Raman studies of three-dimensional foam.

Abstract

The structure of the thin liquid films in a stable three-dimensional foam (shaving cream) has been studied by Raman scattering. Two phases existing in the foam films are identified---lamellar and isotropic. The lamellar phase is an ordered multilayer structure (gel) of stearic acid molecules in all-trans conformation. This is proved by the shape of the Raman spectra in the C-H stretching region and by the values of a characteristic parameter determined from them---the intensity ratio R. The spectra in the C-C stretching and C-H bending regions support this conclusion as they show absence of features characteristic of gauche conformations. An analysis of the Raman spectra in the C-H bending and C-H stretching regions suggests that stearic acid molecules in the foam lamellar phase have a hexagonal packing. On the basis of Raman spectroscopic data we propose a model of foam molecular structure and its evolution. According to it, small bilayer lamellae dispersed in the foam films after foam formation gradually self-organize around the bubbles in large shell-like bilayer structures. This arrangement is induced initially by the gas-liquid interfaces of the bubbles and consequently by the interfaces of the bilayer structures with the foam liquid. The process of organization of small lamellae into large bilayer structures can be followed by changes in the intensity ratio R of the C-H band and in the half-width of the antisymmetric C-H stretching fundamental ${\mathit{d}}^{\mathrm{\ensuremath{-}}}$. The investigations carried out in this work demonstrate the capabilities of Raman spectroscopy to study the microstructure of three-dimensional foams and their dynamics. Raman spectroscopic studies can be employed to study aging of foams on the molecular level and can be possibly extended to characterization of factors affecting foam stability. \textcopyright{} 1996 The American Physical Society.