-hexanol-C(6)SNa and water-penthaethylene glycol-C(6)SNa model systems, it can be argued that the interactions among the hydrophilic heads in the C(6)E(5)-C(6)SNa mixed micelles prevail on the contribution of the hydrophobic tails in ruling the enthalpic properties of the system.

Intradiffusion coefficients, D, of n-alkyltrimethylammonium bromides [CH(3)-(CH(2))(n-1)-N(CH(3))(3)Br, C(n)TAB] (n=6, 8, 10, 12) in mixtures with heavy water were measured by the PGSE-NMR technique at 25 degrees C. The experimental data permitted evaluation of the influence of the alkyl chain length on the surfactant self-aggregation process. For all the surfactants considered, the D trend showed a slope change corresponding to the critical micellar composition (cmc). In the premicellar composition range, D decreased linearly with the square root of the surfactant molality. The D values extrapolated at infinite dilution were related to the limiting mutual diffusion coefficients, determined through the Taylor dispersion technique. In the micellar composition range, solubilized tetramethylsilane (TMS) molecules were used to determine the micelle intradiffusion coefficient, D(M), from which the aggregate radii and the aggregation numbers were obtained. The decreasing trend of D(M) with increasing surfactant molality was interpreted in terms of interparticle electrostatic repulsion. D(M) values allowed evaluation of the Gouy-Chapman layer thickness. The solvent intradiffusion coefficient in the heavy water-C(n)TAB mixtures, D(w), was also measured. It decreased with increasing surfactant molality. For n=8, 10, 12 the D(w) trend presented a slope change at the cmc, which could be ascribed to the strong decrease in hydration of surfactant molecules upon micellization. Because of its short hydrophobic tail, C(6)TAB exhibited peculiar aggregation behavior. Its cmc, which is poorly marked, is lower than the value predicted by extrapolating the cmc values obtained for the other terms of the series. The C(6)TAB aggregates do not solubilize TMS molecules; the estimated aggregation number is extremely low ( approximately 3). Finally, no abrupt slope change in the solvent intradiffusion coefficient trend was detected. This evidence suggests that C(6)TAB molecules do not micellize in aqueous solution, but form trimers in which the surfactant hydrophobic tails are not hidden from contact with water molecules. Copyright 2001 Academic Press.

Intradiffusion coefficients of sodium alkylsulfonates [CH3-(CH2)n-1SO-3 Na+, CnSNa] (n = 5-9, 11) in mixtures with heavy water were measured by the PGSE-NMR technique at 25 degrees C. A slope change in the experimental trends permits the determination of the critical micelle concentration (CMC). In the micellar composition range, solubilized TMS molecules were used to determine the micelle intradiffusion coefficient, from which the micelle radii were obtained. Both the monomer surfactant and the micelle intradiffusion coefficients show a sharp decrease above the CMC. These results can be interpreted in terms of the obstruction effect due to the micelles. The electrostatic repulsion among charged particles strongly enhances this effect. A simple approach that permits the computation of the Gouy-Chapman layer thickness from the experimental coefficients has been proposed and the results are briefly discussed. Copyright 1999 Academic Press.

Isothermal mutual diffusion coefficients (D) have been measured for binary aqueous solutions of sodium alkylsulfonates at 25 degrees C. The diffusion coefficient values drop as the concentration of micelles in the system increases. As the length of the hydrocarbon chain increases, the observed drop in D becomes increasingly sharp and shifts to lower concentrations. When the micellization process is treated as a chemical equilibrium, diffusion coefficients lead to the calculation of the thermodynamic parameters association number, n, and the equilibrium constant, K. These parameters are briefly discussed in connection with the alkyl chain length of the surfactants. Copyright 1999 Academic Press.

The densities and viscosities of binary aqueous mixtures of poly(ethylenoxide)hexanols [C6 H13 (OCH2 CH2 )m OH, C6 Em ] (m = 3, 4, and 5) have been studied in the micellar composition range. For the same surfactants the self-diffusion coefficients in mixtures with heavy water have been determined by the spin-echo pulsed field gradient method. The volumetric data are interpreted by means of the phase separation model, and values of the CMC, volume change, and standard free energy change of micellization are obtained. From the viscosity data the hydration numbers of the surfactant hydrophilic head in the micellar state are computed; they are in agreement with those obtained from HDO self-diffusion data. The surfactant self-diffusion data are used to calculate the apparent micelle radius and the aggregation number. The micellization parameters obtained for the different surfactants are compared and discussed.