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ABSTRACT Calcite scaling studies have been mainly focused on physical quantities such as degree of supersaturation, ionic strength and temperature. However, the importance of another parameter, namely, that of surface or interfacial free energy has been largely ignored. Constant composition methods have been used to investigate the mechanisms of crystal growth of calcium carbonate and its inhibition by sodium dodecyl sulfate, dodecyl trimethyl ammonium bromide and benzalkonium chloride. Thin layer wicking methods were used to measure the calcite surface energy changes induced by the presence of these adsorbed additives for the interpretation of the crystal growth kinetics data.
INTRODUCTION Calcium carbonate is of considerable interest due to the formation of scale in numerous industrial settings. Since scale removal can be both time consuming and expensive, it is necessary to determine the most efficient means of inhibiting its formation. While acids are sometimes used in pipelines and boilers they can lead to corrosion problems. Other methods for scale prevention involve "softening" the water by removing "hard" ions, such as calcium, through ion exchange or chelation, or the use of additives that directly inhibit the nucleation and growth of scale minerals ~' 2. Surfactants constitute one common class of additives because of their amphiphilic properties and stability at high temperatures. It follows that the interaction of these surfactants with common industrial scale such as calcite, the thermodynamically most stable calcium carbonate polymorph, is of particular importance.
The present study is concerned with the influence of sodium dodecyl sulfate, SDS, a common industrial anionic surfactant and two cationic surfactants, dodecyl trimethyl ammonium bromide (DTAB) and benzalkonium chloride (BAC) on the kinetics of calcite growth in supersaturated solutions. The cationic surfactant molecules were chosen due to their structural similarities to SDS. DTAB differs from SDS through its head-group while BAC, though significantly bulkier than either DTAB or SDS, contains the same length carbon chain (C12) as SDS and DTAB.
EXPERIMENTAL
Calcium carbonate supersaturated solutions (300ml) were prepared in temperature controlled jacketed Pyrex glass cells (25.0 ± 0.1 °C) by mixing calcium chloride and sodium bicarbonate. Atmospheric CO2 (g) exchange was limited by reducing to a minimum the dead space above the reaction solutions. The ionic strength, 0.075M, and pH, 8.50, were adjusted using sodium chloride and potassium hydroxide solutions, respectively.
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