Stanford Synchrotron Radiation Lightsource, P. O. Box 20450, Stanford, California 94309, USA.
J Chem Phys. 2012 Feb 21;136(7):074507. doi: 10.1063/1.3684893.
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.
我们报告了小角度 X 射线散射数据，证明了在疏水溶剂中疏水效应从小尺寸到大尺寸转变行为的直接实验微观观察。通过增加两亲性四烷基铵（C(n)H(2n+1))(4)N(+)（R(4)N(+)）阳离子在水溶液中的侧链长度，我们观察到在溶质尺寸为 4.4 到 5 Å 之间出现了表明阳离子之间缔合的衍射峰，这些峰表现出主要由疏水吸引力主导的温度依赖性。利用 O K 边 X 射线吸收，我们表明小分子类似降低温度一样影响水中的氢键，而大分子类似升高温度一样影响水。分子动力学模拟支持并进一步深入了解了实验观察结果的起源。
