Effects of fatty acids on polyelectrolyte–surfactant interactions

The effects of fatty acids on oppositely charged polyelectrolyte–surfactant interactions are experimentally investigated. It was observed that dodecanoic (lauric) acid (LA) disrupts the well-established reported interaction between the anionic surfactant sodium dodecylethersulfate (SDES) and the cationic polymer guar modified with grafted hydroxypropyl trimethyl ammonium chloride (Jaguar C13 BF). Surface tension measurements showed that, at the known surfactant Critical Aggregation Concentration (CAC), SDES binds to the partly surface active polymer, which is transformed into a soluble complex. The complex reaches insolubility when the polymer is entirely neutralized by SDES, and is re-solubilized in excess of surfactant micelles. The interaction pattern is relatively similar when the polymer interacts with a mixed SDES-LA system at LA molar fraction below its solubility limit in SDES. Interestingly, when the LA molar fraction exceeds the solubility limit in SDES and solid fatty acid aggregates form, preferential binding of the polymer to the LA in solid form was observed. Similar behavior was seen in emulsion systems containing the same oppositely charged polymer–surfactant pair, where polymer-induced flocculation is expected due to bridging of oil droplets upon dilution. In this case, the preferential adsorption of the polymer to the fatty acid in solid form prevents the polymer from acting as flocculant.