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Chapter: Pharmaceutical Drugs and Dosage: Surfactants and micelles

Pharmaceutical Drugs and Dosage: Surfactants and micelles - Review questions answers


Review questions

10.1 Which of the following dosage forms may utilize surface-active agents in their formulations?

A.      Emulsions

B.      Suspensions

C.      Colloidal dosage forms

D.      Creams

E.       All of the above

10.2 Increasing the surfactant concentration above the critical micellar concentration will result in:

A.      An increase in surface tension

B.      A decrease in surface tension

C.      No change in surface tension

D.      All of the above

10.3 Which of the following surfactants is incompatible with anionic bile salts?

A.      Polysorbate 80

B.      Potassium stearate

C.      Sodium lauryl sulfate

D.      Benzalkonium chloride

E.       All of the above

10.4 Which of the following statements is TRUE?

A.      Most substances acquire a surface charge by ionization, ion adsorption, and ion dissolution

B.      The term surface tension is used for liquid–vapor and solid–vapor interfaces

C.      At the isoelectric point, the total number of positive charges is equal to the total number of negative charges on a molecule

D.      All of the above

E.       None of the above

10.5 A. Enlist three pharmaceutical applications of surfactants.

B. Enlist three different types of surfactants.

C. You formulated an emulsion by using a surfactant with an HLB value of 18. The emulsion was highly unstable. Explain why there was a problem with emulsion stability.

10.6 Define micelles, critical micellization concentration, and aggrega-tion numbers. What are the types of micelles and how do they form? Describe with the help of a diagram.

10.7 Draw a diagram to illustrate the change in surface tension and con-ductivity with increasing the concentrations of sodium lauryl sulfate in water versus that of glucose. Why should the profile be different in two cases?

10.8 Define and differentiate between cloud point and Krafft point. What are the three factors affecting cloud point?

10.9 Define micellar solubilization with the aid of a diagram. Mention any three properties of the drug that are affected by this phenom-enon. How does the alkyl chain length of the surfactant affect the solubilization of a hydrophobic drug?

10.10 Which of the following is a (i) cationic, (ii) anionic, (iii) nonionic, or (iv) ampholytic surfactant?

A.      Cetrimide

B.      Benzalkonium chloride

C.      Sodium lauryl sulfate

D.      Lecithin

E.       Span 20

F.       Tween 80

Answers:

10.1 E. Surface-active agents facilitate emulsion formation by lower-ing the interfacial tension between the oil and the water phases. Adsorption of surfactants on insoluble particles enables these particles to be dispersed in the form of a suspension.

10.2 C. Increasing the surfactant concentration above the critical micellar concentration will result in no change in surface tension.

10.3 D. Benzalkonium chloride is a cationic surfactant and can interact with bile salts.

10.4 D. Most substances acquire a surface charge by ionization, ion adsorp-tion, and ion dissolution. At the isoelectric point, the total number of positive charges is equal to the total number of negative charges.

10.5 A. Surfactants are used as emulsifying agents, solubilizing agents, detergents, and wetting agents.

B. Anionic, cationic, and nonionic surfactants.

C. Because a surfactant having a HLB value of 18 is highly hydro-philic and does not deposit on the interface.

10.6 Micelles are the aggregates of surface-active agents in solution, which may contain 50 or more monomers. Micelles are small spherical structures composed of both hydrophilic and hydrophobic groups. The concentration of monomer at which micelles are formed is called the critical micellization concentration (CMC). The num-ber of monomers that aggregate to form a micelle is known as the aggregation number of the micelle.

Micelles can be of three types: (1) Normal micelles have the lipophilic parts of the surfactant toward the core and hydrophilic parts toward the periphery, or solvent. These micelles are formed in water. (2) Reverse micelles have the hydrophobic groups toward the outside and the hydrophilic parts toward the core. These form in nonaqueous solvents. (3) Lamellar micelles form at concentrations much higher than the CMC. These are present in the physiological membranes (refer to Figure 9.3).

10.7 Sodium lauryl sulfate is a surfactant, glucose is not. Glucose will show very little change in surface tension, whereas sodium lauryl sulfate will display a sharp fall at CMC but an increase in conductiv-ity (refer to Figure 9.4).

10.8 Cloud point is the temperature above which some surfactants begin to precipitate, whereas Krafft point is the temperature above which the solubility of a surfactant becomes equal to the CMC. Three factors affecting the cloud point are organic solubilisates, aliphatic hydro-carbons, and aromatic hydrocarbons. Organic solubilisates generally decrease the cloud point of nonionic surfactants. Aliphatic hydrocar-bons tend to raise the cloud point. Aromatic hydrocarbons or alkanols may raise or lower the cloud point depending on the concentration.

Below the Krafft point, it is possible that even at the maximum solubility of the surfactant, the interface may not be saturated and, therefore, there is no reason for micelles to form. The surfactant has a limited solubility, and below the Krafft point, the solubility is insufficient for micellization. As the temperature increases, solubil-ity slowly increases. At the Krafft point, surfactant crystals melt and are incorporated into the micelles. Above the Krafft point, micelles will form and, owing to their high solubility, there will be a dramatic increase in surfactant solubility.

10.9 Micelles can be used to increase the solubility of materials that are insoluble or poorly soluble in the dispersion medium used. This phe-nomenon is known as solubilization and the incorporated substance is referred to the solubilisate. The three factors affecting micellar solubilization are the nature of surfactants, the nature of solubilis-ates, and the temperature. For a hydrophobic drug solubilized in a micelle core, an increase in the lipophilic alkyl chain length of the surfactant enhances solubility, whereas an increase in the alkyl chain length results in an increase in the micellar radius, reducing pres-sure, resulting in an increase in the entry of the drug into the micelle. For ionic surfactant micelles, increase in the radius of the hydrocar-bon core is the main way to enhance solubilization.

10.10 A. Cationic

B. Cationic

C. Anionic

D. Ampholytic

E. Nonionic

F. Nonionic

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