13 Overview of Application of NIR Spectroscopy to Physical …
303
Self-association of a series of aliphatic alcohols in CCl 4 (0.01–1.00 M) has for
the first time been examined by means of multivariate curve resolution (MCR) in the
NIR region from 1900–2200 nm (5263–4556 cm
−1 ) [19]. The MCR method enabled
to resolve NIR spectra into three components originating from monomers, linear
aggregates, and cyclic aggregates. As shown, the size of the aggregates increases
with increasing concentration. Due to steric hindrance, the branched alcohols tend
to form smaller aggregates as compared to straight chain alcohols. It should be noted
that the formation of the cyclic aggregates is more favorable than theô linear ones.
As a result, at low to moderate concentrations of alcohols, the solutions are expected
to consist mainly of the cyclic aggregates.
The examination of concentration and temperature-induced changes in NIR
spectra of different alcohols lead to the conclusion that the degree of association
in the saturated straight-chain 1–alcohols decreases with an increase in the chain
length [26, 27]. For long-chain alcohols the hydrophobic interactions between the
chains have a stronger effect on the aggregation than the OH···OH interactions. Similarly, an increase in the order of the alcohol also leads to a decrease in the extent of
the self-association as a result of restricted accessibility of the OH group [18, 27,
28]. In spite of using FT–NIR spectroscopy coupled with 2DCOS it was not possible
to identify any differences between the spectra of the optically active and racemic
samples of octan–2–ol [28]. A possible explanation is that the relatively high degree
of freedom of the OH group in octan–2–ol is reducing the chiral discrimination effect.
The combination of NIR spectroscopy with measurements of nonlinear dielectric
effects provided further information on the hydrogen bonding and association of
octyl alcohols in the pure liquid phase and CCl 4 solutions [29]. At low to moderate
concentrations of the alcohol, nonpolar cyclic species dominate. An increase in the
concentration of the alcohol leads to the formation of linear species showing a significant dipole moment, and this tendency is more pronounced in case of straight-chain
alcohols. The branching in the vicinity of the OH group enhances the probability of
the formation of cyclic associates.
Recently, Orzechowski and Czarnecki studied the association of 1–hexanol in
n–hexane by using experimental (nonlinear dielectric effect, NIR spectroscopy) and
theoretical (DFT) methods [30]. Numerical fitting of the dielectric data provided
populations of all species present in the mixture (Fig. 13.5). At lower alcohol content,
the cyclic species dominate, while at higher concentrations of 1–hexanol, the equilibrium shifts toward linear associates. The obtained results do not confirm common
assumption that the cyclic tetramers are the most populated species in liquid alcohols.
It appears that the most abundant cyclic species are trimers and population of the
cyclic associates rapidly decreases with increasing size of the associates. An average
size of the associates depends on the range of concentrations. This observation nicely
explains differences between models of association obtained from various studies.
Differences in the population of the free OH group obtained from dielectric and
NIR measurements may suggest that the bands due to the free OH do not include
contribution from free-terminal OH groups in the open associates. However, this
controversial supposition needs further experimental verification.
303
Self-association of a series of aliphatic alcohols in CCl 4 (0.01–1.00 M) has for
the first time been examined by means of multivariate curve resolution (MCR) in the
NIR region from 1900–2200 nm (5263–4556 cm
−1 ) [19]. The MCR method enabled
to resolve NIR spectra into three components originating from monomers, linear
aggregates, and cyclic aggregates. As shown, the size of the aggregates increases
with increasing concentration. Due to steric hindrance, the branched alcohols tend
to form smaller aggregates as compared to straight chain alcohols. It should be noted
that the formation of the cyclic aggregates is more favorable than theô linear ones.
As a result, at low to moderate concentrations of alcohols, the solutions are expected
to consist mainly of the cyclic aggregates.
The examination of concentration and temperature-induced changes in NIR
spectra of different alcohols lead to the conclusion that the degree of association
in the saturated straight-chain 1–alcohols decreases with an increase in the chain
length [26, 27]. For long-chain alcohols the hydrophobic interactions between the
chains have a stronger effect on the aggregation than the OH···OH interactions. Similarly, an increase in the order of the alcohol also leads to a decrease in the extent of
the self-association as a result of restricted accessibility of the OH group [18, 27,
28]. In spite of using FT–NIR spectroscopy coupled with 2DCOS it was not possible
to identify any differences between the spectra of the optically active and racemic
samples of octan–2–ol [28]. A possible explanation is that the relatively high degree
of freedom of the OH group in octan–2–ol is reducing the chiral discrimination effect.
The combination of NIR spectroscopy with measurements of nonlinear dielectric
effects provided further information on the hydrogen bonding and association of
octyl alcohols in the pure liquid phase and CCl 4 solutions [29]. At low to moderate
concentrations of the alcohol, nonpolar cyclic species dominate. An increase in the
concentration of the alcohol leads to the formation of linear species showing a significant dipole moment, and this tendency is more pronounced in case of straight-chain
alcohols. The branching in the vicinity of the OH group enhances the probability of
the formation of cyclic associates.
Recently, Orzechowski and Czarnecki studied the association of 1–hexanol in
n–hexane by using experimental (nonlinear dielectric effect, NIR spectroscopy) and
theoretical (DFT) methods [30]. Numerical fitting of the dielectric data provided
populations of all species present in the mixture (Fig. 13.5). At lower alcohol content,
the cyclic species dominate, while at higher concentrations of 1–hexanol, the equilibrium shifts toward linear associates. The obtained results do not confirm common
assumption that the cyclic tetramers are the most populated species in liquid alcohols.
It appears that the most abundant cyclic species are trimers and population of the
cyclic associates rapidly decreases with increasing size of the associates. An average
size of the associates depends on the range of concentrations. This observation nicely
explains differences between models of association obtained from various studies.
Differences in the population of the free OH group obtained from dielectric and
NIR measurements may suggest that the bands due to the free OH do not include
contribution from free-terminal OH groups in the open associates. However, this
controversial supposition needs further experimental verification.