Water-caffeine solutions

FIGURE 16 The chromatogram of an injection of a caffeine solution without the column showing the instrumental bandwidth of a Waters Alliance HPLC system with a 966 PDA detector with a standard flow cell. 

Silky white crystals, usually matted together, or a white crystalline powder. It sublimes at about 180°. M.p. 234° to 239°. When crystallised from water, caffeine contains 1 molecule of water of crystallisation, but it is anhydrous when crystallised from ethanol, chloroform, or ether. It is decomposed by strong solutions of caustic alkalis. 

One of the approaches found most suitable to explain the sensorial properties of sweet, bitter, and sweet-bitter substances proves to be the physico-chemical approach especially as concerns hydration and surface properties (DeSimone and Fleck, 1980 Funasaki et al., 1996 Fimasaki et al., 1999 Mathlouthi and Hutteau, 1999). Thus, solution properties of sweet and bitter molecules were found informative on their type of hydration (hydrophobic or hydrophilic) and on the extent of the hydration layer (Fiutteau et al., 2003). Physico-chemical properties (intrinsic viscosity, apparent specific volume, and surface tension) and NMR relaxation rates of the aqueous solutions of sucrose, caffeine, and sucrose-caffeine mixtures were used in the interpretation of the taste modalities of these molecules and to explain the inhibition of caffeine bitterness by sucrose (Aroulmoji et al., 2001). Caffeine molecules were found to form an adsorption layer whereas sucrose induces a desorption layer at the air/water interface. The adsorption of caffeine gradually increases with concentration and is delayed when sucrose is added in the caffeine solution (Aroulmoji et al., 2004). 

It is also worth noting that for caffeine solutions a thermodynamic parameter such as the enthalpy of dimerization of caffeine in water can be determined comparatively by dissolution calorimetry and by high resolution NMR [118,119]. 

After 24-h exposure to caffeine, the LC50 for Daphnia magna was recorded as 683.7 mg/L (Guilhermino et al., 2000) and an EC50 (immobilization) as 161.18 mg/L (Lilius et al., 1995). Freshwater invertebrate species, such as Ceriodaphnia dubia, Pimephales promelas, and Chironomus dilutus, commonly used in water monitoring tests, were exposed to aqueous caffeine solutions under static exposure for 48 h and... 

Tables 20.4.13 to 20.4.17 present the solubility of several high-value compoxmds, namely caffeine, vanillic acid, ferulic acid, caffeic acid and thymol, in liquid ethyl lactate in the temperature range of 293.2-343.2K. The chemical structures of these compoimds are depicted in Figure 20.4.14. Solubilities in both water-saturated (1.4 wt%) and dried (0.03 wt%) ethyl lactate are given in the tables, since the hydroscopic character of ethyl lactate makes important to understand the effect of small amounts of water on solute solubility.

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

Funk et al. [128a] dipped silica gel plates in a 4% solution of caffeine in order to separate six polyaromatic hydrocarbons relevant in monitoring the quality of potable water (Fig. 42). 

Sample mixture. A suitable sample mixture is obtained by weighing out accurately about 0.601 g of aspirin, 0.076 g of phenacetin and 0.092 g of caffeine. Dissolve the mixture in 10 mL absolute ethanol, add 10 mL of 0.5M ammonium formate solution and dilute to lOOmL with de-ionised water. 

In aqueous solution, caffeine associates to form at least a dimer and probably a polymer 12 the molecules are arranged in a stack.13 Caffeine will also associate with purines and pyrimidines either as the free bases or as their nucleosides.13 Caffeine crystallizes from water as a monohydrate [9],... 

Also, hydrates are more soluble in water-miscible solvents than are the corresponding anhydrous forms. For example, the solubility of caffeine hydrate is lower than that of anhydrous caffeine in water but higher in ethanol. The maximum concentration seen may be due to the solubility of the anhydrous crystalline phase or due to a temporary steady state in which the rate of dissolution of the metastable anhydrous form and the rate of crystallization of the stable hydrate are equal. The decreasing concentration represents crystallization of the stable hydrate from a solution supersaturated with respect to it. If the maximum concentration of the solute in the dissolution experiment corresponds to the solubility, then the initial increase in concentration follows the Noyes-Whitney equation [15]. Van t Hoff plots of log solubility versus the reciprocal of temperature give linear relationships (Fig. 16). 

Prepare 50 mL of a stock standard solution that is 2.0 mg/mL in caffeine and 5.0 mg/mL in sodium benzoate. Use an analytical balance to weigh the chemicals and a clean 50-mL volumetric flask for the solution. Use distilled water as the diluent. Shake well. 

The pale yellow chloroform solution is decolorised by shaking, first with a few cubic centimetres of sodium hydroxide solution, then with the same volume of water, and is evaporated to dryness. The residue of crude caffeine is recrystallised from a little hot water. Yield 2-0-2-5 g. Soft, flexible, silky needles containing one molecule of water of crystallisation. 

All chemicals and solvents used were of analytical and spectroscopic grades. Caffeine was obtained from Bilim Pharm, Ind, Turkey, Stock solutions of 100 gg luL of caffeine was prepared in distilled water. 

In analysing the caffeine of energy drinks, an accurately weighed amount of 15 mL of sample to 50 mL volumetric flask containing 25 mL water. Two milliliters of basic lead acetate solution was added to this solution and diluted the mark with distilled water. After filtering, 25 mL of filtrate was taken and 0.25 g of NaHCOj was added to this solution. Then, the solution was filtered. Five milliliters of filtrate was transferred to a 25 mL volumetric flask and adjusted to volume with distilled water. The peak amplitudes of the first-derivative spectra was measured at 287 and 260 nm. The sample preparation procedure was also used for PLS-1 method and the absorbances of this solution were recorded between 240-320 mn. 

Many compounds that are present in plants are weak bases. Caffeine in coffee and piperidine in black pepper are two examples. A weak base, represented by B, reacts with water to form an equilibrium solution of ions. 

Examples of solvent-mediated transformation monitoring include the conversion of anhydrous citric acid to the monohydrate form in water [235,236], CBZ with water [237] and ethanol-water mixtures [238,239], and cocrystallization studies of CBZ, caffeine, and theophylline with water [240]. Raman spectroscopy was used to monitor the crystallization rate and solute and solvent concentrations as griseofulvin was removed from an acetone solution using supercritical CO2 as an antisolvent [241]. Progesterone s crystallization profile was monitored as antisolvent was added [242]. 

Three compounds recovered from parfait columns were also previously tested for breakthrough from 5-mL Teflon beds (6). The capacity factors for these compounds and their recoveries from the Teflon bed of a parfait column showed a rough correlation. Phenanthrene, which was tested in the parfait column only in the presence of humate, was recovered essentially quantitatively from the 5-mL Teflon column and had a capacity factor of 368. About 15 of the caffeine applied to a parfait column in the absence of humate could be recovered from Teflon, and caffeine showed a capacity factor of 22. Only about 2 of the 2,4-dichlorophenol applied to parfait columns could be recovered on Teflon its capacity factor was 5.6. It may therefore be anticipated that compounds following the inverse correlation of solubility with capacity factor and having a capacity factor greater than about 20 should be detectably absorbed to the Teflon bed of a parfait column. Simply increasing the volume of the Teflon bed may also increase the absolute recovery of adsorbable solutes that have modest values of kFor this reason, a 150-mL bed of Teflon per 8 L of water may not be the ideal bed size a larger bed may be better. 

Furfural is such a reactive, volatile, and water-soluble compound that its loss could be due to several causes. Oxidation and volatilization from eluates, especially from the column effluent, are the most likely sources of loss. Humic acid seemed to affect the recovery of only two solutes caffeine and 2,4-dichlorophenol. 

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