The adsorption and competitive adsorption of bile salts and whey protein at the oil–water interface

Abstract

The adsorption behaviour of NaC and NaDC at the decane–water interface was investigated using molecular dynamics simulation. Both NaC and NaDC adsorbed in a conformation where the sterol ring sits in the decane phase, with on average a tilt angle of 49° to the normal of the interface for both bile salts. This is contrary to previous studies for bile salt adsorption to the phospholipid-water interface where the bile salts adopt a flat conformation parallel to the surface. This is explained by the highly ordered nature of the acyl chains in phospholipid layers. Penetration of the less hydrophobic bile salts into a phospholipid layer would lead to disordering of the structure which is unfavourable. In contrast, the decane phase of a decane–water interface is disordered compared to phospholipids. This allows the bile salt sterol ring to penetrate into it without a significant entropy penalty. The free energy of adsorption calculated using umbrella sampling is greater for NaDC (104 kJ mol⁻¹) than for NaC (80 kJ mol⁻¹), reflecting the higher hydrophobicity of NaDC. NaC and NaDC also have a tendency to form clusters at the interface, possibly reverse micelles in the decane phase which are stabilised by hydrogen bonds formed between the hydroxyl groups on the sterol ring. The higher free energy of adsorption for NaDC is expected to lead to differentiation between the surface properties of NaDC compared to NaC, including their ability to compete for interfacial area with other molecules. To confirm this, the ability of NaC and NaDC to displace whey protein from the oil–water emulsion droplet interface was investigated experimentally. As expected it was found that NaDC displaces more protein for a given protein:bile salt molar ratio than does NaC.