Interfaces dilute solution-solid

A more complex line of approach is that involving the use of two-component liquid systems, the dilute component of which is surface active. Thus Fowkes and Harkins [14] measured the contact angles of aqueous solutions of n-butyl alcohol at various solid-air interfaces. The film pressure at the solution-solid interface was then given by ... 

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

The terminology of L-B films originates from the names of two scientists who invented the technique of film preparation, which transfers the monolayer or multilayers from the water-air interface onto a solid substrate. The key of the L-B technique is to use the amphiphih molecule insoluble in water, with one end hydrophilic and the other hydrophobic. When a drop of a dilute solution containing the amphiphilic molecules is spread on the water-air interface, the hydrophilic end of the amphiphile is preferentially immersed in the water and the hydrophobic end remains in the air. After the evaporation of solvent, the solution leaves a monolayer of amphiphilic molecules in the form of two-dimensional gas due to relatively large spacing between the molecules (see Fig. 15 (a)). At this stage, a barrier moves and compresses the molecules on the water-air interface, and as a result the intermolecular distance decreases and the surface pressure increases. As the compression from the barrier proceeds, two successive phase transitions of the monolayer can be observed. First a transition from the gas" to the liquid state. 

Figure 1.5 shows a schematic representation of the double layer at a planar solid-liquid interface. The potential drop across the Helmholz layer is shown as linear (in the presence of specific adsorption, it will not be completely linear), followed by a tailing-off of the potential into the diffuse layer. For concentrated solutions (>0.1 M) the diffuse layer is typically a nanometer or less, while for dilute solutions it may be tens or even hundreds of nanometers. 

Adsorption isotherms represent a relationship between the adsorbed amount at an interface and the equilibrium activity of an adsorbed particle (also the concentration of a dissolved substance or partial gas pressure) at a constant temperature. The analysis of adsorption isotherms can yield thermodynamic data for the given adsorption system. Theoretical adsorption isotherms derived from statistical and kinetic data, and using the described assumptions (see 3.1), are known only for the gas-solid interface or for dilute solutions of surfactants (Gibbs). Those for the system gas-solid are of a few basic types that can be thermodynamically predicted81. From temperature relations it is possible to calculate adsorption and activation energies or rate constants for individual isotherms. Since there are no theoretically founded equations of adsorption isotherms for dissolved surfactants on solids, the adsorption of gases on solides can be used as a starting point for an interpretation. 

The characterization of surface activity of fillers is obtained by use of several analytical techniques [1]. Examples of them are inverse gas chromatography [1, 2], the adsorption of a low molecular weight analog of elastomers [3], the adsorption of elastomer chains fi om dilute solutions [4], the wettability, viscosity of PDMS fluids in the boundary layer at the surface of solids [5], the determination of the specific surface area, and the analysis of surface groups [1]. It should, however, be mentioned that the results obtained by these methods do not provide direct information on the elastomer behavior at the interface, due to the use of small probe molecules or the presence of a solvent in the systems studied. 

Finally, it should be noted that calorimetric measurements can also be used to monitor adsorption phenomena at the solid-liquid interface (in a solvent). This method has been used to measure the adsorption heats evolved upon injection of dilute solutions of pyridine in alkanes ( -hexane, cyclohexane) onto an acidic solid itself in a slurry with -hexane. The amount of free base in solution is measured separately with a UV-Vis spectrometer, leading to an adsorption isotherm that is measured over the range of base addition used in the calorimetric titrations. The combined data from the calorimetric titration and adsorption measurements are analyzed simultaneously to determine equihbrium constants, quantities of sites per gram and acid site strengths for different acid sites on the solid. 

Hesselink FTh. Adsorption of polyelectrolytes from dilute solution. In Parfitt CD, Rochester CH, eds. Adsorption from Solution at the Solid—Liquid Interface. London Academic Press, 1983 277-412. 

Varoqui R. Structure and stability of weakly charged polyelectrolytes at a solid—liquid interface. In Schmitz KS, ed. Macroion Characterization from Dilute Solutions to Complex Fluids. Washington DC Amer Chem Soc, 1994 421-435. 

During equilibrium adsorption of long-chain flexible macromolecules from dilute solution onto a solid surface, only some chain segments are at the solid interface... 

Justification for giving prority to this interface is the fact that both experimental and theoretical studies of adsorption at the solid/liquid interface preceded those from the gaseous phase. Moreover, some equations of isotherms for adsorption at the solid/liquid interface, particularly, those referring to adsorption from the diluted solutions, are derived from the theoretical description of single gases and their mixtures on solid surfaces. 

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