A SIMPLE INTERACTION MODEL OF PARTICLES COVERED WITH POLYELECTROLYTE BRUSH LAYERS IN THE STRONG CHARGING AND STRONG SCREENING REGIME WITH IMPLICATIONS TO MICROBIAL AGGREGATION AND ADHESION

摘要

In order to better understand and predict the influence of main solution parameters (pH and ionic strength) on aggregation/adhesion of "hairy" colloids mimicking bacteria, a simple interaction model is proposed. In this model, the van der Waals interaction energy between two spheres is combined with the electrosteric interaction energy between brush layers of flexible polyelectrolyte chains end-grafted on the spheres. The latter interaction energy component is expressed using simple asymptotic scaling approximations (omitting their numerical prefactors) derived earlier (Pincus, P.: Colloid stabilization with grafted polyelectrolytes, Macromolecules, 1991, 24, 2912-2919) for the mean thickness L and interaction force P of planar polyelectrolyte brushes in the so-called strong charging limit (SCL) and the strong screening limit (SSL) on the assumption that the distribution of monomers throughout the brushes is uniform. Any other interaction components (double-layer, structural or acid-base, hydrophobic, bridging, etc.) are neglected. The nonelectrostatic (excluded volume) interactions are also omitted. Both quenched and annealed polyelectrolyte types (with the constant and pH-dependent charge, respectively) of brushes are considered. The number of monomers per chain N, grafting density σ, and the monomer size a* are proposed as brush parameters. The fraction of elementary charged monomers f is the next parameter which however may depend on the solution conditions for annealed brushes. The above parameters can be estimated by identifying them with the known molecular parameters and/or from the fixed charge density (fc_p = fσN/L) - a parameter obtainable from the soft particle electrophoretic analysis (Ohshima, H.: Electrophoretic mobility of soft particles, J. Colloid Interface Sci., 1994, 163, 474-483). Three sets of the brush parameters are used to model the effect of ionic strength and f on the interaction energy between Gram-negative bacteria in the SSL regime where the quenched and annealed polyelectrolyte brushes are expected to behave identically.Generally, a step-like interaction energy-vs-separation profile is obtained with a pseudo-secondary minimum (no maximum), being in line with the conception of secondary minimum aggregation and adhesion of bacteria. An opposite influence of increasing f and ionic strentgh on the pseudosecondary minimum depth (decrease and increase, respectively) is found which is expected but not predictable by the DLVO-based or steric interaction models. Dramatic changes in the position and depth of that minimum also indicate a necessity of proper evaluation of the brush parameters but, at the same time, reveal a possibility of accounting the wide variability in the bacterial cell wall structures (also inferred from the soft-particle electrophoretic analysis).