Measurement of low interfacial tensions by capillary wave spectroscopy. Study of an oil-water-surfactant system near its phase inversion

Abstract

Dynamic light scattering by small-amplitude thermal fluctuations at liquid-liquid interfaces (capillary wave spectroscopy) is used to measure interfacial tensions in oil + water + nonionic surfactant systems. Spectra for scattering angles between 0.1° and 0.45° (corresponding to surface modes with wave-numbers between 240 cm⁻¹ and 730 cm⁻¹) are detected by the heterodyne technique, using a high-performance signal analyzer. The resulting power spectra are evaluated on the basis of the Herpin-Meunier dispersion relation. Technical details of the experimental setup are given and possible sources of error are discussed. As an illustration of the method, some results for the system decane + water + dodecyl-tetraoxyethylene monoether (C₁₂E₄) in a temperature range above the phase inversion are presented. In this temperature range in which the oil-water interfacial tension varies by more than two orders of magnitude, one observes a transition from spectra with a peak maximum at a finite frequency shift (corresponding to slowly propagating capillary waves) to spectra centered at the frequency of the incident light (corresponding to overdamped surface modes). The analysis of the spectra in this transition region is discussed.