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Interfacial tension ( γ) between aqueous dodecyltrimethylammonium chloride (DTAC) solution and benzene was measured as a function of pressure ( p) and concentration. The ∂γ/∂p was observed to change discontinuously at the critical micelle concentration; this indicates that the micelle formation of DTAC in the aqueous solution coexisting with benzene can be treated like the appearance of a macroscopic phase. It was shown by drawing the π vs. A curves that hydrocarbon, such as benzene, cyclohexane, and hexane, make the adsorbed film of DTAC expand. The volume behavior of the micelle with benzene molecules solubilized was found to bear a strong resemblance to that of the adsorbed film at the water/benzene interface. The difference in the molar volume value of adsorbed DTAC among the coexistent hydrocarbon phases was attributed to the difference in the contribution of the hydrocarbon molecules to the interfacial excess volume; the number of the solubilized hydrocarbon molecules was evaluated to be one or two a micelle. [ABSTRACT FROM AUTHOR]

The adsorbed film of n -tetradecylphosphocholine (C 14 PC) at the tetradecane (C 14 )/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 C 14 PC 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 C 14 PC 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 C 14 PC 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 C 14 /W interface with those at the Air (A)/W one, it was shown that in the monolayer and bilayer states, hydrophobic chains of C 14 PC molecules are more ordered at the C 14 /W than at the A/W interface because of attractive interaction between C 14 PC and C 14 molecules. This leads to smaller value of the partial molar entropy and energy changes associated with the adsorption at the C 14 /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. [ABSTRACT FROM AUTHOR]

Abstract: 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 298.15K 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. [Copyright &y& Elsevier]

Abstract: The surface tension of the aqueous solution of the binary mixture of 1H,1H-heptafluoro-1-butanol (FC4OH) and dodecyltrimethylammonium bromide (DTAB) was measured as a function of the total molality of the mixture and the composition (mole fraction in the surfactant mixture) of DTAB at 298.15K under atmospheric pressure to examine the phase behavior in the adsorbed film. The results of the surface tension measurement were analyzed by the thermodynamic procedure proposed by us and the composition of the mixed adsorbed film in equilibrium with their bulk solution was calculated. Three different phases of the adsorbed film appeared by a subtle balance between the attractive interaction of the polar head groups and weak dispersion interaction of the hydrophobic chains. In the low-concentration regime, FC4OH molecules and DTAB molecules form a gaseous film and mix attractively in the whole composition by the long-range ion–dipole attraction between hydrophilic groups. The effect of the attractive dispersion interaction between Ce:glyph name="sbnd" />F chains became more influential in the expanded film within a restricted composition region, where it should be noted that the interaction between Ce:glyph name="sbnd" />F is weaker than that between Cor between Calone. Furthermore, the adsorbed films at two specific compositions are stabilized by the stoichiometric arrangements of the molecules, which help ion–dipole attraction, in them. [Copyright &y& Elsevier]