Effect of Hydrophobic Interaction on Structure, Dynamics, and Reactivity of Water Surajit Rakshit, Ranajay Saha, Amrita Chakraborty, and Samir Kumar Pal* Department of Chemical, Biological & Macromolecular Sciences, S.N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700098, India ABSTRACT: The effect of hydrophobic interaction on water is still controversial and requires more detailed experimental and theoretical investigation. The interaction between organic-water molecular complexes might be indicative of the perturbation of hydrogen-bond network in the tetrahedral structure of bulk waters, due to hydrophobic effect. In this contribution, femto/picosecond-resolved solvation dynamics techniques have been adopted to explore the dynamical modification of water clusters in hydrophobic solvent methyl tert-butyl ether (MTBE). The dynamical evolution of water molecules at the surface of micelle-like MTBE has also been studied. Dynamic light scattering techniques have been employed to determine the size of the molecular clusters being formed in respective solvents. Fourier transform infrared (FTIR) spectroscopy well measures the changes in O-H vibration frequency of water induced by MTBE. We have also monitored temperature dependent picosecond-resolved solvation dynamics in order to explore the energetics associated with water solvation in bulk MTBE. Using detailed ab initio calculations at the MP2 level, our study attempts to predict the possible structures, energies, and thermochemical parameters of corresponding MTBE-water molecular complexes in more detail. The chemical reactivity of water further confirms the effect of the hydrophobic interaction on water molecules. The results impart an understanding on hydrophobic interaction imposed by a biomolecule on the structure and reactivity of water, significant for the in vivo cellular condition. ■ INTRODUCTION Segregation of nonpolar molecules from water is commonly known as the hydrophobic effect 1 and is the key to many biological processes, including protein folding, formation of various self-assemblies (like lipid bilayers), molecular recog- nition, etc. 2-4 However, the hydrophobic interaction imposed by a biomolecule on water is quite complex, and so forth to the presence of variety of polar and nonpolar side chains. 5 The range of possible interactions is too vast even for experimental studies, that simple organic model systems are usually chosen as an alternative. To this end, great efforts have been directed toward understanding the interactions of water with organic molecules. Especially the organic molecule that contains a hydrophobic backbone along with the hydrophilic group is a good prototype for studying the chemical heterogeneity, without the additional effect of topological disorder, typical of protein surfaces. Notably, in all these systems, the hydrophobic interaction seems to cause clustering of water molecules 6 or hydrophobic units 7,8 in respective solvents. Therefore, such systems provide a rare opportunity for studying the effect of hydrophobic interaction on water molecules, which also mimics the isolated buried water present in biological systems such as the protein interior. 9-11 Hence, there are numerous exper- imental and theoretical studies regarding the organic-water molecular complexes. Infrared spectroscopic studies have been conducted to elucidate the perturbation of hydrogen bonding network of water molecules in presence of nonpolar organic solvents. 12-16 Various studies have been undertaken on the dynamics of such isolated water molecules to see how the translational and rotational dynamics of water molecules changes, by changing the global structural rearrangement of the hydrogen bond network. 17-22 In spite of all these experimental and theoretical works, the effect of hydrophobic interaction on water structures is not clear enough as the solvents so far used are less hydrophobic, for example, methanol, acetonitrile, dimethyl formamide (DMF), and so forth. For better understanding, an organic solvent having hydrophobicity similar to the protein interior should be chosen. Methyl tert-butyl ether (MTBE) is an example of such a solvent having a bulky hydrophobic tert-butyl group, along with a less polar C-O bond. The choice of MTBE also lies in the fact that the molecule is the most common gasoline oxygenate and has become a widespread contaminant in surface water and groundwater 23-29 due to its high water solubility (44 g L -1 at 20 °C). The presence of MTBE in drinking water and groundwater resources causes different physical hazards. Moreover, there is also evidence that MTBE is a possible human carcinogen. 30 Thus, it is very important to investigate the water-MTBE molecular complexes in considerable detail. Received: October 27, 2012 Revised: December 11, 2012 Published: January 11, 2013 Article pubs.acs.org/Langmuir © 2013 American Chemical Society 1808 dx.doi.org/10.1021/la3042583 | Langmuir 2013, 29, 1808-1817