Exam 1 Emulsions

Question 1

What is an emulsion?

Answer 1

a heterogenous system of two immiscible liquids in which one is dispersed as droplets → contains a dispersed phase and a continuous phase (aka external phase) → aka 2 liquids into 1 product

Question 2

What does the continuous phase determine?

Answer 2

it determines the organoleptic properties (taste, smell, feel) of the emulsion

Question 3

What are the three applications of emulsions?

Answer 3

1. oral
2. external
3. intravenous lipid emulsion (O/W)

Question 4

What is O/W and W/O?

Answer 4

O/W is oil suspended in water whereas W/O is water suspended in oil

Question 5

What are the oral applications of an emulsion?

Answer 5

1. O/W to mask the taste of an oil ( example is mineral oil emulsion as a laxative)
2. O/W to enhance absorption of an oil (like vitamins A and D, cod liver oil)

Question 6

What are the external applications of an emulsion?

Answer 6

1. O/W → water washable like a vanishing cream (moisturizing lotion is more water than oil)
2. W/O → for cleansing skin like a cold cream (more oil than water)

Question 7

What are the intravenous lipid (O/W) applications of an emulsion?

Answer 7

1. for parenteral nutrition (TPN)
2. 100 mL of 20% lipid emulsion provides 200 kcal while 100 mL of 5% dextrose provides 20 kcal
3. smallest capillaries are 5 micrometers so it is critical that droplet size is less than 1 micrometer to avoid embolisms → oil is used for injectables as long as it is emulsified so that the oil is dispersed as small particles to avoid clogging blood vessels

Question 8

What is the interfacial phenomena?

Answer 8

1. in the bulk portion of each phase, molecules are attracted to each other equally in all directions
2. at the boundary between the phases, molecules are acted upon unequally because they are in contact with other molecules exhibiting different forces of attraction (water interacts with hydrogen bonds, oil interacts with London dispersion forces)
3. molecules that are situated at the interface experience interaction forces dissimilar to those experiences in each bulk phase → the imbalance leads to spontaneous movement of molecules from the interface to the bulk phase → leaves fewer molecules per unit area at the interface

Question 9

What is reasoning behind interfacial tension or surface tension?

Answer 9

any attempt to reverse the spontaneous movement of molecules at the interface (by increasing the area of contact between phase) causes the interface to resist expansion and behave as though it is under tension everywhere in a tangential direction

Question 10

What is interfacial tension?

Answer 10

the force of this tension per unit length of interface:

1. interfacial tension at liquid-liquid interface → oleic acid and water is 15.6 dynes/cm
2. surface tension at liquid-air interface → water and air is 72.8 dynes/cm (water has high surface tension!) and oleic acid and air is 32.5 dynes/cm

Question 11

What are some important things to know about interfacial tension?

Answer 11

1. the stronger the intermolecular force in the bulk phase, the higher the interfacial tension
2. the greater the tendency to interact (with own molecules), the less the interfacial tension
3. the higher the temperature, the lower the interfacial tension (since intermolecular forces are reduced at higher temperatures)
4. the more dissimilar the molecules are, the greater the interfacial tension since it takes more to stabilize the emulsion (the molecules want to stay apart)

Question 12

What are emulsifying agents?

Answer 12

agents that are added to the emulsion to stabilize the emulsion

Question 13

What are the three types of emulsifying agents?

Answer 13

1. surface active agents
2. hydrophilic colloids (aka hydrophilic polymers)
3. finely divided solid particles

Question 14

What are surfactants (surface active agents)?

Answer 14

a monomolecular film in which molecules that contain both a hydrophilic region and a hydrophobic region will orient at the liquid-liquid or liquid-air interface and lower the interfacial or surface tension → orienting at the interface and making the 2 phases more compatible → essentially like soap

Question 15

What is critical micelle concentration (cmc)?

Answer 15

depicts how efficient a surfactant is because at a high enough concentration, the surfactant forms micelles but there is a limit!

Question 16

What are the different classes of surfactants?

Answer 16

1. anionic → negative charge with a positive counterion → examples include alkyl sulfate and alkylbenzene sulfonate
2. cationic → positive charge with a negative counterion → examples include alkyltrimethylammonium bromide and alkylpyridinium chloride
3. zwitterionic → has both positive and negative charges → examples include alkyl betaine and alkyldimethylamine oxide
4. nonionic → has no charge but still polar since it can hydrogen bond with water → examples include alcohol ethoxylate and the polyoxyethylene block copolymer

Question 17

What is the Rule of Bancroft (1913)?

Answer 17

it states that the type of emulsion is a function of a relative solubility of the surfactant in which the phase in which it is more soluble being the continuous phase → high HLB (>10) means that it is soluble in water (more like water) so it forms O/W emulsion → low HLB (<10) means that it is more like oil so it forms W/O emulsion

Question 18

What is hydrophile-lipophile balance (HLB)?

Answer 18

1. it’s a measure of the relative contributions of the hydrophilic and lipophilic regions of a surfactant
2. calculated according to an empirical formula
3. ranges 0-20 for non-ionic surfactants
4. low HLBs indicate greater lipid solubility → more like oil
5. typically, a mixture of emulsifying agents is used to get a desired HLB → if there is a lot of water, want a surfactant more like water and if there is a lot of oil, want a surfactant more like oil

Question 19

What is the equation to calculate the HLB of a mixture?

Answer 19

aX + (1-a)Y
in which a is the fraction of surfactant 1 in the surfactant mixture, X is the HLB of surfactant 1, and Y is the HLB of surfactant 2

Question 20

When and how do micelles form?

Answer 20

as the concentration of a surfactant increases above a critical concentration (cmc), the surfactant molecules self-associate into small aggregates called micelles → the micelles are formed in water and have the hydrophobic groups (the tails) of the surfactant oriented toward the center (core) of the micelle → the center of the micelle represents a lipid like region that is capable of dissolving water-insoluble drugs (like soap!)

Question 21

What are hydrophilic colloids?

Answer 21

1. used in O/W emulsions
2. long chain of molecules/polymer on surface of the oil that form a multimolecular film at the interface and increase the viscosity of water
3. DOES NOT lower the interfacial tension → does not make the 2 phases more compatible, just builds steric hindrance so the 2 phases don’t merge

Question 22

What are some examples of hydrophilic colloids?

Answer 22

acacia (polysaccharide), tragacanth (polysaccharide), gelatin (protein)

Question 23

What are finely divided solid particles?

Answer 23

fine particles less than a micron that can absorb at the interface which builds a solid particle film which acts as a barrier between the 2 phases since it sticks to the surface of the oil droplets

Question 24

What are examples of finely divided solid particles?

Answer 24

O/W emulsions: bentonite, magnesium aluminum silicate, aluminum hydroxide → hydrophilic
W/O emulsions: charcoal → hydrophobic

Question 25

How do you predict the type of emulsion?

Answer 25

1. the phase in which the emulsifier is most soluble will be the external (continuous) phase
2. phase volume ratio

Question 26

How can you predict which phase the emulsifier is most soluble in?

Answer 26

1. surface active agents (surfactants) → if HLB < 10 then W/O (oil like product), if HLB > 10 then O/W (water like product)
2. hydrophilic colloids (only O/W) → gelatin, acacia, tragacanth
3. finely divided solids → hydrophilic (O/W) if contact angle <90 degrees, hydrophobic (W/O) if contact angle >90 degrees

Question 27

What is phase volume ratio?

Answer 27

volume of oil phase/total volume of the emulsion → 0-26% is O/W only, 26-74% is either O/W or W/O, 74-100% is W/O only

Question 28

What are the three instabilities of emulsions?

Answer 28

1. creaming
2. coalescence
3. phase inversion

Question 29

What is creaming?

Answer 29

1. given by the Stokes Law equation (in which V is velocity of sedimentation, d is diameter of droplets, pi is density of internal phase, pe is density of external phase, n is viscosity of external phase, and 980 is the gravity constant
2. not bad because it is reversible as long as the interfacial film is effective in maintaining the integrity of individual droplets → can be fixed by shaking

Question 30

What is coalescence?

Answer 30

1. droplet size increases because the interfacial film is unable to maintain the integrity of individual droplets → droplets can merge together
2. irreversible and will ultimately lead to a layer of oil and a layer of water (broken emulsion)
3. cannot be fixed by shaking the bottle → must be reformulated and cannot be used since content uniformity is not up to standard

Question 31

What is phase inversion?

Answer 31

1. happens when the phase volume ratio exceeds 74% → want to keep the phase volume ratio <50%
2. O/W emulsion stabilized with sodium stearate can be inverted to W/O type using “hard” water (containing calcium) → conversion of Na stearate (O/W, has high HLB and good at stabilizing oil in water) to Ca stearate (W/O, has low HLB and better at stabilizing water in oil, Ca2+ can bind to the surfactant) → happens when you use hard water instead of ionized water

Question 32

What are the steps in preparing an emulsion?

Answer 32

1. dissolve all water soluble ingredients in water
2. mix all oil soluble ingredients in oil → heat if necessary to melt
3. heat the aqueous solution to the same temperature as the oil solution → makes the molecules more mobile
4. mix oil solution and water solution with mixing
5. cool slowly
6. pass through a colloid mill or homogenizer → to produce finely dispersed emulsion