The effect of surfactant type on protein displacement from the air–water interface

Displacement of the proteins (β-lactoglobulin and β-casein) from air–water interfaces by non-ionic surfactants (Tween 20 and Tween 60) and ionic surfactants (Sodium Dodecyl Sulphate (SDS), Cetyl-Trimethyl-Ammonium Bromide (CTAB) and Lyso-Phosphatidyl-Choline-Lauroyl (LPC-L)) was studied. Interfacial structure was sampled using Langmuir–Blodgett deposition onto mica for imaging by atomic force microscopy. In all cases protein displacement was found to occur through an orogenic mechanism. For the non-ionic surfactants displacement involved nucleation and growth of surfactant domains, leading to failure of the protein network, and subsequent loss of protein into the bulk phase. The surface pressure dependences of surfactant domain growth, and the failure of the network, were found to be the same for Tween 20 and Tween 60. This suggests that protein network breakdown was dominated by the mechanical properties of the network, and unaffected by any protein–surfactant binding. Displacement of protein by ionic surfactants was found to be dominated by nucleation of surfactant domains, with little domain growth prior to failure of the network. The size of the domains formed by the ionic surfactants was found to be determined by the level of the intermolecular repulsive forces. Electrostatic screening of these charges led to an increase in the size of the domains. The surface pressure at which the networks failed was found to be dependent on the type of ionic surfactant and, in all cases, to occur at higher surface pressures than that required for non-ionic surfactants. This has been attributed to surfactant-protein binding that strengthens the protein network.