Abstract: MATH/CHEM/COMP 2002, Dubrovnik, June 24-29, 2002

Precipitation of CaCO₃ at model monolayers

with nucleation centers

Susanne Hacke and Dietmar MOEbius

Max-Planck-Institut fuer biophysikalische Chemie, D-37070 Goettingen, Germany

Many organisms are capable of controlling crystallization of CaCO₃ including the different modifications to a remarkable extent. We intend to investigate the primary steps and structural requirements of biomineralization using monolayers at the air-water interface as simple model systems. The formation of CaCO₃ precipitates at monolayers of various amphiphiles with different hydrophilic groups like e.g. stearic acid has been studied a few years ago¹. We have also used the -COOH group as nucleation center, however in a two-component system with a neutral head group in order to modify the lateral disitribution of nucleation centers and inactive sites. The method for recording the formation and growth of CaCO₃ precipitates is here Brewster angle microscopy (BAM), that is particularly suited for characterizing the interface covered with a monolayer². The reflectivity of domains seen by BAM depends on the refractive index and the thickness of the domains. After spreading the two-component monolayer and compression to a fixed surface pressure, BAM images were taken with increasing time t showing the formation and growth of CaCO₃ crystals as bright domains. The area fraction f_ac of crystals exceeding a fixed level of reflectivity has been determined from the images. Assuming a Gaussian distribution of reflectivity due to the thickness of the crystals we described the time dependence of f_ac by integrating the Gaussian distribution passing the reflectivity limit. The experimental data were reasonably well represented by least squares fits to the error function. The two systems used in this study, octadecanoic acid/methylstearate (C18/SME) and octadecyl-succinic acid/methylstearate (OSS/SME), respectively, exhibit a qualitatively quite different behaviour due to different distribution of nucleation centers in the monolayer. The dependence of the limiting value f_ac,_∞, of f_ac for t → ∞ in the system OSS/SME so far has not been modelled, however, first attempts of Monte Carlo simulations determining the fraction of nucleation centers having only nucleation centers as nearest neighbours seem to be a reasonable approach mimicking a critical size of the nucleation sites.

¹ B. R. Heywood, S. Mann, Adv. Mater. 6 (1994) 9-20.

² D. Hoenig, D. Moebius, J. Phys. Chem. 95 (1991) 4590-4592.