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a. SL12102: The science of medicines > L19 - Drug Solubility And Dissolution Rate 4 > Flashcards

L19 - Drug Solubility And Dissolution Rate 4 Flashcards

1
Q

What do inclusion compounds result from?

A

incorporation of non-polar portion into non-polar cavity of another molecule that is water soluble

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2
Q

What are cyclidextrins?

A

Ezymatically modified starches

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3
Q

What do CDs glucopyranose units form? (3)

A

A ring
- a-CD ring of 6 units
- b-CD ring of 7 units
- g-CD ring of 8 units

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4
Q

Why is the cyclodextrin ring cylindrical? (2)

A
  • outer surface is hydrophilic
  • internal surface of cavity is non-polar
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5
Q

What ratio are liphophilic molecules in with their host?

A

1:1

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6
Q

How are crystalline complexes of the inclusion complexes formed?

A
  • dissolved CD and hydrophobic poorly soluble drug combine
    = form inclusion complex
  • Non polar in hostile environment

= forms crystalline complex

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7
Q

How does the dissolution-dissociation-recrystallisation process of a CD complex of a poorly soluble guest happen? (3)

A
  • crystalline complex dissolves
  • CD recrystalises
  • guest separates
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8
Q

What has the B-cyclodextrin to improve the solubility of ibuprofen shown at lower pH? (2)

A
  • fast and complete release of the drug in 30 min
  • when compared to the long time release observed with the mixtures of ibuprofen without CD
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9
Q

What are some examples of disadvantages of CDs? (3)

A
  • di-O-methyl B-CD has a strong affinity for cholesterol and is a haemolytic
  • 2-hydroxypropyl B-CD increases solubility of progesterone
  • B-cyclodextrins find a way to be used as controlled release of drugs
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10
Q

What does a surface avtive solute have the ability to do?

A

Reduce the surface tension at an interface
- w/o requireing large concs

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11
Q

What could large concs of an active solute do?

A

Blur the distinction between solvent and solute

= the lower the conc require, the better surface-activity properties of a solute

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12
Q

What structure do you need for active solutes? (2)

A
  • one element having a high affinity for the solvent
  • one element having a minimal affinity for the solvent
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13
Q

What is a surfactant?

A

Surface active agent

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14
Q

What is the structure of a surfactant composed of? (2)

A
  • hydrophilic/polar head group (nonionic, ionic)
  • lipophilic/non polar chain
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15
Q

What does the balance of the two regions in a surfactant determine? (3)

A
  • surfactant solubility in water and oil
  • its applications
  • place on the scale of Hydrophile-Lipophile Balance (HLB)
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16
Q

What should a polar region have? (3)

A
  • affinity for water
  • able to pull long hydrocarbon chains into water
  • sufficiently polar to hold the nonpolar region of the surfactant in solution
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17
Q

What are the charges carried by the polar part? (4)

A
  • anionic
  • cationic
  • nonionic
  • zwitterionic
18
Q

What do surfactants solution act like at dilute solutions? (2)

A
  • normal solutes (normal electrolytes)
  • amphiphiles exist separately, such size as sub-colloidal
19
Q

What do surfactants solutions act like at concentrated solutions? (2)

A
  • aggregate over narrow conc ranges = micelles
  • size range as colloidal 50A
20
Q

What is the critical miceele concentration (CMC)?

A

The concentration of monomer at which micelles form

21
Q

What is the aggreagtion number?

A

The number of monomers that aggregate to form a micelle

22
Q

What is micellisation?

A

alternative mechanism to adsorption

23
Q

What does micellisation permit?

A

Strong water-water interactions

24
Q

What is the hydrophobic effect?

A

Prevention of water-water interactions if the surfactant molecule were in solution as single molecules between water molecules

25
Q

What happens as the concentration of surfactants increases? (2)

A
  • adsorb to surface
  • form micelles at CMC
26
Q

What surfactant physical properties can change at CMC? (4)

A
  • osmotic pressure
  • turbidity
  • electrical conductance
  • surface tension
27
Q

What happens to the physical properties of surfactants at CMC? (4)

A
  • Osmotic pressure - slower linear increase
  • Turbidity - increases
  • Surface tension - remain constant
  • Molar conductivity - decreases
28
Q

This behaviour is explained by the formation of micelles or aggregates of the surfactant molecules in which: (2)

A
  • lipophilic chains are orientated towards interior of micelle
  • hydrophilic groups are in contact with the aq mediun
29
Q

When may CMC increase with?

A

Increase in polarity of head group

30
Q

When may CMC decrease with? (4)

A
  • temperature (cloud point)
  • pH (surfactants are weak electrolytes)
  • second surfactant
  • addition of electrolytes and organic matter
31
Q

What are the critical values for micelles? (4)

A
  • CMC
  • Kraft point
  • cloud point
  • critical micelle pH
32
Q

What is the kraft point?

A

T at which the solubility = CMC

33
Q

What happens when T < kraft point?

A

CMC > solubility
- micelles cannot form

34
Q

What happens when T > kraft point?

A

Surfactant form micelles
- self-solubilisation

35
Q

What do unassociated surfactants have compared to micelles?

A

Limited solubility

  • micelles are highly soluble, can accomodate large amount of surfactants
36
Q

What is the cloud point? (3)

A

Increase in temp causes:
- dehydration of POE chains
- decreased water solubility
- formation of large micelles = cloudy solution

37
Q

How to reverse the cloud point? (2)

A
  • cooling
  • formation of small micelles = clarification
38
Q

What are POE chains?

A

Polyethylene oxide
H-[OCH2CH2]n-OH
- increase polarity of non-ionic surfactants

39
Q

What is the critical micelle pH?

A

If ionised form of a compound is surface active and unionised form is surface inactive, then a change in pH can induce micellisation

40
Q

What are the geometric properties of micelles? (5)

A

At high con of surfactants, higher visosity systems may occur:
- cylindrical rods, flattenes disks
- liquid crystal (hexagonal phase, middle phase)
- lamellar phase (neat phase)
- bilayers
- vesicles

a. SL12102: The science of medicines (34 decks)

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