Modification of Electrostatic Interaction by Rhodamine 6G Adsorption on Polystyrene Latex as Assessed by Frequency Domain Photon Migration

To demonstrate assessment of electrostatic interactions using first principles models, the isotropic scattering coefficients of negatively charged polystyrene latex were measured with frequency domain photon migration (FDPM) as the effective surface charge was changed by adsorbing positively charged surfactant, Rhodamine 6G (R6G). Measurements were conducted on dense suspensions (volume fraction = 0.186) at an ionic strength of 5 mM NaCl equivalent with varying amounts of positively charged R6G adsorbed on the particle surface. The FDPM measurements of isotropic scattering were then fitted to a model prediction obtained by solving the Ornstein-Zernike integral equation using the hard sphere Yukawa interaction model with mean spherical approximation as closure. The results show that the parameter estimate of surface charge decreased with adsorbed R6G showing the ability to use multiply scattered light to characterize electrostatic interaction in dense suspensions.