Zeta potential and surface charge density of polystyrene-latex; comparison with synaptic vesicle and brush border membrane vesicle

The electrophoretic mobility of polystyrene — latex (PSL) of diameter 870 Å was determined using a Laser Zee System 3000. This instrument enables automatic measurement of the electrophoretic mobility of fine particles. Effect of pH and ionic strengthI on the PSL mobility was analyzed. It was found that the mobility as a function of pH has a minimum around pH 3–4 atI=0.1, showing no isoelectric point, and that the mobility decreases with increasing ionic strength up toI=0.1 but is almost constant forI=0.1–0.2 at pH=7.34. From the mobility date, we estimated the zeta potential of PSL. We used an approximate mobility formula derived by Ohshima, Healy and White, which is considerably more accurate than Smolchowski's and Henry's formula and is applicable for fine particles with small a ( a10), where is the Debye-Hückel parameter anda is the particle radius. Further, we calculated the surface charge density of PSL using an approximate relationship. It was found that there are 2000–4000 negative charges on the PSL surface at pH=7.34. Comparison is made with the results on synaptic vesicles (SV) from brain cerebrum cortex and brush-border-membrane vesicles (BBMV) from the small intestine.