2e

Question 1

how do we create more interface and what must occur for us to do so

Answer 1

• move bulk molecules to the surface
• break the intermolecular forces of attraction the bulk molecules feel ( majority of these wont be regained)

Question 2

parts of the surfactant include

Answer 2

• polar head ( hydrophilic)
• hydrocarbon chain (hydrophobic)

Question 3

what is a surfactant

Answer 3

a surface active agent

Question 4

what effect does a flouronated surfactant have

Answer 4

a flouronated surfactant reduces surface tension

Question 5

what happens when u put a surfactant in the water

Answer 5

the HC chain points away from the water

the polar head sits in the water

Question 6

in a micture of surfactant and water,, what is there an equilibrium between

Answer 6

equilibrium between the surfactant dissolved in the bulk and the surfactant that adsorbs on the interface

Question 7

describe what happens when u start with pure water and add surfactant to it // keep increasing the conc of surfactant

Answer 7

• surfactant goes to the interface
• more surfactant on the interface and more surfactant in the bulk
• once the cmc has been reached ( critical micelle conc ) the amount of surfactant on the interface stops increasing and micelles form via micellation.

Question 8

CMC can also be explained as

Answer 8

the solubility limit of the surfactant has been reached.

Question 9

why do surfactant monomers first go to the surface

Answer 9

better thermodynamic environment for them

Question 10

what happens when the surfactant solubility limit // critical micelle conc has been reached

Answer 10

its more thermodynamically favoured for the monomers to aggregate tpgether then for more to dissolve.

from this point,, durface tension stays the same as no more surfactant molecules go to the interface,, just just start forming more micelles.

Question 11

diameter of a micelle is usually

Answer 11

1 nm –> 2nm

Question 12

when a droplet of oil is stabilised by a micelle, what does it look like

Answer 12

their tails are in the inside

Question 13

when a droplet of water is stabilised by a miclle what does it look like

Answer 13

the poalr heads are in the middle

Question 14

whats importasnt to rmemeber about the air/water interface

Answer 14

its more of a region,, bc u have water evaporating and u have water condensing back into the water bit.

so its more of a region then a straight line.

its therefore a dynamic environment meaning its changing

Question 15

as the air/water interface is more of a region rather than a straight line,, what do we need to do and how do we do this

Answer 15

we need to define where the tolution is in the air // water interface region!!

we do this using the gibbs dividing surface equation

Question 16

what does the gibbs dividing surface equation tell u

Answer 16

it gives u a plane where everything above it is air,, and everything below it is solution // water

Question 17

what is the gibbs dividing durface equation

Answer 17

gamma = ( -1/nrt ) ( d§ / d ln(c) )

this is the gibbs absorption isotherm

Question 18

what changes when u change conc of surfactant

Answer 18

whwn u change conc of surfactant,, § aka surface tension changes,, this is bc ur changing the types of forces present on the interface.

ur going from mainly hydrogen bonding ,, to adding VDW which are obvs weaker,, reducing the surface tension.

Question 19

why does conc of SF chnage ST

Answer 19

bc ur changing the composition of the interface
which reduced surface tension

Question 20

if our surface tension is lower,,, what does it mean

Answer 20

it means its easier to form more more surface!!!

Question 21

what is surface excess and what is its symbrol

Answer 21

surface excess is the conc of the thing ur interested in ,, aka the SF,, at the surface above that present in the bulk

AKA SURFACE EXCESS IS THE CONC OF SF ADSORBED TO THE INTERFACE.

its symbol is gamma!!!

Question 22

gamma // surface excess equation

Answer 22

gamma = ni / A

aka number of molecules of a given type / area

Question 23

what links surface tension and surface excess

Answer 23

chemical potential

Question 24

what is the equation for surface tension

Answer 24

§ = dG / dA

Question 25

what is chemical potential

Answer 25

it tells us how free energy changes with the composition of the system!!! long u = dG / dn

Question 26

change in surface energy // surface tension depends on what

Answer 26

the amount of material adsorbed at the surface and the change in chem potential that it causes.

Question 27

d§ in terms of surface excess and chem potential

Answer 27

d§ = (- gamma ) ( d long u )

Question 28

if we have multiple components on the surface,, what do we need to take into accound

Answer 28

we need to take into account that theyre all diff and need to take each of them into account

Question 29

the contribution of each specie in the interface is what

Answer 29

its additive aka d§ = - sum of gamma i x chem potential of i

Question 30

when there are 2 components,, what would d§ be

Answer 30

the -gamma x dlong u of solute + the -gamma x dlong u of solvent

Question 31

the gibbs dividing surface equation gives us what

Answer 31

gives us surface excess

Question 32

the SE of the solvent is what

Answer 32

its 0 at the point where there is no excess of solvent

Question 33

if the surface excess,, aka gamma,, of the solvent is 0 ,, what does that tell us about gamma x d long u

Answer 33

that will also be 0 bc ur multiplying by 0

Question 34

due to the surface excess of the solvent being 0,, the d§ is now based on

Answer 34

the -surface excess of the solute x the chemical potential of the solute

Question 35

chemical potential =

Answer 35

chem potential* + RT ln(a or conc)

Question 36

du =

Answer 36

RT d ln(A)

Question 37

change on chemical potential depends on what

Answer 37

it depends on the change in composition aka the change in concentration at the surface. aka at the gibbs dividing surface aka what part of the air/solvent region acc separates the 2 states.

Question 38

therefore gamma aka surface excess ====

Answer 38

(-1/nRT) ( d§/ dln(c) ) where n is the number of absorbed species at the surface,, for uni its always 1. meaning we dont think of counter ions

Question 39

what does surface excess allow us to do

Answer 39

it helps us compare surfactants + their efficiancy // ability to adsorb at the surface

Question 40

by looking at the composition of the interface + how it changes with the change in free energy ,, what can we see

Answer 40

it can tell us about any chnages in concentration.

Question 41

in the graph,, what are the different axes

Answer 41

y axis = § x axis = ln (c) aka conc of surfactant in moldm-3

Question 42

when u plot § against ln(c) what do u get

Answer 42

u get a graph that kinda looks like the bonnet of a car, it its curved down and then stops increasing // decreasing telling us that even tho conc of surfactant is increasing ,, ,the § surface tension remains the same.

Question 43

the line when § is plotted against ln(c) is what

Answer 43

its in the form of a quadratic!!! if we differentiate the equation and put a value of 'x' from the x axis into it,, it gives us the d§/ ln(c) value.

Question 44

where the line stops decreasing is called the

Answer 44

critical micelle conc this is the conc where monomers begin to aggregate together to form micelles instead of adsorbing at the interface

Question 45

the cmc is also what

Answer 45

its aove the surfactant solubility conc

Question 46

after the cmc,, if u increase the conc of the surfactant what happens

Answer 46

increasing the conc gets u more aggregates,, the surface composition doesnt change anymore

Question 47

why does the composition of the interface chnage once u reach the cmc

Answer 47

bc the surface deoends o nthe equilibrium between the individual SF molecules that have been dissolved,, and the surface. by increasing the conc of SF,, the conc of free SF doesnt change,, bc it forms micelles,, meaning the composition of the interface also doesnt change. bc equilibrium is between free dissolved SF and SF adsorbed to the interface. meaning the surface tension also doesnt change

Question 48

okay from the line of best fitt // equation of the line where u have § against ln (c) of SF how can u find the d§/dln(c) at the CMC

Answer 48

- get the first derivation - take a value of x that is equal to the CMC and subs it in - find the value of the first derivative equation by subbing in the x value.

Question 49

when u have the d§/ d ln(c) value,, what can u do to it

Answer 49

u can put it into the surface excess equation : gamma = -1/nrt x d§/dln(c) then divide this number by 1000 which tells us the conc of SF at the interface.