Liquid microcalorimetry

In the case of water pollution, the estimation of adsorption affinity of potential solid adsorbent toward the specific pollutant can be done using the so-called liquid microcalorimetry. The instruments used for this purpose are differential heat flow microcalorimeters modified to allow continuous stirring of liquid samples. The adsorbate is added to both sample and reference cells simultaneously using a programmable twin syringe pump, linked to the calorimeter. The heat evolved as a result of adsorption can be obtained by integration of the area under the calorimeter signal, for each particular injection (dose). The output of typical microcalorimetric experiment of this type is shown in Fig. 10.9. 

An experiment of adsorption from the gas-phase, performed in microcalorimeter coupled with volumetric line can give a profile of Qdi/ versus the amount adsorbed, integral heats of adsorption, adsorption isotherms (adsorbed amounts vs. equilibrium pressure) and irreversibly absorbed amount of a chemisorbed gas the same stands for the adsorption from the liquid-phase, where the adsorbate (titrant) is added to both sample and reference ceUs simultaneously. The profile of differential heats versus the uptake of probe gives the data concCTning the amount, strength and distribution of the active sites. Besides, the values of initial heats of adsorption characterize the strongest sites active in adsorption process. For the sake of acidic/basic characterization of solids surface, the most commonly used gas-phase probes are ammonia, pyridine or some amines for the interaction with acidic sites. SO2 and CO2 are the probes used to notice and characterize the basic sites. In microporous solids, the accessibility of active sites is not the same for the molecules of different sizes. Therefore, many different probes can be applied to study acidity or basicity of same solid materials this approach brings additional information. For example, acidity of zeolites can be characterized by adsorption of ammonia, but also by adsorption of pyridine (from the gas phase) and aniline (from the liquid phase) [20-22], Liquid microcalorimetry can be also used for the determination of acidic character of solid adsorbent the common liquid-phase probe is aniline dissolved in n-decane [40]. 

High-performance liquid chromatography (HPLC) is the usual chemical method employed, although older methods, such as thin layer chromatography (TLC) can be of use. Microcalorimetry can also be of use [52]. 

This chapter will explore the relationship of thermodynamic and kinetic data as it pertains to characterizing the stability of various protein systems in the liquid state. Finally, from the wealth of information generated over the past few decades, it should be possible to assess the practical use of microcalorimetry for predicting stability. This technique used in combination with several other bio-analytical methods can serve as a powerful tool in the measurement of thermodynamic and kinetic phenomena.3-9 Attention will be given to limitations of the technique rendered from different applications as well as to areas where it is advantageous. Ultimately, the practical utility of this technique will rest with those familiar with the art. 

Figure 13.7 A formulation strategy using microcalorimetry aimed at deriving stable liquid candidates.

Quantitative investigation of recognition of this pair of liposomes was performed with isothermal titration microcalorimetry (ITC). It has been found that one-to-one binding between adenine and barbituric acid in the lipid/water/lipid interface occurs. However at T= 58°C, above the main lipid phase transition, the situation is different and no liposomal binding is detected. This is mainly due to the molecular disorder within the bilayer (liquid-disordered/liquid ordered phase coexistence) that limits the capacity of complementary moieties to bind, due to the weakening of the hydrogen bonds at these high temperatures. 

Experimental techniques of immersion microcalorimetry in pure liquid... 

In principle, to carry out immersion microcalorimetry, one simply needs a powder, a liquid and a microcalorimeter. Nevertheless, it was early realized that the heat effects involved are small and the sources of errors and uncertainties numerous. Many attempts have been made to improve immersion microcalorimetric techniques. Before commenting on this type of experiment, we describe the equipment and procedure which has been found by Rouquerol and co-workers to be of particular value for energy of immersion measurements (Partyka et al., 1979). 

The technique of immersing a known mass of outgassed solid, with no dissolution, in a given liquid and measuring the heat evolved, would appear to provide a means of determining A by a single measurement, provided that A u1 0 is known for the liquid-solid system. If the surface of the solid sample in the immersion cell is at least 1 m2, the amount of heat released is not difficult to measure with the microcalori-metric procedure described in Section 5.2.2. Thus, for the routine control of the specific surface areas of a series of well-defined samples, immersion microcalorimetry is a very useful technique. 

A particular advantage of using immersion microcalorimetry for the study of ultramicroporous materials is that the molecular entry into very fine pores takes place much more rapidly from the liquid phase than from a gas. There are two reasons for this difference gaseous diffusion may be slow (thermally activated) -especially at 77 K - and the higher liquid density also favours a more rapid molecular penetration. 

In another study, Mg-Al hydrotalcite catalysts with different Mg A1 molar ratios (0.6, 1.4, 2.2, 3.0) were characterized by microcalorimetry using CO2 in the gas phase and benzoic acid in toluene [95]. The calcined Al-rich sample (Mg A1 molar ratio of 0.6) possesses Lewis acid sites similar in strength to those found on AI2O3, but stronger than those found on the Mg-rich hydrotalcites. The liquid-phase basicity microcalorimetry measurements with benzoic acid in toluene correlated very well with the catalytic achvity for Michael addihons. 

Assessing microporosity by immersion microcalorimetry into liquid nitrogen or liquid argon... 

This study highlights the use of low temperature immersion microcalorimetry into liquid argon and nitrogen with respect to various carbon and silica samples. We suggest that this method gives a closer estimation of the real surface area in the case of microporous... 

We have previously shown that a most interesting information provided by immersion microcalorimetry into organic liquids was, in the case of carbons, a direct assessment of the internal surface area of the micropores [1]. This conclusion was based on calculations of adsorption potentials in micropores, on geometrical considerations and on microcalorimetric measurements on a number of activated carbons. It was only validated for carbons. 

This is why we thought it worthwhile to switch to immersion microcalorimetry into either liquid nitrogen or - even better - liquid argon, by making use of an isothermal, heat-flux, microcalorimeter, initially designed and built in our laboratory for the sake of gas adsorption experiments at 77 or 87 K. 

Combining gas adsorption experiments with immersion microcalorimetry shows the specific interest of the latter approach to assess the area of the surface wetted by the liquid. 

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]. 

In order to demonstrate that it is actually possible to observe thermal phenomena characteristic of supercooled liquids with our microcalorimetry approach, we have conducted measurements of the heat capacity of ASW samples contaminated with acetic acid. Figure 5 compares the thermogram of pure ASW to that of ASW/Acetic Acid mixture (10 1). While the heat capacity of the ASW/Acetic acid mixture undergoes a rapid increase in heat capacity at 175 K, the heat capacity of pure ASW remains nearly equal to that of crystalline ice. 

An adsorption isotherm is a necessary but not sufficient way of describing the thermodynamics of ionic surfactant adsorption because a full description of the phenomenon requires knowledge of mutual interactions between all the components of the system. Such opportunity is offered by flow and batch liquid adsorption microcalorimetry [25-30]. 

Like layer silicates, the porous palygorskite can also be organophilized. X-ray studies, however, do not reveal any structural changes in the organocomplexes, since cationic surfactants are adsorbed only on the external surfaces. The amount of surfactant bound by ion-exchange adsorption and the extent of organophilicity can be quantified by the liquid sorption studies and microcalorimetry [19-21]. 

The second way to determine adsorption enthalpy is the direct measurement by microcalorimetry. Several papers are devoted to the analysis of the various ways to define liquid adsorption enthalpies and to measure them [51-55]. 

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