Phase Transition in an Adsorption Layer of a Soluble Surfactant at the Air–Water Interface

Abstract

In this paper we provide experimental evidence for a phase transition between a liquid- and gas-like phase occurring in an adsorption layer of a soluble surfactant at the air–water interface. The equilibrium surface tension σe versus bulk concentration σe (c) isotherm of surface chemically pure sodium 2-[4-(4-trifluoromethyl-phenylazo) phenoxy]-ethane sulfonate was measured at a temperature of 295 K up to the solubility limit of the amphiphile. The σe (c) isotherm could be fitted by Frumkin's equation of state. The lateral interaction energy is just above the limit for which Frumkin's model predicts a phase transition. The corresponding surface pressure π versus surface area A isotherm possesses striking similarities to first-order phase transitions in the Langmuir monolayer. The fact that the difference in the two-dimensional density is only a factor of 2 indicates that the system is very close to the critical point. The surface phases were further characterized by surface second harmonic generation. The major structural difference between the two surface phases is the amphiphile's molecular orientation. A mean orientation of the amphiphile of about 80° was found in the gas analogous phase, whereas a molecular tilt of 38° has been identified in the liquid-like phase.