Unusual Adsorption at the Air-Water Interface of a Zwitterionic Carboxybetaine with a Large Charge Separation

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

HYPOTHESIS

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.

EXPERIMENTS

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.

FINDINGS

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.

Molecular Organization of an Adsorbed Layer: A Zwitterionic, pH-Sensitive Surfactant at the Air/Water Interface

Neutron and X-ray reflection measurements have been used to study the structure of the adsorbed layer of a chelating surfactant at the air/liquid interface. The chelating surfactant 2-dodecyldiethylenetriaminepentaacetic acid (C₁₂-DTPA) has a large headgroup containing eight donor atoms that can participate in the coordination of metal ions. The donor atoms are also titrating, resulting in an amphoteric surfactant that can adopt a number of differently charged species depending on the pH. Very strong coordination complexes are formed with metal ions, where the metal ion can be considered as part of the surfactant structure, in contrast to monovalent cations that act as regular counterions to the negative net charge. Adsorption was investigated over a large concentration interval, from well below the critical micelle concentration (cmc) to five times the cmc. The most striking result is the maximum in the surface excess found around the cmc, which is consistent with previous indications from surface tension measurements. Adding divalent metal ions has a limited effect on the adsorption at the air/liquid interface. The reason is the coordination of the metal ion, resulting in compensating deprotonation of the complex. Small variations in the headgroup area of different metal complexes are found, correlating to the conditional stability constants. Adding sodium chloride has a significant effect on the adsorption behavior, and the results indicate that the protonation equilibrium is more important than the ionic strength effects. From combined fits of the neutron and X-ray data, a model that consists of a thick headgroup region and a relatively thin dehydrated tail region is found, and it indicates that the tails are not fully extended and that the limiting area per molecule is determined by the bulky headgroup.