M. Silvina Tomassone

Description of Research

As a unifying thread, our group is interested in understanding the most fundamental phenomena controlling properties of composite materials. Our approach combines theory, atomistic simulations of nano and microstructures, continuum-based simulations of meso and macrostructures, and experiments. A few relevant examples are as follows:

1. Surfactant morphology and dynamics: The adsorption of surfactants on solid/liquid and liquid-liquid interfaces is at the center of interest in colloid and surface science. Research on surfactant molecules adsorbed at liquid-liquid and liquid-solid surfaces is extremely important in, for example, the synthesis of organized inorganic matter. The study of different morphologies formed by surfactants on the nanometer scale has recently developed into an important area of modern research. In aqueous solutions, above the critical micelle concentration (cmc), surfactants assemble into micelles, spherical or cylindrical nanostructures that maintain hydrophilic portions of surfactants in contact with water, and the hydrophobic parts within the micellar interior. Further increases in surfactant concentration result in the self-organization of micelles into periodic hexagonal, cubic, or lamellar mesophases. Surfactant self-assembly conducted in aqueous solutions of soluble silica species results in spontaneous co-assembly of ordered silica-surfactant mesophases. Surfactant removal creates periodic mesoporous solids, essentially silica fossils of the liquid crystalline assembly. These silica fossil materials were virtually unknown in ceramics (with the exception of zeolites with pores less than 3nm) until the discovery that self-assembly processes could be used to control their nanostructural design. A necessary condition for eventual commercial applications requires a theoretical understanding of controlled shapes and structures for the synthesis of new material systems. So, understanding of these morphologies is highly desirable. The study of adsorption of surfactants on solid interfaces has also a profound impact in the development of recognition surfaces for chemical and biological assaying and sensing and also in shedding new light on how surface aggregates are formed via adsorption (hemimicelles, admicelles, etc). Research on surfactant adsorption on metal surfaces is also an important topic our group is investigating. This kind of adsorption has been used to stabilize colloidal clusters and nanoparticles in solution and to limit the activity of the electrode surface. There is little work done on adsorption and potential-induced structure. It is not clear how the aggregation of surfactants take place at the metallic electrified substrate, and what are the changes of these structures when varying the electrode potentials. Research on surfactant molecules adsorbed on metal surfaces is extremely important in the electrochemical industry, and it has direct applications in adhesion, lubrication, detergency and corrosion inhibition. We also investigate the enhanced properties of surfactant mixtures in different solvents. Synergism in surfactant mixtures can result in solution properties different from those predicting by linear combination of the properties of pure surfactant components. Mixtures of surfactants are employed in a wide range of domestic, industrial and technological applications. A strong theme underlying much of this research is the relation between surfactant molecular structure, curvature, and rigidity of surfactant monolayers, which strongly influences the phase behavior in surfactant-containing systems.

2. Nanoparticles (clean and in suspension): There is intense activity focused on the synthesis and characterization of nanoparticles, nanocoatings, and nanopatterned surfaces. Essential examples include * Fabrication of microelectronic devices where, for example, the density of surfaces features controls the storage capacity of memory chips and the speed and energy consumption of microprocessors. * Manufacture of ceramics where the ability to intimately mix nanoparticles determines the quality of the finished product. * Synthesis and processing of nanopharmaceuticals where the ability to control the size and state of agglomeration of nanocrystals determines their behavior in the human body. Interaction forces between micron-sized particles is well described by the potentials of Johnson, Kendall and Roberts (JKR), or Derjaguin, Muller and Toporov (DMT). Both models are based on an earlier analysis by Hertz, who considered two elastic bodies in contact under an external load but ignored attractive interparticle forces. In the JKR approach, the effective steady state pressure in the contact circle is assumed to be the superposition of elastic Hertzian pressure and of attractive surfaces forces, which act only on the contact area. DMT also considers non-contact forces in the vicinity of the contact area. However, for nanoparticles, the continuum elasticity breaks down, and the theory needs to be reformulated. A detailed atomic study is needed in order to determine the effective forces between nanoparticles. In fact, other kinds of forces are exceedingly important for nanoparticles, such as van der Waals forces, capillary bridges, and ion exchange forces. We investigate interaction forces for clean nanoparticles and nanoparticles in suspension using molecular dynamics simulations. In the case of nanosuspensions, our simulations include not only nanoparticles, but also solvent and surfactant molecules. We study stability of nanoparticles in various solvent/surfactant mixtures. All species in the system (solvent, surfactants and nanoparticles) are modeled ab initio from their constituent atoms, thus leading to the development of rigorous methods for understanding the effect microstructure and chemistry on macroscopic behavior of nanoparticles/surfactant/solvent mixtures.

3. Particle shape monitoring and control in crystallization processes using MD: Limitation of experimental techniques for the on-line measurement of solid-phase properties have restricted the development and implementation of optimal design, monitoring and control methods for crystallizers and other particle formation processes. Research on particle shape is extremely important to industrial applications. For example, in the pharmaceutical industry, morphology can affect important properties such as dry powder density, cohesion, and flowability, that can have major impact on a company's ability to formulate drug particles into finished products. Moreover, crystal morphology can affect drug dissolution, potentially affecting finished product performance and, in extreme, resulting in a companies loss of the license to making the drg product. Experimental measurement and control of crystal shape usually present several difficulties. Particle shape measurements are affected by unmeasured disturbance variables that cannot be controlled. Very often the images are replete with bad data. Particles sampled from a suspension crystallizer, for example, can contain broken, agglomerated, aggregated, and irregularly grown crystals besides the correctly grown crystals. Particles fuse or particle boundaries overlap. It is also difficult to obtain representative samples and to sample enough images to remove the effects of noise through averaging. In all cases the measurements are replete with bad data and contain significant noise. Experiments show that crystals can display different morphologies depending on which faces grow faster. While it is not clear how to control the rate of growth of each face, it is known that the presence of certain impurities can affect crystal morphology. A reasonable hypothesis is that impurity molecules that bind preferentially to certain crystal faces can be used to block or delay growth along those faces, thus providing the means to control crystal morphology. While some work has been done in this area, providing preliminary confirmation of this hypothesis, at present the pharmaceutical industry lacks the means to select systematically the "impurities" and processing conditions that would yield the desired morphology for a given crystal. Thus, our goals are to understand the fundamental phenomena that control preferential binding of impurities to crystal faces, in order to develop methods for controlling the morphology of crystals as they grow. We will create a library of biocompatible "impurity" molecules, and use Molecular Dynamics to simulate attachment of these molecules to crystal seed faces in order to develop a method for understanding a priori the factors that control the crystal growth process. Binding energies will be determined, and used to fine-tune methods and models. Appropriate case studies will be selected in consultation with industrial partners.

Selected Publications