Moisture sorption measurement

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 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. 

Surface area and moisture uptake have been related to the disintegration properties of excipients such as crosspovidone, starch, and alginic acid [17]. The surface areas of the three materials were measured, and a linear correlation was found between the maximum moisture sorption and specific surface area for the three disintegrants. The greater the surface area of the material, the more numerous were the sites for capillary attraction of water to its surface. It was postulated that the capillary action appears to be responsible for the disintegration properties of the materials. 

Bell, L.N. and Labuza, T. P. 2000. Moisture Sorption Practical Aspects of Isotherm Measurement and Use . 2nd Ed. American Association of Cereal Chemists, St. Paul, MN. 

Alternatively, aw may be measured by isopiestic equilibration. In this method, a dehydrated sorbent (e.g. microcrystalline cellulose) with a known moisture sorption isotherm (section 7.4) is exposed to the atmosphere in contact with the sample in an enclosed vessel. After the sample and sorbent have reached equilibrium, the moisture content of the sorbent can be measured gravimetrically and related to the nw of the sample. 

Moisture sorption kinetics by neat epoxies were measured using gravimetric analysis using a Mettler balance which was accurate to +0.05 mg. This technique was described in detail elsewhere 137). Another method was used to monitor the sorp-... 

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. 

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. 

When wood below the fiber-saturation point interacts with water, heat is evolved, and there are changes in the free energy and entropy of the sorbed water. Furthermore, the wood exerts swelling forces that can be measured. These effects can be treated by classical thermodynamic methods although moisture sorption by wood is not a perfectly reversible process because sorption hysteresis is involved, as was pointed out in the section on Moisture Sorption Isotherms (p. 136). 

The heats of wetting W and sorption Qi are interrelated, but they have different interpretations in terms of moisture sorption. For example, the heat of sorption is presumed to be a measure of the excess energy required to break the bond between bound water and the sorption sites, and the heat of wetting is a measure of the total number of sorption sites accessible to water (JO). 

Moisture content hypromellose phthalate is hygroscopic it takes up 2-5% of moisture at ambient temperature and humidity conditions. For the moisture sorption isotherm of HP-50 measured at 25°C, see Figure 1. 

The effects of temperature and relative humidity on the kinetics of moisture sorption in epoxy materials for microelectronics encapsulation are not generally known. In a previous paper QJ we examined moisture sorption as a function of temperature under conditions of 100 percent relative humidity. Conjugate sorption measurements were combined with mechanical, dielectric and thermal methods of analysis to examine moisture related micro-structural alterations. 

---