Emulsions & Surfactants

Question 1

Gibb-Marangoni effect

Answer 1

• Mass transfer along an interface between two fluids is due to the surface tension gradient
• Liquid with higher surface tension pulls more strongly, so liquid will flow away from regions of low surface tension
• Surfactants will help mediate undulation caused by thermal or mechanical stresses
• Stability of the emulsion (also in liquid forms)

Question 2

Tears on wine glass

Answer 2

• Adding alcohol lowers surface tension
• Capillary action, ethanol evaporates first, surface tension changes and draws more wine up the side of glass
• Forms droplets b/c structure has lowest tension and then falls under its own weight

Question 3

Bancroft’s rule

Answer 3

• The dispersion medium of and O+W emulsion is the phase in which the solubility of the emulsifying agent is higher
• In O/W emulsions – emulsifying agents are more soluble in water than in oil (High Hydrophilic-lipophilic balance (HLB) surfactants).
• In W/O emulsions – emulsifying agents are more soluble in oil than in water (Low HLB surfactants).
• *HLB = hydrophile-lipophile balance (reflects relative affinity to water vs. oi

Question 4

Phase Separation

Answer 4

Phase separation is viscosity dependent - to reduce phase separation add gelling agents to increase viscosity of the continuous phase

Question 5

Ostwald Ripening

Answer 5

Question 6

Surfactants in lungs

Answer 6

• Increases lung compliance, thus reducing work for breathing
• At high lung volume (expanded) surface tension is high (low concentration of surfactant); prevents overexpansion
• At low lung volume surface tension is low (high concentration of surfactant); prevents alveoli collapse

Question 7

Hydrogels

Answer 7

Question 8

Langmuir-Blodgett Films and Trough

Answer 8

Question 9

Trough

Answer 9

Question 10

CMC

Answer 10

• Highly kinetic process, chains switch in an out over time
• Below CMC means need more surfactant to stabilize emulsion and minimize tension
• Always need to be above CMC for stability, can’t change volume - change concentration

Question 11

Factors

Answer 11

Hydrocarbon chain

• Increase chain length, decrease branching, increase saturation : greater packing, more stable, lower CMC

Hydrophilic head

• Nonionic, smaller size leads to higher number : need less to balance repulsion, lower CMC

Counterions

• Increasing the size of counterion decreases CMC.

Excess salt

• Reduces electrostatic repulsions of headgroups decreasing CMC.

Question 12

Packing parameters

Answer 12

Packs to maximize interactions with structure and minimize overall surface area / tension

Question 13

Micellar morphology

Answer 13

Question 14

Packing Factors

Answer 14