**4. Conclusion**

Measurements of surface rheological characteristics are of great importance for the pharmaceutical industry. Many of pharmaceutical processes depend on the cohesive and adhesive interactions between the materials used during the preparation of the product. Understanding and determination of surface free energies of both liquid and solid surfaces plays a key role in characterization of materials during their development, formulation and manufacturing of pharmaceutical applications. The chemical activity, adsorption, dissolution, and bioavailability of a drug may depend on the surface of the molecule.

There are several experimental approaches that one can employ to evaluate interfacial tension and large differences can exist among measurement methods. While one method may be proven useful for a number of applications, there are several restrictions that detract from its applicability in a specific system. The choice of the method depends on the nature of the interface, the rheology of the liquid(s), the range of temperature and pressure, ease of analysis, accuracy, precision, surface age, cost and convenience of the probing instrument. Most equilibrium methods may be used to measure dynamic tension, and there are certain methods by which one can measure solely dynamic tension. Most methods involve measurement of forces, interface shapes, pressure differences, or flow rates. Commonly used methods for measuring interfacial tension of various solutions as well as solid systems are mentioned in this chapter.

To facilitate an in-depth process understanding, a combination of experimental and computational design may be integrated in interfacial tension of compounds. Providing a simple method of correlating and predicting the interfacial properties of materials would be of great interest for pharmaceutical technology. There are various computational techniques in which the surface tension is evaluated through its thermodynamic definition or empirical equations. These methods require input data and several adjustable parameters obtained from multicomponent system and the pure component. Some of these equations on a thermodynamic basis are the two-parameter model for liquid mixtures. With these methods calculation of the free-energy between the two systems is a challenge to be accurately determined and these methods are difficult to implement at relatively high temperature due to stability problems. Empirical equations may be used to correlate and predict surface tension using one or two parameters. Some of these models may have limited range of applicability and may require a lot of experimental data. For practical use it is very important that the surface tension of multicomponent system can be predicted from the composition of the conjugate phases and some predictable physical parameters without any adjustable parameters.

In essence, an attempt has been made in this chapter to review and examine the performance of computational and experimental techniques in which surface tension are evaluated.
