Microstructure and transport properties of biocompatible silica hydrogels.
Mercedes Perullini, Nathanael Levinson, Matías Jobbágy, and Sara A. Bilmes
INQUIMAE-DQIAQF, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires
Ciudad Universitaria, Pab. II, C1428EHA Buenos Aires, Argentina.
Fax: 54(11)4576-3341; Tel: 54(11)4576-3380- ext.130. E-mail:
supplementary information
SAXS characterization of the microstructure
Themicrostructure characterization was performed at the LNLS SAXS2 beamline in Campinas, Brazil, with a wave vector range: 0.09 nm-1 < q < 2.2 nm-1. Although the rigorous interpretation of experimental results as indicating ‘‘fractality’’ requires many orders of magnitude of power-law scaling, particulate silica hydrogelscan bemodeled as a fractal system.[1],[2] The fractal dimension (D) and the radius of gyration of the primary cluster (Rg) evaluated in terms of a mass fractal aggregation of elementary dense particles (with mean diameter a),give valuable information of the microstructure of these systems.In Figure SI-1 a schematic representation of the process of condensation of primary particles to form the silica network is presented along with Scanning Electron Microscopy (SEM) images to illustrate the physical meaning of the parameters obtained by SAXS characterization.
The SEM images correspond to a representative sample (SiO2 total content 7.0 %; pH = 7.5), which was dried with supercritical CO2. Before drying, the sample was immersed in methanol for 2 days to reduce its water content and then it was transferred to a high-pressure quartz cell with fresh methanol. Supercritical CO2 was passed through the cell at a flow rate ca. 100 mL (NPT) min-1 while the pressure was kept constant at 9 MPa by a high-pressure syringe (Teledyne ISCO 100DM). The temperature was maintained at 55 °C during the whole procedure. The complete removal of methanol, realized by the absence of alcohol downstream,was achieved in less than 3 hours. SEM images were taken with a Zeiss LEO 982 GEMINI microscope (CMA, FCEyN-UBA). Samples were not metalized, as metallization with gold was proved to provoke important changes in its microstructure.
Figure SI-1Above: Schematic representation of the process of condensation of primary particles to form the silica network of the hydrogel. Below: Scanning Electron Microscopy images of aerogels obtained by supercritical drying of silica hydrogelsand schemes illustrating the physical meaning of parameters a (mean diameter of elementary particles) and Rg(radius of gyration of primary clusters) obtained by SAXS characterization.The scale bar in each image corresponds to 20 nm.
Considering a sharp interface between silica particles and aqueous pores, a parameter S related to the interfacial area can be derived from the asymptote of the scattering intensity I(q)q4vs. q for large scattering wavenumbers q: . Figure SI-2 shows the SAXS curves obtained for a particular set of hydrogels (with silica content = 7.2 %) synthesized at different pH and the SAXS microstructure parameters derived from them. At the right the plotting of I(q).q4 vs. q is presented. The parameter S is derived from the constant value of this function at high values of q.
Figure SI-2 Left: SAXS curves obtained for a particular set of hydrogels (with silica content = 7.2 %) synthesized at the pH values indicated in the inset legend (5.58, 6.10 and 6.84, respectively). The fractal dimension (D) and the radius of gyration of the primary cluster (Rg) were evaluated in terms of a mass fractal aggregation model. The size of the elementary particles (a) is obtained from the intersection of the mass fractal model fit and the linear fit of the Porod region (at high q). Right: Plotting of I(q).q4 vs. qand fitting to sigmoidal curve with 3 parameters (above) and 4 parameters (below). The parameter S is derived from the constant value of this function at high q, extracted from the fitting to a sigmoidal curve with 4 parameters (S = I0+).
[1]Schaefer, D.W., Keefer, K. D., Fractal geometry of sílica condensation polymers, Phys Rev Lett, 1984, 53(14),1383–1386.
[2]Avnir, D., Biham, O., Lidar, D., Malcai, O., Is the geometry of nature fractal?,Science, 1998,279, 39–40.