Ultrafast Dynamics in Reverse Micelles

Abstract

Recent advances in ultrafast laser technology have spurred investigations of microheterogeneous solutions. In particular, researchers have explored details of reverse micelles (RMs), which present isolated droplets of polar solvent sequestered from a continuous nonpolar phase by a surfactant layer. This review explores recent studies utilizing a variety of ultrafast laser techniques to uncover details about structure and dynamics in various RMs. Using ultrafast vibrational spectroscopy, researchers have probed hydrogen-bond dynamics and vibrational energy relaxation in RMs. These studies have developed our understanding of reverse micellar structure, identifying varying water environments in the RMs. In a plethora of experiments employing probe molecules, researchers have explored the confined environment presented by RMs and their impact on a range of chemical reactions. These studies have shown that confinement, rather than the specific interactions with surfactants, is an important factor determining the impact of the reverse micellar environment on the chemistry.

Reorientation dynamics of nanoconfined water: Power-law decay, hydrogen-bond jumps, and test of a two-state model

“…For example, confinement in silica pores has been observed to lower the crystallization temperature of water (and other liquids), allowing the study of supercooled water at temperatures inaccessible in the bulk. 6 In addition, solvation dynamics measured by time-dependent fluorescence measurements can be considerably slowed upon nanoconfinement of the solvent, sometimes by orders-of-magnitude; 3,7 the motions probed in these experiments are closely related to the reaction coordinates for charge transfer processes, so these results are indicative of the significant effects of confinement on chemistry.…”

“…Understanding the behavior of water confined on nanometer length scales is of great importance for a broad range of fields, from chemistry to biology to engineering, and covering situations as varied as ion channels, 1 reverse micelles, 2,3 fuel-cell membranes, 4 and carbon nanotubes. 5 Among the different confining environments, silica nanopores have received significant attention, motivated by both their fundamental properties and their relevance to practical applications such as separations, sensing, and catalysis.…”

Ultrafast dynamics of solvation: the story so far

“…112,113 The systems (micelles, reverse micelles, vesicles) studied so far with ultrafast spectroscopy share at least three features: these are large, charged and flexible systems. Since these properties are in contrast with those of commonly used solvents, new solute/solvent effects are seen: (i) some of the solvent degrees of freedom are frozen at the solute/solvent interface, (ii) counter-ions may be present near the interface, and (iii) the flexibility of the solvent molecules leads to an ill-defined, "dynamic" interface.…”

Solute location in a nanoconfined liquid depends on charge distribution

“…[9][10][11][22][23][24] Thus, even small changes in reaction complex location can have important consequences for the reaction mechanism, barriers, and rate constants that are relevant to applications of mesoporous materials, for example, for catalysis or sensing. This provides a strong impetus to understand the relationship between solute charge distribution and location in nanoconfined solvents, including the key factors that influence this connection.…”