Aspartic-Acid-Based Ampholytic Amphiphiles: Synthesis, Characterization, and pH-Dependent Properties at Air/Water and Oil/Water Interfaces.

A facile and two-step strategy was employed to synthesize a series of novel aspartic-acid-based ampholytic amphiphiles from sustainable and commercially viable substances as starting materials. The molecular structures of the synthetic compounds were well-identified by mass spectrometry and 1H/13C nuclear magnetic resonance analysis, and the physicochemical, pH-dependent foaming, and emulsifying properties were evaluated by the use of multiple techniques, such as Fourier transform infrared spectroscopy, differential scanning calorimetry, Langmuir-Blodgett study, and fluorescence microscopy imaging. As a result of the co-existence of amino and carboxyl groups in the synthetic compounds, the compounds presented varying charges (cationic, ampholytic, and anionic) depending upon the pH of the medium compared to the dissociation constants (p Ka). Compounds with cationic (pH 1.0) and anionic (pH 9.0) forms had significantly higher γ0.1 and critical micelle concentration values than those with ampholytic forms (pH 7.0). sn-1-Lauroyl- sn-3-aspartic acid (compound 3) at neutral and alkaline conditions displayed comparable foaming properties, including foaming, calcium-tolerant, and temperature-resistant abilities, with commercial sulfonate sodium dodecyl sulfate (SDS), and thus might be a promising alternative to SDS, applied in personal care products and detergent formula. sn-1-Palmtoyl- sn-3-aspartic acid (compound 5a) with an ampholytic structure was proven as the most excellent stabilizer for the preparation of oil-in-water emulsions compared to palmityl aspartic acid (compound 5b), commercial food ingredient diacetyltartaric acid esters of mono- and diglycerides, and glyceride monopalmitate at aqueous phase pH 7.0. Thus, it has promising use as a pH-dependent emulsifying agent in various fields.