1,2-propylene glycol dielectric constant

FIGURE 9.3 Effect of dielectric constant of wateethanol-propylene glycol mixtures on the observed rate constant for zileuton. 

The dielectric constant of a solvent plays a significant role in solubility and stability of the solute. Few studies relating to the dielectric constant of the solvent medium to the photodegradation rate have been undertaken. Thoma and Klimek (8) reported that the improvement in the photostability of nifedipine noted by them is due to an increase in dielectric constant in both ethanol/water and propylene glycol/water mixtures. 

Sorby, D.L. Bitter, R.G. Webb, J.G. Dielectric constant of complex pharmaceutical solvent systems I. Water-ethanol-glycerin and water-ethanol-propylene glycol. J. Pharm. Sci. 1963, 52 (12), 1149-1153. 

Liquid solubilizers act by reducing the dielectric constant properties of the solvent system, thereby reducing the electrical conductance capabilities of the solvent and increasing the solubility of hydrophobic or non-polar drugs. Lanoxin, Valium, and Nembutal are examples of commercially available sterile solutions containing cosolvent solubilizers. A popular combination consists of 40% propylene glycol and 10 /o ethanol in water. 

Table 1 list solvents and cosolvents used in parenteral products. Water for injection is the most common solvent but may be combined or substituted with a cosolvent to improve the solubility or stability of drugs.f The dielectric constant and solubility parameters are among the most common polarity indices used for solvent blending.f Ethanol and propylene glycol are used either alone or in combination with other... 

A series of other sulfur-containing solvents has been investigated for electrolytic use. Of these, propylene glycol sulfite [400], a derivative of 1,3,2-dioxthiol, was found most suitable. It has a dielectric constant of 33, forms solutions with good conductivity with such salts as LiC104, and does not react with lithium. It has not yet been used for organic electrolysis. 

Figure 2 The fraction of carbon monoxide as a function of the dielectric constant of the solvent used. The results were obtained by irradiation of the naked CdS ( ) and Q-TiOj/SiOj photocatalyst ( ). Solvents used were (a) carbon tetrachloride, (b) dichloromethane, (c) 2-propanol, (d) propionitrile, (e) ethylene glycol monoethyl ether, (f) acetonitrile, (g) sulfolane, (h) propylene carbonate, and (i) water.

Fig. 4. Dielectric constant (D) of hydroorganic solvents as a function of percentage in volume. DMSO, dimethyl sulfoxide EGOH, ethylene glycol MeOH, methanol PrOH, propylene glycol MPD, 2-methyl-2,4-pentanediol. Temperature, 20°C. From Douzou (1977b). Reprinted with permission of Academic Press.

Figure 18. Dielectric loss data of poly(propylene glycol) (PPG-4000, molecular weight 4000 Da) at various combinations of temperature and pressure as indicated to demonstrate the invariance of the dispersion of the a-relaxation at constant a-loss peak frequency v or equivalently at constant a-relaxation time ra.

In Eq. (2.1 10), kD= is the rate constant at infinite dielectric constant, ZA, p, and r are ion charge, dipole moment and the shortest ion-dipole distance, respectively, and k is the Boltzmann constant. The term 0 represents the alignment of reactants, and cos 0 is unity in the case of head-on alignment. Thus, as the dielectric constant decreases, the rates of anion-dipole reactions decrease and the rates of cation-dipole reactions increase. As indicated by a linear relationship with a positive slope in log k versus 1/D plots (Fig. 84), the hydrolysis rate constant for chloramphenicol in water-propylene glycol mixtures increases with decreasing dielectric constant, suggesting a hydronium ion-dipole reaction.397... 

When the aqueous solubility of an active substance is too low, it can be dissolved in a different solvent or mixture of solvents, which is compatible with the route of administration. The solubility of a lipophilic substance (a substance which dissolves well in the oil or fat but poorly in water) can be increased by making the dissolution medium (water) less polar by the addition of less polar but water-miscible solvents. Often mixtures of water, ethanol and propylene glycol are used. Also glycerol and macrogol (polyethylene glycol) can be used. The polarity of a solvent can be expressed by its dielectric constant, e (for examples see Table 18.8, the higher the dielectric constant, the higher the polarity). 

TnBP was also studied in a polymeric solvent, poly-(propylene glycol) (PPG) (91). The molecular weight of the PPG liquids were 400, 1000 and 2000. Contrary to expectation, the solute and solvent motions were well-separated with (solute) being about 30 times larger than Te(solvent) As a result, two distinct dielectric loss peaks were obtained. The Kerr-constant of PPG was very small in comparison with that for TnBP so the Kerr-effect rise and decay transients were essentially due to solute. Figure 9 shows the derived plot of logfjn - vs - (T/K) l for solute and solvent processes. 

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