Surface forces solvatation

A major advance in force measurement was the development by Tabor, Win-terton and Israelachvili of a surface force apparatus (SFA) involving crossed cylinders coated with molecularly smooth cleaved mica sheets [11, 28]. A current version of an apparatus is shown in Fig. VI-4 from Ref. 29. The separation between surfaces is measured interferometrically to a precision of 0.1 nm the surfaces are driven together with piezoelectric transducers. The combination of a stiff double-cantilever spring with one of a number of measuring leaf springs provides force resolution down to 10 dyn (10 N). Since its development, several groups have used the SFA to measure the retarded and unretarded dispersion forces, electrostatic repulsions in a variety of electrolytes, structural and solvation forces (see below), and numerous studies of polymeric and biological systems. 

Figure Bl.20.8. DLVO-type forces measured between two silica glass surfaces in aqueous solutions of NaCl at various concentrations. The inset shows the same data in the short-range regime up to D = 10 mn. The repulsive deviation at short range (<2 nm) is due to a monotonic solvation force, which seems not to depend on the salt concentration. Oscillatory surface forces are not observed. With pemiission from [73].

The inner layer (closest to the electrode), known as the inner Helmholtz plane (IHP), contains solvent molecules and specifically adsorbed ions (which are not hilly solvated). It is defined by the locus of points for the specifically adsorbed ions. The next layer, the outer Helmholtz plane (OHP), reflects the imaginary plane passing through the center of solvated ions at then closest approach to the surface. The solvated ions are nonspecifically adsorbed and are attracted to the surface by long-range coulombic forces. Both Helmholtz layers represent the compact layer. Such a compact layer of charges is strongly held by the electrode and can survive even when the electrode is pulled out of the solution. The Helmholtz model does not take into account the thermal motion of ions, which loosens them from the compact layer. 

Surface force apparatus has been applied successfully over the past years for measuring normal surface forces as a function of surface gap or film thickness. The results reveal, for example, that the normal forces acting on confined liquid composed of linear-chain molecules exhibit a periodic oscillation between the attractive and repulsive interactions as one surface continuously approaches to another, which is schematically shown in Fig. 19. The period of the oscillation corresponds precisely to the thickness of a molecular chain, and the oscillation amplitude increases exponentially as the film thickness decreases. This oscillatory solvation force originates from the formation of the layering structure in thin liquid films and the change of the ordered structure with the film thickness. The result provides a convincing example that the SFA can be an effective experimental tool to detect fundamental interactions between the surfaces when the gap decreases to nanometre scale. 

When two such surfaces approach each other, layer after layer is squeezed out of the closing gap (Fig. 6.12). Density fluctuations and the specific interactions then cause an exponentially decaying periodic force the periodic length corresponds to the thickness of each layer. Such forces were termed solvation forces because they are a consequence of the adsorption of solvent molecules to solid surfaces [168], Periodic solvation forces across confined liquids were first predicted by computer simulations and theory [168-171], In this case, however, the experimental proof came only few years afterwards using the surface forces apparatus [172,173]. Solvation forces are not only an important factor in the stability of dispersions. They are also important for analyzing the structure of confined liquids. 

Now we turn to the situation when the electrode is negatively charged as shown in Figure 1.14. It is important to note that the solvated cations lie at the outer Helmholtz plane, unlike the situation in which the anions adsorb on the positive electrode surface. In terms of interaction forces the cation-water interactions are stronger than the negatively charged electrode - water interactions. In terms of more intimate mechanism the water molecules attached to the cation do not exchange with water molecules adsorbed on the electrode surface. The solvated cation is situated at the outer Helmholtz plane. 

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,... 

Measurement of the oscillatory solvation force became possible after the precise surface force apparatus had been constructed. This apparatus allowed to measure the surface forces in thin... 

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. 

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. 

When a metal electrode is placed in an electrolyte solution, an equilibrium difference usually becomes established between the metal and solution. Equilibrium is reached when the electrons left in the metal contribute to the formation of a layer of ions whose charge is equal and opposite to that of the cations in solution at the interface. The positive charges of cations in the solution and the negative charges of electrons in the metal electrode form the electrical double layer [4]. The solution side of the double layer is made up of several layers as shown in Fig. 2.7. The inner layer, which is closest to the electrode, consists of solvent and other ions, which are called specifically adsorbed ions. This inner layer is called the compact Helmholtz layer, and the locus of the electrical centers of this inner layer is called the inner Helmholtz plane, which is at a distance of di from the metal electrode surface. The solvated ion can approach the electrode only to a distance d2. The locus of the centers of the nearest solvated ion is called the outer Helmholtz plane. The interaction of the solvated ion with metal electrode only involves electrostatic force and is independent of the chemical properties of the ions. These ions are called non-specifically adsorbed ions. These ions are distributed in the 3D region called diffusion layer whose thickness depends on the ionic concentration in the electrolyte. The structure of the double layer affects the rate of electrode reactions. 

The main difference between air and liquid is that other forces start to intrude, especially double-layer forces at long range and solvation forces at ranges less than a nanometer. The double-layer forces can be seen quite easily in the surface force apparatus. At pH around 6 in potassium nitrate (KNOj) solutions of various concentrations, the force was a repulsion which increased exponentially as the gap closed. For higher concentrations of salt, the slope increased as expected from DLVO theory (see Chapter 10). Results are shown in Fig. 6.12 for several concentrations ofKNOj, ranging from 10 M to 10 M. 

In the presence of DMSO, the AG values are equal to 5-6 kJ/mol for the majority of samples of native human bone tissue and protein or mineral components of water in DMSO in the interfacial layers under action of the surface forces of the materials. Taking into account that the heat effect of water crystallization is 4 kJ/mol, one can conclude that the solvation energy of water in DMSO (J< 273 K) is higher than the interaction energy of water with functionalities of both protein and mineral components of bone tissue. 

As mentioned before, AFM can measure surface forces using two different operation modes, DC and AC. In the DC mode, one measures the deflection (AZ) of a cantilever as a function of the tip-sample distance (D) that is varied usually hy a piezoelectric transducer (Fig. 3). The tip-sample force is given by Hooke s law in terms of AZ, F = kAZ, where k is the spring constant of the cantilever. This force clearly depends on the tip-sample distance, D, which is given by D = Z — AZ, in the absence of sample deformation. Z in the expression is the displacement of the piezo and the one that can be controlled in the experiment. When the tip is far away from the sample (large D), the force is zero. When the tip approaches the sample, it experiences various forces, electrostatic, van der Waals, double-layer, solvation forces, and so on. This is the regime of interest in... 

When the tip approaches the substrate, it has to break the order or hydrogen-bonded network, so the measurement of solvation forces provides ordering information and discrete nature of solvent molecules near the solid surfaces. The most general type of solvation force is oscillatory, arising from the ordering of the solvent molecules into quasi-discrete layers near the surface. This solvation force can be approximately described by a cosine function with an exponentially decaying amplitude [45],... 

Another technique that was used to estimate the solvent content and the number of solvent molecules per EG monomers of PLL- -PEG coatings was recently developed by Pasche et al. and involves coUoidal-probe APM surface force measurements. The main assumption made in this technique is that the unperturbed PEG layer is compressed by the colloidal probe from a fully solvated state to a solvent-free, dry state. Thus, the decrease in the layer thickness upon compression is likely to reflect the amount of solvent absorbed within the polymer brush. The results of that study are in reasonable agreement with the findings of the present work. 

When we consider the long-range interactions between macroscopic bodies (such as colloidal particles) in liquids, we find that the two most important forces are the van der Waals forces and electrostatic forces, although in the shorter distance, solvation forces often dominate over both. In this section, some important types of surface forces are discussed, and it should be helpful for calculation of surface forces. 

Measurement of the oscillatory solvation force became possible after the precise SFA had been constructed [36]. This apparatus allowed measuring measure the surface forces in thin liquid films confined between molecularly smooth mica surfaces and in this way to check the validity of the DLVO theory down to thickness of about 5 A, and even smaller. The experimental results with nonaqueous liquids of both spherical (CCI4) or cylindrical (linear alkanes) molecules showed that at larger separations the DLVO theory is satisfied, whereas at separations on the order of several molecular diameters an oscillatory force is superimposed over the DLVO force law. In aqueous solutions, oscillatory forces were observed at higher electrolyte concentrations with periodicity of 0.22-0.26 nm, about the diameter of the water molecule [36]. As mentioned earlier, the oscillatory solvation forces can be observed only between smooth solid surfaces. 

Adsorption refers to the accumulation of any species from one of the continuous phases at the interface between two phases. If the solid-liquid (S/L) interface is in question [i.e., adsorption of a dissolved material (solute) is studied], the wetting of solid material (adsorbent) by the liquid (medium in which adsorbent is dispersed) and the solubdilty of solute in the given liquid (here, solvent) have to be considered in addition to adsorption. Simultaneous equilibria of adsorption, wetting, and solubility exist between the components (adsorbent, solvent, and solute). Competition of solvent and solute molecules for surface sites and also competition of surface and solvation forces for solute molecules are always present in the S/L adsorption systems. Therefore, a better understanding of adsorption from solutions requires that the interaction of a solute with a surface be characterized in terms of the frmdamental physical and chemical properties of aU the three components (solute, adsorbent, and solvent) of adsorption. 

The second model is the Gouy-Chapman model developed in 1910 (Gouy, 1903,1906 Chapman, 1913). In this model, the double layer is not as compact as in the Helmholtz rigid layer. The ions are assumed to be able to move in solution owing to thermal forces and thus the electrostatic interactions are in competition with Brownian motion. Figure 5.3 shows the charge distribution and potential from the electrode surface. The solvated ions interact with... 

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