Study On The Size Effect Of Nanoparticles On The Surface Thermodynamic Properties By Solubility Method

Nanoparticles are very different from its corresponding bulk substance inchemical reaction, phase transition, adsorption, electrochemistry and dissolutionbecause of their outstanding surface effect and size effect. These differences areall attributed to the surface thermodynamic properities of nanoparticles. So far,the study about the influence rule of size effect on the surface thermodynamicproperities of nanoparticels is not clear and the experimental methods fordeterming the surface thermodynamic properities of nanoparticles are deficient.In this paper, the theoretical relationships of molar surface thermodynamicproperties and partial molar surface thermodynamic properties with particle sizewere derived on the base of classical thermodynamic theory by introducing thesurface variables，and the influence of particle size on them was discussed. Therelationships of kinetic parameters for dissolution of nanoparticles with thesurface thermodynamic properties were drived on the basis of transition statetheory, which improved nanoparticles’ dissolution kinetics.In experiments, high purity of nano-sized MgO and CaCO3with differentsizes were prepared respectively by the method of hydrothermal homogeneous precipitation and double decomposition, also their particle size and morphology were characterized by X-ray diffracmeter, scanning electron microscopy and transmission electron microscopy. The conductivity datas of nanoparticles with different size dissolved in water were measured at different temperatures. The standard equilibrium constant Kθ, thermodynamic functions (AGθm, AHθm and ASθm)and kinetic parameters (k, Ea and A) of nanoparticles were obtained according to the conductivity. Furthermore, the surface thermodynamic properties of nanoparticles were calculated according to the thermodynamic and datas and knetic datas. The influence rules of particle size on themodynamic functions, kinetic parameters and surface thermodynamic properties were discussed and compared with corresponding theoretical regularity.The study results indicate that:(1) High purity MgO with average particle size of14-55nm and CaCO3with average particle size of17～71nm can be prepared by changing the prepartion conditions use the methods of hydrothermal homogeneous precipitation and double decomposition respectively. The XRD patterns reveal that the prepared MgO are periclase and the prepared CaCO3are vaterite.(2)The size of nanoprarticles has a remarkable influence on the thermodynamic functions of nanoparticles dissolved in water. With the particle size decreasing, the In Kθ increase, wherous△Gθ,△Hθm,△Sθm all decrease and there are good linear relationships between the reciprocal of sizes and the values of In Kθ, AGθm, AHθm, and ASθm, which are in agreement with the theoretical analysis.(3) The particle size has an obvious effect on the kinetic functons ofnanoparticles dissolved in water. The ln k increase, but Eaand ln A decreasewith the particle size reducing. At the same time, ln k, Eaand presentwell liner relationship with the reciprocal of sizes, which are consistent witht4565632he theoretical analysis.(4) The particle size has dramatical influence on the surfacethermodynamic properities of nanoparticles. The surface thermodynamicproperties of nanoparticles all increase as the particle sizes decrease and theyhave a liner relationship with the reciprocal of sizes. These influence rules areconsistent with the surface thermodynamic theory drived above.(5) The surface thermodynamic properities of nanoparticles can be obtainedaccording to both the dissolution thermodynamics datas and the dissolutionkinetics datas which determined by the method of solubility, but the surfacethermodynamic properities obtained through the dissolution thermodynamics ismore reliable.