Surface and interfacial phenomenon:
Liquid interface, surface & interfacial tensions,surface free energy, measurement of surface & interfacial tensions, spreading coefficient, adsorption at liquid interfaces, surface active agents, HLB Scale, solubilisation, detergency, adsorption at solid interface.
Surface tension occurs whenever there is an interface between a liquid, a solid or a gas. Surface tension of water is an important property in situations where small volumes of liquid occur, or the liquid is in contact with small diameter tubes or porous media. The behaviour of
molecules at boundaries between two immiscible phases is different from their behavior in the bulk of the phases, which has implications for the physiology of the human body as well as for pharmacy. Interfacial phenomena affect drug delivery systems. For example, solubilization and dispersion of drugs, suspension or emulsion stability, and adsorption of drugs on different substrates are all affected by the interfacial properties of drugs and their environment.
After studying the contents of the chapter, students are expected to:
• Understand types of interfaces and describe relevant examples.
• Understand the terms surface tension and interfacial tension and their application in pharmaceutical sciences.
• Understand the concept of surface and interface tensions, surface free energy, its changes, work of cohesion and adhesion, and spreading and methods of their measurements.
• Understand the mechanisms of adsorption on liquid and solid interfaces.
• Differentiate between different types of mono-layers and recognize basic methods for their characterization.
1. What are surface, interface and surface?
2. What are units of surface tension and interfacial tension?
3. Explain the terms surface excess and surface pressure.
4. Enlist methods to determine surface tension of liquids and solids.
5. Describe capillary rise method for determination of surface tension of liquids.
6. Describe drop method to determine surface tension of liquid.
7. Why drop of liquid hanging in air is spherical in shape?
8. What is effect of surfactant concentration and solute on surface tension of liquids?
9. Write about first tensiometer developed and used to determine surface tension.
10. Explain bubble pressure method to determine surface tension.
11. Classify surfactants based on their HLB values.
12. Write note on HLB system and its applications.
13. A polyhydric fatty acid ester has saponification number 48 and acid number 280. What
will be HLB value of ester?
14. Draw HLB scale stating different HLB value ranges for surfactant for their application.
15. Enlist factors affecting HLB value of surfactant. Write on drawbacks of HLB.
16. What is surface free energy? Explain methods to determine it.
17. Elaborate the statement ’Surface tension decreases with increase in temperature’.
18. At 20°C the same volume of water and oil produced 20 and 60 drops using
Stalagmometer. If surface tension of water is 72.8 dynes/cm at same temperature, at which density of oil is 0.872 g/mL; calculate surface tension of oil.
19. The surface excess of long chain amphiphile in water was 3 x 10-9 mol/cm2; calculate area
occupied by each molecule at the surface.20. The surface excess of amphiphile is 5.49 × 10-9 mol/cm2
at a bulk concentration of
3 × 10−3 mol/L; calculate area occupied by each amphiphile molecule at the surface.
(N = 6.02 × 1023)
21. What are wetting agents? Explain their mechanism of action.
22. What is critical micelle concentration? Explain the changes observed on properties of
surfactant solutions at CMC.
23. What are pharmaceutical applications of critical micelle concentration?
24. Enlist methods other than surface tension to determine CMC of surfactant solution.
26. What is adsorption? Differentiate between physisorption and chemisorption.
27. Write characteristics of physisorption and chemisorption.
28. What are assumptions of Langmuir’s adsorption study?
29. Describe Langmuir’s adsorption isotherm to determine the constants ‘log k’ and ‘b’ in the
isotherm equation.
30. Explain Freundlich adsorption isotherm.
31. Write short on Langmuir adsorption isotherm.
32. What is spreading coefficient? Obtain expression for the same.
33. Derive an equation of spreading coefficient. What is its significance in pharmacy?
34. How knowledge of surface tension does helps in understanding of spreading coefficient?
35. Addition of solid particles in to a liquid vehicle is critical step in the preparation of pharmaceutical dispersions. Explain this statement with spreading wetting.
36. When two immiscible liquids are mixed together they fail to remain mixed. Explain.
37. Surface tension of water is 77.8 dyne/cm and that of benzene is 27.1 dyne/cm while interfacial tension between them is 35 dynes/cm then what was the initial spreading coefficient? After establishment of equilibrium, surface tension of water reduces to
62.2 dynes/cm and that of benzene it becomes 27 dynes/cm. What was the final spreading coefficient?
38. Draw different type of adsorption curves and discussion their applications.
39. How you will determine cross sectional area per molecule form adsorption studies?
40. Draw schematic of film balance. Explain the concept of surface pressure.
41. A 5 mL of an oil having molecular weight 300 and density 0.9 g/mL is placed on half an acre (2 × 107 cm2) of pond; calculate length and cross sectional area of the oil molecule.
42. Explain phenomenon of wetting and spreading with the help of suitable contact angle measurement.
43. Explain mechanism of Cosolvents in improving solubility of solutes with suitable examples.
44. Solubility of majority of the drugs in water is influenced by the pH of the system. Explain with suitable example.
45. Describe use of surfactant to solubilize insoluble solutes.
46. Altering chemical structure of the molecule changes solubility of solute in the same solvent. Explain.
