Hydration absorbance spectroscopy

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

The range of IR bands that can be utilized to assess hydration alludes to the pervasiveness of water absorbance in an IR spectrum Water is a strong IR absorber and thus can pose a particular difficulty in the spectral examination of aqueous samples. The advent of computer-interfaced FT systems allowing spectral manipulation (such as band subtraction) can circumvent these difficulties, as can the use of specialized accessories such as ATR elements [3] and even alternative vibrational spectroscopic techniques, e.g., FT-Raman spectroscopy. FT-Raman spectroscopy, providing complementary information to IR, has in recent years been employed for the characterization of human SC [185-190] and model SC lipids [191], as well as for the noninvasive monitoring of topically applied compounds [192] and the in vitro evaluation of SC-enhancer interactions [193]. 

Coupling pulse radiolysis with time-resolved spectroscopy also allowed the determination of the transient absorption spectra of hydrated silver atom and of the first silver clusters showing that the absorption properties of silver atoms and metal aggregates in solution are different from that of bare clusters in gas phase. Silver atom presents an absorption maximum at 360 nm in water,while it absorbs at substantially shorter wavelengths in argon (292-310 nm) or in xenon (322-333 nm) atmosphere. ... 

Water can be determined in solid samples by infrared spectroscopy. The water content of calcium sulfate hydrates is to be measured using calcium carbonate as an internal standard to compensate for some systematic errors in the procedure. A series of standard solutions containing calcium sulfate dihydrate and a constant known amount of the internal standard are prepared. The solution of unknown water content is also prepared with the same amount of internal standard. The absorbance of the dihydrate is measured at one wavelength ( sample) along with that of the internal standard at another wavelength (As,d). The following results were obtained. 

Infrared Spectroscopy. An inspection of the infrared spectra of dry or hydrated pure Nafion in the sulfonic add or various cationic salt forms reveals a multiplidty of bands (28, 30, 31) some of which are inconveniently located in close proximity to the aforementioned peaks characteristic of silicon oxide strudures. The Nafion contribution to the composite spectra was subtracted in each case using the 2860 cm band (combination 1140 + 1720 cm, both CF,CF,) as an internal thickness standard. While this band appears weak and may not be an ideal internal standard (Membranes were not available to test absorbance vs. thickness linearity.), it is backbone-related, lies in a region of peak noninterference and the resultant subtractions do appear effective. 

GA, a dialdehyde with the basic structure of pentanedial, exists mostly in a hydrated form at or below room temperature (Korn et al., 1972). Monsan et al. (1975) analyzed commercial preparations of aqueous 25% GA by NMR spectroscopy and found 79% water, 18% polymers and only 3% free GA. The polymers, which regenerate easily at higher temperatures to the free monomeric form (Korn et al., 1972), are formed particularly at higher pH, and will precipitate in alkaline conditions as poly(GA). GA should, therefore, be stored as a slightly acidic solution (pH 3-4). Purification of GA by vacuum distillation does not increase its cross-linking ability (Siess et al., 1971). The monomer absorbs at 280 nm (4.2 M cm ), while its polymers absorb strongly at 235 nm (1.53 x 10 M cm" ). 

The specific role of the absorbed water in polymer electrolytes and the physical state of the water absorbed by polymer electrolytes were studied by IR [172] and NMR spectroscopy (low temperature relaxation time measurements) [173] and DSC [7]. The DSC curve of a hydrated PBI-PS film (73.1 mol %) containing 11.3 H2O molecules per SO3H group is shown in Fig. 21. 

Infrared spectroscopy was used to follow the ageing processes of two epoxy resins. Resin samples were exposed to ionising radiation and the effects at the molecular level, and how the resins modified their hydration mechanisms was discussed. Aromatic amine cured resins are only slightly affected by the radiation, but those cured by alkyl diamines are more sensitive and thus absorb more water.molecules. 14 refs. 

The development of Fourier transform infrared (FTTR) spectroscopy has facilitated the expansion ofthe scope of applications for IR spectroscopy to new fields of research, from forensics to pharmaceutical on-line analysis. The most important requirement of this technique is that the protein contain a distinctive IR-absorbing ftmctional group not present in the hydrogel. An advantage of FTIR spectroscopy is the ability to obtain spectral information for a polymer in the hydrated state, with the use of... 

The study had three objectives first, changes in NIR spectra were correlated to the time aspirin tablets spent in the hydrator (the calibration had a correlation coefficient of 0.95 and SEE of 18.8 h) second, a calibration was developed for the prediction of tablet salicylic acid content (the researchers ensured that prediction of salicylic acid was based on changes in salicylic acid concentration, and not some related process, such as absorption of moisture, by evaluation of loadings spectra from principal component analysis of the data. The HPLC-determined salicylic acid levels ranged from 0.36 to 1.66 mg, and the NIR method allowed prediction of the degradant with a standard error of 144 mg) and third, the mass of water absorbed by the tablets was determined by NIR spectroscopy. 

The thickness of the oxide layer formed on pure magnesium in ambient conditions was calculated by McIntyre and Chen (7) using X-ray photoelectron spectroscopy (XPS). After exposure for only 10 s the oxide layer was measured to be 2.2 0.3 nm (c. seven monolayers of MgO). The oxide layer was found to increase in thickness slowly and linearly with the logarithm of exposure time during a test period of 10 month s exposure to laboratory atmosphere. Hydration by absorbed water molecules contributed to the measured thickening of the air-formed film through hydroxide formation. 

Differential scanning calorimetry and Fourier transform infrared spectroscopy techniques were used to study the structure of water molecules in polyvinyl alcohol and polyethylene grafted acrylate hydrophilic polymers. Varying amounts of water were added to test samples and the samples conditioned to the sorption equilibrium state in sealed containers for 24 hours prior to evaluation. It was concluded that below a threshold water content, depending on the polymers physical and chemical stmcture, water molecules absorbed in hydrophilic polymer cannot form ice crystals in the polymer matrix. Above this threshold content, the water crystallises but below zero. It was also demonstrated that the absorbed water in hydrophilic polymers develops differing hydrogen bonds in the first and second hydration layers. It was concluded that the potential influence of these intermolecular interactions should therefore be taken into account whenever a polymer is used with a solvent. 25 refs. 

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