surface chemistry

Question 1

what is interfacial tension

Answer 1

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

Question 2

why does interfacial phase exist

Answer 2

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

Question 3

drug dissolution in GI tract (interfacial phenomena)

Answer 3

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

Question 4

drug recrystallisation (interfacial phenomena)

Answer 4

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

Question 5

emulsions (interfacial tensions)

Answer 5

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

Question 6

what is surface tension

Answer 6

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

Question 7

relationship between forces and surface tension

Answer 7

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

Question 8

influence of temperature

Answer 8

increase in temperature> decrease in surface tension in linear mannor

Question 9

measurements of surface and interfacial tenison

Answer 9

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

Question 10

spreading (liquid-liquid interfaces)

Answer 10

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

Question 11

spreading coefficient

Answer 11

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

Question 12

work of cohesion

Answer 12

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

Question 13

work of adhesion

Answer 13

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

Question 14

when will spreading occur

Answer 14

if work of adhesion is greater than work of cohesion

S=Wa-Wc

Question 15

wetting (liquid-solid interfaces)

Answer 15

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

Question 16

youngs adapted equation

Answer 16

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

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

Question 17

contact angle for complete wetting

Answer 17

0 degrees

Question 18

things that can be tried to overcome lack of wettability

Answer 18

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

Question 19

surface active agents

Answer 19

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

Question 20

how do surfactants behave

Answer 20

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.

Question 21

what must be achieved fir surfactants to perform required function

Answer 21

correct hydrophilic-lipophilic balance HBL

Question 22

why are surfactants in medicines

Answer 22

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

Question 23

how to improve spreading and wetting

Answer 23

improve spreading- reduce interfacial tension

improve wetting- reduce surface tension

Question 24

importance of improved wetting in formulation of medicines

Answer 24

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

Question 25

how do surfactants reduce interfacial/surface tension

Answer 25

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

Question 26

what is a micelle

Answer 26

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

Question 27

factors affecting CMC

Answer 27

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

Question 28

CMC- hydrophobic tail

Answer 28

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

Question 29

what is aggregation number

Answer 29

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

Question 30

why do surfactants with branching/ unsaturation have higher CMC values

Answer 30

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

Question 31

CMC-hydrophilic head

Answer 31

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

Question 32

CMC- counterions

Answer 32

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

Question 33

CMC-pH

Answer 33

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

Question 34

CMC- organic material

Answer 34

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

Question 35

reason for formation of micelles

Answer 35

the attainment of a state of minimum free energy

Question 36

when does micellization occur

Answer 36

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

Question 37

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

Answer 37

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.

Question 38

micelle structure

Answer 38

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

Question 39

what is adsorption

Answer 39

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

Question 40

polar and non polar adsorbents

Answer 40

polar adsorbents adsorb polar substances | non-polar adsorbents adsorb non polar substances

Question 41

difference between adsorption and absorption

Answer 41

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

Question 42

what is it called if both adsorption and absorption occur

Answer 42

general term sorption

Question 43

what does extent of adsorption depend on

Answer 43

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

Question 44

what is the (Langmuir or frundlich0 adsorption isotherm

Answer 44

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.

Question 45

physisorption

Answer 45

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

Question 46

chemisorption

Answer 46

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

Question 47

what is an isotherm

Answer 47

constant temperatire | 5 types

Question 48

Langmuir isotherm

Answer 48

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

Question 49

type 2 isotherm

Answer 49

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.

Question 50

Type 3 isotherm

Answer 50

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.

Question 51

how is charcoal used as an adsorbent

Answer 51

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.

Question 52

examples of adsorption in pharmacy

Answer 52

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