Experimental techniques to study protein–surfactant interactions: New insights into competitive adsorptions via drop subphase and interface exchange

Protein surfactant (PS) interactions is an essential topic for many fundamental and technological applications such as life science, nanobiotechnology processes, food industry, biodiesel production and drug delivery systems. Several experimental techniques and data analysis approaches have been developed to characterize PS interactions in bulk and at interfaces. However, to evaluate the mechanisms and the level of interactions quantitatively, e.g., PS ratio in complexes, their stability in bulk, and reversibility of their interfacial adsorption, new experimental techniques and protocols are still needed, especially with relevance for in-situ biological conditions. The available standard techniques can provide us with the basic understanding of interactions mainly under static conditions and far from physiological criteria. However, detailed measurements at complex interfaces can be formidable due to the sophisticated tools required to carefully probe nanometric phenomena at interfaces without disturbing the adsorbed layer. Tensiometry-based techniques such as drop profile analysis tensiometry (PAT) have been among the most powerful methods for characterizing protein's and surfactant's adsorption layers at interfaces via measuring equilibrium and dynamic interfacial tension and dilational rheology analysis. PAT provides us with insightful data such as kinetics and isotherms of adsorption and related surface activity parameters. However, the data analysis and interpretation can be challenging for mixed protein–surfactant solutions via standard PAT experimental protocols. The combination of a coaxial double capillary (micro flow exchange system) with drop profile analysis tensiometry (CDC-PAT) is a promising tool to provide valuable results under different competitive adsorption/desorption conditions via novel experimental protocols. CDC-PAT provides unique experimental protocols to exchange the droplet subphase in a continuous dynamic mode during the in-situ analysis of the corresponding interfacial adsorbed layer. The contribution of diffusion/convection mechanisms on the kinetics of the adsorption/desorption processes can also be investigated using CDC-PAT. Here, firstly, we review the commonly available techniques for characterizing protein–surfactant interactions in the bulk phase and at interfaces. Secondly, we give an overview for applications of the coaxial double capillary PAT setup for investigations of mixed protein–surfactant adsorbed layers and address recently developed protocols and analysis procedures. Exploring the competitive sequential adsorption of proteins and surfactants and the reversibility of pre-adsorbed layers via the subphase exchange are the particular experiments we can perform using CDC-PAT. Also the sequential and simultaneous competitive adsorption/desorption processes of some ionic and nonionic surfactants (SDS, CTAB, DTAB, and Triton) and proteins (bovine serum albumin (BSA), lysozyme, and lipase) using CDC-PAT are discussed. Last but not least, the fabrication of micro-nanocomposite layers and membranes are additional applications of CDC-PAT discussed in this work.