Journal of Colloid and Interface Science 226, 189–198 (2000) doi:10.1006/jcis.2000.6747, available online at http://www.idealibrary.com on Precipitation of Self-Organized Copper Oxalate Polycrystalline Particles in the Presence of Hydroxypropylmethylcellulose (HPMC): Control of Morphology Nathalie Jongen, ∗ Paul Bowen, ∗,1 Jacques Lemaˆ ıtre, ∗ Jean-Christophe Valmalette,† and Heinrich Hofmann ∗ ∗ Powder Technology Laboratory, Materials Science and Engineering Department, EPFL, CH-1015 Lausanne, Switzerland; and †Multiphase Materials & Interfaces Laboratory, University of Toulon-Var, BP 132, F-83957 La Garde Cedex, France Received December 14, 1998; accepted January 24, 2000 Copper oxalate particles precipitated from copper nitrate and sodium oxalate are aggregates of small crystallites. The crystallites seem to be self-organized such that they are very well aligned crys- tallographically within the volume of the aggregate. The addition of various cellulose derivatives induces different particle and crystal- lite morphologies. Without additives, particles with a cushion-like morphology are observed. Increasing the concentration of hydro- xypropylmethylcellulose (HPMC) added to the precipitating agents induces a variation of the shape from low axial ratio forms such as cubes to higheraxial ratio rods. The crystallites within these par- ticles show a more elongated shape and smaller size as the HPMC concentration is increased. The polymeradditive seems to influence the three steps of copper oxalate precipitation:nucleation, crystal- lite growth, and aggregation. The presence of HPMC affects the copperoxalate nucleation step whereby more nuclei are created and theirsizes decrease as reflected by the crystallite volume (total num- berof crystallites increases while the precipitate yield is constant). In this paper we describe the characterization of these well-organized aggregates and propose a mechanism forthe influence of the HPMC on the crystallite and aggregate shape. C  2000 Academic Press Key Words: precipitation; aggregation, self-assembly; morphol- ogy, additives. 1. INTRODUCTION Precipitation is one of the oldest chemical engineering op- erations to have been exploited commercially. The increasing emphasis on high added value speciality chemicals has also highlighted the important role of precipitation. The need to con- trol the properties of the precipitated product with respect to its size, shape, chemical composition, and substructure is becom- ing more and more important and much research and progress has been made over the past 10 years to this goal. Particles can precipitate as single crystals or in the form of polycrystals made up of a large number of small crystals. The formation of single crystals is often well described by classic nucleation and growth theory for many systems (1). For the production of micron-sized 1 To whom correspondence should be addressed. particles much effort has been devoted to the production of nar- row or monosized particles that are often spherical in shape but other morphologies have also been illustrated (2). Nanometer- sized particles are often the primary product in precipitation from solution. Such dispersions are inherently unstable and the so-formed small crystallites aggregate to form larger particles. There are many examples of precipitated powders in which the particle is an agglomerate of small crystallites rather than a sin- gle crystal (3–12). The driving forces and detailed mechanisms by which these polycrystalline aggregates form are not well un- derstood and constitute a fascinating area of current research to which this paper will contribute. Matsui and Ohgai (7) report the formation of polycrystalline hydrous zirconia particles during hydrolysis of ZrOCl 2 . These particles are aggregates of primary nanoparticles smaller than 10 nm and electron diffraction analysis has shown that these polycrystals are made up of randomly oriented subunits (8). When precipitated in the presence of phosphate ions, α-Fe 2 O 3 particles aggregate in an ordered manner to give ellipsoidal par- ticles (12). This was interpreted as being due to the net magnetic moment that hematite has along the c axis. In addition, when a magnetic field is applied during the precipitation, the aspect ra- tio of the so-formed particles increases (12). Many authors have in fact prepared a wide variety of particle morphologies and the results are well documented in the literature (e.g., Refs. 1b, 2, 3). In some cases the morphology is related to the chemical compo- sition of the solid, whereas in other cases materials of the same composition precipitate as particles of different shapes, often as a result of some modification of the experimental conditions such as reactant concentrations, pH, and counterions. A major effort has been made concerning the effect of var- ious additives on single crystal growth (13) and some groups also developed accurate models capable of predicting the shape of molecular crystals including the effect of the growth en- vironment (14). For inorganic compounds organic additives such block copolymers of PMMA-b-PEO have been shown to strongly influence the morphology of CaCO 3 precipitates (15). The PMMA blocks specifically interact with the crystal face parallel to the calcite c axis leading to the growth of elongated 189 0021-9797/00 $35.00 Copyright C  2000 by Academic Press All rights of reproduction in any form reserved.