Abstract
Surfactant solutions typically feature tunable nanoscale, internal structures. Although rarely utilized, they can be a powerful platform for probing thermal transport in nanoscale domains and across interfaces with nanometer-size radius. Here, we examine the structure and thermal transport in solution of AOT (Dioctyl sodium sulfosuccinate) in n-octane liquids using small-angle neutron scattering, thermal conductivity measurements, and molecular dynamics simulations. We report the first experimental observation of a minimum thermal conductivity occurring at the critical micelle concentration (CMC): the thermal conductivity of the surfactant solution decreases as AOT is added till the onset of micellization but increases as more AOT is added. The decrease of thermal conductivity with AOT loading in solutions in which AOT molecules are dispersed as monomers suggests that even the interfaces between individual oleophobic headgroup of AOT molecules and their surrounding non-polar octane molecules can hinder heat transfer. The increase of thermal conductivity with AOT loading after the onset of micellization indicates that the thermal transport in the core of AOT micelles and across the surfactant-oil interfaces, both of which span only a few nanometers, are efficient.
Highlights
Surfactant solutions typically feature tunable nanoscale, internal structures
The minimal thermal conductivity can be determined more readily and its measurement can be used to determine the critical micelle concentration (CMC). This fact, together with the simplicity and rapidity of measuring the thermal conductivity of liquids, makes determining CMC via thermal conductivity measurement a useful alternative to the available methods. In this proof-of-concept study, we show that surfactant solutions can be a viable platform for probing nanoscale thermal transport
By systematically varying the loading of AOT surfactant in n-octane liquids, we found that the thermal conductivity of the solution shows a minimum at AOT loading corresponding to the CMC, at which surfactant molecules transition from dispersed monomers to self-assembled micelles inside the solution
Summary
Surfactant solutions typically feature tunable nanoscale, internal structures Rarely utilized, they can be a powerful platform for probing thermal transport in nanoscale domains and across interfaces with nanometer-size radius. We note that since the internal structure of surfactant solutions can be varied systematically from single monomers dispersed in a sea of solvents to self-assembled micelles with well-defined shape and nanoscale dimension, they can be used as a powerful platform for studying nanoscale thermal transport. It allows one to probe the thermal transport at the interface between a single head group of surfactant molecule and its surrounding fluid, within the nanoscale core of micelles, and at the interface between surfactant’s head/tail groups and the solvent, which may have a radius of just a few nanometers. Using ultrafast transient absorption method, Schmidt et al has shown that the transition of the meso-structure of surfactant molecules greatly alters interfacial thermal transport, i.e., the interfacial thermal conductance decreases as the surfactant concentration approaches the critical micelle concentration
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