106
S. Roy et al.
as suggested by the analysis of the hydration shell water of IO 3
– and I
– anions using
Raman multivariate curve resolution (Raman-MCR) spectroscopy [52–54]. Under
ED-approximation, appearance of the anion hydration shell water in SFG signal
suggests that the inversion symmetry of the hydration shell is broken at the interface,
i.e. the anions are asymmetrically hydrated in the interfacial region. Given the sharply
varying number density of water across the interface, asymmetric hydration of anion
in terms of water number density and orientational preference along the surface
normal is quite reasonable.
5.3 Small Amphiphiles at Air-Water Interface
Amphiphilic small molecule has a hydrophilic group along with a hydrophobic
moiety of limited number of carbon atoms (~4 to 5 carbon). In bulk water such small
hydrophobic groups are accommodated through the formation of cavity created by
the water molecules that are strongly interacting with each other at the surface of
the small hydrophobic group, known as ‘hydrophobic hydration’. Although strong
water-water interaction provides an enthalpic favour to hydrophobic hydration, the
cavitation energy (i.e., energy associated with the creation of the cavity) and the
reduced entropy of the hydration water go against the dissolution of the hydrophobic
group. However, at aqueous interface, these small molecules get readily adsorbed by
protruding their hydrophobic part towards the hydrophobic air (i.e. away from the
aqueous phase) while the hydrophilic moiety remains exposed to the aqueous phase,
making the molecule preferentially oriented at the interface. The preferred orientation
renders differential accessibility of the hydrophobic and the hydrophilic groups from
the aqueous and the gas phases. This feature gives rise to interface-specific solutesolute and solute-solvent interactions which strongly contribute to the dynamics and
kinetics of surface reactions. In light of HD-VSFG measurements, preferential orientations of some environmentally and biologically relevant small amphiphiles such as
acetone, dimethyl sulfoxide (DMSO), propylene carbonate (PC), trimethylamine-Noxide (TMAO) at the air-water interface and the associated change of the interfacial
water structure and orientation are discussed below.
Figure 9 shows the Imχ
(2) spectra of the air-water interface in presence and
absence of acetone, dimethyl sulfoxide (DMSO), and propylene carbonate (PC).
For all the amphiphile solutions, the Imχ
(2) spectra show a negative band within
2800–3000 cm
−1 regions (CH stretch region), while for the neat air-water interface,
the Imχ
(2) signal is zero in that region. The Imχ
(2) bands correspond to the methyl
symmetric stretch (CH 3
SS ~2920 cm
−1 for DMSO and acetone and ~2876 cm
−1 for
PC) and Fermi resonance (CH 3 FR~2925 cm
−1 for PC) of the adsorbed amphiphiles.
The negative sign of CH 3
SS band reveals that the methyl groups of the amphiphiles
are oriented towards the air (methyl-up orientation) at the air-water interface. It is
important to note that, opposite to water OH-stretch, the sign of the hyperpolarizability (β ccc ) of CH 3
SS is negative and hence the negative sign of Imχ
(2) indicates
S. Roy et al.
as suggested by the analysis of the hydration shell water of IO 3
– and I
– anions using
Raman multivariate curve resolution (Raman-MCR) spectroscopy [52–54]. Under
ED-approximation, appearance of the anion hydration shell water in SFG signal
suggests that the inversion symmetry of the hydration shell is broken at the interface,
i.e. the anions are asymmetrically hydrated in the interfacial region. Given the sharply
varying number density of water across the interface, asymmetric hydration of anion
in terms of water number density and orientational preference along the surface
normal is quite reasonable.
5.3 Small Amphiphiles at Air-Water Interface
Amphiphilic small molecule has a hydrophilic group along with a hydrophobic
moiety of limited number of carbon atoms (~4 to 5 carbon). In bulk water such small
hydrophobic groups are accommodated through the formation of cavity created by
the water molecules that are strongly interacting with each other at the surface of
the small hydrophobic group, known as ‘hydrophobic hydration’. Although strong
water-water interaction provides an enthalpic favour to hydrophobic hydration, the
cavitation energy (i.e., energy associated with the creation of the cavity) and the
reduced entropy of the hydration water go against the dissolution of the hydrophobic
group. However, at aqueous interface, these small molecules get readily adsorbed by
protruding their hydrophobic part towards the hydrophobic air (i.e. away from the
aqueous phase) while the hydrophilic moiety remains exposed to the aqueous phase,
making the molecule preferentially oriented at the interface. The preferred orientation
renders differential accessibility of the hydrophobic and the hydrophilic groups from
the aqueous and the gas phases. This feature gives rise to interface-specific solutesolute and solute-solvent interactions which strongly contribute to the dynamics and
kinetics of surface reactions. In light of HD-VSFG measurements, preferential orientations of some environmentally and biologically relevant small amphiphiles such as
acetone, dimethyl sulfoxide (DMSO), propylene carbonate (PC), trimethylamine-Noxide (TMAO) at the air-water interface and the associated change of the interfacial
water structure and orientation are discussed below.
Figure 9 shows the Imχ
(2) spectra of the air-water interface in presence and
absence of acetone, dimethyl sulfoxide (DMSO), and propylene carbonate (PC).
For all the amphiphile solutions, the Imχ
(2) spectra show a negative band within
2800–3000 cm
−1 regions (CH stretch region), while for the neat air-water interface,
the Imχ
(2) signal is zero in that region. The Imχ
(2) bands correspond to the methyl
symmetric stretch (CH 3
SS ~2920 cm
−1 for DMSO and acetone and ~2876 cm
−1 for
PC) and Fermi resonance (CH 3 FR~2925 cm
−1 for PC) of the adsorbed amphiphiles.
The negative sign of CH 3
SS band reveals that the methyl groups of the amphiphiles
are oriented towards the air (methyl-up orientation) at the air-water interface. It is
important to note that, opposite to water OH-stretch, the sign of the hyperpolarizability (β ccc ) of CH 3
SS is negative and hence the negative sign of Imχ
(2) indicates