Macro-Ions Near Confining Surfaces: Influence on Colloidal Forces

Yale University, New Haven CT

Investigators

Abstract

Van Tassel, Paul R. Yale University "Macro-Ions Near Confining Surfaces: Influence on Colloidal Forces" Colloidal forces between supra-nanometer sized objects in electrolyte solutions govern many industrially and medically important systems. Significant contributions to the overall colloidal force arise from electric double layer, van der Waals, and depletion interactions. The latter is brought about by an added macromolecular component whose size is larger than the solvent and electrolyte but smaller than the colloidal objects. A net force results from the unbalanced osmotic pressure in the region near contact between two larger objects due to the altered concentration and structure of the added component in this region. Current understanding of the depletion force is based primarily on steric (i.e. hard particle) interactions with spherical additives. However, a much more pronounced effect is possible using charged and/or non-spherical macromolecular additives; this must be understood/controlled if important applications in colloid/protein crystallization, separation, and bioassay are to be realized. This project proposes a combined theoretical and experimental effort toward understanding, predicting, and technologically exploiting the influence of macromolecular additive charge and shape on colloidal force. The system of study consists of a spherical, ca. 10 micron colloidal particle near a smooth flat surface in the presence of macro-ions (ca. 10 nm spherical or rod-like colloidal particles) and micro-ions (atomic-sized counter ions/salt). The focus is on the behavior of the macro-ion component in the gap between the particle and the surface and its relation to colloidal force. The PIs bring to bear state-of-the-art experiment and modern molecular theory to this study of macro-ions in a confined geometry. The first objective is the experimental understanding of the force versus distance profile between a model colloidal particle (polystyrene or silica) and a planar surface (silica) in the presence of macro-ions. The PIs will employ total internal reflection microscopy (TIRM) and atomic force microscopy (AFM) and the significance will be a systematic account of the influence of macro-ion charge, size/shape, and concentration - and micro-ion charge and concentration - on the colloidal force versus distance profile. The second objective is the theoretical prediction of the molecular structure and force profiles of the systems investigated experimentally. The PIs will employ Monte Carlo simulation and density functional theory and the significance will be a molecular understanding/interpretation of our experimental results - and the possibility of investigating systems not readily addressable via experiment - thus leading to a true predictive framework for understanding interacting colloidal systems. The third objective is to exploit this predictive framework to develop two novel applications of macro-ion additives. One is a chromatography-based tool for particle separation and the other a method for improving the effectiveness of bio-analytical tools in, e.g., cell separation or microorganism detection. The significance could be new technologies exploiting macro-ion control of colloidal forces. Broader Social Impact: To broaden the impact of the scientific objectives, the PIs propose the active participation of undergraduate students in the research activities. The PIs propose to further integrate this research into our undergraduate curriculum by developing an appropriate AFM experiment for the Yale undergraduate lab course. Funds for supporting the undergraduate student will be pursued through the NSF REU program. Because the proposed methods (like AFM) are growing in popularity within the research community, they anticipate the student experience will serve as an excellent preparation for subsequent graduate study. Graduate students schooled in these methods will conduct a tutorial, and students will then have an opportunity to measure surface forces using these methods. The PIs anticipate beginning student participation with the force between a particle and a flat surface as a function of added salt and then moving to simple macro-ion additives (like silica) where force oscillations occur. The significance will be an early exposure to state-of-the-art surface analytical methods.