Synergism in binary mixtures of surfactants 12. Mixtures containing surfactants with two hydrophilic and two or three hydrophobic groups

Abstract

Molecular interactions between several anionic sulfonate surfactants containing two hydrophilic groups and two or three hydrophobic groups in the molecule (gemini surfactants) and a zwitterionic surfactant ( N, N-dimethyl-1-tetradecanamine oxide) or a polyoxyethylenated non-ionic surfactant (C 12H 25(OC 2H 4) 7OH or C 12H 25(OC 2H 4) 8OH) were measured at 25°C in aqueous 0.1 M NaCl solution. The strength of the interaction with the zwitterionic surfactant decreases with increase in the minimum area per sulfonate group of the gemini surfactant at the air/water interface. The interaction with the zwitterionic surfactant or the polyoxyethylenated non-ionic surfactant is also weakened by the presence of multiple ether linkages in the hydrophilic group of the gemini surfactant. The latter surfactants with these linkages show a smaller increase, compared with gemini surfactants without them, in their interaction with the zwitterionic surfactant upon reduction of the pH of the solution. They also show an increase with this change in their area per molecule at the air/water interface and an increase in their interaction with an alkanesulfonate surfactant (C 12H 25SO 3Na), increases not shown when neither surfactant has multiple ether linkages in the molecule. It is suggested that the relative ease of protonation of the multiple ether oxygen atoms in the doubly charged anionic gemini molecule accounts for these observations. The interaction in mixed micelles is much weaker than in mixed monolayers at the air/water interface; this is attributed to the difficulty of incorporating a surfactant with multiple hydrophobic groups into a micelle. The compounds studied all show synergism in surface tension reduction efficiency and effectiveness when mixed with the amine oxides, consistent with the synergism parameters. They also show synergism in mixed micelle formation with the amine oxide, except for a compound with three alkyl chains and one with a trioxyethylene group in the hydrophilic part. This is attributed to steric inhibition of micelle formation. As a result of protonation, the anionic gemini surfactants with multiple ether oxygen atoms in the molecule show little or no synergism when mixed with a polyoxyethylenated non-ionic surfactant. Synergism in the surface tension reduction effectiveness for these compounds when mixed with an alkanesulfonate at a low pH, however, is indicated.