Second harmonic generation (SHG) spectroscopy was used to characterize the pH-dependent electrostatic charging behavior of (0001) and (102) crystallographic surfaces of corundum (alpha-Al2O3) single-crystal substrates. The pH value of the point of zero charge (pH(pzc)) for each surface was determined by monitoring the SH response during three consecutive pH titrations conducted with 1, 10, and 100 mM NaNO3 carbonate-free aqueous solutions. The crossing point of the three titration curves, which corresponds to the pH(pzc), occurs at pH 4.1 +/- 0.4 for the (0001) surface and pH 5.2 +/- 0.4 for the (102) surface. SHG measurements that were recorded as a function of NaNO3 concentration at fixed pH values were found to corroborate the pH(pzc) values identified in the pH titrations. A comparison of the SHG results with surface protonation constants calculated using a simple electrostatic model suggests that surface relaxation and bonding changes resulting from surface hydration do not account for differences between experimental observations and model predictions. The measured pH(pzc) values for the alpha-Al2O3 single-crystal surfaces are significantly more acidic than published values for Al-(hydr)oxide particles which typically range from pH 8 to 10. This discrepancy suggests that the charging behavior of Al-(hydr)oxide particles is determined by surface sites associated with defects assuming that differences in surface acidity reflect differences in the coordination environment and local structure of the potential-determining surface groups.

No abstract

The transport rate of an organic cation, malachite green (MG), across a unilamellar bilayer (∼105 nm) of the dioleoylphosphatidylglycerol (DOPG) liposome has been studied by the SHG technique. This is the first time to our knowledge that the effects of anions on molecular cation transport have been observed. Our studies show four results. First, in the presence of sodium chloride (NaCl) or sodium bromide (NaBr), the time constant for transport of MG across the DOPG bilayer increases with the increase in the concentration of the counterion (i.e., Cl -or Br -). Second, with the organic electrolytes, sodium citrate (NaCitrate) and sodium ethanesulfonate (NaEtSO 3 ), the transport rate is independent of the concentration of the counterion (i.e., Citrateand EtSO 3 -). Third, at the same counterion concentration, the transport rate depends on the species of the counterion used. The rate of MG transport is the slowest with Cl -, faster with Br -, and the fastest with the two organic counterions, Citrate -and EtSO 3 -. Last, at the low counterion concentration of 1mM, the transport rates of MG were found to approach the same value for the four anions. A brief discussion of a possible mechanism is presented.

Polarizability response spectroscopy, a two-color optical Kerr effect method, has been developed and employed to study solvent intermolecular polarizability responses to photoexcited solutes. Here, we report solvent intermolecular polarizability responses in ͑dipolar͒ solvation. The time-resolved nonresonant polarizability signals are analyzed in the frequency domain where they are fit to a functional form representing diffusive reorientational, interaction-induced, and librational motions. Diffusive reorientational motion of CHCl 3 was preferentially driven following photoexcitation of Coumarin 153 while interaction-induced motion was mainly driven in CH 3 CN solutions. The mechanism for selective solvent responses involves the relative orientation of the solvent dipole and most polarizable molecular axes and their interaction strength to the solute dipole.