Colloid Thermophoresis in Surfactant Solutions: Probing Colloid-Solvent Interactions Through Microscale Experiments

Thermophoresis describes the directed migration of species in a temperature gradient. Motivated by recent progress in near-infrared photothermal therapy and the pressing need for detecting low-abundance biochemical species, colloid thermophoresis has garnered increasing attention in nanomedicine and biosensing for its rapid accumulation/depletion and high sensitivity.

Surfactants, as amphiphilic molecules, are extensively used to tailor the surface chemistry of colloidal particles and improve their stability in biological media, providing essential benefits for degradation-prone colloidal systems, such as metal-organic frameworks (MOFs). In some cases, surfactant molecules are naturally present in human organs, such as pulmonary surfactants in the lungs, which reduce alveolar surface tension to prevent collapse during exhalation. However, less is known about how surfactants influence colloid thermophoresis at the microscopic level, particularly in aqueous milieus, where electrostatic and hydration entropy interactions play crucial roles.

In this article, we investigate the mechanistic aspects of the thermophoretic response of silica microspheres in both nonionic and ionic surfactant solutions, focusing on how surfactants tune interfacial interactions using capillary electrophoresis-thermophoresis (CET). Throughout the paper, we identify distinct charging mechanisms at the silica-water interface in different surfactant solutions. In particular, we show with experiments and model calculations, how the mean temperature, surfactant type and concentration influence the electrostatic and hydration entropy interactions, yielding a comprehensive picture reminiscent of complex neural networks.

These findings addressed the missing chunk of physics on the role of surfactant molecules in modulating interfacial interactions at the colloid-water interface, with potential applications in pre-concentration and detection of low-abundance biochemical species in aqueous systems.

Publications: Pu D, Panahi A, Natale G, Benneker A. Colloid Thermophoresis in Surfactant Solutions: Probing Colloid-Solvent Interactions Through Microscale Experiments. The Journal of Chemical Physics.  2024. https://doi.org/10.1063/5.0224865

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