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INTRODUCTION

Learning Objectives

After completing this chapter, the reader should be able to:

  • Explain the potential energy diagram.

  • Understand the types of intermolecular forces and their relative magnitudes.

  • Identify the physical characteristics of each state of matter.

  • Distinguish between the gaseous, liquid, and solid states of matter.

  • Relate the heat of vaporization and the boiling point of liquids to the magnitude of intermolecular forces.

  • Relate the heat of fusion and the melting point of solids to the magnitude of intermolecular forces.

  • Demonstrate and understand the physical properties of gases, liquids, and solids.

  • Describe the different types of solid materials used in pharmaceuticals.

  • Differentiate chemical and physical stability.

This chapter describes the three primary states of matter—gas, liquid, and solid—along with some examples of their application to health and pharmacy. It begins with a discussion of the intermolecular forces that determine the nature of the phase, then proceeds with a discussion of the three phases.

INTERMOLECULAR FORCES

The phase in which a particular substance exists will depend on intermolecular forces of the component molecules or atoms. If the forces are sufficiently attractive, this will cause the formation of a more condensed phase. The forces that hold the molecules or atoms together in a condensed phase can be overcome with sufficient kinetic energy, leading to the formation of a less condensed phase.

Intermolecular Binding Forces

When molecules interact with each other, they do so by the forces of both attraction and repulsion. Forces of attraction are essential for molecules to come together. The two types of attractive forces are called cohesive forces and adhesive forces. For example, when like molecules are attracted to each other this represents cohesive forces of attraction. When different molecules are attracted to one another, these are adhesive forces of attraction. A good analogy would be the opposite poles of a magnet, which attract one another when in close proximity. In addition to attracting each other, molecules are acted on by repulsive forces, which act to separate molecules. Using the magnet analogy, bringing two positive ends of a magnet in close proximity leads to repulsion, as would two negative ends. Attractive and repulsive forces between atoms or molecules can be described by their potential energy of interaction, denoted as the Lennard-Jones potential.1 This is described in the appendix of this chapter.

Intermolecular Attractive Forces

There are four main types of intermolecular attractive forces: the van der Waals attractive forces (dipole–dipole, dipole-induced dipole, induced dipole-induced dipole) and the ion–dipole forces. An additional and critical attractive force is the hydrogen bond that exists between an electronegative atom and a hydrogen atom. Hydrogen bonding is considered a uniquely strong type of dipoledipole interaction. It is also responsible for the existence of another attractive force: hydrophobic interactions.

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