Two mechanisms of gel/surfactant binding

The interaction of poly(methacrylic acid) gel with cationic surfactant, cetylpyridinium chloride (CPC), has been studied at pH of the solution ranging from 6.0 to 2.0. It has been shown that the value of pH has a dramatic effect on the swelling behavior of the gel in the surfactant solution: at pH > 2.5 the gel collapses, at lower pH the gel swells. At the same time, the total amount of CPC absorbed by the gel is only weakly dependent on the value of pH. The results were explained by the interplay of two mechanisms of binding of surfactant by the gel: electrostatic and hydrophobic. The former leads to the gel collapse, while the latter one induces the gel swelling. The contribution of each mechanism has been estimated. It has been shown that at high pH the electrostatic forces play an essential role, while with decreasing pH the gel ionization is suppressed and the hydrophobic interactions between the alkyl tails of the surfactant molecules and the gel chains become dominant. Thus, it has been demonstrated that the swelling behavior of the gel upon absorption of charged surfactant can be radically altered as a result of the changeover from electrostatic to hydrophobic mechanism of the surfactant binding. (C) 1999 Elsevier Science Ltd. All rights reserved.