POLYMER- SURFACTANT INTERACTIONS 217 lO 9 8 pH / / ,,' =.- - • 1'5 /' •"" P" 6 i I I © • o,ol o,1 1 % Polymer I I i i i i I 0.001 0,005 0,01 0,05 0,1 0,5 1 5 PERCENT SDS Figure 3. Solution pH of mixtures of SDS/AADD. Lower curve: •), SDS alone. AADD concentration: A, 0.01%. O, 0.1%. O, 0.5%. [], 1%. Inset: O, pH of AADD alone in water. content, the more SDS is required to induce the pH change. The limiting pH value reached is seen to be close to 10. The increase in pH of the mixed solution seems to indicate that the addition of SDS to an AADD solution promotes the protonation of the polymer, with water molecules being the proton donor, thus producing hydroxide ions in the bulk solution, giving rise to the measured increase in pH. Protonation of the amine in the polymer converts the macromolecule to its cationic form. It interacts with SDS through the attraction of the anionic headgroup to the positively charged amine group in the polymer. A similar behavior of induced protonation of the cationizable group in the presence of anionic surfactant has been reported in systems containing SDS and long-chain dimethylamine oxides (17). van der Berg et al. (18) has also reported for systems containing polyeth- yleneimine (PEI) polymers an observed shift in the bulk pH on addition of SDS to the polymer solution. They interpreted their results as the formation of uncharged com- plexes between the negatively charged dodecyl sulfate ions and the positively charged monomeric groups of PEI. The onset of pH change is seen to vary with polymer concentration. This is expected since at any given pH, an acid-base equilibrium exists between the protonated and nonprotonated sites in the polymer. Addition of SDS to the polymer results in the complexation of the cationic sites while initially a region of constant pH is maintained.

218 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS This region of constant bulk pH activity widens with increasing amounts of the polymer. The rise in pH becomes apparent when the amount of SDS required to exhaust these sites has been exceeded, thereby promoting further protonation of other ionizable sites in the polymer. Since the degree of the cationic functionality of the polyamine varies with pH, as indi- cated by its acid-base titration results, different complexation behavior with SDS due to different degree of protonation is expected to result with increasing pH: the charge- charge interaction decreases in its contribution, while the hydrophobic and dipole-ion interactions increase. In this experiment, the various polymer/SDS mixtures were pre- pared without any adjustment in the pH. In these mixtures, turbidity and eventual precipitation are expected to occur when enough anionic surfactant adsorbs onto the positively charged polymer in a head-to-head configuration. The polymer charge is eventually neutralized, with a concomitant increase in its hydrophobicity. Maximum precipitation occurs at or near the stoichiometric ratio where the particles in suspension have zero or little charge (16,19). We observed in the solubility studies that maximum precipitation appears when the ratio of the polymer to SDS is in the neighborhood of 2:1 (w/w). (We note, however, that the manufacturer is uncertain about the exact copolymer ratio in this product. Moreover, we have not yet established a precise relationship between pH and the degree of ionization. Thus, the exact stoichiometric ratio could not be ascertained.) The re- sulting insoluble nondispersible gum precipitate is indicative of an effective neutraliza- tion of positive charges in the polymer by SDS. It is expected that particles above this ratio will have a positively charged surface because of incomplete charge neutralization. As the concentration of surfactants is increased beyond that required for maximum precipitation, resolubilization of the precipitate occurs due to the adsorption of a second layer of surfactant onto the neutralized polymer (8,20), forming a polyanion, with the solution eventually becoming clear. Dubin et al. (4), in their study of the complex formation between anionic polymer and cationic/nonionic surfactant systems, pointed out that there are three forms of associations that may be encountered: soluble aggre- gates, coacervate, or solid precipitate. And these phase boundaries are dependent on the mole fraction of the ionic surfactant. From Figure 2, it is clear that the same type of dependency also applies to polycation/anionic surfactant systems. A somewhat different solubilization behavior is reported for Cartaretin F-4, the higher charge density version of AADD (6). Here, the addition of excess surfactant did not resolubilize the precipitate aggregates. Many factors affecting the redissolution of pre- cipitated polyelectrolyte complexes have been studied by Goddard (8). Surfactant struc- ture, such as chain branching or introduction of polyethoxy chains, may render the dissolution incomplete. Most important is the charge density along the backbone of the polymer. If this happens to be very high, the redissolution of the precipitated complex is not going to occur. The difference in the resolubilization behavior of both Cartaretin polymers is an excellent example of this effect. The surface tension data at pH 2.5 are presented in Figure 4. The polymer, fully cationic at this pH, shows little surface activity, except at 1%, where the surface ten- sion drop becomes more pronounced. This observed sharp reduction is most likely due to surface film formation. SDS shows its usual behavior with a cmc at 6 x 10 -3 M. With the addition of a small amount of SDS, a strong interaction occurs that reduces