Relation of foam stability to solution and surface properties of gemini cationic surfactants derived from arginine

The dynamic surface tensions, under constant area and pulsating area are reported for one cationic monomeric surfactant LAM (Nα-lauroyl arginine methyl ester) and three related bicationic gemini surfactants Cn(LA)2, or bis (Args), with a spacer chain of n=3, 6, or 9 methylene groups. The tension equilibrates faster the higher the concentration, even beyond the cmc, indicating a substantial micellar contribution to tension dynamics. Both LAM and the gemini surfactants form more stable foams at 25°C as the concentration increases up to about half the cmc of LAM or half the second cmc of the geminis. The geminis are about twenty times more efficient foam stabilizers than LAM. The foam stability correlates strongly with tension equilibration time. The logarithm of the time t50f, for the initial foam height to drop by 50%, is a linear function of t50γ, which is the time for the tension to drop 50% of its equilibrium change. At comparable concentrations, the gemini surfactant with the shorter spacer chain (n=3 or 6) equilibrate faster and form more stable foams than the one with n=9.