Drop profile analysis tensiometry with drop bulk exchange to study the sequential and simultaneous adsorption of a mixed β-casein /C12DMPO system

Abstract

The formation of mixed protein/surfactant adsorption layers is studied by the drop profile analysis tensiometry equipped with a special tool for drop volume exchange during experiments. This arrangement allows investigating in the traditional way by simultaneous adsorption from a mixed solution and also by a subsequent adsorption of the protein followed by surfactant. The experiments are performed for β-casein as the protein in the presence of different amounts of the non-ionic surfactant C 12 DMPO. The surface layers formed via the two routes show similar equilibrium surface properties. However, the dynamics of desorption of the protein complexes into the pure buffer solution deviate significantly, which is explained by the different locations of the protein/surfactant interaction. Although in both cases the complex formation is based on hydrophobic interaction, the accessibility of the hydrophobic parts of pre-adsorbed proteins due to unfolding is more favourable by the surfactant than in the solution bulk. Therefore, the amount desorbed from surface layers formed from mixed solutions is significantly less as compared to the displacement of proteins by subsequently injected surfactants interacting at the surface.

Demulsification Mechanism of Asphaltene-Stabilized Water-in-Oil Emulsions by a Polymeric Ethylene Oxide–Propylene Oxide Demulsifier

“…3), and the measured decrease in the pressure after wash-off corresponds to an increase in the interfacial tension. Such an increase indicates that the molecules at the interface can desorb [52][53][54] :…”

Mixed Interfaces of Asphaltenes and Model Demulsifiers, Part II: Study of Desorption Mechanisms at Liquid/Liquid Interfaces

“…Third, it is possible to follow the IFT while surface active species are being adsorbed/desorbed maintaining the drop/bubble volume constant [21][22][23] .…”

“…Such device was successfully validated and used to follow the desorption of a non-ionic surfactant (Triton X-100) and a series of n-alkyl dimethyl phosphine oxide surfactants by Ferri et al 18 . Similarly, Kotsmár et al 23 followed the desorption kinetics of a surfactant-protein complex (C 12 -dymethil phosphine oxide and βcasein) concluding that displacement by the formation of complex-like structures was the main cause of desorption.…”

Subphase exchange experiments with the pendant drop technique

“…The group of Reinhard Miller in Golm reported for the first time a direct comparison of the behaviour of adsorption layers formed by simultaneous and sequential adsorption in [43]. Since then, they have deepened into this aspect by reporting competitive and sequential adsorption of proteins (BCS, BLG and lysozime) and surfactants (ionic and non-ionic) at different interfaces, including interfacial rheology and theoretical considerations [1,3,11,13,41,[43][44][45]. In general, nonionic surfactants displace the proteins from the interface following an orogenic mechanism [42] and do not provide large differences between competitive and sequentially formed films.…”

“…Namely, the replaced amount of mucin simultaneously is higher than that replaced sequentially, because the mucin-surfactant complex formed simultaneously is more surface active than the sequentially formed at the interface. Conversely, when the displacing surfactant is non-ionic, the authors found that sequential and simultaneous adsorption isotherms are identical [43]. This behaviour is attributed to different protein-surfactant interactions: hydrophobic type for non-ionic surfactants and combined hydrophobic/electrostatic for ionic surfactants.…”