Enhanced stabilization of aerosol-OT surfactant monolayer upon interaction with small amounts of bovine serum albumin at the air-water interface

An investigation is made of the influence from small amounts of the protein bovine serum albumin (BSA) on the lateral organization of low molecular weight surfactant sodium bis-2-ethylhexyl sulfosuccinate (AOT) at the air-water interface. Surface pressure (pi - A), surface potential (deltaV - A) and Brewster angle microscopy (BAM) experiments were carried out, with particular emphasis on the monolayer stability under successive compression-expansion cycles. AOT monolayer is not stable at the air-water interface, which means that the majority of AOT molecules go into the aqueous subphase as monomers and/or normal micelles. When a waiting time elapses between spreading and compression, the surfactant monolayer tends to reorganize partially at the air-water interface, with a monolayer expansion being observed for waiting times as large as 12 h. The incorporation of very small amount of BSA (10(-9)M) at the interface, also inferred from BAM, increases the monolayer stability as revealed by pi - A and deltaV - A results. For a waiting time of circa 3 h, the mixed monolayer reaches its maximum stability. This must be related to protein (and/or protein-surfactant complexes) adsorbed onto the AOT monolayer, thus altering the BSA conformation to accommodate its hydrophobic/hydrophilic residues. Furthermore, the effects from such small amounts of BSA in the monolayer formation and stabilization mean that the AOT monolayer responds cooperatively to BSA.