Influence of the nonionic surfactant structure on surfactant-surfactant interactions

This paper presents the influence of the structure of nonionic surfactant on interactions between a cationic surfactant and the micelle of a non-ionic surfactant in aqueous solution at 25 degrees C. Cationic.surfactant dodecyltrimethylanomoilitIM bromide (DTA) and seven nonionic surfactants of different structure Brij 30 (BB30), Brij 35 (ER35), Brij 56 (BR56), Brij 58 (BR58), Brij 700 (BR.700), Triton X.-100 (TA IOW and Tween 20 (TW20)---- were used in this research. Interactions have been studied by means of pm tentiometry, based on the use of ion selective membrane electrode sensitive to free DTA surfactant cations. The equilibrium constant, cc,, and the standard free energy change, Delta G:degrees, for surfactant-surfactant complex formation, were determined. The results show that the strength of binding of DTA surfactant cations into the micelle of nonionic surfactant is dependent upon the length and shape of both hydrophilic and hydrophobic groups of the nonionic surfactant. A nonionic surfactant with shorter hydrophilic polyoxyethytene group,forms weaker interactions with a DIA surfactant cation than a nonionic surfactant with a longer hydrophilic group, with the length of their hydrophobic groups being equivalent. A nonionic surfactant with a longer hydrophobic chain also forms stronger interactions with a DIA surfactant cation than a nonionic surfactant with a shorter hydrophilic chain, assuming the magnitude of their hydrophilic groups is the same. Branching of both the hydrophilic and hydrophobic group of a nonionic surfactant reduces the strength of intermolecular interactions. It can thus be concluded that the electrostatic interactions occurring between a quaternary ammonium atom of a DTA surfactant and polyoxyethylene group of a nonionic surfactant as well as the hydrophobic interactions occurring among hydrophoium chains of both surfactants are important for incorporation of DTA cations into the micelle of a nonionic surfactant.