Key concepts in colloids and interfaces

Question 1

Define an interface

Answer 1

2 phases mutually insoluble form an interface (interfacial interactions are dominant to the behaviour of colloidal systems)

Question 2

Define surface tension

Answer 2

Unbalanced forces at the surface lead to a net inward force of attraction on the surface of molecules that tends to minimise surface area

Question 3

Units of surface tension and surface free energy

Answer 3

Nm-1

Question 4

What are the 2 methods to measure surface tension?

Answer 4

wilhelmy plate

capillary rise

Question 5

Explain wilhelmy plate

Answer 5

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

Question 6

Explain capillary rise

Answer 6

Tendency of liquids to rise up the tube as a consequence of surface tension

Question 7

Define a surfactant

Answer 7

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)

Question 8

Classify surfactants

Answer 8

Draw these
Anionic - negative head
Cationic - positive head
Non-ionic- no charge on head
zwitterionic - positive and negative head

Question 9

Define critical micelle concentration

Answer 9

Concentration at which micelles form in surfactant solution

Question 10

Explain graph of surface tension Vs log concentration (and draw)

Answer 10

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

Question 11

How to calculate surface excess and define

Answer 11

gradient of tangent / RT

the amount of concentration at an interface with units molm-2

Question 12

How to calculate area per molecule

Answer 12

1/ Na x surface excess

Na =. Avogadro

Question 13

Traubes rule

Answer 13

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)

Question 14

Why does a surfactant lower surface tension

Answer 14

a surfactants tendency for an interface favours expansion of that interface and reduces surface tension

Question 15

how / why do monomolecular films form

Answer 15

surfactants with high hydrophobic content and insoluble in water can form insoluble monolayers. (1 molecule thick)

Question 16

Describe3 physical states of monomolecular films

Answer 16

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)

Question 17

Differences between chemisorption and physisorption

Answer 17

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

Question 18

Difference between adsorption and absorption

Answer 18

Contact angles
Adsorption - accumulation of material at a surface - to relieve unbalanced forces within a system

absorption - absorbs into the surface

Question 19

Draw 3 types of adsorption isotherms

Answer 19

monolayer adsorption
multilayer adsorption
multilayer adsorption (rapid)

Question 20

how does detergency work

Answer 20

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

Question 21

4 factors affecting adsorption

Answer 21

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

Question 22

Define a polymer

Answer 22

Substance with high molecular weigh, build from a large number of repeating units of a monomer

Question 23

Architecture of a polymer

Answer 23

linar, branched, star, dendrimer

Examples- PE, PMM, PEO - block co-polymer

Question 24

Define a block co-polymer

Answer 24

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)

Question 25

what are polymers in solution?

Answer 25

lyophilic colloids

Question 26

Define colloid

Answer 26

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

Question 27

Differentiate dimensions of solutions, colloids, suspensions

Answer 27

Solutions <1nm Colloid 1-1000nm Suspension >1000nm (affected by gravity)

Question 28

Explain why formation of micelles is entropy driven

Answer 28

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

Question 29

Internal factors affecting CMC

Answer 29

length of hydrocarbon chain (tail) - longer the chain, lower the CMC. Non ionic - length of head group Ionic - nature of head group

Question 30

External factors affecting CMC

Answer 30

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

Question 31

Lyophobic colloidal stability

Answer 31

thermodynamically unstable due to tendency to aggregate. Stability determined by interactions between particles - kinetic stability, electrostatic or steric hindrance.

Question 32

what are lyophobic colloids

Answer 32

2 phase systems, dispersed phase 1-100nm. thermodynamically unstable- don't form spontaneously apparent kinetic stability due to kinetics of collapse

Question 33

methods to form lyophobic

Answer 33

dispersion methods - colloidal mills | condensation methods

Question 34

Describe the 3 electric double layer models

Answer 34

Helmholtz- adsorbed ions balance charge Guoy-chapman- diffuse layer balances surface charge stern - adsorbed layer and diffuse layer

Question 35

define DLVO theory

Answer 35

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

Question 36

what is total attractive force dependent on?

Answer 36

1- hamaker constant (itself is dependent on the chemical nature of particles) 2- radius of particles 3- distance between particles

Question 37

What is repulsive forces dependent on? i.e electric double layer

Answer 37

1. surface charge (potential) 2. electrolyte concentration (reduces thickness) ALSO have steric hindrance (DLVO doesn't consider) - via polymer chains and repulsion between hydrated surfaces

Question 38

what are colloidal dispersions stabilised by?

Answer 38

Mainly charge at particle surface

Question 39

Differences between colloidal and coarse suspensions (3)

Answer 39

Colloidal dispersed phase 1-1000nm Coarse dispersed phase >1000nm Coarse is a larger size meaning they are affected by gravity and thus sedimentation

Question 40

ideal properties of a suspension (3)

Answer 40

should not settle too rapidly or too slowy should be readily dispersed on shaking not be too viscous

Question 41

what is sedimentation governed by?

Answer 41

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

Question 42

how to avoid cake systems and stabilise

Answer 42

controlled flocculation - DLVO - deep secondary minimum | Steric stabilisation - attractive forces

Question 43

Define critical coagulation concentration

Answer 43

electrolyte concentration at which coagulation will occur

Question 44

define schulz-hardy rule

Answer 44

efficiency of flocculation increases with charge number of counter ion of the added electrolyte

Question 45

how does addition of electrolytes affect the CMC of non-ionic and anionic surfactants

Answer 45

(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)