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GI > Drug Solubility > Flashcards

Drug Solubility Flashcards

1
Q

definition of solution

A

dispersion formed by two or more compounds which form a homogenous system

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2
Q

definition of solvent

A

component that determines the phase of the solution - usually the largest proportion

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3
Q

definitions of solutes

A

other components of the solution and are dispersed as molecules or ions throughout the solvent

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4
Q

definition of saturated solution

A

solute is in equilibrium with solute

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5
Q

definition of solubility

A

concentration of a solute in saturated solution at a certain temperature that dissolves to form a homogenous phase

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6
Q

definition of unsaturates/subsaturated solution

A

the dissolved solute is in concentration below that necessary for complete saturation at a definite temperature

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7
Q

definition of supersaturated solution

A

contains more dissolved solute than normal

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8
Q

semi quantative expressions of solubility

A 
very soluble (<1part)
freely soluble (1-10 parts)
soluble (10-30 parts)
sparingly soluble (30-100 parts)
slightly soluble (100-1000 parts)
very slightly soluble (1000-10000 parts)
insoluble (>10000 parts)
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9
Q

quantative expressions of solubility

A 
molarity
% w/w
% w/v
mg/ml
g/100ml
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10
Q

what is a diffusion layer

A

when molecules in surface layer of drug dissolve to form a saturated solution around particles
dissolved drug molecules pass through dissolving fluid to contact absorbing mucosa
replenishment of diffusing layer by further drug dissolution

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11
Q

Factors affecting dissolution rate

A 
surface area of undissolved particles
solubility in dissolution medium
conc in bulk of solution
diffusion coefficient
thickness of boundary layer
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12
Q

Reduction in particle size

A

inclreases dissolution rate - exposure of increasing amounts of solute to the solvent
solubility increased with decreasing particle size

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13
Q

to increase solubility

A

salt form of drug used

aqueous solubility can be reduced by esterification of prent drug

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14
Q

effect of esterification

A

masks the taste and protects from degredation in GI

can facilitate absorption in GI tract

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15
Q

polymorphism

A

crystallisation conditions -> molecules/ions arrange differently in lattice -> stable polymorph (less soluble) or metastable polymorph (more soluble, less stabe)

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16
Q

counterions used in salt formations

A

HCl, sulphate, mesylate, phosphate, maleate, salicylate, tartrate, lactate, citrate, succinate, acetate, sodium, potassium, calcium, lithium, magnesium, inc, choline, aluminium

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17
Q

weak bases in GI tract

A

high dissolution rate in stomach

dissolution rate falls as pH of GI tract rises

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18
Q

weak acids in GI tract

A

minimal dissolution in the stomach

dissolution rate increases down the gut

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19
Q

salt dissolution

A

dissolution of the salt form is greater than the free form

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20
Q

effect on use of salt form

A

modifies pH of diffusion layer
salt of weak acid increases pH of diffusion layer
salt of weak base decreases pH of diffusion layer
salt is preferable to acid as dissolution is better controlled
salt form controls pH of diffusion layer indepentdly of position of GI tract, this improves dissolution rates

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21
Q

co-solvent advantage

A

increases solubility

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22
Q

when does precipitation occur

A

when pH of solution is adjusted to produce mostly unionised molecules, exceeding solubility of their form

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23
Q

how to improve solubility of weak electrolytes or non-polar compounds

A

adding water-miscible solvents in which the compound is soluble

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24
Q

what are co-solvents

A

vehicles used in combination with water to increase solubility
the solubility in mixed systems is greater than that of individual solvents

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25
Q

Cosolvency

A

obtains aqueous-based systems in which the drug solubility is higher than aqueous solubility; it formulates higher concentrations of the drug
it improves stability of formulation

26
Q

cosolvents

A

organic compounds
miscible with water
better solvents than water for a drug(H-bond acceptors/donors, small hydrocarbon regions)
most cosolvents are liquids (ethanol, glycerol, propylene glycol)
some are solids that are highly soluble in water (urea, PEG, PVP)

27
Q

effect of cosolvents decreasing H-bond density of aqueous systems

A

reduces cohesive interactions of water, which reduces polarity of solutions, so the solution is less effective than water in squeezing out nonpolar structures solubilisation by cosolvency

28
Q

cosolvency

A

water-ethanol has more cosolvency than water-glycerin with non-polar drugs
cosolvents increase solubility of non-polar and semipolar solute in water
as solutes become more polar, cosolvency becomes less efficient; it will decrease solubility of a polar solute in water

29
Q

what are cyclodextrins

A

inclusion compounds that enhance solubility of drugs that are poorly soluble in water
they are enzymatically modified starches with a ring of glycopyranase units
the centre of the ring accommodates lipophilic molecules (outer ring is hydrophlic and inner is non-polar)

30
Q

what are surfactants

A

surface active agents that enhance solubility

31
Q

what do cyclodextrins do

A

they incorporate the non-polar portion of one polar molecule into the non-polar cavity of another molecule or group of molecules that is water soluble
driving force is similar to micellar solubilisation - it reduces the non-polar water interfacial area by inserting the solute into the complexing agent
the complex formed increases solubility initially but decrease in solubility as drug is released

32
Q

beta cyclodextrins

A

useful solubilisers used in the controlled release of drugs

33
Q

use of surfactants

A

have the ability to reduce surface tension at an interface without requiring large concentrations
lower conc required = better surfactant
highest reduction of interfacial tension obtained with solutes that combine in their molecular structure, one element having a high afinity for the solvent and one having minimal affinity for a solvent

34
Q

role of surfactants

A

resolves interfacial tension between two immiscible liquids

more surface tension = less thermodynamically stable

35
Q

structure of surfactants

A

polar head group and lipophilic/non-polar chain

can be presented in different proportions, which determines the surfactant solubility in water or oil

36
Q

what is HLB

A

hydrophile/lipophile balance

37
Q

solutions of surfactants

A

unusual properties at dilute solutions
act as normal solutes and normal electrolytes
amiphiles exist separately and are of such a size to be sub-colloidal
as concentrated solutions, they aggregate to form a micelle

38
Q

what is critical micelle concentration

A

the narrow concentration range which causes surfactants to aggregate to form a micelle

39
Q

what is micellisation

A

alternative mechanism to adsorption
permits strong water-water interations that would be prevented if the surfactant molecules were in solution as single molecules (hydrophobic effect)

40
Q

physical properties that change with CMC

A

osmotic pressure, tubidity, electrical conductivity, surface tension
CMC can increase with increase of polarity of surfactant head group
CMC can decrease with temp, pH, additional surfactants or electrolytes

41
Q

what is Kraft point

A

temp/time where solubility is equal to CMC
when kraft point is larger than temperature, the CMC is larger than solubility and micelles cannot form
when temp is larger than kraft point, the surfactant forms micelles (self solubilisation)

42
Q

what is the cloud point

A

for non-ionic surfactants
increase in temp causes dehydration of POE chains, decreased solubility, formation of large micelles so the solution becomes cloudy

43
Q

reversing cloudiness of non-ionic surfactants

A

by cooling

this forms small micelles - called clarification

44
Q

what is critical micelle pH

A

ionised form of a compound is surface active and unionised form is surface inactive (or has lower CMC than ionised form), a change in pH can cause micellisation

45
Q

geometric properties of micelles

A

at high conc of surfactants, high viscosity systems may occur
cylindrical rods, flattened discs, liquid crystals
lamella phase
bilayers
vesicles

46
Q

groups on non-ionic surfactants

A

hydroxy
ether

less polar than ioninsed groups

47
Q

what is a POE number

A

number of monomeric polyoxyethylene groups in the molecule

48
Q

polyoxyethylene

A

chains with 20+ ether groups linked to non-polar moiety

several chains then linked to cyclic sugar and alkyl group

49
Q

surfactant applications

A

anionic: cheap, toxic so only used externally
cationic: disinfectant/preservatice properties, o/w emulsifiers
nonionic: o/w, w/o emulsifiers, low toxicity and low irritancy, parenteral use

50
Q

parenteral applicaitons for ionic surfactants

A

haemolysis of RBC and desrtuction of T-cells

51
Q

parenteral applications for non-ionic surfactants

A

phospholipids, polysorbates, cremophor EL anaphylactic shock, toxicity related to ethylene oxide

52
Q

what is solubilisation

A

process by which water=-insoluble or patially soluble substances are brought into aqueous solution by incorporation into micelles

53
Q

solubilisation with micelles

A

site of solubilisation depends on chemical nature of solubilisate
more polar solute, more likely to be solubilised closer to the surface
polar region of non-ioninc micelle larger than polar region of ioninc micelle

54
Q

what is solubilisation capacity

A

quantifies solubilisation
measure the ability of a surfactant to solubilise a solute
molar solubilisation capacity (k)is the number of moles of solute that can be solubilised by one mole of micellar surfactant

55
Q

to decrease CMC

A

increase hydrocarbon chain length
introduce polar region on molecule or double bond

branched surfactants will produce smaller micelles

56
Q

semipolar solutes

A

surface and palisade region largely unaffected by nonpolar region

57
Q

factors of surfactant selection

A

amount of surfactant required
ability to solubilise a solute
increased chain length for reduced CMC and reduced solubility

58
Q

Lundelieu’s rule

A

any factors that decreases solubility of surfactant promotes surface activity

59
Q

HLB values

A

higher HLB = more hydrophilic
100% lipophilic = HLB1
100% hydrophilic = HLB20

60
Q

what is phase intervention temperature (PIT)

A

HLB varies with temp because relative solubilities of lipophile and hydrophile parts vary with temperature
this is more pronounced with non-ionic surfactants becuase their solubility depends on H-bonding

61
Q

temperature on H-bonding

A

higher temps weaken H-bonds and emulsifiers are less soluble in water
common non-ionic emulsifiers are water soluble at low temps to stabalise o/w emulsions
oil soluble at high temps to stabalise w/o emulsions

62
Q

PIT of emulsifier

A

temp at which it changes from o/w to w/o

hydrophilic and lipophilic nature balance

GI (21 decks)

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