Adsorption of Dodecylammonium Chloride to the Muscovite Mica Basal Plane in Acid Aqueous Solution

Abstract

The adsorption of dodecylammonium chloride (DAC) to the basal plane of muscovite mica was investigated together with that of fused silica by means of zeta potential, adsorption isotherms, and contact angle measurements at pH 3 where the zeta potential for the mica (-75 mV) was more negative than that for the silica (-25 mV). Marked changes in zeta potential, adsorption, and contact angle occurred at critical concentrations of 10-5 and 2 × 10-4M DAC for mica and silica, respectively, implying that the difference in the inner Helmholtz potential between mica and silica is -85 mV. The negative surface charges for mica and silica were neutralized at the adsorption densities of 8.4 × 10-11 and 8.6 × 10-12 mol/cm2, respectively. The interaction between hydrocarbon chains in adsorption energy gave the values of -0.87 and -0.53 kT per CH2 group for the mica and the silica, respectively. The contact angles for mica gave the maximum value of 55°, which was higher than the value of 25° for silica at charge-neutralized surfaces. At a concentration of 10-2M DAC, the zeta potential for mica showed the value of 100 mV which was higher than the one of 20 mV for silica, and the ionized fraction of DA+ ions in the second layer gave the values of 0.08 to 0.18 for both the mica and the silica. These values agreed well with that of 0.14 for micelles. These observations supported the difference in zeta potential between mica and silica at pH 3.