(403g) Theoretical Investigation of Polarizability of Soft Biological Particles

Theoretical investigation of polarizability of soft biological particles

Naga Neehar Dingari and Cullen R. Buie

Abstract

We present a detailed theoretical model to investigate the effect of ‘soft’ polyelectrolyte layers on the polarization of biological particles such as bacteria. Building off of the work done by Zhao et.al¹, we model the soft layer of biological particles including dissociation of ionogenic charged groups and specific interactions with the background electrolyte. We model ionic transport around the soft particle with a modified Poisson-Nernst-Plank equation.² The fluid flow around the particle is modeled by a modified Stokes equation accounting for permeability of the soft layer.  To demonstrate our model we consider two test cases: fibrillated and unfibrillated S. salivarius bacteria whose electrophoretic mobilities were analyzed theoretically and experimentally by Duval et.al³.We explore a wide range of bulk electrolyte concentrations to consider both thin and thick double layer limits. The results demonstrate an interesting interplay between soft layer conductivity and double layer conductivity on the polarizability of these two strains of S. salivarius. The soft layer conductivity is a function of pH, pKa of ionogenic groups and bulk electrolyte concentration whereas double layer conductivity is a function of bulk electrolyte concentration. Under low concentration regimes, double layer conductivity dominates over soft layer conductivity and these two strains exhibit widely different dipole moment coefficients. The dipole moment coefficients are also stronger functions of pH at low bulk electrolyte concentrations whereas they approach a constant value of -0.5 at higher concentrations. As a consequence of our analysis, we highlight electrolytic and pH conditions favorable for manipulation of biological particles using dielectrophoresis and electrophoresis depending on the nature of their soft layers.

(1)      Uppapalli, S.; Zhao, H. Langmuir : the ACS journal of surfaces and colloids 2012, 28, 11164–72.

(2)      Duval, J. F. L.; Ohshima, H. Langmuir : the ACS journal of surfaces and colloids 2006, 22, 3533–46.

(3)      W., Norde; Duval, J. F. L  Langmuir: the ACS journal of surfaces and colloids  2005, 21, 11268.