## 2 Materials and Methods
### Modelling  
Unbiased molecular dynamics simulations using all atoms were performed for the investigation of the feasibility of growing NaCl crystals. The initial amorphous PVDF and PP structures with a density of 1.9 and 0.9 g/cc, respectively, were constructed using the amorphous cell module in the commercial software Material Studio package (version 7.0) (Dassault Systemes BIOVIA, San Diego, 2013) [35] and the COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) force field [36]. The polymeric models were grown in a 3D cubic box under periodic boundary conditions at 298 K using the algorithm developed by [PERSON] and [PERSON] [17] and the scanning method proposed by [PERSON] [38]. All MD simulations, both considering only the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Fiber Type** & \begin{tabular}{c} **Outer** \\ **Diameter** \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} **Inner** \\ **Diameter** \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} **Thickness** \\ **(mm)** \\ \end{tabular} & \
\begin{tabular}{c} **Pore Size** \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} **Porosity (\%)** \\ **Angle (\({}^{\circ}\))** \\ \end{tabular} \\ \hline PVDF & 1.75 & 0.94 & 0.40 & 0.47 & 80.8 & 91 \\ PP & 1.80 & 1.56 & 0.12 & 0.20 & 70.0 & 115 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Structural parameters of the utilized membranes. bulk solution and with the polymeric membrane models, were then performed using the GROMACS (Groningen MAchine for Chemical Simulation, developed at the University of Groningen, Groningen, The Netherlands) software package, version 5.1.4 [39]. The force field parameters of the PVDF polymer developed by [PERSON] and [PERSON] [40] were used in this study and are listed in Table 2.  
The size of the PVDF model was 9.72 \(\times\) 6.18 \(\times\) 6.2 nm, that of the PP model was 5.61 \(\times\) 5.61 \(\times\) 10.1 nm, whereas the size of the bulk NaCl solution was 5.83 \(\times\) 5.83 \(\times\) 7.7 nm. The simulation boxes of three models were filled with 6919 SPC/E water molecules [42]. Slightly supersaturated conditions were considered with a concentration of solution of 5.94 \(\pm\) 0.2 M (crystallization of NaCl to occur at sodium chloride concentrations larger than 5:4 mol/L [43]) with almost 741 pairs of Na\({}^{+}\) and Cl\({}^{-}\) ions chosen in order to simulate the condition near a membrane surface during crystallization experiments. The ion parameters with the OPLS force field [44] were used. All pair interactions consisted of Lennard-Jones (LJ) and Columbic terms, and the Lorentz-Berthelot combining rules [45] were chosen to generate LJ parameters between different types of atoms. Other LJ parameters utilized in this study are shown in Table 3. The total energy of the materials calculated is given by [39; 40]  
\[ \begin{array}{ll}V&=\sum\limits_{ij}\frac{1}{2}k_{ij}^{b}(r_{ij}-b_{ij})^{2}+ \sum\limits_{i,j,k}\frac{1}{2}k_{ijk}^{\theta}\left(\theta_{ijk}-\theta_{ijk}^ {0}\right)^{2}+\sum\limits_{i,j,k}\sum\limits_{n}\frac{1}{2}k_{ijkl}^{\phi}( n)\left(1+\cos\left(n\phi_{ijkl}-\phi_{s}\right)\right)\\ &+\sum\limits_{i>j}4\varepsilon_{ij}\left(\left(\frac{\sigma_{ij}}{r_{ij}} \right)^{12}-\left(\frac{\sigma_{ij}}{r_{ij}}\right)^{6}\right)+\sum\limits_ {i>j}\frac{1}{4\pi\epsilon_{0}}\left(\frac{q_{i}q_{j}}{\epsilon r_{ij}}\right) \end{array}  \tag{4}\]  
where \(r_{ij}\), \(\theta_{ijk}\), and \(\phi_{ijkl}\) are the distance, angle, and torsional angle between the bonded atoms \(i\) and \(j\), \(i\), \(j\) and \(k\), \(i\), \(j\), \(k\), and \(l\), respectively. \(\phi_{s}\) is 180\({}^{\circ}\). \(\varepsilon_{ij}\) and \(\sigma_{ij}\) are the well depth and size of Lennard-Jones. \(\varepsilon_{0}\) is the permittivity of vacuum, and \(\varepsilon_{r}\) is the relative dielectric constant, and in this case, it is 1. \(q_{i}\) and \(q_{j}\) are the respective charges of atoms \(i\) and \(j\). Other LJ parameters utilized in this study are shown Table 3. Periodic boundary conditions were applied.