A Hydrodynamic Atomic Force Microscopy Flow Cell for the Quantitative Measurement of Interfacial Kinetics:  The Aqueous Dissolution of Salicylic Acid and Calcium Carbonate

Abstract

A novel liquid flow cell allows atomic force microscopy (AFM) images to be obtained under defined hydrodynamic flow conditions, enabling reaction fluxes calculated from proposed heterogeneous reaction mechanisms to be compared with those determined experimentally. The cell employs an inclined jet to direct a fluid flow at the sample surface to cover the area under investigation including the AFM scanning cantilever tip. The flow pattern and velocity were calculated by using the finite element fluid dynamics program FIDAP and confirmed by placing an electrode 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 confirmed by the direct measurement of the dissolution rate of calcite exposed to a flow 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 constant k 1 = 0.035 cm s-1 obtained for the dissolution step was in excellent agreement with the value (0.043 ± 0.015) cm s-1 found from independent channel flow cell experiments. The dissolution of the (1̄10) and (110) faces of salicylic acid (SA) single crystals in water and in solutions of salicylic acid was studied as a function of flow rate and was found to be consistent with a model combining a constant 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 = 3 × 10-9 mol cm-2 s-1 and = 1.74 × 10-4 cm s-1 for the (1̄10) face and = 1.5 × 10-8 mol cm-2 s-1 and = 1.06 × 10-3 cm s-1 for the (110) face.

Determination of Outer Layer and Bulk Dehydration Kinetics of Trehalose Dihydrate Using Atomic Force Microscopy, Gravimetric Vapour Sorption and Near Infrared Spectroscopy

“…As has been shown before,4–8 the data presented here demonstrate that AFM is a suitable technique for the investigation of the kinetics of surface processes. Using this technique, it was shown that the outer layer kinetics of the dehydration of trehalose dihydrate at 27–28°C and <1.5% RH followed a different model with a different rate constant when compared with the kinetics of dehydrating the whole sample.…”

“…The development of atomic force microscopy (AFM) has had a major impact on the understanding of surfaces 3. AFM experiments can be carried out on unprepared samples under various environmental conditions, an advantage which has been exploited by various authors to study the kinetics of surface processes, such as dissolution,4 the formation of lipid bilayers5 and the desorption of Langmuir–Blodgett films 6. There are also pharmaceutical examples of the use of AFM to study surface kinetics, both of which examined the crystallisation of amorphous lactose 7, 8.…”

Magnesium inhibition of calcite dissolution kinetics

“…We did not attempt to establish hydrodynamic descriptions of these flow environments (e.g., Coles et al, 1998;Higgins et al, 2002), and have instead employed sufficiently high flow rates to measure dissolution rates that reflect surface control. This strategy is consistent with many of the published AFM experiments that address both calcite dissolution (Liang et al, 1996;Liang and Baer, 1997) and growth (e.g., Teng et al, 1998).…”

Quantitative Plane-Resolved Crystal Growth and Dissolution Kinetics by Coupling In Situ Optical Microscopy and Diffusion Models: The Case of Salicylic Acid in Aqueous Solution

“…15 However, for salicylic acid, only dissolution kinetics have been investigated using in situ AFM. [16][17][18] In water, the (110) and (110) faces were studied 16 and dissolution rates determined.…”