water molecules when combined with Na
+ and Li
+ cations, and no more than one
water molecule when combined with weakly hydrated cations like Cs
+
, K
+ and
NH 4
þ . It points at the formation of the solvent separated ion pair between carboxylate
anions and Na
+ and Li
+ cations. What is more, the observed propensity of cations
forming solvent separated ion pairs with formate anion ðCs
þ
\K
þ
; NH 4
þ
\Li
þ
\Na
þ
Þ followed well Hofmeister series for monovalent cations. The specificity of ions interactions with macromolecules in water was reviewed by Liu and Liu
in [122], and specific ion interactions with polycationic brushes were described in
detail by Willott et al. [123].
Nonlinear spectroscopy studies on aqueous polymer pH-responsive systems are
rather limited. A particular potential of such studies was certainly shown by VSFG
spectroscopy, which is a surface-specific method providing spectra of the layer a
few molecules thick. The surface specificity is inherent to this method, as VSFG
measures the second-order nonlinear susceptibility v
(2) which can be generated only
in non-centrosymmetric media. The net signal from the bulk of the material cancels
out and only a few molecular layers present at an interface contribute to a VSFG
spectrum [124]. Initial studies on the surface propensity of the pH-responsive poly
(acrylic acid) [125, 126] and poly (methacrylic acid) [126] were performed by the
Richmond’s group. They observed a strongly pH-dependent multi-step process of
surface absorption of both polyacids. Further studies on the surface propensity of
polyacrylic acid were performed by Balzerowski et al. [127] with the use of
heterodyne-detected VSFG. The heterodyne scheme of detection of this method
allows for disentangling real and imaginary parts of v
(2) as a function of the frequency and the sign of imaginary part of the spectrum is directly connected to the
absolute orientation of molecules at the interface. Thus, it may be easily deduced
from spectra which chemical groups point towards and which outward an interface.
Balzerowski et al. observed that the polyacrylic acid has the increased surface
propensity below the degree of ionization of 0.203, and above this value, the
polymer is well dissolved in the bulk. Moreover, it was shown that the ionization of
the carboxylic acid groups is accompanied by an increase in the VSFG signal of
interfacial water molecules as a result of enhanced orientation of water.
The high surface specificity of VSFG makes it an excellent tool for studying
systems such as stimuli-responsive brushes, carpets. Kondo et al. [128] investigated
zwitterionic poly(carboxymethylbetaine) brushes prepared with the use of atom
transfer radical polymerisation (ATRP) method on the fused quartz substrate. With
the use of VSFG, they studied orientation of brushes on a surface by polarizationresolved analysis of the polymer C–H stretching bands. They also noticed only tiny
orientation of water molecules located in the vicinity of a zwitterionic brush. One can
easily imagine numerous applications of VSFG to studies of stimuli-responsive layers
and interfaces. It should be kept in mind, however, that VSFG signal is generated
through focusing two femtosecond beams of high pulse peak power on the very
interfacial layer. A strong optical field often leads here to thermal decomposition of
organic molecules present at the interface. Hence, an VSFG experiment on the layer
with high concentration of a polymer would be difficult to perform.
238
M. Kozanecki et al.
+ and Li
+ cations, and no more than one
water molecule when combined with weakly hydrated cations like Cs
+
, K
+ and
NH 4
þ . It points at the formation of the solvent separated ion pair between carboxylate
anions and Na
+ and Li
+ cations. What is more, the observed propensity of cations
forming solvent separated ion pairs with formate anion ðCs
þ
\K
þ
; NH 4
þ
\Li
þ
\Na
þ
Þ followed well Hofmeister series for monovalent cations. The specificity of ions interactions with macromolecules in water was reviewed by Liu and Liu
in [122], and specific ion interactions with polycationic brushes were described in
detail by Willott et al. [123].
Nonlinear spectroscopy studies on aqueous polymer pH-responsive systems are
rather limited. A particular potential of such studies was certainly shown by VSFG
spectroscopy, which is a surface-specific method providing spectra of the layer a
few molecules thick. The surface specificity is inherent to this method, as VSFG
measures the second-order nonlinear susceptibility v
(2) which can be generated only
in non-centrosymmetric media. The net signal from the bulk of the material cancels
out and only a few molecular layers present at an interface contribute to a VSFG
spectrum [124]. Initial studies on the surface propensity of the pH-responsive poly
(acrylic acid) [125, 126] and poly (methacrylic acid) [126] were performed by the
Richmond’s group. They observed a strongly pH-dependent multi-step process of
surface absorption of both polyacids. Further studies on the surface propensity of
polyacrylic acid were performed by Balzerowski et al. [127] with the use of
heterodyne-detected VSFG. The heterodyne scheme of detection of this method
allows for disentangling real and imaginary parts of v
(2) as a function of the frequency and the sign of imaginary part of the spectrum is directly connected to the
absolute orientation of molecules at the interface. Thus, it may be easily deduced
from spectra which chemical groups point towards and which outward an interface.
Balzerowski et al. observed that the polyacrylic acid has the increased surface
propensity below the degree of ionization of 0.203, and above this value, the
polymer is well dissolved in the bulk. Moreover, it was shown that the ionization of
the carboxylic acid groups is accompanied by an increase in the VSFG signal of
interfacial water molecules as a result of enhanced orientation of water.
The high surface specificity of VSFG makes it an excellent tool for studying
systems such as stimuli-responsive brushes, carpets. Kondo et al. [128] investigated
zwitterionic poly(carboxymethylbetaine) brushes prepared with the use of atom
transfer radical polymerisation (ATRP) method on the fused quartz substrate. With
the use of VSFG, they studied orientation of brushes on a surface by polarizationresolved analysis of the polymer C–H stretching bands. They also noticed only tiny
orientation of water molecules located in the vicinity of a zwitterionic brush. One can
easily imagine numerous applications of VSFG to studies of stimuli-responsive layers
and interfaces. It should be kept in mind, however, that VSFG signal is generated
through focusing two femtosecond beams of high pulse peak power on the very
interfacial layer. A strong optical field often leads here to thermal decomposition of
organic molecules present at the interface. Hence, an VSFG experiment on the layer
with high concentration of a polymer would be difficult to perform.
238
M. Kozanecki et al.