Foam and thin-liquid-film studies of alkyl glucoside systems

The foaming behavior of nonionic alkyl glucosides, in terms of foamability and foam stability, has been investigated using various methods. Three different foam tests, two dynamic methods (the Bikerman cylinder and the Watkins funnel) and one static method, have been employed for measuring the foamability and foam stability of nonionic surfactant solutions made from either a linear octyl beta-glucoside, a linear octyl alpha-glucoside, or a branched octyl alpha-glucoside, 2-ethylhexyl alpha-glucoside. At low surfactant concentration there is only a small difference in foamability between the surfactants, which remains when the salt concentration increases. At all surfactant concentrations tested octyl beta-glucoside is observed to be the best foaming agent. The stability of the individual foam lamellae has also been studied with the thin-film-balance (TFB) technique. The stabilizing forces in these nonionic foam films are, in most cases, electrostatic double-layer forces. The thickness dependence of the repulsive forces is consistent with Poisson-Boltzmann theory. As the salt concentration is increased, the range of the double-layer force observed below the cmc decreases, as expected. This trend correlates with a reduction in foam stability whereas the foamability is only marginally affected by the salt concentration. For the octyl beta-glucoside close to the cmc, only a very weak double-layer force is observed, but for this surfactant Newton black films stabilized by steric/hydration forces are formed.