Surface adsorption and solution aggregation of a novel lauroyl-l-carnitine surfactant

l-carnitine plays a crucial role in the cellular production of energy by transporting fatty acids into mitochondria. Acylated l-carnitines are amphiphilic and if appropriate physical properties were demonstrated, they could replace many currently used surfactants with improved biocompatibility and health benefits.

This work evaluated the surface adsorption of lauroyl-l-carnitine (C₁₂LC) and its aggregation behavior. The size and shape of the aggregates of C₁₂LC surfactant were studied at different temperatures, concentrations, pH and ionic strength by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). Surface tension measurements were carried out to determine the critical micellar concentration (CMC) of C₁₂LC. Combining with the Gibbs equation, the surface excess at different concentrations could be determined. Neutron reflection (NR) was used to determine the structure of the adsorbed layer at the air/water interface with the help of isotopic contrast variations.

At pH 7, the limiting area per molecule (A_CMC) of the zwitterionic C₁₂LC adsorbed layer at the air/water interface was found to be 46 Å² from surface tension and neutron reflection, smaller than the values of C₁₂PC, C₁₂E₅, DTAB, C₁₂C₄betaine and C₁₂C₈betaine but close to that of SDS. A pronounced surface tension minimum at pH 2 at the low ionic strength was linked to a minimum value of area per molecule of about 30 Å², indicating the competitive adsorption from traces of lauric acid produced by hydrolysis of C₁₂LC. As the concentration increased, area per molecule reached a plateau of 37-39 Å², indicating the dissolution of the more surface-active lauric acid into the micelles of C₁₂LC. DLS and SANS showed that the size and shape of micelles had little response to temperature, concentration, ionic strength or pH. The SANS profiles measured under 3 isotopic contrasts could be well fitted by the core-shell model, giving a spherical core radius of 15.7 Å and a shell thickness of 10.5 Å. The decrease of pH led to more protonated carboxyl groups and more positively charged micelles, but the micellar structures remained unchanged, in spite of their stronger interaction. These features make C₁₂LC potentially attractive as a solubilizing agent.