Recent experimental results suggest that stacked layers of graphene oxide exhibit strong selective permeability to water. To construe this observation, the transport mechanism of water permeating through a membrane consisting of layered graphene sheets is investigated via nonequilibrium and equilibrium molecular dynamics simulations. The effect of sheet geometry is studied by changing the offset between the entrance and exit slits of the membrane. The simulation results reveal that the permeability is not solely dominated by entrance effects; the path traversed by water molecules has a considerable impact on the permeability. We show that contrary to speculation in the literature, water molecules do not pass through the membrane as a hydrogen-bonded chain; instead, they form well-mixed fluid regions confined between the graphene sheets. The results of the present work are used to provide guidelines for the development of graphene and graphene oxide membranes for desalination and solvent separation.

We investigate the response of molecular fluids to temperature gradients. Using non equilibrium molecular dynamics computer simulations we show that non polar diatomic fluids adopt a preferred orientation as a response to a temperature gradient. We find that the magnitude of this thermomolecular orientation effect is proportional to the strength of the temperature gradient and the degree of molecular anisotropy, as defined by the different size or mass of the molecular atomic sites. We show that the preferred orientation of the molecules follows the same trends observed in the Soret effect of binary mixtures. We argue this is a general effect that should be observed in a wide range of length scales.Thermal gradients are responsible for a wide range of non equilibrium effects, electron transport (thermoelectricity) [1], mass transport in suspensions (thermophoresis) [2][3][4][5][6], mass separation in liquid mixtures [7][8][9] and nucleation and growth of colloidal crystals [10]. Recently it has been shown that temperature gradients can induce orientation in polar fluids, a physical effect that is supported by Non Equilibrium Thermodynamics Theory (NET) and that can be explained in terms of the coupling of a polarization field and a temperature gradient [11]. The polarization of the fluid results in sizable electrostatic fields whose strength scales linearly with the temperature gradient.In this Letter we show that temperature gradients can also induce the molecular orientation of non polar fluids. We call this effect thermomolecular orientation (TMO). The physical origin of this effect cannot be discussed in terms of the polarization field / heat flux coupling, although it is important to note that there is not a principle within non equilibrium thermodynamics theory that precludes observing molecular orientation in non polar fluids. To investigate this hypothesis we use boundary driven non equilibrium molecular dynamics (NEMD) simulations. NEMD has been successfully employed before to investigate thermodynamic and transport properties of atomic and molecular fluids as well as simple ionic liquids [12][13][14][15]. In this work we focus on diatomic fluids. The advantage of using such a model is that it provides simplicity, the necessary "flexibility" to change the molecular anisotropy in a controlled way, and as we will see below, it is possible to establish a direct connection with binary mixtures, making it possible to investigate correlations between TMO and the Soret effect [16,17].We have investigated the TMO effect using a diatomic molecule consisting of two tangent spheres of diameters σ 1 , σ 2 , masses m 1 , m 2 and bond length σ = (σ 1 + σ 2 )/2. The bond length was kept constant using a rigid bond through the Rattle algorithm [18]. The interaction between the particles is completely repulsive, U ij (r) = 4ε (σ ij /r) 12 , where ε is the interaction strength, which we use to define the reduced temperature, T * = k B T /ε, and σ ij = (σ i + σ j )/2 is defined in terms of the diameters of sites i and j. ...

Large scale molecular simu lations to model the formation of polyamide membranes have been carried out using a procedure that mimics experimental interfacial polymerization of trimesoyl chloride (TMC) and metaphenylene diamine (MPD) monomers. A coarse - grained representation of the m onomers has been developed to facilitate these simulations, which captures essential features of the stereochemistry of the monomers and of amide bonding between them. Atomic models of the membranes are recreated from the final coarse - grained representatio ns. Consistent with earlier treatments, membranes are formed through the growth and aggregation of oligomer clusters. The membranes are inhomogeneous, displaying opposing gradients of trapped carboxyl and amine side groups, local density variations, and r egions where the density of amide bonding is reduced as a result of the aggregation process. We observe the interfacial polymerization reaction is self - limiting and the simulated membranes display a thickness of 5 – 10 nm. They also display a surface roughn ess of 1 – 4 nm. Comparisons are made with recently published experimental results on the structure and chemistry of these membranes and some interesting similarities and differences are found

We report non-equilibrium molecular dynamics simulations (NEMD) of water under temperature gradients using a modified version of the central force model (MCFM). This model is very accurate in predicting the equation of state of water for a wide range of pressures and temperatures. We investigate the polarization response of water to thermal gradients, an effect that has been recently predicted using Non-Equilibrium Thermodynamics (NET) theory and computer simulations, as a function of the thermal gradient strength. We find that the polarization of the liquid varies linearly with the gradient strength, which indicates that the ratio of phenomenological coefficients regulating the coupling between the polarization response and the heat flux is independent of the gradient strength investigated. This notion supports the NET theoretical predictions. The coupling effect leading to the liquid polarization is fairly strong, leading to polarization fields of ~10(3-6) V m(-1) for gradients of ~10(5-8) K m(-1), hence confirming earlier estimates. Finally we employ our NEMD approach to investigate the microscopic mechanism of heat transfer in water. The image emerging from the computation and analysis of the internal energy fluxes is that the transfer of energy is dominated by intermolecular interactions. For the MCFM model, we find that the contribution from hydrogen and oxygen is different, with the hydrogen contribution being larger than that of oxygen.