Sahu Pooja, Ali Sk M, Shenoy K T
Chemical Engineering Division, Bhabha Atomic Research Center, Mumbai 400085, India.
J Chem Phys. 2015 Feb 21;142(7):074501. doi: 10.1063/1.4908051.
Thermodynamic properties of the fluid in the hydrophobic pores of nanotubes are known to be different not only from the bulk phase but also from other conventional confinements. Here, we use a recently developed theoretical scheme of "two phase thermodynamic (2PT)" model to understand the driving forces inclined to spontaneous filling of carbon nanotubes (CNTs) with polar (water) and nonpolar (methane) fluids. The CNT confinement is found to be energetically favorable for both water and methane, leading to their spontaneous filling inside CNT(6,6). For both the systems, the free energy of transfer from bulk to CNT confinement is favored by the increased entropy (TΔS), i.e., increased translational entropy and increased rotational entropy, which were found to be sufficiently high to conquer the unfavorable increase in enthalpy (ΔE) when they are transferred inside CNT. To the best of our knowledge, this is the first time when it has been established that the increase in translational entropy during confinement in CNT(6,6) is not unique to water-like H bonding fluid but is also observed in case of nonpolar fluids such as methane. The thermodynamic results are explained in terms of density, structural rigidity, and transport of fluid molecules inside CNT. The faster diffusion of methane over water in bulk phase is found to be reversed during the confinement in CNT(6,6). Studies reveal that though hydrogen bonding plays an important role in transport of water through CNT, but it is not the solitary driving factor, as the nonpolar fluids, which do not have any hydrogen bond formation capacity can go inside CNT and also can flow through it. The associated driving force for filling and transport of water and methane is enhanced translational and rotational entropies, which are attributed mainly by the strong correlation between confined fluid molecules and availability of more free space for rotation of molecule, i.e., lower density of fluid inside CNT due to their single file-like arrangement. To the best of our information, this is perhaps the first study of nonpolar fluid within CNT using 2PT method. Furthermore, the fast flow of polar fluid (water) over nonpolar fluid (methane) has been captured for the first time using molecular dynamic simulations.
已知纳米管疏水孔内流体的热力学性质不仅不同于本体相，也不同于其他传统受限环境。在此，我们使用最近开发的“两相热力学（2PT）”模型理论方案，来理解促使极性（水）和非极性（甲烷）流体自发填充碳纳米管（CNT）的驱动力。发现碳纳米管受限环境对水和甲烷在能量上都是有利的，导致它们在CNT(6,6)内部自发填充。对于这两个系统，从本体转移到碳纳米管受限环境的自由能受熵增加（TΔS）的青睐，即平移熵增加和转动熵增加，当它们转移到碳纳米管内部时，这些熵增加被发现足够高，足以克服焓（ΔE）的不利增加。据我们所知，这是首次确定在CNT(6,6)中受限期间平移熵的增加并非水状氢键流体所特有，在甲烷等非极性流体中也观察到了这种情况。从碳纳米管内流体分子的密度、结构刚性和输运方面解释了热力学结果。发现在本体相中甲烷比水扩散得快，但在CNT(6,6)受限期间情况相反。研究表明，尽管氢键在水通过碳纳米管的输运中起重要作用，但它不是唯一的驱动因素，因为没有任何形成氢键能力的非极性流体也能进入碳纳米管并在其中流动。水和甲烷填充及输运的相关驱动力是平移和转动熵的增加，这主要归因于受限流体分子之间的强相关性以及分子有更多自由旋转空间，即由于它们单排排列导致碳纳米管内流体密度较低。据我们所知，这可能是首次使用2PT方法对碳纳米管内非极性流体进行的研究。此外，首次使用分子动力学模拟捕捉到了极性流体（水）比非极性流体（甲烷）快速流动的情况。
