Brownian dynamics simulations of Laponite colloid suspensions
G. Odriozola,* M. Romero-Bastida,† and F. de J. Guevara-Rodríguez‡
Programa de Ingeniería Molecular, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, 07730 México, Distrito Federal, Mexico (Received 29 January 2004; published 31 August 2004) Colloidal suspensions of Laponiteclay platelets are studied by means of Brownian dynamics simulations. The platelets carry discrete charged sites which interact via a Yukawa potential. As in the paper by S. Kutter et al. [J. Chem. Phys. 112, 311 (2000)], two models are considered. In the ﬁrst one all surface sites are identically negative charged, whereas in the second one, rim charges of opposite sign are included. These modelsmimic the behavior of the Laponite particles in different media. They are employed in a series of simulations for different Laponite concentrations and for two values of the Debye length. For the equilibrium states, the system structure is studied by center-to-center and orientational pair distribution functions. Long-time translational and rotational self-diffusion coefﬁcients are computed by twodifferent methods, which yield very similar results. DOI: 10.1103/PhysRevE.70.021405 I. INTRODUCTION PACS number(s): 61.20.Ja, 61.20.Lc
Clay colloidal suspensions appear in several industrial processes. Examples are found in the ceramic, paint, cosmetic, and petroleum industries, among others [1,2]. In many of their applications, the interest in them is driven by the diversity of behaviorsthey show. These go from a Newtonian liquid up to a viscoelastic gel; they may even form a ﬂocculated dispersion, depending on their type, size, shape, concentration, and medium composition [3–6]. Hence, understanding their behavior is both a challenge and a necessity. Clays are lamellar mineral crystals. In particular, Laponite particles are three-layer synthetic clays composed of a centralmagnesium sheet sandwiched by two silica sheets [7,8]. This structure forms thin platelet-shaped lamellas of diameters close to 25 nm and thicknesses of 1 nm, which can be separated to form a water dispersion. Due to existing isomorphous substitutions of a fraction of divalent magnesium ions by monovalent lithium ions, the net charge of the Laponite ﬂat surface is negative. The edge surfaces of theLaponite particles, however, behave quite differently. Here, the tetrahedral silica sheets and the octahedral magnesia sheets are disrupted, leading to the adsorption of speciﬁc ions which rule the surface charge. Hence, depending on the media composition, the edge surface may be negatively or positively charged. We should mention here that the regular size and shape that characterizes thissynthetic clay make it very convenient for experimental studies. In fact, lately there has been a lot of experimental work on Laponite dispersions by means of scattering and rheological techniques [5–7,9,10]. On the other hand, although the one-dimensional swelling of hydrated clays has been very well studied by computer simulations [11–14], a theoretical description of their suspensions is not verydeveloped. This is at least partially due to the fact
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that the highly anisotropic shape of the particles makes the interparticle potential extremely complicated. There are, however, three very interesting theoretical works by Dijkstra et al. and Kutter et al.[15,16]. In the ﬁrst two, the Laponite particles are modeled by platelets which carry a constant electrostatic quadrupole moment. This approach, although crude, was capable of predicting a sol-gel transition, in good agreement with experimental observations. The third work, a molecular dynamics study, presents a much more realistic model, where the platelets carry a given number of charged sites...