Lars Wadsö

On application of an isothermal sorption microcalorimeter

A wide scope of application of a novel isothermal sorption microcalorimeter is presented. With the technique, it is possible to simultaneously and independently in one experiment obtain the sorption isotherm of a sample along with the corresponding differential enthalpies of sorption. The method is suited for measurements with water vapor as well as organic vapors and has been tested at temperatures between 25 and 40°C. The technique is suitable for the thermodynamic characterization of the sorption process. It is demonstrated that the method can be applied to the study of a wide range of physico-chemical phenomena associated with the uptake of vapor by a substance/material including capillary condensation, crystallization, lyotropic phase transitions and hydrate/solvate formation. The calorimetric data are reproducible and agree well with the results of well-established techniques such as solution calorimetry, differential scanning calorimetry, sorption microbalance and osmotic stress measurements.

Describing non-Fickian water-vapour sorption in wood

Moisture transport and sorption in wood may not be accurately described by Ficks law of diffusion. The problem of making a model of non-Fickian behaviour (NFB) for wood is discussed. Some measurements in which NFB in wood is clearly seen are also reviewed. Four criteria, which must be satisfied by a model describing sorption in wood cell walls, are presented: (1) the model should not only describe the response to step changes in vapour pressure; (2) it should be able to predict sorption with more than one time scale; (3) the sorption rate should not depend on the thickness of the cell wall; (4) small rapid changes in vapour pressure should give slower fractional weight change than large rapid changes. A review of models of NFB in synthetic polymers indicates that there is presently no model of NFB which fulfills the above criteria. More measurements of the sorption behaviour of the cell wall are needed to construct such a model for wood. This model can then probably be used, together with a Fickian diffusion model, to model the sorption behaviour of whole wood.

A new method for determination of vapour sorption isotherms using a twin double microcalorimeter

A new twin heat conduction microcalorimeter for the study of vapour sorption isotherms, sorption enthalpies and the kinetics of sorption has been developed. Each calorimeter has two calorimetric vessels which are connected by a steel tube. The upper vessel (a vaporization chamber) is charged with a vapour forming liquid and the lower vessel (a sorption chamber) contains the sorbent sample. An identical double calorimeter serves as a reference. During a measurement, the liquid in the upper vessel will vaporize and is allowed to diffuse to the sorption vessel. Thermal powers of vaporization and sorption are measured separately. The rate of vapour flow is obtained from a known value for the enthalpy of vaporization. Assuming equilibrium conditions and applying Ficks law of diffusion makes it possible to calculate the sorption isotherm. Results from test measurements of water vapour sorption on samples of crystalline NaCl and KCl in saturated aqueous solution and on cotton wool are presented.

An Experimentally Simple Method for Measuring Sorption Isotherms

Abstract We describe a simple, yet practical and precise, way of measuring sorption isotherms with each sample in its own glass jar with a saturated salt solution. The measurements are done with below-balance weighing and with the sample kept inside the closed jar during the whole measurement period, providing constant relative humidity (RH) conditions. The technique has been tested on microcrystalline cellulose and bentonite clay. The agreement with literature values was good; the differences seen for bentonite at high RH are discussed in terms of the slow attainment of equilibrium for this material.

Isothermal Microcalorimetry. Current problems and prospects

A brief survey is given of recent developments and current activities in isothermal microcalorimetry. The discussion focuses on new methods in areas where the techniques have proved to be particularly useful or are promising to be so, in a near perspective. Some problems and limitations with current methods are also discussed.

Small polar molecules like glycerol and urea can preserve the fluidity of lipid bilayers under dry conditions

Glycerol and urea are examples of small, water-soluble molecules with low vapor pressure that can protect lipid membranes upon dehydration. Both are a part of the Natural Moisturizing Factor in human skin, and are also present in other organisms, where they prevent drying due to osmotic stress. This study was conducted in order to understand the mechanism of such protection. We have selected two ternary systems: dimyristoylphosphatidylcholine (DMPC)–glycerol–water and DMPC–urea–water, as models to investigate the molecular mechanisms behind this protective effect with a focus on factors that control the solid to liquid phase transition in the phospholipid bilayers. By combining a number of experimental techniques, including solid-state NMR, sorption microbalance and DSC, the structure and the phase transitions have been characterized at low water content and in excess solution. It was discovered that both glycerol and urea stabilize the liquid crystalline bilayers at low relative humidities (down to 75% RH at 27 °C), whereas for the pure DMPC–water system, a solid gel phase is induced at 93% RH. This demonstrates the protective effect of glycerol and urea against osmotic stress. It is further concluded that for lipid systems with limited access to solvent, the phase behavior is determined by solvent volume, irrespective of the composition. The observation that glycerol and urea have a similar effect on the lipid phase behavior under dry conditions, together with the lack of evidence of specific interactions between the lipids and glycerol or urea, implies a general mechanism, which might also be applicable to other, similar solutes.

A method to simultaneously determine sorption isotherms and sorption enthalpies with a double twin microcalorimeter

Sorption of vapors of water, ethanol, and other liquids on solids like pharmaceuticals, textiles and food stuffs are of both practical and theoretical importance. In this article we present a technique to simultaneously measure sorption isotherms and sorption enthalpies. The sample is contained in one end of a sorption vessel. In the other end a vaporizable liquid is introduced to start the measurement. Mass transfer from the liquid to the sample is by vapor diffusion and the rate of mass transfer is calculated from the measured thermal power of vaporization. Simultaneously, the thermal power of sorption is measured and from this one may calculate the differential enthalpy of sorption. The thermal power measurements are made by inserting the sorption vessel in an isothermal double twin microcalorimeter.

A double twin isothermal microcalorimeter

The design and properties of a double twin heat conduction microcalorimeter are described. In this instrument two twin microcalorimeters are placed close together, one on top of the other. The size of the instrument is the same as that of a commercial single twin microcalorimeter and each of the twin parts has similar properties as one normal twin microcalorimeter. The cross-talk between the calorimeters can be made low; we measured <0.1% of the signal generated in one calorimeter in the other calorimeter. This figure is, however, dependent on how well the two sides of the instrument are thermally balanced. The paper also contains a general discussion of the use of a reference in reducing the effect of temperature changes in the heat sink. The advantage with a double calorimeter is that one may easily perform two related calorimetric experiments at the same time and in close proximity to each other, e.g. both sorption isotherms and sorption enthalpies may be measured simultaneously, or the heat production rate of a biological process may be monitored at the same time as the CO2 production is measured. (Less)

Effects of modified atmosphere on shelf-life of carrot juice

The effects of de-aeration and modification of the headspace atmosphere on carrot juice shelf-life were investigated. The shelf-life was defined by microbial growth, determined by isothermal calorimetry and plate counts. Compared with storage under air, de-aeration of carrot juice by helium or nitrogen bubbling, followed by flush packaging had no effects on shelf-life. Similar treatment with carbon dioxide, giving a carbonated drink, prolonged the shelf-life by up to 250%. This effect could not be explained by the change in pH to 5.6, since the shelf-life was not prolonged by the addition of hydrochloric acid until pH <4.5. Both the concentrations of H+ and of dissolved CO2 were correlated to the shelf-life of fresh carrot juice; higher concentrations giving shorter shelf-lives. The concentration of dissolved CO2 was also linearly correlated to the peak thermal power

Microcalorimetric measurements of metabolic activity of six decay fungi on spruce wood as a function of temperature

Microcalorimetry, i.e., the measurement of heat production rate was used to assess the metabolic activity of six wood-decay fungi growing on moist wood particles as a function of the temperature. Four brown-rot fungi increased their activity with increasing temperature in the temperature range 5-35 C, and one brown-rot and the white-rot fungus had a lower activity at 35 C than at 25 C. It may be possible to estimate the relative growth rate of a fungus in relation to different environmental factors by measurements of the production of heat if there are no transient effects. The possible use of the results is discussed for three applications: hazard evaluation of large piles of moist biological fuels, brown-rot degradation of wood constructions, and detoxification by white rot fungi of soils containing recalcitrant toxic chemicals. (Less)