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PHAR124 > Colloids > Flashcards

Colloids Flashcards

1
Q

What is a dispersed system?

A

a system in which one substance (the disperse phase) is distributed as particles throughout another phase (the dispersion medium or continuous phase)
disperse phase=solid, liquid, gas
dispersion medium=solid, liquid, gas

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2
Q

What are examples of dispersed systems?

A

molecular dispersions (solute molecules are homogenously distributed throughout the solvent)
-solutions
other dispersed systems contain undissolved or immiscible drug distributed throughout the vehicle:
-colloid
-gels
-suspensions
-emulsion
-lotion, ointment, and creams

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3
Q

Which physicochemical feature is most important to define the type of the dispersed system?

A

size of the dispersed particles

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4
Q

Describe general facts of colloidal systems.

A

particle size 1-500nm (too small to see)
excellent candidate for research regarding novel nano-drug delivery systems
large surface area

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5
Q

What are unique properties of colloidal systems?

A

platinum is effective as a catalyst only as platinum black (colloidal form)
cellular uptake of nanoparticles is different from micro/macroparticles or true molecular dispersions-drug delivery, toxicity

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6
Q

What is the shape of colloidal systems?

A

more than one shape
may occur as:
-globules
-rods
-flakes
-threads
-branched structures

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7
Q

What does the shape and size of a colloidal system determine?

A

flow
sedimentation
osmotic pressure

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8
Q

Which dispersion does not exist for colloidal dispersions?

A

gas in gas dispersion

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9
Q

Why are colloidal dispersions made in pharmacy?

A

increase surface area of absorption
-greater therapeutic effect like colloidal copper and platinum
used in chemotherapy
-platinum has a strong catalytic effect because it acts by
absorbing reactants onto its surface
reduce side effects of the drug due to low retention time and exposure to mucosal membrane
-silver chloride and silver iodide are bactericidal but these
salts are irritants if not used in colloidal form
stability and solubility enhancement
-colloidal electrolytes in suspensions and emulsions

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10
Q

What are the types of colloids?

A

lyophillic or hydrophilic colloids
lyophobic or hydrophobic colloids
association or amphiphilic colloids

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11
Q

What is the definition of a colloid?

A

particles are medium and dispersed but do not settle

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12
Q

Describe association or amphiphilic colloids.

A

surface active agents
aqueous attracting region and non-aqueous attracting region
partition at interface of the liquid and forms micelles
micelles are similar in size as colloidal particles

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13
Q

What is the critical micelle concentration?

A

the concentration of the surfactant at which micelles are formed
below that concentration the surfactant remains at the surface as monomers

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14
Q

What is the difference between lyophilic and lyophobic colloids?

A

lyophilic: solvent loving
-true solutions
-gels
-particulate dispersions
lyophobic: solvent hating
-little or no attraction between the particle and vehicle
if the solvent is water, these terms change to hydrophilic and hydrophobic

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15
Q

What is it called when you take a flash light to a colloid and see the line through the vial?

A

Tyndall effect

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16
Q

Describe lyophilic/hydrophilic colloids.

A

affinity to the dispersion medium
become hydrated when dispersed in water
hydration causes swelling and increased viscosity of the system–>better stability
spontaneous dispersion–>thermodynamically stable

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17
Q

What is solvation?

A

attraction between the dispersion medium and dispersion phase

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18
Q

What is the most common form of colloidal dosage form?

A

gels

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19
Q

Describe gels.

A

semi solid or semi rigid systems
particles forms an intercalating three-dimensional network–>restrict the movement of dispersion medium
extensive physical and chemical crosslinking–>twisted strands held by strong Van der Waals forces
macromolecules are distributed throughout the liquid

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20
Q

What is the sol–>gel transition?

A

conversion of liquid state to gel state
*change in temperature or through agitation–>sol to gel transition is possible (when by agitation–>thixotropy)

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21
Q

What are the two possible natures of colloids for gels?

A

organic
inorganic

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22
Q

What are inorganic colloids?

A

also called particulate colloids, magmas or milks
contain small discrete particles (two phase system)

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23
Q

Whats an example of an inorganic colloid?

A

milk of magnesia (thixotropic)

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24
Q

What are organic colloids?

A

polymer macromolecules that dissolve in water like tragacanth, carbomer, poloxamer, methyl cellulose (single phase system)

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25
Q

What is an example of an organic colloid?

A

cindoxyl gel

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26
Q

Describe thixotropic agitation.

A

viscous when prepared–>solvate when agitated–>viscous upon standing
inorganic gels: gel-sol-gel transition

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27
Q

What are the natures of the solvents for colloids?

A

hydrogels
organogels

28
Q

Describe hydrogels.

A

dispersible in water

29
Q

What are examples of hydrogels?

A

milk of magnesia
cindoxyl gel

30
Q

Describe organogels.

A

not dispersible in water
include vegetable and animal fat, soap base greases, hydrocarbons

31
Q

What are examples of organogels?

A

petrolatum
cocoa butter
carbowaxes (PEG ointments)
PlastiBase

32
Q

Compare colloids to suspensions.

A

colloids:
-small size (invisible)
-filtration not possible
-DO NOT SETTLE but sometimes lyophobic colloids show
visible sedimentation at bottom merely due to aggregation of
colloidal particles
suspensions:
-large size (visible)
-filtration possibly easily
-ALWAYS SETTLE at bottom upon standing

33
Q

What are gelling agents?

A

pharmaceutical excipient employed in making gels
chemical substances capable of undergoing high degree of cross-linking and entanglement in dispersion medium
increases the overall viscosity of the formulation

34
Q

What is the difference between a gelling agent that is a two-phase system compared to one-phase system?

A

two-phase system:
-mostly inorganic gels–>tendency to cross-link and solidify
upon standing and liquefy under stress like shaking
one-phase system (common gels in pharmacy):
-linear or branched polymer macromolecules that dissolve in
water are used as gelling agents

35
Q

What is the concentration used for gelling agents?

A

usually between 0.5% to 5% with some agents requiring slightly higher concentrations (up to 10%)
higher the gelling agent–>stiffer (more viscous) the gel

36
Q

What are types of gelling agents?

A

natural polymers (acacia and tragacanth)
semi-synthetic polymers (cellulose derivates like methylcellulose)
synthetic polymers (carbomers or polaxamers)
they are all polymers

37
Q

Describe aginic acid as a natural polymer for a gelling agent.

A

obtained from seaweeed
used in 1-5% and takes about 30 minutes to get dispersed in water
sodium alginate–>10%
prone to microbial growth–>must have preservatives like parabens

38
Q

Describe tragacanth as a natural polymer for a gelling agent.

A

used for gels that are stable at pH 4-8
requires addition of preservatives
powdered tragacanth tends to form lumps in water–>aqueous dispersions are prepared or wetting agent like glycerin are used

39
Q

At what pH are semi-synthetic cellulose derivates used as gelling agents and why?

A

pH 3-11 except carboxymethyl cellulose (pH 7-9)
maintain viscosity

40
Q

Describe methylcellulose as a semi-synthetic cellulose derivative for a gelling agent.

A

makes thinner gels and hydrates into hot water
high tolerance for added drugs and salts
good solvent comparability with water, alcohol, and propylene glycol
cooling for an hour improves clarity and viscosity

41
Q

Describe HPMC as a semi-synthetic cellulose derivative for a gelling agent.

A

makes thicker gels
compatible with water and alcohol
disperse good in cool water
good gelling agent for time release preparations

42
Q

Describe carbomers as a synthetic for a gelling agent.

A

high bulk density and forms acidic aqueous solutions (pH 3)
they thicken at higher pH of 5-6, 1000x their original volume
sub-divided based on the viscosity of gel it generates
sprinkled in medium with rapid stirring to avoid clumping
when dispersed–>solution pH is low–>neutralize to increase pH–>increase viscosity

43
Q

Describe poloxamers as a synthetic for a gelling agent.

A

copolymers of polyethylene and polyoxypropylene
poloxamer gel base widely used for extemporaneous compounding
it is an absorption enhancing topical vehicle
forms reverse thermal gels

44
Q

What are reverse thermal gels.

A

dissolve in cold water or by cooling overnight
used in 15-50% concentration

45
Q

What is PLO gel?

A

poloxamer combined with lecitihin and isopropyl palmitate
common compound: diclofenac 10% in PLO gel

46
Q

What is the organic phase and aqueous phase of PLO gel?

A

organic phase: lecithin
aqueous phase: pluronic

47
Q

What are some compounding considerations regarding gels?

A

patients prefer clear, water-washable and non-greasy gels–>can be prepared when all ingredients are soluble in the dispersion medium–>NOT always possible, therefore some gels are turbid
although each gelling agent is unique in characteristics there are some generalizations in use that can be applied:
-too rapid addition of gelling agent–>clump which may take
several days to become homogenized
-pre adjust the temperature of water based on the solubility
profile of the polymer–>some dissolve in hot water better
some dissolve hot water better some dissolve in cold water
-some gelling agents require pH adjustments (use carbamer)
-most gelling agents require 24-48h to completely hydrate and
reach maximum viscosity and clarity

48
Q

Why does clumping occur in gels when the gelling agent is added too rapidly?

A

too rapid addition without adequate mixing–>outer molecules of the gelling agent contact the medium first–>they hydrate forming a layer with the gelled surface that is difficult for medium to penetrate–take more time for hydration

49
Q

How can you minimize clumping from occurring when compounding gels?

A

sift the powder into the vortex of stirring medium
levigate the powder with water miscible non solvent like alcohol, glycerin or propylene glycol
use a blender to homogenously mix the powder and solvent

50
Q

Should the gel first be formed and then the active drug added, or should the active drug be added and then the gel be formed?

A

the active drug may be added before or after the gel is formed, two things to consider:
-drug does not interfere with the gelling process
-drug is stable at the conditions of temperature and pH
changes induced
preference: prior to forming the gel–>it is easier and result in more uniform dispersion

51
Q

Describe lyophobic colloids.

A

no or little affinity to the dispersion medium (water)
no change in systems viscosity
maintain dispersion due to mutual repulsion and Brownian movement
generally inorganic molecules
thermodynamically unstable
-changes in the environment lead to aggregation/precipitation
-sensitive to electrolytes
-addition of electrolytes–>precipitation salting out

52
Q

What is Brownian Movement?

A

the zig-zag movement of colloidal particles continuously and randomly
arises due to the uneven distribution of the collisions between colloid particle and the solvent molecules
more rapid for smaller particles
decreases with increase the viscosity of the medium

53
Q

What are examples of lyophobic colloids?

A

colloidal gold (Sol)
-under research for applications such as contrast agent for
targeted tumor diagnosis gene delivery
colloidal sulfur
-used in some specialty dermatological preparations as an
antimicrobial, anti-acne mask
colloidal silver
-used as an antimicrobial, research does not support its use
for most ailments initially identified, toxic

54
Q

How do particles in a liquid medium become charged?

A

selective absorption of a particular ionic species present in the dispersion
ionization of groups on the particle surface (e.g. COO-)
adsorption of ionic surfactant molecules to the particle

55
Q

What does the electrical charge do for the stability of dispersed systems?

A

the charge on the particle surface will attract oppositely charged ions (counter ions) to achieve electrical neutrality: this causes the formation of an electrical double layer around the particle
the electrical double layer determines the distance between adjacent particles in the dispersion–>directly affects stability of system

56
Q

What is the DLVO theory?

A

Derjaguin/Landau-Verwey/Overbeek
describes a quantitative approach to estimating the stability of hydrophobic sols
reflects a balance between net attractive and repulsive forces
VT=VA+VR

57
Q

What does each component of the following equation stand for: VT=VA+VR

A

VT=total potential energy of interaction (attraction or repulsion)
VA=van der Waals attraction
VR=electrostatic repulsion

58
Q

Differentiate between secondary minimum, primary maximum, and primary minimum.

A

secondary minimum: easy dispersion
primary maximum: repulsion is greater than attraction–>particles are going to avoid each other–>stay more dispersed
primary minimum: indicates that the aggregated state is of the lowest-energy condition and this is where we would expect the particles to reside

59
Q

What is the ideal state of dispersion?

A

primary maximum

60
Q

Provide a summary of the DLVO theory.

A

sums attractive and repulsive forces
explains the stability of dispersed systems
at primary maximum repulsion between particles supersedes attraction–>ideally uniformly dispersion
two regions where forces of attraction predominates:
1. small distances (primary minimum)
2. long distances (secondary minimum)

61
Q

What are some states of stability problems?

A

at primary minimum–>irreversible aggregation occurs–>not preferred for any dispersed sytem
-coagulation particles cannot be re-dispersed
at secondary minimum–>reversible loose aggregation of particles occurs–>can be overcome by shaking–>preferred for suspension
-flocculation a loose aggregation

62
Q

What is the importance of packed settling in the stability of a dispersed system?

A

to maintain safety, therapeutic or diagnostic effectiveness

63
Q

What are protective colloids?

A

sometimes hydrophilic colloids when adsorb on the hydrophobic colloids–>prevent destabilizing effects of the electrolytes in the medium–>reduce settling

64
Q

What are examples of protective colloids?

A

gelatin or methylcellulose

65
Q

What are other ways to stabilize a dispersed system?

A

changing the viscosity of the system–>restricts particle movement
addition of buffer salts to maintain pH required for product stability
addition of electrolytes after compatibility testing for type and concentration of electrolytes

66
Q

What is the zeta potential?

A

quantitative measures of the charges on the particles within a dispersion

67
Q

Differentiate between flock and cake.

A

flock: rapid sediment but reversible
cake: slow sediment but irreversible

PHAR124 (12 decks)

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