2e Flashcards
how do we create more interface and what must occur for us to do so
- move bulk molecules to the surface
- break the intermolecular forces of attraction the bulk molecules feel ( majority of these wont be regained)
parts of the surfactant include
- polar head ( hydrophilic)
- hydrocarbon chain (hydrophobic)
what is a surfactant
a surface active agent
what effect does a flouronated surfactant have
a flouronated surfactant reduces surface tension
what happens when u put a surfactant in the water
the HC chain points away from the water
the polar head sits in the water
in a micture of surfactant and water,, what is there an equilibrium between
equilibrium between the surfactant dissolved in the bulk and the surfactant that adsorbs on the interface
describe what happens when u start with pure water and add surfactant to it // keep increasing the conc of surfactant
- 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.
CMC can also be explained as
the solubility limit of the surfactant has been reached.
why do surfactant monomers first go to the surface
better thermodynamic environment for them
what happens when the surfactant solubility limit // critical micelle conc has been reached
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.
diameter of a micelle is usually
1 nm –> 2nm
when a droplet of oil is stabilised by a micelle, what does it look like
their tails are in the inside
when a droplet of water is stabilised by a miclle what does it look like
the poalr heads are in the middle
whats importasnt to rmemeber about the air/water interface
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
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
we need to define where the tolution is in the air // water interface region!!
we do this using the gibbs dividing surface equation
what does the gibbs dividing surface equation tell u
it gives u a plane where everything above it is air,, and everything below it is solution // water
what is the gibbs dividing durface equation
gamma = ( -1/nrt ) ( d§ / d ln(c) )
this is the gibbs absorption isotherm
what changes when u change conc of surfactant
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.
why does conc of SF chnage ST
bc ur changing the composition of the interface
which reduced surface tension
if our surface tension is lower,,, what does it mean
it means its easier to form more more surface!!!
what is surface excess and what is its symbrol
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!!!
gamma // surface excess equation
gamma = ni / A
aka number of molecules of a given type / area
what links surface tension and surface excess
chemical potential
what is the equation for surface tension
§ = dG / dA
what is chemical potential
it tells us how free energy changes with the composition of the system!!!
long u = dG / dn
change in surface energy // surface tension depends on what
the amount of material adsorbed at the surface and the change in chem potential that it causes.
d§ in terms of surface excess and chem potential
d§ = (- gamma ) ( d long u )
if we have multiple components on the surface,, what do we need to take into accound
we need to take into account that theyre all diff and need to take each of them into account
the contribution of each specie in the interface is what
its additive
aka d§ = - sum of gamma i x chem potential of i
when there are 2 components,, what would d§ be
the -gamma x dlong u of solute + the -gamma x dlong u of solvent
the gibbs dividing surface equation gives us what
gives us surface excess
the SE of the solvent is what
its 0 at the point where there is no excess of solvent
if the surface excess,, aka gamma,, of the solvent is 0 ,, what does that tell us about gamma x d long u
that will also be 0
bc ur multiplying by 0
due to the surface excess of the solvent being 0,, the d§ is now based on
the -surface excess of the solute x the chemical potential of the solute
chemical potential =
chem potential* + RT ln(a or conc)
du =
RT d ln(A)
change on chemical potential depends on what
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.
therefore gamma aka surface excess ====
(-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
what does surface excess allow us to do
it helps us compare surfactants + their efficiancy // ability to adsorb at the surface
by looking at the composition of the interface + how it changes with the change in free energy ,, what can we see
it can tell us about any chnages in concentration.
in the graph,, what are the different axes
y axis = §
x axis = ln (c) aka conc of surfactant in moldm-3
when u plot § against ln(c) what do u get
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.
the line when § is plotted against ln(c) is what
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.
where the line stops decreasing is called the
critical micelle conc
this is the conc where monomers begin to aggregate together to form micelles instead of adsorbing at the interface
the cmc is also what
its aove the surfactant solubility conc
after the cmc,, if u increase the conc of the surfactant what happens
increasing the conc gets u more aggregates,, the surface composition doesnt change anymore
why does the composition of the interface chnage once u reach the cmc
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
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
- 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.
when u have the d§/ d ln(c) value,, what can u do to it
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.
