Hydration force, interfacial water

It is important to propose molecular and theoretical models to describe the forces, energy, structure and dynamics of water near mineral surfaces. Our understanding of experimental results concerning hydration forces, the hydrophobic effect, swelling, reaction kinetics and adsorption mechanisms in aqueous colloidal systems is rapidly advancing as a result of recent Monte Carlo (MC) and molecular dynamics (MO) models for water properties near model surfaces. This paper reviews the basic MC and MD simulation techniques, compares and contrasts the merits and limitations of various models for water-water interactions and surface-water interactions, and proposes an interaction potential model which would be useful in simulating water near hydrophilic surfaces. In addition, results from selected MC and MD simulations of water near hydrophobic surfaces are discussed in relation to experimental results, to theories of the double layer, and to structural forces in interfacial systems. 

Experimental studies of the thermodynamic, spectroscopic and transport properties of mineral/water interfaces have been extensive, albeit conflicting at times (4-10). Ambiguous terms such as "hydration forces", "hydrophobic interactions", and "structured water" have arisen to describe interfacial properties which have been difficult to quantify and explain. A detailed statistical-mechanical description of the forces, energies and properties of water at mineral surfaces is clearly desirable. 

What is the likely future use of MC and MD techniques for studying interfacial systems Several promising approaches are possible. Continued investigation of double layer properties, "hydration forces", "hydrophobic effects", and "structured water" are clearly awaiting the development of improved models for water-water, solute-water, surface-water, and surface-solute potentials. 

The SM / SB model for the ion hydration force is compared with the traditional model for ion hydration (based on the Bom energy of ions) and with a recent model due to Bostrom, Kunz and Ninham [Bostrom, M. Kunz, W. Ninham, B. W. Langmuir 2005, 21, 2619], which accounts for the decrease in hydration energy caused by the water density decrease in the vicinity of the air/water interface. Even when the ion-dispersion force is included in the latter model, it leads to an interfacial depletion of both... 

Since then, the depletion of ions from the air/water interface became the commonly accepted representation of the surface tension of electrolyte solutions. However, this common picture is not always true near a charged interface, there is an accumulation of counterions, predicted by the Poisson—Boltzmann treatment, which at very low electrolyte concentrations (less than 1CT3 M) can dominate the interfacial depletion of ions due to ion hydration forces,6 and consequently, the surface tension of aqueous electrolytes can decrease with increasing ionic strength (The Jones-Ray effect).7 Even more significant, the surface tension of acids decreases with increasing... 

M. Manciu, O. Calvo, E. Ruckenstein Polarization model for poorly-organized interfacial water Hydration forces between silica surfaces, ADVANCES IN COLLOID AND INTERFACE SCIENCE 127 (2006) 29-42. 

Polarization model for poorly-organized interfacial water Hydration forces between silica surfaces... 

Getting stractural information on the molecular supports of electropermeabilization was not easy. A key property of biological membranes is their dynamics. At a substructural level, 3IP NMR spectroscopy showed that a tilt of the orientation of the phospholipid polar head region was present in the electropermeabilized state of the membrane [45, 46]. The consequence of the interfacial water organization was proposed to be associated with a decrease of the hydration forces and the observed fusogenic state of electropermeabilized surfaces. At a more collective level, phospholipid flip-flop between the two faces of the plasma membrane was observed in the case of electropermeabilized erythrocytes [47]. 

Interpretation of the Hofmeister Series Many different explanations of the Hofmeister series of ions have been proposed over the years. Specific ion interactions with specific sites of the biomolecules must be taken into account and a subtle balance of several competing evenly matched interactions, such as differences in hydration strength, dispersion forces, polarization, ion size effects, and the impact on interfacial water structure, is involved according to Koelsch et al. [116] and Tobias and Hemminger [117]. 

Fluctuations of interfaces are directly relevant to a number of interfacial phenomena. One example, ion transfer across a liquid-liquid interface, will be discussed in Section 6.1. Another example is the behavior of monolayers of surfactants on water surfaces. Surface fluctuations are also fundamental to several processes in water-membrane systems, such as unassisted ion transport across lipid bilayers and the hydration forces acting between two membranes. Here, however, the problem is more complicated because not only capillary waves but also bending motions of the whole bilayer have to be taken into account. Furthermore, the concept of the surface tension is less clear in this case. This topic is discussed in Molecular Dynamics Studies of Lipid Bilayers. 

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