Journal of Non-Crystalline Solids 147&148 (1992) 67-73 North-Holland ]OURNA L OF NON-CffSTALLINESOLIDS Effect of electrolyte on the gelation and aggregation of SnO 2 colloidal suspensions Renato Satoshi Hiratsuka, Celso Valentim Santilli, Dileize Valeriano Silva and Sandra Helena Pulcinelli Instituto de Qufmica de Araraquara, PO Box 355, CEP 14800, Araraquara, SP, Brazil Electrolytes may modify the physical-chemical characteristics of colloidal particle interfaces in suspension, which can favour gel or aggregate formation. The influence of NH4CI loading on the aggregation and gelation of SnO z colloidal suspensions was investigated using measurements of rheology, turbidity and infrared spectra. A rapid aggregate growth for samples with CI- > 20 mM was observed. With increasing age, gelation was observed due to formation of interaggregate bonds. For concentration of CI- between 20 and 9 raM, the aggregation process was slower allowing the formation of gel with a network which was not destroyed as the gel was submitted to a small rate of shear. As aging continues, the condensation reaction between OH groups gave rise to the formation of Sn-O bonds, irrespective of the electrolyte loading. 1. Introduction 2. Experimental The preparation of SnOe-based compounds by the sol-gel technique has received some atten- tion in the past few years. Pathways to gel prepa- ration from tin (II or IV) alkoxides or inorganic salts have been investigated [1-3]. Less attention has been paid to modeling the formation of gels from a structural and chemical point of view, despite their importance as precursors in the preparation of thin layers and coatings. Amorphous or crystalline SnOz-based materi- als containing different degrees of hydration can be prepared from SnC14 water solutions. Struc- tural differences may be due to variability of preparation conditions, of physical-chemical characteristics of the solid-solution interface, and of the hydrolysis and condensation reaction rate. Otherwise, zero charge pH between 3 and 7 has been reported [4]. The wide dispersion of data has been attributed to sample pretreatment and specific adsorption of cations. In this study, conditions of hydrogel formation from particulate tin oxide sol were investigated. The effect of the electrolyte loading on aggrega- tion and gelation of dispersed sols was analyzed from rheology and spectroscopy. Colloidal SnO 2 was prepared by precipitation at pH 11, adding ammonium hydroxide to a SnCI 4 aqueous solution (1.0 N). The precipitate was stored for 24 h inside the mother-liquid at 25°C. The final electrolyte concentration (NH~-, CI-) of suspensions was adjusted by the control of dialysis time. Suspensions were aged in dialysis vessels in equilibrium with the final wash water. During aging a decrease of pH of suspensions was observed due to the evaporation of excess ammonia. The electrolyte loading after dialysis, determined from the C1- concentration, and the values of the pH measured immediately before and after aging, are given in table 1. The evolution of chemical and physical charac- teristics during aging of suspensions were fol- lowed by turbidity, Fourier transform infrared spectra (FTIR) and steady rotational flow mea- surements. Turbidities were determined using a photoelectric colorimeter (Micronal-B340), mon- ochromatic, 410 nm wavelength light. FTIR ab- sorption spectra were measured between 100 and 4600 cm-~ using the Nicolet System with resolu- tion less than 2 cm-~. The samples were injected into an IR cell consisting of two KRS5 windows. 0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved