Effect of emulsifier type against the action of bile salts at oil-water interfaces

Bile salts (BS) are important agents in lipid digestion and absorption. This biological process involves high amounts of BS micelles and this could induce depletion flocculation of oil-in-water (O/W) emulsions by non-adsorbed micelles that are excluded from the interstitial space. In a previous work we observed that the emulsifier type - Pluronic F68 (non-ionic) and phospholipids Epikuron 145 V (anionic) - provided different emulsion stability in the presence of a bile salt (sodium taurodeoxycholate, NaTDC). Namely, Pluronic provided higher stability than Epikuron against the action of BS. In order to elucidate these results observed in O/W emulsions, the aim of the current study is to probe the effect of NaTDC on the interfacial behavior of such emulsifiers. The interfacial properties were measured with a pendant drop film balance equipped with a subphase exchange technique, which allows adding the bile salt directly into the subphase once the surfactant has been pre-adsorbed onto the oil-water interface. We can hence monitor in-situ the effect of the BS on the interfacial layer by comparing with the behavior of the individual systems. Interfacial tension showed lower adsorption rates for BS onto Pluronic-covered interface, as compared to pre-adsorbed Epikuron layer. Finally, in order to gain structural information we have fitted the experimental data with the Frumkin adsorption Isotherm by using software package IsoFit. As a result, we get that the molecular area and molecular interaction decrease in the following order: Pluronic > NaTDC > Epikuron within the interfacial layer. Therefore, we prove that the Pluronic adsorbed layer is more resistant to bile salt adsorption than the Epikuron interfacial layer. This correlates with the physicochemical properties of O/W emulsions. The use of interfacial techniques provides new insight into the action of BS on O/W emulsion, which constitutes one of the main challenges in order to clarify the mechanisms involved in lipid digestion and absorption.

►Pluronic F68 provides higher emulsion stability than phospholipids against bile salts. ►We analyze the effect of a bile salt on the interfacial behavior of such emulsifiers. ►Thermodynamic and mechanical properties are analyzed by surface tension techniques. ►Structural information is obtained by a thermodynamic model. ►Pluronic adsorbed layer more resistant to bile salt adsorption than phospholipid one.