Interactions between polyampholytes and ionic surfactants

The interactions of two partially charged ampholytic terpolymers [consistin g of acrylamide, sodium 2-acrylamido-2-methylpropanesulphonate, and 2-(meth acryloyloxyethyl)trimethylammonium chloride segments with molar composition s 80/12/08 and 80/08/12] and two fully charged ampholytic copolymers (conta ining only the two latter comonomers with molar compositions of 80/20 and 5 0/50), with cationic surfactants [tetradecyltrimethylammonium bromide (TTAB ) and cetyltrimethylammonium bromide (CTAB)] and the anionic surfactant sod ium dodecylsulphate (SDS). are investigated. The studies include phase beha viour (swelling, solubilisation, precipitation), viscometry, electrical con ductivity, and potentiometry (bromide ion and surfactant ion-specific elect rodes). The 80/08/12 and 80/12/08 polyampholytes swell in water and are sol ubilised in the presence of cationic or anionic surfactants above a particu lar surfactant concentration that is proportional to the polymer concentrat ion. The polyampolyte 80/20 is soluble in water but precipitates in the pre sence of TTAB, whereas 50/50 is insoluble in water and in the presence of T TAB, but is solubilised upon addition of SDS. The results indicate that TTA B binds to 80/12/08 with little or no cooperativity. Solubilisation appears to be the result of the increasing polyelectrolyte character of the polpam pholyte upon neutralisation of its charged sites by bound surfactant ions o f opposite charge. The binding of TTAB by the 50/50 polyampholyte is very w eak and non-cooperative. In contrast, 80/20 binds TTAB cooperatively, much like a true polyelectrolyte-surfactant system of opposing charges. In parti cular, the binding is characterised by the existence of a critical aggregat ion concentration. A partial phase diagram for this system has been determi ned from the TTA(+)-electrode potential data. The behaviour of true polyele ctrolytes and polyampholytes, with respect to their interaction with surfac tants, is discussed.