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中国物理学会期刊

基于固液界面水分子重排的短程疏水力作用机制研究

Study on the mechanism of short-range hydrophobic force based on water molecule rearrangement at solid-liquid interfaces

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  • 短程疏水力是气泡-颗粒黏附、蛋白质折叠以及分子自组装等众多界面物理化学与生命科学过程的关键驱动力。但其物理起源至今尚未形成统一理论,主要原因是对分子层面水分子行为的认识尚不全面。本文通过原子力显微镜(AFM)和大尺度牵引分子动力学模拟深入探究十八烷基三氯硅烷疏水化体系下短程疏水力的作用机制。AFM力测试结果表明,疏水引力可观测起始距离约为20 nm,其作用强度随距离增加呈指数衰减,拟合得到本征衰减长度为1-2 nm,且与探针的接近次数无关。此外,试验发现气体溶解量及较低浓度无机盐离子对短程疏水力无显著影响。大尺度牵引分子动力学模拟结果显示,当疏水颗粒与基板靠近至临界距离时,弹簧势力表现为引力,同时受限区域发生水分子重排及氢键网络重构,水分子密度骤降。该现象可归因于,疏水表面难以与水分子形成氢键,从而诱导邻近水分子形成高密度氢键网络,当两表面靠近时,氢键网络结构被破坏,其间有序的水分子释放至体相水中,使得体系水分子重排诱导短程疏水力的产生。本文通过AFM试验与大尺度牵引分子动力学模拟相结合,揭示了基于固液界面水分子重排的短程疏水力作用机制,为从分子层面理解并调控疏水作用提供了坚实的理论依据。

    The short-range hydrophobic interaction serves as a critical driving force in numerous interfacial physicochemical and biological processes, such as bubble-particle attachment, protein folding, and molecular self-assembly. However, a unified theoretical framework concerning its physical origin remains elusive, primarily due to the incomplete understanding of interfacial water behavior at the molecular level. In this study, we combined atomic force microscopy (AFM) with large-scale steered molecular dynamics (SMD) simulations to investigate the mechanism of the short-range hydrophobic force in an octadecyltrichlorosilane (OTS) self-assembled monolayer system with a water contact angle of approximately 100°. AFM force measurements reveal that the observable onset distance of the hydrophobic attraction is approximately 20 nm. Based on the EDLVO theory and a stepwise nonlinear least-squares fitting strategy, our analysis shows that the force magnitude decays exponentially with separation distance. The decay length is determined to be 1-2 nm, a characteristic that remains independent of the number of probe approach cycles. Furthermore, comparative experiments between air-saturated and partially degassed water demonstrate that variations in dissolved gas content do not alter the intrinsic parameters of the short-range hydrophobic force; additionally, experiments indicate that low concentrations of inorganic salt ions have no significant effect. Large-scale SMD simulations further demonstrate that when the hydrophobic particle approaches the substrate to a critical distance, the steering force manifests as a net attraction. Simultaneously, the confined region undergoes significant water molecule rearrangement and hydrogen bond network restructuring, accompanied by a precipitous drop in water density. This phenomenon is attributed to the inability of hydrophobic surfaces to form hydrogen bonds with adjacent water molecules, which induces the formation of a high-density, ordered hydrogen bond network. As the two surfaces approach, this structured network is disrupted, and the ordered water molecules are released into the bulk phase. This rearrangement of interfacial water molecules consequently gives rise to the short-range hydrophobic force. By integrating macroscopic AFM experiments with large-scale SMD simulations, this work reveals the mechanism of the short-range hydrophobic force based on solid-liquid interfacial water rearrangement, providing a solid theoretical foundation for understanding and modulating hydrophobic interactions at the molecular level.

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