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The surface freezing transition of a mixed adsorbed film containing cetyltrimethylammonium chloride (CTAC) and n-hexadecanol (C16OH) was utilized at the dodecane-water interface to control the stability of oil-in-water (O/W) emulsions. The corresponding surface frozen and surface liquid mixed adsorbed films were characterized using interfacial tensiometry and X-ray reflectometry. The emulsion samples prepared in the temperature range of the surface frozen and surface liquid phases showed a clear difference in their stability: the emulsion volume decreased continuously right after the emulsification in the surface liquid region, while it remained constant or decreased at a much slower rate in the surface frozen region. Compared to the previously examined CTAC-tetradecane mixed adsorbed film, the surface freezing temperature increased from 9.5 to 25.0 °C due to the better chain matching between CTAC and C16OH and higher surface activity of C16OH. This then renders such systems much more attractive for practical applications.

A condensed film formation of surfactants with a charged head group at the oil/water interface was achieved by mixing surfactants of different geometric shapes to control molecular packing at the interface. The adsorbed films of mixed tetradecyltrimethylammonium bromide (C14TAB)-cholesterol (Chol) and tetradecylphosphocholine (C14PC)-Chol systems at the hexane/water interface were examined by interfacial tension and X-ray reflectivity measurements. The interfacial tension versus Chol concentration curves have break points because of the expanded-condensed phase transition of the adsorbed film. A two dimensional (2D) phase diagram, phase diagram of adsorption, indicated that 1:1 mixing in the condensed film is energetically favorable because of stronger mutual interaction between different molecules than between the same ones. The electron density profile normal to the interface manifested that the packing of C14TAB (or C14PC) and Chol molecules is like a 2D solid in the condensed state. As C14TAB and C14PC molecules take a corn shape with a large head group (critical packing parameter: CPP ≈ 1/3) and Chol takes an inverted corn shape with a bulky sterol ring (CPP1), the mixing of corn shape and inverted corn shape molecules produces well-ordered packing to promote solid-like molecular packing at the interface by energy gain because of vdW interaction between hydrophobic chains in addition to attractive ion-dipole interaction between head groups. Furthermore, the heterogeneous feature in the adsorbed film of the C14TAB-Chol system is explained by an interplay between contact energy and dipole interaction, which contribute to line tension at the domain boundary.

Competitive binding of binary mixed counterions to the headgroups of adsorbed surfactant films has been investigated at solution surfaces by total reflection X-ray absorption fine structure (XAFS) spectroscopy. The obtained extended XAFS χ spectra for bromide counterions are linear combinations of the spectra of fully hydrated bromide ions (free Br) and partially dehydrated bromide ions bound to the headgroups of the surfactant ions (bound Br). From the fraction of bound Br in counterion mixed systems, two series of the relative strengths of counterion binding are proposed for the trimethylammonium (TA+) and 3-methylimidazolium (MIM+) headgroups: (a) TA–SO4 < TA–Cl < TA–Br < TA–BF4 and (b) MIM–Br < TA–Br < TA–BF4 < MIM–BF4. For the TA headgroup, matching the hydration of the headgroups and counterions gives series (a) according to Collins’ law, which states that the tendency of contact ion pair formation becomes larger when the absolute values of the hydration enthalpies of the ions match. For the MIM hea...

We investigated the surface dilational viscoelasticities of aqueous solutions of dihexanoyl-phosphastidylcholine (DC6PC) and dioctanoyl-phosphastidylcholine (DC8PC), dodecyltrimethylammonium bromide (DTABr) and DTABr and sodium bromide (NaBr), dodecyltrimethylammonium tetrafluoroborate (DTABF4) and bisdodecyltrimethylammonium sulfate (DTA2SO4), and nonionic surfactants with oxyethylene units (C10E5). We measured surface quasi-elastic light scattering (SQELS) spectra and analyzed the solutions by both surface rheological and phenomenological fitting approaches. For the DC6PC-DC8PC mixture, the electrocapillary wave (ECW) method was also applied. The molecular interactions between DC8PC and DC6PC in the adsorbed film clearly influenced the relaxation process on the order of several milliseconds (ECW). However, the effects were less influential on the order of several hundreds of microseconds (SQELS). The characteristic frequencies for relaxation owing to the staggered structure formation likely occurred in the range from 150 to 300 Hz. The dilational elasticity at 250–400 kHz of DTABr suggested that the main relaxation process influencing the dilational properties was neither diffusion of surfactants from the bulk phase to the subsurface nor an adsorption-desorption of surfactant ions between the adsorbed layer and subsurface. However, the diffusion of counterions between the Stern and diffuse layers occurred on a time scale of several microseconds. Studies of C10E5 suggested that the relaxation process captured by the SQELS studies was neither related to surfactant diffusion nor surfactant reorientation, but rather was likely to be a relaxation process where distortion of the hydration sphere and/or hydration-dehydration to/from the hydration sphere was caused by diffusion of water molecules.

The surface freezing (SF) of liquid 6-perfluorohexyl hexanol (F6H6OH)–1-dodecanol (C12OH) mixture was studied by surface tension, external reflection absorption FTIR (ERA-FTIR), and X-ray reflectivity (XR) measurements. The surface tension vs. temperature curve of pure C12OH liquid shows a sharp break point at the surface liquid (SL)–SF phase transition temperature Ts. The surface tension of pure F6H6OH, on the other hand, decreases monotonically with increasing temperature. The entropy of surface formation in the SF state of C12OH is large negative due to a well-ordered structure at the surface. The electron density profile determined by XR showed that the C12OH molecules form bilayer stabilized by the hydrogen bonding between hydroxyl groups facing each other through water molecule intercalated in between them. In the F6H6OH–C12OH system, the estimation of surface composition indicated that the SL state is enriched in F6H6OH compared to bulk liquid. The negative excess entropy in the SL layer suggests that F6H6OH molecules form domains at the surface. Furthermore, the surface composition and electron density profile in the SF state indicated that a small amount of F6H6OH mix with C12OH in the SF bilayer in which F6H6OH molecules preferentially exist in the upper layer of the bilayer.

The effect of oil on condensed film formation in the adsorbed film of hexadecyltrimethylammonium bromide (C16TAB) at the tetradecane (C14)/water (W) interface was examined by interfacial tension and X-ray reflectivity measurements. The interfacial tension vs temperature curves have break point due to the expanded?condensed phase transition of the adsorbed film. The partial molar entropy of C16TAB at the interface changes discontinuously, whereas the interfacial density changes almost continuously at the phase transition point. The electron density profile normal to the interface manifested that the condensed film is regarded as a two-dimensional (2D) solid rotator phase in which C16TAB and C14 molecules are densely packed with perpendicular orientation. Combining the interfacial tension and X-ray reflectivity data, the mixing ratio of C16TAB to C14 in the solid film was determined to be 2:3 and thus the film is enriched in oil molecules than surfactant ones. Furthermore, the partial molar entropy change of C14 associated with solid film formation was found to be largely negative and very close to that of surface freezing of liquid alkane, manifesting that C14 molecules are well ordered to form a 2D solid film by mixing with C16TAB molecules at the interface. The solid film formation of the present system is driven by effective vdW interactions between adsorbed C16TAB and intercalated C14 molecules. The morphology of the condensed domain observed during phase transition suggested that the contact energy is more predominant than the dipole repulsion at the domain boundary, which promotes coalescence of small domains into large ones during phase transition.

Intermolecular forces are classified into three categories; (1) purely electrostatic Coulomb force, (2) polarization force, and (3) quantum mechanical force. The interaction between charges, permanent dipoles, etc. belongs to the first category. The polarization force is the interaction arises from the dipole moments induced in atoms and molecules by the electric field of charges and permanent dipoles.

The adsorbed film of n-tetradecylphosphocholine (C14PC) at the tetradecane (C14)/aqueous solution (W) interface was studied by interfacial tensiometry and X-ray reflection (XR). The interfacial tension γ was measured as a function of molality m C 14 PC and temperature T under atmospheric pressure. The interfacial density of C14PC Γ C 14 PC H increases with increasing m C 14 PC and converges into 3 μmol m−2 (first saturation) at m C 14 PC ≈ 0.08 mmol kg − 1 and into 4.2 μmol m−2 (second saturation) at the critical micelle concentration (CMC). The electron density profile determined by XR indicated that C14PC molecules form monolayer with relatively loose packing at the first saturation. In the second saturation, on the other hand, the molecules form bilayer in which charge separated phosphocholine groups take upside-down arrangement to interact attractively between neighbors. The positive entropy changes associated with adsorption both from monomeric and from micellar states result from that the contribution of dehydration around C14PC molecules, especially around PC groups, by the adsorption from aqueous solution overcomes that of a comparatively ordered molecular orientation at the interface. By comparing the results at the C14/W interface with those at the Air (A)/W one, it was shown that in the monolayer and bilayer states, hydrophobic chains of C14PC molecules are more ordered at the C14/W than at the A/W interface because of attractive interaction between C14PC and C14 molecules. This leads to smaller value of the partial molar entropy and energy changes associated with the adsorption at the C14/W than at the A/W interface. Furthermore the entropy of micelle formation changes from positive to negative with increasing T, suggesting that the entropy gain due to the dehydration exceeds the entropy loss by the aggregation at low temperatures, while the gain is less than the loss at high temperature.

In this review we examine the influence of the line tension τ on droplets and particles at surfaces. The line tension influences the nucleation behavior and contact angle of liquid droplets at both liquid and solid surfaces and alters the attachment energetics of solid particles to liquid surfaces. Many factors, occurring over a wide range of length scales, contribute to the line tension. On atomic scales, atomic rearrangements and reorientations of submolecular components give rise to an atomic line tension contribution τatom (∼1 nN), which depends on the similarity/dissimilarity of the droplet/particle surface composition compared with the surface upon which it resides. At nanometer length scales, an integration over the van der Waals interfacial potential gives rise to a mesoscale contribution |τvdW| ∼ 1–100 pN while, at millimeter length scales, the gravitational potential provides a gravitational contribution τgrav ∼ +1–10 μN. τgrav is always positive, whereas, τvdW can have either sign. Near wetting, for very small contact angle droplets, a negative line tension may give rise to a contact line instability. We examine these and other issues in this review.

The effect of molecular orientation on the miscibility and structure of the adsorbed film of the 1H,1H,10H,10H-perfluorodecane-1,10-diol (FC10diol)-1H,1H,2H,2H-perfluorodecanol (FC10OH) mixture at the hexane/water interface were examined by interfacial tension and X-ray reflectivity measurements. The interfacial tension and X-ray reflectivity at the hexane solution/water interface were measured as a function of total molality m and composition of FC10OH in the mixture X2 under atmospheric pressure at 298.15 K. The interfacial pressure vs mean area per molecule curves showed that two kinds of condensed monolayers (C1 and C2) and multilayer (M) states appeared depending on m and X2. In the pure component systems, it was found that FC10OH forms condensed monolayer in which the molecules orient almost normally to the interface, and FC10diol orients parallel and is densely packed in the condensed monolayer and then piles spontaneously to form multilayer. In the mixed system, the phase diagram of adsorption indicated that FC10OH molecules are richer in C2 than in C1 state. The X-ray reflectivity measurements manifest that the condensed monolayer below X2 = 0.985 is heterogeneous in which the normal- and parallel-oriented domains coexist at the interface (C1 state), and that above X2 = 0.985 seems to be homogeneous with normal molecular orientation (C2 state). The structure of M state depends on those of condensed monolayers, on which the molecules pile spontaneously. The heterogeneous structure in C1 state is compared to that previously observed in the mixed system of FC10diol-FC12OH (1H,1H,2H,2H-perfluorododecanol), where FC12OH has longer fluorocarbon chain length than FC10OH and is discussed in terms of domain line tension.

Alkane droplets on aqueous solutions of surfactants exhibit a first-order wetting transition as the concentration of surfactant is increased. The low-concentration or “partial wetting” state corresponds to an oil lens in equilibrium with a two-dimensional dilute gas of oil and surfactant molecules. The high-concentration or “pseudo-partial wetting” state consists of an oil lens in equilibrium with a mixed monolayer of surfactant and oil. Depending on the combination of surfactant and oil, these mixed monolayers undergo a thermal phase transition upon cooling, either to a frozen mixed monolayer or to an unusual bilayer structure in which the upper leaflet is a solid layer of pure alkane with hexagonal packing and upright chains while the lower leaflet remains a disordered liquid-like mixed monolayer. Additionally, certain long-chain alkanes exhibit a surface freezing transition at the air–oil interface where the top monolayer of oil freezes above its melting point. In this review, we summarize our previous studies and discuss how these wetting and surface freezing transitions influence the line tension of oil lenses from both an experimental and theoretical perspective.

The composition of the adsorbed film and the excess Gibbs energy of adsorption $$ {\widehat{g}}^{\mathrm{H},\mathrm{E}} $$ were evaluated from thermodynamic analysis of surface tensions for the 1-decyl-3-methylimidazoulium bromide (C10mimBr)–tetraethylene glycol monooctyl ether (C8E4) and 1-decyl-3-methyl-imidazolium tetrafluorobrorate (C10mimBF4)–C8E4 systems, where the counter anion of imidazolium salts is different from each other. The higher miscibility of two components compared to an ideal mixing and thus negative $$ {\widehat{g}}^{\mathrm{H},\mathrm{E}} $$ were observed in the former, which comes from the ion–dipole interaction between imidazolium cation and the oxyethylene group of C8E4. On the other hand, the lower miscibility and thus positive $$ {\widehat{g}}^{\mathrm{H},\mathrm{E}} $$ were observed for the latter. Such differences were attributed to that BF4 − forms two hydrogen bonds and has stronger affinity with the cationic head group of C10mim+ than Br−. This results in that the ion–dipole interaction between C8E4 and C10mim+ cation is diminished in the C10mimBF4–C8E4 system.

We investigated miscibility of ionic liquids in the adsorbed film by surface tensiometry from the viewpoint of effect of the hydrogen bonding between cation and anion. The ion pair formation accompanied by two hydrogen-bond between cation and anion was indicated in 1-hexyl-3-methylimidazolium tetrafluoroborate (C6MIMBF4) system and also in hexylpyridinium tetrafluoroborate (C6PyBF4) system, the ion pairs of which structure is similar to that of C6MIMBF4 systems was suggested. The phase diagram of adsorption (PDA) of the ionic liquid–ionic liquid mixed system (C6PyBF4–C6MIMBF4) showed an ideal mixing. On the other hand, those of ionic liquid–alcohol mixed systems (C6MIMBF4–1-butanol, C6PyBF4–1-butanol) showed positive deviation in the adsorbed film. Judging from the values of excess Gibbs energy of adsorption of these systems, we concluded that the magnitude of the two hydrogen-bonds of C6PyBF4 and C6MIMBF4 is not significantly different, but the one between C6MIM+ and BF4− is slightly larger than that between C6Py+ and BF4−. The difference should be caused by the magnitude of difference of atomic charge distributions in the molecules owing to nitrogen atom in the ions, especially in the C(2)–H.

In the present work, a new copper surfactant complex [Cu(BrCl2)(C12H25N(CH3)3)] was synthesized and characterized. Its organization at the air–water interface and in bulk aqueous medium was investigated. The aggregation behavior and adsorption pattern of the metallosurfactant revealed important information about the molecular organization and counter ion distribution, which until now was unknown. The critical micellization concentration and optical thickness of the copper surfactant were determined using surface tension, cyclic voltammetry and ellipsometry techniques. The X-ray absorption fine structure technique was applied to the copper surfactant solution under both total reflection (TRXAFS) and transmission conditions to understand the ion organization at the interface and in the bulk, respectively. The Br and Cu K-edge absorption revealed the presence of Br− ions and the absence of Cu2+ ions at the interface and in the inner core of micelles. Furthermore, Br− ions were found to exist in two solvation states, designated as “free-Br” and “bound-Br”. Even the surface excess concentration of the ions was precisely determined from the adsorbed film of copper surfactant using external reflection absorption (ERA)-FTIR spectroscopy.

The temperature-dependent conformational states of a monolayer film of ethylene glycol monododecyl ether (C12E1) at the air/water interface have been investigated using ellipsometry, surface tension, external reflection–absorption FTIR spectroscopy and two-dimensional infrared (2DIR) correlation analysis. The ellipticity coefficients and the entropy associated with C12E1 adsorption changed almost discontinuously at certain temperatures, which manifested the interfacial phase transitions. The phase transition and coexistence of two phases were further clarified using 2DIR correlation analysis with temperature perturbation. The asynchronous correlation maps revealed that both bands of asymmetric and symmetric C–H stretching vibration in one-dimensional IR were split into two components, which confirmed the coexistence of two phases at the interface.

The purpose of this study was to find the primary factor determining the mixing of the counter anions of 1-alkyl-3-methylimidazolium-based ionic liquid (IL, CnmimX) at the air/water interface. First, the properties of the adsorbed films of the C10mimBr–C10mimBF4 mixtures were estimated by using the thermodynamic analysis of their surface tension data. The mole fraction of BF4− was lower in the adsorbed films than in the ideal state and the excess Gibbs energy of adsorption was negative, which was in marked contrast to those of the C6mimBr–C6mimBF4 mixtures. Meanwhile, the possibility of forming side-by-side ion pair of BF4− and imidazolium ring at the adsorbed films, as we found in the C6mimBr–C6mimBF4 system, was greatly reduced in the present case. These changes were mainly attributed to the reduction in the hydrogen-bonding interaction between BF4− and C10mim+, as the carbon number in the hydrophobic part of Cnmim+ was increased from 6 to 10. In addition, it was found that the values of excess Gibbs energy of adsorption were less negative in the C10mimBr–C10mimBF4 system than in the dodecyltrimethylammonium (C12TA)Br–C12TABF4 system, where the long-range Columbic interaction solely exists between the counter anions and surfactant cations. Thus, we concluded that the short-range hydrogen-bonding interaction played an essential role in the interfacial activity of ILs.

X-ray reflectivity measurements were applied to hexadecane wetting film on aqueous solutions of hexadecyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB). In both cas...

The aggregate formation and surface adsorption of aqueous binary mixture of short-chain phospholipids, dihexanoyl-phosphatidylcholine (DC6PC), and dioctanoyl-phosphatidylcholine (DC8PC), were exami...

The total reflection X-ray absorption fine structure (TR-XAFS) technique was applied to adsorbed films at the surface of aqueous solutions of surfactant mixtures composed of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium tetrafluoroborate (DTABF4). The obtained XAFS spectra were expressed as linear combinations of two specific spectra corresponding to fully hydrated bromide ions (free-Br) and partially dehydrated bromide ions adsorbed to the hydrophilic groups of surfactant ions (bound-Br) at the surface. The ratio of free- and bound-Br ions was determined as a function of surface tension and surface composition of the surfactants. Taking also the results in our previous studies on the DTAB – dodecyltrimethylammonium chloride (DTAC) and 1-hexyl-3-methylimidazolium bromide (HMIMBr) – 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF4) mixed systems into consideration, the relation between counterion distribution and miscibility of counterions at the solution surface was deduced for the surfactant mixtures having common surfactant ions but different counterions.

The line tension of tetradecane lens on the aqueous solutions of cationic surfactant, tetradecyltrimethylammonium bromide were measured as a function of surfactant concentration at the wetting transition between partial wetting and pseudo-partial wetting and as a function of temperature at the freezing transition of the pseudo-partial wetting film. In both experiments, the reversal of the line tension sign was observed; from positive to negative in the former and from negative to positive in the latter. These observations were analyzed based on the interface displacement model and explained by the change in the free energy of unit area of the wetting film at the air–water interface.

The surface tension γ of the aqueous solution of 1H,1H,6H,6H-perfluorohexane-1,6-diol (FC₆diol) and 1H,1H,8H,8H-perfluorooctane-1,8-diol (FC8diol) mixtures was measured as a function of total molality m and composition of FC₈diol in the mixture X₂ at 293.15 K under atmospheric pressure. The γ vs m curves except at X₂ = 0 and 0.05 have a distinct break point due to a phase transition in the adsorbed film. The surface pressure π vs mean area per adsorbed molecule A curves consist of two parts connected by a discontinuous change. The curve was almost vertical just below the phase transition, and the variation of the A value with film composition X(2)(H) was linear corresponding to the fact that FC₆diol and FC₈diol molecules orient parallel to the surface and are densely packed with the same areas of individual condensed films. Above the phase transition, the A value further decreases to around 0.12-0.19 nm² which is much smaller than the cross-sectional area of the fluorocarbon chain, indicating the multilayer formation at the surface. The phase diagram of adsorption (PDA) in the condensed film showed that the m vs film composition X(2)(H) curve is almost linear and the excess Gibbs energy of adsorption g(HE)/RT is at most 0.01, manifesting the ideal mixing of molecules. This is in contrast to a positive deviation (g(HE)/RT ~0.12) observed in the condensed film of the mixture of 1H,1H,2H,2H-perfluorodecanol (FC₁₀OH) and 1H,1H,2H,2H-perfluorododecanol (FC₁₂OH) with perpendicular molecular orientation. The loss of dispersion interaction between different species having different chain lengths is more appreciable in the perpendicular condensed films and thus leads to less miscibility of FC₁₀OH and FC₁₂OH. In the parallel condensed film, on the other hand, FC₆diol and FC₈diol molecules can arrange their position as close as possible to minimize the loss of dispersion interaction. The X(2)(H) value in the multilayer is close to unity, and thus, the multilayer consists of almost FC₈diol molecules which form a multilayer in the single-component system. Furthermore, the condensed monolayer-multilayer phase transition was accompanied by a large increase in surface density of FC₈diol and a small decrease in that of FC₆diol, indicating that FC₈diol molecules pile preferentially to form a multilayer.

The adsorption behavior of ionic liquids based on the 1-alkyl-3-methylimidazolium cation with different anions to the air–water interface was investigated through surface tension measurements. From the thermodynamic analysis of the surface tension data, a physical model for the structure of the adsorbed film is proposed: the HMIM + and BF 4 − ions occupy the same level of the surface due to ion-pairing in the HMIMBF 4 system, whereas the Br − ions are mainly distributed in the diffuse electrical double layer below the top surface in the DeMIMBr system. In the former case, the entropy change associated with adsorption increases with increasing concentration when the surface density is close to saturation because the hydrated water is released during the ion-pairing of HMIM + and BF 4 − . Furthermore, the structure of the adsorbed film is almost the same as a neat ionic liquid surface; this is probably because both the anions and the cations are highly concentrated in the adsorbed film.

A first-order transition from partial wetting to pseudo-partial wetting of hexadecane droplets on aqueous solutions of surfactant, hexadecyltrimethylammonium bromide, has been investigated by using...

We have performed ellipsometry and surface tensiometry at tetradecyltrimethylammonium bromide (TTAB) aqueous solution surface coexisting with tetradecane lens as a function of the molality of TTAB and the temperature under atmospheric pressure. From the theoretical analysis of the coefficient of ellipticity, it was clarified that the liquid monolayer comprising the surfactant and alkane is formed at higher surfactant concentrations by the wetting transition of tetradecane lens on the aqueous solution, and the solid monolayer is formed by lowering temperature (freezing transition). The results of the surface tension measurement support the occurrence of wetting transition and the freezing transition. A phase diagram of the wetting film was constructed by ellipsometry and surface tensiometry, of which the mixed solid monolayer had never been reported before. From the thermodynamic analysis of the phase diagram, it is also demonstrated that the TTAB surface density decreases accompanied with the freezing transition, which agrees with surface densities of TTAB calculated from surface tension vs. concentration curves.

The surface tension of aqueous mixtures of dodecyltrimethylammonium tetrafluoroborate (DTABF4) and sodium tetrafluoroborate (NaBF4) was measured as a function of total molality and composition of DTABF4 at 298.15 K. The results were analyzed by originally developed thermodynamic equations and compared with those of dodecyltrimethylammonium bromide (DTAB)–sodium bromide (NaBr) mixed system. It was indicated that BF 4 − ions reduce the repulsion between DTA+ ions more effectively than Br− ions in the adsorbed film. To investigate this difference more closely, the surface tension of DTAB–NaBF4 and DTABF4–NaBr mixed system was also measured. The data analysis revealed that BF 4 − ions are adsorbed positively even for the pure NaBF4 system and preferentially to Br− ions in these mixtures. Furthermore, it was concluded that the side-by-side arrangement suggested in the adsorbed film of 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF4) is due to not only the positive adsorption of BF 4 − ions but also the capability of hydrogen bond formation between imidazolium ion and BF 4 − ions.

The interfacial tension γ of the hexane solution of 1H,1H,10H,10H-perfluorodecane-1,10-diol (FC10diol) and 1H,1H,2H,2H-perfluorododecanol (TFC12OH) mixture against water was measured as a function of the total molality m and composition of TFC12OH in the mixture X2 at 298.15 K under atmospheric pressure. The interfacial pressure π vs. mean area per adsorbed molecule A curves and the phase diagram of adsorption (PDA) were constructed in order to shed light on the effect of the molecular orientation on the state of and the miscibility of FC10diol and TFC12OH in the adsorbed film. The π vs. A curves demonstrated that the mixed system exhibits three kinds of film states, parallel-condensed, normal-condensed, and multilayer states, depending on m and X2. The PDA indicated that film composition X2H in the parallel-condensed state never exceeds 0.14 even when X2 is increased to 0.9. This limited miscibility is probably due to a microphase separation that FC10diol and TFC12OH molecules are not dispersed molecularly but the parallel-condensed domains of FC10diol and the normal-condensed domains of TFC12OH are mixed at the interface. In the normal-condensed state, on the other hand, the FC10diol molecules may stand upright and be mixed with TFC12OH molecules. The multilayer is much richer in FC10diol than in TFC12OH when the multilayer is evolved from the parallel-condensed monolayer below X2 = 0.9, while the fraction of TFC12OH increases very rapidly when it is evolved from the normal-condensed monolayer above X2 = 0.925. Below X2 = 0.9, the X2H value in the multilayer is similar to that in the parallel-condensed monolayer. Thus it is plausible that FC10diol molecules are preferentially piled with parallel orientation in the upper layer of the multilayer. Above X2 = 0.925, the multilayer may not be homogeneous but heterogeneous in thickness. These views were confirmed from the X2 dependence of molality mieq of each component at the phase transition points.

The thermodynamic and X-ray reflectivity studies were applied to the adsorbed films of 1-eicosanol, partially hydrogenated perfluorodecanol, and their mixtures at the hexane/water interface and clearly demonstrated the existence of domains. The thermodynamic and FTIR studies on ethyleneglycol monododecyl ether system and the thermodynamic and total reflection XAFS studies on dodecyltrimethylammonium bromide system at air/water interfaces confirmed that the inhomogeneous structure is rather generally observed in the adsorbed films. The knowledge from the thermodynamic and structure studies has been combined and further utilized in the mesoscopic thermodynamic formulation on the Gibbs films at soft interfaces.

The surface tension of aqueous solutions of tetraethyleneglycol octyl ether (C8E4) and octyl-β-d-maltopyranoside (OM) mixture was measured as a function of the total molality of surfactants and the composition of OM under atmospheric pressure at 298.15 K by drop volume technique. The results of surface tension measurements were analyzed by originally developed thermodynamic equations, then phase diagrams of adsorption and micelle formation were constructed. From the analysis of the surface tension data, it was found that the C8E4 and OM molecules interact attractively in the adsorbed film and the excess Gibbs energy of adsorption can be compared with those observed in typical cationic–nonionic surfactant systems; nevertheless, they are mixed almost ideally in the mixed micelle. Judging from a negative excess surface area calculated by differentiating the excess Gibbs energy by the surface tension, we concluded that the attraction between C8E4 and OM molecules is a short-range one originated in the hydrogen bonding between them which favors the planar configuration.

The adsorbed film of 1-decyl-3-methylimidazolium bromide (DeMIMB) at the air/water interface was investigated employing the surface tension measurement and the x-ray absorption fine structure method under the total reflection condition (TRXAFS). From the surface tension measurement, the surface excess concentrations of ions were determined. From the XAFS measurement, two solvation states of bromide ion were found in the adsorbed film, which were assigned to be "free-Br" and "bound-Br". The hydration number of the former was estimated to be 6 while that of the latter was estimated to be 4. The results based on the XAFS analysis provided significant information on the formation of domains in the adsorbed film; the most conceivable situation is that the adsorbed molecules are definitely not homogeneously dispersed, but domains (islands or clusters) are dispersed in the adsorbed film. A regular and rather tight stacking of immidazolium rings may be formed in the domains.

Aggregate formation of a didodecyldimethylammonium bromide (DDAB) and didodecyldimethylammonium chloride (DDAC) mixture in aqueous solution was investigated. The concentration vs composition diagram of aggregate formation was constructed by analyzing the surface tension, turbidity, and electrical conductivity data. The cryogenic transmission electron microscopy was applied to several representative points in the diagram and provided information of the morphology of aggregates. The sequence of monomer (m) - m + small aggregate (A) - m + A + vesicle (V) - m + V was concluded with increasing total concentration of surfactants at all mixing ratios. The compositions of counterions in A and V were estimated on the basis of thermodynamic consideration and examined from the viewpoint of asymmetry of constituents and uneven distribution between outer and inner monolayers of a vesicle bilayer. Vesicle surfaces were suggested to abound in chloride ions compared to bulk solution, which is opposite to spherical micelle surfaces.

The total-reflection XAFS measurement possessing bromide ion selectivity at the interfacial region was applied to the adsorbed film of hexadecyltrimethylammonium chloride (HTAC) and dodecyltrimethylammonium bromide (DTAB) mixture. The surface compositions X H of individual ions j (j = HTA + , Cl - , DTA + , and Br - ) were evaluated by combining the surface excess concentration of Br∼ estimated from the XAFS with the surface composition of the respective surfactants from the surface tension results. It is clearly shown that HTA + and Br - are preferentially adsorbed to DTA + and Cl - at the air/water interface. The preferential adsorption was estimated numerically in terms of activity coefficient fh H,p i± of component i and excess Gibbs energy of adsorption g H pr ·Then, the magnitude of g H,E pr was compared with that of g H,E in attributable to intrinsic interaction between ions.

The adsorption of lH,lH,10H,10H-perfluorodecane-1,10-diol (FC 10 diol) at the hexane/water interface was investigated by the measurement of temperature dependence of interfacial tension and the thermodynamic data analysis in order to know the effect of two hydroxyl groups at both ends of the hydrophobic chain and the rigidity of the hydrophobic chain on the adsorption of fluorocarbon alcohol at the interface. The curves of interfacial tension versus temperature and concentration show break points corresponding to the phase transitions in the adsorbed FC 10 diol film. The interfacial pressure versus mean area per adsorbed molecule curve shows three kinds of states connected by two discontinuous changes. The area value after the first phase transition is very close to the calculated cross-sectional area of the FC 10 diol molecule along its major axis, and thus the FC 10 diol molecules form a condensed monolayer with molecular orientation parallel to the interface. Another noticeable point is that the value after the second phase transition point decreases furthermore to 0.12 nm 2 , which is much smaller than the cross-sectional area of the fluorocarbon chain, 0.28 nm 2 , with increasing interfacial pressure. This suggests that FC 10 diol molecules pile spontaneously and successively form a multilayer above the second phase transition. Furthermore, the partial molar entropy and energy change of adsorption in the expanded and condensed states were evaluated and compared to those of 1H,1H,2H,-2H-perfluorodecanol (TFC 10 OH), which orients almost perpendicular to the interface. In addition to the contact of two hydroxyl groups with hexane in the bulk solution, the results are explained by the dependence of partial molar entropy and energy at the interface on the following factors resulting from the parallel orientation of FC 10 diol at the interface; (a) hydrogen bonding of two hydroxyl groups with water molecules, (b) hydrogen bonding between two hydroxyl groups facing each other, and (c) the fluorocarbon chain-water contact. The adsorbed FC 10 diol film is stabilized by factors a and b, which overwhelm the energetic disadvantage caused by factor c. Furthermore, the entropy change of adsorption As in the multilayer is compared to the Δs cal calculated on the assumption that the condensed monolayer piles to form the multilayer. It was suggested that FC 10 diol molecules are not so densely packed in the multilayer compared to the first condensed monolayer and therefore the multilayer is not simply formed by the piling of condensed monolayers.

The mixed adsorbed film of lH,lH-perfluorooctanol (DFC 8 OH) and lH,lH,2H,2H-perfluorodecanol (TFC 10 OH) at the hexane/water interface was studied on the basis of interfacial tension measurement and its thermodynamic data analysis. An adsorbed film at any composition of the mixed system as well as those of pure DFC 8 OH and TFC 10 OH systems exhibits three states: the gaseous, expanded, and condensed states. Construction of the phase diagram of adsorption clarified that DFC 8 OH and TFC 10 OH mix almost ideally in the gaseous and expanded states. On the contrary, the excess Gibbs energy of adsorption g H,E value evaluated in the condensed state was positive. These results are explained by considering the following two factors: (1) The mixing of binary alcohols is accompanied by the loss of dispersion interaction energy due to the difference in extent of fluorination of hydrophobic chains and in their chain length and increases the g H,E value. (2) Since the interchange energy concerning the interaction between dipoles with different dipole moments is negative, the mixing of these alcohols reduces the repulsive force between hydrophilic groups and thus leads to a decrease in the g H,E value. In the gaseous and expanded states, both of above two factors are not effective. On the other hand, the positive g H,E value in the condensed state is attributable to more effective dispersion interaction than the dipole-dipole interaction in short molecular distance, and so factor 1 becomes dominant. Comparison of the g H,E value of the present system with that of the homologous TFC 10 OH-TFC 12 OH mixture leads us to a conclusion that the hydrogenation on /?-carbons in hydrophobic chains affects appreciably the balance of interactions between hydrophilic and hydrophobic groups which governs the mixing of molecules in adsorbed films.

The surface tension of aqueous solutions of dodecyltrimethylammonium bromide (DTAB) and octyl-β- d -glucopyranoside (OG) mixture was measured as a function of the total molality of surfactants and the relative proportion of OG under atmospheric pressure at 298.15 K by drop volume technique. The results of surface tension measurements were analyzed by originally developed thermodynamic equations then phase diagrams of adsorption and micelle formation were constructed. From the analysis of surface tension data, it was suggested that the addition of relatively small amount of DTAB molecules helps the denser monolayer formation compared with the ideal mixing. Furthermore, it was also suggested that the phase diagram of micelle formation has a negative azeotropic point and that the interaction between different components is stronger in the mixed micelle than in the adsorbed film from the view point of the excess Gibbs energy. From these results, it is concluded that the hydrogen bond formation between OG molecules is the main contribution to determine the physical properties in the adsorbed film whereas the ion–dipole interaction between head groups of different components becomes main cause of attractive interaction in the micelle.

We measured the line tension of the air/hexadecane/aqueous solution of the dodecyltrimethylammonium bromide (DTAB) system as a function of the molality of DTAB aqueous solution at 298.15 K. The experimental values of the line tension were 10−10 to 10−12 N, and they coincided with the theoretical estimates. Furthermore, it was found that the line tension changes in sign from positive to negative at around 0.750 mmol kg−1. This concentration is close to the point of transition from partial to frustrated-complete wetting. Taking into account the profiles of the free energy of the air/water surface, previously developed by Indekeu to understand the interrelationship between the wetting transition and line tension, it is suggested that the sign reversal of the line tension can be attributed to the transition from partial to frustrated-complete wetting.

The adsorption of phase transfer catalysts, 18-crown-6 and dicyclohexano-18-crown-6, at the air/water and the hexane/water interfaces were investigated. Interfacial tension σ decreased by increasing concentrations of these compounds and therefore both of these crown ethers are accumulated at interfaces. The variation of σ with concentration for both compounds follows the Szyszkowski equation very well, from which the values of saturated surface densities and interaction parameters have been evaluated. On the basis of occupied surface area of each molecule, the orientation of each of molecules at the air/water and the hexane/water interfaces have been proposed. The present results show that dicyclohexano-18-crown-6 has the higher tendency not only to dissolve into the hexane phase but also to adsorb at the hexane/water interface than 18-crown-6 and that the Starks extraction mechanism was suggested for the present phase transfer catalysis systems.

We measured the interfacial tension and the density of air/n-hexane, n-decane, 1-perfluorohexane/1-hexyl-3-methyl-imidazolium hexafluorophosphate systems as a function of temperature. From the air/ionic liquid surface tension values, it was suggested that Coulombic interaction between imidazolium cations and counter anions are not so much different between the surface and bulk. The density values indicated that the decrease of surface tension by saturating organics was closely correlated to the mutual solubility between ionic liquid and organics. Interfacial tension at the oil/ionic liquid interfaces suggested that ionic liquid molecules were more ordered at the oil/ionic liquid interfaces compared to the air/ionic liquid interfaces, but the decrease of the entropy due to the interfacial orientation of ionic liquid was compensated by the increase of the entropy due to the contact of different chemical species. The initial spreading coefficients and the Hamaker constants indicated that all the oil phases spread at the air/ionic liquid interfaces spontaneously, and form the complete wetting films.

The dynamic surface tension of the aqueous solutions of tetraethyleneglycol monooctyl ether (C8E4), a nonionic surfactant, was measured at different concentrations and temperatures. Present data at 298.15 K clearly indicate that the mechanism of adsorption is purely diffusion controlled at low concentrations (0.1~0.4 mmol/kg), and there is a switchover in adsorption mechanism to the mixed diffusion-kinetic control at higher concentrations. The calculated activation energies increase with concentration, and thus, with surface density, but decrease with temperature. The magnitude of activation energy and its increase with surface density suggest that the barrier is due to the free surface site formation by overcoming mainly the attractive van der Waals forces between the chain of adsorbed C8E4 molecules.

Wetting behavior of butylcyclohexane on aqueous solutions of dodecyltrimethylammoniumbromide (DTAB) was studied by interfacial tensiometry, ellipsometry, and dihedral angle measurement. The air–water surface tension is the same with and without butylcyclohexane at low surfactant concentrations (partial wetting) but lowered by the presence of hexadecane with increasing surfactant concentration (pseudo-partial wetting). From ellipsometric measurements, it is confirmed that the transition between partial wetting and pseudo-partial wetting is the first-order and the thickness of the film in the pseudo-partial wetting regime is monomolecular level. The contact angle decreases rapidly with increasing surfactant concentration in the partial wetting regime then gradually increases in the pseudo-partial wetting regime, then approaches to a saturation value around the critical micelle concentration (cmc). These behaviors are qualitatively the same as those observed in our previous studies on the hexadecane system but quantitatively it is found that the wetting transition is promoted by the introduction of the cyclohexane ring to the molecular structure of oil compared to linear chain alkanes. The effect of molecular structure of oil on the formation of a microscopic oil film is rationalized by dispersion interactions between molecules in the film and in the bulk oil phase.

The phase transition of the adsorbed films of long chain fluorocarbon alcohol, hydrocarbon alcohol, and their mixtures at the hexane/water interface was reviewed mainly from the viewpoint of pressure effect on them. It is shown that the bulk concentration at the phase transition is lowered substantially by hydrostatic pressure. The phase transition is further examined in terms of the volume associated with adsorption and the composition of alcohols at the interface, and also by the X-ray reflectivity measurements.

The surface tension of the 1H,1H-heptafluoro-1-butanol (FC4OH)–dodecyltrimethylammonium chloride (DTAC) mixed aqueous solution was measured as a function of the total molality of the mixture and the composition of DTAC at 298.15 K under atmospheric pressure. The phase diagram of adsorption (PDA) that gives the composition relation between the aqueous solution and adsorbed film was constructed. It was suggested that the subtle balance between the attractive surfactant cation-OH dipole interaction and the weak dispersion interaction between C–H and C–F chains is crucial for the phase behavior. The phase diagram of adsorbed film (PDAF) showing the composition relation between the different state of adsorbed films demonstrated the phase behavior is significantly dependent on the degree of counter ion binding. Moreover, the possible surface structures at the azeotropes are suggested.

The phase transition and domain formation of the adsorbed film of two kinds of hybrid alcohols (CF3(CF2)m-1(CH2)nOH, FmHnOH), 2-perfluorooctylethanol (F8H2OH) and 2-perfluorohexylhexanol (F6H6OH), as a mixture at the hexane/water interface was investigated by interfacial tensiometry and X-ray reflection. The interfacial tension γ versus total molality m curve of pure F8H2OH has a break point at high concentration, and thus, the mean area per molecule A changes discontinuously at high interfacial pressure π, corresponding to the phase transition between expanded and condensed films. The Fresnel divided reflectivity R/RF versus Qz plots in the expanded state was well-fitted by the domain model for incoherent interference to determine the interfacial coverage, which is the fraction of the interface covered by the condensed phase. This indicates that the expanded film is heterogeneous and consists of a condensed F8H2OH domain, the size of which is larger than the X-ray coherence length (∼5 μm). In the mixed system, the discontinuous change in A at the phase transition point becomes small with increasing the bulk composition of F6H6OH X2 in the mixture, and eventually the A value changes continuously; i.e, the phase transition becomes obscure in X2 ≥ 0.6. This behavior was linked to an increase in interfacial coverage with X2. Furthermore, the R/RF versus Qz plot was fitted by the domain model for coherent interference, suggesting that the size of the domain is smaller than 5 μm. These results are probably due to the reduction of domain line tension by preferential adsorption of F6H6OH at the F8H2OH domain boundary.

Alkanes on surfactant solutions can form three distinct phases at the air-solution interface, a liquid phase (L), a solid monolayer phase (S1), and a hybrid bilayer phase (S2). Phase coexistence between any two, or all three, of these phases has been observed by Brewster angle microscopy of tetradecane, hexadecane, and their mixtures on solutions of tetradecyltrimethylammonium bromide. The morphologies of the domains depend on the competition between line tension and electrostatic interactions, which are essentially different depending on the pair of phases in contact. Domains of S1 in the L phase are long and thin; however, long, thin domains of L in an S1 phase are not stable but break up into a string of small circular domains. The bilayer S2 domains are always circular, owing to the dominance of line tension on the morphology.