Polymer–surfactant Interactions

Surfactants and polymers are two of the important components in detergent formulations. The interactions between polymers and surfactants in aqueous media give rise to the formation of association structures, thereby modifying the solution and interfacial properties. The morphologies of association complexes depend on the molecular properties of the polymer and the surfactant. In this paper, we examine in detail, the association structure that forms in the presence of nonionic polymers, based on a quantitative theory. First we show that a variety of experimental observations are suggestive of the proposed structure of the complex. Then a detailed thermodynamic treatment is formulated to examine the competition between the formation of polymer-free surfactant aggregates and polymer-surfactant association complexes. The molecular features controlling this competition are identified to be the steric interactions between surfactant head groups and polymer segments, the shielding of micelle-water contact by the polymer, and the polymer hydrophobicity. The theory is used to explain why the presence of the polymer lowers the critical micelle concentration, reduces the size of spherical micelles, allows formation of complexes with mixed surfactants, and transforms large rodlike micelles and vesicles into smaller globular micellar aggregates. We conclude by alluding to the impact of the polymer on other properties such as solubilization and microemulsification. BIOGRAPHICAL INFORMATION Nagarajan received his Ph.D. in chemical engineering from the State University of New York at Buffalo. He joined The Pennsylvania State University in 1979, where he now holds the rank of Professor of Chemical Engineering. His research interests span the fundamentals and applications of surfactant and block copolymer systems. A major part of his research concerns the creation of molecular theories to predict a priori the self-assembling behavior of surfactants and block copolymers in a variety of aqueous, organic solvents and solvent mixtures. Phenomena such as micelles, mixed micelles, solubilization, microemulsions, surfactant-polymer interactions, adsorption at solid-liquid interfaces are among the problems addressed in his work. The other major aspect of his research pertains to developing applications for surfactant and block copolymer systems. Examples of past work include new methods of chemical separations based on selective micellar solubilization, forward and backward extraction of proteins using reverse micelles, removal of fermentation products exploiting phase separation of block copolymers, steroid conversion and hydrolysis of natural oils using enzymes immobilized in colloidal media, deinking of laser-printed paper using block copolymers and enzymes, synthesis, extraction and stabilization of metal nanoparticles using surfactant and block copolymer solution media. Nagarajan is a Fellow of the American Institute of Chemical Engineers and the American Institute of Chemists, Program Chairman of American Chemical Society Division of Colloid and Surface Chemistry, and editorial board member of the Journal of Colloid and Interface Science. He teaches the short courses "Surfactants and Block Copolymers: Self-Assembly" and "Surfactants and Block Copolymers: Colloidal and Interfacial Properties". Nagarajan, R. Polymer-Surfactant Interactions. In “New Horizons: Detergents for the New Millennium Conference Invited Papers”, published by American Oil Chemists Society and Consumer Specialty Products Association, Fort Myers, Florida (2001).