Structural features of interfacially adsorbed acyl-l-carnitines.

Hypothesis: Acyl-l-carnitines (CLCs) are potentially important as biosurfactants in drug delivery and tissue engineering due to their good biocompatibility. However, little is currently known about the basic interfacial behavior underlying their technological applications. Following our previous characterization of their solution aggregation and adsorption at the air/water interface, this work examines how they adsorb at the hydrophilic solid/liquid interface.

Experiments: As the SiO/water interface has served as the model substrate for many interfacial adsorption studies, so it has been used in this work as the solid substrate to facilitate dynamic adsorption by spectroscopic ellipsometry (SE) and structural determination of the adsorbed layers by neutron reflection (NR) under different conditions at the SiO/water interface from a group of CLC (n = 12, 14, and 16).

Findings: CLC surfactants are zwitterionic at neutral pH. They reached saturated adsorption above their critical micellar concentrations (CMCs) and formed a sandwich bilayer with a head-tail-head structure at the hydrophilic SiO/water interface. The total thicknesses of the adsorbed layers at CMC were found to be 33 ± 2, 35 ± 2, and 37 ± 2 Å for CLC, CLC, and CLC, respectively, with their inner and outer head layers remaining similar but the thickness of the interdigitated middle layer increasing with acyl chain length. As the solution becomes acidic, the carboxyl groups become protonated and the l-carnitine heads are net positively charged, resulting in increased repulsion between the head groups. In this situation, the CLC surfactants are adsorbed as distinct aggregates to reduce repulsive interaction, resulting in reduced surfactant volume fraction and layer thickness. However, a high ionic strength can screen the repulsive interaction and enhance the adsorbed amount, effectively diminishing the impact of pH. This information provides a useful basis for exploring the technological applications of CLCs involving a solid substrate.