Hydrophobic Interactions and Dewetting between Plates with Hydrophobic and Hydrophilic Domains

We study by molecular dynamics simulations the wetting/dewetting transition and the dependence of the free energy on distance between plates that contain both hydrophobic and hydrophilic particles. We show that dewetting and strength of hydrophobic interaction is very sensitive to the distribution of hydrophobic and hydrophilic domains. In particular, we find that plates characterized by a large domain of hydrophobic sites induce a dewetting transition and an attractive solvent-induced interaction. On the other hand, a homogeneous distribution of the hydrophobic and hydrophilic particles on the plates prevents the dewetting transition and produces a repulsive solvent-induced interaction. We also present results for a kind of Janus interface in which one plate consists of hydrophobic particles and the other of hydrophilic particles showing that the inter-plate gap remains wet until steric constraints at small separations eject the water molecules. Our results indicate that the Cassie equation, for the contact angle of a heterogeneous plate, can not be used to predict the critical distance of dewetting. These results indicate that hydrophobic interactions between nanoscale surfaces with strong large length-scale hydrophobicity can be highly cooperative and thus they argue against additivity of the hydrophobic interactions between different surface domains in these cases. These findings are pertinent to certain protein-protein interactions where additivity is commonly assumed.Comment: 28 pages, 6 figure

Aggregation and Dispersion of Small Hydrophobic Particles in Aqueous Electrolyte Solutions

The effect of salts on the solvent-induced interactions between hydrophobic particles dispersed in explicit aqueous solution is investigated as a function of the salt’s ionic charge density by molecular dynamics simulations. We demonstrate that aggregates of the hydrophobic particles can be formed or dissolved in response to changes in the charge density of the ions. Ions with high charge density increase the propensity of the hydrophobic particles to aggregate. This corresponds to stronger hydrophobic interactions and a decrease in the solubility (salting-out) of the hydrophobic particles. Ions with low charge density can either increase or decrease the propensity for aggregation depending on whether the concentration of the salt is low or high, respectively. At low concentrations of low charge density ions, the aggregate forms a “micelle-like ” structure in which the ions are preferentially adsorbed at the surface of the aggregate. These “micelle-like ” structures can be soluble in water so that the electrolyte can both increase the solubility and increase aggregation at the same time. We also find, that at the concentration of the hydrophobic particles studied (0.75 m), the aggregation process resembles a first-order transition in finite systems. I

Strings-to-rings transition and antiparallel dipole alignment in two-dimensional methanols

Structural order emerging in the liquid state necessitates a critical degree of anisotropy of the molecules. For example, liquid crystals and Langmuir monolayers require rod- or disc-shaped and long-chain amphiphilic molecules, respectively, to break the isotropic symmetry of liquids. In this Letter we present results from molecular dynamics simulations demonstrating that in two-dimensional liquids, a significantly smaller degree of anisotropy is sufficient to allow structural organization. In fact, the condensed phase of the smallest amphiphilic molecule, methanol, confined between two, or adsorbed on, graphene sheets forms a monolayer characterized by long chains of molecules. Intrachain interactions are dominated by hydrogen bonds, whereas interchain interactions are dispersive. Upon a decrease in density toward a gaslike state, these strings are transformed into rings. The two-dimensional liquid phase of methanol undergoes another transition upon cooling; in this case, the order–disorder transition is characterized by a low-temperature phase in which the hydrogen bond dipoles of neighboring strings adopt an antiparallel orientation

Bilayer ice and alternate liquid phases of confined water

We report results from molecular dynamics simulations of the freezing and melting, at ambient temperature (T=300 K), of a bilayer of liquid water induced by either changing the distance between two confining parallel walls at constant lateral pressure or by lateral compression at constant plate separation. Both transitions are found to be first order. The system studied consisted of 1200 water molecules that were described by the TIP5P model. The in-plane symmetry of the oxygen atoms in the ice bilayer was found to be rhombic with a distorted in-registry arrangement. Above and below the stability region of the ice bilayer we observed two bilayer phases of liquid water that differ in the local ordering at the level of the second shell of nearest neighbors and in the density profile normal to the plane, yielding two liquid phases with different densities. These results suggest the intriguing possibility of a liquid-liquid transition of water, confined to a bilayer, at regions where the ice bilayer is unstable with respect to either of the liquid phases. In addition, we find that under the same conditions, water confined to 3-8 layers remains in the liquid phase (albeit stratification of the transverse density profile) with values of the lateral diffusion coefficient comparable to that of the bulk. (C) 2003 American Institute of Physics

Divergence of the Long Wavelength Collective Diffusion Coefficient in Quasi-one and Quasi-two Dimensional Colloid Suspensions

We report the results of experimental studies of the short time-long wavelength behavior of collective particle displacements in quasi-one-dimensional and quasi-two-dimensional colloid suspensions. Our results are represented by the behavior of the hydrodynamic function H(q) that relates the effective collective diffusion coefficient, D_e(q) with the static structure factor S(q) and the self-diffusion coefficient of isolated particles D_0: H(q)=D_e(q)S(q)/D_0. We find an apparent divergence of H(q) as q->0 with the form H(q) proportional to q^-gamma, 1.7<gamma<1.9, for both q1D and q2D colloid suspensions. Given that S(q) does not diverge as q=>0 we infer that D_e(q) does. We provide evidence that this divergence arises from the interplay of boundary conditions on the flow of the carrier liquid and many-body hydrodynamic interactions between colloid particles that affect the long wavelength behavior of the particle collective diffusion coefficient in the suspension. We speculate that in the q1D and q2D systems studied the divergence of H(q) might be associated with a q-dependent partial slip boundary condition, specifically an effective slip length that increases with decreasing q. We also verify, using data from the work of Lin, Rice and Weitz (J. Chem. Phys. 99, 9585 (1993)), the prediction by Bleibel et al (arXiv:1305.3715), that D_e(q) for a monolayer of colloid particles constrained to lie in the interface between two fluids diverges as 1/q as q->0. The verification of that prediction, which is based on an analysis that allows two-dimensional colloid motion embedded in three-dimensional suspending fluid motion, supports the contention that the boundary conditions that define a q2D system play a very important role in determining the long wavelength behavior of the collective diffusion coefficient

Reduced Graphene Oxide/Polymer Monolithic Materials for Selective CO2 Capture

Polymer composite materials with hierarchical porous structure have been advancing in many different application fields due to excellent physico-chemical properties. However, their synthesis continues to be a highly energy-demanding and environmentally unfriendly process. This work reports a unique water based synthesis of monolithic 3D reduced graphene oxide (rGO) composite structures reinforced with poly(methyl methacrylate) polymer nanoparticles functionalized with epoxy functional groups. The method is based on reduction-induced self-assembly process performed at mild conditions. The textural properties and the surface chemistry of the monoliths were varied by changing the reaction conditions and quantity of added polymer to the structure. Moreover, the incorporation of the polymer into the structures improves the solvent resistance of the composites due to the formation of crosslinks between the polymer and the rGO. The monolithic composites were evaluated for selective capture of CO2. A balance between the specific surface area and the level of functionalization was found to be critical for obtaining high CO2 capacity and CO2/N2 selectivity. The polymer quantity affects the textural properties, thus lowering its amount the specific surface area and the amount of functional groups are higher. This affects positively the capacity for CO2 capture, thus, the maximum achieved was in the range 3.56–3.85 mmol/g at 1 atm and 25 °C.Spanish Government (CTQ2016-80886-R; BES-2017-080221), Basque Government (GV IT999-16) and NATO (SfP project G4255) are gratefully acknowledged for their financial support. The authors would like to acknowledge the contribution of the COST Action CA 15107

Size and branching effects on the fluorescence of benzylic dendrimers possessing one apigenin fluorophore at the core

Different generations of dendrimers incorporating one fluorescent core of apigenin and three Fréchet benzylic dendrons have been prepared. The chief geometric features of these dendrimers have been obtained by Molecular Dynamics simulations. These computational data suggest that the asphericities of dendrimers belonging to the third and fourth generations are considerably larger than those associated with lower radii of gyration. Fluorescence spectra of high generation dendrimers evolve along time and quantum yields show an appreciable lowering for the fourth generation dendrimer. All these data suggest aggregation phenomena and lower quantum yields for nonspheric dendrimers in solution.Financial support by the Spanish Ministry of Economy and Competitiveness, with the participation of European Union (MINECO, projects CTQ2010-16959/BQU, CTQ2012- 35535 and Consolider-Ingenio CSD2007-00006), from the University of the Basque Country (UPV/EHU, UFI11/22 QOSYC), from the Basque Government (GV/EJ, grant IT-324-07), from the Donostia International Physics Center (DIPC), from the Ministry of Education, Youth and Sports of the Czech Republic (grant MSM6046137305), and Czech Science Foundation (projects 304/10/1951, P503/11/0616) is acknowledged. M. d. B. thanks the CSIC for the JAE-Pre contract funding for her PhD. The authors also thank the SGI/IZO-SGIker UPV/EHU and the DIPC for generous allocation of computational resources.Peer reviewe

Binding Reactions at Finite Systems

A perpetual yearn exists among computational scientists to scale-down the size of physical systems, a desire shared as well with experimentalists able to track single molecules. A question then arises whether averages observed at small systems are the same as those observed at large, or macroscopic, systems. Utilizing statistical-mechanics formulations in ensembles in which the total numbers of particles are fixed, we demonstrate that system\u27s properties of binding reactions are not homogeneous functions. That means averages of intensive properties, such as the concentration of the bound-state, at finite-systems are different than those at large-systems. The discrepancy increases with decreasing numbers of particles, temperature, and volume. As perplexing as it may sound, despite these variations in average quantities, extracting the equilibrium constant from systems of different sizes does yield the same value. The reason is that correlations in reactants\u27 concentrations are ought be accounted for in the expression of the equilibrium constant, being negligible at large-scale but significant at small-scale. Similar arguments pertain to the calculations of the reaction rate-constants, more specifically, the bimolecular rate of the forward reaction is related to the average of the product (and not to the product of the averages) of the reactants\u27 concentrations. Furthermore, we derive relations aiming to predict the composition of the system only from the value of the equilibrium constant. All predictions are validated by Monte-Carlo and molecular dynamics simulations. An important significance of these findings is that the expression of the equilibrium constant at finite systems is not dictated solely by the chemical equation but requires knowledge of the elementary processes involved

Refinement of the OPLSAA Force-Field for Liquid Alcohols

We employ the popular all-atom optimized potential for liquid simulations, OPLSAA, force-field to model 17 different alcohols in the liquid state. Using the standard simulation protocol for few hundred nanosecond time periods, we find that 1-octanol, 1-nonanol, and 1-decanol undergo spontaneous transition to a crystalline state at temperatures which are 35–55 K higher than the experimental melting temperatures. Nevertheless, the crystal structures obtained from the simulations are very similar to those determined by X-ray powder diffraction data for several <i>n</i>-alcohols. Although some degree of deviations from the experimental freezing points are to be expected, for 1-nonanol and 1-decanol, the elevation of the freezing temperature warrants special attention because at room temperature, these alcohols are liquids; however, if simulated by the OPLSAA force-field, they will crystallize. This behavior is likely a consequence of exaggerated attractive interactions between the alkane chains of the alcohols. To circumvent this problem, we combined the OPLSAA model with the L-OPLS force-field. We adopted the L-OPLS parameters to model the hydrocarbon tail of the alcohols, whereas the hydroxyl head group remained as in the original OPLSAA force-field. The resulting alcohols stayed in the liquid state at temperatures above their experimental melting points, thus, resolving the enhanced freezing observed with the OPLSAA force-field. In fact, the mixed-model alcohols did not exhibit any spontaneous freezing even at temperatures much lower than the experimental values. However, a series of simulations in which these mixed-OPLSAA alcohols started from a coexistence configuration of the liquid and solid phases resulted in freezing transitions at temperatures 14–25 K lower than the experimental values, confirming the validity of the proposed model. For all of the other alcohols, the mixed model yields results very similar to the OPLSAA force-field and is in good agreement with the experimental data. Thus, for simulating alcohols in the liquid phase, the mixed-OPLSAA model is necessary for large (7 carbons and above) hydrocarbon chains