**1. Introduction**

The adsorption and accumulation of fouling organisms on surface of materials, i.e., marine biofouling, is a major problem faced by ships and offshore facilities [1,2]. The annual cost of increased fuel consumption, cleaning, maintenance, and repair of ships caused by marine biofouling is as high as billions of dollars [3,4]. Early marine antifouling coatings mainly used biotoxic tributyltin (TBT) antifouling paints, which killed marine organism larvae or spores through the release of antifouling agents to achieve antifouling purposes [5,6]. However, traditional antifouling paints are highly toxic for many aquatic organisms and have caused severe damage to the environment. The development of ecofriendly antifouling coatings is gradually becoming a research hotspot in this field [7–9].

Among them, protein-resistant antifouling material that inhibits the settlement of proteins is a relatively promising one [10], such as poly (ethylene glycol) (PEG), zwitterionic polymers [11] (poly (Sulfobetaine methacrylate), pSBMA, or poly (Carboxybetaine methacrylate), pCBMA). For example, Jiang's group [12–14] has been engaged in biofouling research for a long period and synthesized a series of zwitterionic polymers. On the one hand, they used molecular simulation methods to reveal the antifouling mechanism of materials on the microscopic level. On the other hand, they carried out application research on this basis to design and synthesize new antifouling materials. Zheng and coworkers [15–17] investigated the antifouling properties of zwitterionic polymer brushes, polyacrylamide, and hydroxyalkyl acrylamides using combined molecular dynamics and

**Citation:** Zhang, H.; Zheng, J.; Lin, C.; Yuan, S. Molecular Dynamics Study on Properties of Hydration Layers above Polymer Antifouling Membranes. *Molecules* **2022**, *27*, 3074. https://doi.org/10.3390/ molecules27103074

Academic Editor: Danilo Roccatano

Received: 8 April 2022 Accepted: 9 May 2022 Published: 11 May 2022

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steered molecular dynamics, believing that the carbon space and anionic groups have distinct effects on their antifouling performance. The state key laboratory of marine corrosion and protection in China has also synthesized a series of antifouling coatings by grafting zwitterionic sulfobetaine methacrylate (T4-SB) or anionic sulfonate methacrylate (T4-SP), which have the property of inhibiting adsorption of proteins on the surface of polysiloxane material (T4). These materials have a good antifouling effect on fouling organisms such as diatoms. We found that the static adsorption number of diatoms in the T4-SP antifouling material is 15/mm<sup>2</sup> (4% of the T4 antifouling material) in the experiment; for T4-SB, the static adsorption number of diatoms is 9/mm<sup>2</sup> (2% of the T4 antifouling material), which significantly improved the antifouling performance of the silicone material.

The adsorption of protein on surface is affected by many factors [18–21], among which the factors favorable for adsorption mainly include the enthalpy loss from the van der Waals and electrostatic attraction between protein and surface, and the entropic gain from the removal of hydration layer at the surface of material and protein. The disadvantages include the enthalpy gain required for the dehydration of surface and protein, protein's conformation adjustment, as well as the entropic loss from protein adsorption and exposure of hydrophobic regions. The hydration layer above the surface of the antifouling material plays a crucial role from the antifouling perspective [22] because it provides the physical and energy barriers that must be overcome during protein adsorption. To confirm the structure of the hydration layer above the surface of antifouling materials, many experimental studies have been carried out. For example, Leng et al. [23,24] confirmed that there is a tightly bounded and regularly ordered hydration layer above zwitterionic antifouling membrane compared with polymer membrane without antifouling ability using sum frequency generation (SFG) vibrational spectroscopy. Paul et al. [25] directly observed the structure of hydration layer above the surface of epoxy organosilane modified silica nanoparticles and unmodified silica nanoparticles by frequency modulation−atomic force microscopy. Combined with molecular dynamics simulations, a more continuous and thicker hydration layer structure was found on the surface of modified silica particles, which endows the material with a better antifouling ability.

In this work, we will compare the antifouling ability of three polymer antifouling membranes (T4-DM, T4-SP, T4-SB) using molecular dynamics simulation at the molecular level through the hydration layer. We hope this work will provide theoretical support for the subsequent design and optimization of related antifouling materials.

### **2. Simulation Method**

### *2.1. Model*

Three antifouling membranes were constructed according to their molecular structures (Figure 1). The T4 substrate was neglected considering the main differences between different antifouling membranes focusing on the grafted polymers. The modeling process of T4-DM system is illustrated in Figure 2 as an example. The polymer chains with a degree of polymerization of 15 (Figure 2b) were built from their repeat unit (Figure 2a) using the Visualizer module in Materials Studio. This was repeated 10 times in the x and y directions to derive the initial configuration of antifouling membrane in Figure 2c. The initial configurations of the antifouling membranes were then subject to a 21-step molecular dynamics compression and relaxation [26] to obtain the equilibrium packing structure (which might not be the optimal one) in Figure 2d. The procedure of the 21-step MD simulation protocol is listed in Table S1. The simulation boxes were then enlarged two times along the z-axis to accommodate solvent molecules (Figure 2e). As a comparison, antifouling membranes without water were also studied (Figure 2g). Finally, all systems were subject to equilibrium molecular dynamics simulations to derive equilibrium structures (Figure 2f,h).

**Figure 1.** Chemical structure of three nonfouling membranes (**a**) T4-DM, (**b**) T4-SP, (**c**) T4-SB. **Figure 1.** Chemical structure of three nonfouling membranes (**a**) T4-DM, (**b**) T4-SP, (**c**) T4-SB. **Figure 1.** Chemical structure of three nonfouling membranes (**a**) T4-DM, (**b**) T4-SP, (**c**) T4-SB.

**Figure 2.** Modeling process and Simulation protocol of T4-DM system. (**a**) Repeat unit of DM; (**b**) single polymer chain of DM in simulation box (side view), (**c**) enlarged 10 times in x and y directions of (**b**) (top view); (**d**) compressed and relaxed configuration of DM membrane (top view); (**e**) initial configuration of DM with water system (side view); (**f**) final configuration of DM with water system (side view); (**g**) initial configuration of DM without water system (side view); (**h**) final configuration of DM without water system (side view). **Figure 2.** Modeling process and Simulation protocol of T4-DM system. (**a**) Repeat unit of DM; (**b**) single polymer chain of DM in simulation box (side view), (**c**) enlarged 10 times in x and y directions of (**b**) (top view); (**d**) compressed and relaxed configuration of DM membrane (top view); (**e**) initial configuration of DM with water system (side view); (**f**) final configuration of DM with water system (side view); (**g**) initial configuration of DM without water system (side view); (**h**) final configuration of DM without water system (side view). **Figure 2.** Modeling process and Simulation protocol of T4-DM system. (**a**) Repeat unit of DM; (**b**) single polymer chain of DM in simulation box (side view), (**c**) enlarged 10 times in x and y directions of (**b**) (top view); (**d**) compressed and relaxed configuration of DM membrane (top view); (**e**) initial configuration of DM with water system (side view); (**f**) final configuration of DM with water system (side view); (**g**) initial configuration of DM without water system (side view); (**h**) final configuration of DM without water system (side view).

*2.2. Simulation Details*

*2.2. Simulation Details*

### *2.2. Simulation Details*

The repeat unit of each polymer was calculated at B3LYP/def2SVP// B3LYP/def2TZVP level using Gaussian 16 [27]. Then, RESP charges were derived from Multiwfn 3.8 [28]. All molecular dynamics simulations were performed using Gromacs 2019.3 software package [29]. Gromos 54a7 force field was used [30]. The total potential energy was given as a combination of valence terms, including bond stretching, angle bending, torsion, and nonbonded interactions. The nonbonded interactions between atoms were described by the Lennard-Jones potential, and the standard geometric mean combination rules were used for the van der Waals interactions between different atom species. Water molecules used the SPC model [31].

In the simulations, each of the systems was initialized by minimizing the energies of the initial configurations using steepest descent method. Following the minimization, a 50 ns MD simulation under NPT ensemble was carried out for each system, with a time step of 2 fs. In all simulations, the temperature was kept constant at 298 K by the v-rescale thermostat algorithm [32]. The pressure was kept constant at 1 atm by the Berendsen algorithm [33]. Bond lengths were constrained using the LINCS algorithm and periodic boundary conditions were applied in all directions [34]. Short-range nonbonded interactions were cut off at 1.2 nm, with long-range electrostatics calculated using the particle mesh Ewald method [35]. Trajectories were stored every 2 ps and visualized using VMD 1.9.3 [36].

### **3. Results and Discussion**

### *3.1. Properties of Antifouling Membranes*

### 3.1.1. Density Profiles

The simulated configurations of three antifouling membranes at dry and hydrated states are illustrated in Figure S1. We can clearly see that there are no significant differences between T4-DM membrane under dry and hydrated states, while for T4-SP and T4-SB membranes, many side chains extend to water phase. This indicates that the side chains of T4-SP and T4-SB have a better hydrophilicity. Besides this, the compression of these chains during adsorption of foulant would reduce the conformation possibility, which is entropically unfavorable, subsequently causing steric repulsion and preventing adsorption [10].

To quantitatively study the structure of three antifouling membranes, the density profile along z-axis was calculated, as shown in Figure 3. The results were derived from the last 5 ns trajectory. The density profile was symmetrized around the membrane center to obtain a better result. The density profiles of T4-DM in dry and hydrated states almost overlapped. As for T4-SP and T4-SB membranes, the density profile of hydrated state broadened compared with that of dry state (more obvious for T4-SB membrane), which is consistent with the configurations in Figure S1. The density of water in T4-SB is higher than that of T4-SP, and even higher than that of T4-DM, which indicates that the side chains of T4-SB can attract extra water molecules compared with those of T4-SP and T4-DM. We then deduce that the hydrophilicity of the antifouling membranes follows the order of T4-SB > T4-SP > T4-DM.

### 3.1.2. Surface Roughness

Since the density profile is a statistical average of the entire membrane layer, it cannot reflect the local specific structural information of membranes. To further analyze the detailed surface structure, contour maps of the upper surface of three antifouling membranes in hydrated states were sketched, as shown in Figure 4. To define the surface of membrane, the simulation box was divided into grids with 0.4 nm × 0.4 nm resolution in xy plane. Atoms with the largest or smallest z-axis were selected as the top atoms to define the membrane surface. It can be seen from Figure 5 that T4-DM membrane's surface is relatively flat, while T4-SP has more peaks and valleys than T4-DM. As for T4-SB, the contour lines are the densest, indicating that the order of surface roughness is T4-SB > T4-SP > T4-DM.

**Figure 3.** Density profiles along z-axis. (**a**) T4-DM, (**b**) T4-SP, (**c**) T4-SB. Black dashed lines and red lines represent density of antifouling membranes under dry state or hydrated states. Dotted blue lines represent density of water. **Figure 3.** Density profiles along z-axis. (**a**) T4-DM, (**b**) T4-SP, (**c**) T4-SB. Black dashed lines and red lines represent density of antifouling membranes under dry state or hydrated states. Dotted blue lines represent density of water.

**Figure 4.** Contour maps of three antifouling membrane surfaces. (**a**) T4-DM, (**b**) T4-SP, (**c**) T4-SB. **Figure 4.** Contour maps of three antifouling membrane surfaces. (**a**) T4-DM, (**b**) T4-SP, (**c**) T4-SB.

**Table 1.** Root-mean-square roughness of three antifouling membranes. **Root-Mean-Square Roughness R \*** To quantify the surface roughness of three antifouling membranes, the root mean square roughness R was introduced [37]:

$$\mathbf{R} = \sqrt{\frac{\sum\_{i=1}^{N} \left(Z\_i - \overline{Z}\right)^2}{N}}$$

T4-SP 3.73 ± 0.050 3.83 ± 0.056 2.68 ± 0.037 2.41 ± 0.037 T4-SB 4.28 ± 0.068 4.20 ± 0.059 2.94 ± 0.042 2.98 ± 0.032 \* Data derived from the last 1 ns trajectory. where *Z<sup>i</sup>* is the z-coordinate of the atoms exposed in the outermost layer in each grid point and *Z* is the average value of the z-coordinates of all the atoms exposed on the outermost surface. Both the up and down surfaces of three antifouling membranes in dry and hydrated states are calculated and listed in Table 1. The data suggested there is little difference between dry and hydrated states for T4-DM. The roughness in hydrated state follows the order of T4-SB > T4-SP > T4-DM, which is consistent with Figures 3 and 5. Obviously, the greater the roughness of the surface, the more hydrophilic sites were exposed, and the more water molecules could be bound.

In addition to the influence of surface roughness on surface hydration, the hydrophilicity and hydrophobicity of the surface determine the surface hydration ability directly. The hydrophilic and hydrophobic surface area of each antifouling membrane were calculated from the last 5 ns trajectory, as shown in Figure 5. During calculation, the atomic charge between −0.2 and 0.2 was considered as the hydrophobic surface area, and the other is the hydrophilic surface area. The hydrophilic surface area and its proportion of all three antifouling membranes increased in hydrated state. The total surface area does not change much between dry and hydrated states, which is consistent with the configuration in Figure S1. The total surface area, especially the hydrophilic surface area, of T4- SP and T4-SB both increased significantly when immersed in water, which suggests that

**Figure 5.** Solvent accessible surface area including hydrophobic and hydrophilic part of three antifouling membranes. Numbers above the bar means the proportion of hydrophilic area. **Figure 5.** Solvent accessible surface area including hydrophobic and hydrophilic part of three antifouling membranes. Numbers above the bar means the proportion of hydrophilic area.


*3.2. Properties of Surface Hydration Layer* **Table 1.** Root-mean-square roughness of three antifouling membranes.

calculated the cosine of the angle between dipole of water and z-axis at different distances \* Data derived from the last 1 ns trajectory.

### from the surface, as shown in Figure 6. Obviously, for a random distribution, the cos*θ* 3.1.3. Hydrophilicity

3.1.3. Hydrophilicity

they have a strong hydration ability.

should be close to 0 [38]. In the T4-DM membrane system, only water molecules close to membrane have a certain orientation, while water molecules farther away are randomly distributed. In the T4-SP system, the dipole orientation of surface water molecules slightly decreased to 0 after 2 nm, while in the T4-SB system, there is still a long-distance In addition to the influence of surface roughness on surface hydration, the hydrophilicity and hydrophobicity of the surface determine the surface hydration ability directly. The hydrophilic and hydrophobic surface area of each antifouling membrane were calculated from the last 5 ns trajectory, as shown in Figure 5. During calculation, the atomic charge between −0.2 and 0.2 was considered as the hydrophobic surface area, and the other is the hydrophilic surface area. The hydrophilic surface area and its proportion of all three antifouling membranes increased in hydrated state. The total surface area does not change much between dry and hydrated states, which is consistent with the configuration in Figure S1. The total surface area, especially the hydrophilic surface area, of T4-SP and T4-SB both increased significantly when immersed in water, which suggests that they have a strong hydration ability.
