Interaction between carboxymethylchitin and tetradecyltrimethylammonium bromide: phase separation, surface tension and stability of microscopic foam films and foams

Abstract

The bulk and interfacial properties of water soluble nonstoichiometric surfactant-polyelectrolyte complexes (SPEC) between oppositely charged tetradecyltrimethylammonium bromide (TDAB) and carboxymethylchitin (CMCh) were investigated by tensiometry, monolayer technique, viscosimetry and by the method of contact interactions, allowing us to measure the stability to rupture of microscopic foam films (MFF) formed between gaseous bubbles in liquid media. The calculated standard free energies of adsorption Δ adG o 298 and aggregation Δ aggG o 298 per one mol of CH 2-groups of alkyl radicals of cationic surfactant molecules bound to anionic macro-ions were found to be equal to −2.4 kJ mol −1, that is much greater than for free ionic surfactants in the solution (−1.8 kJ mol −1). The adsorption of SPEC is highly cooperative, and the area per one surfactant molecule belonging to SPEC at the air-water interface does not depend on the surfactant polyelectrolyte composition of the solution, Z=[TDAB]/[CMCh], in the range 0.001–1. The effect of the concentrations of TDAB and CMCh on the life-time before rupture of MFF was found to be highly synergistic. The reversible transfer from expanded to more compacted (aggregated) conformations of nonstoichiometric water soluble SPEC occurs at low CMCh concentration (less than 10 −4 base-mol dm −3) for a SPEC critical composition (cc) equal to φ ∗≈0.1 . The bulk and interfacial properties (viscosity, adsorption activity, stabilizing ability for MFF) of SPEC undergo a sharp variation in the course of this transfer.