RESUMO
In the present study, the intermolecular interactions between a water-insoluble phospholipid (DOPC) and water-soluble protein (myoglobin) and the interaction among themselves were investigated at the air-water interface using the Langmuir and Langmuir-Blodgett techniques. The effects of changes in physicochemical factors, like pH and temperature, on these interactions were also examined. Surface pressure-molecular area (π-A) isotherms of the DOPC monolayer at the air-water interface, with and without myoglobin (Myo) revealed the evolution of various physical properties, such as elastic, thermodynamic, and hysteric properties, in response to changes in subphase pH and temperature. With the increment of subphase pH from 5 to 7 at a fixed temperature (20 °C), the DOPC isotherm expanded, and the in-plane elasticity (CS-1) decreased, but no significant presence of hysteresis was encountered in either of the pH values. On the other hand, a diminution of temperature (from 20 to 5 °C) leads to an expansion of monolayers yielding low elasticity and significant hysteresis. The incorporation of Myo molecules within the DOPC monolayer decreased the CS-1 value of the DOPC monolayer. Such a decrement in CS-1 was also encountered while increasing the pH and decreasing the temperature (T) of the subphase in the absence of Myo. Systematic expansion of DOPC isotherm and increased hysteric area with the increase in Myo proportion were observed and the atomic force microscopy (AFM) observations suggested a strong conjugation between Myo and DOPC in the mixed monolayer. The denaturation effect of Myo molecules was studied using AFM at different temperatures. Furthermore, the Myo molecules were found to be most surface active at pH = 7, which is very close to its isoelectric point. These observations come up with the interaction mechanism between biomolecules under dynamically varied conditions.
RESUMO
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).
RESUMO
The growth of highly crystalline rubrene thin films for organic field effect transistor (OFET) application remains a challenge. Here, we report on the vapor-deposited growth of rubrene films on the substrates made of cadmium arachidate (CdA) multilayers deposited onto SiO2/Si(100) via the Langmuir-Blodgett technique. The CdA films, containing 2n+1 layers, with integer n ranging from 0 to 4, are surface-terminated identically by the methyl group but exhibit the thickness-dependent morphology. The morphology and structure of both CdA and rubrene films are characterized by X-ray reflectivity (XRR), X-ray diffraction (XRD), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and atomic force microscopy (AFM). Crystalline rubrene films, evidenced by XRD and marked by platelet features in AFM images, become observable when grown onto the CdA layer thicker than 5L. XRD data show that vertical ordering, that is, ordering along surface normal, of CdA multilayer substrates exerts a strong influence in promoting the crystalline growth of rubrene films. This promoted growth is not due to the surface energy of CdA layer but derived from the additional interaction localized between rubrene and CdA island sidewall and presumably strengthened by a close dimensional match between the a-axis of rubrene lattice and the layer spacing of CdA multilayer. The best OFET mobility is recorded for 9L CdA substrate and reaches 6.7 × 10(-2) cm(2) V(-1) s(-1), presumably limited by the roughness of the interface between CdA and rubrene films.
RESUMO
Differential charging in XPS was recently shown to be a novel technique for studying in-depth structural information of discrete cadmium layers in Langmuir-Blodgett (LB) multilayer films. Here we report structural modification in multilayer LB films after sulfidation using differential charging in angle-dependent XPS that are not observable by the x-ray reflectivity technique. An AFM study suggests less modification in compact LB films in comparison to the non-compact ones. The differential charging in the LB multilayers changes after sulfidation due to the formation of cadmium sulfide nanostructures in the cadmium arachidate LB matrix, which was reflected prominently in the differential charging of Cd 3d(5/2) XPS peaks. It was found that, even after sulfidation, the compact multilayer LB films were differentially charged in the out-of-plane (in-depth) direction, whereas this kind of differential charging was not apparent in rough and non-compact films. Our results clearly indicate that for LB films with compact structure a partial layered structure survives the impact of particle formation, whereas for non-compact films the modification is large and no specific conclusion could be drawn. While x-ray reflectivity cannot provide specific information about the internal structure of the post-sulfidation films it shows that the total thickness of the films reduces in all cases.
