Entire Range Of Water Content Research Articles

WATER PLASTICIZATION EFFECTS ON CRYSTALLIZATION BEHAVIOR OF LACTOSE IN A CO-LYOPHILIZED AMORPHOUS POLYSACCHARIDE MATRIX AND ITS RELEVANCE TO THE GLASS TRANSITION

ABSTRACT Crystallization of lactose in a co-lyophilized amorphous polysaccharide matrix was investigated under various hydration conditions to test the possible relation between the ability of the polymer to raise the glass transition temperature (Tg) of the lactose-pullulan blend, relative to pure lactose, and the crystallization kinetics. Both calorimetric (DSC) non-isothermal measurements and x-ray diffraction analysis of samples stored at a constant temperature revealed a marked retardation in lactose crystallization in the presence of pullulan; i.e., the rate constant values, as determined by the Avrami analysis, declined and the ‘half-time’ (t1/2c) for lactose crystallization increased with decreasing ratio of lactose/pullulan. The inhibitory action of pullulan on crystallization of lactose could not be solely attributed to Tg-related effects on molecular mobility of the composite systems. At a pullulan weight fraction range of 0.25–0.33 (w/w of total solids) the influence of the polymeric additive on Tg was marginal over the entire water content range examined, although crystallization was delayed as compared with pure lactose. Modeling of the temperature-dependence of t1/2c for the combined lactose-pullulan systems with the Williams-Landel-Ferry (WLF) equation was feasible only when the coefficients C1 and C2 were allowed to vary instead of assuming their ‘universal’ values.

Stability analysis of the unsaturated water flow equation: 2. Experimental verification

In this study, we verify the stability analysis of the unsaturated flow equation presented in paper 1. We compare finger sizes measured in laboratory experiments with predictions by the stability model. Using measured unsaturated hydraulic properties which are fitted and extrapolated over the entire water content range, the model enables us to predict finger sizes over a wide range of nonponding infiltration rates. The model predicts an increase of finger diameter at high and low infiltration rates for all soils. Such increases were also observed in our experiments. The model yields reliable predictions of finger diameters at high and intermediate infiltration rates but does not perform as well at infiltration rates <1 cm/h. This is caused by experimental limitations in accurately measuring hydraulic conductivities at low infiltration rates. A sensitivity analysis for initial water content and the shape parameter of the hydraulic conductivity curve showed that a small change of the parameters resulted in a relatively small change of finger size at high and intermediate infiltration rates but at low infiltration rates the finger size change was large, i.e., several orders of magnitude. We conclude that the stability model presented in paper 1 can be used for the assessment of wetting front instabilities in homogeneous soils over a wide range of nonponding infiltration rates.

Use of Sorptivity to Determine Field Soil Hydraulic Properties

AbstractConvenient and reliable techniques for estimating intact soil hydraulic properties are required for predictions of soil‐water flow in the environment. The dependence of sorptivity, S, on water supply potential, Ψ0, is used here to find the dependence of the intact field soil hydraulic properties soil‐water diffusivity, D(θ), hydraulic conductivity, K(θ), and soil‐water characteristic Ψ(θ) on water content, θ. The approximations used in deconvoluting sorptivity are examined critically using a simple parametric D(θ) that gives analytic solutions applicable to most known soil behavior. We show that the Parlange approximation differs from the exact solution by <5% over a very wide range of D(θ) and over the entire water content range. A rapidly convergent iterative scheme in which this approximation forms the initial estimate is introduced for situations where greater accuracy is required. The technique is tested for a repacked soil, Ustochrept, and the D(θ) derived from S(Ψ0) agrees excellently with conventional measurements. We use the disc permeameter to find S(Ψ0) at 20 sites for an intact field soil, Haplustalf. The D(θ) derived from these measurements is not strongly θ‐dependent. This is attributed to the shape of Ψ(θ), which, in contrast to repacked samples, shows that as θ approaches saturation, Ψ approaches zero gradually. The capillary length found from the K(Ψ) measurements is only 23 mm. Both findings are attributed to the presence of biopores in the field soil. Sorptivity may be used to determine reliably intact soil hydraulic properties whose magnitudes reflect the intricacies of field soil structure.

Interrelationships between water and cellular metabolism in Artemia cysts. XI. Density measurements.

Cysts of the crustacean Artemia are a useful model for studies on intracellular water because they are capable of essentially complete and reversible desiccation. We have used a variety of techniques on this system, the present work being an attempt to estimate the density of intracellular water (rho w). The density of individual cysts was evaluated from sedimentation velocity. Heptane displacement methods were used to determine the volume of a known mass of cysts, from which the density was calculated. The two methods produce comparable results. It was shown that the densities and water contents of large masses of cysts accurately reflect those of individual cysts. Cyst densities (rho c) were determined over the entire range of water content from 0 to 0.63 weight fraction of water (Wf), and temperature dependence was measured for 0.61 Wf over 2-41 degrees C. The following refer to 25 degrees C. No marked change was detected in rho c until the water content exceeded 0.15 Wf, at which rho c decreased as a linear function of Wf to maximum water content. However, the cyst does not behave ideally in the sense that the densities of the nonaqueous components and added water are not additive as a function of Wf. The partial specific volume of water in cysts at maximum hydration was estimated to be 3% larger than that of pure water. These observations are compared with density measurements on other systems, and with previous findings on the physical properties of water in this system.