Adsorption and Dilational Rheology of Mixed β-Casein/DoTAB Layers Formed by Sequential and Simultaneous Adsorption at the Water/Hexane Interface

Abstract

The interfacial behavior of β-casein (βCS) has been investigated in presence of the cationic surfactant dodecyl trimethyl ammonium bromide (DoTAB) at the water/hexane interface and compared to that obtained for the water/air interface. The used experimental technique is a drop profile analysis tensiometer specially equipped with a coaxial double capillary, which allows investigation of sequential adsorption of individual components besides the traditional simultaneous adsorption of two species. This method also provides the dilational rheological measurements based on low frequency harmonic drop oscillations. The tensiometric results show that the equilibrium states of the mixed βCS/DoTAB layers built up on the two different routes do not differ significantly, that is, the general compositions of the mixed layers are similar. However, the results of dilational rheology for the two adsorption strategies are remarkably different indicating different dynamic characteristics of the adsorbed layers. These findings suggest that the respective mixed layers are more proteinlike if they are formed via sequential adsorption and more surfactant-like after simultaneous adsorption. In contrast to the W/A interface, at the W/H interface proteins remain at the interface once adsorbed and cannot be displaced just by competitive adsorption of surfactants.

Surfactant induced complex formation and their effects on the interfacial properties of seawater

“…The elasticity modulus shows a pseudo-plateau for the lowest surfactant concentration, dropping with the increase of the surfactant concentration to a nearly zero value in the proximity of the CTAB cmc. The latter dependence is similar to that found by Dan et al for the rheological response of ␤-casein and dodecyl trimethyl ammonium bromide [38]. It is noticeable that the mod(E) values in the two cases are about overlapping at large concentration, which confirms the same feature observed for the -c isotherms.…”

“…In fact, the plateau in the low-concentration range is similar to those found in other systems where similar effects are speculated, such as polyelectrolytes with opposite charged surfactants [33][34][35][36] or proteins with surfactants [37] and in particular what observed [38] for a similar cationic surfactants (DTAB) interacting with proteins (␤-casein).…”

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

“…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].…”

“…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.…”