On the Importance of the Direct Measurements of the Specific Refractive Index for Microgels and Micelles

Specific refractive index (dn/dc) is the change in solutions' index of refraction as concentration of a solution changes. When static light scattering (SLS) is used to determine structural properties of scatterers in a solution, a separate measurement of dn/dc on the same solutions is crucial to obtain dependable results for molecular weight as a 5% dn/dc error causes a 10% Mw error according to scattering theory. This project is focused on dn/dc measurements for microgels of various crosslinking density, ELP micelles of various composition, and polystyrene spherical standards of various sizes using a Brice-Phoenix Refractometer. The dn/dc values measured were shown to depend on size and concentration of particles, composition of microgels and micelles, and temperature of solutions tested. Here we present how dn/dc results affect Mw values deduced from SLS for microgels and ELP micelles highlighting the critical importance of direct dn/dc measurements for solutions of interest in light scattering.