1 Introduction

The evaporation of nanoscale amounts of water on various surfaces is of great interest in many areas, from nature to industry; for instance, evaporation is responsible for huge water losses through soil salinization [1], inorganic salt transport in plants [2], and carbon dioxide capture to mitigate climate change [3]. In the industrial field, evaporation is also a crucial process in printable electronics fabrication [4], spray cooling [5, 6], gold nanorod (GNR) assembly [7], and protein folding [8, 9].

The evaporation of water can be affected by many factors, e.g., surface characteristics [10, 11], temperature [12, 13], and humidity [14, 15]. Recent studies show that the evaporation of small amounts of water is greatly affected by the solid surface wettability and show different characteristics with respect to the evaporation from bulk water surfaces [16-18]. Our previous work showed that the evaporation of small volumes of water on patterned graphene oxide (GO) surfaces is faster than that on homogeneous graphene oxide surfaces [19]. He et al. found that a chemically patterned surface can enhance evaporation by extending the contact lines [20]. Last year, Guo et al. discovered that the evaporation rate of nanoscale water on uniformly complete wetting surfaces slowly increases with increasing temperature [21].

Ionic species, which are ubiquitous in nature [22-32], have also been shown to affect the evaporation properties of bulk water and droplets [33-35]. Many studies showed that the presence of impurities slows down the evaporation of macroscopic water in atmospheric or industrial conditions [36-39]. However, the specific influence of ionic species on the evaporation of small volumes of water on nanoscale surfaces remains unclear.

Herein, using molecular dynamics (MD) simulations, we investigated the influence of sodium and chlorine ions on the evaporation of nanoscale water on GO surfaces. We find that the evaporation rate shows a higher decrease on patterned GO surfaces than that on homogeneous ones. The total evaporation rate on patterned GO surfaces originates from the corresponding rates of both oxidized and unoxidized regions. The decrease in the evaporation rate on patterned GO can be attributed to the obvious decrease in water evaporation from the unoxidized regions. The analysis shows that the increased amount of Na⁺ ions near the contact lines significantly reduces the number of water molecules diffusing from the oxidized to the unoxidized regions. Furthermore, the water evaporation from patterned GO does not change significantly until the C/(OH) ratio reaches 10.7: ions and water molecules can no longer be excluded from the unoxidized region, and the evaporation rate drops. These findings could be useful for achieving controllable evaporation of liquids on graphene oxide surfaces.

2 Computational Methods

We investigated patterned and homogeneous graphene oxide nanosheets with dimensions of 14.7 nm × 12.9 nm (Fig. 1(a)) and carbon-carbon bond lengths of 0.142 nm. A graphene oxide ceiling of the same size was placed 8.7 nm above the substrate. The homogeneous GO and the oxidized regions of patterned GO were built based on a C₃₂O_n(OH)_n molecular formula (Lerf-Klinowski structural model [40]), which means that n epoxy and n hydroxyl groups were decorated on the graphene plane every 32 carbon atoms, where n = 3, 4, 5, 6, or 8 [41]. The unoxidized regions of patterned GO were solid surfaces without oxidative functional groups (Fig. 1(b)). The oxidized regions have hydrophilic properties associated with the polar oxygen functional groups [42], while the unoxidized regions remain hydrophobic, reflecting the wetting properties of graphene.

Fig. 1Full-screen View

(Color online) Water evaporation on patterned and homogeneous GO surfaces. (a) Geometry of the patterned and homogeneous surfaces. The blue and black rectangles outline the unoxidized and oxidized regions, respectively. (b) Detailed geometry of the unoxidized and oxidized regions. (c) Snapshot of water and ions on patterned and homogeneous GO surfaces. (d) Evaporation rate on GO surfaces with different NaCl concentrations. The black circles and red squares represent the evaporation rates of patterned and homogeneous GO, respectively.

Initially, 2256 TIP3P [43] water molecules were arranged in a box placed 0.5 nm above the homogeneous and patterned GO substrates. The initial size of the box was 13.0 nm × 11.0 nm × 0.5 nm. In order to prevent the water molecules from returning to the surface, an accelerating region was set from 1.85 to 4.35 nm above the lower substrate. An upward force of 1.0 kcal mol^-1 Å^-1 (force vector (0, 0, 1)) was applied to the oxygen atom when a water molecule entered the accelerating region. These nonequilibrium settings are equivalent to the conditions applied in other literature studies, in which molecules evaporate into an infinite vacuum [44]. The Na⁺ and Cl^- ions were randomly distributed on the surfaces. The ionic concentrations were set to 0.5, 1.0, and 1.5 M, with corresponding total number of ions of 20, 38, and 60, respectively, equally divided between Na⁺ and Cl^- ions. Due to the periodic boundary conditions, the evaporated water molecules were absorbed by the solid ceiling and the water molecules under it. The evaporation rate R is defined as the number of water molecules moving from the substrate into the accelerating region per nanosecond.

MD simulations were carried out in a box with initial size of 14.7 nm × 12.9 nm × 11.0 nm, using NAMD 2.10 [45]. The time step of the simulation was 1 fs. Periodic boundary conditions were applied to all Cartesian coordinates. The Lennard-Jones parameters of the carbon atoms were σ = 0.358 nm and ε = 0.0663 kcal mol^-1. Charges of 0.2 and 0.266 e were employed for carbon atoms attached to the epoxy and hydroxyl groups, respectively. The Charmm27 force field [46] was used in the simulations. The particle-mesh Ewald method [47] was employed to treat long-range electrostatic interactions, and Langevin dynamics was applied to maintain the system at 300 K. The velocities and coordinates were collected every 500 fs. For all systems, after an equilibrium MD simulation of 2 ns, we applied the accelerating region to five independent systems for 7 ns, and data extracted from the last 5 ns of these simulations were averaged for the subsequent analysis.

3 Results and Discussion

Fig. 1(c) shows snapshots of water molecules and ions on both patterned and homogeneous GO surfaces (the oxidized regions were built based on a C₃₂O₆(OH)₆ composition). The figure shows that both water molecules and ions are distributed only on the oxidized regions. As shown in Fig. 1(d), as the concentration of NaCl increases from 0 to 1.5 M, the total evaporation rate decreases for both patterned and homogeneous GO surfaces. In order to understand this decrease, we analyze the averaged total interaction energy E_Total of the outermost water molecules, which acts as an energy barrier preventing water molecules from evaporating [48].

In the following, E_Total denotes the sum of the water-surface (E_{Water-surface}), water-water (E_Water-water), water-Na⁺ (E_Water-Na⁺), and water-Cl^- (E_Water-Cl^-) interaction energies. As shown in Fig. 2(a,b), the total interaction energy E_Total shows a similar increasing trend on patterned and homogeneous GO. Thus, as the concentration of NaCl increases, the energy barrier associated with the outermost water molecules becomes larger, and the evaporation rate decreases.

Fig. 2Full-screen View

Changes in average interaction energy per outermost water molecule and water film thickness with NaCl concentration. (a) Average interaction energy per outermost water molecule on patterned GO surfaces. (b) Average interaction energy per outermost water molecule on homogeneous GO surfaces. The blue down-triangles, blue up-triangles, blue circles, and black solid squares represent the average interaction energy between water and Cl^- (E_Water-Cl^-), water and Na⁺ (E_Water–Na⁺), water and GO surface (E_{Water–surface}), and two water molecules (E_{Water–water}), respectively, while the black hollow squares denote the average total interaction energy (E_Total). (c) Thickness of water film on patterned and homogeneous GO surfaces with different NaCl concentrations. The black circles and red squares represent the water film thickness on patterned and homogeneous GO, respectively.

For patterned GO surfaces (Fig. 2(a)), E_Water-Na⁺ and E_Water-Cl^- show an obvious increase as the concentration of NaCl increases, whereas E_Water-water and E_{Water-surface} only exhibit slight changes; these variations result in an overall increase in E_Total. A similar trend is observed for homogeneous GO surfaces (Fig. 2(b)), except for the slight decrease in E_{Water-surface}.

E_{Water-surface} is mainly affected by the distance between the GO surface and the outermost water molecules (thickness of the water film). Fig. 2(c) shows that, as the concentration of NaCl increases, the thickness of the water film on the patterned GO surface remains almost constant, because the film is relatively thick. In contrast, the thickness of the relatively thin water film on the homogeneous GO surfaces exhibits an obvious decrease.

It has been reported that water evaporation is enabled by the concerted and ultrafast H-bond dynamics of interfacial water [12]. Thus, we examined the hydrogen bonding characteristics of the outermost water molecules. In particular, two water molecules were considered hydrogen bonded when their O–O distance and O…O–H bond angle were less than 3.3 Å and 30°, respectively [49]. The average lifetime of the H-bonds, τ_H-bond, was determined by fitting the exponential decay function y(t) = e^-t/τH-bond (the result of double exponential fittings of the H-bonds autocorrelation function can be found in the Electronic Supplementary Information, ESI^†). As shown in Fig. 3(a,b), as the concentration of NaCl increases, the average number of H-bonds per outermost water molecule shows only a slight decrease on both patterned and homogeneous GO surfaces, while τ_H-bond shows a significant increase. The presence of the Na⁺ and Cl^- ions results in an obvious increase in the τ_H-bond value for the outermost water molecules, and therefore reduces the water evaporation rate.

Fig. 3Full-screen View

Hydrogen bonding characteristics of outermost water molecules on patterned and homogeneous GO surfaces with different NaCl concentrations. (a) Average number of H-bonds for outermost water molecules on patterned and homogeneous GO surfaces with different NaCl concentrations. The black circles and red squares represent the average numbers of H-bonds per outermost water molecule on patterned and homogeneous GO surfaces, respectively. (b) Average lifetime of H-bonds for outermost water molecules on patterned and homogeneous GO surfaces with different NaCl concentrations. The black circles and red squares represent the average H-bonds lifetimes for outermost water molecules on patterned and homogeneous GO, respectively.

As shown in Fig. 1(d), as the concentration of NaCl increases from 0 to 1.5 M, the total evaporation rate on the patterned GO surfaces decreases by 4.3 ns^-1 (from 23.3±1.1 to 19.0±0.8 ns^-1), while that on the homogeneous GO surfaces only decreases by 2.3 ns^-1 (from 15.9±0.7 to 13.6±0.6 ns^-1). Since the total evaporation rate of patterned GO surfaces derives from the evaporation rates of the oxidized and unoxidized regions [50], we calculated the evaporation rate on both regions with different NaCl concentrations, as shown in Fig. 4. The figure shows that the reduced evaporation from patterned GO surfaces is mainly due to the obvious decrease in evaporation rate from the unoxidized regions.

Fig. 4Full-screen View

Evaporation rate on patterned GO surfaces with different NaCl concentrations. The black, red, and blue bars represent the total evaporation rate, the evaporation rate from the oxidized regions, and the evaporation rate from the unoxidized regions, respectively.

In order to understand the decrease in evaporation rate on the unoxidized regions, we examined the distribution of ions and water molecules on patterned GO surfaces with different concentrations of NaCl. The results are illustrated in Fig. 5. The reference plane (z = 0) for all calculations is the lower one.

Fig. 5Full-screen View

Distribution of Na⁺, Cl^-, and water molecules on patterned GO surfaces with different NaCl concentrations. (a–c) Distribution probability of Na⁺, Cl^-, and water molecules along the z direction for NaCl concentrations of 0.5 M (a), 1.0 M (b), and 1.5 M (c). (d–f) Distribution probability of Na⁺/Cl^- ions and surface density of water molecules vs. distance to contact line for NaCl concentrations of 0.5 M (d), 1.0 M (e), and 1.5 M (f). The black solid squares, black hollow squares, and red circles represent Na⁺, Cl^-, and water molecules, respectively.

Fig. 5(a–c) shows the distribution probabilities of Na⁺, Cl^-, and water molecules along the z-direction, for NaCl concentrations of 0.5, 1.0, and 1.5 M. Throughout the concentration range investigated, the figures show that the Na⁺ ions are closer to the lower plane than the Cl^- ones. Fig. 5(d–f) shows the Na⁺ and Cl^- distribution probabilities and the surface density of water molecules as a function of the distance to the contact line, for NaCl concentrations of 0.5, 1.0 and 1.5 M. Compared with Cl^-, the distribution of Na⁺ ions is found to be closer to the contact lines.

As shown in Fig. 6(a), as the NaCl concentration increases, the average number of H-bonds shows a slight decrease on patterned GO surfaces, which is consistent with the slight decrease of the water-water interaction energy. However, the average total interaction energy per outermost water molecule still increases, due to the increase in NaCl concentration (Fig. 2a). As shown in Fig. 6(b), the τ_H-bond values show an obvious increase with the concentration of NaCl, which can be attributed to the clear increase in the average H-bond energy per outermost water molecule (the average H-bond energies per outermost water molecule can be found in the ESI^†). The influence of Na⁺ ions on the H-bonds lifetime of the outermost water molecules is more marked than that of Cl^- ions. The closer proximity of Na⁺ ions to the contact lines improves the stability of the H-bonds involving the outermost water molecules, which hinders the diffusion of water across the contact lines, and reduces the number of water molecules evaporating from the unoxidized regions.

Fig. 6Full-screen View

Hydrogen bonding characteristics of outermost water molecules on patterned GO surfaces with different NaCl concentrations. (a) Average number of H-bonds for different NaCl concentrations. The black solid and black hollow squares represent the average number of H-bonds per outermost water molecule under the influence of Na⁺ and Cl^- ions, respectively, on patterned GO surfaces with different NaCl concentrations. (b) Average lifetime of H-bonds for different NaCl concentrations. The black solid and black hollow squares represent the average lifetime of H-bonds per outermost water molecule under the influence of Na⁺ and Cl^- ions, respectively, on patterned GO surfaces with different NaCl concentrations.

As the oxidized regions of patterned GO can have different oxidation degrees [51], we further investigated the evaporation of water with 1.0 M NaCl concentration on patterned GO surfaces with different oxidation degrees. Fig. 7(a) shows that the evaporation rate does not change significantly when the C/(OH) ratio increases from 4 to 8. However, when C/(OH) reaches 10.7, the water evaporation rate shows a sharp drop. Fig. 7(b) reveals that, as C/(OH) reaches 10.7, water and ions spread onto the unoxidized region, and Fig. 7(c) highlights a corresponding drop in the water thickness. Fig. 7(d) shows that the number of H-bonds per outermost water molecule remains almost constant when the oxidation degree changes. The τ_H-bond values show an obvious increase for C/(OH) = 10.7, which is mainly due to the reduction in water thickness. These results are highly consistent with our previous work with no ions included in the system [19], which shows that when the oxidation degree varies in a certain range, the effect of the Na⁺ and Cl^- ions on the water evaporation is less significant.

Fig. 7Full-screen View

(Color online) Evaporation of 2256 water molecules on patterned GO surfaces with 1.0 M NaCl concentration under different oxidation degrees. (a) Evaporation rate on patterned GO with and without 1.0 M NaCl. The black and red squares represent the total evaporation rate with and without 1.0 M NaCl, respectively. The black and red solid circles represent the evaporation rate on oxidized regions (R_oxidized) with and without 1.0 M NaCl, respectively. The black and red hollow circles represent the evaporation rate on unoxidized regions (R_unoxidized) with and without 1.0 M NaCl, respectively. (b) Snapshot of water and ions on patterned GO at C/(OH) = 10.7. The black dashed rectangle outlines a water film formed on the unoxidized region. (c) Water thickness on patterned GO with 1.0 M NaCl under different oxidation degrees. (d) Hydrogen bonding characteristics of outermost water molecules in the oxidized regions of patterned GO with 1.0 M NaCl, under different oxidation degrees. The black squares and the red triangles represent the average number and lifetime of H-bonds, respectively, per outermost water molecule.

4 Conclusion

In conclusion, we carried out MD simulations to investigate the influence of Na⁺ and Cl^- ions on the evaporation of nanoscale water from graphene oxide surfaces. We found that the evaporation rate on both patterned and homogeneous GO surfaces decreases as the concentration of NaCl increases from 0 to 1.5 M, because the presence of the ions extends the H-bond lifetimes of the outermost water molecules. Compared with the homogeneous GO surfaces, the evaporation rate on patterned GO shows a higher decrease. Further analysis shows that the obvious decrease in evaporation rate on the unoxidized regions can be attributed to the increased amount of Na⁺ ions near the contact lines. The proximity of Na⁺ ions greatly extends the H-bonds lifetime of the outermost water molecules and reduces the number of water molecules diffusing from the oxidized to the unoxidized regions. Moreover, the effect of the Na⁺ and Cl^- ions on the water evaporation is less significant when the oxidation degree varies in a certain range. Our findings indicate that the effect of the Na⁺ and Cl^- ions on the evaporation of nanoscale water can be affected by the morphology of the solid surfaces; these results may find potential applications in achieving controllable evaporation of nanoscale liquids on a solid surface.