Liquid-phase adsorptions adsorbent characteristics

Liquid-phase adsorption characteristics examined by atomic force microscopy (AFM) were compared for two pyridine base molecules, pyridine and d-picoline. on (010) surfaces of two natural zeolites, heulandite and stilbite. These adsorption systems formed well-ordered. two-dimensioncJ (quasi-)hexagonal adlayers. The 2D lattice structures of the ordered adlayers w ere dependent on the adsorbate/substrate combinations. Although there existed certain habit in the orientation of the 2D lattice unit vector of the adsorbed phase with respect to the substrate(OlO) lattice vectors, the molecular arrays w ere incommensurate with the substrate atomic arrangements. 

Liquid phase zeoHtic separation includes two main events adsorption and desorption. Adsorption of an adsorbate (liquid component being adsorbed by solid) onto zeolitic adsorbent is dictated by the characteristics of the adsorbate-adsorbent interaction. A zeolitic adsorbent is a crystalline porous solid having particular characteristics (see Chapter 2). When immersed in a liquid mixture, the porous... 

Adsorption from solution involves the transfer of soluble species from the liquid phase to the surface of an adsorbent, a transfer which is governed by specific system dynamics and which results in a characteristic equilibrium distribution, or phase partitioning, of a solute. In the case of microporous adsorbents such as activated carbon, the uptake of solute consists of the four basic steps illustrated schematically in Figure 4 I) advective or bulk... 

Mary variables have to be considered during the design and operation of liquid phase sorption systems. Generally, both data on the trratment conditions (concentration of adsorbate, temperature, pH, flow rates, and pressure drop), and characteristics of adsorbate (molecular mass, soliirility, polarity) should be taken into account. Additionally, the adsorption isotherms, costs aralysis, and possibility of reactivation should be taken into consideration during the selection of adsmbent of the optimum efficiency. Adsorption behaviors for a variety of organic cempounds are summarized in Table 5. 

It is also convenient to combine studies of polymer interactions with solid substrates with studies of the adsorption characteristics of the organic components themselves. Such an approach has much to offer in adhesion research and the basis of studies of adsorption from a liquid phase and its applicability in adhesion has been discussed in detail elsewhere [7] so it will not be treated in depth here. A brief overview will, however, provide a background to this approach. The determination of gas-phase adsorption isotherms is a well-known methodology in surface chemistry in this manner it is possible to describe adsorption as following Langmuir or other characteristic adsorption types. The conventional method of studying the adsorption of molecules from the liquid phase is to establish the depletion of the adsorbate molecule from the liquid phase. However, as first pointed out by Castle and Bailey [8], with the advent of surface analysis methods it is now... 

There is a sharp separation between adsorbed water and liquid water, in which an adsorbent may be suspended and dissolved. In adsorbed water the structure is imposed by the adsorption field, while in liquid water the (dis)order is the one characteristic of the bulk phase. This state of affairs suggests that the adsorbed-to-liquid transition is a phase transition. On another side, water on many surfaces continuously undergoes adsorbed-to-liquid transition and vice versa. Familiar examples of adsorbents where the state of water frequently cycles between the adsorbed and liquid phases are soils and the skin of terrestrial mammalians. It is also noted that the formation of liquid water in clouds may occur via heterogeneous nucleation (i.e., via multilayer adsorption), either spontaneously on dust particles or artificially on Agl crystals formed by condensation of sublimated Agl,... 

Further simplification is possible when it is assumed that the two components, a and b, have similar adsorption characteristics on the adsorbent employed. For instance, the intraparticle diffusion coefficients, Dt, of both components may be approximated as the same when the two peaks are adjacent, then resolution for liquid phase can be written as... 

Activated carbons are widely used as adsorbents in either the gas or the liquid phase, and also as catalysts and catalyst support. In some cases their adsorption behaviour depends basically on their textural characteristics, i.e., porous structure and pore volume. As an example carbon molecular sieves (CMS) are a kind of activated carbons that make use of a narrow pore size distribution, of a few angstroms in diameter, to selectively separate gas mixtures [1], such as N2/O2, CO2/CH4, C3H6/C3H8 and some others. But also the surface chemistry can condition in many cases [2,3] the adsorption behaviour, as well as the activity as catalyst and catalyst support [4, 5]. This means that the adsorption properties of the activated carbons cannot be easily explained only on the basis of textural characteristics (surface area and pore size distribution) the nature of the chemical surface must also be taken into account. Therefore, for a complete characterization of the activated carbon surface, textural and chemical characteristics must be assessed. 

For most adsorption experiments the temperature at which the measurements are made is less than the triple point of the gas being used but above its freezing point. This being the case, one would normally expect that the adsorbate characteristics resemble the liquid phase rather than the solid phase of the adsorptive. This is the normal assumption used for most adsorption theories. The principle measurement performed as an adsorption experiment is the measurement of the adsorption isotherm. The adsorption isotherm is the measurement of amount adsorbed versus adsorptive pressure at constant temperature. This is the easiest measurement to make. Another type of measurement is calorimetry. One form of calorimetry measures the amount of heat evolved as the adsorptive is adsorbed. Another form measures the heat capacity of the adsorbate. There are various forms of calorimetry but the most accurate methods are very difficult to perform and only a few examples are available in the literature. Another form of calorimetry, which is easier to perform, is scanning calorimetry. This calorimetry form is a good tool to determine qualitative features of the adsorption and to yield a fair indication of the physical quantities. 

Performance of the adsorption dependents on the type of adsorbent used. The materials are versatile. This versatility allows the sorbent to be used under different forms, from insoluble beads, to gels, sponges, capsules, films, membranes or fibers. Materials are available in a variety of stmc-tures with a variety of properties. The surface of contact between any sorbent and the liquid phase plays an important role in the phenomena of sorption. The efficiency of adsorption depends on physicochemical characteristics such as porosity, surface specific area and particle size of sorbents. A fundamentally important characteristic of good adsorbents [188, 117] is their high porosity and consequent larger surface area with more specific adsorption sites. 

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