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PM1B medicines discovery and development > Key concepts in colloids and interfaces > Flashcards

Key concepts in colloids and interfaces Flashcards

1
Q

Define an interface

A

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

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2
Q

Define surface tension

A

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

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3
Q

Units of surface tension and surface free energy

A

Nm-1

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4
Q

What are the 2 methods to measure surface tension?

A

wilhelmy plate

capillary rise

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5
Q

Explain wilhelmy plate

A

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

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6
Q

Explain capillary rise

A

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

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7
Q

Define a surfactant

A

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)

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8
Q

Classify surfactants

A 
Draw these
Anionic - negative head
Cationic - positive head
Non-ionic- no charge on head
zwitterionic - positive and negative head
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9
Q

Define critical micelle concentration

A

Concentration at which micelles form in surfactant solution

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10
Q

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

A

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

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11
Q

How to calculate surface excess and define

A

gradient of tangent / RT

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

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12
Q

How to calculate area per molecule

A

1/ Na x surface excess

Na =. Avogadro

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13
Q

Traubes rule

A

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)

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14
Q

Why does a surfactant lower surface tension

A

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

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15
Q

how / why do monomolecular films form

A

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

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16
Q

Describe3 physical states of monomolecular films

A

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)

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17
Q

Differences between chemisorption and physisorption

A

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

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18
Q

Difference between adsorption and absorption

A

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

absorption - absorbs into the surface

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19
Q

Draw 3 types of adsorption isotherms

A

monolayer adsorption
multilayer adsorption
multilayer adsorption (rapid)

20
Q

how does detergency work

A

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

21
Q

4 factors affecting adsorption

A

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

22
Q

Define a polymer

A

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

23
Q

Architecture of a polymer

A

linar, branched, star, dendrimer

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

24
Q

Define a block co-polymer

A 
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)
25
Q

what are polymers in solution?

A

lyophilic colloids

26
Q

Define colloid

A

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
27
Q

Differentiate dimensions of solutions, colloids, suspensions

A

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

28
Q

Explain why formation of micelles is entropy driven

A

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

29
Q

Internal factors affecting CMC

A

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

30
Q

External factors affecting CMC

A

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

31
Q

Lyophobic colloidal stability

A

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

32
Q

what are lyophobic colloids

A

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

33
Q

methods to form lyophobic

A

dispersion methods - colloidal mills

condensation methods

34
Q

Describe the 3 electric double layer models

A

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

35
Q

define DLVO theory

A

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

36
Q

what is total attractive force dependent on?

A

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

37
Q

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

A
  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

38
Q

what are colloidal dispersions stabilised by?

A

Mainly charge at particle surface

39
Q

Differences between colloidal and coarse suspensions (3)

A

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

40
Q

ideal properties of a suspension (3)

A

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

41
Q

what is sedimentation governed by?

A

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

42
Q

how to avoid cake systems and stabilise

A

controlled flocculation - DLVO - deep secondary minimum

Steric stabilisation - attractive forces

43
Q

Define critical coagulation concentration

A

electrolyte concentration at which coagulation will occur

44
Q

define schulz-hardy rule

A

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

45
Q

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

A

(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)

PM1B medicines discovery and development (13 decks)

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