Liquid adsorption equilibrium measurement

Liquid chromatography (LC) has been, in the past decade, increasingly used for the measurement of liquid adsorption equilibrium (14,... 

Equilibrium behavior of adsorbents is usually determined as constant tenperature isotherms. Valenzuela and Myers (1984,1989) present the most extensive conpilations of isotherm data, and are the best entry point into the very large literature on adsorption equilibrium measurements and theories. Basmadjian (1986) has extensive data on water isotherms from gases and liquids. Dobbs and Cohen 119801 and Faust and Aly (19871 present adsorption data for common pollutants. 

The maximum-likelihood method is not limited to phase equilibrium data. It is applicable to any type of data for which a model can be postulated and for which there are known random measurement errors in the variables. P-V-T data, enthalpy data, solid-liquid adsorption data, etc., can all be reduced by this method. The advantages indicated here for vapor-liquid equilibrium data apply also to other data. 

Adsorption equilibrium has an extra degree of freedom compared to conventional vapor-liquid equilibrium. This extra degree of freedom increases the difficulty in experimental measurements. It is difficult to find enough experimental data on binary equilibria in the adsorption literature. On the other hand, pure component isotherm measurement is so common that commercial push-button systems are available in the market for over a decade [1]. 

Adsorption Chromatography. The principle of gas-solid or liquid-solid chromatography may be easily understood from equation 35. In a linear multicomponent system (several sorbates at low concentration in an inert carrier) the wave velocity for each component depends on its adsorption equilibrium constant. Thus, if a pulse of the mixed sorbate is injected at the column inlet, the different species separate into bands which travel through the column at their characteristic velocities, and at the outlet of the column a sequence of peaks corresponding to the different species is detected. Measurement of the retention time (tl) under known flow conditions thus provides a simple means of determining the equilibrium constant (Henry constant) ... 

But the application of the LC technique for the measurement of liquid diffusion in molecular sieves was rather limited (18, 19). The recently developed LC technique using a commercial HPLC system (13), with many advantages over the conventional batch techniques, enables us to determine liquid phase diffusion and adsorption equilibrium in molecular sieve crystals in a simpler, more accurate and rapid way. 

This method obviously does not provide an isotherm, because the adsorption is measured at only one equilibrium point—saturation adsorption—and this point has very little practical significance. The saturation adsorption, when expressed as milliliters of liquid, is reported to be almost identical for a number of different substances.1 2... 

AH estimations of the pore size of a material from its gas adsorption isotherm measurements are based on the well-known Kelvin equation suggested more than 100 years ago. It considers an equilibrium between the vapor phase and the bulk liquid at a constant temperature and relates the relative vapor pressure p/p to the radius r of the convex (plus) or concave (minus) spherical meniscus of the Hquid placed in a capillary ... 

In this chapter, we will discuss how the chemical and physical properties of substances at interfaces differ from those in the bulk. For quantitative description, quantities like surface tension and surface energy have to be introduced. With the help of these quantities, phenomena known from everyday life like the lotus effect can be explained. However, perhaps you are more interested to learn how detergents clean Then have a look at Sect. 16.3 which deals with the adsorption on liquid surfaces. The next section covers the adsorption on solid surfaces and the variation of the extent of coverage with pressure or concentration of the substance to be adsorbed. Langmuir s isotherm, the simplest description of such an adsorptiOTi process, is deduced by kinetic interpretation of the adsorption equilibrium. Alternatively, it can be derived by introducing the chemical potential of free and occupied sites and cmisideiing the equilibrium condition. In the last part of the chapter, some important applications such as surface measurement and adsorption chromatography are discussed. 

To measure an adsorption Isotherm, the oscillation frequency was monitored as the computer adjusted the partial pressure p of Nj In the gas phase over the device at 77 K. This was done by controlling the relative flow rates of a He mix-down stream and a Nj carrier stream. Since He Is nonadsorbing at liquid Nj temperature (16), p -0 for the mix-down stream, while for the carrier stream, p - p, since the device temperature Is maintained at the boiling point of N2. The value of p/p, was incremented every 3 sec from a value of 0 at the start of the run to a value around 0.95 and then back to 0. It was found that two hours for a full adsorption Isotherm was sufficiently slow to maintain adsorption equilibrium (10). 

Because the measurement of a contact angle must involve some movement of the wetting line, it is possible, or even probable, that the act of spreading of the hquid will displace certain surface equilibria that will not be reestablished over the time frame of the experiment. For example, the displacement of a second fluid may result in the estabhshment of a nonequilibrium situation in terms of the adsorption of the various components at the solid-liquid, solid-fluid 2, and liquid-fluid 2 interfaces. Time will be required for adsorption equilibrium to be attained, and it may not be attained during the time of the contact angle measurement if the transport and adsorption-desorption phenomena involved are slow. The kinetic effect may be especially significant for solutions containing surfactants, polymers, or other dissolved adsorbates. 

Most spraying processes work under dynamic conditions and improvement of their efficiency requires the use of surfactants that lower the liquid surface tension yLv under these dynamic conditions. The interfaces involved (e.g. droplets formed in a spray or impacting on a surface) are freshly formed and have only a small effective age of some seconds or even less than a millisecond. The most frequently used parameter to characterize the dynamic properties of liquid adsorption layers is the dynamic surface tension (that is a time dependent quantity). Techniques should be available to measure yLv as a function of time (ranging firom a fraction of a millisecond to minutes and hours or days). To optimize the use of surfactants, polymers and mixtures of them specific knowledge of their dynamic adsorption behavior rather than equilibrium properties is of great interest [28]. It is, therefore, necessary to describe the dynamics of surfeictant adsorption at a fundamental level. The first physically sound model for adsorption kinetics was derived by Ward and Tordai [29]. It is based on the assumption that the time dependence of surface or interfacial tension, which is directly proportional to the surface excess F (moles m ), is caused by diffusion and transport of surfeictant molecules to the interface. This is referred to as the diffusion controlled adsorption kinetics model . This diffusion controlled model assumes transport by diffusion of the surface active molecules to be the rate controlled step. The so called kinetic controlled model is based on the transfer mechanism of molecules from solution to the adsorbed state and vice versa [28]. 

The positive adsorption of a solute at the S—MO interfaee results whenever the ehemieal potential ehange (at eonstant temperature and pressure) is negative for a solute moved from a bulk solution and bonded to or adsorbed at a S—MO interfaee. The ehemieal potential imbalanee diminishes as solute adsorption and eoneentration ehanges at the S—MO interfaee. The eomplete disappearanee of a ehemieal potential differenee (at eonstant temperature and pressure) for the solute in the bulk liquid and the S—MO interfaee serves as the definition of adsorption equilibrium for the solute. This singular definition of adsorption has produeed a large number of adsorption models whieh strive to translate the ehemieal potential model of equilibrium adsorption into measurable variables and eonstants. Many of these adsorption equations will be examined in detail later in this ehapter. 

As a technique for selective surface illumination at liquid/solid interfaces, TIRF was first introduced by Hirschfeld(1) in 1965. Other important early applications were pioneered by Harrick and Loeb(2) in 1973 for detecting fluorescence from a surface coated with dansyl-labeled bovine serum allbumin, by Kronick and Little(3) in 1975 for measuring the equilibrium constant between soluble fluorescent-labeled antibodies and surface-immobilized antigens, and by Watkins and Robertson(4) in 1977 for measuring kinetics of protein adsorption following a concentration jump. Previous rcvicws(5 7) contain additional references to some important early work. Section 7.5 presents a literature review of recent work. 

Typically, adsorption isotherms are generated using a batch experiment at a fixed temperature and a fixed feed composition. These experiments include exposing a known amount of adsorbent to a known concentration of adsorbate at a constant temperature. Once equilibrium is established, the net adsorbate concentration in the liquid is measured. This process is repeated at multiple adsorbate concentrations and temperatures. A plot of adsorbate loading (g adsorbate/g adsorbent) versus adsorbate concentration reveals the adsorption isotherm with the shape of the isotherm determining the suitability of a particular adsorbent for a particular system [20]. 

Adsorption Methods. Five grams of hematite were first conditioned in 0.001 M NaCl at pH 4.1. After the SDS had been added to the slurry and the pH adjusted as required, the samples were conditioned on a rotating shaker for two hours. The solutions were then centrifuged, and the supernatant liquid analyzed for its SDS content. The amount of SDS adsorbed was calculated as the difference between the initial amount added and the residual amount measured. Experimental results showed that two hours was sufficient time for equilibrium to be reached. Somasundaran ( ) observed similar equilibrium adsorption times for sulfonate adsorption on aluminum oxide. 

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