Because
of the antioxidant activity of vitamin C (Vit C) polar heads, they
can be used as a protective agent for fatty acids. Hence, the study
on the growth of Vit C/stearic acid (SA) mixed binary films at air–water
interface (known as Langmuir monolayer) and air–solid interface
(known as Langmuir–Blodgett films) is of paramount interest.
Although Vit C is situated at subsurface beneath SA molecules and
interacts via hydrogen bonding between the hydroxyl groups of Vit
C and SA, several Vit C molecules may infiltrate within SA two-dimensional
matrix at the air–water interface. The increased mole fraction
of Vit C (0.125–0.5) and the reduction of temperature (from
22 to 10 °C) of the subphase water result in an increase in the
amount of adsorbed Vit C at the air–water interface. The surface
pressure (π)–area (
A
) isotherms illustrate
that such inclusion of Vit C provokes a spreading out of Vit C/SA
binary monolayers, which leads to an alteration of different physicochemical
parameters such as elasticity, Gibbs free energy of mixing, enthalpy,
entropy, interaction energy parameter, and activity coefficient. However,
being polar in nature, the transfer of pure Vit C on substrates gets
affected. It can be transferred onto substrate by mixing suitably
with SA as confirmed by infrared spectra. Their structures, extracted
X-ray reflectivity, and atomic force microscopy (topography and phase
imaging) are found to be strongly dependent on the nature of the substrate
(hydrophilic and hydrophobic).
We
study the structure and elastic properties of the bi-heterocyclic
azo compound at the air–water interface through surface pressure
(π)–area (
A
) isotherm recording followed
by transferring them on hydrophilic and hydrophobic Si surfaces by
the Langmuir–Blodgett (LB) deposition method. A substantial
change in the area/molecule is observed as a function of subphase
pH and temperature. Such parameters strongly influence intramolecular
interactions within azo molecules and the interactions between azo
molecules and water that manifested in higher surface activity at
low temperature and high pH, which in turn modifies the elasticity
of azo assembly at the air–water interface. A large pH-dependent
hysteresis with negative change in entropy, indicating molecular rearrangements,
is observed. Molecular assembly formed at the air–water interface
is then transferred onto hydrophilic and hydrophobic Si surfaces at
two different surface pressures (5 and 30 mN/m) by the LB technique.
The structural analysis performed by X-ray reflectivity and atomic
force microscopy techniques suggests that the LB films exhibit an
abrupt layered structure on hydrophilic Si, whereas an overall rough
film is formed on hydrophobic Si. The coverage and compactness of
individual layers are found to increase with the deposition pressure
(5 to 30 mN/m).
We report here the swelling and relaxation properties of confined poly(n-butyl methacrylate) (PBMA) films having thicknesses of less than 70 nm under supercritical carbon dioxide (scCO2) using the X-ray reflectivity technique. Swellability is found to be dominant in thinner films compared to thicker ones as a consequence of the confinement-induced densification of the former. Swellability is proportionately increased with the density of the film. PBMA films exhibit a more significant swelling than do PS films, and their differences become more prominent with the increase in film thickness. A comparison between the results obtained for polystyrene (PS) and PBMA ultrathin films reveals that the swellability is dependent upon the specific intermolecular interaction between CO2 and the chemical groups available in the polymers. Owing to strong Lewis acid-base interactions with scCO2 and the lower glass-transition temperature (bulk Tg ≈ 29 °C), PBMA films exhibit a greater amount of swelling than do PS films (bulk Tg ≈ 100 °C). Though they reach to the different swollen state upon exposition, identical relaxation behavior as a function of aging time is evidenced. This unprecedented behavior can be ascribed to the strong bonding between trapped CO2 and PBMA that probably impedes the release of CO2 molecules from the swollen PBMA films manifested in suppressed relaxation.
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