Nonequivalent adsorption of pH-responsive dicephalic sugar surfactants at the air/solution interface

The present work contributes to the analysis of the effect of electrolyte and pH on the adsorption of dicephalic N-dodecyl-N,N-bis[(3-D-aldonylamido) propyl]amines (aldonyl = gluconyl and lactobionyl) at the air/water interface. The headgroup of these surfactants consists of two amide groups connected by a central tertiary amine, which may be protonated in an aqueous environment. We performed measurements of surface tension isotherms of aqueous solutions of the studied dicephalics in the presence of NaCl and NaOH and applied the STDE model for evaluation of the experimental results. They showed that the adsorption of the surfactants at the air/water interface was pH- and electrolyte- dependent, since addition of NaCl and NaOH had different effects on the surface activity and critical micelle concentration values (CMC). The theoretical model reflected well the surface tension isotherms obtained for a broad range of surfactants concentration, confirming the theoretical predictions concerning the protonation of tertiary amines, which was additionally supported by the measurements of solution pH.