Chapter 1 - Water chemistry and its role in industrial water systems

Abstract

Water is a universal solvent. The polarity of water molecules is very significant not only for the hydration of ion species but also for solid surfaces. The interaction of water with solid surfaces is expressed by the contact angle, the values of which range from 0 degree (fully water wet) to 180 degree (hydrophobic or oil wet). Surface roughness is directly related to the magnitude of the respective contact angle. Surfaces encountered in real processes and applications (e.g., oil recovery and production) are either water wet or neutral wet (rather hydrophobic). In several cases, the surface of solids contains both hydrophobic and hydrophilic domains. The arrangement of water molecules on these two types of surfaces, in case the aqueous phase is supersaturated with respect to a sparingly soluble salt, is expected to affect the nucleation and crystal growth processes. In the present work, the precipitation of calcium carbonate was investigated in microchips with a microchannel in which the supersaturated solutions were prepared by direct mixing of CaCl2 and NaHCO3 solutions, through Y-shaped junction microchannels, and the appearance and growth of crystals was monitored through a microscope equipped with a programmable camera, taking snapshots at preset time intervals. From the observation of the time of appearance of the first crystallite (defined as induction time) and from the size increase as a function of time, it was possible to measure the linear rates of crystals in the microchannels. Neutral-wet microchannels resulted in faster nucleation (shorter induction times) and lower crystal growth rates. Precipitation in hydrophilic (water-wet) microchannels gave longer induction times and higher crystal growth rates, while precipitates consisted of calcite exclusively. The difference was explained by a model of surface complexation that promoted the crystal growth through the development of locally high supersaturation. In the case of neutral-wet microchannels, the nucleation was higher, resulting in a large number of small crystals that formed large agglomerates clogging the microchannel, while at the initial stages of precipitation, the formation of aragonite was favored.