Chapter 14: Molecular Interactions

The discussion encompasses charge-dipole attractions, dipole-dipole interactions, induced dipole phenomena, London dispersion forces arising from instantaneous fluctuations in electron density, and hydrogen bonding as a special case of dipole interaction. These interactions are quantitatively modeled using the Lennard-Jones potential, which describes the balance between attractive van der Waals forces at moderate distances and repulsive forces at short ranges. For liquid systems, the chapter introduces the radial distribution function as a tool for characterizing short-range molecular ordering and explores surface behavior through the concepts of surface tension and surface pressure, demonstrating how surfactant molecules reduce interfacial energy through their amphiphilic structure. The chapter then shifts to macromolecular architecture, presenting the hierarchical organization of polymers across primary, secondary, tertiary, and quaternary structural levels. Random coil modeling provides a statistical mechanical framework for understanding polymer conformations, with key metrics including contour length and root-mean-square end-to-end separation that relate molecular structure to physical dimensions. The mechanical behavior of stretched polymers is explained through changes in conformational entropy rather than bond stretching, revealing the entropic elasticity of polymer systems. Finally, the chapter addresses self-assembly phenomena in colloids and micelles, emphasizing the hydrophobic interaction as the primary driving force in biological self-assembly. This interaction is fundamentally entropy-driven, arising from the thermodynamic cost of maintaining an ordered hydration shell around nonpolar molecules, making hydrophobic aggregation a spontaneous process mediated by solvent reorganization rather than direct molecular attraction.