EFFECT OF CTAB ADSORPTION ON SURFACE-CHARGE AND STABILITY OF AN AEROSIL

Potentiometric titrations of an aerosil in CTAB solutions with a suppo rting SolUtion of 10(-3) M KCl have shown that as the CTAB concentrati on increases in solution there is a steady increase in the negative ch arge on the SiO2 surface until it reaches a constant value in the ccm region. The limiting surface charge density in CTAB solutions at pH = 6 is -62 muC/cm2, much higher than its value in pure KCl solution wher e it is -0.32 muC/cm3 at the same pH. This indicates that in CTAB solu tions the surface charge on the aerosil does not arise simply as a res ult of adsorption of potential-determining ions (OH-) but is due mainl y to bond formation between neutral SiOH groups and CTA+ ions adsorbed in the first (or Stem) layer. The observed effect enables the results from continuous potentiometric titrations to be used to calculate the adsorption isotherm of CTA+ in the first layer together with the adso rption potential of CTA+ ions,found to be -9.8 kT. Results from the ad sorption studies have been compared with those for stability studies o n aerosil dispersions in CTAB solutions.