Phase-diagrams of Aqueous-solutions of Polycarboxylates in the Presence of Divalent-cations

Phase separation and gelation phenomena induced by addition of divalent cations in aqueous solutions of synthetic and natural polycarboxylates have been investigated by comparison of their phase diagrams. Three major types of behavior may be identified on the basis of the position and shape of the demixing line in the phase diagram, regardless of the chemical structure of the polymer. We propose a model for quantitative prediction of the demixing and sol-gel transition. The relevant parameters introduced are the neutral units, the charge density, and the cation/polyanion affinity, all of which are responsible for inter- and intramolecular complexes. Besides this general scheme, the local configuration of the backbone in natural polymers seems to promote intermolecular cross-links, leading to a large gel phase

Small angle neutron scattering and viscoelasticity study of the colloidal structure of aqueous solutions and gels of a globular protein

Small-angle neutron scattering and viscoelasticity measurements were performed on β-lactoglobulin solutions and heat-set gels at pH 9 and low ionic strength. The rheological properties of the solutions were characterized by antithixotropy and viscoelasticity.In the case of aggregates and gels, a peak in the scattering curves was observed; qmax varied as C0.5. This correlation peak vanished with increasing ionic strength. The dilution of the aggregates showed that the correlations were lost and that the aggregates displayed the same internal structure as the gels. Gels formed at low ionic strength indicated a liquid type viscoelastic behaviour. At high ionic strength, fractal structures of the aggregates would be observed between 400 and 40 Å with df= 1.3 at low protein concentration and 1.75 at a higher concentration. Subsequent dilution did not change the fractal dimension of these structures

Physico-chemical properties and rheology of alginate gel beads formed with various divalent cations

The physico-chemical parameters influencing the behaviour of alginate beads have not been deeply studied in the literature. In this communication, the formation and the mechanical and structural properties of alginates with different molecular weights and M/G ratio were investigated. The results obtained showed that (1) to produce spherical beads, the product of alginate concentration and intrinsic viscosity C[eta] must be higher than a threshold value and the accurate value is a function of the gelling cation type; (2) the nature of the cation, the polymer and cation concentration, and the ionic strength influence the mechanical properties of alginate beads; (3) critical point drying and cryogenic preparation are equivalent and less disturbing methods than lyophilization for studying the structural properties of alginate beads in SEM; and (4) calcium-alginate beads are smooth and uniform whereas copper-alginate beads are rough and seem to be formed of associations of independent microgels. (C) 1999 Elsevier Science Ltd. All rights reserved

The effect of different storage temperatures on the physical properties of pectin solutions and gels

The stability (in terms of viscosity and gel strength) of pectin solutions and gels potentially plays an important role in their behaviour and functional properties in a wide range of applications and therefore any changes over time must be understood. The gel strength of pectin gels and intrinsic viscosity of pectin solutions at different temperatures (4 °C, 25 °C and 40 °C) have been investigated using a "rolling ball" viscometer and a texture analyser respectively. Both the intrinsic viscosity ([η]) and gel strength decrease with increased storage time, although this more pronounced at elevated temperatures. The changes in intrinsic viscosity with storage time and temperature were used to determine the depolymerisation constant (k). Pectin storage conditions and particularly temperature have an influence on depolymerisation, particularly elevated storage temperatures, but whether or not this will be detrimental to its intended application will depend on the functional significance of the changes that occur. In this case based on the previous diffusion studies on a model drug (paracetamol) we conclude that the decreases in viscosity and gel strength within the range observed have no detrimental effect on the drug release properties.© 2010 Elsevier Ltd. All rights reserved

Effect of Drug Loading Method on Drug Content and Drug Release from Calcium Pectinate Gel Beads

Drug-loaded calcium pectinate gel (CaPG) beads were prepared by either mixing, absorption, or swelling method. The effects of drug loading method as well as the drug loading factors (i.e., drug concentration, soaking time in drug solution, type of solvent) on drug content and drug release were investigated. The amount of drug uptake (i.e., drug content) into CaPG beads increased as the initial drug concentration increased and varied depending on the loading method. The in vitro release studies in 0.1 N hydrochloric acid (HCl) and pH 6.8 buffer indicated that the drug loading method affected drug release and release parameter, time for 50% of drug release (T50). The mixing method provided a faster drug release and lower T50 than the absorption method and swelling method, respectively. This is probably due to higher drug content in CaPG beads. The increased concentration of drug in soaking solution and soaking time resulted in higher drug content and thus faster drug release (lower in T50 values). When using 0.1 N HCl as solvent for soaking instead of water, the drug release was slower owing to the increase in molecular tortuosity of CaPG beads. The drug release was also affected by pH of the release medium in which drug release in 0.1 N HCl was faster than in pH 6.8 buffer