Key concepts in colloids and interfaces Flashcards
Define an interface
2 phases mutually insoluble form an interface (interfacial interactions are dominant to the behaviour of colloidal systems)
Define surface tension
Unbalanced forces at the surface lead to a net inward force of attraction on the surface of molecules that tends to minimise surface area
Units of surface tension and surface free energy
Nm-1
What are the 2 methods to measure surface tension?
wilhelmy plate
capillary rise
Explain wilhelmy plate
Thin plate - completely wetted by a liquid. Force acting on the plate due to wetting is measured by a balance and is proportional to the surface tension
Explain capillary rise
Tendency of liquids to rise up the tube as a consequence of surface tension
Define a surfactant
Surface active agent -
Non polar hydrophobic tail - derives the formation of self assembled aggregates or colloids
Polar hydrophilic head - ensures water solubility
- Different types - anionic, cationic, non-ionic, zwitterionic
(DRAW IT)
Classify surfactants
Draw these Anionic - negative head Cationic - positive head Non-ionic- no charge on head zwitterionic - positive and negative head
Define critical micelle concentration
Concentration at which micelles form in surfactant solution
Explain graph of surface tension Vs log concentration (and draw)
Gibbs adsorption is the gradient of the curve of surface tension Vs Log Conc.
Can then calculate surface excess and area per molecule of the adsorbed species.
- the gradient at concentration just below the CMC is where the molecules at the surface are closely packed
How to calculate surface excess and define
gradient of tangent / RT
the amount of concentration at an interface with units molm-2
How to calculate area per molecule
1/ Na x surface excess
Na =. Avogadro
Traubes rule
For solutions of a surfactant series, the molar concentrations required to produce equal lowering of surface tension of water decreases threefold for each additional CH2 group in the hydrocarbon chain
(increase in chain length = increase surface activity)
Why does a surfactant lower surface tension
a surfactants tendency for an interface favours expansion of that interface and reduces surface tension
how / why do monomolecular films form
surfactants with high hydrophobic content and insoluble in water can form insoluble monolayers. (1 molecule thick)
Describe3 physical states of monomolecular films
1- condensed - molecules closely packed
2- expanded - films coherent but occupy larger area than condensed
3- gaseous - separate and move about the surface independently
(draw them if easier)
Differences between chemisorption and physisorption
Chemisorption - adsorbate interacts with surface to form strong covalent bond
specific, localised, irreversible, has maximum adsorption, activated by high temperature
Physisorption - adsorbate is bound to surface via non covalent interactions.
Non specific, mobile, reversible, can form multiple layers, favoured by low temps
Difference between adsorption and absorption
Contact angles
Adsorption - accumulation of material at a surface - to relieve unbalanced forces within a system
absorption - absorbs into the surface
Draw 3 types of adsorption isotherms
monolayer adsorption
multilayer adsorption
multilayer adsorption (rapid)
how does detergency work
process of dirt removal from solid surfaces by surface chemical means.
- good wetting
- remove dirt in bulk solution
- prevent dirt from re-attaching
made up of surfactants and builders
4 factors affecting adsorption
solubility - inversely proportional to solubility as solute-solvent bonds must be broken for adsorption
pH - affects ionisation & solubility so suppress ionisation = increase adsorption
nature of adsorbent surface - interactions, hydrophobicity
temperature - increase temp. increase solubility, reduce adsorption
Define a polymer
Substance with high molecular weigh, build from a large number of repeating units of a monomer
Architecture of a polymer
linar, branched, star, dendrimer
Examples- PE, PMM, PEO - block co-polymer
Define a block co-polymer
Blocks of polymers linked together Takes structure- A-A-A-A-B-B-B-B diblock AAA-BBBB-AAAAA trick example - non ionic surfactant pluronic PPO-PEO-PPO (also draw)
what are polymers in solution?
lyophilic colloids
Define colloid
Mixture in which very small particles of a substance are distributed evenly throughout another substance
- they form spontaneously
- 1-1000nm
- they scatter light so can use Tyndall effect to recognise colloids
- large surface area
Differentiate dimensions of solutions, colloids, suspensions
Solutions <1nm
Colloid 1-1000nm
Suspension >1000nm (affected by gravity)
Explain why formation of micelles is entropy driven
Despite the apparent ordering of surfactant molecules in micelle, it is entropically driven by hydropobic interactions.
- governed by the limited movement and caging of water molecules around hydrophobic domains that works to minimise contact of water with hydrophobic region of molecules
Internal factors affecting CMC
length of hydrocarbon chain (tail) - longer the chain, lower the CMC.
Non ionic - length of head group
Ionic - nature of head group
External factors affecting CMC
Temperature
addition of electrolytes - reduces repulsion, CMC reduced and micelle formation favoured
Addition of organic molecules
(A) alcohol - reduces repulsions and steric hindrance, lowers CMC
(b) sugars - structure makers, decrease CMC
(c) urea formaldehyde - structure breakers, increase CMC
Lyophobic colloidal stability
thermodynamically unstable due to tendency to aggregate. Stability determined by interactions between particles - kinetic stability, electrostatic or steric hindrance.
what are lyophobic colloids
2 phase systems, dispersed phase 1-100nm.
thermodynamically unstable- don’t form spontaneously
apparent kinetic stability due to kinetics of collapse
methods to form lyophobic
dispersion methods - colloidal mills
condensation methods
Describe the 3 electric double layer models
Helmholtz- adsorbed ions balance charge
Guoy-chapman- diffuse layer balances surface charge
stern - adsorbed layer and diffuse layer
define DLVO theory
DLVO helps describe colloidal dispersion stability.
The total interaction energy between 2 approaching particles is given by the sum of the attractive and repulsive forces
Repulsive - electrical double layer of steric repulsion
attractive - VDW dispersion forces
what is total attractive force dependent on?
1- hamaker constant (itself is dependent on the chemical nature of particles)
2- radius of particles
3- distance between particles
What is repulsive forces dependent on? i.e electric double layer
- surface charge (potential)
- electrolyte concentration (reduces thickness)
ALSO have steric hindrance (DLVO doesn’t consider) - via polymer chains
and repulsion between hydrated surfaces
what are colloidal dispersions stabilised by?
Mainly charge at particle surface
Differences between colloidal and coarse suspensions (3)
Colloidal dispersed phase 1-1000nm
Coarse dispersed phase >1000nm
Coarse is a larger size meaning they are affected by gravity and thus sedimentation
ideal properties of a suspension (3)
should not settle too rapidly or too slowy
should be readily dispersed on shaking
not be too viscous
what is sedimentation governed by?
particle size - reduce by pestle and mortar
density - increase by adding polyethylene glycol, glycerine, sorbitol, sugar
Viscosity- increase by adding suspending agents - methyl cellulose, acacia
how to avoid cake systems and stabilise
controlled flocculation - DLVO - deep secondary minimum
Steric stabilisation - attractive forces
Define critical coagulation concentration
electrolyte concentration at which coagulation will occur
define schulz-hardy rule
efficiency of flocculation increases with charge number of counter ion of the added electrolyte
how does addition of electrolytes affect the CMC of non-ionic and anionic surfactants
(Define them)
Non ionic is NOT affected by electrolyte.
Anionic CMC is lowered because the counter ion balances the charge of head group while reducing the impact of electrostatic repulsion of head groups in micelle (schulz-hardy)
