Synthesis and interface behaviors of amino acid surfactants with naturally derived branched hydrophobic chains

Abstract

Amino acid surfactants play a crucial role in many personal care products and pharmaceuticals. Their significance arises from their unique characteristics, including diverse molecular structures, low skin irritation, and excellent biodegradability. The structure of amino acid surfactants, particularly the structure of their hydrophobic chains, plays a pivotal role in determining their interfacial properties. It is proposed that the steric hindrance effect stemming from the presence of branched hydrophobic chains can exhibit a profound influence on both the interfacial adsorption behavior and the overall performance of amino acid surfactants. We synthesized a range of novel amino acid surfactants featuring varying lengths of branched chains, derived from natural terpenoid alcohols. Several characterization techniques, including surface tension measurements, dynamic light scattering, foam volume assessments, demulsification time evaluations, and contact angle measurements were used to reveal the substantial influence of branched chains and the chain length on the surfactant performance. The investigation shows how the presence of branched chains influences their interfacial properties, their propensity to form larger aggregates above the critical micelle concentration and the impact of pH on the surfactants performance. Within the examined pH range, surfactants featuring natural branched farnesol chains exhibit critical micelle concentration ranging from approximately 0.8 to 3.8 mM. Those values are significantly lower when compared to surfactants possessing similar length of linear chains. Simultaneously, the conversion of linear hydrophobic chains into branched chains enhances the foam stability promoted by the surfactants by approximately 10 %. These findings emphasize the collective impact of hydrophobic interactions and steric hindrance of the hydrophobic chains on surfactant surface packing. The distinctive interfacial behavior exhibited by branched surfactants shows great potential in establishing a theoretical foundation for formulation research in the development of highly efficient detergents and premium cosmetics.