Polymer-surfactant interactions studied by titration microcalorimetry: Influence of polymer hydrophobicity, electrostatic forces, and surfactant aggregational state

Isothermal titration microcalorimetry has been applied to investigate the interactions between hydrophobically-modified water-soluble polymers and surfactants. The following polymers were used in this study: poly(sodium acrylate-co-n-alkyl methacrylate) (A), where n-alkyl = C9H19, C12H25, and C18H37 (percentage of n-alkyl methacrylate to total monomer content ranging from 0 to 8), and poly(acrylamide-co-n-alkyl methacrylate) (B), where n-alkyl = C12H25 (percentage of lauryl methacrylate to total monomer content ranging from 0 to 5). The surfactants were a cyclic (mono-) n-dodecyl sodium phosphate (1) (CMP), a cyclic di-n-dodecyl sodium phosphate (2) (CDP), n-dodecyltrimethylammonium bromide (3) (DTAB), and di-n-dodecyldimethylammonium bromide (4) (DDAB). The following factors were found to influence the interactions between polymers and surfactants: electrostatic forces, polymer hydrophobicity (both the length of the hydrophobic moiety and the degree of hydrophobic modification), and the aggregational states of the amphiphilic molecules, which are micellar for the single-tailed surfactants and vesicular for the double-tailed amphiphiles. We provide evidence that, in the case of the single-tailed surfactants, individual amphiphilic molecules adsorb onto existing polymeric microdomains. This is in strong contrast with 'classical' polymer-surfactant interactions, where cooperative aggregation of single-tailed amphiphiles in the presence of homopolymers like poly(ethylene oxide) or poly(propylene oxide) was found at concentrations lower than the critical micelle concentration in pure water. In the case of vesicle-forming surfactants, the hydrophobic side chain of the polymer anchors into the bilayers of the vesicles. Non-hydrophobically-modified polymers do not interact at all with the vesicle bilayers. Interestingly, the interactions between single-tailed surfactants and hydrophobically-modified polymers are governed by different factors than the binding of hydrophobically-modified polymers to vesicular bilayers. In the former case, the number and strength of existing (inter)polymeric associations is of importance, and it is particularly the length of the hydrophobic moieties that is decisive. However, for favorable polymer-bilayer interactions it is sufficient that the hydrophobic moieties are long enough to be able to anchor. If this is the case, the number of hydrophobic anchors per polymer molecule further determines the effectiveness of the interaction. Finally, it appears that electrostatic repulsions can be easily overcome by hydrophobic interactions, but added salt facilitates the interactions between equally charged polymers and surfactants