Surfactant binding to polyelectrolytes

The cationic charge on a polymer affects its behavior considerably when surfactants are included in formulations. Cationic polymers (a member of the broader class of polymers termed polyelectrolytes) usually interact strongly with anionic surfactants, weakly with cationic surfactants, and unpredictably with nonionic and amphoteric surfactants (14,167-169). Anionic surfactant binds to cationic polymers at concentrations well below the critical micelle concentration (cmc). The low surfactant concentration where polymer and surfactant begin to interact is known as the critical aggregation concentration (cag). 

Although the electrostatic force plays the main role in the interactions of oppositely charged surfactants and polymers, hydrophobicities of polymer and surfactant also play a role. Zana and Benrraou (2000) studied the interaction of two polyelectrolytes PSl and PS4 (copolymers of disodium maleate and methyl or butyl vinyl ether, respectively), and quaternary ammonium bromide surfactants (3-dodecyldimethyl(alkyl)ammonium bromides and two dimeric surfactants of the polymethylene-a,fi)-bis(dodecyldimethylammonium bromide) type). They used the surfactant-binding isotherms method and spectrofluorometry using pyrene as a fluorescent probe to detect the onset of binding. Their results showed that surfactant binding to the polymer is more pronounced when the polymer is more hydrophilic for a given surfactant. 

It is known that interactions between ionic surfactants and polyions with the opposite charge lead to the formation of soluble colloidal complexes. The polyelectrolyte chain binds to surfactant molecules through Coulombic attractions, and the hydrophobic moieties of the surfactant molecules stabilize the complexes due to hydrophobic interactions in the aqueous solution (Morris and Jennings, 1976 Satake and Yang, 1976 Osica etal., 1977 Fendler, 1982 Hayakawa et al., 1983 Jonsson et al, 1998). 

A similar physical picture of counterion binding can be adopted for systems containing surfactant counterions, although in this case some additional effect may be expected. The main factors that influence the binding of ionic surfactants to polyelectrolytes with opposite charge are (1) the charge density of the polyion, A, (2) the hydrophobic character of the surfactant (the length of its hydrocarbon chain), (3) the additional attractive forces between the... 

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