title: Microscopic probing of the size dependence in hydrophobic solvation.
abstract: We report small angle x-ray scattering data demonstrating the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in hydrophobic solvation. By increasing the side chain length of amphiphilic tetraalkyl-ammonium (C(n)H(2n+1))(4)N(+) (R(4)N(+)) cations in aqueous solution we observe diffraction peaks indicating association between cations at a solute size between 4.4 and 5 Å, which show temperature dependence dominated by hydrophobic attraction. Using O K-edge x-ray absorption we show that small solutes affect hydrogen bonding in water similar to a temperature decrease, while large solutes affect water similar to a temperature increase. Molecular dynamics simulations support, and provide further insight into, the origin of the experimental observations.
authors.affiliation: Stanford Synchrotron Radiation Lightsource, P. O. Box 20450, Stanford, California 94309, USA.
authors.forename: Ningdong
authors.initials: N
authors.lastname: Huang
authors.forename: Daniel
authors.initials: D
authors.lastname: Schlesinger
authors.forename: Dennis
authors.initials: D
authors.lastname: Nordlund
authors.forename: Congcong
authors.initials: C
authors.lastname: Huang
authors.forename: Tolek
authors.initials: T
authors.lastname: Tyliszczak
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Weiss
authors.forename: Yves
authors.initials: Y
authors.lastname: Acremann
authors.forename: Lars G M
authors.initials: LG
authors.lastname: Pettersson
authors.forename: Anders
authors.initials: A
authors.lastname: Nilsson
mesh_terms: D057927:Hydrophobic and Hydrophilic Interactions; D056004:Molecular Dynamics Simulation; D000644:Quaternary Ammonium Compounds / Q000737:chemistry; D053838:Scattering, Small Angle; D012995:Solubility; D012997:Solvents / Q000737:chemistry*; D013696:Temperature; D014961:X-Ray Diffraction
chemical_list: D000644:Quaternary Ammonium Compounds; D012997:Solvents