title: First-order phase transition during displacement of amphiphilic biomacromolecules from interfaces by surfactant molecules.
abstract: The adsorption of surfactants onto a hydrophobic interface, already laden with a fixed number of amphiphilic macromolecules, is studied using the self consistent field calculation method of Scheutjens and Fleer. For biopolymers having unfavourable interactions with the surfactant molecules, the adsorption isotherms show an abrupt jump at a certain value of surfactant bulk concentration. Alternatively, the same behaviour is exhibited when the number of amphiphilic chains on the interface is decreased. We show that this sudden jump is associated with a first-order phase transition, by calculating the free energy values for the stable and the metastable states at both sides of the transition point. We also observe that the transition can occur for two approaching surfaces, from a high surfactant coverage phase to a low surfactant coverage one, at sufficiently close separation distances. The consequence of this finding for the steric colloidal interactions, induced by the overlap of two biopolymer + surfactant films, is explored. In particular, a significantly different interaction, in terms of its magnitude and range, is predicted for these two phases. We also consider the relevance of the current study to problems involving the competitive displacement of proteins by surfactants in food colloid systems.
authors.affiliation: Food Colloids Group, School of Food Science and Nutrition, University of Leeds, Leeds LS2 9JT, UK.
authors.forename: Rammile
authors.initials: R
authors.lastname: Ettelaie
authors.forename: Eric
authors.initials: E
authors.lastname: Dickinson
authors.forename: Luis
authors.initials: L
authors.lastname: Pugnaloni
mesh_terms: D000327:Adsorption; D001704:Biopolymers / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D008958:Models, Molecular; D044367:Phase Transition; D011506:Proteins / Q000737:chemistry*; D013499:Surface Properties; D013501:Surface-Active Agents / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D001704:Biopolymers; D011506:Proteins; D013501:Surface-Active Agents