Pure-solvent calorimetry

VDW terms (C , Pure solvent properties [56] Vapor pressure, calorimetry,... 

In the batch calorimetry method, the adsorbent is initially kept in suspension in the pure solvent by means of continuous stirring. The solution is then introduced by... 

In flow calorimetry the enthalpy change that is measured is that which occurs as the adsorbant changes from being in equilibrium with a known fluid, e.g., pure solvent, to that of a second known fluid, e.g., a solution of an adsorbate in solvent. Therefore, the initial and final states are relatively well defined and the enthalpy can be more easily attributed to known adsorption-desorption processes taking place at the surface of the adsorbent. 

If the immersion experiment is repeated at different solution concentrations, the enthalpy isotherm of immersion, A Hvs. Ci, is obtained. For immersion calorimetry, the presentation of the results is often more appropriate in terms of the relative enthalpy of immersion, AnH, which, for dilute solutions, is equal to the integral enthalpy of displacement A2 Hi, more generally obtained Ifom batch or flow sorption microcalorimetric measurements. The relative enthalpy of immersion is defined as the enthalpy of immersion in the solution minus the enthalpy of immersion in pure solvent 2 ... 

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. 

Another method to obtain enthalpies of formation of compounds is by solute-solvent drop calorimetry. This method was pioneered by Tickner and Bever (1952) where the heat formation of a compound could be measured by dissolving it in liquid Sn. The principle of the method is as follows. If the heat evolved in the dissolution of compound AB is measured, and the equivalent heat evolved in the dissolution of the equivalent amount of pure A and B is known or measured, the difference provides the enthalpy of formation of the compound AB. Kleppa (1962) used this method for determining enthalpies of formation of a number of Cu-, Ag- and Au-based binaries and further extended the use of the method to high-melting-point materials with a more generalised method. 

The enthalpies of solution were measured with a LKB 8700-1 precision calorimetry system. The experimental procedure and test of the instrument have been given before (6,7). EC (Fluka, purissimum) was distilled under reduced pressure and the middle fraction was stored over molecular sieves (4 A) for at least 48 hr. ACN (Merck, pro analysis) was dried over molecular sieves and used without further purification. The purity of both solvents (determined shortly before use), as deduced from GLC, was always better than 99.8%. The volume fraction of water, determined by K. Fischer titration (8) was always less than 3.10-4. The mixed solvents were prepared by weight as shortly as possible before the measurements. AH° of Bu4NBr in W-ACN mixtures have been measured at 25°C while those in W-EC are at 45°C, which is above the melting point of pure EC. 

Figure 7.53. Differential scanning calorimetry (DSC). Shown are (a) schematic of the heat-flux sample chamber (b) an example of a DSC thermogram, showing endothermic eventsbDf (c) DSC thermogram of a poly(vinyUdene fluoride)-ethyl acetoacetate polymer-solvent system, showing two melting events for the polymer due to its intermolecular interactions with solvent molecules. The inset shows a comparison between the pure polymer (b) and the polymer-solvent (a). Reproduced with permission from Dasgupta, D. Mahk, S. Thierry, A. Guenet, J. M. Nandi, A. K. Macromolecules 2006, 39,6110.

Enrichment in S/L interfaces is of great importance in numerous industrial purification processes (solvent purification, separation, water treatment, decoloriza-tion, flotation, oil recovery, detergency, and so on). The surface area of industrial adsorbents is also often derived from S/L adsorption isotherms. Adsorption at S/L interfaces can be divided into two types, namely adsorption from pure liquids and adsorption firom solutions. Interaction with pure liquids is often characterized by immersion calorimetry. 

The present chapter does not pretend to be an exhaustive record of Solid-Liquid calorimetry applications in Surface Science and Technology. It should be rather regarded as an introductory course with some illustrative examples. It is important to realise that the individual author s experience in the field has been the principal criterion for selection of specific instruments and their uses, without any intention of neglecting other contributions. The presentation of calorimetry methods will be restricted only to interfacial systems composed of a pure liquid or a dilute binary, at the most, solution in contact with a solid which does not dissolve in the liquid phase. This formalism may be still employed in the case of solutions which are not strictly binary but may be viewed as such (e.g., solutions containing ionizable solutes, background electrolytes or other additives that may be lumped together as constituting a mean solvent or a mean solute). 

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