Acid-induced dissolution and erosion of materials is important in a wide range of areas including the earth sciences,1 the life sciences, 2 pharmaceutical sciences 3 and in technological applications, such as scale removal.4 Methods which can probe and provide a quantitative understanding of acid-induced erosion are thus of considerable value. This is particularly true in the area of dental science where knowledge of the rate of dental enamel dissolution is very important in the context of acid erosion and in understanding how this process can be inhibited.5-7 Dental erosion is a product of modern diets rich in highly acidic foods and beverages, although other factors can contribute.5-8
Enamel is a complex mineral, comprising of over 95 % (by weight) calcium hydroxyapatite (HA), typically as aligned rods in domains 4 – 8 µm across, while the remaining material is a matrix of organics and water.5-10 The net result of the attack of protons on HA can be represented by9:
+ 2+ 2-
10 4 6 2(s) (aq) (aq) 4 (aq) 2
Ca (PO ) (OH) + 8H 10Ca + 6HPO + 2H O (3.1)
Many techniques have been used to investigate the effect of dissolution on enamel including
ex-situ surface techniques such as scanning electron microscopy (SEM),9-15 atomic force microscopy (AFM),9, 13, 16-20 profilometry 9, 21 and, more recently, nanoindentation. 7, 9, 13, 22 Measurements of dissolution rates have tended to involve the chemical analysis of solutions during enamel dissolution.9, 15, 23-25 These studies have shown that the extent of enamel loss via acidic dissolution is closely linked to the (bulk) pH of the solution,10, 20-22, 24, 26-29 with low pH clearly promoting the erosion process, and the degree of saturation with respect to calcium and phosphate (with higher degrees of saturation correlating with slower erosion).19
As for all chemical processes at solid/liquid interfaces, acid-induced erosion involves mass transport and coupled chemical reactions in solution and at the surface.30 Understanding the contributions of each of these processes to the net rate of dissolution is important to gain a true understanding of the process. This requires that experiments are carried out under conditions of well defined and quantifiable mass transport. Gray31 incorporated the effects of mass transport through studies investigating the effect of stirring and temperature, concluding that dissolution was diffusion-controlled and similar conclusions have been reached in more recent studies.32, 33 However, it is important to point out that these studies were typically carried out at low transport rates, which were also not particularly well defined. While the rotating disc method provides very well-defined mass transport,1, 16, 34, 35 it has not been applied to the rather low pH regimes characteristic of enamel erosion.
This chapter endeavours to determine a definitive rate constant for the acid-induced dissolution of bovine dental enamel, through the use of scanning electrochemical microscopy (SECM). This technique is capable of delivering a reagent to a surface with high rates of mass transport, so allowing the characterisation of fast surface processes.36-39 In the context of the studies herein, SECM has proved powerful for studying the kinetics of dissolution and related processes at crystals and minerals.37-47 We have previously studied dissolution kinetics by SECM chronoamperometry, which both induces and monitors dissolution rates.42-
49 In these studies the ultramicroelectrode (UME) tip potential was stepped from a value
where no reaction occurred to one which initiated dissolution by depleting one or more ion types, making the solution undersaturated. The current response, measured as a function of time and/or distance between the UME and surface, provided direct information on dissolution fluxes, which could be quantified because such experiments were underpinned by well-defined mass transport models.42-49 A further advantage of the SECM approach is that measurements are made on the microscopic scale, so that it is possible to make multiple
dissolution measurements on one sample, providing good statistics and the ability to determine heterogeneity in reaction rates.
It is not always possible to both perturb and monitor dissolution by simple electrolysis (as in previous SECM crystal dissolution studies) at the UME probe and this is the case for acid- induced dissolution. Although one can generate protons electrochemically 41 and one could collect these back, in principle, in a second potential step, as in double potential step SECM studies, 50, 51 the system would be complicated for the low proton concentrations that one might expect, and by the presence of oxygen (an interferent) in the generation and detection of protons.52 The technique employed in this chapter thus uses the UME to quantitatively produce protons,53 necessary to dissolve the enamel surface (equation 3.1), in a highly controlled manner, but in contrast to previous dissolution studies we use longer etch periods and analyse the pit shape that results in the surface quantitatively. SECM has been used previously to create etch pits in various solid materials,44, 45, 54, 55 but these features have not generally been analysed quantitatively to provide kinetic data. By developing a moving boundary finite element model for the acid-induced dissolution process, we show that it is possible to obtain highly quantitative information on the acid erosion process by geometric analysis of the pit shape as a function of proton flux and time. Our studies provide considerable insights on the kinetics of the acid-induced erosion process and allow the rate of acid attack to be quantified.