Liquid flow calorimetry

Most carbon blacks have a low affinity for water, i.e. they are hydrophobic. However, the level of hydrophobicity is reduced by the presence of chemisorbed oxygen and certain functional groups (Walker and Janov) 1968 Bradley et al., 1995). The relative extents of the polar and hydrophobic areas of carton blacks have been studied by various methods (Boehm, 1994), including energy of immersion measurements (Barton and Harrison, 1975) and by liquid flow calorimetry (Groszek,... 

It is evident that liquid flow calorimetry should be regarded as a complementary -rather than an alternative- approach to the BET method. Indeed, there is considerable scope for further work on the characterization of the adsorption site energy distribution of a wide range of adsorbents and advanced materials (e.g. catalysts, ceramics and molecular sieves). 

No other readily available method can offer the same scope, reliability and accuracy as gas adsorption. Adsorption from solution measurements are relatively easy to carry out, but often difficult to interpret they cannot be recommended for general use, but are essential for some applications involving treatment of liquid media. Although enthalpies of immersion are more difficult to determine, they can provide useful information provided that isotherm data are also available. Liquid flow calorimetry is becoming popular for the characterization of powders and porous solids, but the technique requires refinement if it is to be used to obtain thermodynamic data. 

The first case certainly corresponds to the study of the thermodynamic reversibility of adsorption onto solids from binary solutions. Liquid-flow calorimetry measurements usually provide clear, unambiguous arguments for irreversible character of the phenomenon in numerous systems. An example of such systems is illustrated in Fig.6.25. With non-porous Graphon possessing a very small number of surface polar sites, the adsorption of Geo fullerene from toluene is completely reversible. In the case of porous active carbons, the phenomenon is only partially reversible, the degree of reversibility being evaluated from the difference between the values of Adpih measured for the adsorption and desorption stage. 

In the liquid-flow calorimetry experiment, the purified adsorbent bed remains in contact with a stock solution of constant composition. It is clear that the environment of the liquid phase does not change during the measurement. This is an important advantage of the flow calorimetry, especially in the case of solid-solution systems containing electrified interfaces. The study of ions adsorption from aqueous solutions... 

Figure 6.27 presents the thermogram resulting from adsorption of a heavy metal cation from aqueous solution on the negatively charged surface of Spherosil registered during liquid-flow calorimetry measurements. 

Flow calorimetry is another technique which has been developed in recent years for the study of interactions at the liquid/solid interface and has been used also as an indirect means of determining surface areas. Some flow calorimeters are of relatively simple design whereas others are more sophisticated it is not surprising to find that the quality of the thermal data is dependent on the type of equipment and the modus operandi. 

Heat Flow calorimetry In a flow calorimeter, the sample is a flowing liquid. 

Liquid-flow microcalorimefry is a reliable method to measure simultaneously the enthalpy changes and amounts of adsorption under dynamic conditions. Calorimetry experiments may be carried out in two different ways by following a pulse or saturation operating mode [64, 78-83]. In the pulse mode, small aliquots of a stock solution at a known concentration are injected into the carrier liquid (pure solvent) flowing through the adsorbent bed placed inside the calorimetric cell. In this case, the calorimetric system contains an additional loop injection facility (a manual injection valve with appropriate injection loops). The interpretation of the enthalpy data obtained is straightforward only when the whole amount of the solute injected is irreversibly adsorbed on the solid surface. 

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... 

Exceptions to the practice of not adding buffer to plant tissue samples are numerous. Root tissues must be maintained moist and consequently are examined while in close contact with buffer-saturated filter paper disks [34, 35]. Immersing root tissue in water does not yield satisfactory data. Callus tissue cells and cell cultures are commonly measured while being maintained on agar or in liquid media. Marine tissues may be examined in water suspension by flow calorimetry, but unstirred samples rapidly settle to the bottom of the ampule and become O2 limited during batch calorimetry. 

Solution calorimetry covers the measurement of the energy changes that occur when a compound or a mixture (solid, liquid or gas) is mixed, dissolved or adsorbed in a solvent or a solution. In addition it includes the measurement of the heat capacity of the resultant solution. Solution calorimeters are usually subdivided by the method in which the components are mixed, namely, batch, titration and flow. 

Various flow calorimeters are available connnercially. Flow calorimeters have been used to measure heat capacities, enthalpies of mixing of liquids, enthalpy of solution of gases in liquids and reaction enthalpies. Detailed descriptions of a variety of flow calorimeters are given in Solution Calorimetry by Grolier [17], by Albert and Archer [18], by Ott and Womiald [H], by Simonson and Mesmer [24] and by Wadso [25]. 

Recent developments m calorimetry have focused primarily on the calorimetry of biochemical systems, with the study of complex systems such as micelles, protems and lipids using microcalorimeters. Over the last 20 years microcalorimeters of various types including flow, titration, dilution, perfiision calorimeters and calorimeters used for the study of the dissolution of gases, liquids and solids have been developed. A more recent development is pressure-controlled scamiing calorimetry [26] where the thennal effects resulting from varying the pressure on a system either step-wise or continuously is studied. 

Figure 6.3. Levitation of a molten metal in a radio-frequency field. The coil consists of water-cooled copper tubes. The counter winding above the sample stabilizes levitation. The same coils (and possibly additional ones) act as the induction heater. This technique has been applied to container-less melting and zone refining of metals and for drop calorimetry of liquid metals. It can be also used to decarburize and degas in ultrahigh vacuum mono-crystalline spheres of highly refractory metals (adapted from Brandt (1989)). The arrows indicate the instantaneous current flow directions in the inductors.

Other approaches for measuring thermal resistance or conductivity of fibers and fabrics include the use of calorimetry (28), thermal flow through a heat sink of known emissivity (2 ), immersion of fibers or fabrics in liquids of known thermal conductivity (30), and measurement of the rate of cooling of textiles or insulating materials by a Cenco-Fitch apparatus (2). 

Calorimetry the science of measuring heat flow. (9.4) Capillary action the spontaneous rising of a liquid in a narrow tube. (16.2)... 

The porous structure of active carbons can be characterized by various techniques adsorption of gases (Ni, Ar, Kr, CO ) [5.39] or vapors (benzene, water) [5,39] by static (volumetric or gravimetric) or dynamic methods [39] adsorption from liquid solutions of solutes with a limited solubility and of solutes that are completely miscible with the solvent in all proportions [39] gas chromatography [40] immersion calorimetry [3,41J flow microcalorimetry [42] temperature-programmed desorption [43] mercury porosimetry [36,41] transmission electron microscopy (TEM) [44] and scanning electron microscopy (SEM) [44] small-angle x-ray scattering (SAXS) [44] x-ray diffraction (XRD) [44]. 

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