Makoto Aratono

Thermodynamic consideration of the mixed micelle of surfactants

In conformity with the conclusion obtained previously, the mixed micelle formation of surfactants was treated thermodynamically as the appearance of a macroscopic bulk phase with the aid of the excess thermodynamic quantities similar to those used for the adsorbed film. The composition of surfactant in the mixed micelle and the thermodynamic quantities of micelle formation were found to be evaluated by applying the thermodynamic equations derived. These equations were extended so as to be applicable to any kind of surfactant mixture. It was shown that the critical micelle concentration vs. composition of surfactant curves form a diagram analogous to the phase diagram of binary mixture. Applying the equation to the published data on typical surfactant systems, this thermodynamic approach was proved to be useful to clarify the miscibility of surfactants in the micellar state.

Thermodynamic studies on adsorption at interfaces. II. One surface-active component system: Tetradecanol at hexane/water interface

Abstract In order to show that the thermodynamic [ J. Colloid Interface Sci. 64 , 348 (1978)] treatment developed in Part I is useful in studying the adsorption of surface-active substances at interfaces, comparison has been made with a system consisting of water and dilute solution of tetradecanol in hexane. As the system is trivariant, we have chosen the temperature, pressure, and mole fraction of tetradecanol in hexane for the thermodynamic independent variables and measured the interfacial tension as their functions. By means of the thermodynamic equations derived in Part I, the interfacial concentration of tetradecanol and thermodynamic quantities of interface formation have been determined from the experimental results. Further, the partial molar thermodynamic quantity changes at the adsorption have been calculated. It has been observed that the thermodynamic quantities of interface formation decrease, while the interfacial concentration increases with increasing the mole fraction of tetradecanol. The results are explicable in terms of negative values of the partial molar quantity changes.

Phase transition in the adsorbed films at water/air interface

Abstract The surface tension at the aqueous solution/air interface has been measured for eight surface active substances at 298.15°K under atmospheric pressure. It has been found that each surface tension versus concentration curve has a distinct break at a relatively low concentration and a relatively high surface tension. This result implies that the two states of adsorbed monolayer coexist in equilibrium at the break point. The surface excess numbers of moles of surface active substances and the surface pressure versus mean area per adsorbed molecule curves confirmed that the phase transformation takes place between the gaseous and the expanded states. It has also been shown that the hydrocarbon chain length, and the ionic nature and size of polar head group of a surface active substance have a great influence on the properties inherent in the phase transition.

Thermodynamic studies on adsorption at interfaces: VII. Adsorption and micelle formation of binary surfactant mixtures

Abstract By extending the thermodynamic method developed in this series, a thermodynamic formulation has been proposed to account for the behavior of the adsorbed films of binary surfactant mixtures at interfaces. Further, this formulation has been extended so as to be applicable to the mixed adsorbed films in equilibrium with mixed micelles. The surface tension of the aqueous solution of a decylammonium bromide (DeAB)-dodecylammonium chloride (DAC) mixture has been measured as a function of the total molality of DeAB and DAC and the mole fraction of DAC in the total surfactant at 298.15 K under atmospheric pressure. The total surface density and the compositions in the adsorbed film and micelle have been evaluated by applying the thermodynamic equations derived to the experimental results. It has been concluded on the basis of the surface tension-composition and critical micelle concentration (CMC)-composition diagrams that DeAB and DAC mix slightly nonideally with each other in both the adsorbed film and the micelle.

Thermodynamic studies on adsorption at interfaces

Abstract The thermodynamic treatment of adsorption developed in Part I has been extended so as to include the case of strong electrolytes. By applying the thermodynamic relations to experimental results obtained for the system consisting of aqueous solution of sodium dodecyl sulfate (SDS) and hexane, the thermodynamic quantities of interface formation and partial molar quantities of the adsorption have been evaluated. Their values have been compared with the corresponding ones of tetradecanol described in Part II. It has been observed that SDS has a negative entropy of interface formation which is smaller than tetradecanol, while it has a larger interfacial concentration. This value is closely related to a larger positive value of the volume of interface formation. The above view has been confirmed by changes in the mean partial molar entropy and volume at adsorption. It has also been found from the energy of interface formation and partial molar energy of the adsorption that the adsorption of SDS from aqueous solution is inferior energetically to that of tetradecanol from hexane solution.

Thermodynamic studies on adsorption at interaces: V. Adsorption from micellar solution

Abstract The thermodynamic treatment of the preceding paper has been extended so as to be applied to the adsorption of ionic surfactant from its micellar solution at the water/air interface. The surface tension of dodecylammonium chloride solution has been measured as a function of temperature at concentrations around its critical micelle concentration (CMC). The derivative of surface tension with respect to temperature has been observed to change abruptly at the CMC and then decrease very slowly with increasing concentration. It has been asserted by comparing the thermodynamic relations with experimental results that the chemical potential of micelle depends only on temperature and pressure in a limited concentration range, and the derivative of the surface tension of micellar solution with respect to temperature is related directly to the entropy change associated with the adsorption of surfactant from the micellar state. The micelle has been concluded to be similar in thermodynamic behavior to the adsorbed film.

Thermodynamic study on the adsorption of dodecylammonium chloride at water/hexane interface

Abstract The adsorption of dodecylammonium chloride (DAC), which is a cationic surfactant, at water/hexane interface has been studied by measuring the interfacial tension as functions of temperature and pressure at various concentrations of DAC in the aqueous solution. By applying the thermodynamic treatment developed previously to the experimental results, the thermodynamic quantity changes associated with interface formation and corresponding partial molar quantity changes of DAC have been evaluated. These values have been compared with those of sodium dodecyl sulfate (SDS), which is an anionic surfactant. It has been observed that the dependence of the entropy change on temperature and that of the volume change on pressure are remarkable. It has also been observed that DAC and SDS differ appreciably in the values of the partial molar entropy and energy of adsorption while they have similar values in the partial molar volume change of adsorption under atmospheric pressure.

Thermodynamic study on the adsorption of sodium chloride at the water/hexane interface

Abstract The negative adsorption of sodium chloride at the water/hexane interface has been investigated by measuring interfacial tension as a function of pressure and concentration. By analyzing the results thermodynamically, the interfacial densities and the distance between the two dividing planes introduced so as to make the excess numbers of moles of water and hexane zero were evaluated. The results lead us to the view that the ions and hexane molecules distribute further apart from each other in the interfacial region. Furthermore, the volume of interface formation, Δυ, was evaluated; Δυ was found to be positive and independent of the concentration. In addition, the entropy, Δυ, and energy, Δυ, of interface formation were evaluated by measuring interfacial tension as a function of temperature. It was seen that Δυ, like Δυ, is independent of concentration, whereas Δ s decreases slightly with increasing molality. This discrepancy was explained as resulting from the contribution of the entropy of mixing.

Mixed adsorbed film of dodecylammonium chloride with decylammonium chloride at water/air interface

Abstract The surface tension of the aqueous solution containing dodecylammonium chloride and decylammonium chloride has been measured as a function of total concentration at 298.15°K under atmospheric pressure. It has been observed that the surface tension decreases with increasing the composition of dodecylammonium chloride at a fixed total concentration and the surface tension vs concentration curve has a distinct break point. By applying the thermodynamic relations developed, the surface densities of surfactants have been evaluated numerically. Examining the relation of the composition in the bulk phase to the one in the adsorbed film at constant surface tension, the difference in the surface activities of surfactants has been found to be pronounced at high surface tension. It has been proved that the break point corresponds to the phase transformation of the adsorbed film from a gaseous to an expanded state. It has been shown that the phase transition is expressed by a two-dimensional phase diagram.

Thermodynamic studies on adsorption at interfaces. VI: Interface between cyclohexane-benzene mixture and water

Abstract The thermodynamic method developed in this series has been extended to be applicable to the interface between two fluid mixtures. The interfacial tension of cyclohexane-benzene mixture against water has been measured as a function of temperature, pressure, and composition. By applying the equations derived, the excess numbers of moles of cyclohexane and benzene per unit area and the thermodynamic quantities of interface formation have been evaluated numerically. It has been concluded that the affinity of the π-electron of benzene for water causes the density of benzene in the interfacial region to be considerably larger than that in the bulk phase. This conclusion has been confirmed by estimating the mole fraction of benzene at the interface defined on the basis of the assumption that the interfacial region is monomolecular.