A Comparison of Energy Flow in Micelle and Vesicle Structures Fredy Pratama and G. J. Blanchard* Michigan State University Department of Chemistry 578 S. Shaw Lane East Lansing, Michigan 48824, United States * S Supporting Information ABSTRACT: We have investigated the vibrational population relaxation dynamics and state-dependent orientational relaxation behavior of perylene in micelles and vesicles formed using the same amphiphile(s). Decanoic acid and its conjugate base sodium decanoate can form either micelle or vesicle structures in aqueous solution depending on amphiphile concentration and solution pH. The issue of interest in this work is whether or not different assemblies of a given amphiphile manifest different efficiencies with the dissipation of energy. Vibrational population relaxation data show that initial energy flow from the chromophore to the amphiphile aliphatic chains is more efficient in micelles than in vesicles. Longer time scale relaxation, gauged by transient local heating induced by the dissipation of excess energy from perylene shows that the local environment formed by micelles experiences greater temperature change than the local environment formed by vesicles. This finding suggests that the strength of coupling between the bath and the amphiphiles differs for the two structural motifs. ■ INTRODUCTION Understanding the molecular basis for and mediation of intermolecular energy flow is important from both fundamental and applied perspectives. Intermolecular energy transfer plays a key role in the dissipation of excess energy, a property of direct importance in areas such as the design of coolants and lubricants. We focus here on intermolecular vibrational energy transfer (IVET) in the solution phase. There have been a number of simulations and experimental examinations of IVET in liquids, 1-9 but more remains to be understood. In this work we focus on how the conformation of the local environment of a molecule influences initial vibrational energy transfer efficiency as well as longer-term dissipation. The specific system we have chosen to evaluate is decanoic acid/sodium decanoate (Figure 1a) in water. The decanoic acid/decanoate conjugate base system is capable of forming either micelles or vesicles, depending on solution pH and amphiphile concentration. We have chosen to use the chromophore perylene (Figure 1b) as a probe because of its well understood spectroscopic properties and its insolubility in water. 3,5,8,10 Perylene partitions selectively into solution regions dominated by amphiphile aliphatic chains. The local environ- ment of perylene formed by micelles is relatively confined and of nominally spherical symmetry. In contrast, the perylene local environment formed in vesicles is not spherical. Decanoate micelles and decanoate/decanoic acid vesicles have been investigated previously, and their behavior is well under- stood. 11-14 In solutions of pH higher than 8.5, the amphiphile is fully deprotonated and the decanoate molecules form micelles. In the pH range of 6.4 to 7.8, where both decanoic acid and decanoate exist in solution, vesicles can be formed. 11 We note that the pK a of decanoic acid is 4.9 and bilayer is typically stable at pH values near the pK a of the amphiphiles. The difference between the pK a and the pH is due to the difference of the pH at the bilayer surface and the pH of the bulk solution. The role of electrostatic effects on the pH differences between the bilayer surface and the bulk solution have been discussed elsewhere. 11,15-17 Essentially, the cause of the discrepancy is the polyanionic surface of the bilayer attracting protons from the bulk solution, causing a local [H + ] gradient in the vicinity of the bilayer surface. Perylene has been used to study the local environment formed by DMPC vesicles. For DMPC vesicles with diameters less than 800 nm, perylene was found to locate in the acyl chain region of one of the vesicle leaflets, and for larger diameter vesicles the chromophore located in the interleaflet gallery. 18 The size of the vesicles used in this work was ca. 400 nm diameter and, while they are different from DMPC vesicles, the curvature of decanoic acid/decanoate vesicles is likely to play a role in determining how perylene is accommodated. The location of perylene in micellar structures is within the acyl chain region, but more detailed information is not available at this point. In addition to serving as a probe of local organization, perylene also serves as the site for energy deposition in these experiments. We measure two points in the energy relaxation process. The first point is the initial transfer of (vibrational) energy from a specific perylene vibrational mode to the Received: November 21, 2014 Revised: January 14, 2015 Article pubs.acs.org/JPCB © XXXX American Chemical Society A DOI: 10.1021/jp511676r J. Phys. Chem. B XXXX, XXX, XXX-XXX