Surface forces hydration

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]. 

Calcium siHcate hydrate is not only variable ia composition, but is very poody crystallised, and is generally referred to as calcium siHcate hydrate gel or tobermorite gel because of the coUoidal sizes (<0.1 fiva) of the gel particles. The calcium siHcate hydrates ate layer minerals having many similarities to the limited swelling clay minerals found ia nature. The layers are bonded together by excess lime and iatedayer water to form iadividual gel particles only 2—3 layers thick. Surface forces, and excess lime on the particle surfaces, tend to bond these particles together iato aggregations or stacks of the iadividual particles to form the porous gel stmcture. 

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]. 

In filtration, the particle-collector interaction is taken as the sum of the London-van der Waals and double layer interactions, i.e. the Deijagin-Landau-Verwey-Overbeek (DLVO) theory. In most cases, the London-van der Waals force is attractive. The double layer interaction, on the other hand, may be repulsive or attractive depending on whether the surface of the particle and the collector bear like or opposite charges. The range and distance dependence is also different. The DLVO theory was later extended with contributions from the Born repulsion, hydration (structural) forces, hydrophobic interactions and steric hindrance originating from adsorbed macromolecules or polymers. Because no analytical solutions exist for the full convective diffusion equation, a number of approximations were devised (e.g., Smoluchowski-Levich approximation, and the surface force boundary layer approximation) to solve the equations in an approximate way, using analytical methods. 

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. 

Fig. 1. Change in surface tension at low electrolyte concentrations (the Jones-Ray effect). Circles experimental data from Ref. 112] line predicted surface tension by the SM/SB model which accounts for OH adsorption on the surface, ion hydration and image forces within the Poisson Boltzmann approach (Ref. [5]).

Another clear failure of the Poisson-Boltzmann approach was provided by the experiments regarding the force between neutral lipid bi layers [11], The repulsion required to explain their stability was determined to have an almost exponential dependence, with a decay length of about 2—3 A [11], and neither this decay length nor the magnitude of the interactions were dependent on the ionic strength. This interaction was initially attributed to the structuring of water near the surface (the hydration of the surfaces) and it is usually called hydration force [12]. The microscopic origins of this interaction are still under debate. 

While the hydration force was associated with the structuring of water in the vicinity of a lyophilic surface [12], there is no consensus about its microscopic origin. This incertitude is probably due to the apparent contradictory experimental results for phospholipid bilayers, the hydration force is apparently independent of the electrolyte concentration and has a decay length of about 2 A [11], while for mica surfaces the hydration is strongly dependent not only on the electrolyte concentration, but also on the nature of the cation (cation-specific effects) [17] and has a decay length of about 10 A. 

Recent surface force measurements revealed a similar trend (20). Comparing steam-treated to flame-treated silica sheets using site-dissociation/site-binding model, a decrease in silanol surface sites and apparent decrease in average pKa was observed upon heat treatment. Furthermore, a repulsive force other than double-layer and van der Waals forces was observed 15 A from the surface. This repulsion was attributed to hydration of the surface and was found to be independent of surface treatment and electrolyte concentration. In Bums treatment, an arbitrary plane of shear was introduced to provide a best model fit (l 3). A value of 9 A from the surface for the plane of shear was determined from electro-osmosis measurements. 

Another AFM-based technique is chemical force microscopy (CFM) (Friedsam et al. 2004 Noy et al. 2003 Ortiz and Hadziioaimou 1999), where the AFM tip is functionalized with specific chemicals of interest, such as proteins or other food biopolymers, and can be used to probe the intermolecular interactions between food components. CFM combines chemical discrimination with the high spatial resolution of AFM by exploiting the forces between chemically derivatized AFM tips and the surface. The key interactions involved in food components include fundamental interactions such as van der Waals force, hydrogen bonding, electrostatic force, and elastic force arising from conformation entropy, and so on. (Dther interactions such as chemical bonding, depletion potential, capillary force, hydration force, hydrophobic/ hydrophobic force and osmotic pressure will also participate to affect the physical properties and phase behaviors of multicomponent food systems. Direct measurements of these inter- and intramolecular forces are of great interest because such forces dominate the behavior of different food systems. 

Using a surface forces apparatus, Israelachvili determined the force law for two molecularly smooth charged mica surfaces immersed in an aqueous solvent (see Israelachvili and Marra, 1986, and references cited therein). The repulsive hydration force is oscillatory (Fig. 8A). It is understood to reflect the geometry and local structure of the solvent and... 

Hydration and other surface forces are important for the approximation of aggregates with large areas. It is possible that the concept of... 

In the last 40 years, techniques to directly measure surface forces and force laws (force vs. separation distance between surfaces) have been developed such as the surface forces apparatus (SFA) [6] and AFM. Surface forces are responsible for the work required when two contacting bodies (such as an AFM tip in contact with a solid surface) are separated from contact to infinite distance. Although the physical origin of all relevant surface forces can be derived from fundamental electromagnetic interactions, it is customary to group these in categories based on characteristic features that dominate the relevant physical behavior. Thus, one speaks of ionic (monopole), dipole—dipole, ion—dipole interactions, electrostatic multipole forces (e.g., quadrupole), induced dipolar forces, van der Waals (London dispersive) interactions, hydrophobic and hydrophilic solvation, structural and hydration forces,... 

Another aspect of current interest associated with the lipid-water system is the hydration force problem.i -20 When certain lipid bilayers are brought closer than 20-30 A in water or other dipolar solvents, they experience large repulsive forces. This force is called solvation pressure and when the solvent is water, it is called hydration pressure. Experimentally, hydration forces are measured in an osmotic stress (OS) apparatus or surface force apparatus (SFA)2o at different hydration levels. In OS, the water in a multilamellar system is brought to thermodynamic equilibrium with water in a polymer solution of known osmotic pressure. The chemical potential of water in the polymer solution with which the water in the interlamellar water is equilibrated gives the net repulsive pressure between the bilayers. In the SEA, one measures the force between two crossed cylinders of mica coated with lipid bilayers and immersed in solvent. 

Surface Forces. Surface forces can be measured with thin film balances where freely suspended horizontal films are submitted to controlled pressures. If the film drainage has been smooth enough, so that no early rupture has occurred, a regime where interactions between the two sides of the film become significant can be attained this is in a thickness region of about 100 nm and less. Typical interactions are van der Waals forces (attractive) and electrostatic forces (repulsive). Short range forces (steric, hydration) are also present. 

Experimental Results. The DLVO theory, which is based on a continuum description of matter, explains the nature of the forces acting between membrane surfaces that are separated by distances beyond 10 molecular solvent diameters. When the interface distance is below 10 solvent diameters the continuum picture breaks down and the molecular nature of the matter should be taken into account. Indeed the experiment shows that for these distances the forces acting between the molecularly smooth surfaces (e.g., mica) have an oscillatory character (8). The oscillations of the force are correlated to the size of the solvent, and obviously reflect the molecular nature of the solvent. In the case of the rough surfaces, or more specifically biomembrane surfaces, the solvation force displays a mono tonic behavior. It is the nature of this solvation force (if the solvent is water, then the force is called hydration force) that still remains a puzzle. The hydration (solvation) forces have been measured by using the surface force apparatus (9) and by the osmotic stress method (10, II). Forces between phosphatidylcholine (PC) bilayers have been measured using both methods and good agreement was found. 

Since the first measurements of the electrostatic double-layer force with the AFM not even 10 years ago, the instrument has become a versatile tool to measure surface forces in aqueous electrolyte. Force measurements with the AFM confirmed that with continuum theory based on the Poisson-Boltzmann equation and appKed by Debye, Hiickel, Gouy, and Chapman, the electrostatic double layer can be adequately described for distances larger than 1 to 5 nm. It is valid for all materials investigated so far without exception. It also holds for deformable interfaces such as the air-water interface and the interface between two immiscible liquids. Even the behavior at high surface potentials seems to be described by continuum theory, although some questions still have to be clarified. For close distances, often the hydration force between hydrophilic surfaces influences the interaction. Between hydrophobic surfaces with contact angles above 80°, often the hydrophobic attraction dominates the total force. 

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