結合DLVO理論與粗格化分子動力學模擬單壁奈米碳管於離子活性劑水溶液之研究

Abstract

單壁奈米碳管於水溶液中的分散穩定性一直以來都是相當重要的議題，若要同時了解溶液的巨觀性質與微觀分子等級的分散機制是相當具有挑戰的。在本研究中，我們結合粗格化分子動力學模擬與 Derjaguin−Landau−Verwey−Overbeek (DLVO) 理論來預測碳管於水溶液中的穩定性。在分散模擬中我們添加不同濃度的活性劑十二烷基硫酸鈉(SDS)。接著藉由 Langmuir isotherm 模型來了解其吸附於碳管上與懸浮在溶液中的關係。隨著 SDS 離子吸附量增加，平均表面電荷密度也會隨之增加，進而強化可防止碳管聚集的電雙層斥力。然後利用 DLVO 理論計算不同管徑碳管在不同 SDS 活性劑濃度下的能量障壁，藉此預測碳管於溶液的分散行為。由研究結果發現在特定活性劑濃度下，碳管具有最佳的分散效果，此結果與實驗趨勢相符合。不僅如此，溶液的特性，如碳管半徑與活性劑的吸附，都能準確地利用此計算模型預估出來。此外，亦可探討 SDS 離子於碳管上的結構型態，以及其於溶液中自組裝形成微胞的構型與界面特性。本研究亦發現 SDS 離子受熱熵與熱焓影響下的分布還會進一步改變抗衡鈉離子於溶液中的分布概率。我們的研究成果預期可以對碳管分散實驗的 SDS 活性劑濃度添加量與碳管管徑設計提供方向。

The dispersion of the single-walled carbon nanotubes (SWCNTs) in aqueous solutions has been an important issue. However, it is a challenging task since it requires the understanding to both macroscopic properties of the solution and the colloidal mechanism in the molecular level. In this work, a simulation framework which combines the coarse-grained molecular dynamics (MD) simulations and the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, is developed to predict the stability of the SWCNTs in aqueous solutions. The surfactant used here is sodium dodecyl sulfate (SDS). The adsorption of the solutions with different concentration of SDS is simulated. Next, the Langmuir isotherm model is used to find the relationship between the amount of the adsorbed SDS and the bulk SDS concentration. With the increasing number of the SDS covered on SWCNTs, the surface charge density is also enhanced, and thus, greater electrical double layer repulsion is achieved to prevent the aggregation of the SWCNTs. The potential energy barrier as a function of SDS concentration and radius of the SWCNT then can be obtained by using DLVO theory. It can predict the dispersion of the SWCNTs in the solution. The results suggest an optimal surfactant concentration which can stabilize the SWCNTs in the solution. The finding have good agreement with those in experiments. Many features of the solution, such as the relationship between radius of the SWCNTs and surfactant adsorption, are accurately predicted. We not only investigate the structural configuration of SDS ions absorbed on the SWCNT surface, but also discuss the configuration and interface characteristics of SDS self-assembled micelles. In addition, the distribution of SDS ions can affect the distribution of the sodium counterions in the solution. The current framework is expected to provide the guidance for the design of the concentration of SDS surfactants and the radius of SWCNTs in dispersed experiments.