(64e) Synergistic Effects of Nanoparticles and Surfactants on the Stability of Oil-Water Interface Under Compression

Oil-water emulsions play important roles in many applications including pharmaceutical processes, enhanced oil recovery, material synthesis [1]. The emulsions are typically stabilized by surfactants and/or nanoparticles (NPs). These stabilizers prevent the emulsions from coalescence or flocculation by either decreasing the interfacial tension (surfactants) [2] or forming an aggregated layer (NPs) that stops the droplets from merging [3]. During the breakup of emulsion droplets, the interface goes through large deformations in the form of compressions or shear stresses. The stability of the emulsions thus highly depends on the behavior of the stabilizers under deformations [4].

In this study, we focus on distinguishing the behavior of different types of NPs on the oil-water interface under compression with the presence of surfactants. Homogeneous NPs with different wettability and Janus NPs are used with octaethylen glycol monododecyl ether (C₁₂E₈) on water-dodecane interfaces. We employ a computational approach in this investigation based on dissipative particle dynamics simulation [5]. The model parameters are validated by matching the simulated results with experimental data. The change in oil-water interfacial tension as a function of surfactant concentration, obtained from computations, was used to verify the correct interaction coefficients for surfactant. The parameters for NPs are validated by their experimentally measured contact angles at the oil-water interface. Under compression, without surfactants, the homogeneous NPs form a multilayer of NPs adsorbed on the interface while the Janus particles form a monolayer that collapses via buckling and folding, in agreement with experimentally reported data in the literature [6]. This difference in collapse behavior is attributed to the higher adsorption energy of JPs and their more restricted rotational freedom on the interface compared to homogeneous NPs. Depending on the NPs surface properties, different collapse mechanisms are observed for the particle-laden interface in presence of surfactants. Homogeneous NPs are expulsed to the bulk phase under compression. Janus particles, on the other hand, remain on the interface and do not appear to be affected by the addition of surfactants. Our results provide insights into the synergistic effect of NPs and surfactants under compression that could be beneficial in the design of emulsions.

ACKNOWLEDGEMENTS

Acknowledgment is made to the donors of The American Chemical Society Petroleum Research Fund for partial support of this research through grant PRF # 58518-ND9, and to NSF for grant CBET 1934513. The use of computing facilities at the University of Oklahoma Supercomputing Center for Education and Research (OSCER) and at XSEDE (under allocation CTS-090025) is gratefully acknowledged.

REFERENCES

1. Rosen, M. J.; Kunjappu, J. T., Surfactants and Interfacial Phenomena; John Wiley & Sons, 2012.
2. Vu, T. V.; Papavassiliou, D. V., Synergistic Effects of Surfactants and Heterogeneous Nanoparticles at Oil-Water Interface: Insights from Computations. Journal of Colloid and Interface Science 2019, 553, 50-58.
3. Binks, B. P., Particles as Surfactants—Similarities and Differences. Current Opinion in Colloid & Interface Science 2002, 7, 21-41.
4. Razavi, S.; Cao, K. D.; Lin, B.; Lee, K. Y. C.; Tu, R. S.; Kretzschmar, I., Collapse of Particle-Laden Interfaces under Compression: Buckling Vs Particle Expulsion. Langmuir 2015, 31, 7764-7775.
5. Groot, R. D.; Warren, P. B., Dissipative Particle Dynamics: Bridging the Gap between Atomistic and Mesoscopic Simulation. The Journal of Chemical Physics 1997, 107, 4423-4435.
6. Razavi, S.; Lin, B.; Lee, K. Y. C.; Tu, R. S.; Kretzschmar, I., Impact of Surface Amphiphilicity on the Interfacial Behavior of Janus Particle Layers under Compression. Langmuir 2019, 35, 15813-15824.