Mixed protein–surfactant adsorption layers formed in a sequential and simultaneous way at water–air and water–oil interfaces

Tensiometry and dilational rheology of mixed β-lactoglobulin/ionic surfactant adsorption layers at water/air and water/hexane interfaces

“…They are progressively replaced from the surface by increasing surfactant concentrations and washed out along with the free surfactant from the system [18]. The adsorption isotherm of BLG/surfactant mixed solutions did not show an intermediate maximum as in the case of b-casein/surfactant ones [14,19,20] which is the most striking difference when comparing binding of surfactant to a globular or a random-coil protein. However, some peculiarities were observed in the wash out results from simultaneous and sequential experiments as detected also for b-casein/ SDS mixed layers [14,15,19,20].…”

“…At a fixed pH the surfactant charge significantly influences the conformation and hydrophobic/hydrophilic balance of the formed protein/surfactant complexes [40,41,43], i.e. their surface activity, and subsequently the adsorption, desorption and rheological characteristics of the mixed adsorption layer [15,19,20,28,32]. At pH 7 the SDS/BLG complexes are in an intensive competition for the surface area with increasing amount of unbound surfactant molecules [26,27,36].…”

“…The interpretation of the data aims at further elucidating the mechanisms of formation and adsorption of the protein/surfactant complexes under different conditions such as the surface activity of different types of complexes, competitive adsorption of free surfactant and replacement of the complexes from the interface [14,15,18,20,[26][27][28]32,36,42]. At a fixed pH the surfactant charge significantly influences the conformation and hydrophobic/hydrophilic balance of the formed protein/surfactant complexes [40,41,43], i.e.…”

“…Schematics of the experimental protocols used in this study are shown in Fig. A1 in the supplementary data [15,19,20]. Briefly, they consist of several stages, namely simultaneous adsorption (Sim): (I) adsorption from mixed protein/surfactant solution; and (II) washing out with pure buffer; sequential adsorption (Seq): (I) adsorption from protein solution; (II) washing out with pure buffer; (III) drop volume exchange with surfactant solution; and (IV) washing out with pure buffer.…”

“…Recently mixed interfacial layers of b-casein (BCS) and an ionic surfactant SDS or DoTAB at the W/A and water/hexane (W/H) interfaces were studied using simultaneous and sequential adsorption protocols followed by a procedure of washing out the bulk solution with pure buffer thus probing the resistance of the mixed interfacial layer against desorption [14,19,20]. These studies showed that above a sufficiently high SDS concentration self-association of the complexes occurs in the bulk that controls the adsorption of free surfactant molecules at the interface, which in turn leads to an intermediate maximum in the simultaneous adsorption isotherm unlike in the case of the sequential adsorption route.…”