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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