Abstract: π-A isotherms were studied for mixed monolayers formed from each of two cationic gemini surfactants(dissymmetrical gemini surfactant 12-2-16 and symmetrical gemini surfactant 12-2-12) with octadecyl alcohol(C₁₈H₃₇OH) on air-water interface. The mixed surface excess free energy, ΔG_M^exo, was analyzed for each mixed monolayer with various mixed molar ratios at different surface pressures under the point of phase separation, to analyze the compatibility of each of the gimini surfactants and the fatty alcohol in the monolayer. The results show that ΔG_M^exo changes with the nature of the mixed molecules, mixed molar ratio, and surface pressure. Mixed monolayers of 12-2-16 and C₁₈H₃₇OH were considered as incompatible because ΔG_M^exo>0 in all mixed molar ratios. While for a system where 12-2-12 is mixed with C₁₈H₃₇OH, the compatibility of the mixed monolayer lies mostly on the mixed molar ratio. The values of ΔG_M^exo augment with increasing the C₁₈H₃₇OH molar ratio, changing from a net attractive interaction to a net repulsive interaction between 12-2-12 and C₁₈H₃₇OH with a critical point of C₁₈H₃₇OH molar ratio 0.65 added in the mixed system. When the mixing system is thermodynamically favored, increasing surface pressure will promote the mixing; In contrast, increasing surface pressure for a repulsive mixed system will be more thermodynamically unfavorable.

Key words: Quaternary ammonium Gemini surfactant, Octadecyl alcohol, Mixed monolayer, Mixed surface excess free energy