Sol-gel Transition Process Research Articles

Formation of thermally reversible maltodextrin gels as revealed by low resolution H-NMR

Pulsed low resolution proton NMR (20 MHz) was applied to follow the gelation process in different thermally reversible gelling maltodextrin-water systems. Results are expressed in terms of the solidus ratio, which is the ratio of proton systems with high and low relaxation time after the application of a radio frequency pulse. The sol-gel transition process may be followed from the onset until equilibrium is reached after several days. A marked change in the signal occurs on the formation of reversible gels, which depends on the temperature, concentration, time and structural peculiarities. This behaviour is paralleled with the development of the shear modulus as well as ΔH° for melting. A linear relation between the solid-liquid ratio from NMR and X-ray crystallinity suggests that the physical basis of the NMR results is the formation of highly ordered domains as essential constituents of the gel network. Non-gelling solutions of amylopectin as well as of acetylated maltodextrins do not give ‘solid’ signals. Amylose is necessary for initiation and acceleration of the sol-gel transition. However, crystalline amylose in the precipitated, retrograded state gives solid signals too. It is to be concluded that the method applied is suitable for following sol-gel transitions if they are based on a highly ordered (crystalline) structure.

The Optical Behavior of Organic and Organometallic Molecules in Sol-Gel Matrices

ABSTRACTThe low temperatures involved in sol-gel processing have enabled organic and organometallic molecules to be incorporated in gel matrices. These molecules serve as optical probes to characterize gel chemistry and structure. The present paper describes the use of luminescence spectroscopy to detail matrix rigidity effects and protonation/deprotonation reactions during the sol-gel transition and subsequent aging processes. In separate experiments, the addition of a laser dye, rhodamine 6G, has enabled us to demonstrate lasing and optical gain in a silica gel.