When two phases meet, the boundary between them is called an interface. Drugs, to have a pharmacological effect must either interact with or pass through a number of interfaces. In some cases the dosage form itself contains a number of interfaces. The way the dosage form releases its active ingredients, its stability and its functionality are all affected by the way these boundaries interact.

Ansel has two chapters which deal with some important heterogeneous systems. Section III Chapter 9, page 244, deals with Ointments, Creams, Lotions etc.and Section V Chapter 13 page 346 deals with dispersed systems like suspensions and emulsions.  Martin covers the topic in Chapter 14 "Interfacial Phenomena" on page 362. This week we will examine what is going on at the various interfaces in each of these pharmaceutically important dosage forms. I suggest that you read over these chapters for general background now and then revisit them as we develop our concepts over the next two weeks. In the laboratories over the rest of the semester we will be making a number of these products (with the exception of transdermal patches).
 

Solid - Solid systems are relatively nonreactive because of the nature of solids. All the other systems are very dynamic and deserve our interest. We can refer to all of these boundaries as interfaces. In some context the interface between a liquid and a gas can also be referred to as a surface. Hence surface tension can be defined for a liquid - air interface but not for a liquid - liquid, interface while interfacial tension can be defined for all cases. An understanding of the molecular activity at an interface will help us to understand the behavior of the heterogeneous systems we use as dosage forms.
 

What is different about a molecule at the surface of a liquid from one that is in the bulk of the liquid?
How can our understanding of Gibbs free energy help us understand the characteristics of a liquid surface?
Definition - Surface or interfacial tension can be defined as the force needed to oppose the natural pull of the molecules in the surface or interface to minimize the size of that surface or interface.
What does that working definition mean to you? Can you explain it in a way that will help us all understand the nature of this force?

Definition - Surface or Interfacial Free Energy - This is a more exact definition. It is the work done to increase the surface by one unit of area. The units of surface tension are dynes/cm, while the units of surface free energy are ergs /cm squared. You will notice that both of these are the same value. (Multiply both by one unit of area and the answer comes out to be ergs.)
The symbol for surface tension is a gamma. (I can't figure out how to put one in my web site yet). Surface tension or surface free energy can be substituted for the Gibbs free energy term in the equation G=H-TS. Write that equation.
What are the implications of this equation? How does it help us understand the impact of surface tension?
 

There are several ways to measure surface tension. We will use the DuNouy tensiometer in laboratory.  An additional method is the capillary rise method. We will not perform this method but it does introduce some additional ways of looking at the impact of surface free energy.
What are the forces and/or energy that is(are) involved when you blow a soap bubble??
Bring a balloon to class and we can do an experiment to help us understand the relationship between pressure and surface free energy.
What is the Young-Laplace equation? How does it help us understand this phemonenon?

The capillary rise method uses the natural tendency of some liquids to rise in a capillary. These liquids form a hemisphere inside the capillary. The pressure across the curved surface acts against gravity to cause the liquid to rise. The length of the rise can be used to determine the surface tension.
What is hydrostatic pressure and how does it apply to this method of determining surface tension?