Heterogeneous Nucleation of Calcium Oxalate on Native Oxide Surfaces
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(1)
Critical parameters for film formation including the nucleation induction time (t), nucleation density (N), and particle growth rate (dr/dt) are also controlled by S, y,and T: In (l/t) = In B - 16nta3v2/[3(kT)3(ln S)2)]
(2)
In N
=
In P - Qpa3v2/[(kT)3(ln S)2]
(3)
dr/dt
=
K(S- 1)n
(4)
where K, 0, P, Q, B, and n are constants, k is Boltsmann's constant, and v is the molecular volume of the precipitating phase. The key to successful biomimetic processing is to modify the substrate surface so that the net interfacial energy for nucleation on the surface is lower than the interfacial energy (a) operated for the U. S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. 223 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
associated with homogeneous nucleation and precipitation within the bulk solution. The purpose of this investigation is to use nucleation experiments to measure effective interfacial energies for both homogeneous and heterogeneous nucleation to try to determine factors that control interfacial energies. Once the factors that contribute to lowering the nucleation barrier are understood, it will be easier to modify substrate surfaces to promote film formation. Calcium oxalate (CaOx) was selected as a model nucleating system because its solubility is well known [3] and is constant between pH 4 and pH 11. Supersaturation levels are relatively easy to control for CaOx over a wide range of solution conditions. The main substrates investigated include colloidal particles of the simple oxides silica, titania, and alumina. The above oxides each exhibit unique surface charge characteristics in the pH window of interest (Fig. 1) [4]. The surface charge can control the degree to which Ca2 + and Ox 2 - ions are adsorbed or complexed in addition to controlling the contribution that the electrical double layer makes to the net interfacial energy. Each oxide also exhibits different lattice spacings and elastic constants. The interfacial strain energy for CaOx growing on each oxide is expected to be different, allowing investigation of the role of lattice mismatch in mediating surface nucleation. 60 A12 0 3 40-
S20-
TiO2
"0-S~~~SiO 2
.. •
S-2 0 -401. 1'1.. 1. 11 .. 6 8 10 12 pH Figure 1. Zeta potentials of A120 3 , TiO2 and Si0 2 (0.5v/v%) as a function of pH. T= 25oC, in 0.01M NaC1. -60 -.. 0
.. .' will 2 4
EXPERIMENTAL The commercial powders a-A12 0 3 (AKP-30, Sumitomo, surface area = 6.8 m2 /gm), 15 m 2 /gm), TiO 2 (P25, Degussa, 57 m 2/gm) and BaSO 4 2 (Baker,6.8m /gm) were used in Constant Composition (CC) experiments. Colloids were dispersed ultrasonically in 0.01 M NaCl solutions adjusted to the desired pH using HC1 or NaOH and equilibrated for at least three days prior to CC experiments. Stock solutions of calcium chloride and potassium oxalate were prepared from reagent grade chemicals (Fisher) using deionized, C0 2-free water and were passed through 0.2 pn Millipore filters before use. CC experiments were initiated by adding the des
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