Hydrophobic Plates Research Articles

Dependence of the number of hydrogen bonds per water molecule on its distance to a hydrophobic surface and a thereupon-based model for hydrophobic attraction

A water molecule in the vicinity of a hydrophobic surface forms fewer hydrogen bonds than a bulk molecule because the surface restricts the space available for other water molecules necessary for its hydrogen-bonding. In this vicinity, the number of hydrogen bonds per water molecule depends on its distance to the surface. Considering the number of hydrogen bonds per bulk water molecule (available experimentally) as the only reference quantity, we propose an improved probabilistic approach to water hydrogen-bonding that allows one to obtain an analytic expression for this dependence. (The original version of this approach [Y. S. Djikaev and E. Ruckenstein, J. Chem. Phys. 130, 124713 (2009)] provides the number of hydrogen bonds per water molecule in the vicinity of a hydrophobic surface as an average over all possible locations and orientations of the molecule.) This function (the number of hydrogen bonds per water molecule versus its distance to a hydrophobic surface) can be used to develop analytic models for the effect of hydrogen-bonding on the hydration of hydrophobic particles and their solvent-mediated interaction. Presenting a model for the latter, we also examine the temperature effect on the solvent-mediated interaction of two parallel hydrophobic plates.

Orientational dynamics of water trapped between two nanoscopic hydrophobic solutes: A molecular dynamics simulation study

We investigate thoroughly the effect of confinement and solute topology on the orientational dynamics of water molecule in the interplate region between two nanoscopic hydrophobic paraffinlike plates. Results are obtained from molecular dynamics simulations of aqueous solutions of paraffinlike plates in the isothermal-isobaric ensemble. An analysis of survival time auto correlation function shows that the residence time of the water molecule in the confined region between two model nanoscopic hydrophobic plates depends on solute surface topology (intermolecular distance within the paraffinlike plate). As expected, the extent of confinement also changes the residence time of water molecules considerably. Orientational dynamics was analyzed along three different directions, viz., dipole moment, HH, and perpendicular to molecular plane vectors. It has been demonstrated that the rotational dynamics of the confined water does not follow the Debye rotational diffusion model, and surface topology of the solute plate and the extent of confinement have considerable effect on the rotational dynamics of the confined water molecules.

Role of electrostatics in modulating hydrophobic interactions and barriers to hydrophobic assembly.

Hydrophobic effects continue to be an active area of research due to implications for a wide range of physicochemical phenomena. Molecular dynamics simulations have been used extensively in the study of such effects using various water potential models, with few studies addressing the differences between models. In particular, studies considering the explicit treatment of water polarizability are underrepresented in the literature. We present results from molecular dynamics simulations that systematically compare the dependence of large-scale hydrophobic effects on the water model. We consider three common nonpolarizable models (SPC/E, TIP3P, and TIP4P) and two common polarizable models (TIP4P-FQ and SWM4-NDP). Results highlight the similarities and differences of the different water models in the vicinity of two large hydrophobic plates. In particular, profiles of average density, density fluctuations, orientation, and hydrogen bonding show only minor differences among the water models studied. However, the potential of mean force for the hydrophobe dimerization is significantly reduced in the polarizable water systems. TIP4P-FQ shows the deepest minimum of approximately -54(+/-3) kcal/mol compared to -40(+/-3), -40(+/-2), -42(+/-3), and -45(+/-5) kcal/mol for TIP4P, TIP3P, SPC/E, and SWM4-NDP (all relative to the dissociated state). We discuss the relationship between hydrophobic association and the strength of water-water interactions in the liquid phase. Results suggest that models treating polarizability (both implicitly and explicitly) influence a stronger driving force toward hydrophobic assembly. Implications of these results, as well as prospectives on future work, are discussed.

Liquid−Vapor Oscillations of Water Nanoconfined between Hydrophobic Disks: Thermodynamics and Kinetics

We use extensive molecular dynamics simulations with the monatomic model of water (mW) to characterize the thermodynamics and kinetics of the liquid-vapor (wetting-drying) equilibrium of water confined between nanoscopic hydrophobic plates. The transition in confined water is first-order-like, with two well-defined states (wet and dry) separated by a free energy barrier. Different from its bulk counterpart, the confined system oscillates between liquid and vapor: the two phases coexist in time but not in space. Also different from the phase behavior in bulk, there is a finite range of the thermodynamic variables (e.g., temperature or separation between the plates) for which the liquid and vapor state coexist in dynamical equilibrium. We determine the range of temperatures and plate separations for which reversible oscillations can be observed between a stable and metastable phase, compute the time scales of the phase transition along the equilibrium coexistence line, and investigate the pathway for drying along simple collective coordinates that describe the opening of a vapor bubble. The results of the simulations are compared with a simple capillary model for the thermodynamics and transition state theory for the kinetics of phase oscillations.

Competition of electrostatic and hydrophobic interactions between small hydrophobes and model enclosures.

The binding affinity between a probe hydrophobic particle and model hydrophobic plates with different charge (or dipole) densities in water was investigated through molecular dynamics simulation free-energy perturbation calculations. We observed a reduced binding affinity when the plates are charged, in agreement with previous findings. With increased charge density, the plates can change from "hydrophobic like" (pulling the particle into the interplate region) to "hydrophilic like" (ejecting the particle out of the interplate region), demonstrating the competition between hydrophobic and electrostatic interactions. The reduction of the binding affinity is quadratically dependent on the magnitude of the charge for symmetric systems, but linear and cubic terms also make a contribution for asymmetric systems. Statistical perturbation theory explains these results and shows when and why implicit solvent models fail.

Enhanced Production of Red Blood Cells in Suspension by Electrostatic Interactions with Culture Plates

Enucleation of erythroblasts, a critical step in the generation of red blood cells (RBCs), occurs at a low rate without cocultured stromal cells. Previously, the surface properties of the cell culture plate were not considered in the enucleation process, because the cells exist in suspension. Here, we show that a significantly higher rate of enucleation of erythroblasts occurred on the positively charged plates than on the negatively charged surfaces or the both negatively and positively charged plates. Also, the negatively and positively charged plate group showed a significantly higher enucleation than did the hydrophobic plates. Therefore, the plates fully coated with amine groups generated 1.88 times more enucleated RBCs than did the hydrophobic plates. This study suggests an important insight into the effect of surface characteristics of cell culture plates on suspension cell culture. Further, this simple and inexpensive procedure could contribute to a more efficient RBC production system, eliminating the need for robust and expensive coculture procedures.

Hydrophobic interaction of two large plates: An analysis of salting-in/salting-out effects

The decrease in water accessible surface area drives the association of two large hydrophobic plates in both water and aqueous salt solutions: the contact configuration of the two plates possesses the minimum Gibbs energy in all cases. High charge density ions, having strong electrostatic attractions with waters, render more costly the process of cavity creation, as determined by classic scaled particle theory calculations, and strengthen pairwise hydrophobic interaction. Low-charge density ions, being preferentially bound to plate surfaces due to dispersion interactions, have to be removed to arrive at the contact configuration and weaken pairwise hydrophobic interaction.

Long-Range Effects of Confinement on Water Structure

Molecular dynamics simulations were performed for water confined between two extended hydrophobic surfaces, which were represented by either planar surfaces or, more realistically, linear alkane monolayers, separated up to hundreds of angstroms. These simulations show that the confinement significantly affects the structure and density distribution of water, as well as the hydration of the confining surfaces. The results are largely independent of model surfaces used. At large separation distances (up to 800 A) between the confining hydrophobic surfaces, it was found that the water distribution is inhomogeneous: the water density is high near the surfaces and low in the middle of the water slab. Using a simple free energy density functional approach, these features seen from simulations were reproduced when a nonlocal free energy density term was introduced. The confinement was also found to have a noticeable effect on the hydrogen bond network of water. For water confined between two hydrophobic plates separated at distances in the range of 100-800 A, the tetrahedrality parameter q was found to be more liquid-like near the surface (distribution shifts toward random arrangements) and more ice-like in the middle (distribution shifts toward more perfectly tetrahedral bonding). These rather long-range effects are expected to vanish at even longer distances, which shall be simulated in the future with the state of the art computational resources. On the other hand, confinement was found to have little effects on water orientational relaxation dynamics.

Highly luminescent Langmuir–Blodgett films of amphiphilic Ir(iii) complexes for application in gas sensing

Four members of mixed ligand cyclometalated iridium(III) complexes, fac-[Ir(ppy)nL3−n] (ppy = 2-phenylpyridine; L = 2-(3-octadecyloxyphenyl)pyridine; n = 0–3), were prepared in a one-pot reaction between [Ir(ppy)2(O⁁O)] (O⁁O = acetylacetonato) and LH in glycerol at 170 °C. Each compound was purified and optically resolved to Δ- and Λ-enantiomers chromatographically on a chiral column. A film of fac-[Ir(ppy)nL3−n] floating at an air–water interface was transferred horizontally onto a hydrophobic glass plate as a Langmuir–Blodgett (LB) film. The emission from a single layered LB film was monitored under the atmosphere of various kinds of gases. As a result, molecules with functional groups such as –OH, CO and –CN were found to quench efficiently excited fac-[Ir(ppy)nL3−n] in a film. The effects of isotope and molecular chirality on energy transfer were also investigated, comparing the quenching ability between H2O and D2O or between R-2-butanol and S-2-butanol on a racemic or chiral film of fac-[Ir(ppy)2L], respectively.

Dimerization Thermodynamics of Large Hydrophobic Plates: A Scaled Particle Theory Study

The temperature dependence of the association thermodynamics of two large hydrophobic plates has been determined by means of molecular dynamics simulations ( Zangi , R. ; Berne , B. J. J. Phys. Chem. B 2008 , 112 , 8634 - 8644 ). This offers the opportunity to test the ability of an approach grounded on the basic concept that the reduction in the solvent-excluded volume plays a fundamental role and on a suitable application of scaled particle theory to reproduce such association thermodynamic functions over an extended temperature range. The results indicate that the Gibbs energy change is always largely negative and little dependent on temperature. The enthalpy change is positive at 0 degrees C, small negative at 25 degrees C, and largely negative at 100 degrees C. The entropy change is largely positive at 0 degrees C, passes through zero at about 85 degrees C, and is negative at 100 degrees C. The strong temperature dependence of the enthalpy and entropy changes, due to a large negative heat capacity change, is almost exactly compensating, so that the Gibbs energy change is little affected. The most part of the present results proves to be in more than qualitative agreement with those obtained by means of molecular dynamics simulations.

Molecular dynamics investigation of hydration of nanoscopic hydrophobic paraffin-like plates

The effect of surface characteristics on the hydration behavior of various paraffin-like plates has been investigated. Structure and orientation characteristics of the water molecules in the solvation shells of various nanoscopic paraffin-like plates differing from each other in the intermolecular spacing have been extensively studied using molecular dynamics simulation in isothermal-isobaric ensemble. Single particle density distribution of water molecules around the plate reveals well defined solvation shells around each of the paraffin-like plates studied here. A sharp first peak in the density profile in each of the plates signifies no visible dewetting around the paraffin plate. Instantaneous density of water molecules around the plate also reveals that the plate is sufficiently hydrated and there is no intermittent fluctuation in water density in the first hydration shell leading to short lived dewetted state for any of the model plates within the two nanosecond time span. This is in contrast to the hydration behavior of the intersolute region, where intersolute dewetting has been observed for some of the model plates. Thus the present results demonstrate that dewetting in the intersolute region of nanoscopic hydrophobic plates does not stem from drying interface of the individual solute. No significant effect of surface topology on the orientational structure of water molecules as revealed through distributions of dipole moment as well as oxygen-hydrogen bond vectors of a water molecule in different solvation shells has been observed.

Thermodynamics of supercooled water in solutions

Molecular dynamics simulations are performed on bulk TIP4P water and on a low concentration aqueous sodium chloride solution. The thermodynamic properties of the systems are studied by analysing the isotherms and the isochores planes. The comparison of the thermodynamics planes of the two systems reveals that modifications beyond a trivial shift occur in the solution. Moreover the limit of mechanical stability and the temperatures of maximum density line can be calculated from the analysis of the thermodynamic planes. While the limit of mechanical stability results unaffected by the presence of the ions, the temperatures of maximum density line is both mildly shifted in temperature and modified in shape in the solution, with respect to bulk water. Signatures of the presence of liquid-liquid coexistence are found in the aqueous solution. The results are also compared to water in a hydrophobic environment and to water confined between hydrophobic plates.

Effect of Temperature on the Structure and Phase Behavior of Water Confined by Hydrophobic, Hydrophilic, and Heterogeneous Surfaces

We perform molecular dynamics simulations of water confined between atomically detailed hydrophobic, hydrophilic, and heterogeneous (patchy) nanoscale plates. We study the effects of temperature 220 <or= T <or= 300 K on confined water's behavior at various pressures -0.2 <or= P <or= 0.2 GPa and plate separations 0.5 <or= d <or= 1.6 nm. Combining this with our earlier results on the same system [Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2006, 73, 041604; Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G. J. Phys. Chem. C, 2007, 11, 1323], where pressure was varied at constant temperature, allows us to compare water's behavior in nanoscale confinement, upon isobaric cooling and isothermal compression, corresponding to paths of interest in protein denaturation. At a fixed temperature, water confined between hydrophobic plates can form vapor, liquid, or crystal (bilayer ice) phases, depending on the values of P and d. The P-d phase diagrams at T = 300 K and T = 220 K show that cooling suppresses the vapor phase and stabilizes the liquid and crystal phases. The critical separation d(c)(P), below which vapor forms, shifts to lower values of d and P upon cooling. The density profiles show that, upon cooling, water approaches the hydrophobic plates. Hence, the effective hydrophobicity of the plate decreases as T decreases, consistent with the suppression of the vapor phase upon cooling. However, both the orientation of water's molecules at the interface and the water contact angle on the hydrophobic surface show practically no temperature dependence. Simulations of water confined by heterogeneous plates decorated with hydrophobic and hydrophilic patches reveal that cooling leads to appreciable blurring of the differences between water densities at hydrophobic and hydrophilic surfaces. This observation, together with remarkable similarities in confined water's response to isobaric cooling and to isothermal compression, suggests that the invasion of hydrophobic cavities by water is an important mechanism underlying both pressure and cold denaturation of proteins.

Efficient immobilization of a ligand antibody with high antigen-binding activity by use of a polystyrene-binding peptide and an intelligent microtiter plate

A method for immobilization of ligand antibody to improve the efficiency and sensitivity of a sandwich enzyme-linked immunosorbent assay (ELISA) was investigated by the use of anti-TNF-α monoclonal antibody chemically conjugated with a polystyrene-binding peptide (PS-tag) and an intelligent microtiter plate with large surface area. We compared both adsorption and antigen-binding activity of the ligand antibody (mAb) and mAb with the PS-tag (mAb-PS-tag) on 3 different PS plates: a hydrophobic PS plate (PS-F-1 plate), a hydrophilic PS plate (PS-A plate), and an intelligent microtiter plate packed with PS beads (PS-E plate). Contact areas of the PS-E plate toward ligand antibody solutions were 7-fold larger than those of conventional PS-F-1 and PS-A plates and consequently, both mAb and mAb-PS-tag were efficiently immobilized on the surface of the PS-E plate due to the significantly enhanced surface area. In particular, when the non-ionic surfactant, Tween20, was present during adsorption of the ligand antibodies, the mAb-PS-tag was site-specifically immobilized on the surface of the PS-E plates and exhibited the highest specific antigen-binding activity. High specific antigen-binding activities were preserved, even though the density of the mAb-PS-tag immobilized on the PS-E plate was reduced with application of smaller volumes. Consequently, the decline in the signal intensities detected from the sandwich ELISA on the PS-E plate was negligible, even when one-fourth the original volume of mAb-PS-tag solution was used to coat the plate. The ligand antibody was immobilized with retention of high antigen-binding activity due to the large surface area of the PS-E plate, specific recognition of the PS-tag, and minimal non-specific interaction of the ligand antibody due to use of Tween 20. Thus, the immobilization method developed in this study resulted in an efficient and highly sensitive sandwich ELISA system that requires smaller amounts of ligand antibodies. This method may be useful for fabrication of protein-based biochips, such as antibody chips.

고분자 전해질 연료전지 유로의 수분배출 특성의 실험 및 해석적 평가

Polymer electrolyte membrane (PEM) fuel cells are a promising technology for short-term power generation required in residential and automobile applications. Proper management of water has been found to be essential for improving the performance and durability of PEM fuel cells. This study investigated the liquid water exhaust capabilities of various flow channels having different geometries and surface properties. Three-pass serpentine flow fields were prepared by patterning channels of 1 mm or 2 mm width onto hydrophilic Acrylic plates or hydrophobic Teflon plates, and the behaviors of liquid water in those flow channels were experimentally visualized. Computational fluid dynamics (CFD) simulations were also conducted to quantitatively assess the liquid water exhaust capabilities of flow channels for PEM fuel cells. Numerical results showed that hydrophobic flow channels have better liquid water exhaust capabilities than hydrophilic flow channels. Flow channels with curved corners showed less droplet stagnation than the channels with sharp corners. It was also found that a smaller width is desirable for hydrophobic flow channels while a larger width is desirable for hydrophilic ones. The above results were explained as being due to the different droplet morphologies in hydrophobic and hydrophilic channels.

Urea’s Action on Hydrophobic Interactions

For more than a century, urea has been commonly used as an agent for denaturing proteins. However, the mechanism behind its denaturing power is still not well understood. Here we show by molecular dynamics simulations that a 7 M aqueous urea solution unfolds a chain of purely hydrophobic groups which otherwise adopts a compact structure in pure water. The unfolding process arises due to a weakening of hydrophobic interactions between the polymer groups. We also show that the attraction between two model hydrophobic plates, and graphene sheets, is reduced when urea is added to the solution. The action of urea is found to be direct, through its preferential binding to the polymer or plates. It is, therefore, acting like a surfactant capable of forming hydrogen bonds with the solvent. The preferential binding and the consequent weakened hydrophobic interactions are driven by enthalpy and are related to the difference in the strength of the attractive dispersion interactions of urea and water with the polymer chain or plate. This relation scales with square root(epsilon(b)), where epsilon(b) is the Lennard Jones (LJ) energy parameter for each group on the chain. Larger values of epsilon(b) increase the preferential binding and result in a larger decrease of the hydrophobic interactions, with a crossover at very weak dispersions. We also show that the indirect mechanism, in which urea acts as a chaotrope, is not a likely cause of urea's action as a denaturant. These findings suggest that, in denaturing proteins, urea (and perhaps other denaturants) forms stronger attractive dispersion interactions with the protein side chains and backbone than does water and, therefore, is able to dissolve the core hydrophobic region.

A Catalytic Role of Heparin within the Extracellular Matrix

We investigated the mechanism by which heparin enhances the binding of vascular endothelial growth factor (VEGF) to the extracellular matrix protein fibronectin. In contrast to other systems, where heparin acts as a protein scaffold, we found that heparin functions catalytically to modulate VEGF binding site availability on fibronectin. By measuring the binding of VEGF and heparin to surface-immobilized fibronectin, we show that substoichiometric amounts of heparin exposed cryptic VEGF binding sites within fibronectin that remain available after heparin removal. Measurement of association and dissociation kinetics for heparin binding to fibronectin indicated that the interaction is rapid and transient. We localized the heparin-responsive element to the C-terminal 40-kDa Hep2 domain of fibronectin. A mathematical model of this catalytic process was constructed that supports a mechanism whereby the heparin-induced conformational change in fibronectin is accompanied by release of heparin. Experiments with endothelial extracellular matrix suggest that this process may also occur within biological matrices. These results indicate a novel mechanism whereby heparin catalyzes the conversion of fibronectin to an open conformation by transiently interacting with fibronectin and progressively hopping from molecule to molecule. Catalytic activation of the extracellular matrix might be an important mechanism for heparin to regulate function during normal and disease states.

Chemical Denaturants Inhibit the Onset of Dewetting

The mechanism by which the aqueous cosolvents guanidinium chloride and urea denature proteins is a matter of controversy. Here, we use all-atom molecular dynamics simulations to study the effect of both denaturants on the dewetting of water confined between nanoseparated hydrophobic plates. It is found that the denaturants inhibit the onset of dewetting, so that it occurs at shorter interplate distances than in pure water. Our results support a role for urea and guanidinium in assisting in the solvation of nonpolar surfaces, thereby weakening hydrophobic effects known to be important for protein stability.

Pseudo 3-D simulation of a falling film microreactor based on realistic channel and film profiles

A falling film microreactor has demonstrated in the past high potential for sustainable chemical processes, e.g. by better use of resources (selectivity), enabling direct routes (saving of waste), or smaller reactor footprint (space-time yield). Due to the extremely high liquid based specific area (up to 20 , 000 m 2 / m 3 ) it is especially equipped to carry out fast exothermic and mass transfer limited reactions. However, to maximize the process intensification in the falling film microreactor there is a need to characterize and investigate the design parameters of the reactor. In general, the major rate limiting steps occur on the liquid side. Therefore a realistic description of the liquid film is needed which requires the use of a 3-D reactor model. In the current study we present a so-called pseudo 3-D computational fluid dynamic (CFD) model. Based on the realistic channel geometry profiles we compute liquid menisci, flow velocities, species transport, and reactions. The reactor model was developed and validated experimentally by the absorption of CO 2 in NaOH aqueous solution. This 3-D model allows investigating the effects of channel fabrication precision, liquid flow distribution, gas chamber height, and hydrophilic–hydrophobic plate material. Result shows that fabrication imprecisions of the investigated microchannels by 11% in channel width and 6% in channel depth has only a 2% impact on the reaction conversion. Moreover we show that a liquid flow mal distribution, in the parallel microchannels assembled on plate, with a relative standard deviation of 0.37 lowers the reaction conversion by about 2%. A reduction of gas chamber height slightly improves the conversion and gas phase mass transfer limitation can be overcome. Moreover the material of the reactor plate has to provide sufficient wetability for the liquid falling film.

Water structure near single and multi-layer nanoscopic hydrophobic plates of varying separation and interaction potentials

We have performed a series of molecular dynamics simulations of water containing two nanoscopic hydrophobic plates to investigate the modifications of the density and hydrogen bond distributions of water in the vicinity of the surfaces. Our primary goal is to look at the effects of plate thickness, solute-solvent interaction and also interplate separation on the solvent structure in the confined region between two graphite-like plates and also near the outer surfaces of the plates. The thickness of the plates is varied by considering single and triple-layer graphite plates and the interaction potential is varied by tuning the attractive strength of the 12-6 pair interaction potential between a carbon atom of the graphite plates and a water molecule. The calculations are done for four different values of the tuning parameter ranging from fully Lennard-Jones to pure repulsive pair interactions. It is found that both the solvation characteristics and hydrogen bond distributions can depend rather strongly on the strength of the attractive part of the solute-water interaction potential. The thickness of the plates, however, is found to have only minor effects on the density profiles and hydrogen bond network. This indicates that the long range electrostatic interactions between water molecules on the two opposite sides of the same plate do not make any significant contribution to the overall solvation structure of these hydrophobic plates. The solvation characteristics are primarily determined by the balance between the loss of energy due to hydrogen bond network disruption, cavity repulsion potential and offset of the same by attractive component of the solute-water interactions. Our studies with different system sizes show that the essential features of solvation properties, e.g. wetting and dewetting characteristics for different interplate separations and interaction potentials, are also present in relatively smaller systems consisting of a few hundred atoms.