Correlation of surfactant/polymer phase behavior with adsorption on target surfaces

In this contribution, the phase behavior of a surfactant/polymer mixed system is related to the adsorption of a complex derived from the mixture onto a target surface. The phase map for the system sodium dodecyl sulfate (SDS, a model anionic surfactant)/pDMDAAC (poly(dimethyl diallyl ammonium chloride), a cationic polymer) shows behavior very typical of surfactant/oppositely charged polyelectrolyte mixtures. The predominant feature is a broad, two-phase region in the phase map which lies asymmetrically around the 1:1 stoichiometry of surfactant charge groups to polymer charge units. The overall controlling principle driving the phase separation is charge compensation. Excess of polymer yields an isotropic solution, as does a great excess of surfactant (termed resolubilization). The phase separating in the SDS/pDMDAAC system is characterized by a positive zeta-potential when the polymer is in excess and a negative zeta-potential when the surfactant is in excess. The surface charge properties of the precipitated phases are essentially identical to those of target particles (ground borosilicate glass) dispersed at the same approximate position in the phase map, suggesting that the surfactant/polymer complex at the precipitation boundary is the same as that adsorbing onto the pigment particle. This conclusion is confirmed by depletion studies which allow the polymer adsorption density to be determined. For polymer-rich systems, essentially all of the surfactant adsorbs along with the polymer via a high-affinity isotherm with a plateau coverage of about 0.8 mg polymer/m (2). Surfactant-rich systems adsorb with a similar affinity, despite the mismatch of the complex charge matching that of the particle surface. Once adsorbed, these complexes are not readily removed by rinsing, though complexes adsorbed from SDS-rich systems will lose excess surfactant upon extreme dilution. Over a wide range of surfactant-rich compositions, from 1:1 stoichiometry out toward the resolubilization zone, a chemical analysis reveals that the surfactant/polymer precipitate species consists of a 1:1 charge complex with the addition of about 0.25 mol of surfactant/mol of complex. Resolubilization of these sparingly soluble species is achieved simply by dilution to below their solubility limit.