Predicting the foamability of N-acyl amino acid surfactants via noncovalent interactions

A rich, creamy lather is the desired property of N-acyl amino acid surfactants (AAAS), which depends on unknown noncovalent interactions varied with amino acid structures. Herein, a small library of AAAS was established by molecular design and synthesis of seventeen AAAS. Self-assembly parameters of AAAS in the aqueous solution, micelle, and adsorption monolayer were determined, for example, the critical micelle temperature (CMT) obtained by the low-temperature solubility method, the (I1/I3)CMC of pyrene by fluorescence molecular probe, the pC20 (the adsorption efficiency at the air/water surface) and the critical micelle concentration (CMC) by tensiometry. The foaming capacity (H0) and the foam stability (H5) of AAAS were determined by the Ross- Miles foam test. It is found that better foamability is associated with greater micellization/adsorption competitive tendency (CMC/C20) of AAAS than that of SLGly. We also conducted a systematic study on sorting out the noncovalent interactions of AAAS (such as hydrophobic interactions, intra/intermolecular hydrogen bonding, electrostatic repulsion, solvation, etc.) through the specific combination mode of the CMT and the (I1/I3)CMC, by which the foamability is predicted and estimated. Both the CMT and the (I1/I3)CMC of AAAS lower than those of SLGly are favorable for enhancing the foamability by strengthening the hydrophobic effect or blocking the intermolecular hydrogen bonding of SLGly. Furthermore, the CMT decreased while the (I1/I3)CMC increased indicates the electrostatic repulsion or the solvation effect highlight, conversely the intramolecular hydrogen bonding instead, and these interactions are less favorable for the foamability.