Solution aggregation and interfacial adsorption of novel acyl-L-carnitine surfactants

Abstract

Solution aggregation and interfacial adsorption of novel acyl-L-carnitine surfactants L-carnitine has been widely used as a nutrition supplement to help with weight loss, as it plays an important role in the generation of energy by the transport of fatty acids into mitochondria. Acyl-L-carnitines are biosurfactants with amphiphilic behavior and potential in antimicrobial and personal care applications. However, the understanding of their physicochemical properties remains lacking, limiting the development of their practical applications. The work in this thesis has focused on the physicochemical properties including solubility, stability, solution aggregation and interfacial adsorption (air/liquid and solid/liquid interfaces) of this novel series of biosurfactants. The aqueous solubility of acyl-L-carnitines (CnLC, n=12, 14 and 16) is influenced by temperature, pH and acyl chain length as investigated by dynamic light scattering (DLS). Our studies showed that C12LC has the highest solubility followed by C14LC and C16LC. Higher temperature and lower pH could improve their solubility. The hydrodynamic radii of the micelles of CnLC were firstly measured by DLS. The size and shape of the micelles were further investigated by small angle neutron scattering (SANS) with protonated and deuterated samples. Our studies have revealed that the micelles of CnLC have a core-shell spherical structure. The thickness of the shell of the three CnLC micelles is similar as the shell consists of the same head groups (L-carnitines) and water molecules. The micellar shape of CnLC micelles remained unchanged, but pH-responsive carboxyl groups in head groups switched the zwitterionic surfactants at neutral pH into cationic surfactants in an acidic environment. As the pH decreases, CnLC micelles carry more positive charges on the micellar surface leading to stronger electrostatic repulsion between micelles. However, this repulsive interaction could be screened under high ionic strength. Interfacial adsorption was studied at two typical interfaces including air/liquid and solid/liquid interfaces. At the air/water interface, surface tension measurements were used to determine their critical micelle concentrations (CMCs). The surface excess and area per molecule of CnLC at the air/water interface were calculated in combination with the Gibbs equation. The structure of the adsorbed layer was examined by neutron reflection (NR) by taking advantage of deuterium labelled CnLCs. Our studies showed that the CnLC molecules form a monolayer at the air/water interface with the tilted acyl chains in the air. The thickness of the layer, adsorbed amount, area per molecule and tilting angle are dependent on concentration, acyl chain length, pH, and ionic strength. At the SiO2/water interface, the adsorption dynamics and equilibrium adsorption amount were firstly investigated by spectroscopic ellipsometry (SE) and NR was used to provide detailed structural information. CnLC molecules tended to form surface aggregates at the interface which then became into the surfactant bilayer as the concentration increased. This project provided a detailed investigation of the solution aggregation and interfacial adsorption of CnLC under different environmental conditions. These features make CnLCs potentially more attractive biosurfactants than many synthetic ones.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jian Lu (Supervisor) & Henggui Zhang (Supervisor)