Molecular Interactions at Interfaces

Abstract

The study on molecular organization and structure formation at the nanometer length scale is important due to its vast application in the field of nanoscience and nanotechnology. Molecular interactions play a pivotal role in the process of molecular assembly. The properties of materials can be maneuvered precisely by manipulating the structures at the nanometer length scale. The field of thin films science and technology has been growing remarkably due to its enormous industrial applications. The properties of thin films depend on the nature of the adsorbate and the structures on the surface. The structures of the thin films on a surface leads to the growth of bulk material, and hence the material properties can be controlled by manipulating the structures of the thin films. The form of such structures depends on the molecule-substrate and intermolecular interactions. The development of thin films science and technology has influenced the field of nanoscience and nanotechnology significantly. In this chapter, we discuss the role of molecular interactions in ultrathin films at air-water (A-W) and air-solid (A-S) interfaces. We form monomolecular thick films on the surface of water and study the film stability, surface phases, and other thermodynamical parameters. We found that the stability of the films at the A-W interface primarily depends on the molecular-surface and intermolecular interaction. Amphiphilic molecules, when spread on the water surface, form a monomolecular thick film at A-W interface. Such monomolecular thick film is known as Langmuir monolayer. An amphiphilic molecule has two parts : hydrophilic (water loving) and hydrophobic (water hating) part. When such molecules with a proper balance between hydrophilic and hydrophobic parts are dispersed on water surface, the hydrophilic part gets anchored to the water surface whereas the hydrophobic part stays away from the water surface. Under such condition, the anchored molecules are constrained to move on the two dimensional smooth water surface. The surface density can be varied and a corresponding change in surface tension is recorded. A Langmuir monolayer has proved to be an ideal two dimensional system not only for studying the thermodynamics but also for depositing the films on different types of substrates by vertical deposition mechanism in a highly controlled manner. Such films at A-S interface are known as Langmuir-Blodgett (LB) films