| Abstract | Hydrophobic effect is important in diverse phenomena particularly the behaviors of proteins. Changes in structures and dynamics of water
induced by apolar group are believed to play a key role in protein folding, ligand binding and formation of biological membrane. Moreover length scale dependence of hydrophobic effects from fundamental molecular lever has been revealed theoretically and by simulations. Small apolar solutes are accommodated in water with little
perturbation to the hydrogen bonding network; while hydration of apolar solutes of nanometer or larger size will break hydrogen bonds and form a liquid-vapor like interface around it. Although the crossover length has been discussed intensively by theory and simulations [Rajamani], however it hasn't been observed directly by
experiments. Here we report experimental evidences for the cross over behavior of hydrophobic efforts in amphiphilic aqueous solutions. In addition to ionic effects, tetraalkyl ammonium (CnH2n+1)4N+ (TAA) cations also show hydrophobic effects on the structure of water. For cations with shorter side hydrocarbon chains, hydrophobic hydration
enhances the hydrogen bonds (HBs) in liquid water and separates solutes with solvents; while cations with chains about 5 A will weaken
HBs and tend to attract each other. Thus TAA cations provide direct model system to further investigate the small to large crossover under influences such as temperature, pressure and so on. |
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