Probing interfacial structure and electrostatic interactions in mixed surfactant systems using HD VSFG spectroscopy Aswathi Vilangottunjalil, Jan Versluis, Huib J. Bakker AMOLF, Amsterdam, Netherland The study of surfactants and their interactions at the interface is crucial for promoting sustainable technologies, manipulating interfacial properties, and designing organic semiconductrs1. Fueled by such a vast range of applications, a lot of efforts were made to understand the intermolecular surfactant interaction and their adsorption both in bulk and at the interface. The Langmuir adsorption model explains the behavior of these surfactants at the interface, indicating that in a binary mixture, particles compete for surface sites to adsorb2. This is based on assumptions of non-interacting particles, a homogeneous surface, and the exclusion of simultaneous occupation of a specific site by both particles. However, there is still limited understanding of how surfactant interactions impact this adsorption in binary mixtures. Our primary objective is to gain deeper understanding of electrostatic interactions and their impact on the adsorption mechanism, with a focus on modifying Langmuir isotherm to account for these interactions. To investigate these electrostatic interactions and the interfacial structure we employ, heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG) on a binary mixture composed of sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium bromide (DTAB) which are two oppositely charged ionic surfactants. Using SFG we can probe the surface coverage and the surface potential simultaneously. A tool that can probe the interfacial potential which in turn, is dependent on the surface charge density and ionic strength of the solution is the water alignment. The surface coverage in the horizontal plane can be understood by analyzing the orientation of the hydrophobic tails of these surfactants, which we can extract from the CH signals. Through a series of experimental investigations, we aim to understand the intricate interplay between the surfactant molecules, their electrostatic interactions, and their adsorption behavior at the interface. Our findings indicate that because of strong electrostatic interaction between the oppositely charged surfactants, the surface tends to maintain a state of neutrality, regardless of the differences in their bulk concentration ratios. By analyzing the CH stretch vibrations, we conclude that the surface occupancy of both the surfactants remains constant irrespective of bulk concentration. Furthermore, we see decrease in water alignment due to the slight difference in surface charge density resulting in a decrease in interfacial electrostatic potential which we explain in our modified Langmuir isotherm incorporating the surface potential term from Poisson-Boltzmann theory of the diffuse double layer model3 References 1. Rahmanudin, Aiman, et al. "Organic Semiconductors Processed from Synthesis to Device in Water." Advanced Science 7.21 (2020): 2002010. 2. Masel, Richard I. Principles of adsorption and reaction on solid surfaces. Vol. 3. John Wiley & Sons, 1996. 3. Butt, H. J., K. Graf, and M. Kappl. "Physics and Chemistry of Interfaces, WILEYVCH GmbH & Co." KGaA: Weinheim (2003). 4. Gragson, D. E., B. M. McCarty, and G. L. Richmond. "Ordering of interfacial water molecules at the charged air/water interface observed by vibrational sum frequency generation." Journal of the American Chemical Society 119.26 (1997): 6144-6152.
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