Investigation Of Different Polymorphs And Morphologies Of Calcium Carbonate Based On Biomineralization

The biomineralization is quite different from the common mineral crystallization in nature.The mineral crystallization is regulated by the organic matrix that the biology excretes,and then the organic matrix can induce the inorganic materials to crystallize and grow.The organic matrix can be defined for the local area face and interface constituted by organism component,it is the medium of biomineralization,also come to a decision the mineral pellets space orientation and construction.Organic matrixes play the significant role in biomineralization.However,the mechanism research of biomineralization is still in the initial stage.Among biominerals,calcium carbonate is most widespread in nature,the compositions of which include minor organic matrix in addition to inorganic materials.The organic matrix can be divided into two classes: water-soluble(SM) and water-insoluble(IM) matrix.SM is often rich in anionic groups such as carboxylate,phosphonate and sulphate groups and plays important role in regulating the crystallization of calcium carbonate.It is generally accepted that the control of organic matrix over CaCO₃ morphology and polymorphism is due to the molecular recognition between organic-inorganic interface such as electrostatic attraction,lattice geometery matching and stereochemical complimentary.At the negative charged interface between organic monolayer and inorganic phase,the electrostatic attraction results in the composition departure from lattice ion stoichiometry and pH lowering which are considered as the main factors to promote inorganic mineral nucleation.Lattice geometry matching and stereochemical complimentary between organic matrix and calcium carbonate effectively reduce the activation energy of nucleation of minerals along such matching faces to result in the oriented nucleation and growth of minerals.Though there is little understanding of detailed control mechanism of biogenic calcium carbonate by matrix,the research will shed lights on material science and provide new pathways to fabricate advanced organic-inorganic composite materials.The contents of this dissertation are as follow:(1) polymorph of calcium carbonate depended on carboxymethyl chitosan and temperature;(2) CTAB-induced crystallization of amorphous calcium carbonate.In Chapter 1,we briefly introduce the control mechanism of organic matrixes on biomineralization process,and research progress of calcium carbonate in biomineralization.In Chapter 2,we have demonstrated that polymorph of calcium carbonate is closed related to carboxymethyl chitosan and temperature.As a heterogeneous nucleator and stabilizing agent,CMCS changes the nucleation and growth of calcium carbonate from thermodynamic into kinetic control.Thermodynamic and kinetic analyses reveal that vaterite has the highest Gibbs free energy of the three polymorphs of calcium carbonate at various temperatures,but has the fastest nucleation rate at low temperatures.Calcite has the least Gibbs free energy of three polymorphs at various temperatures,but has the slowest nucleation rate at various temperatures.The Gibbs free energy of aragonite is calculated to be higher than that of calcite at various temperatures,but is lower than that of vaterite.Below 327 K the nucleation rate of aragonite is higher than calcite,but lower than vaterite.The nucleation rate of aragonite turns out to be the highest when the temperature is above 327 K.On the base of the thermodynamic and kinetic characteristics of the polymorphs,it can be seen that calcite is formed in thermodynamic control while vaterite and aragonite are formed in kinetic control at different temperatures.In this work,because CMCS molecules change the nucleation and growth reaction of calcium carbonate from thermodynamic into kinetic control,the vaterite spheres formed at 25℃and pure bar-shaped aragonite obtained at 95℃in CMCS solution.In Chapter 3,based on the basic principles of biomineralization,amorphous calcium carbonate was synthesized under the control of sodium citrate.Then as a template,CTAB control the crystalization and growth of amorphous calcium carbonate. This study not only provides a novel way to prepare the appropriate polymorph, structure and size material of calcium carbonate,but also is helpful in understanding of biomineralization mechanisms.