Mixed micellar aggregates of nonionic and anionic surfactants with short hydrophobic tails: a microcalorimetric study

Apparent molar relative enthalpies were measured for the nonionic ethoxylated surfactant CH3-(CH2)5-(OCH2-CH2)5OH (C6E5) in aq. soln. at const. molality of the ionic surfactant CH3-(CH2)5-SO-3Na+ (C6SNa) at 25°C. The exptl. data obtained by a stepwise diln. process allowed evaluation of the C6E5 first interaction parameter at several const. molalities of C6SNa. The C6E5 crit. micelle compn. as a function of the C6SNa molality was also estd. The exptl. calorimetric data, together with the mixed micelles compn. computed in the past by some of us [Ciccarelli et al., Langmuir 14, 7130 (1998)], allowed computation of the ΔhMic of micellization. The exptl. data are compared to those predicted by the ideal soln. model and regular soln. model of mixed micellization. From a calorimetric study performed on the water-hexanol-C6SNa and water-penthaethylene glycol-C6SNa model systems, it can be argued that the interactions among the hydrophilic heads in the C6E5-C6SNa mixed micelles prevail on the contribution of the hydrophobic tails in ruling the enthalpic properties of the system.