## 3 Results and Discussion  
Crystals obtained from crystallization experiments reported in this work show the characteristic cubic block-like form in accordance with the expected geometry of the NaCl crystals (Figure 2). The molecular view of the crystals was reached observing the crystal obtained by molecular simulations both in the bulk solution of NaCl and in the presence of PVDF and PP membrane surfaces. Crystals achieved from bulk solutions are indicated in Figure 3, wherein the lattice of Na\({}^{+}\) and Cl\({}^{-}\) ions is visible. Crystal growth was observed within 100 ns from supersaturated solutions (about 6 M). The salt concentration was set at a relatively high value because it facilitates nucleation during the MD simulations. NaCl crystallized into a face-centered cubic lattice, a cubic with space group Fm3m and Z = 4 (four formula units per cell). The lattice parameter is a = 5.6601 A at room temperature, slightly larger than the experimental value of 0.564 nm determined by X-ray diffraction measurements by [PERSON] et al. [53] on synthetic NaCl powders. This is due to small inaccuracies in the interaction potentials and the finite size of the crystal. Similar regular structures of face-centered cubic NaCl crystals appeared when the supersaturated solutions (about 6 M) were in contact with polymeric PVDF and PP membranes, in agreement with the crystal forms obtained from experimental screening (Figure 2). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Atom Type** & \(\sigma\) **(Å)** & \(\varepsilon\) **(kJ/mol)** & **Charge (e)** \\ \hline O\({}_{\text{w}}\) & 3.166 & 0.6501 & \(-\)0.847600 \\ H\({}_{\text{w}}\) & 0.0 & 0.0 & +0.423800 \\ Na\({}^{+}\) & 3.330 & 0.0116 & \(+\)1 \\ Cl\({}^{-}\) & 4.417 & 0.4928 & \(-\)1 \\ C\({}_{\text{PVDF, H}}\) & 3.500 & 0.2763 & \(-\)0.650250 \\ C\({}_{\text{PVDF, F}}\) & 3.500 & 0.2763 & +0.765000 \\ H\({}_{\text{PVDF}}\) & 2.500 & 0.1256 & +0.225875 \\ F\({}_{\text{PVDF}}\) & 2.983 & 0.2512 & \(-\)0.283250 \\ \hline \hline \end{tabular} \({}^{\text{a}}\) The O\({}_{\text{w}}\) and H\({}_{\text{w}}\) parameters are for water and from [42], and the Na\({}^{+}\) and Cl\({}^{-}\) parameters are from [50, 51], respectively. The parameters of C\({}_{\text{PVDF, H}}\), C\({}_{\text{PVDF, F}}\), H\({}_{\text{PVDF}}\), and F\({}_{\text{PVDF}}\) are for PVDF and from [52].  
\end{table} 
Table 3: Nonbonded force field parameters and charges of the Lennard-Jones potential for water, ion, and PVDF \({}^{\text{a}}\). Figure 2: NaCl crystalline habit. Sample using polypropylene (PP) membrane. Magnification: \(\times\)10. Crystal growth is a relatively slow process but is accessible to MD simulations for simple systems. Nucleation, on the other hand, is an activated process belonging to a class of rare event phenomena [13]. The simulations indicated that crystallization in both the NaCl solution and PVDF and PP membrane surfaces follow an Ostwald-like process [54; 55; 56; 57; 58]. Firstly, local regions of relatively high salt concentration in the supersaturated solution appear prior to visible spatial ordering with the early-stage salt nucleus containing a significant amount of water, followed by a second step that includes nucleation and development of spatial ordering of the ions. This is due to the fact that formation of several small nuclei is entropically favorable [59; 60]. Na\({}^{+}\) and Cl\({}^{-}\) ions, both in the bulk solution and with PVDF and PP membrane surfaces, during the simulation time organize themselves in regions of highly concentrated disordered ions and then the initial formation of ordered clusters and growth followed the Ostwald-like process during the simulation time, as observed for the NaCl solution at different concentrations by [PERSON] and [PERSON] [34]. Figure 4 gives a qualitative overview of crystal nucleation of NaCl crystals from the bulk solution and in the presence of the PP membrane surface. Small size aggregates of ions appeared in both systems and combined gradually to form larger aggregates (Figure 4a,c).