Why is hydrocarbon hydrophobic

The development overcomes major limitations of future hydrate use in an industrial-scale natural gas storage process. Surfactants as Hydrate Promoters?

Energy Fuels , 14 , — , DOI: Natural gas hydrates from different types and concns. An anionic, a cationic, and a nonionic surfactant were used to prep.

Although the thermodn. The hydrate formation rate is increased with the use of anionic surfactant for all concns. Hydrate formation rate is also increased at low concns. The effect of nonionic surfactant is less pronounced compared to anionic and cationic ones. Adsorption of Surfactants on Two Different Hydrates. Langmuir , 24 , — , DOI: The interaction between surfactants and hydrates provides insight into the role of surfactants in promoting hydrate formation.

This work aims at understanding the adsorption behavior of sodium dodecyl sulfate SDS on cyclopentane CP hydrates and its deriv. Cyclopentane CP is a hydrophobic former whereas tetrabutylammonium bromide TBAB is a salt that forms semiclathrate hydrates.

The adsorption on these two hydrates was studied by zeta potential and pyrene fluorescence measurements. CP hydrates have a neg. Then, it increases with further increased SDS concn. The adsorption d. Colloid Interface Sci.

Adsorption of Gemini surfactants onto clathrate hydrates. The first crit. The coarse-grained model mols. The bulk viscosity of SDS was calcd. The effect of the size of the simulation box on the obtained CMC has been investigated, as well as the effect of the no.

The CMC, which was obtained from a graph of the calcd. The formation of a spherical micelle-like aggregate is obsd. The advantage of using coarse-grain mol. Mechanisms of Gas Hydrate Formation and Inhibition. Fluid Phase Equilib. Koh, C. Elsevier Science B. The formation of gas hydrates in gas and oil subsea pipelines often results in blockage and shutdown of these pipelines.

Modern control methods depend on understanding the mechanisms through which gas hydrates form. This paper reports our recent studies of clathrate hydrate formation and inhibition mechanisms using neutron diffraction, differential scanning calorimetry DSC and a multiple cell photo-sensing instrument.

The structural transformations of water around methane during methane hydrate formation have been studied using neutron diffraction with isotope substitution over the temp. The hydration sphere around methane in the liq. The hydration shell is disordered during methane hydrate formation, with ordering of solvent sepd. The effects of the addn. The QAB inhibitor exhibits the greatest hydrate crystal growth control, while VC is most effective at inhibiting hydrate nucleation.

Insight into the perturbations on host and guest mols. Energy Fuels , 20 , — , DOI: A review. Low dosage hydrate inhibitors LDHIs are a recent and alternative technol.

LDHIs are divided into two main categories, kinetic inhibitors KHIs and anti-agglomerants AAs , both of which are successfully being used in field applications. This paper reviews the research and development of LDHIs with emphasis on the chem. The structure, thermodn. A three-phase mol. The decrease in the three-phase coexisting temps. NaCl solns.

The growth rate of methane hydrates in NaCl soln. The kinetic inhibition of NaCl is found to be a result of the reduced water repelling at the growing interface due to the strong hydration of ions. Individual ions or NaCl ion pairs can replace water mols. The distortion of water cages due to the presence of ions may result in a reduced fraction of occupation of methane in the cage cavities.

Our results provide useful insights into the mechanism of growth of methane hydrates in seawater and the desalination. Traditionally, the effect of pressure on hydrate formation has been described using the Clausius-Clapeyron equation, which assumes that only vapor phase vol.

According to this assumption, equil. But both data and theory shows that this is not true at higher pressures. In this research, equil. These measurements were done using a new exptl. The data show significant deviations from linear Clausius-Clapeyron behavior. Both the Clapeyron relation and van der Waals and Platteeuw hydrate theory has been used.

Both models represent the data well, but show minor deviations at high salt concn. This review aims to introduce the chem. The review covers both kinetic hydrate inhibitors and anti-agglomerants from the point of view of structure-function relationships, focussing on recent refinements in mechanistic understanding and chem.

Anderson, Brian J. Within the framework of a proposed two-step mechanism for hydrate inhibition, the energy of binding of four inhibitor mols. One key feature of this proposed mechanism is that the binding of an inhibitor mol. It is found through the mol. Furthermore, the effect of an inhibitor mol. Two mol. Equipped with this mol.

Energy Fuels , 27 , — , DOI: Plugging of flow lines by gas hydrates is a costly and challenging problem for the oil and gas industry. LDHIs are injected in much lower concn. Quaternary ammonium surfactants currently dominate the AA market. The authors report the 1st systematic study of lab. The effect on AA performance by varying the alkyl chain length from 8 to 18 carbon atoms and the effect of salinity were studied.

There is an optimum alkyl tail chain length for the anti-agglomerating efficacy of this surfactant class and increased salinity improves the AA performance. The performance of the AAs could also be further improved by the addn. Elsevier B. Anti-agglomeration is a promising soln. Currently ineffectiveness at high water to oil ratios limits such applications.

We present exptl. We find that our surfactant at a very low concn. We propose a mechanism different from the established water-in-oil emulsion theory in the literature that the process is effective without the oil phase. There is no need to emulsify the water phase in the oil phase for hydrate anti-agglomeration; with oil-in-water emulsion and without emulsion hydrate anti-agglomeration is presented in our research.

We expect our work to pave the way for broad applications in offshore natural gas prodn. Antifreeze Proteins as Gas Hydrate Inhibitors. Walker, Virginia K. Alireza; Alavi, Saman; Englezos, Peter. Canadian Science Publishing. Certain organisms survive low temps. Several of these proteins have been purified and shown to also inhibit the crystn.

They have been found to be effective against structure II sII hydrates formed from the liq. For the most part, AFPs are equal to or more effective than the com. Enclathrated gas anal. KHIs and results in properties that should make these proteins more valuable in some field applications. Efforts to overcome the difficulties of recombinant protein prodn. KHIs for hydrocarbon recovery and transport with all the attributes of these AFP "green inhibitors", including their benefits for human and environmental safety.

Antifreeze proteins AFPs prevent ice growth by binding to a specific ice plane. Some AFPs have been found to inhibit the formation of gas hydrates which are a serious safety and operational challenge for the oil and gas industry. Each binding set is composed of the Me side chain of threonine and two alanine residues, four and seven places further down in the sequence of the protein.

Understanding the principle of action of AFPs can lead to the rational design of green hydrate inhibitor mols. Nature Publishing Group. Natural gas hydrates are solid hydrogen-bonded water crystals contg. The amt. Here, we report how the presence of amino acids in water induces changes in its structure and thus interrupts the formation of methane and natural gas hydrates. The perturbation of the structure of water by amino acids and the resulting selective inhibition of hydrate cage formation were obsd.

A strong correlation was found between the inhibition efficiencies of amino acids and their physicochem. Further, the interactions between amino acids and water are essential to the equil. Energy Fuels , 28 , — , DOI: Clathrate Hydrate Growth and Inhibition.

THF and ethylene oxide EO solns. Adding the same inhibitors to the sI system caused rapid small-scale branching of the crystals. The highly branched sI specimens were found still to be single crystals.

Higher concns. These concns. A phys. This hypothesis appeals to a structural fit of the inhibitor polymers on the hydrate surface and strong bonding in specific configurations. American Institute of Physics. We have used small-angle neutron scattering to characterize the polymer conformations of four nonionic water sol. The last three of these are able to kinetically suppress hydrate crystn. This attribute is of significant com. The dil. Each soln.

Utilizing results from our previous contrast variation study, we show that this arises from polymer adsorbed to the hydrate-crystal surface. Furthermore, we find a strong correlation between the scattering intensity at low q values and the effectiveness of the inhibitor polymer.

We suggest this is an indication that as surface adsorption increases, the inhibitor's blocking of growth sites increases. Also measured for one of the kinetic-inhibitor polymers was the dil. This system shows addnl. This may affect hydrate nucleation behavior and offer a second mechanism for kinetic hydrate inhibition.

The mechanism by which a hydrate growth inhibitor, i. The formation of hydrates, e. The topol. The adsorption sites were located around the large partial cavities, at positions detd. There is an added van der Waals attraction between the NEP and the hydrate surface which is optimal when the pyrrolidone occupies the partial large hedra cavities on the hydrate surface.

New York Academy of Sciences. The conformation and particle size distribution of PVP bound to a hydrate surface and their role on the adsorption energy were studied. The results were correlated with the behavior of the vinylpyrrolidone monomer. For PVP and the monomer the same adsorption sites are evident. In the case of PVP, adsorption of any one unit is less optimal than was found for the monomer, but the trains of adsorbed polymer involve partial occupancy of a succession of monomer adsorption sites.

Kinetic Inhibitor of Hydrate Crystallization. We present the results of a combined theor. The inhibitors are based on quaternary ammonium zwitterions, and were identified from a computational screen. These simulations show that the inhibitor is bifunctional, with the hydrophobic end being compatible with the water structure present at the hydrate interface, while the neg.

The compd. In this paper we present the results of a mol. Direct mol. In the case of wax inhibition we present a simple model for predicting the activity of comb-like polymers, which is likely to be useful for identifying the most active inhibitor from within a given class. The method is applied to a set of random copolymers and shown to correlate well with exptl.

Classical mol. Vinylpyrrolidone, vinylvalerolactam, L-proline, 1-formylpyrrolidine, and dimethylaminoethyl methacrylate are used as models of clathrate formation kinetic inhibitors. The goal is to get new insights into the behavior of these composite systems in the early stages of clathrate formation, where the inhibitor is thought to perform its main inhibition effect. Before performing the simulations for the clathrate systems, evidence was searched to find if the chosen combination of force fields reproduced the cage effect for an infinitely dild.

The systems are analyzed via visualization of the trajectories and examn. The inhibitors can be grouped into hydrophilic and hydrophobic types, and their behavior toward water is analyzed on the basis of their structure and intermol. At the initial stages of clathrate formation, the hydrophobic inhibitors may act by blocking the surface of the nascent crystals, whereas the hydrophilic ones act mainly by disruption of the water structure.

Faraday Discuss. Clathrate hydrates are important in both industrial and geol. They give rise to many technol.

In all of these applications there is a need to exert a high degree of control on the crystn. This crystn. The net effect is that the mechanisms for hydrate formation and growth are still poorly understood, with the consequence that development of additives to control nucleation and growth is still largely governed by trial-and-error approaches. In this paper we show how classical mol. The results show that the two additives work by very different mechanisms, with PVP increasing the surface energy of the interfacial region and PDMAEMA adsorbing to the surface of hydrate nanocrystals.

The surface energy effect is a mechanism that has not previously been considered for hydrate inhibitors. The adsorption of guest and kinetic inhibitor mols. The free energy profile for transferring a solute mol. Spherical solutes with a diam.

The range of the attractive force is subnanoscale, implying that this force has no effect on the macroscopic mass transfer of guest mols. The adsorption mechanism of a kinetic hydrate inhibitor is examd. A monomer of the kinetic hydrate inhibitor is strongly adsorbed on the hydrate surface. However, the hydrogen bonding between the amide group of the inhibitor and water mols.

The preferential adsorption of both the kinetic inhibitor and the spherical mols. The dependence of surface affinity on the size of adsorbed mols. Steered and equil. The size of both the hydrate particle and the water droplet is comparable to that of the aq.

The simulations were repeated in the presence of various quaternary ammonium chloride surfactants. The authors studied the effects due to different groups on the quaternary head group e. Me vs. Bu groups , as well as different hydrophobic tail lengths e. Visual inspection of sequences of simulation snapshots indicates that when the water droplet is not covered by surfactants it is more likely to approach the hydrate particle, penetrate the protective surfactant film, reach the hydrate surface, and coalesce with the hydrate than when surfactants are present on both surfaces.

Force-distance profiles obtained from steered mol. These results qual. The simulation results provide addnl. Langmuir , 33 , — , DOI: The liq.

The anti-agglomerants considered were surface-active compds. The length of the hydrophobic tails and the surface d.

The results were analyzed in terms of the preferential orientation of the anti-agglomerants, d. At low surface densities the hydrophobic tails do not show preferred orientation, irresp. At sufficiently high surface densities, our simulations show pronounced differences in the structure of the interfacial film depending on mol. Some anti-agglomerants are found to pack densely at the interface and exclude methane from the interfacial region.

At these conditions the anti-agglomerants film resembles a 'frozen interface'. The hydrophobic tails of the anti-agglomerants that show this feature are of length comparable to that of the n-dodecane in the liq.

It is possible that the structured interfacial layer is in part responsible for detg. The simulation results are qual. It was found that the anti-agglomerants for which our simulations suggest evidence of frozen interface at sufficiently high surface densities are those that show excellent performance in rocking cell expts.

The authors used mol. From the mol. In the aq. To characterize the extent of surface adsorption in each liq. The surface adsorption in the liq. The extent of surface adsorption is much larger in the liq. Finally, the authors examine the effect of the inhibitor on the H2O structure in the liq. The structure of a subset of water mols. By portraying the mean HB dynamics of water as a doubly periodic event, it was demonstrated that the orientational retardation factor is effectively defined by the ratio of the HB acceptor switch period in the hydration layer and bulk.

Excluded vol. The orientational mobility of water populations with long HB lifetimes is also probed, and although a relaxation plateau is obsd. The proposed mol. Nature , , — , DOI: The authors report combined polarized, isotopic, and temp. Raman-MCR that explore hydrophobic hydration by mapping the vibrational spectroscopic features arising from the hydrophobic hydration shells of linear alcs. This structure disappears with increasing temp.

The unique structural, dynamical and chem. For example, nonhydrogen-bonded dangling' OH groups-which create surface defects in water's hydrogen bonding network and are exptl. However, how the size, curvature or charge of the exposed hydrophobic surface influences water's propensity to form dangling OH defects has not yet been established quant. Here the authors use Raman multivariate curve resoln.

The authors thus expose an important difference between the chem. Perera, P. National Academy of Sciences. We report the exptl. The results are obtained by combining vibrational Raman spectroscopy and multivariate curve resoln.

MCR , to reveal a high-frequency OH stretch peak arising from the hydration shell around nonpolar hydrocarbon solute groups. The frequency and width of the obsd. The area of the obsd. The origins of the hydrophobic effect are widely thought to lie in structural changes of the water mols.

The spatial distribution functions of the water mols. In addn. The position data alone, however, do not describe the majority of orientational changes in the water mols. In this paper, we measure the changes in orientation of the water mols. These data show that the water mols. Water mols. The structure of water mols. Here, we re-examine the subtle issue by a combination of thermodn. Both the structural and thermodn.

Hydrogen-bond formation is slightly hindered near a hydrophobic solute due to the restriction of the degree of freedom for water mols. As predicted by the scaled-particle theory, both Henry's consts. Hydrophobicity plays an important role in numerous physicochem. The classical view of hydrophobic hydration is that, in the presence of a hydrophobic solute, water forms transient microscopic "icebergs" arising from strengthened water hydrogen bonding, but there is no exptl.

Here, we have used the red shifts and line shapes of the isotopically decoupled IR oxygen-deuterium O-D stretching mode of HDO water near small purely hydrophobic solutes methane, ethane, krypton, and xenon to study hydrophobicity at the most fundamental level. We present unequivocal and model-free exptl. The water mols. The no. Ab initio mol. We show the absence of intercalating water mols.

Our results confirm the classical view of hydrophobic hydration. Entropy in binary liquid mixtures; partial molal entropy in dilute solutions; structure and thermodynamics in aqueous electrolytes.

The ideas of parts I and II, which make it possible to interpret entropy data in terms of a phys. These equations are tested against data obtained by various authors on mixts.

Good general agreement is found, and it is concluded that in such solns. As would be expected, permanent gases, when dissolved in normal liqs. Entropies of vaporization from aq. The nature of the deviations found for nonpolar solutes in water, together with the large effect of temp.

Such icebergs are apparently formed also about the nonpolar parts of the mols. The entropies of hydration of ions are discussed from the same point of view, and the conclusion is reached that ions, to an extent that depends on their sizes and charges, may cause a breaking down of water structure as well as a freezing or satn.

Various phenomena recorded in the literature are interpreted in these terms. The effect of temp. It appears that the salting-out phenomenon is at least partly a structural effect. It is suggested that structural influences modify the distribution of ions in an electrolyte soln.

An example is given of a possible addnl. Patel, Amish J. Water near extended hydrophobic surfaces is like that at a liq. Here we use mol.

Consequently, water near these surfaces is sensitive to subtle changes in surface conformation, topol. Our work also resolves the long-standing puzzle of why some biol. We study the properties of water at the surface of an antifreeze protein with femtosecond surface sum frequency generation spectroscopy.

We find clear evidence for the presence of ice-like water layers at the ice-binding site of the protein in aq. Decreasing the temp. Our observations indicate that not the protein itself but ordered ice-like water layers are responsible for the recognition and binding to ice. Water Dynamics in the Hydration Shells of Biomolecules. The structure and function of biomols. Structural fluctuations and mol. Recent progress in theory and mol. Here, we review recent advances in both theory and expt.

Fuel , , — , DOI: Elsevier Ltd. Anti-agglomerants AAs can be effective mols. Salt and water cut ratio of water vol.

The understanding of the effect of salt and water cut on hydrate anti-agglomeration is limited. We use a condensate liq. The effect of NaCl includes lowering of water to hydrate conversion and decreasing the soly. The measurements over a broad range of water cuts reveal formation of water-in-oil and oil-in-water emulsions as well as flocculation. To shed light on the mechanisms we perform emulsion studies and measure AA partitioning between the aq.

This study presents the mechanisms that their understanding may be essential in application of AA in a broad range of water cuts. Petroleum fluids may form hydrate crystals with water at conditions often encountered. Hydrate formation in large pieces is a serious problem in flow assurance and oil capture from the seabed. Use of functionalized mols. The literature suggests water-in-oil emulsion for hydrate antiagglomeration, which limits the application because of requirement of large amts.

In a recent article, the authors demonstrated hydrate antiagglomeration of methane in water and brine by a new surfactant mol. However, in a natural gas contg. CO2, the same surfactant loses effectiveness. The authors offer a revised formulation consisting of the surfactant, small amts. The base adjusts the pH, and the alkane serves as a defoamer. The effects of each component are systematically discussed and a synergetic effect is found.

The new formulation provides effective antiagglomeration in a broad range of conditions. Also, the formulation has three other beneficial effects including kinetic inhibition, redn. Gas hydrates may plug hydrocarbon flowlines even at low hydrate vol. Anti-agglomeration is perhaps the most effective approach for flow assurance. An effective anti-agglomerant AA can prevent gas hydrate particles from sticking together by lowering of the water-oil interfacial tension and increase of the contact angle of water on hydrate surface.

In a recent work, we have introduced a new AA formulation which is effective; it forms hydrate slurry over a broad range of oil to water ratio. In this study, we introduce a modified chem. The formation of hydrate powders has not been reported in the literature. The powders flow readily in rocking cells. The AA effectiveness reported in this study is much higher than the past work. A new anti-agglomeration mechanism is also proposed.

The gas hydrate powder formation may have applications not only in flow assurance, but also in energy storage. Salts and additives can significantly affect the strength of water-mediated interactions in soln. We present results from mol. Whether these effects are enthalpic or entropic in origin is not fully known. Here we perform temp. We find that in TMAO solns. In contrast, in NaCl solns. The resoln. Our results are consistent with exptl.

In combination with recent work by Zangi, Hagen, and Berne and the exptl. Although NaCl strengthens hydrophobic interactions at both small and large length scales, that effect is enthalpy-dominated at small length scales and entropy-dominated for large solutes and interfaces. Our results have implications for understanding of additive effects on water-mediated interactions, and on biocompatibility of osmolyte mols.

In particular the effect of temp. The salt effect on methane's solvation free energy, solvation enthalpy, and entropy, as well as their temp. To distinguish the influence of local in close proximity to ions and global effects, the salt solns.

The chem. This observation is correlated with systematic structural alterations similar to water under pressure. The obsd. The influence of temp. On the basis of the model of the water mol. The following are inferred in a qual.

Salting-out of hydrophobic solutes in aq. Although salt effects on hydrophobic transfer have been well studied, to our knowledge, no quant. Here we present quant.

Specifically, we quantify the effect of NaCl on the potential of mean force between mol. In both cases, we observe relative stabilization of compact conformations that is accompanied by a clear depletion of salt d.

We show that the structural details of salt exclusion can be related to the salt-induced free energy changes using preferential interaction coeffs. We also test the applicability of surface-area-based models to describe the salt-induced free energy changes. These models provide a useful empirical description that can be used to predict the effects of salt on conformational equil.

However, we find that the effective increase in the surface tension of the solute-aq. These calcns. The hydrophobic polymer also provides a useful model for studies of effect of thermodn. Dielectric Properties of the Hydrates of Argon and Nitrogen. Classical Electrodynamics , 3 rd ed. Fluids of charged particles act as the supporting medium for chem.

The local structure in an electrolytic background is deformed by micro- and nanoscopic polarizable objects. Vice versa, the forces between the objects are regulated by the cohesive properties of the background. The authors study here the range and strength of these forces and the microscopic origin from which they emerge. The forces to be sensitively dependent on the material properties of the charged fluid and the immersed solutes were found.

The induced interactions can be varied over decades, offering high tunability and aided by accurate theory, control in expts. To distinguish correlational effects from simple ionic screening, the authors describe electrolyte-induced forces between neutral objects.

The interplay of thermal motion, short-range repulsions, and electrostatic forces is responsible for a soft structure in the fluid. This structure changes near polarizable interfaces and causes diverse attractions between confining walls that seem well-exploited by microbiol. For parameters that correspond to monovalent electrolytes in biol. Interfaces and the Driving Force of Hydrophobic Assembly. The hydrophobic effect - the tendency for oil and water to segregate - is important in diverse phenomena, from the cleaning of laundry, to the creation of micro-emulsions to make new materials, to the assembly of proteins into functional complexes.

This effect is multifaceted depending on whether hydrophobic mols. Despite the basic principles underlying the hydrophobic effect being qual.

Water as an Active Constituent in Cell Biology. Here, the author reviews the case for considering water to be a versatile, adaptive component of the cell that engages in a wide range of biomol.

The author highlights the distinctions between generic and specific behaviors of biol. Physical Chemistry: Oil on Troubled Waters.

The nature of the boundary between water and oil is crucial to many nanometer-scale assembly processes, including protein folding. But until now, what the interface really looks like remained in dispute. Transferable Potentials for Phase Equilibria. United-Atom Description of n-Alkanes. A new set of united-atom Lennard-Jones interaction parameters for n-alkanes is proposed from fitting to crit.

Configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to det. Nature , , ], and TraPPE. TraPPE model were studied by simulations in the isobaric-isothermal and canonical ensembles, resp.

It is found that one set of Me and methylene parameters is sufficient to accurately describe the fluid phases of all n-alkanes with two or more carbon atoms.

Whereas other n-alkane force fields employ Me groups that are either equal or larger in size than the methylene groups, it is demonstrated here that using a smaller Me group yields a better fit to the set of exptl. As should be expected from an effective pair potential, the new parameters do not reproduce exptl. Classical force-field parameters have been developed for amines primarily by fitting to exptl. The resultant parameters were used to calc. The calcns. The proper balance of hydrogen-bond strengths, which is achieved with the OPLS-AA force field, is essential for correct ordering of the free energies of hydration.

MC simulations for the pure liqs. These simulations covered aliph. It is apparent that the prior problems with classical force fields for amines were simply a result of nonoptimal parametrization rather than to a crit. The parametrization and testing of the OPLS all-atom force field for org.

Parameters for both torsional and nonbonded energetics have been derived, while the bond stretching and angle bending parameters have been adopted mostly from the AMBER all-atom force field.

The torsional parameters were detd. The quality of the fits was high with av. The force-field results for mol. The nonbonded parameters were developed in conjunction with Monte Carlo statistical mechanics simulations by computing thermodn. The Monte Carlo simulations included sampling all internal and intermol. It is found that such non-polar and monofunctional systems do not show significant condensed-phase effects on internal energies in going from the gas phase to the pure liqs.

The functions provide good descriptions of the structures and energetics for isolated ion-water complexes in comparison with results of ab initio MO calcns. However, the principal testing involved Monte Carlo statistical mechanics simulations of dil. The computed results are in accord with exptl.

We have developed a united atom force field able to accurately describe the adsorption properties of linear alkanes in the sodium form of FAU-type zeolites. This force field successfully reproduces exptl. The force field reproduces the sodium positions in dehydrated FAU-type zeolites known from crystallog. The cations in the sodalite cages are significantly more sensitive to the n-alkane loading than those in the supercages.

We provide a simple expression that adequately describes the n-alkane Henry coeff. This expression affords an adequate substitute for complex configurational-bias Monte Carlo simulations. The applicability of the force field is by no means limited to low pressure and pure adsorbates, for it also successfully reproduces the adsorption from binary mixts. Statistical Mechanics of Fluid Mixtures. Expressions for the chem.

Molecular-Dynamics with Coupling to an External Bath. Berendsen, H. In mol. A method is described to realize coupling to an external bath with const. The method is easily extendable to other variables and to gradients, and can be applied also to polyat. The influence of coupling time consts. A leap-frog algorithm is presented for the general case involving constraints with coupling to both a const. Canonical dynamics: Equilibrium Phase-Space Distributions. A: At. Physical review. A parallel message-passing implementation of a mol.

Simulation with communication to and from left and right neighbors, but can run on any parallel system onto which a a ring of processors can be mapped and which supports PVM-like block send and receive calls. The MD program can handle rectangular periodic boundary conditions with temp.

The interactions that can be handled without modification are variable non-bonded pair interactions with Coulomb and Lennard-Jones or Buckingham potentials, using a twin-range cut-off based on charge groups, and fixed bonded interactions of either harmonic or constraint type for bonds and bond angles and either periodic or cosine power series interactions for dihedral angles. Special forces can be added to groups of particles for non-equil. The parallelism is based on particle decompn.

Interprocessor communication is largely limited to position and force distribution over the ring once per time step. Simulation that was developed at the University of Groningen, The Netherlands, in the early s.

The software, written in ANSI C, originates from a parallel hardware project, and is well suited for parallelization on processor clusters.

The program is versatile, as force routines can be added by the user, tabulated functions can be specified, and analyses can be easily customized. Interfaces with popular quantum-chem. The package includes about utility and anal.

When the same standard free energies of transfer are plotted as a function of the accessible surface areas of the hydrocarbons, the data do not fall upon the same line nor are the standard free energies of transfer directly proportional to the accessible surface areas. An examination of the standard free energies of transfer for the different classes of hydrocarbons from the gas phase to water and from the gas phase to hexadecane reinforces the conclusion that the number of hydrogen-carbon bonds in a molecule rather than its accessible surface area is the basis of the hydrophobic effect.

Consequently, estimates of the contribution of different functional groups to the hydrophobic effect providing the free energy of folding of a molecule of protein or providing the free energy of dissociation for the association of two proteins or the association of a ligand with a protein should be made by counting the number of hydrogen-carbon bonds excluded from water rather than computing the accessible surface areas excluded from water.

Abstract The property of a molecule that most reliably determines the magnitude of the hydrophobic effect that it will experience is the number of hydrogen-carbon bonds it contains not the accessible surface area of its nonpolar portions.

Publication types Review.