Dowding Catherine E, Borda Michael J, Fey Martin V, Sparks Donald L
Department of Soil Science, University of Stellenbosch, South Africa.
J Colloid Interface Sci. 2005 Dec 1;292(1):148-51. doi: 10.1016/j.jcis.2005.05.075. Epub 2005 Jul 14.
Although it is understood that the chemical environment at a drying surface is likely to be quite different from that at a fully hydrated surface, the difficulty of quantitative measurement has meant that this potentially crucial aspect of surface chemistry has gone largely overlooked. As a result, most of our understanding comes from measurement before and after drying, with a gray region of speculation in between. An interesting natural example is the paradoxical reduction of Mn oxides in moist soils as they dry, because drying is usually considered an oxidative process. This phenomenon indicates that important chemical changes are occurring during drying and an approach is needed to probe the chemistry of drying surfaces. Here we show the suitability of attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy for real-time, in situ investigation of the drying solid-water interface, using the change in surface pH as an example. This was achieved by adsorbing thymol blue pH indicator (pK(a)=1.65) onto a natural Mn-rich clay and observing the real-time pH change, which dropped from pH 5 to below pH 1.65 with the removal of free water from the surface.
尽管人们明白干燥表面的化学环境可能与完全水合表面的化学环境大不相同，但定量测量的困难意味着表面化学这一潜在关键方面在很大程度上被忽视了。因此，我们的大部分理解来自干燥前后的测量，中间存在一片推测的灰色地带。一个有趣的自然例子是潮湿土壤在干燥时锰氧化物出现的反常减少，因为干燥通常被认为是一个氧化过程。这种现象表明干燥过程中发生了重要的化学变化，需要一种方法来探测干燥表面的化学性质。在此，我们以表面pH值变化为例，展示了衰减全反射傅里叶变换红外光谱（ATR-FTIR）用于实时原位研究干燥固-水界面的适用性。这是通过将百里酚蓝pH指示剂（pK(a)=1.65）吸附到天然富锰粘土上并观察实时pH值变化来实现的，随着表面自由水的去除，pH值从5降至1.65以下。
