Phase diagrams of non-ionic microemulsions containing reducing agents and metal salts as bases for the synthesis of bimetallic nanoparticles

Phase diagrams of microemulsions containing metal salt(s) and reducing agent, respectively, were studied in detail. The microemulsions were based on non-ionic surfactants, namely pure tetraethyleneglycol monododecylether, C12E4, and technical grade Brij30. We studied the influence of the metal salts H2PtCl6, Pb(NO3)2, Bi(NO3)3, H2PtCl6 + Pb(NO3)2 (1:1 mixture), and H2PtCl6 + Bi(NO3)3 (1:1 mixture) as well as of the reducing agent NaBH4 on the location of the phase boundaries. The focus was on the water emulsification failure boundary (wefb) where the aqueous phase forms spherical droplets. The temperature shifts of the wefb, which were caused by the presence of the salt(s), are directly related with the shift of the clouding points of the corresponding oil-free systems. The location of the wefb is affected in a complex manner by the pH (the lower the pH the higher the temperature at which the wefb occurred), the ionic strength and by specific salting-in or salting-out effects of the electrolyte ions. The desired overlap of the wefb of the microemulsions containing the metal salt(s) and the reducing agent, respectively, could be achieved by adding NaOH to the C12E4-based microemulsions and by titrating 1-octanol to the Brij30-based microemulsions, respectively.