title: Observation of a liquid-gas phase transition in monolayers of alkyltrimethylammonium alkyl sulfates adsorbed at the air/water interface.
abstract: The equilibrium adsorption layers of symmetric chain alkyltrimethylammonium alkyl sulfates (Cn+.Cn- for n = 8, 12) were investigated at the air/water interface by sum-frequency vibrational spectroscopy in the function of the bulk surfactant concentration. To ensure the surface purity of the solutions investigated, an improved version of the foam fractionation method was used for the purification of the constituent ionic surfactants and the surface purity of the solutions was also checked. In the monolayer of the C12+.C12- surfactant, a two-dimensional first-order gas/liquid phase transition was observed. At surfactant bulk concentrations just exceeding the concentration corresponding to the phase transition, the monolayer is conformationally disordered, liquidlike, but with increasing bulk surfactant concentration the conformational order of the monolayer increases. The SFG spectra of the C8+.C8- monolayer did not indicate the occurrence of phase transition at room temperature.
authors.affiliation: Department of Colloid Chemistry, Eötvös University, P.O. Box 32, H-1518 Budapest 112, Hungary.
authors.forename: Imre
authors.initials: I
authors.lastname: Varga
authors.forename: Tamas
authors.initials: T
authors.lastname: Keszthelyi
authors.forename: Róbert
authors.initials: R
authors.lastname: Mészaros
authors.forename: Orsolya
authors.initials: O
authors.lastname: Hakkel
authors.forename: Tibor
authors.initials: T
authors.lastname: Gilanyi