Elsevier

Journal of Membrane Science

Volume 487, 1 August 2015, Pages 32-39
Journal of Membrane Science

Understanding water and ion transport behaviour and permeability through poly(amide) thin film composite membrane

https://doi.org/10.1016/j.memsci.2015.03.052Get rights and content

Highlights

  • The behaviours of water molecules through a polyamide thin film were revealed.

  • Water–polyamide and ion–polyamide intermolecular interactions were discovered.

  • Barrier energies were determined for water permeation and ion rejection.

  • A significant difference in water behaviours under a pressure gradient was found.

Abstract

Molecular dynamics (MD) together with the adaptive biasing force (ABF) and metadynamics free energy calculation methods was used to investigate the permeation properties of salt water through poly(amide) thin film composite reverse osmosis membranes. The thin films were generated by annealing an amorphous cell of poly(amide) chains through an MD method. The MD results showed they have typical structural properties of the active layer of thin film composite membranes and comparable water diffusivity (2.13×10−5 cm2/s for the film with a density of 1.06 g/cm3) and permeability (9.27×10−15 cm3 cm/cm2s Pa) to experimental data. The simulations of water permeation through the films under different transmembrane pressures revealed the behaviours of water molecules in the thin films and the dynamic regimes of water permeation, including Brownian diffusion, flush and jump diffusion regimes. The intermolecular interactions of water and ions with poly(amide) chains showed a strong dependence on the local structure of films. The attraction between water and ploy(amide) molecules can be up to 8.5 kcal/mol in dense polymer regions and 5 kcal/mol in the pores of about 3 nm. The ABF and metadynamics simulations produced the profiles of free energy potential of water and ions along the depth of the thin films, which provided important information for quantitatively determining the barrier energy required for water permeation and rejection of ions. The thin film with a density of 1.06 g/cm3 and a thickness of 6 nm offers a rejection to Na+ but a slight absorption of Cl (0.25 kcal/mol) at 0.3–0.4 nm distance to its surface. Water molecules must overcome 63 kcal/mol energy to move to the centre of the film. The dependences of the barrier energy and the water–polymer interaction energy on the local free volume size in the thin film were analysed. The simulations of water permeation under high transmembrane pressures showed a nonlinear response of the concentration and distribution of water molecules in the film to the imposed pressure. Compaction of the film segments close to the porous substrate and water congestion in dense regions significantly influenced the water permeation when the membrane was operated under pressures of more than 3.0 MPa.

Introduction

Thin film composite membranes [1], [2], [3] are currently applied in commercial water treatment and water pollution control worldwide. They offer advantages over single-material asymmetric cellulose acetate membranes in high flux, high rejection to salt and some low molecular weight organics, and stability at higher temperature and over a larger pH range [4], [5], as the top-barrier layer formed in situ and the bottom porous substrate can be easily controlled both in chemistry and structure to maximise the overall membrane performance. The top-active layers are often a thin layer of poly(amide) polymer made by interfacial polymerisation of an aromatic polyamine, such as m-phenylenediamine (MPD), with aromatic poly(acyl)-halides, such as trimesoyl chloride (TMC) [4], [6]. The layer is essential for the performance of the membranes as it predominantly determines the separation properties, while the substrate as a support layer gives the necessary mechanical properties. The water permeation and ion rejection properties of the thin film poly(amide) membranes are strongly affected by a considerable number of parameters. The most important are the properties of the skin layer, including polymer composition [7], [8], [9], [10], molecular structure [11], [12], [13], free volume and its distribution [14], morphology [15], [16], [17] and electric potential on surface [18].

Extensive experimental investigations have been performed with significant interest in developing high water flux and energy efficiency, high contaminant selectivity, and high chemical-stable thin-film composite membranes. The theoretical interpretation based on the available experiments [18], [19], [20], [21] has provided useful qualitative information in understanding water and ion transport through poly(amide) membranes. Nonetheless, many aspects of the molecular mechanism governing the membrane performance are still unclear, such as the dependence of water and ion diffusions on polymer structure and dynamics and salt and water transport in the active layer of poly(amide) TFC membranes.

Molecular dynamics (MD) is an effective tool to probe molecular interaction in polymer penetrant systems. MD simulations of the structures and diffusions of water, Na+, and Cl− in poly(amide) membranes have suggested that H2O transport occurs by a jump-diffusion process with each jump ~3 Å in length [22], [23]. Equilibrium MD simulations of a hydrated poly(amide) membrane were performed to determine the density and diffusivity of water within the membrane [24]. MD simulation technique was incorporated with positron annihilation lifetime spectroscopy (PALS) to understand the fine-structure of polyamide active layers of thin-film composite membranes in a dry and a wet condition [13] and the structural properties of a polydimethylsiloxane (PDMS) membrane and the feed transport behaviours of ethanol and water during a pervaporation process [25], [26], [27]. The effects of residual solvent in a 6FDA-mPDA PI membrane on the gas sorption and permeation of the poly(amide) membrane and on the flexibility of the polymer segments were analysed based on the energy data from MD simulations [28]. Two atomistic models were proposed for ODPA–ODA amorphous phase to study the effects of skin layer of a glassy polyimide on gas permeation [29] and to investigate the gas permeation in bulk models of the glassy oxydiphthalic anhydride and oxydianiline (ODPA–ODA) polyimide [30].

The knowledge of the intermolecular interactions of water and ion with molecules constituted a thin film and the free energies of water and ions during permeation are essential prerequisite to understand the performance of the barrier layer. Most of MD studies [22], [23], [24], [28], however, were based on equilibrium MD simulations at a constant pressure and no data for free energy of water and ion through poly(amide) thin film have been provided. Reverse osmosis membrane desalination is operated under a pressure gradient, and the membrane structure and water concentration in the membrane will be different from that at a constant pressure. To the best of our knowledge, there are few reports on the water and ion permeations across poly(amide) membrane under pressure gradient at the nanoscale. The current work aims to provide the basic information of water and ions through poly(amide) membranes for understanding the permeation or rejection mechanisms, by investigating the behaviours of water molecules and ions through different poly(amide) membranes and the intermolecular interaction with MD simulation. The free energies were calculated by both metadynamics [31] and adaptive biasing force (ABF) [32], [33] methods. The motion features of water molecules in the membrane under a transmembrane pressure were studied. Diffusivity and permeability of water were determined and critically compared with experimental values.

Section snippets

Materials and structure of thin films

Unsupported poly(amide) thin films were created for the MD simulations. As this work focuses on the properties of the active skin layer of polymer membranes, the porous substrate is excluded in the system. Each molecule chain of the thin films is composed of 39 mphenylenediamine (MPD) monomers and 19 benzene 1,3,5-tricarboxylic acid chloride (TMC) monomers. The polymerisation of poly(amide) membranes has been described in the literature [22], [34], [35]. TMC and MPD monomers and the repeat unit

Characteristics of water and ion transport

The irregular assembly of polymer chains with van der Waals and electric interactions in the active layer of polymer thin films leads to polymer-free void spaces or pores where water could pass through but ions could not, as shown in Fig. 1. The water molecules entering the free volume spaces in a shallow depth move to the lower pressure side through the relative dense centre layer of the membrane, where the free volumes may be scattered or connected. There is no doubt that water molecules

Conclusions

A method was proposed to generate fully atomistic poly(amide) thin films having typical structural features of the active layer of thin film composite reverse osmosis membranes. With the MD simulations of water and ion permeation through the thin films, the water penetration by Brownian diffusion, flush and jump diffusion, as well as its dependence on the free volume size, was confirmed by the behaviours of water molecules and quantitatively characterised by the diffusivity, moving velocity,

Acknowledgement

This work was supported by the CSIRO׳s Water for a Healthy Country Flagship and the computations were performed with the facilities at Victorian Partnership for Advanced Computing (VPAC).

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