Thermodynamic study of the acid-base equilibrium on the surface of a synthetic clay, laponite.: Adsorption of dimethyldodecylamine-N-oxide onto laponite at various pHs, in aqueous media, at 298 K

Adsorption isotherms were determined for dimethyldodecylamine-N-oxide (DDAO) onto Laponite dispersed in water. The isotherms were obtained at pH 3.5 and 4.5 and at various NaCl concentrations (0.1, 0.2, and 0.3 M) by potentiometry, using an electrode sensitive to the nonionic surfactant. The stability of the synthetic clay at the pH values and for the durations of the measurements used was initially verified in preliminary tests. At the acidities used, DDAO in aqueous solution is bound in an acid-base equilibrium with its protonated form (DDAOH(+)). For a given pH, adsorption isotherms, determined for different ionic strengths, showed a "common intersection point". Coordinates of this point were compared to those obtained for its cationic homologue, dodecyltrimethylammonium (DTA(+)), in the same conditions. We used these data to deduce the value of the acid-base constant of DDAOH(+)/DDAO pairs on the Laponite surface: (sigma)pK(a) = 3.9. The acid, DDAOH(+), seems to be stronger when adsorbed than when in solution (pK(a) = 4.95).