Gel changes

Change in ionic strength —> change in concentration of ions inside gel —> change in swelling —> change in release of drug... 

Silica gel is probably the adsorbent which is best known. Small sachets of gel are often included in packages of material that might deteriorate in a damp atmosphere. Sometimes a dye is added in the manufacturing process so that the gel changes colour as it becomes saturated. 

The swelling behavior of poly(N-isopropylacrylamide) has been studied extensively [18,19]. It has been shown that this gel has a lower critical point due to the hydrophobic interaction. Such a swelling curve is schematically illustrated in Fig. 9. The gel is swollen at a lower temperature and collapses at a higher temperature if the sample gel is allowed to swell freely in water. The volume of the gel changes discontinuously at 33.6°C. The swelling curves obtained in this way correspond to the isobar at zero osmotic pressure. On the other hand, the friction coefficient is measured along the isochore, which is given in Fig. 9,... 

Finally, combining Mossbauer spectroscopy with IR spectroscopy, Hobert and Arnold (228) studied the interaction of amines with the surface of a-Fe203 supported on silica gel. Changes in both the Mossbauer and IR... 

It is known13,14 that both thermal (T) and hydrothermal treatments (HTT) of silica gels change their pore structure. The specific surface area diminishes and the pore size increases depending on treatment temperature. This occurs due to hydrolysis of Si-O-Si bonds, transferring of Si(OH)4 and larger complexes with... 

Autoclave Synthesis of VPI-5. Table I shows how the properties of the gel change after various heating times. The TBA that is present in the starting gel is rapidly expelled into solution upon heating and is not found in any of solid samples collected. The pH was found to rise sharply after 45 minutes and reaches a final pH of 7.0 after 23 hours. The solid Al/P ratio decreases to a ratio of 1.06 over the course of the reaction while the liquid Al/P ratio rises to a value of 0.9. 

Brannon Peppas and Peppas [23] found swelling ratios up to 250 for systems where the swelling solution had an ion concentration comparable to the charged groups in the gel. For a preswollen gel, changes in swelling ratio can be computed from V2.S values obtained at appropriate pH values. 

The mechanical properties of the gel change with acrylamide concentration, and can be modified within limits by adjusting the relative number of cross-links. Most workers use a commercial 20 1 mixture of acrylamide and methylene fcisacrylamide, called Cyanogum 41, which is satisfactory over a wide range, but not at very low or at very high concentration of polymer. 

Another observation pointing in the same direction has been described in Glaser (115). Sometimes, during the anodic oxidation of TICIO4 in acidic aqueous solution, the anode solution coagulated to a yellowish-white gel. The electrolysis was then stopped and the stopcock between the anodic and cathodic compartments was closed. After some days, the gel changed back into a clear, colorless solution. In addition. 

Relatively inexpensive portable colorimetric indicators are now commonly used to detect the presence of a variety of contaminant gases in the atmosphere. These are normally specific for one or perhaps a series of gases. For example, the NBS colorimetric detector uses an indicator tube that contains a 15-mm section of yellow palladous silico-molybdate gel the gel changes color when exposed to carbon monoxide at concentrations as low as 0.001 vol. % in air. Figure 52 shows one version of a commercial unit used for carbon monoxide as well as carbon dioxide, the unsaturated hydrocarbons, and a variety of other gases this unit can also be used to determine the approximate concentrations of contaminants in liquid oxygen. Precise determinations can be made with a variety of analytical tools, including infrared... 

---