Liquid-liquid phase equilibrium and the effect of a water-soluble polymer on the interaction between droplets in water-in-oil microemulsions

Significance 

Generally, microemulsions are formed by mixing together oil, water, and surfactants. Owing to their excellent properties and characteristics, application of water-in-oil microemulsions have tremendously increased in the recent past. Among the available methods for obtaining water-in-oil microemulsions, a mixture of water, an alkaline and sodium bis-(2-ethylhexyl) sulfosuccinate is widely used. Recent studies have shown that the interactions between the droplets and the structures of the microemulsions droplets are significantly affected by the addition of water-soluble additives like polyethylene glycol. Unfortunately, the effects of the additives, especially on the stability of the droplet system and the nature of the interaction between the droplets, is still missing.

Alternatively, several methods have been proposed as a solution. For instance, the liquid-liquid phase equilibrium analysis of the microemulsion solutions has proved useful for examining the nature of the interaction between the droplets. Consequently, it has also been identified as a promising technique for evaluating the influence of additives on the droplet interactions.

To this note, a group of researchers at East China University of Science and Technology: Dr. Xue Wang, Dr. Doudou Liu, Dr. Mingjie Wang, Dr. Zhiyun Chen, Dr. Tianxiang Yin and Dr. Weiguo Shen investigated the liquid-liquid equilibrium curves of water-in-oil microemulsions solutions with various polyethylene glycol concentrations in water and molar ratio of water to AOT set at 37.9. In particular, they evaluated the feasibility of using pseudo binary droplet solution for multiple microemulsion by determining the critical exponent taking into consideration the width of the coexistence curves and comparing the results to the predicted 3D-Ising values. Their work is published in the journal, Soft Matter.

Briefly, the research team commenced their work by cross-examining the previously used thermodynamic method and further improved it to suit the analysis of the liquid-liquid coexistence curves of multi microemulsion systems. Next, the effects of polyethylene glycol 200 additives on the properties of the droplets were identified. Lastly, they determined the enthalpy and entropy interaction parameters and their dependences on additives and polyethylene glycol concentrations.

The authors observed that the critical temperature, as well as the interaction entropy and enthalpy, were decrease due to the addition of polyethylene glycol 200 additive. On the other hand, the interaction enthalpy and entropy decreased initially followed by a linear increase with the increase in the polyethylene glycol concentration. For instance, a minimum concentration value of 25g L-1 was inconsistent with the initially obtained results using isothermal titration methods. Additionally, it was worth noting that the phase separations were majorly dominated by the entropy effects. Furthermore, the contribution of enthalpy and entropy to the microemulsion instability was significant at a concentration less than 25g L-1  and greater than 25g L-1 respectively.

In summary, Xue Wang and colleagues successfully demonstrated the effects of the interaction enthalpy and entropy on phase separations as well as their dependence on polyethylene glycol concentrations. In general, their work will pave way for better understanding of the droplets interactions and separation mechanisms in numerous water-in-oil systems.

Reference

Wang, X., Liu, D., Wang, M., Chen, Z., Yin, T., & Shen, W. (2018). Liquid–liquid phase equilibrium and the effect of a water-soluble polymer on the interaction between droplets in water-in-oil microemulsionsSoft Matter14(48), 9950-9958.

Go To Soft Matter

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