Why molecules move along a temperature gradient
Why molecules move along a temperature gradient
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DOI：
10.1073/pnas.0603873103
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发表时间：
2006-12-26
影响因子：
11.1
通讯作者：
Braun, Dieter
中科院分区：
文献类型：
--
作者：
Duhr, Stefan;Braun, Dieter
分子沿着温度梯度漂移，这种效应称为热泳效应、索雷特效应或热扩散。在液体中，它的理论基础是一个长期争论的主题。通过使用一个全光学microfluiclic荧光方法，我们目前的实验结果，DNA和聚苯乙烯珠在很大范围的粒径，盐浓度和温度。这些数据支持了基于溶剂化熵的统一理论。简单地说，Soret系数由负溶剂化熵除以kT给出。该理论预测的聚苯乙烯珠和DNA的热扩散没有任何自由参数。我们假设溶剂分子在分子周围的局部热力学平衡。对于低于波动标准的中等温度梯度，该假设得到满足。对于DNA和聚苯乙烯珠，热泳运动在较低温度下改变符号。这种向较低温度的嗜热性是由于一个不断增加的正熵的水合作用，而一般占主导地位的疏水性解释的负熵的离子屏蔽。对热扩散的理解为详细探测胶体和生物分子的溶剂化性质奠定了基础。例如，我们成功地确定了DNA和珠子在电泳无法达到的尺寸范围内的有效电荷。
Molecules drift along temperature gradients, an effect called thermophoresis, the Soret effect, or thermodiffusion. In liquids, its theoretical foundation is the subject of a long-standing debate. By using an all-optical microfluiclic fluorescence method, we present experimental results for DNA and polystyrene beads over a large range of particle sizes, salt concentrations, and temperatures. The data support a unifying theory based on solvation entropy. Stated in simple terms, the Soret coefficient is given by the negative solvation entropy, divided by kT. The theory predicts the thermodiffusion of polystyrene beads and DNA without any free parameters. We assume a local thermodynamic equilibrium of the solvent molecules around the molecule. This assumption is fulfilled for moderate temperature gradients below a fluctuation criterion. For both DNA and polystyrene beads, thermophoretic motion changes sign at lower temperatures. This thermophilicity toward lower temperatures is attributed to an increasing positive entropy of hydration, whereas the generally dominating thermophobicity is explained by the negative entropy of ionic shielding. The understanding of thermodiffusion sets the stage for detailed probing of solvation properties of colloids and biomolecules. For example, we successfully determine the effective charge of DNA and beads over a size range that is not accessible with electrophoresis.