Structure and Elastic Properties of an Unsymmetrical Bi-Heterocyclic Azo Compound in the Langmuir Monolayer and Langmuir–Blodgett Film

Abstract

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).

Physicochemical Properties of a Bi-aromatic Heterocyclic-Azo/BSA Hybrid System at the Air–Water Interface

“…These two terms are nothing but the area under the compression and expansion isotherm curves, respectively. Total work done for compressing 1 mol of molecules is given by 14 , 51 Hence the difference in work done is or Δ W can be expressed as …”

“…Since a significant hysteresis associated with the rearrangement of the film-forming molecules is observed, the entropy of the assembly may differ in each cycle. Applying fundamental laws of thermodynamics in this isothermal ( T is constant around 20 °C) compression–expansion cycle of a 2D monolayer system, one can calculate the change in entropy using the following equation: 14 …”

“…The structural characterization is easier in the case of solid-supported LB films than that of Langmuir films . Thus, a good alternative to in situ characterization of Langmuir monolayer in a LB trough is to imitate the assembly on the substrate using LB deposition. , Monolayer (upstroke), bilayer (downstroke–upstroke), and trilayer (upstroke–downstroke–upstroke) depositions of floating molecules were chosen for the preparation of LB films on desired Si surfaces at surface pressure π = 30 mN/m.…”

“… 11 − 13 But the amount of literature on mixed N-heterocyclic azo compounds is limited in literature. 14 − 16 In these azo-biheterocyclic compounds, the presence of a −N=N–C=N- azo–imine, functional group between two N-heterocyclic substances promotes their activity. 17 , 18 Azo-benzimidazole compounds are of significant importance due to their use as potent antibacterial, antitubercular, antitumor, and antifungal substances.…”

“…N-Heterocyclic back bones are found in imidazole, pyridine, and benzimidazole derivatives, which are actively used in pharmaceutical fields. − N-Heterocyclic compound polarizability favors their solubility in many polar synthetic and natural solvents. , The majority of N-heterocyclic compounds having different physical properties are soluble in water. − Moderate solubility of benzimidazole in water allows its use in synthesis of useful drugs. Azo-heterocyclic compounds have been tremendously developed in the last few decades having valuable activity in biological fields. − But the amount of literature on mixed N-heterocyclic azo compounds is limited in literature. − In these azo-biheterocyclic compounds, the presence of a −NN–CN- azo–imine, functional group between two N-heterocyclic substances promotes their activity. , Azo-benzimidazole compounds are of significant importance due to their use as potent antibacterial, antitubercular, antitumor, and antifungal substances. , Azo-benzimidazole also shows some particular physical properties, such as nonlinear optical and thermal properties. , The presence of active N-hydrogen in the imidazole part of benzimidazole leads to a tendency to form H-bonds with several amino acids of different protein moieties. In contrast, one benzene ring fused within the imidazole (i.e., benzimidazole) decreases the solubility in hydrophilic solvent compare to the imidazole derivatives. − Essentially, one of these N-heterocyclic azo-imidazole compounds can exhibit many biological − and physical properties. , …”