ABSTRACT
I. INTRODUCTION The association between polyelectrolytes and oppositely charged surfactants has been extensively studied, and many recent reviews and books covering different aspects can be found [1-7]. An overwhelming majority of the studies on the subject are concerned with association in bulk solution. One rationale for this is the importance of polyelectrolyte-surfactant systems as rheology modifiers [7,8], gelation agents [9], and solubilizers for sparingly soluble substances such as dyes [10,11], perfume, and pollutants [12,13]. The association between ionic surfactants and oppositely charged polyelec trolytes in bulk solution is primarily affected by electrostatic and hydropho bic forces [14-19]. However, the architecture of the polyelectrolyte, such as branching, cross-links, and chain stiffness, also influences the association behavior [20-22]. In particular, it is found that in bulk solution the initial binding of a charged surfactant to an oppositely charged polyelectrolyte is electrostatically driven. At a higher concentration a cooperative association takes place that also is driven by the hydrophobic interaction (mainly be tween the surfactant tails). The surfactant concentration at which this co operative association step occurs, the critical association concentration, is lower than the critical micelle concentration. This is largely due to a differ ence in the change in counterion entropy following formations of free mi celles and surfactant aggregates associated with poly electrolytes [20,22]. The structures formed are sometimes characterized as micellelike structures
bound along the poly electrolyte chain in a bead-and-necklace structure [18], or sometimes as ordered mesomorphous phases with quite complex topology [23-27]. When association between surfactants and hydrophobically modi fied polyelectrolytes are considered, it is often fruitful to regard the associ ation as a formation of mixed micelles [28]. Already at very low concentra tions the surfactant is incorporated in the hydrophobic microdomains formed by the hydrophobically modified polyelectrolyte. This uptake of surfactant is less cooperative than for polyelectrolytes without hydrophobic side chains [29], and the concept of a critical association concentration becomes less appropriate. Most studies have been performed at surfactant concentrations below the cmc of the surfactant. However, Dubin and coworkers have made important contributions to our understanding of polyelectrolyte surfactant association above the cmc [2,30-33].
