Quantitative Characterization of Adsorption Isotherms Using Isothermal Microcalorimetry

Abstract

The integral heat of adsorption of water vapor on sodium benzoate samples was determined at various partial vapor pressures using a heat conduction microcalorimeter. An equation is presented to describe the calorimetric integral heat response (mJ/g of solid) as a function of relative humidity. This equation, although similar in principle to the well-known BET equation, relates the heat evolved (rather than volume or mass of gas adsorbed) upon adsorption to the partial pressure of the gas. It qualitatively describes the shape of the calorimetric isotherm and quantitatively allows the calculation of "monolayer capacity" or the apparent surface area with water as the adsorbate. The modified BET equation was applied to the calorimetric adsorption data available in the literature. The surface area or the monolayer coverage values of the solid samples used in these studies were calculated from data-fitted parameter estimates. Good agreement was found between Vm or surface area values obtained by the application of the model to the calorimetric data and those reported by the authors using conventional gravimetric or volumetric measurement of adsorption. The model satisfactorily described the experimental calorimetric data of water vapor adsorption on sodium benzoate. The model equation and the use of isothermal microcalorimetry provide a means to obtain the water adsorption surface area of solid materials. The method may also be useful in comparing the surface properties of drugs and excipients obtained by different methods or from different sources. The microcalorimetric method to characterize adsorption is more sensitive and convenient in comparison with some of the conventional techniques.

Thermodynamics of Water Interaction with Human Stratum Corneum I: Measurement by Isothermal Calorimetry

“…Calorimetry has long been used to study the interactions of liquids and gases with solid substrates 4–10. Much early and often very high quality work employed adiabatic methods, exemplified by Morrison' studies of water sorption in wool and other natural fibers 4–6.…”

Textural Characterization and Energetics of Porous Solids by Adsorption Calorimetry

“…This result could be comprehended as an indication of possible reaction between adsorbed nitrous oxide and carbon monoxide from the gas phase. In order to observe the consistency between the amount adsorbed and the heat produced in the adsorption establishing a relationship like that proposed Pudipeddi, et al [32] in which it is proposed that the amounts of heat as a function of relative pressure, allows one to find the amount adsorbed in the monolayer, V m , and enthalpy that occurs in the adsorption of the first layer of adsorbate H 1 . Figure 4 shows the heat curves for the of amount ammonia adsorbed depending on the relative pressure, observing that the tendency of the curves is similar and from these you can find the values given for the case specific determinations were made for adsorption of ammonia on the sample ACM is a value for the constant C which is equal to exp [(H 1 − H L )/RT] of 10.61 and the amount adsorbed in the monolayer 330.6 μmol/g, which corresponds to the sample with higher adsorption for ammonia.…”

Excipient Selection in Oral Solid Dosage Formulations Containing Moisture Sensitive Drugs

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“…enthalpy. These enthalpy changes can be used as a measure of the rates at which materials interact with water vapor (Pudipeddi et al 1996;Buckton 2000;Lechuga-Ballesteros et al 2003;Roskar and Kmetec 2005). In an isothermal heat-conduction microcalorimeter, the heat produced by a sample is exchanged with the surroundings at a rate that is proportional to the water adsorption rate.…”