Perry Amelia R, Lazenby Robert A, Adobes-Vidal Maria, Peruffo Massimo, McKelvey Kim, Snowden Michael E, Unwin Patrick R
Department of Chemistry, University of Warwick Gibbet Hill Road Coventry CV4 7AL UK
CrystEngComm. 2015 Jun 26;17(41):7835-7843. doi: 10.1039/c5ce00138b. eCollection 2015 Oct 12.
Dissolution kinetics of the (110) face of salicylic acid in aqueous solution is determined by hopping intermittent contact-scanning electrochemical microscopy (HIC-SECM) using a 2.5 μm diameter platinum ultramicroelectrode (UME). The method operates by translating the probe UME towards the surface at a series of positions across the crystal and inducing dissolution the reduction of protons to hydrogen, which titrates the weak acid and promotes the dissolution reaction, but only when the UME is close to the crystal. Most importantly, as dissolution is only briefly and transiently induced at each location, the initial dissolution kinetics of an as-grown single crystal surface can be measured, rather than a surface which has undergone significant dissolution (pitting), as in other techniques. Mass transport and kinetics in the system are modelled using finite element method simulations which allows dissolution rate constants to be evaluated. It is found that the kinetics of an 'as-grown' crystal are much slower than for a surface that has undergone partial bulk dissolution (mimicking conventional techniques), which can be attributed to a dramatic change in surface morphology as identified by atomic force microscopy (AFM). The 'as-grown' (110) surface presents extended terrace structures to the solution which evidently dissolve slowly, whereas a partially dissolved surface has extensive etch features and step sites which greatly enhance dissolution kinetics. This means that crystals such as salicylic acid will show time-dependent dissolution kinetics (fluxes) that are strongly dependent on crystal history, and this needs to be taken into account to fully understand dissolution.
采用直径为2.5μm的铂超微电极（UME），通过跳跃间歇接触扫描电化学显微镜（HIC-SECM）测定水杨酸（110）面在水溶液中的溶解动力学。该方法的操作过程是，将探针超微电极在晶体表面的一系列位置上向着表面平移，并通过将质子还原为氢气来诱导溶解，这会滴定弱酸并促进溶解反应，但只有当超微电极靠近晶体时才会发生。最重要的是，由于在每个位置上溶解只是短暂且瞬时地被诱导，因此可以测量刚生长的单晶表面的初始溶解动力学，而不像其他技术那样测量已经经历显著溶解（点蚀）的表面。使用有限元方法模拟对系统中的传质和动力学进行建模，从而可以评估溶解速率常数。研究发现，“刚生长”晶体的动力学比经历了部分体相溶解的表面（模拟传统技术）要慢得多，这可归因于原子力显微镜（AFM）所确定的表面形态的显著变化。“刚生长”的（110）表面向溶液呈现出延伸的平台结构，显然溶解缓慢，而部分溶解的表面具有广泛的蚀刻特征和台阶位点，这大大提高了溶解动力学。这意味着像水杨酸这样的晶体将表现出强烈依赖于晶体历史的时间依赖性溶解动力学（通量），要全面理解溶解过程就需要考虑到这一点。
