Stability of nano-emulsions produced by phase inversion temperature (PIT) method

In this study, the feasibility of preparing nano-emulsions under laboratory conditions by phase inversion temperature (PIT) method was investigated. Nano-emulsions were prepared using 19.2 wt% n-Dodecane (C12H26) or n-Decane (C10H22) oils as the dispersed phases, 4 wt% Brij 30 (C12E4) as the surfactant and 76.8 wt% ultrapure water or brine as the aqueous phase. The brine used in this work was prepared by dissolving different salts in ultrapure water. The salts used in this study were sodium chloride (NaCl), potassium chloride (KCl), anhydrous and hexahydrate magnesium chloride (MgCl2). PIT points were determined by measuring the electrical conductivity of the mixture as a function of temperature. The stability of nano-emulsions was determined by measuring the mean size, polydispersity index (PDI) and size distribution of droplets using dynamic light scattering (DLS) technique and monitoring them as a function of time. Cryo-SEM technique was also used for verifying the droplet size measurements obtained from DLS measurements. Nano-emulsions were characterized also using refractive index (RI), surface tension, and pH measurements. The efficiency of reversibility test on both fresh and aged systems was also investigated.

Experimental results indicate that the effect of salt on PIT value depends on the type and concentration of the salt used. Nano-emulsion systems with 1.0 M NaCl solution lead to the lowest PIT value among the ones with n-Dodecane as the oil phase. However, nano-emulsion systems with 1.0 M KCl solution lead to the lowest PIT value among the ones with n-Decane as the oil-phase. In general, n-Dodecane based nano-emulsions have higher PIT values than n-Decane based nano-emulsions. Nano-emulsions with anhydrous MgCl2 solution exhibited higher initial conductivity values compared to systems with other salts because this salt is the strongest electrolytes used in this study.

DLS results show that ageing and increasing salt concentration lead to increases in the average droplet size, PDI, and droplet growth rate for all the systems studied over a period of 5 days. Both fresh and single reversed nano-emulsion systems with 0.1 M KCl solution are found to have the smallest droplet size. These results confirm that reversibility test is efficient on both fresh and aged systems in reproducing nano-emulsions. However, the droplet size growth rate in reversed systems is found to be higher than that in fresh systems. Nano-emulsion systems with 1.0 M MgCl2 solution are found to have the longest stability based on visual observation. Both RI and pH values for all nano-emulsions increase with ageing. However, the pH value decreases and RI value increases with increasing salt concentration for all systems. Surface tension values for the reversed system are found to be lower than those for fresh systems. They are, however, found to decrease with increasing salt concentration.

Nano-emulsions with no salt are found to be the most stable within the study period used. Addition of salt, however, is found to help in the reversibility process. This is evident from the smaller droplets and long term stability observed for nano-emulsions prepared with lower salt concentrations.