Structural features of interfacially adsorbed acyl-l-carnitines

Huayang Liu; Ke Fa; Xuzhi Hu; Zongyi Li; Lin Zhang; Kun Ma; Giovanna Fragneto; Peixun Li; John R P Webster; Jordan T Petkov; Robert K Thomas; Jian Ren Lu

doi:10.1016/j.jcis.2022.05.024

Structural features of interfacially adsorbed acyl-l-carnitines

J Colloid Interface Sci. 2022 Oct:623:368-377. doi: 10.1016/j.jcis.2022.05.024. Epub 2022 May 7.

Authors

Huayang Liu¹, Ke Fa¹, Xuzhi Hu¹, Zongyi Li¹, Lin Zhang¹, Kun Ma², Giovanna Fragneto³, Peixun Li², John R P Webster², Jordan T Petkov⁴, Robert K Thomas⁵, Jian Ren Lu⁶

Affiliations

¹ Biological Physics Laboratory, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK.
² ISIS Neutron Facility, Rutherford Appleton Laboratory, STFC, Chilton, Didcot, Oxon OX11 0QX, UK.
³ Institut Laue-Langevin, 71 Avenue des Martyrs, CS20156, 38042 Grenoble Cedex 9, France.
⁴ Arxada, Hexagon Tower, Delaunays Road, Blackley, Manchester M9 8ZS, UK.
⁵ Physical and Theoretical Chemistry, University of Oxford, South Parks, Oxford OX1 3QZ, UK.
⁶ Biological Physics Laboratory, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK. Electronic address: j.lu@manchester.ac.uk.

PMID: 35594595
DOI: 10.1016/j.jcis.2022.05.024

Abstract

Hypothesis: Acyl-l-carnitines (C_nLCs) 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 SiO₂/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 SiO₂/water interface from a group of C_nLC (n = 12, 14, and 16).

Findings: C_nLC 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 SiO₂/water interface. The total thicknesses of the adsorbed layers at CMC were found to be 33 ± 2, 35 ± 2, and 37 ± 2 Å for C₁₂LC, C₁₄LC, and C₁₆LC, 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 C_nLC 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 C_nLCs involving a solid substrate.

Keywords: Antimicrobial surfactant; Biosurfactant; Compatibility; Interfacial adsorption; Neutron reflection; acyl-l-carnitine; pH responsive.

MeSH terms

Adsorption
Carnitine
Silicon Dioxide* / chemistry
Surface-Active Agents* / chemistry
Water / chemistry

Substances

Surface-Active Agents
Water
Silicon Dioxide
Carnitine