Determination of miniemulsion droplet size via soap titration

The application of the well-known soap titration method, originally develop ed to determine the size of latex particles, to the characterization of the average droplet size of octadecyl methacrylate (ODMA) and styrene (St) min iemulsions was investigated. Control experiments were first performed using individual monodisperse polystyrene latexes of known size and then mixture s of these to create broad (multimodal) distributions. The surfactant (soap ) titration method was applied to determine the effect of the particle size distribution on the accuracy of the method. The average particle sizes (D- vs, volume-surface average diameter) obtained were in good agreement with b oth the experimental (capillary hydrodynamic fractionation, CHDF) and calcu lated particle sizes. Next, the droplet size of ODMA miniemulsions (with an d without hexadecane as costabilizer) was characterized by both soap titrat ion and CHDF. The two measurements were found to be in good agreement, prov ing the viability of the technique in sizing emulsion droplets. Various sty rene miniemulsions were then characterized by soap titration. By increasing the SLS concentration used in the preparation of the miniemulsions, the dr oplet size decreased as expected. It was also found that the presence of Ti O2 particles dispersed within the monomer phase prior to forming the miniem ulsion droplets leads to an increase in the droplet size. In addition, an i ncrease in the TiO2 content resulted in an increase in the average droplet size, indicating a greater number of TiO2 particles within each droplet. Th ese measurements are important for assessing the extent of droplet nucleati on that; might be expected in subsequent polymerizations. Low free-surfacta nt concentrations (less than critical micelle concentration) and small drop lets (large in number) favor nucleation in droplets, as opposed to homogene ous or micellar nucleation.