The interfacial tension of the hexane solution of 1,1,2,2-tetrahydrotridecafluorooctanol CF3(CF2)5(CH2)2OH (FC8OH) against water was measured as a function of pressure at various concentrations m1 and 298.15 K. There are three series of the break points, two of which result from a phase transition in the adsorbed film at the hexane/water interface and the other one results from a solubility limit of FC8OH in hexane. The curve of the interfacial pressure vs area per adsorbed molecule of FC8OH was drawn and compared with those of the 1,1,2,2-tetrahydroheptadecafluorodecanol CF3(CF2)7(CH2)2OH (FC10OH) and 1,1,2,2-tetrahydrohenicosafluorododecanol CF3(CF2)9(CH2)2OH (FC12OH) systems previously reported. It was concluded that the adsorbed film of FC8OH reveals the first-order phase transitions between a gaseous and an expanded state and that between an expanded and a condensed state. The value of the volume change Δv associated with the adsorption in the gaseous and expanded state decreases rapidly with an increase in m1, while that in the condensed state decreases very slowly. It was described from the curves of Δv vs m1 for FC8OH, FC10OH, and FC12OH at 80 MPa that the expanded state region in the interfacial film structure strongly decreases with increasing the chain length and disappears at the FC12OH film and that the Δv value in the condensed state rapidly decreases with an increase in the chain length. We can explain that the pressure dependence of Δv in the condensed state is produced by the pressure dependence of the partial molar volume of FC8OH in the hexane solution. We concluded that the deposition is not a pure liquid but FC8OH-rich phase containing small quantity of hexane.
The aqueous solutions of hydrochloric acid−tetraethylene glycol monooctyl ether (C8E4) and sodium chloride−C8E4 mixtures were investigated to examine the interaction between inorganic ions and nonionic surfactants both in the adsorbed film and micelle. Their surface tension was measured as a function of the total molality of the inorganic electrolyte and C8E4 mixture and the composition of C8E4. The results of surface tension measurement were analyzed by our thermodynamic procedure, and the phase diagrams of adsorption and micelle formation were drawn. By comparing the phase diagrams of these systems, it was been that the Na+ ions do but Cl- ions do not interact with the ethylene oxide group of the C8E4 molecule. This finding leads us to the conclusion that the attractive interaction observed in the dodecylammonium chloride−C8E4 system reported previously is caused by ion−dipole interaction or hydrogen bonding between the dodecylammonium ion and oxygen atom of the ethylene oxide group of the C8E4 molecule.
The enthalpy of the mixing of water and tetraethylene glycol monooctyl ether (C8E4) was measured precisely as a function of concentration of surfactant and temperature by using the high-accuracy isothermal titration calorimeter. The differential enthalpies of solution of monomer and micelle and the enthalpy of micelle formation were evaluated by applying the thermodynamic relations to the enthalpy of mixing. It was shown that the dissolution of pure liquid C8E4 is exothermic and the micelle formation is endothermic, respectively. It was also shown that the micellization process is exothermic from the viewpoint of the partial molar enthalpy of water. The heat capacity changes accompanied by the dissolution and micellization processes were calculated. The partial molar heat capacity change vs molality curves for C8E4 and water were found to have a sharp maximum or minimum, and both its height and width are strongly temperature-dependent.
In a three-phase equilibrium of H2O –oil–amphiphile mixtures, the middle amphiphile rich phase may or may not wet the water/oil interface. For nonwetting middle phases, theory predicts a nonwetting→wetting transition upon approaching either one of the two critical endpoints. With respect to an experimental confirmation of this prediction, the situation appears to be controversial. In this paper, we have, therefore, studied the wetting behavior of the middle phase as it depends on the amphiphilicity of nonionic amphiphiles. We find that in mixtures with short-chain amphiphiles, the middle phase wets the water/oil interface in the entire three-phase interval, whereas with long-chain amphiphiles it (apparently) never wets. For medium-chain amphiphiles, however, one does find a nonwetting→wetting transition. On the basis of this result, we suggest that there exist four cases for the wetting behavior as a consequence of the dependence of the relations between the three interfacial tensions on amphiphilicity. The wetting behavior can be correlated with the evolution of the three-phase bodies from a tricritical point. Upon increasing amphiphilicity, their characteristic properties pass through maxima in the range of medium-chain amphiphiles, coinciding with the transition from always wetting to never wetting.
The interfacial tension γ of the hexane solution of 1H,1H-perfluorononanol (FDFC9OH) and its ω-hydrogenated analogue, 1H,1H,9H-perfluorononanol (HDFC9OH), against water was measured as a function of pressure and concentration at 298.15 K in order to clarify the effect of ω-dipole on the orientation of fluorononanol molecules from the viewpoint of volume. The adsorbed films of both alcohols exhibit two kinds of phase transitions among three different states: the gaseous, expanded, and condensed states. The partial molar volume changes of adsorption − in the expanded and condensed states were evaluated and compared between the two systems. The − values of both alcohols are negative, and thus the alcohol molecules have smaller volume in the adsorbed film than in the bulk solution. Furthermore, the value was obtained through the evaluation of by the density measurement of the bulk hexane solution. It was found that the value of HDFC9OH is smaller than that of FDFC9OH in the condensed state. On the basis of three matters concerning the molecular structure of alcohols, the occupied area at the interface, and the orientation of FDFC9OH in the adsorbed film deduced from the earlier results of X-ray reflectivity measurement, the mean tilt angle of HDFC9OH from the interface normal in the condensed film was estimated to be 15°. The thermodynamic estimation demonstrated here is highly valuable one to provide structure information on an adsorbed film.
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.
