Dissolu'on of Calcite Microcrystals Harriet Pearce, Maria Adobes-­‐Vidal and Prof. Patrick Unwin Department of Chemistry, University of Warwick CALCITE MICROCRYSTALS INTRODUCTION •  Calcium carbonate (calcite) has a wide range of industrial applica:ons. It is a common system of study in the field of biomineralisa:on and a complete understanding of the dissolu:on process is necessary. •  Dissolu:on studies were carried out using Op:cal Microscopy (OM) combined with Bias Modulated Scanning Ion Conductance Microscopy (BM-­‐SICM)1. EXPERIMENTAL z y x Crystals grown on different surfaces: Forma:on of calcite 2 Ca2+ + HCO3-­‐ è CaCO3 + H+ Func:onalised Glass (PLL) -­‐ posi:ve Func:onalised Glass (PGA) -­‐ nega:ve Glass – slightly nega:ve SEM image 3 Hemispherical diffusion profile 30 μm < Length < 40 μm FAST DIFFUSION enhances visualisa:on of surface processes OM images were used to measure the perimeter (x and y displacement) of the crystal over :me whilst BM-­‐SICM in hopping mode tracked the height (z displacement) of the crystal over :me. Plas:c -­‐ neutral Single isolated microcrystal (Op:cal Microscope image at 200x magnifica:on) Op:cal Microscope images at 100x magnifica:on Experimental Setup: Nanopipe]e •  Dissolu:on condi:ons: 50 mM KCl and HCl at pH 3.95, 3.10 and 3.02. •  ImageJ was used to analyse the images and measure the perimeter of the crystal at each :me point. Current Amp. Ag/AgCl electrode Control system The :p scans over the sample, approaching and retrac:ng. As the :p gets close to the surface, the ion current decreases 1. RESULTS t = 0 Dissolu:on rate (DR) calculated from crystal perimeter changes pH 3.95 > 6 hours ~ 1 hour pH 3.02 Op'mum for BM-­‐SICM pH 3.10 Non Linear – DR non constant over :me Linear t Time lapse Op:cal Microscope images at 400x magnifica:on. BM-­‐SICM -­‐ Dissolu'on of calcite microcrystals at pH 3.10: OM -­‐ Dissolu'on of calcite microcrystals at different pH values: Graph to show the height of the crystal with respect to the x posi:on over :me during dissolu:on. Experiment Average DR (mol m-­‐2 s-­‐1) OM pH 3.02 (2.6 ± 0.4) × 10-­‐3 OM pH 3.10 (1.8 ± 0.4) × 10-­‐3 BM -­‐ SICM pH 3.10 (3.2 ± 0.6) × 10-­‐3 Effect of surface on dissolu:on rate: Linear ~ 15 mins Graphs to show the displacement of crystals from their star:ng perimeter changing with :me during dissolu:on at different pH values. Dissolu:on :mes are given. Graph to show the average height of the crystal over :me during dissolu:on. pH 3.10 t = 45 mins Surface Average DR (mol m-­‐2 s-­‐1) Plas:c (2.5 ± 0.8) × 10-­‐3 PLL + (2.0 ± 1.2) × 10-­‐3 PGA -­‐ (3.2 ± 1.1) × 10-­‐3 Crystals > 40 μm, not studied further Glass DR consistent between surfaces so BM-­‐SICM used only crystals grown on plas:c. FURTHER WORK CONCLUSIONS •  Results have shown that :me lapse Op:cal Microscopy combined with BM-­‐SICM is suitable for tracking the dimensions of a single calcite microcrystal during dissolu:on and calcula:ng the dissolu:on rate in different dimensions. •  Regrowth of calcite microcrystals aFer par'al dissolu'on •  Studying dissolu:on kine:cs with simula:ons and Finite Element Modelling in COMSOL •  Dissolu:on of groups of calcite microcrystals References: 1 – K. McKelvey, D. Perry, J.C. Byers, A.W. Colburn and P.R Unwin, Anal. Chem., 2014, 86, 3639-­‐3646. 2 – B. P. Nadappuram, K. McKelvey, R. Al-­‐Botros, A.W. Colburn and P.R. Unwin, Anal. Chem., 2013, 85(17), 8070-­‐8074. 3 – E.L. Sharp, H. Al-­‐Shehri, T.S. Horozov, S.D. Stoyanov and V.N. Paunov, RSC Adv., 2014, 4, 2205-­‐2213.