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.

Adsorption of poly(ethylene oxide) at the air/water interface: A dynamic and static surface tension study

“…A detailed description of both the applied apparatus and its operative principles has been given elsewhere [21]. The drops were formed in a closed, temperature-controlled chamber at 25.00 ± 0.05 • C. In order to avoid the evaporation of the pendant drop, the sidewalls of the chamber were covered with wetted filter paper (the volume decrease of the drop did not exceed 1% during several hours).…”

“…The time required for the formation of a pendant drop was approximately 1 s. After the formation of the third pendant drop, the syringe pump was stopped and the monitoring of the drop shape started (t = 0). A picture of the pendant drop was taken at every 5 s, and the recorded sequential digital images were used for the calculation of the surface tension values on the basis of the Laplace equation [21]. The accuracy and reproducibility of the determined surface tension values were ±0.1 mN/m.…”

Computer simulations of catanionic surfactants adsorbed at air/water interfaces. II. Full coverage

“…There is a large body of experimental information available [10][11][12][13][14][15] that shows that the dynamic and equilibrium properties of the resulting interfaces are also very much dependent on the branching and chain lengths of the hydrophobic segments of the individual surfactants. In contrast, from the theoretical side, the information available on these complex mixtures is not that abundant and have mainly focused on predictions of their thermodynamic properties based on classical theories of liquid mixtures.…”

Study of FeOx/Au inverse model catalysts by in situ sum frequency generation vibrational spectroscopy

“…In our laboratory the method was successfully applied for the investigation of the adsorption behavior of catanionic surfactant monolayers at the air/water interface [7]. The results -along with surface tension measurementssuggested a two-dimensional first order phase transition in the case of the solution of a long alkyl chain surfactant (C 12 + .…”