A HYDRODYNAMIC ATOMIC-FORCE MICROSCOPY FLOW CELL FOR THE QUANTITATIVEMEASUREMENT OF INTERFACIAL KINETICS - THE AQUEOUS DISSOLUTION OF SALICYLIC-ACID AND CALCIUM-CARBONATE

A novel liquid flow cell allows atomic force microscopy (AFM) images t o be obtained under defined hydrodynamic flow conditions, enabling rea ction fluxes calculated from proposed heterogeneous reaction mechanism s to be compared with those determined experimentally. The cell employ s an inclined jet to direct a fluid flow at the sample surface to cove r the area under investigation including the AFM scanning cantilever t ip. The now pattern and velocity were calculated by using the finite e lement fluid dynamics program FIDAP and confirmed by placing an electr ode at the sample position and measuring the limiting current for the one-electron oxidation of potassium hexacyanoferrate(II) in water as a function of flow rate. The operation of the cell has been further con firmed by the direct measurement of the dissolution rate of calcite ex posed to a now of 0.98 mM aqueous HCl by deducing the rate of removal of the surface from the change of the z-piezo voltage. The rate consta nt k(1) = 0.035 cm s(-1) obtained for the dissolution step was in exce llent agreement with the value (0.043 +/- 0.01.5) cm s(-1) found from independent channel flow cell experiments. The dissolution of the ((1) over bar 10) and (110) faces of salicylic acid (SA) single crystals i n water and in solutions of salicylic acid was studied as a function o f flow rate and was found to be consistent with a model combining a co nstant rate of dissolution with a simultaneous reprecipitation having a first-order dependence on [SA](0), with the flux J = k(d) - k(p)[SA] (0) where the parameters are k(d)((1) over bar 10) = 3 x 10(-9) mol cm (-2) s(-1) and k(p)((1) over bar 10) = 1.74 x 10(-4) cm s(-1) for the ((1) over bar 10) face and k(d)(110) = 1.5 x 10(-8) mol cm(-2) s(-1) a nd k(p)(110) = 1.06 x 10(-3) cm s(-1) for the (110) face.