Ester-Based Quaternary Ammonium Salts: A Surface Tension and Small-Angle X-ray Scattering Study

Conventional synthesis of ester-based quaternary ammonium salts is complex reactions and poor atomic utilization. This paper presents an optimized synthetic route via Michael addition to achieve high atom economy and operationally simple synthesis. Three ester-based quaternary ammonium salts (EQ-MA, EQ-BA, EQ-TBA) differing only in the ester-adjacent linker (methyl, n-butyl, t-butyl) were synthesized. Surface tension isotherms give similar CMC values (0.40-0.45 mmol L^-1) but distinct linker-dependent adsorption: EQ-BA shows the highest pC₂₀, while saturated interfacial packing becomes denser from EQ-MA to EQ-TBA, evidenced by increasing maximum surface excess concentration (Γ_max) and decreasing minimum area per molecule (A_min). SAXS (Guinier and PDDF) indicates predominantly spherical micelles for all three surfactants, with different concentration responses: EQ-MA changes little up to 2.0 mM and grows at 4.0 mM; EQ-BA reorganizes most strongly toward smaller, more compact, and more uniform aggregates at higher concentration; EQ-TBA shows the same trend but with weaker changes. Time-dependent SAXS at 1.0 mM (up to 7 days) shows no low-q upturn or new features, confirming dispersion stability. Foaming tests at 1.0 mM show higher initial foam heights for EQ-BA and EQ-TBA than for EQ-MA, while the foam retention ratios over 5 min are similar (0.95-0.96). Overall, the data suggest that linker geometry influences interfacial packing and the concentration/time response of micellar structure.