EFFECT OF POLYMER SIZE ON THE POLYELETROLYTE-SURFACTANT INTERACTION

The interaction between alkylpyridinium (C-12 and C-14) chloride and s odium poly(aspartate) with different molecular weights is investigated in the presence of 10 mM. NaCl by means of a potentiometric titration method using a surfactant-selective electrode at 25 degrees C. The ef fect of polymer size on the binding process is especially focused on: the critical aggregation concentration increases and the apparent coop erativity of binding decreases with the decreasing of polyelectrolyte molecular weight. Binding isotherms are analyzed by direct calculation of a matrix expressing the partition function and compared with the b ehavior in other polyelectrolyte-surfactant systems. It is found that the binding constant prominently depends on the polymer chain length w hen the number of binding sites is less than ca. 35. Such an end effec t may be associated with the less effective superimposition of electro static potentials around the polymer chain in small-sized polymers. Mo del calculation is carried out by both the matrix method and the Satak e-Yang equation, while the matrix one fits the experimental data bette r for the short-chain polyelectrolyte.