Monolayer dispersion adsorption

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

Acidity, 27 284, 285 catalytic performance, 30 121 crystalline titanium silicates, 41 319-320 estimating, 37 166 heteropoly compounds, 41 139-150 ion exchange and, zeolites, 31 5-6 sulfate-supported metal oxides, 37 186-187 surface, monolayer dispersion, 37 34-35 tin-antimony oxide, 30 114-115, 125-1256 Acids, see also specific compounds adsorption of, on oxide surfaces, 25 243-245... 

Monocoordination, versus dicoordination, BOC-MP, 37 125-127 Monolayer-dispersed, 37 2-4 adsorption, 37 33-34 capacities, 37 13-14... 

The phenomenon of monolayer dispersion described above may well be attributable to solid/solid adsorption. However, for the phenomenon of monolayer dispersion, we wonder whether the analogy between liquid/ solid and solid/solid is so close as to justify borrowing a term such as wetting (27-31). 

The ethylene adsorption capacity increases with the content of CuCl and reaches the highest value at the point near the monolayer dispersion threshold. Similar plots have been reported by Zhao et al. (57) in the investigation of ethylene adsorption on Cu0/y-Al203 and by Duan et al. 

Langmuir and Freundlich adsorption models can be used to describe the adsorption behavior of dispersants on an abrasive surface. The Langmuir adsorption isotherm model assumes that the particle surface is homogeneously covered by the monolayer dispersant. It is expressed as follows ... 

The comparison of obtained data shows that at a given radius of aerosol particles of SAS solution, the rate adsorption constants of this aerosol at its adsorption from air are essentially higher than the rate adsorption constants from water bulk. This allows one to realize technically the flotation process using the close-pack ionized adsorption monolayer of surfactants, specially obtained at air bubble surface by aerosol dispersion and, thus, to control this process efficiently. 

Keywords surfactant, interface, surface, adsorption, monolayer, miscibility, compatibility, anti-static agent, diffusion, lubricant, foam, foam control agent, wetting agent, compatibilizer, dispersant, stabilizer, surfactant aggregates. 

Next, we consider the flexural properties of surfactant adsorption monolayers, which are important for the formation of small droplets, micelles, and vesicles in the fluid dispersions. The contributions of various interactions (van der Waals, electrostatic, steric) into the interfacial bending moment and the curvature elastic moduli are described. The effect of interfacial bending on the interactions between deformable emulsion droplets is discussed. 

In addition to the surface forces, hydrodynamic forces play an important role for the interactions in the real liquid films and colloidal dispersions. The hydrodynamic force is due to the viscous friction accompanying the expulsion of the liquid from the gap between two particles. When the particles are fluid (drops, bubbles), the fluidity of their surfaces (determined by the properties of the surfactant adsorption monolayers) can significantly... 

The modification of the fluid interfaces due to surfactant adsorption strongly influences the interactions between fluid particles (droplets, bubbles) in dispersions. Frequently a thin liquid film is formed in the zone of contact of two fluid particles. The contact angle at the periphery of such a film is a measure for the interaction of the two opposite surfactant adsorption monolayers. When the latter adhere to each other, a hysteresis of the contact angle is observed, irrespective of the fact that the fluid interfaces are molecularly smooth. The properties of the thin liquid films are important for the flocculation in dispersions and the deposition (attachment-detachment) of particles at surfaces see Sec. V. 

The presence of a surfactant also affects the hydrodynamic interactions between fluid particles in dispersions. When the surfactant is soluble in the continuous phase, the adsorption monolayers immobilize the drop (bubble) surfaces and decelerate the approach of two such particles. On the contrary, if the surfactant is dissolved in the droplets (in the disperse phase), it efficiently damps the surface tension gradients and accelerates the approach and (eventually) coalescence of the droplets. The processes of surfactant transfer from the continuous toward the disperse phase, or vice versa, also influence the droplet-droplet hydrodynamic interactions see Sec. Vn. 

Techniques applied to study polymer adsorption have to be sensitive enough to detect small mass ( l-5 mg/m ) included within adsorbed polymer monolayers. One way to increase the sensitivity of the measurements is to study polymer adsorption using dispersed particles. Sensitive techniques are also now available to study thin adsorbed polymer layers at planar surfaces. 

A monolayer can be regarded as a special case in which the potential is a square well however, the potential well may take other forms. Of particular interest now is the case of multilayer adsorption, and a reasonable assumption is that the principal interaction between the solid and the adsorbate is of the dispersion type, so that for a plane solid surface the potential should decrease with the inverse cube of the distance (see Section VI-3A). To avoid having an infinite potential at the surface, the potential function may be written... 

The strength of dispersion interaction of a solid with a gas molecule is determined not only by the chemical composition of the surface of the solid, but also by the surface density of the force centres. If therefore this surface density can be sufficiently reduced by the pre-adsorption of a suitable substance, the isotherm may be converted from Type II to Type III. An example is rutile, modified by the pre-adsorption of a monolayer of ethanol the isotherm of pentane, which is of Type II on the unmodified rutile (Fig. 5.3, curve A), changes to Type III on the treated sample (cf. Fig. 5.3 curve B). Similar results were found with hexane-l-ol as pre-adsorbate. Another example is the pre-adsorption of amyl alcohol on a quartz powder... 

The simplest mode of IGC is the infinite dilution mode , effected when the adsorbing species is present at very low concentration in a non-adsorbing carrier gas. Under such conditions, the adsorption may be assumed to be sub-monolayer, and if one assumes in addition that the surface is energetically homogeneous with respect to the adsorption (often an acceptable assumption for dispersion-force-only adsorbates), the isotherm will be linear (Henry s Law), i.e. the amount adsorbed will be linearly dependent on the partial saturation of the gas. The proportionality factor is the adsorption equilibrium constant, which is the ratio of the volume of gas adsorbed per unit area of solid to its relative saturation in the carrier. The quantity measured experimentally is the relative retention volume, Vn, for a gas sample injected into the column. It is the volume of carrier gas required to completely elute the sample, relative to the amount required to elute a non-adsorbing probe, i.e. 

Surfactants greatly improve the performance of trans-cinnamaldehyde as a corrosion inhibitor for steel in HCl [741,1590,1591]. They act by enhancing the adsorption at the surface. Increased solubility or dispersibility of the inhibitor is an incidental effect. N-dodecylpyridinium bromide is effective in this aspect far below its critical micelle concentration, probably as a result of electrostatic adsorption of the monomeric form of N-dodecylpyridinium bromide. This leads to the formation of a hydrophobic monolayer, which attracts the inhibitor. On the other hand, an ethoxylated nonylphenol, a nonionic surfactant, acts by incorporating the inhibitor into micelles, which themselves adsorb on the steel surface and facilitate the adsorption of trans-cinnamaldehyde. 

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

Of special interest in liquid dispersions are the surface-active agents that tend to accumulate at air/ liquid, liquid/liquid, and/or solid/liquid interfaces. Surfactants can arrange themselves to form a coherent film surrounding the dispersed droplets (in emulsions) or suspended particles (in suspensions). This process is an oriented physical adsorption. Adsorption at the interface tends to increase with increasing thermodynamic activity of the surfactant in solution until a complete monolayer is formed at the interface or until the active sites are saturated with surfactant molecules. Also, a multilayer of adsorbed surfactant molecules may occur, resulting in more complex adsorption isotherms. 

Before our theory was fully developed, extensive work by J. Koral in cooperation with R. Ullman (15) confirmed in detail and with considerable accuracy all previously known features. They ascertained, in addition, the particulars of the adsorption isotherms for a number of polymers and dispersed adsorbates and established the remarkable degree to which most isotherms could be approximated by 2-parameter equations, like Langmuir s isotherm for monolayers of small molecules. They found the dependence of the adsorption on MW to be weak and determined the area per adsorbed molecule. 

The charge density, Volta potential, etc., are calculated for the diffuse double layer formed by adsorption of a strong 1 1 electrolyte from aqueous solution onto solid particles. The experimental isotherm can be resolved into individual isotherms without the common monolayer assumption. That for the electrolyte permits relating Guggenheim-Adam surface excess, double layer properties, and equilibrium concentrations. The ratio u0/T2N declines from two at zero potential toward unity with rising potential. Unity is closely reached near kT/e = 10 for spheres of 1000 A. radius but is still about 1.3 for plates. In dispersions of Sterling FTG in aqueous sodium ff-naphthalene sulfonate a maximum potential of kT/e = 7 (170 mv.) is reached at 4 X 10 3M electrolyte. The results are useful in interpretation of the stability of the dispersions. 

---