Moisture sorption determination

The Clausius-Clapeyron equation implies that if we plot the natural log of the pressure of the gas phase versus inverse temperature, the slope of the resulting line is the heat of vaporization divided by the gas constant (R). A plot of In P (vapor pressure of water) versus inverse temperature is given in Figure 3. The calculated heat of vaporization (determined by multiplying the slope by R) is 10,400 cal/mol. The important aspect of Eq. (10) with regard to moisture sorption is the fact that increasing the temperature also increases the vapor pressure. 

In any study of the moisture sorption-desorption properties of modified wood, the moisture content of the samples is determined using gravimetric methods. Invariably, the EMC of the sample is based upon the oven-dry weight of the modified wood ... 

Infrared Crystallinity Index. A measure of the relative degree of crystallinity was determined from the infrared spectra obtained in the preceding section by reporting the ratio of the optical density of the band at 1372"1 (7.29/mi) to that at 2900 cm 1 (3.45/xm). According to Nelson and O Connor (15), the crystallinity index measured in this way compares well with data from x-ray diffraction, density measurements, and accessibility measured by moisture sorption. 

Water vapor at room temperature will not penetrate well-defined crystallites but will be adsorbed in the amorphous regions. Consequently, moisture sorption measured gravimetrically at a given relative vapor pressure and temperature has been used to determine order in cellulosic materials. In the case of Valentine [252] and Jeffries [253], the fraction of ordered material was obtained by correlating moisture sorption with values obtained by the deuterium... 

The influence of sorbed moisture on chemical stability and the flow and compaction of powders and granulations is well established. The moisture content and hygroscopicity of excipients is particularly important as total product processing as well as finished product stability can be affected. Hygroscopicity, moisture-sorption isotherms, and equilibrium moisture content can be determined by thermogravimetric analysis and Karl Fisher titration methods. 

Effect of Relative Humidity and Sorption History. An indirect method for estimating wood moisture content is to measure its equilibrium relative vapor pressure h. This is related to wood moisture content by a sorption isotherm. The percent relative humidity (H) or relative vapor pressure (h) (H = 100 h) is the most important factor in determining the EMC for wood. A curve showing EMC as a function of percent relative humidity or relative vapor pressure at constant temperature is called a moisture sorption isotherm. 

For lyophilized dosage forms, there is sometimes an optimum moisture content for stability, especially for proteins. The choice of excipient can determine the moisture content of the lyophilized cakes based on the moisture sorption levels.12 Controlling moisture levels to this optimum also requires that the dosage form be protected from environmental moisture, usually by packaging in glass. 

Dynamic moisture sorption, in particular, provides an excellent opportunity to study solid form conversion Fig. 18.6 depicts a typical sorption curve of an antiarrhythmic compound that shows the conversion of an anhy-drate to a monohydrate. Moisture uptake by the anhydrous form is very small on the moisture uptake curve until a critical humidity of about 70% is achieved. At this point, rapid moisture uptake occurs and a hydrate form containing 10%moisture is generated. Subsequent reduction in the humidity (desorption) shows the hydrate to remain until approximately 5% RH, when it spontaneously converts to the anhydrous form. It is important to recognize, however, that conversion between solid forms is very time dependent. The relative humidities at which conversion was seen in Fig. 18.6 are significantly dependent on the length of time the solid material was equilibrated. For the material shown in Fig. 18.6, conversion from the anhydrous to the hydrate "at equilibrium" will occur somewhere between 10 and 70% RH. More precise determination of the critical humidity at which conversion occurs may be determined as described in Section 2.3.1. 

The automation of moisture sorption measurements is a relatively recent innovation (Marshall et al. 1994). Prior to this advance, moisture sorption of compounds ( 10 mg) was determined by exposing weighed amounts of compound in dishes placed in sealed desiccators containing saturated salt solutions. Saturated solutions of salts that give defined relative humidities (as a function of temperature) have been reported by Nyqvist (1983). A typical range at 25°C is given in Table 3.13. The samples are then stored at a selected temperature and analyzed at various time points for moisture and stability, usually by TGA and HPLC, respectively. 

Moreover, the heat transfer process has significant impact on the evaporation process in cotton fabrics but not in polyester fabrics. The process of moisture sorption is largely affected by water vapor diffusion and liquid water diffusion, but not by heat transfer. When there is liquid diffusion in the fabric, the moisture sorption of fibers is mainly determined by the liquid transport process, because the fiber surfaces are covered by liquid water quickly. Meanwhile, the water content distributions in the fibers are not significantly related to temperature distributions. 

Deuteration has also been used as a gravimetric method for determining accessibility, the results for cotton being in exact agreement with those of Valentine. The linear relationship which exists between moisture sorption and the fraction of material found to be amorphous to infrared has been confirmed for thirteen different t5qies of cellulose, and it is suggested that a difference in the ratio of moisture regain to amorphous content between samples of cellulose I and cellulose II reflect differences in their sorption behavior and, hence, in the structure of their amorphous... 

Determine the solvation/desolvation behaviour depending on the vapour pressure of the solvent (hydrates moisture sorption/desorption isotherm). 

X-ray diffraction methods (powder, single crystal) Spectroscopy (UV, IR, Raman, solid-state NMR) Thermal analysis, microcalorimetry, solubility determination, vapour pressure determination Moisture sorption Microscopy and micromeritics... 

To determine the behavior of a product, it must be stored at known conditions for a period of time and its properties measured. In the case of oxidation, for example, some method must be available to determine the amount of reaction with oxygen that the product has undergone. This is often done by measuring peroxide values for oil-containing products, or hexanal values for products that have hexanal as the end degradation product for oxidation. For moisture sorption, the product can be stored over a saturated salt solution until moisture uptake is at equilibrium. Then taste or texture is often the measured parameter to determine the end-point of shelf life. For pharmaceuticals, the true end-point is determined by the bioavailability of the drug. 

Of particular importance is the timescale over which diffusion occurs under various conditions of relative humidity (RH) and temperature. The RH determines the equilibrium moisture concentration, whereas higher temperatures will accelerate the moisture sorption process. In order to predict the moisture profile in a particular structure, it is assumed that Fickian diffusion kinetics operate. It will be seen later that many matrix resins exhibit non-Fickian effects, and other diffusion models have been examined. However, most resin systems in current use in the aerospace industry appear to exhibit Fickian behaviour over much of their service temperatures and times. Since the rate of moisture diffusion is low, it is usually necessary to use elevated temperatures to accelerate test programmes and studies intended to characterize the phenomenon. Elevated temperatures must be used with care though, because many resins only exhibit Fickian diffusion within certain temperature limits. If these temperatures are exceeded, the steady state equilibrium position may not be achieved and the Fickian predictions can then be inaccurate. This can lead to an overestimate of the moisture absorbed under real service conditions. 

Moreover, the isotherms determine the proper storage environment and the packaging conditions, especially for foods. Through the isotherms, the isosteric heat of sorption can be determined and, hence an accurate prediction can be made of the energy requirements for the drying of a solid. The utility of the isotherm is extended to the determination of the moisture sorption mechanism as well as to the degree of bound water. 

Spiess, W.E.L. and Wolf, W., Critical evaluation of methods to determination of moisture sorption isotherms, in Water Activity Theory and Applications to Food, Rockland, L.B. and Beuchat, L.R. (eds.), Marcel Dekker, New York, 1981. 

To determine the solubility, excess amounts of 3 CyDs were added to water and shaken at 25, 40, 55, and 70 C. After equilibrium was attained, the concentrations were measured by polarimeter (Jasco DIT-4). In the moisture sorption studies, approximately 2g of powder was placed in the desicator maintaining the various relative humidities (R.H.) at 25°C for 2 days. Reported values (in Figure 2) represent the average moisture content of the 3 measurements, on a dry weight basis. The surface tension of test solutions (0.1 mM of 3 CyDs) were measured using a Shimadzu Du Noliy Surface Interfacial Tensionmeter. 

Figure 6. Schematic of experimental set up to determine equilibrium and transient moisture sorption behavior of paper materials. The experimental chamber is a constant humidity and temperature chamber in which the paper sample is hung from a balance.

From sorption isotherms such as those in Figure 4 it is possible to transform the relative humidity x-axis of Figure 3 to a moisture ratio axis (utilizing functions of moisture content as a function of temperature at various relative humidities). The results of such calculations are shown in Figure 5 where the softening point measured as the logarithm of the load frequency is plotted versus the moisture ratio for different temperatures. As seen in the figure, there is a somewhat lower scatter in the data when moisture content is used. The 95% confidence interval of the moisture ratio determinations was 0.35%. 

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