Review questions answers

Review questions

17.1 Coalescence can be reduced by

A.      Decreasing the difference between the density of the dispersed phase and the density of the medium

B.      Adding an agent that reduces the viscosity of the medium

C.      Increasing the droplet size of the dispersed phase

D.      All of the above

17.2 When compounding an emulsion that contains a flavoring agent, the flavoring agent should be in the

A.      Continuous phase

B.      Discontinuous phase

C.      Aqueous phase

D.      Oil phase

E.       Emulsifier

17.3 Define and differentiate between the following:

A.      Creaming and breaking

B.      Creaming and sedimentation

C.      Coalescence and aggregation

D.      Phase inversion and self-emulsification

E.       Multiple emulsions and microemulsions

F.       SEDDS and SMEDDS

17.4 Explain how sedimentation and creaming in emulsions can be minimized. 17.5 Why is a surfactant needed to make stable emulsions? Explain which properties of a surfactant are important in formulating emulsions. Enlist two factors that determine whether an emulsion is o/w or w/o.

17.6 List the three mechanisms of emulsification.

17.7 What are emulsifying agents? List the three types of emulsifying agents and differences in their mechanism of stabilization of an emulsion, for example, in terms of the type of film formed around the dispersed phase and the zeta potential on the dispersed phase.

17.8 Which surfactants will you select for o/w and w/o emulsification.

17.9 Identify the type of self-emulsifying system most appropriate for the following statements (SEDDS or SMEDDS):

A.      Has lower dispersed-phase globule size after emulsification

B.      Has higher content of oil

C.      Has higher content of cosolvent

D.      Is transparent in appearance after emulsification

E.       Is likely to have higher oral bioavailability

17.10 Which of the following surfactants is suitable for the formulation of a o/w emulsion?

A.      Surfactant with an HLB value of 1–3

B.      Surfactant with an HLB value of 3–6

C.      Surfactant with an HLB value of 6–9

D.      Surfactant with an HLB value of 9–12

E.       Surfactant with an HLB value of 12–15

F.       Surfactant with an HLB value of 15–18

17.11 Which of the following surfactants is suitable for the formulation of a w/o emulsion?

A.      Surfactant with an HLB value of 1–3

B.      Surfactant with an HLB value of 3–6

C.      Surfactant with an HLB value of 6–9

D.      Surfactant with an HLB value of 9–12

E.       Surfactant with an HLB value of 12–15

F.       Surfactant with an HLB value of 15–18

Answers:

17.1 A.

17.2 A.

17.3 A. Creaming and breaking: Creaming is the upward movement of dispersed droplets relative to the continuous phase and it is a reversible process. In contrast, breaking is irreversible. When breaking occurs, simple mixing fails to resuspend the globules in a stable emulsified form, since the film surrounding the particles has been destroyed and the oil tends to coalesce.

B. Creaming and sedimentation: Creaming is the upward movement of dispersed droplets relative to the continuous phase, whereas sedimentation is the downward movement of particles.

C. Coalescence and aggregation: Coalescence is the process by which the emulsified particles merge with each other to form large par-ticles. Coalescence is an irreversible process because the film that surrounds the individual globules is destroyed. In aggregation, dispersed droplets come together but do not fuse. Aggregation is to some extent reversible.

D. Phase inversion: An emulsion is said to invert when it changes from an o/w to a w/o emulsion or vice versa. Phase inversion can occur by the addition of an electrolyte or by changing the phase:volume ratio. Monovalent cations tend to form o/w emul-sions, whereas divalent cations tend to form w/o emulsions. An o/w emulsion stabilized with sodium stearate can be inverted to a w/o emulsion by adding calcium chloride to form calcium stearate.

17.4 Creaming is the upward movement of dispersed droplets relative to the continuous phase, whereas sedimentation is the downward movement of particles. Factors that influence the rate of creaming are similar to those involved in the rate of sedimentation. According to Stokes’ law,

where:

v is the velocity of creaming

d is the globule diameter

ρ and ρ₀ are the densities of dispersed phase and dispersion medium, respectively

η is the viscosity of the dispersion medium (poise)

g is the acceleration of gravity (981 cm/s²)

According to this equation, we can minimize sedimentation and creaming phenomena by

·           A reduction in the globule size

·           A decrease in the density difference between the two phases

·           An increase in the viscosity of the continuous phase

17.5 A. Increased free energy at the interface occurs because the increase in surface area of dispersed phase is responsible for the instability of the emulsion. The surfactants deposit on the interface between the two liquid phases and reduce the interfacial tension and free energy at the interface.

B. HLB value of surfactant and relative concentration of the two phases.

17.6 Emulsifying agents form a film around the dispersed globules to pre-vent coalescence and thus avoid the separation of two immiscible liq-uids used for emulsion formation. Emulsifying agents aid in forming emulsions through three different approaches: (1) reduction of inter-facial tension, (2) formation of a rigid interfacial film, and (3) forma-tion of an electrical double layer. The film can act as a mechanical barrier to the coalescence of the globules. An electrical double layer minimizes coalescence by producing electrical forces that repulse approaching droplets. Emulsifying agents can be divided into three groups: (1) surfactants, (2) hydrophilic colloids, and (3) finely divided solid particles.

·           Surfactants are adsorbed at oil–water interfaces to form mono-molecular films and reduce interfacial tensions.

·           Hydrophilic colloids are used as emulsifying agents. These include proteins (gelatin and casein) and polysaccharides (acacia, cellulose derivatives, and alginates). These materials adsorb at the oil–water interface and form multilayer films around the dispersed droplets of oil in an o/w emulsion. Hydrated lyophilic colloids differ from sur-factants as they do not cause an appreciable lowering in interfacial tension.

·           Finely divided solid particles are adsorbed at the interface between two immiscible liquid phases and form a film of par-ticles around the dispersed globules. Finely divided solid par-ticles are concentrated at the interface, where they produce a particulate film around the dispersed droplets so as to prevent coalescence.

17.7 In general, o/w emulsions are formed when the HLB of the surfactants is within the range of about 9–12, and w/o emulsions are formed when the range is about 3–6.

17.8 A surfactant with a high HLB value (~9–12) is used as an emulsifier to form o/w emulsions; and a surfactant of low HLB value (~3–6) to form w/o emulsions.

17.9 A. SEDDS

B. SEDDS

C. SMEDDS

D. SMEDDS

E. SMEDDS

17.10 D.

17.11 B.