Not sure why you need the dielectric constant for volume/number weighted size distribution, but the following article measured it for DMEM and DMEM/FCS (where I believe FCS=FBS). It was essentially the same as water, 78 +/- 0.5. They did not specify a temperature. But values in that range will have little to no effect on zeta potential measurements. Here's the link to the paper: Article Cytotoxicity and genotoxicity of nano - and microparticulate...