INFERENCE OF SURFACE EXCESS AND ITS HIGH-ACTIVITY LIMIT IN MONOLAYER ADSORPTION FROM INTERFACIAL-TENSION AND ACTIVITY DATA

For dilute aqueous solutions of organic adsorbates it has been common to assume adsorbate activity proportional to its concentration (Henry's law) to avoid activity measurements. It is shown that for concentrations up to 1 M the Henry's law "constant" K can be represented by ln K = ln K(H)-degrees - beta-c, that a surface excess based on Henry's law can hence be in error by a factor (1 - beta-c), and that this error can be as high as 30% for a saturated butanol-1 solution. Methods of estimating this correction are given. Methods for using the regular localized monolayer or Frumkin equation model to extend the experimental data range useful for estimating limiting surface excess are given. A study of the adsorption of butanol-1 and butanol-2 from saturated aqueous Na2SO4 at the mercury-electrolytic solution interface polarized to the potential of zero charge in the base electrolyte solution, based on interfacial tensions measured as a function of butanol activity, is used to illustrate these principles. A limiting surface excess 10(10)-GAMMA-m = 6.0 mol/cm2 was thus found for butanol-1, agreeing excellently with Hommelen's results for CH3(CH2)n-1OH, 6 less-than-or-equal-to n less-than-or-equal-to 10, and substantially higher than earlier reported values based on the assumption of Henry's law and simple graphical estimation of limiting tangent.