Tunable Thermal Transport in Phase Change Materials Using Inverse Micellar Templating and Nanofillers S. A. Angayarkanni and John Philip* SMARTS, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam603 102, India ABSTRACT: We report extremely large tunable thermal conductivity (k) in alkanes using inverse micellar templating and nanofillers. The thermal properties of n-hexadecane containing inverse micelles of different volume fractions (ϕ) have been studied during freezing and melting. The k enhancement between the solid and liquid phase in the presence of oleic acid, dioctyl sodium sulfosuccinate, and sorbitan oleate inverse micelles (size ∼1.5−6 nm) are found to be 185, 119, and 111%, respectively. Unlike the conventional nanofluids, the k enhancement in micellar templated alkanes is perfectly reversible under repeated thermal cycling owing to the monodispersity and nonaggregating nature of micelles. Our results suggest that during the first-order phase transition, the inverse micelles with highly packed linear chain surfactant are pushed to the intercrystal boundaries of alkanes, thereby reducing the interfacial thermal resistance. The k contrasts in surface modified graphite nanofibers and multiwalled carbon nanotube in n-hexadecane at 15 °C for a ϕ ∼ 0.0039 are found to be 161 and 157%, respectively. The surface modified nanofillers dispersed in alkanes showed a higher thermal contrast compared to bare ones, owing to their uniform dispersibility in intercrystal regions. Our findings of the large thermal contrast using inexpensive surfactant micelles in alkane should have interesting applications in heat management. 1. INTRODUCTION The use of organic phase change materials (PCMs) as heat transfer media has attracted much attention in the recent years due to the advantages of ability to freeze without much supercooling. 1 The crystallization of the n-alkane chain releases a large amount of latent heat, which is the key to heat transfer applications. Another material of focus for such cooling applications over the past decade was dispersions of nanoma- terials, popularly known as nanofluids. 2 The intense research on various nanofluids led to the conclusion that traditional nanofluids show only modest thermal conductivity enhance- ment. 3−7 During the last decade, several new promising approaches to achieve extremely large thermal conductivity enhancement using carbon nanotubes, 8 graphene, 9−12 magnetic materials, 13−15 and composites 9,16−18 have been demonstrated. The recent finding of reversible tuning of electrical and thermal conductivities using first-order phase transitions in percolated composite materials 19−22 have attracted much interest among the nanofluid research community because of their important applications in heat management in various industrial sectors. Zheng et al. 19 have observed large contrasts in the electrical and thermal conductivities at the phase transition temperature in graphite/water and carbon nanotube/ hexadecane suspensions, which was attributed to the modulations in the electrical and thermal contact resistances due to the internal stress generated during a phase transition. Harish et al. 20 reported a large enhancement in the thermal conductivity (k) in the solid phase (∼250%) of an alkane (n- octadecane) containing 0.25 wt % of single-walled carbon nanotubes, compared to nominal enhancement in the liquid state (∼10%). Schiffres et al. 22 demonstrated tunable electrical and thermal conductivities by controlling the crystal growth through freezing rate control in solution-based nanocomposites where nanoparticles are driven into concentrated intercrystal regions to increase the percolation pathways and to reduce the internanoparticle resistance. Sun et al. 21 studied the room temperature electrical and thermal switching in CNT/ hexadecane composites and found 5 orders of electrical and 3 times of k variations at the phase change point of hexadecane. Though significant enhancements in thermal and electrical conductivities are observed in nanocomposite during phase change, the reversible switching under long repeated cycling is strongly affected due to agglomeration of nanomaterials due to strong van der Waals interaction and high reactivity of nanomaterials. Also, aggregation not only hampers the long- term stability of such nanocomposites but also reduces their switching time. Here we disclose a novel strategy to overcome these difficulties by templating alkanes with inverse micelles. Since the agglomeration issue is negligible in micellar systems due to steric interactions, such systems offer promising heat transfer applications. For these studies, we prepare soft systems of inverse micelles with size ranging from 1.5 to 6 nm, containing different head groups. We also compare the k tunability of the micellar systems with dispersions of multi- Received: April 1, 2014 Revised: May 20, 2014 Published: May 28, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 13972 dx.doi.org/10.1021/jp503209y | J. Phys. Chem. C 2014, 118, 13972−13980