Theoretical study of the dimerization of calcium carbonate in aqueous solution under natural water conditions
Theoretical study of the dimerization of calcium carbonate in aqueous solution under natural water conditions
复制标题
DOI：
10.1016/j.gca.2009.06.003
复制
发表时间：
2009-09-15
影响因子：
5
通讯作者：
de Leeuw, Nora H.
中科院分区：
文献类型：
--
作者：
Di Tommaso, Devis;de Leeuw, Nora H.
第一性原理计算已被用来研究水合碳酸氢钙单体在模拟水环境中的缩合反应。采用COSMO介电连续介质模型，在密度泛函理论PBE水平上计算了碳酸氢钙二聚过程的反应路径，模拟了水合环境。结果表明，碳酸氢钙二聚体通过缔合机制形成：第一步涉及七重碳酸氢钙中间体，然后从钙的第一配位壳层损失一个水分子。这两个步骤的特征在于约2千卡摩尔（-1）的低能垒，这表明二聚过程在水溶液中不受动力学阻碍。然而，使用PBE和mPW 1B 95密度泛函理论水平（气相组分）和UAHF-CPCM溶剂化模型（水合作用贡献）计算的缩合反应形成碳酸氢钙二聚体和Ca（HCO 3）（2）（H2O）（4）、Ca（HCO 3）（3）（H2O）（3）（-）和Ca-2（HCO 3）（H2O）（10）（3+）的吉布斯自由能，均为阳性，这表明这些早期碳酸氢钙簇的形成在水溶液中是有害的。因此，我们的计算表明，在我们的模拟中考虑的条件下，即T = 298 K和中性pH，碳酸钙的低聚在水中不是自发的，这表明当碳酸钙-碳酸氢根离子对是水性环境中CaCO 3的主要来源时，碳酸钙的成核不能通过均相过程发生。(C)2009爱思唯尔有限公司保留所有权利。
First principles calculations have been used to investigate the condensation reactions of hydrated calcium bicarbonate monomers in a simulated aqueous environment. The reaction pathway for the calcium bicarbonate dimerization process has been computed at the density functional theory-PBE level with the COSMO dielectric continuum model to simulate the hydrated environment. The results indicate that calcium bicarbonate dimers form via an associative mechanism: the first step involves a sevenfold calcium bicarbonate intermediate followed by the loss of one water molecule from the first coordination shell of calcium. Both steps are characterised by a low energy barrier of approximately 2 kcal mol(-1), suggesting that the dimerization process is not kinetically hindered in aqueous solution. However, the Gibbs free energies for the condensation reactions to form the calcium bicarbonate dimers and the species Ca(HCO3)(2)(H2O)(4), Ca(HCO3)(3)(H2O)(3)(-) and Ca-2(HCO3)(H2O)(10)(3+), computed using the PBE and mPW1B95 density functional theory levels for the gas-phase component and the UAHF-CPCM solvation model for the hydration contribution, are all positive, which indicates that the formation of these early calcium bicarbonate clusters is thermodynamically unfavourable in aqueous solutions. Our calculations therefore suggest that the oligomerization of calcium carbonate is not spontaneous in water, at the conditions considered in our simulations, i.e. T = 298 K and neutral pH, which indicates that the nucleation of calcium carbonate cannot occur through a homogeneous process when calcium-bicarbonate ion pairs are the major source of CaCo3 in the aqueous environment. (C) 2009 Elsevier Ltd. All rights reserved.