Effect of polymer size on the polyeletrolyte-surfactant interaction

The interaction between alkylpyridinium (C-12 and C-14) chloride and sodium 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 effect of polymer size on the binding process is especially focused on: the critical aggregation concentration increases and the apparent cooperativity 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 behavior in other polyelectrolyte-surfactant systems. It is found that the binding constant prominently depends on the polymer chain length when the number of binding sites is less than ca. 35. Such an end effect may be associated with the less effective superimposition of electrostatic potentials around the polymer chain in small-sized polymers. Model calculation is carried out by both the matrix method and the Satake-Yang equation, while the matrix one fits the experimental data better for the short-chain polyelectrolyte.