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Droplets of alkanes on aqueous solutions of the cationic surfactants CnTAB (CH3(CH2)n-1N+(CH3)3Br-) exhibit a first-order wetting transition as the concentration of the surfactant is increased. A theoretical model is presented in which the surface free energy is broken down into a long-range dispersion interaction and a short-range interaction described by a 2D lattice gas, taking into account the interaction between oil and surfactant molecules. The model provides quantitative agreement with the observed wetting transitions and the variation in composition of the wetting film with bulk surfactant concentration. The behavior of oil drops on large reservoirs of dilute surfactant is discussed.

The surface tensions (γ) of the aqueous solutions of tetradecyltrimethylammonium bromide (TTAB) and dodecyltrimethylammonium bromide (DTAB) were measured as a function of the total molality of surfactants (&mcirc;) and the relative proportion (composition) of DTAB (&Xcirc;2) at 298.15 ± 0.05 K under atmospheric pressure. The effect of the difference in the hydrophobic chain length between hexadecyltrimethylammonium bromide (HTAB) and DTAB on the synergism was examined. This synergism was observed in the miscibility at the surface of a mixture of these two compounds. The excess Gibbs energy of adsorption of the TTAB−DTAB system was positive in contrast to the HTAB−DTAB system. This indicates that there are certain restrictions on the difference in the hydrophobic chain length for the synergism to be brought about in homologous cationic surfactant mixtures. This mechanism was explained by the theory of a staggered structure formation at the air/water interface. A similar argument successfully applied to the hexadecyltrimethylammonium chloride (HTAC)−dodecyltrimethylammonium chloride (DTAC) and tetradecyltrimethylammonium chloride (TTAC)−DTAC mixtures also.

The mole fraction of chloride ion of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium chloride (DTAC) mixture in the adsorbed film %@mt;sys@%%@ital@%X%@rsf@%%@sx@%C%@be@%H%@sxx@%%@mx@% was estimated not only by the thermodynamic analysis of the surface tension data but also by analyzing the Br K-edge jump of the XAFS spectrum under the total reflection condition (TRXAFS method). The phase diagrams of adsorption (PDA) at several surface tensions from the two methods were in good agreement. On the basis of the PDA obtained, it was clearly shown that the criterion of an ideal mixing for the DTAB−DTAC system is not given by the linear relation between the total molality of surfactant mixture m and %@mt;sys@%%@ital@%X%@rsf@%%@sx@%C%@be@%H%@sxx@%%@mx@% , m = %@mt;sys@%%@ital@%m%@rsf@%%@sx@%B%@be@%0%@sxx@%%@mx@% + (%@mt;sys@%%@ital@%m%@rsf@%%@sx@%C%@be@%0%@sxx@%%@mx@% − %@mt;sys@%%@ital@%m%@rsf@%%@sx@%B%@be@%0%@sxx@%%@mx@% )%@mt;sys@%%@ital@%X%@rsf@%%@sx@%C%@be@%H%@sxx@%%@mx@% , but by the one between m² and %@mt;sys@%%@ital@%X%@rsf@%%@sx@%C%@be@%H%@sxx@%%@mx@% , m² = (%@mt;sys@%%@ital@%m%@rsf@%%@sx@%B%@be@%0%@sxx@%%@mx@% )² + [(%@mt;sys@%%@ital@%m%@rsf@%%@sx@%C%@be@%0%@sxx@%%@mx@% )² − (%@mt;sys@%%@ital@%m%@rsf@%%@sx@%B%@be@%0%@sxx@%%@mx@% )²]%@mt;sys@%%@ital@%X%@rsf@%%@sx@%C%@be@%H%@sxx@%%@mx@% . Furthermore, it was demonstrated that the theoretical approach that provides the latter relation draws a distinction between the criteria for an ionic surfactant mixture without a common ion and that for an ionic surfactant mixture with a common ion.

The X-ray absorption fine structure technique was applied to the aqueous dodecyltrimethylammonium bromide solution surface under the total reflection condition at 275 K. The Br K-edge absorption jump (J) value was evaluated and compared to the surface density (%@mt;sys@%Γ%@sx@%1%@be@%H%@sxx@%%@mx@% ) of surfactants calculated from the surface tension versus concentration curve at 275.15 K. An excellent correlation between the J and %@mt;sys@%Γ%@sx@%1%@be@%H%@sxx@%%@mx@% values was found, and their dependences on concentration were not of a simple Langmuir type but of the Frumkin type. This should be indicative that a phase transition between the gaseous and expanded states takes place in the adsorbed film, claimed previously by us. Furthermore, the J value shows a stepwise increase at around the critical micelle concentration (cmc). This may be attributed to a change of the structure of the adsorbed film, such as staggered arrangement of surfactant ions close to the cmc to minimize the electric repulsion between their headgroups, or that of the electrical double layer. The results obtained in this study are compared to those evaluated from the radiotracer method applied to the aqueous tritiated sodium dodecyl sulfate solution surface.

The interfacial tensions and dihedral angles of hexadecane lenses on aqueous solutions of dodecyltrimethylammonium bromide (DTAB) were measured in air as a function of the molality of DTAB at 298.15 K and atmospheric pressure. The air/water interfacial tension of DTAB solution shows a break point that shifts to a lower concentration in the presence of a hexadecane lens. Ellipsometric measurements showed a continuous variation in ellipticity with concentration for pure DTAB solutions, but a discontinuous change in the ellipticity of the air/water interface in the presence of a hexadecane lens. This behavior can be viewed either as a first-order wetting transition driven by the adsorbed surfactant, or as a gaseous-expanded phase transition in the surfactant adsorbed film induced by the hexadecane. The formation of a microscopic oil film in equilibrium with a lens is rationalized by an attractive long-range potential across the thin film. From the measurement of dihedral angles, it was found that there was a concentration region where a lens exists stably even when complete wetting was expected from measured values of interfacial tensions.

The interfacial tension γ of the hexane solution of oleyl alcohol against water was measured as a function of temperature T and molality m1 under atmospheric pressure. The entropy change associated with the adsorption Δs was dependent on both temperature and molality below about 35 mmol kg^-1 while independent of both those above about 35 mmol kg^-1. The former is responsible for the contact of the double bond of oleyl alcohol with water at the hexane/water interface, but the latter is responsible for the similarity of the aggregates, which are formed by the alcohol molecules in their hexane solution, to the adsorbed films in the situation that hydrogen bonds are formed between the alcohol molecules. Considering the aggregate formation and the thermodynamic equation used, it was found that the decrease of the interfacial density Γ1^H at a high concentration region is an artifact introduced by the assumption of the ideal solution at that region. Furthermore, by drawing the interfacial pressure π versus the mean area per adsorbed molecule A curves, the onset of the phase transition comes out at high temperatures and also oleyl alcohol does not form the condensed film because of the steric hindrance of the hydrocarbon chain of the alcohol molecules. The experiments by using other oils suggested that the alkene systems obviously exhibit the phase transition. Taking notice of the affinty between water and π-electrons and the occupied area just below and above the phase transition, it was concluded that the phase transition in this case is accompanied by the detachment of the double bond of the alcohol molecules from the interface, and therefore the driving force is the water−π-electrons interaction.

The enthalpies of mixing of oleyl alcohol and oils (cyclohexane, benzene) were measured as a function of the concentration at fixed temperatures by the use of a high-accuracy isothermal calorimeter. By applying the thermodynamic equations to the enthalpy of mixing, the differential enthalpies of solution of oleyl alcohol in the oil solution were evaluated. The enthalpies are positive and decrease with increasing concentration in both oils, indicating that the aggregate formation is exothermic. However, the enthalpy of aggregate formation in the cyclohexane solution is much larger than that in the benzene solution and has a magnitude corresponding to the hydrogen bond formation, which was supported by the infrared spectra. Furthermore, the infrared spectra of the benzene solution showed that the hydroxyl group of alcohol attractively interacts with benzene molecule. These experimental results suggested that oleyl alcohol molecules associate themselves through the hydrogen bond formation between the hydroxyl groups in the cyclohexane solution, whereas with keeping interaction between hydroxyl group and benzene molecule in the benzene solution.

The surface tension of the aqueous solutions of sodium dodecyl sulfate (SDS) and tetraethylene glycol monooctyl ether (C8E4) was measured as a function of the total molality of the surfactants and the composition of C8E4 at constant temperature under atmospheric pressure. The results of the surface tension measurement were analyzed by our thermodynamic procedure. Both the phase diagram of adsorption and that of micelle formation were found to have an azeotropic point, and the stronger energetic stabilization was suggested. The extent of this nonideal mixing was estimated quantitatively in terms of the excess Gibbs energy, ĝ^H,E, in the adsorbed film and of that in the micelle, ĝ^M,E. The dependence of ĝ^H,E on the surface tension was also utilized to elucidate the mechanism of the attraction between the different species in these aggregates in detail. By combining the results of the present study with that of the previous one, we concluded that the attractive interaction acting in the adsorbed film and micelle is the indirect interaction between dodecyl sulfate ions (DS^-) and the ether oxygen atom of C8E4 molecules through the Na⁺ countercations. Sodium ions interact with the DS^- ion and C8E4 simultaneously, probably through the electrical attraction.

The interfacial tensions of the hexane solution of fluorooctanols (1,1,2,2-tetrahydrotridecafluorooctanol, TFC8OH, and 1,1-dihydropentadecafluorooctanol, DFC8OH) against water were measured as a function of temperature and molality under atmospheric pressure. By drawing the interfacial pressure π vs mean area per adsorbed molecule A curves, it was concluded that the adsorbed film of TFC8OH exhibits a first-order phase transition between the gaseous and expanded states and that of DFC8OH shows the two types of phase transitions from the gaseous to the expanded state and from the expanded to the condensed one at the hexane/water interface. The comparison of the π vs A curve between TFC8OH and DFC8OH shows that the intermolecular interaction is enhanced by the substitution of fluorine for hydrogen on the β-carbon of TFC8OH. Furthermore, the difference in the transition pressure between DFC8OH and TFC10OH (1,1,2,2-tetrahydroheptadecafluorodecanol) is explained by the differences in London dispersion force between hydrophobic chains and the dipole moment of their hydroxyl group. The partial molar entropy &smacr;s^H − ss^O and energy ūs^H − us^O changes of adsorption were evaluated and compared to those of TFC10OH. The &smacr;s^H − ss^O value is negative and therefore alcohol molecules have smaller entropy at the interface than in the solution, which is attributable to the orientation of the molecules at the interface. The phase transition from the expanded to the condensed state in the adsorbed TFC10OH film causes larger decrease in partial molar entropy than that in the DFC8OH one. This may arise from the larger partial molar entropy of TFC10OH molecules due to the larger entropy of mixing of longer fluorocarbon chain with hexane in the expanded state and the smaller entropy of TFC10OH due to the stronger attractive interaction in the condensed state than that of DFC8OH molecules. The ūs^H − us^O value is less negative for DFC8OH than for TFC10OH and therefore the energetical stabilization of DFC8OH accompanied by the adsorption from the solution is less than that of TFC10OH. Furthermore, it was concluded that the DFC8OH molecules are stabilized by forming the condensed film at the interface because of the strong molecular interaction between them, and the TFC8OH molecules form mainly tetramers in the hexane solution to lower the energetical state of the system.

The tension of the three interfaces and the dihedral angle of the lens of the air/hexadecane lens/aqueous solution of tetramethylammonium dodecyl sulfate (TMADS) system were measured as a function of the surfactant concentration. The air/water surface tension vs concentration curves both with and without an oil lens exhibited a break point corresponding to the phase transition of the adsorbed films. From the thermodynamic analysis, it was shown that the phase transition drives the spreading of hexadecane molecules into the adsorbed film. The dihedral angle decreased very rapidly from about 50° at zero concentration to about 6° at the phase transition point and then increased slowly to about 20° at the critical micelle concentration (cmc), suggesting the very drastic change in wetting properties at the phase transition point. The plausible gap was observed between the dihedral angles measured and those calculated from the interfacial tension values at the concentration range from just above the phase transition to near the cmc. This suggests a case where the Neumann relation does not hold even when a lens exists stably. One of the possibilities may be that the wetting film is rather thick just above the phase transition point.

The temperature effect on the adsorption at air/water interface and micelle formation of poly(ethylene glycol) monoalkyl ether (CiEj) nonionic surfactant was investigated on the basis of the thermodynamic strategy. From the results of surface tension experiments and after applying the thermodynamic treatment, surface density and thermodynamic quantity changes associated with the adsorption and micelle formation were obtained. It has been found that the entropy of adsorption from monomeric state of CiEj is positive and depend on both concentration and temperature in the same trend; the entropy increases with increasing adsorption of surfactant and increasing temperature. This phenomenon is against that of ionic surfactant. The difference was analyzed in term of the competition between the dehydration effect and the orientation effect. For CiEj nonionic surfactant systems, the large amount of dehydration at high temperature dominates the controversial results of surface density and entropy changes. Moreover, the effect of hydrophobic chain length on the surface adsorption and micelle formation was included in this study using three CiE5's with i = 8, 10, and 12. Those gave outstanding aspects with respect to the thermodynamic property changes.

The thermodynamic equations necessary to obtain the enthalpy of micelle formation for binary mixed surfactant systems from the titration calorimetry of the aqueous solution of surfactant mixture were developed. They were applied to the experimental results of the following mixtures:  pentaethylene glycol monodecyl ether(C10E5), pentaethylene glycol monooctyl ether(C8E5), and C10E5-tetraethylene glycol monooctyl ether(C8E4). It was found that C10E5 and C8E5 molecules are ideally mixed in the micelle from the viewpoint of the enthalpy as well as the Gibbs energy, whereas C10E5 and C8E4 molecules mix in a slightly nonideal fashion due to the positive excess enthalpy. Furthermore, it was shown that the excess entropy of mixed micelle formation of the C10E5-C8E4 system is positive.

The interfacial tension γ of the hexane solution of a mixture of 1,1,2,2-tetrahydroheptadecafluorodecanol (FC10OH) and 1,1,2,2-tetrahydrohenicosafluorododecanol (FC12OH) against water was measured as a function of the total molality m and composition of FC12OH X2 at 298.15 K under atmospheric pressure. The γ vs m curve has one or two distinct break points depending on X2. By plotting the γ and m values at the break points and drawing the interfacial pressure π vs mean area per adsorbed molecule A curves, it was suggested that the triple point of adsorbed film, at which the three kinds of states (gaseous, expanded, and condensed states) coexist simultaneously, exists at a middle composition. The phase diagrams of adsorption (PDA) were constructed and the excess Gibbs energy of adsorption g^H,E was calculated to examine the miscibility of FC10OH and FC12OH molecules in the adsorbed film. It was found that these alcohols mix almost ideally both in the gaseous and expanded states and nonideally in the condensed state. Furthermore, the mixing of FC10OH and FC12OH in the condensed film was accompanied by a positive g^H,E value. This suggests that the difference in the magnitude of mutual interaction between the same species affects appreciably the miscibility of molecules in the condensed state where the adsorbed molecules are assumed to be closely packed and arranged regularly.

The surface tension of aqueous solutions of dodecylammonium chloride (DAC) and tetraethylene glycol monooctyl ether (C8E4) was measured as a function of the total molality of surfactants and the composition of C8E4 at constant temperature under atmospheric pressure. The results of the surface tension measurements were analyzed by our thermodynamic procedure, and phase diagrams of adsorption and micelle formation were drawn. The miscibility of the surfactants and their intermolecular interaction in both the adsorbed film and the micelles were examined. Deviation from ideal mixing was observed and an attractive interaction was suggested. Furthermore, the phase diagram of micelle formation was shown to have an azeotropic point, and a stronger energetic stabilization was suggested in the mixed micelle formation than in the adsorbed film at the cmc. This difference suggests that the structures of both the surfactant molecules and the aggregates significantly affect the intermolecular interaction between surfactants.