Testing Molecular Mechanisms for Growth and Dissolution Reactions on Calcite Surfaces

测试方解石表面生长和溶解反应的分子机制

• 批准号: 0643139
• 负责人: Andrew Stack
• 金额: $ 21.94万
• 依托单位: Georgia Tech Research Corporation
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-08-01 至 2011-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0643139&HistoricalAwards=false
• 关键词: Testing; Molecular; Mechanisms; Growth; Dissolution

Intellectual Merit: Kinetic models of mineral dissolution and growth are hampered by our inability to precisely define the mechanisms of relevant reactions at the molecular scale. The net reaction for many mineral surface processes are known, but the pathway(s) and mechanism(s) through which they occur are not well constrained. Calcite is a particularly important mineral in that it is: a dominant buffer of acidity in soils and groundwater, one of several materials used to understand crystal growth generally, and studied widely to understand biomineralization processes. In the abiological dissolution and growth of calcite, specific, molecular-scale, limiting reaction mechanisms have been proposed, but have not been tested.Similarly, during biomineralization it is known that certain molecules and proteins such as aspartates can influence growth morphology, but quantitative estimates of their kinetic effects remain elusive. The main goal of this proposal is to test proposed reaction mechanisms for calcite dissolution and growth by directly comparing formation and activation energies for step movement measured experimentally to reactions simulated from first principles. Classical crystal growth theories will form the framework to relate the experimentally measured quantities to the simulated chemical mechanisms. The reactions that will be tested are the mechanisms of kink site formation thought to control the movement of steps at circum-neutral pH, atmospheric pCO2, and saturations near equilibrium as well as growth inhibition by a model compound.The proposed research will take place over three years and will address three main hypotheses that will form the bulk of a Ph.D. dissertation:*Hypothesis 1: Step movement on calcite surfaces observed by AFM can be fit by the same model for both growth and dissolution, where kink site formation and propagation are limiting.*Hypothesis 2: Accurate kink site formation and activation energies can be simulated from first principles calculations and simulations.*Hypothesis 3: Detachment of the carboxylic acid functional group of aspartate from calcium ions on the step edge controls its ability to poison growth.The activation and formation energies for kink sites will be estimated from atomic force microscopy measurements of step velocity as a function of saturation and temperature. These will be compared to direct observations of kink site concentrations in equilibrium solutions using high resolution AFM. Simulations will conducted after ensuring the best fit to existing crystal truncation rod date of water structure at calcite interfaces. The mechanism of the reactions will be then be tested by simulating kink site formation and activation energies using ab initio density functional theory and molecular dynamics umbrella sampling. The possible importance of the dissociation of water in the reaction will be explored by protonating surface carbonate and hydroxylating surface calcium ions. Finally, the same AFM measurements and simulation techniques will be made in the presence of aspartate and the experimentally estimated activation energy for growth poisoning will be compared to the simulated activation energy for detachment of aspartate from a calcium on a step edge. The proposed research will combine experimental and computational techniques in a novel way to explicitly test our understanding of growth and dissolution reactions on calcite. This information could be used subsequently in larger-scale rate measurements where assignation of physical meaning to measured rate constants is ambiguous.Broader Impacts: This project will form the bulk of a graduate student's Ph.D. dissertation. Additionally, in year two, a K-12 teacher will be awarded a Georgia Intern-Fellowship for Teachers (GIFT) and work in the laboratory of the PI for six weeks. The goal is to provide experience for the teacher so that he or she can better incorporate modern environmental science into their K-12 Earth Science class. The results from this work could result in some long-term benefits to science as well. These include an enhancement of our ability to predict the kinetics of mineral surface reactions in environmental settings, the design of new growth inhibitors to tailor crystallization techniques to industrial applications, creation of nano-devices and improved heterogeneous catalysis techniques. These are areas where the current lack of understanding makes it difficult to predict a priori what the effect added components will have on a system. This in turn makes design of new growth inhibitors or modifiers for a given system or application difficult, and the quantitative prediction of the rates of mineral surface reactions is often beyond reach.This project is supported jointly with the Ceramics Program in the Division of Materials Research.

智力优势：矿物溶解和生长的动力学模型由于我们无法在分子尺度上精确定义相关反应的机制而受到阻碍。许多矿物表面过程的净反应是已知的，但它们发生的途径和机制却没有得到很好的约束。方解石是一种特别重要的矿物，因为它是土壤和地下水中酸性的主要缓冲物质，是用于了解晶体生长的几种材料之一，并被广泛研究以了解生物矿化过程。在方解石的非生物溶解和生长中，已经提出了特定的、分子尺度的、限制性的反应机制，但尚未进行测试。同样，在生物矿化过程中，已知某些分子和蛋白质（如天冬氨酸）可以影响生长形态，但其动力学效应的定量估计仍然难以捉摸。本文的主要目的是通过直接比较实验测量的台阶运动的形成和活化能与第一原理模拟的反应，来测试方解石溶解和生长的反应机制。经典晶体生长理论将形成一个框架，将实验测量的量与模拟的化学机制联系起来。将被测试的反应是被认为在环中性pH值、大气二氧化碳分压和接近平衡的饱和度以及模型化合物的生长抑制下控制步骤运动的扭结位点形成机制。拟议的研究将持续三年，并将解决三个主要假设，这些假设将构成博士论文的大部分内容：*假设1：原子力显微镜观察到的方解石表面上的台阶运动可以用生长和溶解的相同模型来拟合，其中结点的形成和传播是有限的。*假设2：通过第一性原理计算和模拟可以模拟出准确的扭结位点形成和活化能。*假设3：天冬氨酸羧酸官能团与台阶边缘钙离子的分离控制了其毒性生长的能力。扭结位点的活化能和形成能将通过原子力显微镜测量的台阶速度作为饱和度和温度的函数来估计。这些将与使用高分辨率原子力显微镜在平衡溶液中直接观察到的扭结位点浓度进行比较。在确保最适合现有方解石界面水结构的晶体截断棒日期后进行模拟。然后，通过从头算密度泛函理论和分子动力学伞式采样模拟扭结位点形成和活化能来测试反应的机理。水在反应中解离的重要性将通过表面碳酸盐的质子化和表面钙离子的羟基化来探讨。最后，将在天冬氨酸存在的情况下进行相同的AFM测量和模拟技术，并将生长中毒的实验估计活化能与天冬氨酸从台阶边缘的钙分离的模拟活化能进行比较。提出的研究将实验和计算技术结合起来，以一种新颖的方式来明确地测试我们对方解石生长和溶解反应的理解。该信息可用于随后的更大规模的速率测量，其中物理意义分配到测量速率常数是不明确的。更广泛的影响：该项目将构成研究生博士论文的主体。此外，在第二年，一名K-12教师将获得乔治亚州教师实习奖学金（GIFT），并在PI的实验室工作六周。目标是为教师提供经验，以便他或她能够更好地将现代环境科学融入K-12地球科学课程。这项工作的结果也可能给科学带来一些长期的好处。其中包括提高我们在环境环境中预测矿物表面反应动力学的能力，设计新的生长抑制剂以使结晶技术适应工业应用，创建纳米器件和改进的多相催化技术。在这些领域，由于目前缺乏了解，很难预先预测添加的组件将对系统产生什么影响。这反过来又使得为给定系统或应用设计新的生长抑制剂或改性剂变得困难，并且矿物表面反应速率的定量预测往往无法实现。本项目由材料研究部陶瓷项目联合支持。