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Molecular and collective reorientations in interfacial water are by-and-large decelerated near surfaces subjected to outgoing electric fields (pointing from surface to liquid, i.e., when the surface carries positive charge). In incoming fields at negatively charged surfaces, these rates show a nonmonotonic dependence on field strength where fastest reorientations are observed when the field alignment barely offsets the polarizing effects due to interfacial hydrogen bonding. This extremum coincides with a peak of local static permittivity. We use molecular dynamics simulations to explore the impact of background static field on high frequency AC permittivity in hydration water under an electric field mimicking the conditions inside a capacitor where one of the confinement walls is subject to an outgoing field and the other one to an incoming field. At strong static fields, the absorption peak undergoes a monotonic blue shift upon increasing field strength in both hydration layers. At intermediate fields, however, the hydration region at the wall under an incoming field (the negative capacitor plate) features a red shift coinciding with maximal static-permittivity and reorientation-rate. The shift is mostly determined by the variation of the inverse static dielectric constant as proposed for mono-exponentially decaying polarization correlations. Conversely, hydration water at the opposite (positively charged) surface features a monotonic blue shift consistent with conventional saturation. The sensitivity of absorption peaks on the field suggests that surface charge densities could be deduced from sub-THz dielectric spectroscopy experiments in porous materials when interfaces accommodate a major fraction of water contained in the system. 

Kinetics of molecular and collective reorientations at solid/water interfaces are known to depend on local electric field. Deceleration is observed near surfaces subjected to an outgoing static field (pointing from surface to liquid) as is the case when the solid carries positive charge. In an incoming field, both the reorientation rates and local permittivity in hydration layer show a nonmonotonic dependence on field strength with fastest reorientations and highest permittivity observed when the field alignment barely offsets the orienting bias at the wall. (Mulpuri and Bratko, J. Chem. Phys. 158,134716, 2023). Here, we use Molecular Dynamics simulations to explore the impact of background field (or, equivalently, surface charge density) on high frequency (GHz to THz) AC permittivity in hydration water inside a nanosized aqueous film under perpendicular DC field. Our model system mimics conditions inside a capacitor where one of the confinement walls is subject to outgoing and the other one to incoming field. In very strong static fields, the frequency corresponding to the maximal imaginary part of AC permittivity, features a blue shift with increasing field strength in both hydration layers. At intermediate fields, however, the hydration region at the wall under ingoing field (adjacent to the negative capacitor plate) features a red shift, which is especially pronounced at the field strength corresponding to the maxima of static-permittivity and reorientation-rate. The shift reflects the variation of the inverse static dielectric constant in normal direction, (Gekle and Netz, J. Chem. Phys. 137, 104704, 2012) with marginal effect of librational motions on the local AC permittivity. Hydration water at the opposite surface (closer to the positive capacitor plate), on the other hand, features a monotonic blue shift consistent with conventional saturation. The sensitivity of imaginary peaks on the field suggests surface charge densities could be deduced from THz dielectric spectroscopy experiments in a porous material where hydration layers comprise a major fraction of water contained in the system. 

Forcible wetting of hydrophobic pores represents a viable method for energy storage in the form of interfacial energy. The energy used to fill the pores can be recovered as pressure–volume work upon decompression. For efficient recovery, the expulsion pressure should not be significantly lower than the pressure required for infiltration. Hysteresis of the wetting/drying cycle associated with the kinetic barrier to liquid expulsion results in energy dissipation and reduced storage efficiency. In the present work, we use open ensemble (Grand Canonical) Monte Carlo simulations to study the improvement of energy recovery with decreasing diameters of planar pores. Near-complete reversibility is achieved at pore widths barely accommodating a monolayer of the liquid, thus minimizing the area of the liquid/gas interface during the cavitation process. At the same time, these conditions lead to a steep increase in the infiltration pressure required to overcome steric wall/water repulsion in a tight confinement and a considerable reduction in the translational entropy of confined molecules. In principle, similar effects can be expected when increasing the size of the liquid particles without altering the absorbent porosity. While the latter approach is easier to follow in laboratory work, we discuss the advantages of reducing the pore diameter, which reduces the cycling hysteresis while simultaneously improving the stored-energy density in the material. 

Molecular dynamics simulations are used to study nonlinear dielectric responses of a confined aqueous film in a planar nanopore under perpendicular electric fields at varied voltages between confining graphene sheets. Dielectric saturation reminiscent of the bulk phase behavior is prevalent at very strong fields, whereas we observe a nonmonotonic permittivity dependence on electric field at intermediate strengths where field-alignment and spontaneous polarization of interfacial water are of comparable magnitudes. The coupling between the two effects results in distinct dielectric responses at opposite confinement walls. The normal component of both the differential dielectric constant and dielectric difference constant tensors averaged over the region closer to the wall under incoming electric field (field pointing from the liquid to the solid phase) initially increases with the strength of the imposed field. The differential permittivity peaks at a field strength previously shown to offset the surface-induced orientation bias of hydration molecules at this wall. Further strengthening of the field results in conventional saturation behavior. At the opposite wall (subject to outgoing field) and in the central region of water slab, the nonlinear dielectric response resembles bulklike saturation. The conditions at the permittivity extremum coincide with the window of accelerated reorientation rates of interfacial water molecules under incoming field uncovered in earlier molecular dynamics analyses. 

Correction for ‘Reversible electrowetting transitions on superhydrophobic surfaces’ by D. Vanzo et al. , Phys. Chem. Chem. Phys. , 2021, 23 , 27005–27013, DOI:10.1039/D1CP04220C. 

An electric field applied across the interface has been shown to enable transitions from the Cassie to the Wenzel state on superhydrophobic surfaces with miniature corrugations. Molecular dynamics (MD) simulations manifest the possibility of reversible cycling between the two states when narrow surface wells support spontaneous expulsion of water in the absence of the field. With approximately 1 nm sized wells between the surface asperities, the response times to changes in the electric field are of O(0.1) ns, allowing up to GHz frequency of the cycle. Because of the orientation preferences of interfacial water in contact with the solid, the phenomenon depends on the polarity of the field normal to the interface. The threshold field strength for the Cassie-to-Wenzel transition is significantly lower for the field pointing from the aqueous phase to the surface; however, once in the Wenzel state, the opposite field direction secures tighter filling of the wells. Considerable hysteresis revealed by the delayed water retraction at decreasing field strength indicates the presence of moderate kinetic barriers to expulsion. Known to scale approximately with the square of the length scale of the corrugations, these barriers preclude the use of increased corrugation sizes while the reduction of the well diameter necessitates stronger electric fields. Field-controlled Cassie-to-Wenzel transitions are therefore optimized by using superhydrophobic surfaces with nanosized corrugations. Abrupt changes indicate a high degree of cooperativity reflecting the correlations between the wetting states of interconnected wells on the textured surface. 

Permeation of water in a poorly wettable material results in a conversion of pressure/volume work into surface free energy representing a novel form of energy storage. The addition of salt increases the amount of stored energy and can reduce the hysteresis of the infiltration−expulsion cycle. Our molecular simulations provide a theoretical perspective into the mechanisms involved in the process and underlying structures and interactions in compressed nanoconfined solutions. We consider aqueous NaCl in nanosized confinements at pressures of up to 3 kbar. Open ensemble Monte Carlo simulations utilizing fractional exchanges of molecules for efficient addition−removal of ions have been utilized in conjunction with pressure-dependent chemical potentials to model bulk phases under pressure. Confinements open to these pressurized bulk, aqueous electrolyte phases show reversibility at narrow pore sizes and irreversibility in wider ones, consistent with experiment. The addition of salt increases in the solid−liquid interfacial tension in narrower pores and associated infiltration and expulsion pressures. These changes are consistent with strong desalination effects at the lower pore size observed irrespective of external pressure and initial concentration. 

Wetting experiments show pure graphene to be weakly hydrophilic, but its contact angle (CA) also reflects the character of the supporting material. Measurements and Molecular Dynamics simulations on suspended and supported graphene often reveal a CA reduction due to the presence of the supporting substrate. A similar reduction is consistently observed when graphene is wetted from both sides. The effect has been attributed to transparency to molecular interactions across the graphene sheet, however, the possibility of substrate-induced graphene polarization has also been considered. Computer simulations of CA on graphene have so far been determined by ignoring the material’s conducting properties. We improve the graphene model by incorporating its conductivity according to the Constant Applied Potential Molecular Dynamics. Using this method, we compare the wettabilities of suspended graphene and graphene supported by water by measuring the CA of cylindrical water drops on the sheets. The inclusion of graphene conductivity and concomitant polarization effects lead to a lower CA on suspended graphene but the CA reduction is significantly bigger when the sheets are also wetted from the opposite side. The stronger adhesion is accompanied by a profound change in the correlations among water molecules across the sheet. While partial charges on water molecules interacting across an insulator sheet attract charges of the opposite sign, apparent attraction among like charges is manifested across the conducting graphene. The change is associated with graphene polarization, as the image charges inside the conductor attract equally signed partial charges of water molecules on both sides of the sheet. Additionally, by using a non-polar liquid (diiodomethane), we affirm a detectable wetting translucency when liquid-liquid forces are dominated by dispersive interactions. Our findings are important for predictive modeling toward a variety of applications including sensors, fuel cell membranes, water filtration, and graphene-based electrode materials in high-performance supercapacitors.