Adsorption from solution, heat

The adsorption of nonelectrolytes at the solid-solution interface may be viewed in terms of two somewhat different physical pictures. In the first, the adsorption is confined to a monolayer next to the surface, with the implication that succeeding layers are virtually normal bulk solution. The picture is similar to that for the chemisorption of gases (see Chapter XVIII) and arises under the assumption that solute-solid interactions decay very rapidly with distance. Unlike the chemisorption of gases, however, the heat of adsorption from solution is usually small it is more comparable with heats of solution than with chemical bond energies. 

There are numerous references in the literature to irreversible adsorption from solution. Irreversible adsorption is defined as the lack of desotption from an adsoibed layer equilibrated with pure solvent. Often there is no evidence of strong surface-adsorbate bond formation, either in terms of the chemistry of the system or from direct calorimetric measurements of the heat of adsorption. It is also typical that if a better solvent is used, or a strongly competitive adsorbate, then desorption is rapid and complete. Adsorption irreversibility occurs quite frequently in polymers [4] and proteins [121-123] but has also been observed in small molecules and surfactants [124-128]. Each of these cases has a different explanation and discussion. 

In writing the present book our aim has been to give a critical exposition of the use of adsorption data for the evaluation of the surface area and the pore size distribution of finely divided and porous solids. The major part of the book is devoted to the Brunauer-Emmett-Teller (BET) method for the determination of specific surface, and the use of the Kelvin equation for the calculation of pore size distribution but due attention has also been given to other well known methods for the estimation of surface area from adsorption measurements, viz. those based on adsorption from solution, on heat of immersion, on chemisorption, and on the application of the Gibbs adsorption equation to gaseous adsorption. 

Surface Area Determination The surface-to-volume ratio is an important powder property since it governs the rate at which a powder interacts with its surroundings. Surface area may be determined from size-distribution data or measured directly by flow through a powder bed or the adsorption of gas molecules on the powder surface. Other methods such as gas diffusion, dye adsorption from solution, and heats of adsorption have also been used. It is emphasized that a powder does not have a unique surface, unless the surface is considered to be absolutely smooth, and the magnitude of the measured surface depends upon the level of scrutiny (e.g., the smaller the gas molecules used for gas adsorption measurement the larger the measured surface). 

The thermodynamic characteristic of adsorption from solutions can be determined from the dependence of adsorption on temperature. But the determination of adsorption isotherms from solution at different temperatures is the rather complicate problems. Liquid chromatography may be very useful method for the determination of thermodynamic characteristics of adsorption at small coverage [11] because of the measurement of retention volume (the Henry constant) at different temperatures of the chromatographic columns makes it possible to calculate the heats of adsorption and the differential standard change entropy of adsorption from ... 

While for hydrophobic groups the heats of adsorption are positive, for most of hydrophilic groups the heats of adsorption are negative and serves to measure the heat of desorption from the surface under a given eluent composition. With the increasing of ethanol concentration in eluent the heats of adsorption of hydrophobic groups decrease as well as for hydrophilic groups the heats of desorption. These results is particularly attractive because it is impossible to experimentally measure these values in adsorption from solutions. 

Two questions are raised by the tide of this main section and deserve being answered immediately, i.e., (i) why are we deahng with calorimetry and (ii) is immersion calorimetry reserved to pure hquids The answers are that (i) the heat exchanged on wetting is a precious data to be exploited, for sure (as we shall see), whereas (ii) the way devised to carry out a clean and precise immersion calorimetry experiment requests a pure hquid and is not adapted for the study of adsorption from solutions. In this section, we should certainly pay tribute to Zettlemoyer [2], who, with his coworkers, was the first to extensively apply immersion calorimetry for the study of adsorbents. 

Eltekov, Pavlova, Shikalova, Bogacheva and co-workers (Moscow State University Institute of Physical Chemistry, the U.S.S.R. Academy of Sciences, Moscow) (17, 290, 297-307) investigated the effect of the chemical nature and the dimensions of the pores in silica adsorbents on the adsorption from solutions. The adsorption of benzene from n-hexane solutions on silica gels with a hydroxylated and a dehydroxylated surface was studied. Dehydroxylation sharply lowered the heat of adsorption of benzene. A comparison was made of the adsorption isotherms for a series of n-hexane solutions of aromatic hydrocarbons on hydroxylated silica gel. The intermolecular interaction of aromatic hydrocarbons with this adsorbent is stronger than the interaction of saturated hydrocarbons because aromatic hydrocarbons form hydrogen bonds with the silanol groups on the SiC>2 surface. 

Because of the almost infinite number of structural variations that protein and solvent molecules may undergo during adsorption, an ab initio statistical computation of A gS is practically impossible. At constant pressure, which is usually the case for adsorption from solution, equals the heat of adsorption ... 

FIGURE 4.15 Immersion heats of Carbosieve-s as a function of precoverage. A = water = methanol O = 2-propanol. (After Zettlemoyer, A.C., Pendelton, R, and Micale, F.J., in Adsorption from Solution, R.H. Ottewill, C.H. Rochester, and A.L. Smith, eds.. Academic Press, New York, 1983, p. 113. With permission.)... 

Adsorption enthalpies were measured in a Thermal Activator Monitor (TAM), an isothermal number 2277 microcalorimeter from LKB, Sweden. It contains a 25 ml stainless steel titration cell, fitted into a single detector measuring cylinder. The cell, with a reference ampoule, was especially designed for mixing liquids and adsorption from solution. For more details, and the execution of the measurements, see [6]. Basically, the heat evolved is measured by adding the surfactant solution to the kaolinite dispersion, where the heat of dilution is subtracted as the blank. In this way a plot q isiT) of the heat evolved as a function of the amount adsorbed F is obtained. 

The theory presented here was the basis for the investigations dealing with numerous problems coimected with adsorption from binary solutions the determination of energy distribution function from the excess adsorption isotherms [154,215], the role of adsorbent heterogeneity and molecular interactions in the adsorption process at Uquid-solid interface [202,203,216-219], the influence of the difference in molecular sizes of components on the adsorption equilibrium [212,220,221], the multilayer effects in adsorption from solutions [222-225], the relations between heats of immersion and excess adsorption isotherms [216,219,225], and many other interesting subjects. 

Mjcracalorimetry, also nanocalorimetry to follow the recent trends in thermal instrumentation and analysis, is a measuring technique that can be used to study interfacial phenomena occurring at the Solid-Liquid interface. Immersion of a solid in a pure liquid or a solution, wetting of a solid initially in contact with a gas or vapour by a liquid, adhesion between two condensed phases upon their molecular contact are examples of exothermic phenomena which are accompanied by significant heat evolvement. Competitive adsorption from solution is an important exception to the exothermicity of interfacial phenomena. This is because certain components of the... 

R. Denoyel, F. Rouquerol, J. Rouquerol, Interest and requirements of liquid-flow microcalorimetry in the study of adsorption from solution in the scope of tertiary oil recovery, in Adsorption from Solution, ed. by C. Rochester (Academic Press, London, 1982), pp. 1-10 G.W. Woodbury Jr, L.A. NoR, Heats of adsorption from flow calorimetry relationships between heats measured by different methods. CoUoids Surf. 28, 233-245 (1987). doi 10. 1016/0166- 6622(87)80187-7... 

Adsorptive Properties. Substances such as silica gel and activated charcoal can be used to collect (adsorb) certain solids from solution. The adsorber bed may be discarded when depleted or recycled by washing and heating. 

Alternatively, peak asymmetry could arise from thermal effects. During the passage of a solute along the column the heats of adsorption and desorption that are evolved and adsorbed as the solute distributes itself between the phases. At the front of the peak, where the solute is being continually adsorbed, the heat of adsorption will be evolved and thus the front of the peak will be at a temperature above its surroundings. Conversely, at the rear of the peak, where there will be a net desorption of solute, heat will be adsorbed and the temperature or the rear of the peak will fall below its surroundings. 

Mills, A. C., and J. W. Biggar, Adsorption of 1,2,3,4,5,6-hexachlorocyclohexane from solution The differential heat of adsorption applied to adsorption from dilute solutions on organic and inorganic surfaces , J. Coll. Int. Sci., 29, 720-731 (1969b). 

In reality, additional sources of zone broadening include the finite width of the injected band (Equation 23-32), a parabolic flow profile from heating inside the capillary, adsorption of solute on the capillary wall (which acts as a stationary phase), the finite length of the detection zone, and mobility mismatch of solute and buffer ions that leads to nonideal elec-... 

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