POLYMER- SURFACTANT INTERACTIONS 219 7O L• 68 64 0,01 0,1 1 % Polymer 4O 5 4 3 2 1 -Log [SDS] Figure 4. Surface tension of the systems SDS/AADD at pH 2.5, O, SDS alone. AADD concentration: O, 0.01%. •l, 0.1%. A, 1%. Inset: O, surface tension of AADD alone in water. the surface tension drastically. The greatest reduction occurs below the cmc of the surfactant. At 1% polymer, a constant surface tension is observed throughout the SDS concentration region, which indicates surface saturation. At 0.01 and 0.1% polymer, a minimum in surface tension is obtained in the zone of high precipitation. Further addition of SDS causes the surface tension to increase slightly, with concomitant clearing of the mixture. The low surface tension obtained demonstrates the formation of a highly surface-active complex. Surface-active complex and precipitated material result from the head-to-head interaction between the surfactant anion and the quaternium ion

220 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS in the polymer. Solubilization of the complex is due to subsequent tail-to-tail adsorp- tion of a second layer of surfactant ions, leading to a reversal of charges from a polycat- ionic to a polyanionic state (16). Accompanying these surface tension reductions is a change in the bulk behavior of the system similar to that depicted in the solubility diagram. At 0.01% polymer, the mixtures appear clear up to [SDS] = 5 x 10 -4 M, with turbidity and precipitation ensuing thereafter. Resolubilization of the precipitate and clarification of the mixture occur at [SDS] • 5 x 10 -3 M, and sustainable foam is achieved as well. At 0.1% polymer, the systems remain clear up to [SDS] = 5 x 10-3 M. Here, a small amount of nondispersible rubbery material is obtained in a clear liquid phase with sustainable foam. This rubbery material disappears with further addition of SDS as the solution turns turbid in appearance, and the precipitate becomes more powdery in nature. A clear system is obtained at [SDS] = 5 X 10-2 M. Increasing polymer concentration to 1% still results in clear mixtures up to [SDS] = 5 x 10 -4 M. Precipitation begins with higher SDS concentration followed by eventual clearing. Again, a rubbery solid is obtained at [SDS] = 5 x 10 -2 M. It is noted that this type of precipitate only occurs at the specific ratio of polymer to SDS = 1:1.4 (w/w) at the present pH of 2.5. Figure 5 shows the surface tension data taken at pH = 9.5. At this pH, the polymer is partially charged. Measurements of AADD alone in water show that the polymer is only weakly surface-active. A strong interaction between the polymer and surfactant is ob- served, as indicated by the significant reduction in surface tension. Surface tension minimum in the premicellar region of SDS is observed in all three cases. The surface tension of the mixtures remains lower than that of the SDS above the cmc of the latter. It is also interesting to note that the mixtures exhibit lower surface tension in the submi- cellar region at this pH than their low pH counterpart. This seems to indicate that the polymer/surfactant complex formed at high pH is more surface-active than the polymer/surfactant complex formed at low pH. This higher surface activity may be a result of the presence of more nonprotonated sites on the polymer, thus rendering the whole complex more hydrophobic. The bulk solution also exhibits some changes. At 0.01% polymer, the mixtures appear clear, with no sign of precipitation. Sustainable foam is obtained at [SDS] = 1 X 10-3 M, which coincides with the observed minimum in the surface tension curve. The appearance and foam characteristics of 0.1% polymer solutions resemble those of the 0.01%, except at [SDS] = 1 x 10 -3 M where a nondispersible solid material is formed in the solution. Further addition of SDS resolubilizes the solid. At the highest polymer concentration investigated, i.e., 1%, the onset of haze/turbidity and formation of non-redispersible precipitate appears at [SDS] = 5 x 10 -4 M, reaching a maximum in haze/turbidity and precipitation at [SDS] = 5 X 10 -3 M. Further increase in SDS concentration increases the clarity of the mixture, with some of the precipitate resolubi- lized into the solution. In a highly alkaline medium, the polymer becomes nonionic, but interaction with an ionic surfactant species is still possible, principally through hydrophobic and/or di- pole-ion interactions. Figure 6 refers to the surface tension data of the polymer at pH 11.5. At this pH, AADD is neutral. Very little surface activity is encountered with the polymer by itself, while a lowering of the surface tension is still observed upon addition of a small amount of SDS. The general features of the surface tension curves are very