Electric birefringence and emf measurements on polymer/surfactant complexes

Abstract

Electromotive force measurements with a surfactant-sensitive electrode and transient electric birefringence measurements have been carried out to study the interaction between the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the neutral copolymer poly (vinyl alcohol-co-vinyl acetate) (PVAA). The results show that binding of CTAB on the polymer starts abruptly at a critical surfactant concentration c 1 far below the c.m.c. of CTAB. The uptake of surfactant ions by the neutral polymer is accompanied by a dramatic increase of the Kerr constant B. With increasing CTAB concentration, the Kerr constant approaches a saturation value which increases linearly with the polymer concentration. The relaxation of the electric birefringence after removal of the electric field had to be fitted with two time constants τ 1 and τ 2. The relaxation times showed, in contrast to the Kerr constants, no significant variation with polymer or surfactant concentration, which implies that no expansion of the macromolecular coil due to binding takes place. It seems that the long relaxation time τ 2 corresponds to the overall rotational motion of the whole macromolecular coil. The critical association concentration c 1 is, in a large polymer concentration range, nearly independent of the polymer concentration. However, c 1 is markedly depressed, as expected, by addition of NaCl. A linear relationship between the logarithm of the critical concentrations c 1 and the c.m.c. and the logarithm of the counter-ion concentration was found with a smaller slope for c 1. It was concluded that in the CTAB concentration range studied two different association processes take place which differ with respect to their cooperativity: a first non- or only slightly cooperative association process after binding starts, followed by a second highly cooperative association process. The nature of the two different processes is discussed.