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Foundations in medicine design > surface chemistry > Flashcards

surface chemistry Flashcards

1
Q

what is interfacial tension

A

the force “tension” per unit length existing at an interface between two immiscible liquids, or two solids, or liquid/ solid interfaces
boundary between two phases called an interface e.g. liquid/gas

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2
Q

why does interfacial phase exist

A

molecules at the surface/interface experience different forces from those in bulk
study of interfaces is surface chemistry

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3
Q

drug dissolution in GI tract (interfacial phenomena)

A

due to interfacial tension between the drug particle and the liquid it is in, a diffusion layer forms around the outside. the drug particle must be wetted, dissolve, and pass across the diffusion layer before it can be absorbed from GI tract

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4
Q

drug recrystallisation (interfacial phenomena)

A

drug particle size affected by rate of crystal growth. interfacial tension between nucleus and recrystallisation fluid creates a barrier to drug molecules joining nuclei

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5
Q

emulsions (interfacial tensions)

A

creams- if an oil and water are stirred together at a fast enough speed one will form a dispersion in the other
however due to the interfacial tension between the two this is not stable and the two phases will separate out again, unless something is added to reduce interfacial tension

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6
Q

what is surface tension

A

in a liquid there are cohesive forces between adjacent molecules, molecules at the surface can only develop attractive intermolecular cohesive forces with other liquid molecules that are situated below and adjacent to them
therefore molecules at the surface of the liquid experience a net inward force towards the bulk. the force pulls the molecules at the interFace together and therefore contracts the surface resulting in surface tension
defined as for e per unit length

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7
Q

relationship between forces and surface tension

A

the stronger the forces between molecules the greater the surface tension and greater the contraction

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8
Q

influence of temperature

A

increase in temperature> decrease in surface tension in linear mannor

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9
Q

measurements of surface and interfacial tenison

A

the du nuoy tensiometer
the wilhelmy plate method (surface and interfacial)
capillary rise method

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10
Q

spreading (liquid-liquid interfaces)

A

two immiscible liquids (water and oil) if a small quantity of one is placed on the surface of the second it may spread to cover the surface with a dilm, or remain as a drop
oil will spread on water if adhesive forces between oil and the water molecules is greater than the cohesive forces within the oil. extent of spreading determined by spreading coefficient

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11
Q

spreading coefficient

A

S =lambda A - (lambda B + lambda A/B)
liquid A is water
therefore surface tension of A must be grater than the sum of s.t. of B plus interfacial tension between a and b for spreading of b and a to occur

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12
Q

work of cohesion

A

cohesion refers to the forces within one liquid
for a single liquid- the work required to pull apart a column of liquid of unit cross sectional area and create two liquid air interfaces
work= surface tension x unit area change
two new surfaces are created with a surface tension of lambda B therefore
Wc=2lambda B

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13
Q

work of adhesion

A

adhesion refers to the forces between two liquids
for 2 immiscible liquids; work required to separate unit cross sectional area of L/L interface to form 2 different zL/G interfaces
Wa= lambda A + lambda B - lambda A/B

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14
Q

when will spreading occur

A

if work of adhesion is greater than work of cohesion

S=Wa-Wc

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15
Q

wetting (liquid-solid interfaces)

A

a drop of liquid adsorbed on to solid will exhibit a definitive contact angle
can get values of 0-180 degrees
higher contact angle>poorer wetting

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16
Q

youngs adapted equation

A

Wa=Wsl=lambda LV(1+cos theta)

wsl is the work of adhesion between a solid and a liquid
contact angle increases, wetting decreases

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17
Q

contact angle for complete wetting

A

0 degrees

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18
Q

things that can be tried to overcome lack of wettability

A

chemical modification of structure, reduce particle size, coat with hydrophilic polymer and use wetting agent (surfactant) to lower contact angle by reducing interfacial tension betweenh drug and body fluid/surface tension of body fluid/drug

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19
Q

surface active agents

A 
surface active/wetting agents (surfactants, amphiphiles) are molecules adsorbed at an interface, have affinity  for polar and non polar solvents
surfactant can by hydrophobic, hydrophilic or reasonable balance
either ionic (carries charge) or non ionic
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20
Q

how do surfactants behave

A

lipophilic chains are directed into the air and hydrophilic groups in the water. if too hydrophilic remains in aqueous phase, I too lipophilic remains in oil phase.

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21
Q

what must be achieved fir surfactants to perform required function

A

correct hydrophilic-lipophilic balance HBL

22
Q

why are surfactants in medicines

A

because they are adsorbed at interfaces they do 2 important things: surfactants allow low/non polar liquids and water to mix- improves spreading
they also reduce contact angle between solids and water- improve wetting

23
Q

how to improve spreading and wetting

A

improve spreading- reduce interfacial tension

improve wetting- reduce surface tension

24
Q

importance of improved wetting in formulation of medicines

A

in granulation, in coating tablets, on administration, in suspensions, in eye drops

25
Q

how do surfactants reduce interfacial/surface tension

A

when a surfactant is added to a liquid it reduces the surface tension. it does this as it is adsorbed as a monolayer at the l/g interface. there is an equilibrium between the surfactant molecules at the surface and those in the bulk solution
as conc of surfactant is increased, surface tension decreases as more molecules of surfactant accumulate a the liquid surface. a point is reached when this layer approaches saturation and no other molecules can enter (critical micelle concentration-CMC) at this point surfactant molecules self associate into micelles

26
Q

what is a micelle

A

self associated aggregates of surfactant molecule
they are in dynamic equilibrium with free individual molecules in solution. micelles are continuously breaking down and reforming

27
Q

factors affecting CMC

A

hydrophobic tail, hydrophilic head, counterions, pH, presence of organic compounds

28
Q

CMC- hydrophobic tail

A

increase in tail length- decrease in CMC
only applicable up to around 16
aggregation number increases with chain length as more molecules caan pack into each micelle as more hydrophobic interactions increase

29
Q

what is aggregation number

A

the number of surfactant molecules that come together to form a single micelle. higher for more hydrophobic surfactants
therefore decrease in CMC, micelle size increases

30
Q

why do surfactants with branching/ unsaturation have higher CMC values

A

the bulky nature of these surfactants interfering with the chain arrangement into micelles- steric hinderance

31
Q

CMC-hydrophilic head

A

non ionic surfactants have lower values for CMC than ionic surfactants, as well as higher aggregation numbers, ionic surfactants need stronger hydrophobic interactions than non-ionic to form micelles die to electrostatic repulsion between head groups. more ionic higher the CMC

32
Q

CMC- counterions

A

the more counterions present or the higher the charge on the ions the greater the ion pairing and the lower the CMC
changes in charge on surfactant head groups done by measuring zeta potential

33
Q

CMC-pH

A

has little effect on non-ionic surfactants
has an effect on ionic surfactants with weak acid/base groups in head
weak acid ionisation changed with pH- alters electrostatic repulsion, CMC lower

34
Q

CMC- organic material

A

co solvents in small volumes have little effect on CMC, in large volumes they increase CMC by increasing solubility of tail in aqueous solution.

35
Q

reason for formation of micelles

A

the attainment of a state of minimum free energy

36
Q

when does micellization occur

A

micellization occurs once saturation at surface is reached 1i.e. only way to attain minimum free energy

37
Q

why is micellization accompanied with an increase in entropy of the system

A

This is because below the CMC, the hydrocarbon tail constitutes a cavity in the water structure, this cavity is lined by water molecules which differ in their organisation from that of the bulk water.
Water is normally highly structured due to H-bonding; the presence of surfactants disrupts this.Furthermore, the hydrocarbon tail is less free to move because of the surrounding water. In other words the entropy of the system is negative (low) and so the free energy is high. In order to overcome this and return the system to minimum free energy the monomers migrate to the L/G interface. However, at the CMC, the interface has become saturated with monomers. At this point the only way to return the system to minimum free energy is to form micelles.

38
Q

micelle structure

A

sausage shaped and spherical, depends on concentration. at low concs, they are spherical as more molecules pack in they elongate becoming ellipsoidal before extending to sausage shape

39
Q

what is adsorption

A

When a gas or liquid is brought into contact with a solid, some atoms or molecules become attached to the solid surface in the form of a layer. Process is called adsorption.
the solidis the adsorbent, the gas/liquid is the adsorbate and the resulting layer is the adsorbed layer
if adsorbate is liquid, this would be wetting

40
Q

polar and non polar adsorbents

A

polar adsorbents adsorb polar substances

non-polar adsorbents adsorb non polar substances

41
Q

difference between adsorption and absorption

A

adsorption- surface only phenomenon, if diffusion occurs I is absorption

42
Q

what is it called if both adsorption and absorption occur

A

general term sorption

43
Q

what does extent of adsorption depend on

A

temperature, pressure (or concentration) and the effective surface area. the best adsorbents are porous solids e.g. charcoal and silica gel
grater SA- grater adsorption
higher P/C- greater adsorption
higher temp-lower adsorption

44
Q

what is the (Langmuir or frundlich0 adsorption isotherm

A

The relationship between the amount of gas/liquid physically adsorbed on a solid and the pressure (for a gas) or concentration (for a liquid) at constant temperature
Main difference is that Langmuir states there is a limit to the volume of gas/liquid that can be adsorbed.

45
Q

physisorption

A

adsorption may be relatively weal non specific forces called physical adsorption (physisorption)
it is rapid, reversible and multi-layered
e.g. gas adsorbed on charcoal

46
Q

chemisorption

A

adsorption may ne stronger specific valence forces (covalent or ionic bonding) known as chemical adsorption (chemisorption)
chemisorption is specific, requires activation energy and therefore slow, not readily reversible and only monolayered
e.g. oxygen is adsorbed on metals causing formation of oxides

47
Q

what is an isotherm

A

constant temperatire

5 types

48
Q

Langmuir isotherm

A

indicates that a monolayer is being formed e.g. as with charcoal. found in chemisorption and physisorption
where there is a fin pore structure in the solid
graph- a rapid rise in adsorption with pressure soon reaching a limiting value

49
Q

type 2 isotherm

A

Commonest in physical adsorption. They are sigmoidal in shape and occur when gases undergo physical adsorption onto non-porous solids to form a monolayer, followed by multilayer formation, e.g. with fumed silica.
graph- the first inflection point represents the formation of a monolayer, the continued adsorption with increasing pressure indicates subsequent multilayer formation.

50
Q

Type 3 isotherm

A

Similar to Type II and typical of adsorption onto porous solids.
Multilayer formation and condensation within the pores of the solid are thought to be responsible for the further adsorption, which reaches a limiting value before the saturation vapour pressure is attained, e.g. as with bentonite.
graph- the first inflection point again represents the amount of gas required to form a monolayer on the solid surface.

51
Q

how is charcoal used as an adsorbent

A

Activated charcoal is used as an adsorbent. Produced at elevated temperatures by either:
Oxidation, or
Chemical activation
Activated charcoal has a highly porous structure as it is more disordered (after activation) and so has a larger surface area for adsorption.

52
Q

examples of adsorption in pharmacy

A

important in packaging of dosage forms, e.g. glyceryl trinitrate tablets, insulin infusions
improving water solubility of hydrophobic drugs
anti irritants/diarrhoea
taste masking

Foundations in medicine design (9 decks)

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