Hydration forces

While evidence for hydration forces date back to early work on clays [1], the understanding of these solvent-induced forces was revolutionized by Horn and Israelachvili using the modem surface force apparatus. Here, for the first time, one had a direct measurement of the oscillatory forces between crossed mica cylinders immersed in a solvent, octamethylcyclotetrasiloxane (OMCTS) [67]. 

B1.20.3.1 MEASURING SHORT-RANGE SOLVATION AND HYDRATION FORCES... 

A typical force curve showing the specific avidin-biotin interaction is depicted in figure Bl.20.10. The SFA revealed the strong influence of hydration forces and membrane undulation forces on the specific binding of proteins to membrane-bound receptors [81]. 

Israelachvili J N and Pashley R M 1983 Molecular layering of water at surfaces and origin of repulsive hydration forces Nature 306 249-50... 

Butt, H.J., Measuring electrostatic. Van der waals, and hydration forces in electrolyte-solutions with an atomic force microscope. Biophys. J., 60(6), 1438-1444 (1991). 

The surface potential can play an important role in the behavior of liposomes in vivo and in vitro (e.g.. Senior, 1987). In general, charged liposomes ai e more stable against aggregation and fusion than uncharged vesicles. However, physically stable neutral liposomes have been described (e.g.. Van Dalen et al., 1988). They are sufficiently stabilized by repulsive hydration forces, which counteract the attractive van der Waals forces. 

This strnctnring of liqnids into discrete layers when confined by a solid surface has been more readily observable in liquid systems other than water [1,55]. In fact, such solvation forces in water, also known as hydration forces, have been notoriously difficult to measure due to the small size of the water molecule and the ease with which trace amounts of contamination can affect the ordering. However, hydration forces are thought to be influential in many adhesive processes. In colloidal and biological systems, the idea that the hydration layer mnst be overcome before two molecules, colloidal particles, or membranes can adhere to each other is prevalent. This implies that factors affecting the water structure, such as the presence of salts, can also control adhesive processes. 

The colloid probe technique was first applied to the investigation of surfactant adsorption by Rutland and Senden [83]. They investigated the effect of a nonionic surfactant petakis(oxyethylene) dodecyl ether at various concentrations for a silica-silica system. In the absence of surfactant they observed a repulsive interaction at small separation, which inhibited adhesive contact. For a concentration of 2 X 10 M they found a normalized adhesive force of 19 mN/m, which is small compared to similar measurements with SEA and is probably caused by sufactant adsorption s disrupting the hydration force. The adhesive force decreased with time, suggesting that the hydrophobic attraction was being screened by further surfactant adsorption. Thus the authors concluded that adsorption occurs through... 

Water is a special liquid that forms unique bonds involving protons between the oxygen atoms of neighboring molecules, the so-called hydrogen bond. The solvation forces are then due not simply to molecular size effects, but also and most importantly to the directional nature of the bond. They can be attractive or hydrophobic (hydration forces between two hydrophobic surfaces) and repulsive or hydrophilic (between two hydrophilic surfaces). These forces arise from the disruption or modification of the hydrogen-bonding network of water by the surfaces. These forces are also found to decay exponentially with distance [6]. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

The first term is related to the van der Waals interaction, with A being the Hamaker constant. The second term includes other forces that decay exponentially with distance. As discussed, these may include double-layer, solvation, and hydration forces. In our data analysis, B and C were used as fitting variables the Hamaker constant A was calculated using Lifshitz theory [6]. 

Suematsu, N. J., Nishimura, S. and Yamaguchi, T. (2008) Release and transfer of polystyrene dewetting pattern by hydration force. Langmuir, 24, 2960-2962. 

Gur, Y. Ravina, I. Babchin, A. J., On the electrical double layer theory. II. The Poisson-Boltzman equation including hydration forces, J. Colloid Inter. Sci. 64, 333-341... 

The DLVO theory, with the addition of hydration forces, may be used as a first approximation to explain the preceding experimental results. The potential energy of interaction between spherical particles and a plane surface may be plotted as a function of particle-surface separation distance. The total potential energy, Vt, includes contributions from Van der Waals energy of interaction, the Born repulsion, the electrostatic potential, and the hydration force potential. [Israelachvili (13)]. 

Perera, L., Essmann, U. and Berkowitz, M. L. (1996). Role of water in the hydration force acting between lipid bilayers, Langmuir, 12, 2625-2629. 

Rand, R. P. and Parsegian, V. A. (1989). Hydration forces between phospholipid bilayers, Biochim. Biophys. Acta, 988, 351-376. 

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. 

Further, the extensive change of ionic charges (if present) have been found to be very prominent. The simple picture that bilayers in aqueous medium are principally stabilized by the competition between hydration forces-vflft der Wa a Is- electrostatic interactions seem to be the most plausible basis. 

The hydration forces arise from the hydrogen bond formation between the polar groups of the surfactant and the water molecule. Van der Waals forces are attraction forces between all molecules (these are short range). 

These various relationships between force and particle separation imply that the attractive force between particles will become infinite when they touch. In reality, other short-range forces will modify this relationship when r is very small, in particular the repulsion from overlap of atomic orbitals. The van der Waals attraction will then be balanced by this overlap repulsion. At these short distances (a few tenths of a nanometer), the van der Waals attraction will be strong enough to hold the particles fairly strongly together. This balance between van der Waals forces of attraction and overlap repulsion forces is shown schematically in Fig. 1.4, where the very steep repulsive interaction at atomic distances is due to the overlap repulsion. Hydration forces (see section 1.3.3) may also result in repulsion between surfaces at somewhat greater separations. 

There are other close-range forces related to entropy changes, including various interactions between solution species and a solid surface, such as solvation (in water, hydration) forces. Hydration forces can occur when hydrated cations are adsorbed at interacting surfaces. As these surfaces approach each other closely, loss of water of hydration is necessary in order to allow closer approach. While these forces can be repulsive, attractive or oscillating, they are most likely to be repulsive under the conditions of CD. Such forces may be very important for CD, which is almost always carried out in the presence of a high ionic concentration. For example they could be a cause of poor adhesion of some CD films. Solvation forces are treated in detail in Israelachvili s book—see Further Reading at the end of this chapter, Forces subsection. 

So-called solvation/structural forces, or (in water) hydration forces, arise in the gap between a pair of particles or surfaces when solvent (water) molecules become ordered by the proximity of the surfaces. When such ordering occurs, there is a breakdown in the classical continuum theories of the van der Waals and electrostatic double-layer forces, with the consequence that the monotonic forces they conventionally predict are replaced (or accompanied) by exponentially decaying oscillatory forces with a periodicity roughly equal to the size of the confined species (Min et al, 2008). In practice, these confined species may be of widely variable structural and chemical types — ranging in size from small solvent molecules (like water) up to macromolecules and nanoparticles. 

---