6e Flashcards
whats diff about solid particle dispersion and normal particle dispersion
in solid particle dispersion the 2 solids cannot agglomerate into one single big particle
whereas oil droplets can
describe solid particle dispersion and what happens to it overtime
- cup of solvent // water with the solid particles floating in the liquid
- overtime the particles relax at the bottom of the liquid
what 3 forces cause the solid particles to relax at the bottom of the solvent
- gravity (macroscopic phase transition)
- brownian motion (stabilisation force when they move around)
- viscous drag force ( the solvent needs to move out the way so the particle can move and have its browning motion)
in the 3 forces that act on the solid particles which ones oppose eachother
- viscous drag force
- browning motion
when is it easier to keep the dispersion stable
when the particle is super small
this is why colloidal dispersion systems are easier to stabilise than solid dispersion
bc their particles are smaller
which of the 3 forces are size dependent
the gravity
the viscous drag force (bc more particles would have to move out the way if the particle is bigger)
what force is size independent
the browning motion force
which force dominates and when
when the particles are small,,, the browning motion force dominates bc its size independent,, whereas the graity and viscous drag would decrease
what attractive forces are seen between solid particles
VDW : infuced dipole,, just dipole etc
what part of the solid particle do we care about when we talk about particles getting closer etc
we talk about their centre of mass
what is the attractive interaction between solid particles dominated by
theyre dominated by their centres of mass!!
which is a destabilising interaction bc if particles come together,, they aggregate and stay in contact,, this reduces entropy
why is aggregation in larger particles so bad
bc aggregatiion increases their size
which means that gravity and viscous drag has a larger effect on it.
and the browning motion is less able to overcome it
when is there normally a less stable dispersion
theres a less stable dispersion when the particles are larger!!!
bc more gravity and viscous drag,, browning motion finds it harder to overcome them
how can we increase the stability of dispersion
reduce the size of the particle
introduce charge at the particles surface (by adding pH dependent groups to it which will become protonated // deprotonated at different pH’s)
can adsorb surfactants on particles ( the SF can be ionic meaning its polar head will be charged,, its a permanent charge)
introduce steric stabilisation: adsorb a polymer on the surface : allows a polymer coated // fuzzy particle
why does adding a charge on the particle either using ionic surfactants or using pH dependent groups
bc if all the particles hva ethe same charge on their surface ,, they will repel eachother
increasing the distance between eachother
increasing the stability of th dispersion bc the solid stays dispersed in the solvent for longer !! yay
what happens when we change the ionic strangth of a solution
u can neutralise some of the surface charge
reducing the charge present on the surface reduces the distance at which repulsion occurs by!!
what happens when u neutralise the charge on solid particles by changing the ionic strength of the solvent
bc it neutralises some of the surface charge
this minimises the repulsion the particles feel towards themselves bc theyre less charged meaning less repulsion!!!
meaning they can get closer together!!
it reduces the distance that repulsion occurs at
property of polymers that makes them good for adsorbing onto solid particles to increase the stability of the dispersion
its a long chain
it has a large molecular weight
when will polymers prefer to adsorb to the particle surface
when they have a low solubility in the solvent!!
when they dont like the solvent they will adsorb onto the particles surface
when a poymer is adsorbed onto a surface,, describe the diffferent structural parts you see
u have the trails ( polymer is adsorbed flat onto the particle)
loops ( polymer forms a hill on top of the particles surface)
tails ( polymer is sticking off the particles surface)
what does the polymer provie the particle with
it provides it with a solvated bilayer
aka that part is hydrated meaning theres a bunch of polymers and water on the surface
this is kinda bouncy and prevents particles from getting too close to eahcother.
as 2 particles covered in polymers come closer together,, what happens to the area inbetween them
the area inbetween them will have a higher conc of of polymers
when particles with polymers get closer and closer,, the amount of polymer inbetween them increases,, but what else increases
the chemical potential of that region also increases
an increase in chemical potential also means u increase the gibbs free energy of that area
increase in gibbs free energy in that area also means that thermodynamically them getting closer is unfavourable
equation that links gibbs, chemical potential and number of moles
chemical potential = gibbs / number of moles
bc the gibbs free energy increases when two particles with polymers gets closer,, what happens
theres a thermodynamic driving force for them to get further aparttttt
if two particles with polymers acc get close enough to have a physical interaction, what happens
they touch and theres a restriction in their polymer conformation which reduces the entropy of the system
this is bc they polymer gets squished and restricted meaning it can not move around as much,, meaning its entropy decreases.
theres also some displacement of solvent from the polymer layer
the polymer is less able to change its conformation
whenever theres a reduction in entropy,, an increase in chemical potential or gibbs free energy,, what is there said to be
there is said to be a thermodynamic driving force for the opposite action to occur.
aka if the particles getting closer led to that change,, the opposite will have the thermodynamic driving force,, meaning the gibbs will decrease and that opposite process will be favoured and spontaneous!!!
aka particles will separate, and this separation will will be favoured and occur spontaneously!!!
gibbs free energy total = what
gibbs attractive
+
gibbs electrostatic
+
gibbs steric interactions
what accounts for gibbs attractive
the VDW forces which move around for them to be attractive
whats gibs electrostatic
the surface charge bits
whats G steric interactions
the G mix
( the solvent and polymer in the interparticle region) aka when the particles are still kinda far apart
+
the G volume restriction ( when the polymers cant move bc theyre physically interacting and so the entropy decreases) aka G when the particles make physical contant
when conc is increased,, how does it afcect gibbs free energy
an increase in conc increases the free energy
meaning the reaction will be thermodynamically unfavored!!!
aka not spontaneous
when particles separate,, what happens to their interparticle region
the conc of polymers in the interparticle region decreases
this reduces the gibbs free energy of this area
this means that the separation of the particles is a favoured process as the gibbs free energy will be favoured
what relationship should we always think about
decrease in entropy = increase in gibbs
increase in entropy = decrease in gibbs
explain the graphs that we draw to understand the attractive and repulsive forces what occur when different solid particles interact : describe the main graph
y axis = interaction energy
x axis = centre of mass - centre of mass interaction !!
repulsion is upper level
attraction is lower level
with the graphs,, whats diff about them to other ones
u kinda start on the RHS ,, aka where the distance is between the centres is larger
as u go from right to left ur distance is reduced and they can either repel or attract until they reach the hard sphere repulsion ( where he come into contact bc 2 solid particles cannot aggregate together )
the hard sphere repulsion occurs at what distance
at 2r
bc this is the centre of mass to centre of mass when the particles make physical contant
the curve on the graph represents what
curve = total interaction energy
the sum of the attractive and repulsive forces
in the graph,, whats the first /// primary minimum
this is where they aggregate together - this is where the 2 phase separation is!!
this is where 2r is
its normally more to the LHS of the graph
in the graph,, what does the polymer particle have that the bare particle doesnt ,,
the polymer particle has Gmix and Gvr
wheras the bare one doesnt have this
it just means that when u draw the average curve,, u need to draw it in between the Gmix and Gvr curvesss (these are normally repulsive bc they slope up)
whats the Ea
the energy needed to overcome the attractive interactions and move the particles apart to where there is no interaction
thicker polymer layers normally have greater attraction : yes or no
yes a thicker polymer layer = lower attraction
a thinner polymer = a larger attraction
when can solid particles remain stable when u have the Ea value of the graph (from peaks // the total interaction energy : sum of attraction and repulsion)
when Ea < 3/2KT
particles remain stable when the Ea is smaller than the 3/2 KT bc KT is the kinetic energy u have due to browning motion
whats (3/2) KT
the kinetic energy u get from browning motion
when u have a charged particle,, what happens in the graph
u have charge charge repulsion even when the distance between the centre of masses for both particles is large.
the graph goes up at first and then swoops down : going from RHS to LHS
when u have a charged particle have there is repulsive forces even when the distance between particles is large,, what Ea do u have
the Ea that prevents aggregation state from forming ,,, aka the energy u need to prevent aggregation from occurring. the energy u need for the particles to attract themselves.
whats the primary and secondary minimum
primary minimum = the big trough : this is the one u have from attractive to the hard sphere repulsion
the secondary minimum is the 2nd trough
when u have a charge particle u have 2 minimums,, and the 2* minimum has 2 different Ea’s ,, what are these
the small ea from the trough to the line. this is the Ea for dispersion : normally just shaking th solution provides it with enough energy for this to be overcome.
it also has the Ea needed for aggregation ,, the more charged the ion is,, the larger this value,, bc a larger charge means more repulsion
systems with a secondary minimum are considered what
flocculated systems
this is when particles are loosely associated with eachother
u can redisperse it by shaking
its a semi stable state of dispersion !!! aka little energy is needed for it to be dispersed.
