Fundamentals of ATR-FTIR Spectroscopy and Its Role …
25
Fig. 13 a Adsorbate ATR-FTIR vibrational profile of the n-propanol and water binary mixture with
varying mole fraction (y) values of n-propanol (1–0). The CH-region (2750–3000 cm −1 ) intensities
are amplified by 5 for clear representation. b Adsorbed effective thickness measured from the peak
intensity values obtained from ATR-FTIR spectra separately of n-propanol and water molecules
over SiO 2 . Adapted with permission from [68]. Copyright, 2012, American Chemical Society
different thermal properties, which distinguishes it from pure solutions [55, 57–60].
These characteristics are influenced by several external factors like temperature, ionic
strength, mechanical disturbances, etc. [61–63]. Comprehension of these parameters
offers useful insights about binary mixtures in the domains of chromatographic purification or adulteration processes, biomolecular coupling reactions, drug designing,
microfluidic devices and hydrodynamic cavitation in microchannels [62, 64–66].
Several studies have been performed aiming at the interaction processes occurring
among the water molecules and different organic liquids, including alcohols, acetone,
dimethylsulfoxide (DMSO), acetonitrile, esters, ethers, dimethylformamide (DMF),
and amine-based solvents [57, 59, 61–63, 67]. Barnette and Kim [68] probed the
adsorbate thickness of the binary mixture of n-propanol and water layered on the
fused silica surface. Panel a of Fig. 13 display the ATR-FTIR spectra of n-propanol
with water as a function of varying n-propanol vapor mole fraction (y). The observed
results depicted that the spectral intensity of alkyl stretching mode stays relatively
even for the y propanol between 1 and 0.4. Below y propanol value of 0.36, the intensity gets
significantly decreased. However, contrastingly in the higher wavenumber region, the
OH-stretch vibrational mode observed at ~3550 cm
−1 is seen to show a slight red
shift along with a rise in the intensity in its vibrational mode towards peak centered
at ~3400 cm
−1 as y propanol decreases beyond 0.6. Thus, as evident from the spectra,
the presence of water molecules becomes dominant. The authors utilized the ATR
intensity profile of the n-propanol-water binary mixture to evaluate the effective
thickness of the adsorbed layer. Figure 13 (panel b) discretely shows that as the
y propanol decreases beyond 0.4 value, the effective thickness of the n-propanol layer
over the crystal surface also decreases reaching ~0.23 nm thick layer (equivalent
to 0.4 mL of n-propanol). It could be conversely realized for an increase in the
multilayer formation of water molecules which rises to ~6.5 nm (>20 mL) of the
adsorbed layer over the crystal surface. Therefore, considering a direct association
25
Fig. 13 a Adsorbate ATR-FTIR vibrational profile of the n-propanol and water binary mixture with
varying mole fraction (y) values of n-propanol (1–0). The CH-region (2750–3000 cm −1 ) intensities
are amplified by 5 for clear representation. b Adsorbed effective thickness measured from the peak
intensity values obtained from ATR-FTIR spectra separately of n-propanol and water molecules
over SiO 2 . Adapted with permission from [68]. Copyright, 2012, American Chemical Society
different thermal properties, which distinguishes it from pure solutions [55, 57–60].
These characteristics are influenced by several external factors like temperature, ionic
strength, mechanical disturbances, etc. [61–63]. Comprehension of these parameters
offers useful insights about binary mixtures in the domains of chromatographic purification or adulteration processes, biomolecular coupling reactions, drug designing,
microfluidic devices and hydrodynamic cavitation in microchannels [62, 64–66].
Several studies have been performed aiming at the interaction processes occurring
among the water molecules and different organic liquids, including alcohols, acetone,
dimethylsulfoxide (DMSO), acetonitrile, esters, ethers, dimethylformamide (DMF),
and amine-based solvents [57, 59, 61–63, 67]. Barnette and Kim [68] probed the
adsorbate thickness of the binary mixture of n-propanol and water layered on the
fused silica surface. Panel a of Fig. 13 display the ATR-FTIR spectra of n-propanol
with water as a function of varying n-propanol vapor mole fraction (y). The observed
results depicted that the spectral intensity of alkyl stretching mode stays relatively
even for the y propanol between 1 and 0.4. Below y propanol value of 0.36, the intensity gets
significantly decreased. However, contrastingly in the higher wavenumber region, the
OH-stretch vibrational mode observed at ~3550 cm
−1 is seen to show a slight red
shift along with a rise in the intensity in its vibrational mode towards peak centered
at ~3400 cm
−1 as y propanol decreases beyond 0.6. Thus, as evident from the spectra,
the presence of water molecules becomes dominant. The authors utilized the ATR
intensity profile of the n-propanol-water binary mixture to evaluate the effective
thickness of the adsorbed layer. Figure 13 (panel b) discretely shows that as the
y propanol decreases beyond 0.4 value, the effective thickness of the n-propanol layer
over the crystal surface also decreases reaching ~0.23 nm thick layer (equivalent
to 0.4 mL of n-propanol). It could be conversely realized for an increase in the
multilayer formation of water molecules which rises to ~6.5 nm (>20 mL) of the
adsorbed layer over the crystal surface. Therefore, considering a direct association