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(a)-(c) Mean-square displacement of the water molecules in various layers that are in parallel of the surface for μ = 11.5, 13.6, and 16.3 D, respectively. (d) In-plane self-diffusion coefficient (D xy ) of water molecules in various layers.
Source publication
Phase transitions of water molecules are commonly expected to occur only under extreme conditions, such as nanoconfinement, high pressure, or low temperature. We herein report the disordered-ordered phase transition of two-dimensional interfacial water molecules under ambient conditions using molecular-dynamics simulations. This phase transition is...
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Context 1
... further characterize the phase transitions of the water structures, we calculate the probability distribution of the water dipole orientation angle ϕ, which is the angle between the projection of a water molecule dipole orientation onto the xy plane and a crystallographic direction (see three typical examples in Fig. S4 in SM) [42,43]. Accordingly, we calculated the fluctuations of the probability (denoted as S) of the water monolayer dipole orientation angle ϕto measure the degree of ordered ...
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... 4 Å < z layer2 7.5 Å as the second layer, 7.5 Å < z layer3 11 Å as the third layer, and 11 Å < z layer4 15 Å as the fourth layer. Three typical systems with various dipole moments dipole moment μ = 11.5, 13.6, and 16.3 D that correspond to the dipole length l = 0.120, 0.142, and 0.170 nm, respectively, are chosen and the results are shown in Figs. 4(a)-4(c). One can observe that there is a clear linear relationship between the water mean-square displacement and the time t. As the water molecule approaches the solid surfaces, the MSD of water molecules becomes smaller. Accordingly, we can obtain the self-diffusion coefficient D xy of water molecule by measuring MSD of water molecules for ...
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... the water mean-square displacement and the time t. As the water molecule approaches the solid surfaces, the MSD of water molecules becomes smaller. Accordingly, we can obtain the self-diffusion coefficient D xy of water molecule by measuring MSD of water molecules for various water layers for systems with different solid surfaces. As shown in Fig. 4(d), the self-diffusion coefficient gradually increases as the water molecule is far away from the solid surface. Clearly, the diffusion constant of the ordered water phase is less than one half of the other two disordered water phases. These two self-diffusion values are much smaller than the bulk water self-diffusion value of 2.95 × 10 ...
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Citations
... [41] So far, the molecular understanding and the physical insights into this system with hydroxylated silica and water are still illusive. [42,43] In addition, water molecules exhibit versatile ordered structures under ambient conditions dependent on the lattice constant, which may impact the surface wetting behavior [44][45][46][47][48][49][50] and other properties. [51][52][53][54][55] So far as we know, the relationship between the lattice strain of the hydroxylated silica surface and the interfacial water structure is not fully understood. ...
Using molecular dynamics simulations, we have revealed a novel wetting phenomenon with a droplet on composite structures formed by embedded water into (111) surface of β-cristobalite hydroxylated silica. This can be attributed to the formation of a composite structure composed of embedded water molecules and the surface hydroxyl (-OH) groups, which reduces the number of hydrogen bonds between the composite structure and the water droplet above the composite structure. Interestingly, a small uniform strain (±3%) applied to the crystal lattice of the hydroxylated silica surface can result in a notable change of the contact angles (>40◦) on the surface. The finding provides new insights into the correlation between the molecular-scale interfacial water structures and the macroscopic wettability of the hydroxylated silica surface.
... Generally, the microscopic friction is usually determined by measuring the contact angle [24][25][26][27][28][29][30][31][32][33][34] ; a large contact angle indicating a hydrophobic surface is associated with low surface friction, and vice versa 35 . However, the contact angle as a macroscopic surface wetting property is not always consistent with the microscopic viewpoint, even for nano-scale wetting behaviors themselves on polar surfaces [32][33][34]36 . This complicates the relationship between surface friction and surface hydrophobicity/ hydrophilicity. ...
Generally, the interface friction on solid surfaces is regarded as consistent with wetting behaviors, characterized by the contact angles. Here using molecular dynamics simulations, we find that even a small charge difference (≤0.36 e) causes a change in the friction coefficient of over an order of magnitude on two-dimensional material and lipid surfaces, despite similar contact angles. This large difference is confirmed by experimentally measuring interfacial friction of graphite and MoS2 contacting on water, using atomic force microscopy. The large variation in the friction coefficient is attributed to the different fluctuations of localized potential energy under inhomogeneous charge distribution. Our results help to understand the dynamics of two-dimensional materials and biomolecules, generally formed by atoms with small charge, including nanomaterials, such as nitrogen-doped graphene, hydrogen-terminated graphene, or MoS2, and molecular transport through cell membranes.
The nature of the hydrophobicity found in rare‐earth oxides is intriguing. The CeO 2 (100) surface, despite its strongly hydrophilic nature, exhibits hydrophobic behaviour when immersed in water. In order to understand this puzzling and counter‐intuitive effect we performed a detailed analysis of the water structure and dynamics. We report here an ab‐initio molecular dynamics simulation (AIMD) study which demonstrates that the first water layer, in immediate contact with the hydroxylated CeO 2 surface, is responsible for the effect behaving as a hydrophobic interface with respect to the rest of the liquid water. The hydrophobicity is manifested in several ways: a considerable diffusion enhancement of the confined liquid water as compared with bulk water at the same thermodynamic condition, a weak adhesion energy and few H‐bonds above the hydrophobic water layer, which may also sustain a water droplet. These findings introduce a new concept in water/rare‐earth oxide interfaces: hydrophobicity mediated by specific water patterns on a hydrophilic surface.
The nature of the hydrophobicity found in rare‐earth oxides is intriguing. The CeO2 (100) surface, despite its strongly hydrophilic nature, exhibits hydrophobic behaviour when immersed in water. In order to understand this puzzling and counter‐intuitive effect we performed a detailed analysis of the confined water structure and dynamics. We report here an ab‐initio molecular dynamics simulation (AIMD) study which demonstrates that the first adsorbed water layer, in immediate contact with the hydroxylated CeO2 surface, generates a hydrophobic interface with respect to the rest of the liquid water. The hydrophobicity is manifested in several ways: a considerable diffusion enhancement of the confined liquid water as compared with bulk water at the same thermodynamic condition, a weak adhesion energy and few H‐bonds above the hydrophobic water layer, which may also sustain a water droplet. These findings introduce a new concept in water/rare‐earth oxide interfaces: hydrophobicity mediated by specific water patterns on a hydrophilic surface.
Hypothesis:
Numerous hydrocarbon and fluorine-based hydrophobic surfaces have been widely applied in various engineering and bioengineering fields. It is hypothesized that the hydrophobic interactions of hydrocarbon and fluorinated surfaces in aqueous media would show some differences.
Experiments:
The hydrophobic interactions of hydrocarbon and fluorinated surfaces with air bubbles in aqueous solutions have been systematically and quantitatively measured using a bubble probe atomic force microscopy (AFM) technique. Ethanol was introduced to water for modulating the solution polarity. The experimental force profiles were analyzed using a theoretical model combining the Reynolds lubrication theory and augmented Young-Laplace equation by including disjoining pressure arisen from the Derjarguin-Landau-Verwey-Overbeek (DLVO) and non-DLVO interactions (i.e., hydrophobic interactions).
Findings:
The experiment results show that the hydrophobic interactions were firstly weakened and then strengthened by increasing ethanol content in the aqueous media, mainly due to the variation in interfacial hydrogen bonding network. The fluorinated surface exhibited less sensitivity to ethanol than hydrocarbon surface, which is attributed to the presence of ordered interfacial water layer. Our work reveals the different hydrophobic effects of hydrocarbon and fluorinated surfaces, with useful implications on modulating the interfacial interactions of relevant materials in various engineering and bioengineering applications.
Interfacial water has been estimated to mediate the thermal coupling at the interface between biological tissues and graphene/graphene oxide (GO)-based bio-nano devices, while the interfacial energy transfer is limited by the extreme thermal resistance between graphene and water due to the inherent vibration mismatch and the weak interaction. Oxygen-containing functional groups on the surface of GO form hydrogen bonds (H-bonds) with water, which enhances interfacial interaction and promotes thermal transport at the interface, thereby GO/water model is used to investigate the effects of H-bonds on the thermal boundary conductance (TBC). The results reveal that both the density and distribution of hydroxyl groups affect the interfacial H-bonds and further affect the thermal transport at interface. TBC increases initially with the increased H-bond density and then reaches a plateau when H-bond density reaches saturation. The homogeneously distributed hydroxyl groups form more H-bonds with water molecules than the clustered pattern, and results in more efficient interfacial thermal transfer. The variation of TBC with oxidation concentration can be explained by the mass density depletion length and the density of interfacial H-bonds. Our study highlighted the key role of H-bonds in regulating interfacial thermal transfer and provides theoretical basis and guiding methodology for thermal dissipation of graphene and GO-based bioelectronic devices.
Despite considerable effort, the dielectric constant of interfacial water at solid surfaces is still not fully understood, thus hindering our understanding of the ubiquitous physical interactions in many materials and biological surfaces. In this study, we used molecular dynamics simulations to show that the parallel dielectric constant at the solid/water interface depends on solid-water interactions as well as the interfacial water structure on various solid crystal faces. In particular, ordered water structures can lead to a significant reduction (∼44%) in the parallel dielectric constant at the solid/water interface compared with that of bulk water. This sharp decrease in the parallel dielectric constant can be attributed to the specific antiferroelectric ordered structure of interfacial water molecules, which significantly suppresses the amplitude of the dipolar fluctuation associated with both the number of hydrogen bonds and the degree of order of interfacial water.
