極低濃度NaCl水溶液表面張力的理論研究

Theoretical Study on the Surface Tension of NaCl Aqueous Solution at Extremely Low Concentrations

林子新(Tzu-Hsin LIN)

指導教授 ： 林祥泰

國立臺灣大學/工學院/化學工程學系/碩士(2023年)

https://doi.org/10.6342/NTU202301888

摘要

電解質水溶液的表面張力在眾多化學工程過程中起著至關重要的作用。經典靜電理論預測，隨著電解質濃度的增加，表面張力會單調增加。然而，瓊斯和雷(Jones and Ray)發現，電解質溶液在非常低濃度（約1 mM）時的表面張力比純水低，這種現象經典理論無法解釋。迄今為止，這一有趣現象的原因仍不清楚且存在爭議。

本研究利用分子動力學（MD），混和分子動力學-蒙地卡羅（hybrid MDMC）以及並行退火(REMD)模擬，研究了不同濃度下氯化鈉水溶液的表面張力。通過壓力和表面過剩濃度計算表面張力，並研究了離子分佈和表面水分子方向。我們模擬得到的高濃度區域的表面張力與實驗值相似，並遵循實驗數據的趨勢。在低濃度區域，儘管直接計算表面張力存在較大的統計不確定性，但鹽類在表面的聚集傾向可能表示了基於吉布斯吸附方程式(Gibbs adsorption equation)的降低表面張力現象。

關鍵字

表面張力 ； 電解質水溶液 ； 分子動力學模擬 ； 蒙特卡羅模擬 ； 液-氣界面 ； 表面過剩濃度

並列摘要

The surface tension of an electrolyte aqueous solution plays a crucial role in numerous chemical engineering processes. Classical electrostatic theory predicts a monotonic increase of surface tension with electrolyte concentration. However, Jones and Ray observed that the surface tension of electrolyte solutions is lower at very low concentrations (~1 mM) compared to that of pure water, which cannot be explained by classical theory. Until now, the reasons behind this interesting phenomenon remain unclear and controversial.

In this study, molecular dynamics (MD), hybrid molecular dynamics and Monte Carlo (hybrid MDMC), and replica-exchange molecular dynamics (REMD) simulations are used to investigate the surface tension of aqueous NaCl solutions at different concentrations. The surface tension was calculated using stress and Gibbs adsorption equation, while ion distribution and surface water molecular orientation were also studied. The surface tensions obtained from our simulations in the high concentration region are similar to the experimental values and follow the same trend as the experimental data. In the low concentration region, while the direct calculating of surface tension is subject to large statistical uncertainty, the preferred accumulation of NaCl near the surface may suggest a lowered surface tension based on Gibbs adsorption isotherm.

並列關鍵字

Surface tension ； Aqueous electrolyte solution ； Molecular dynamic simulation ； Monte Carlo simulation ； Liquid-air interface ； Surface excess concentration

參考文獻

Berry, M. V. (1971). The molecular mechanism of surface tension. Phys. Educ., 6, 79.

F.J. Zuiderweg and A. Harmens (1958), "The influence of surface phenomena on the performance of distillation columns," Chemical Engineering Science, vol. 9, no. 2, pp. 89-103.

Saheed Olawale Olayiwola and Morteza Dejam (2019), "A comprehensive review on interaction of nanoparticles with low salinity water and surfactant for enhanced oil recovery in sandstone and carbonate reservoirs," Fuel, vol. 241, pp. 1045-1057.

Daniela Georgieva (2009), Alain Cagna, and Dominique Langevin, "Link between surface elasticity and foam stability," Soft Matter, vol. 5, pp. 2063-2071.

Heydweiller, A. (1910). “Concerning the physical characteristics of solutions in correlation. II. Surface tension and electronic conductivity of watery salt solutions.” Ann. Phys, 33, 145-185.