Small-Angle X-ray Scattering Studies of Fe-Montmorillonite Deposits during Ultrafiltration in a Magnetic Field Fre ´de ´ric Pignon,* ,† Ayse Alemdar, † Albert Magnin, † and Theyencheri Narayanan ‡ Laboratoire de Rhe ´ ologie, Universite ´ Joseph Fourier Grenoble I, Institut National Polytechnique de Grenoble, CNRS UMR 5520, BP 53, 38041 Grenoble Cedex 9, France, and European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France Received January 21, 2003. In Final Form: July 21, 2003 This paper presents a small-angle X-ray scattering (SAXS) characterization of the structural organization of deposits of Fe-montmorillonite dispersions formed during frontal ultrafiltration in a magnetic field. The mineral colloidal dispersions are made up of platelike montmorillonite clay particles with average thickness and diameter of about 2 and 500 nm, respectively. A newly developed X-ray-compatible frontal ultrafiltration cell allowed simultaneously applying a transmembrane pressure (5 × 10 4 Pa) and a magnetic field. The Fe-montmorillonite dispersions obtained by cation exchange reaction displayed regular ordering of the particles with an anisotropic arrangement when subjected to a uniform magnetic field of strengths ranging up to 1.4 T. The degree of anisotropy and the particle concentration profiles as a function of the distance from the filter membrane were deduced from the measured SAXS intensity. For the same volume of filtered permeate (0.26 mL), the deposit formed in a magnetic field of 1 T exhibits an anisotropic arrangement of the platelike particles with their faces aligned parallel to the membrane. In the absence of a magnetic field, the deposits are composed of randomly oriented particles. The application of an external magnetic field produced uniaxially oriented deposits with a higher concentration of particles, and resulted in a higher permeation flow than in the absence of the field. Introduction Understanding the mechanisms involved in the struc- tural organization of colloidal particles subjected simul- taneously to filtration and an external magnetic field is relevant in many processing applications involved in microelectronics, ceramics, bio- and agro-industries, sludge treatment, and so forth. The packing and the orientation of colloidal particles during the processing (involving deposition, ultrafiltration, extrusion, etc.) have a direct influence on the final macroscopic optical, mechanical, or electrical properties of the fabricated material as well as the performance of the processing. For example, in the case of membrane separation, an important factor which controls the efficiency of the filtration is the structural characteristics of the deposits near the separating mem- brane, in relation to the magnitude of the permeate flow rate. During the past few years, many experimental works 1,2 and theoretical models 3-5 have been reported. Nevertheless, few attempts have been made to character- ize the structural organization of the deposits, which controls the performance of the filtration process. 6-9 The goal of this paper is first to characterize the structural organization of deposits of Fe-montmorillonite dispersions as a function of time and second to control the structural arrangement of the particles by the application of an external magnetic field during the filtration process. The consequences are several: controlling the particle organization can be used to fabricate new types of dense colloidal materials with anisotropic structural, mechan- ical, or electrical properties, and understanding the structural organization of the deposit helps in enhancing filtration performance. To probe the mechanism of ar- rangement of colloidal particles under the influence of a magnetic field, in situ small-angle X-ray scattering (SAXS) measurements were performed during the frontal ultra- filtration process. Using SAXS and a newly developed cell filtration it became possible to monitor and fine-tune the structural organization of the deposits during the filtration process in a magnetic field. The material used here consisted of an aqueous Fe-montmorillonite dispersion, composed of platelike particles of average thickness and diameter about 2 and 500 nm, respectively. Synchrotron SAXS measurements using a highly collimated beam (0.1 mm × 0.3 mm) provided time-resolved structural infor- mation as a function of distance from the filtration membrane with a resolution of 0.1 mm. This paper reports measurements performed both under static conditions (in a capillary) and with a differential air pressure of 5 × 10 4 Pa in the filtration cell. With controlled physicochemical parameters (pH, ionic strength) and a fixed filtration pressure, external magnetic fields of different strengths (B) were applied. In static conditions, without the magnetic field (B ) 0), the system consists of randomly oriented particles. In a magnetic field, B ranging from 0.01 to 1.43 T, particles orient uniaxially with increasing anisotropy with the field. Using a filtration cell without a magnetic field, measurements showed that the deposits are composed of randomly oriented particles * Corresponding author. E-mail: Pignon@ujf-grenoble.fr. † Universite ´ Joseph Fourier Grenoble I. ‡ European Synchrotron Radiation Facility. (1) Bacchin, P.; Aimar, P.; Sanchez, V. AIChE J. 1995, 41, 368-376. (2) Waite, T. D.; Scha ¨fer, A. I.; Fane, A. G.; Heuer, A. J. Colloid Interface Sci. 1999, 212, 264-274. (3) Bowen, W. R.; Jenner, F. Chem. Eng. Sci. 1995, 50, 1707-1735. (4) Bowen, W. R.; Mongruel, A.; Williams, P. M. Chem. Eng. Sci. 1996, 51, 4321-4333. (5) Jo ¨nsson, A. S.; Jo ¨nsson, B. J. Colloid Interface Sci. 1996, 1801, 504-518. (6) Pignon, F.; Magnin, A.; Piau, J. M.; Cabane, B.; Aimar, P.; Meireles M.; Lindner, P. J. Membr. Sci. 2000, 174, 189-204. (7) Su, T. J.; Lu, J. R.; Cui, Z. F.; Thomas, R. K.; Heenan, R. K. Langmuir 1998, 14, 5517-5520. (8) Stefanopolous, J. L.; Romanos, G. E.; Mitropolous, A. Ch.; Kanellopoulos, N. K.; Heenan, R. K. J. Membr. Sci. 1999, 153,1-7. (9) Antelmi, D.; Cabane, B.; Meireles, M.; Aimar, P. Langmuir 2001, 17, 7137-7144. 8638 Langmuir 2003, 19, 8638-8645 10.1021/la030020p CCC: $25.00 © 2003 American Chemical Society Published on Web 09/06/2003