Micelle formation in non-aqueous solvents: calorimetric study of the association of poly(ethylene oxide) alkyl ethers and hexadecyltrimethylammonium bromide in formamide

Abstract

The association in formamide of three poly(ethylene oxide) alkyl ethers (C₁₂E₄, C₁₂E₆ and C₁₆E₈) and hexadecyltrimethylammonium bromide (CTAB) has been studied by calorimetric solution and dilution experiments. Differential enthalpies of solution were determined as functions of concentration for C₁₂E₄ and C₁₂E₆ and differential enthalpies of dilution of concentrated C₁₆E₈ solutions were measured at 25 and 40 °C. Values for the enthalpies of micelle formation, c.m.c.s and aggregation numbers were derived from the microcalorimetric titration results. Differential enthalpies of solution of CTAB were measured at 45 and 55 °C. The c.m.c. for the C_nE_x surfactants are a thousandfold higher in formamide than in water. The enthalpies of micelle formation are small and vary only slightly with temperature. The aggregation numbers for C₁₂E₄ and C₁₂E₆ are significantly lower in formamide than in water. Association also takes place in the CTAB system but the aggregates are small. It appears that surfactant aggregation in water and formamide solution shows the same basic features but the association tendency is reduced in formamide mainly resulting from a reduced solvophobic effect of the alkyl groups.