Enhancing the Hydrophobic Effect in Confined Water Nanodrops Palla Venkata Gopala Rao, K. S. Gandhi, and K. G. Ayappa* Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India ReceiVed July 28, 2007. In Final Form: October 25, 2007 The distribution of hydrophobic solutes, such as methane, enclosed in a nanosized water droplet contained in a reverse micelle of diameter 2.82 nm is investigated using Monte Carlo simulations. The effect of the hydrophobic solute’s atomic diameter on the solute-solute potential of mean force is also studied. The study reveals that confinement has a strong influence on the solute’s tendency to associate. The potential of mean force exhibits only a single minimum, indicating that the contact pair is the only stable configuration between solutes. The solvent-separated pair that is universally observed for small solutes in bulk water is conspicuously absent. This enhanced hydrophobic effect is attributed to the lack of sufficient water to completely hydrate and stabilize the solvent-separated configurations. The study is expected to be important in understanding the role of hydrophobic forces during protein folding and nucleation under confinement. 1. Introduction The interactions between hydrophobic entities mediated by water have been investigated widely over many decades, and several reviews, 1-3 cover this vast subject and its implications for self-assembly and protein folding. Early theoretical 4 and simulation studies, 5 coupled with more recent and extensive molecular simulations, have shed light on the subtle interplay between enthalpic, entropic, and size effects when a hydrophobic entity is surrounded by water. 6-12 These studies aim to provide a molecular interpretation of the well-accepted signatures of the hydrophobic effect illustrated by weakly soluble apolar solutes, namely, the positive solvation free-energy change, negative entropy of solvation change, positive heat capacity, and solubility minimum with temperature. A measure of the strength of the interaction between a solute pair in water is obtained from the potential of mean force (PMF). The PMF represents the mean force between a pair of solute particles after averaging over all possible solvent configurations. When two apolar solutes such as methane are immersed in water, the PMF reveals two minima. The first minimum indicates the stability of the contact solute pair, and the second, weaker minimum indicates the presence of a solvent-separated pair. Although the thermodynamics of the hydrophobic effect has been investigated widely for solutes in bulk water, the influence of confinement on hydrophobic interaction has received little attention. Recent simulations of solutes in water confined in nanometer (1-4 nm)-sized hydro- phobic cavities 13 reveal that apolar solutes such as methane prefer to reside at the surface of the cavity and the solvent-separated minimum observed in bulk water is not observed. In this communication, we study the interaction between small apolar solutes immersed in water confined within a reverse micelle of radius 14.1 Å. Reverse micelles are formed in oil-water-surfactant mixtures where oil forms the continuous phase, with polar surfactant head groups forming a hydrophilic cavity encasing water. The radius of the reverse micelle depends on the molar ratio of water to surfactant. The model for the sodium bis(2-ethylhexyl) sulfo- succinate or Na-AOT reverse micelle used in this study is similar to that used in earlier simulation studies where the structure and dynamics of water as a function of the reverse micelle size and counterions in the reverse micelle 14,15 as well as the hydration and exchange of ions have been investigated. 16 Confinement or “crowding” is increasingly known to play an important role in protein folding and is significant while elucidating folding pathways in environments that resemble the geometric and functional characteristics of living cells. 17 In this regard, studies on the folding and unfolding of proteins encapsulated in reverse micelles 18-20 have been carried out. The confined environment in reverse micelles can force fold proteins that are unfolded in free solution 21 and cause the refolding of protein aggregates. 22 In this communication, we show that when small hydrophobic solutes such as methane are confined in a reverse micelle only a single contact minimum is found in the PMF. The secondary minimum in the PMF that corresponds to the solvent-separated solute is conspicuously absent. This suggests that confinement induces a tendency for the hydrophobic particles to associate, and aggregation is observed to occur within the aqueous core of the reverse micelle. A study conducted by varying the van der Waals radii of the solute particles with fixed interparticle interactions reveals that the formation of a stable contact pair or the tendency to aggregate increases with particle size. * Corresponding author. E-mail: ayappa@chemeng.iisc.ernet.in. Tel: 011-91-80-22932769 (3600085). (1) Pratt, L. R. Annu. ReV. Phys. Chem. 2002, 53, 409-436. 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