Adsorption behaviors of branched cationic gemini surfactants and wettability in quartz-solution-air systems.

The adsorption and wetting on quartz surfaces by aqueous solutions of xylyl-substituted biquaternary ammonium salt gemini surfactants with different spacer groups (C and C), have been investigated. The interfacial properties of surfactant solutions such as contact angle, adhesional tension (γ cos θ), quartz-water interfacial tension (γ) as well as adhesion work (W) have been estimated. The obtained results show that C and C have similar adsorption behavior on quartz surfaces. Before critical micelle concentration (cmc) is reached, the contact angles of gemini surfactants slowly increase with the increasing concentration, and the adsorption amount at the water-air interface is almost the same as those at a quartz-water interface. After reaching cmc, the gemini surfactant Cn molecules form a more compact adsorption film through bending the flexible spacer chain, instead of forming a bi-layer. As a result, a further increase in quartz-liquid interfacial tension (γ) and a consequent increase in contact angle have been observed after cmc. Gemini C shows a stronger ability towards hydrophobic modification at a quartz surface than C, demonstrating the contribution of the longer methylene spacer to the hydrophobic modification of the quartz surface.