L38, 39 Colloids

Question 1

38

What are colloidal dispersions?

Answer 1

Colloidal dispersions are biphasic systems where one phase (dispersed phase) is distributed in another (continuous/dispersing phase), with particle sizes between 1–1000 nm.

Question 2

38

What are the three types of dispersions in terms of particle size?

Answer 2

Molecular dispersions: <1 nm

Colloidal dispersions: 1–1000 nm

Coarse dispersions: 10–50 µm

Question 3

38

What are the two classification types of colloids based on physical state and affinity?

Answer 3

Physical state: Sols (low viscosity), Gels (high viscosity)

Affinity: Lyophilic (high affinity for the dispersion medium), Lyophobic (low affinity)

Question 4

38

What do hydrosols and hydrogels refer to?

Answer 4

Dispersions where the continuous phase is water.

Question 5

38

What characterises a lyophobic colloid?

Answer 5

Low affinity for the dispersion medium

Poor solvation

Thermodynamically unstable

Requires energy to form

Decrease in entropy and increase in free energy

Question 6

38

How are lyophobic colloids typically prepared?

Answer 6

Mechanical dispersion: Physically breaking down larger particles into fine colloids.

Controlled aggregation: Controlling particle aggregation by altering conditions.

Chemical reactions: Changing the chemical structure of materials (oxidation, reduction) to form colloids.

Solvent change: Switching from a good solvent to a bad solvent to induce colloidal formation.Mechanical dispersion

Controlled aggregation

Chemical reactions (oxidation, reduction, hydrolysis)

Solvent change (good to bad solvent)

Question 7

38

Give an example of a lyophobic colloid used pharmaceutically.

Answer 7

Hydrophobic colloidal anhydrous silica BP, used in emulsions, gels, and semi-solid preparations.

Question 8

38

What defines a lyophilic colloid?

Answer 8

High affinity for the dispersion medium

Forms spontaneously

Thermodynamically stable

Increase in entropy and decrease in free energy

Question 9

38

Give an example of a lyophilic colloid.

Answer 9

Bentonite, used in Calamine Lotion BP and clay masks.

Question 10

38

What are association colloids?

Answer 10

Colloids formed by self-association of amphiphilic molecules (e.g., surfactants, phospholipids) into structures like micelles or liposomes.

Question 11

38

What is a micelle?

Answer 11

A spherical structure formed by surfactants in solution, where hydrophobic tails face inwards and hydrophilic heads face the solvent.

Question 12

38

What is the Critical Micelle Concentration (CMC)?

Answer 12

The minimum concentration of surfactant above which micelles form in solution.

Question 13

38

What drives micelle formation?

Answer 13

Entropy – release of ordered water molecules around hydrophobic tails upon micelle formation increases system entropy.

Question 14

38

Do all amphiphilic molecules form micelles?

Answer 14

No, only some amphiphiles have the necessary structure and conditions to form micelles.

Question 15

38

What is the Krafft point?

Answer 15

The temperature above which the solubility of a surfactant increases sharply, allowing micelle formation.

Question 16

38

What is the cloud point?

Answer 16

The temperature above which a surfactant’s solubility decreases, leading to precipitation due to dehydration of polar heads. This is a reversible process.

Question 17

38

What are the pharmaceutical applications of colloidal dispersions?

Answer 17

Drug Delivery:

• Sustained release: Gradual drug release over time, reducing dosing frequency.
• Targeted delivery: Delivers drugs specifically to desired tissues, reducing side effects.

Improved Solubility:

• Colloids like micelles and nanosuspensions enhance the solubility of poorly water-soluble drugs, improving absorption.

Controlled Release Formulations:

• Long-term, controlled drug release for stable therapeutic effects and reduced side effects.

Topical and Transdermal Products:

• Liposomes and nanoemulsions improve drug penetration through the skin, allowing controlled release for local or systemic effects.

Parenteral Formulations:

• Nanosuspensions for injectable formulations improve bioavailability and stability of poorly soluble drugs. Can provide long-acting injections.

Question 18

38

What is the difference between thermodynamic and kinetic stability in colloids?

Answer 18

Thermodynamic stability: Whether a system forms spontaneously and remains dispersed (true for lyophilic)

Kinetic stability: Resistance to aggregation or sedimentation over time, even if not thermodynamically stable (seen in lyophobic colloids)

Question 19

38

What types of molecular interactions stabilise lyophilic colloids?

Answer 19

Hydrogen bonding: Between colloid and solvent, enhancing dispersion.

van der Waals forces: Gentle attractive interactions supporting solubility.

Solvation layers: Solvent molecules form a stabilising barrier around particles, preventing aggregation.

Question 20

38

What are the methods used to identify colloidal systems?

Answer 20

Light scattering (Tyndall effect)

Ultracentrifugation

Viscosity measurements

Electron microscopy

Question 21

38

Why is entropy important in colloidal stability and micelle formation?

Answer 21

Increased entropy favours spontaneous formation of micelles and stability in lyophilic colloids due to disordering of structured water molecules.

Question 22

38

What is the Tyndall effect?

Answer 22

The scattering of light by colloidal particles, making the path of light visible in colloidal systems.

Question 23

38

What happens to a surfactant solution below the Krafft point?

Answer 23

The surfactant is poorly soluble and cannot form micelles; the solution may remain cloudy or precipitate.

Question 24

38

How does temperature influence micelle formation?

Answer 24

Below Krafft point: No micelles form

Above Krafft point: Micelles form if concentration exceeds CMC

Above cloud point (for non-ionic surfactants): Micelles precipitate out due to dehydration

Question 25

# 38 What are some examples of lyophilic colloidal excipients?

Answer 25

Gelatin Acacia Tragacanth Methylcellulose Used to stabilise emulsions or suspensions, or increase viscosity.

Question 26

# 38 What makes amphiphilic molecules suitable for micelle formation?

Answer 26

A balance between hydrophilic head and hydrophobic tail Chain length, ionic nature, and temperature all influence this ability

Question 27

# 38 What is the difference between a sol and a gel?

Answer 27

Sol: A colloidal dispersion with fluid-like consistency Gel: A semi-solid colloidal system where the dispersed phase forms a 3D network throughout the dispersion medium

Question 28

# 38 Why are lyophobic colloids more difficult to prepare than lyophilic colloids?

Answer 28

They require special preparation methods due to low affinity for the medium and are thermodynamically unstable.

Question 29

# 38 What are common methods for preparing lyophobic colloids via chemical reactions?

Answer 29

Oxidation (e.g. forming sulphur sols) Reduction (e.g. gold sols from chloroauric acid) Hydrolysis (e.g. ferric hydroxide sols)

Question 30

# 38 What is the role of surfactants in pharmaceutical colloids?

Answer 30

Stabilise emulsions Enhance solubility Assist in drug delivery Reduce interfacial tension between phases

Question 31

# 38 How are association colloids classified?

Answer 31

Micelles (spherical, above CMC) Reverse micelles (in non-polar solvents) Vesicles (bilayer structure, e.g. liposomes)

Question 32

# 38 What are vesicles and how do they differ from micelles?

Answer 32

Vesicles are bilayered structures that enclose an aqueous core Micelles have a monolayer structure with no inner core

Question 33

# 38 What factors affect the CMC of a surfactant?

Answer 33

Temperature Presence of electrolytes Nature of hydrophilic and hydrophobic groups Molecular structure

Question 34

# 38 How does the presence of electrolytes influence colloidal stability?

Answer 34

Electrolytes can reduce repulsion between particles, leading to aggregation or flocculation, especially in lyophobic colloids.

Question 35

# 38 What happens when a surfactant is heated above its cloud point?

Answer 35

The solution becomes turbid or separates due to loss of hydration of the hydrophilic heads—reversible upon cooling.

Question 36

# 38 Which of the following best explains why lyophobic colloids require stabilisers for long-term use in pharmaceutical formulations? A. They have a strong affinity for the dispersion medium B. They exhibit high thermodynamic stability C. They lack an electrical double layer, which prevents micelle formation D. They tend to aggregate due to poor solvation and weak intermolecular repulsion

Answer 36

✅ Correct Answer: D Explanation: Lyophobic colloids are not well solvated, making them prone to aggregation unless stabilised.

Question 37

# 38 Which condition is most likely to increase the CMC of a surfactant? A. Adding a small amount of NaCl B. Replacing a methyl group in the tail with an ethyl group C. Increasing temperature beyond the Krafft point for an ionic surfactant D. Shortening the hydrophobic tail

Answer 37

✅ Correct Answer: D Explanation: A shorter hydrophobic tail decreases hydrophobic interactions, increasing the CMC.

Question 38

# 38 Which of the following best differentiates association colloids from lyophilic colloids? A. Association colloids are always thermodynamically stable B. Lyophilic colloids only form above the cloud point C. Association colloids consist of amphiphilic molecules that form micelles above CMC D. Lyophilic colloids consist of particles larger than 1 μm

Answer 38

✅ Correct Answer: C Explanation: Association colloids form micelles from amphiphilic molecules, unlike lyophilic colloids that are often polymer-based and stable in solvents.

Question 39

# 38 Which of the following scenarios would most likely prevent micelle formation in an aqueous surfactant solution? A. Temperature below the Krafft point B. Concentration slightly above the CMC C. pH within optimal range for head group ionisation D. Addition of a co-surfactant

Answer 39

✅ Correct Answer: A Explanation: Below the Krafft point, surfactant solubility is too low for micelle formation.

Question 40

# 38 A student observes a bluish beam of light passing through a sample. What conclusion can be drawn about the nature of the sample? A. It is a true solution B. It contains molecules smaller than 1 nm C. It is a colloidal dispersion exhibiting the Tyndall effect D. It has sedimented particles

Answer 40

✅ Correct Answer: C Explanation: The Tyndall effect is characteristic of colloidal particles which scatter visible light.

Question 41

# 38 What is the most likely result of increasing electrolyte concentration in a lyophobic colloidal system? A. Decreased particle size due to flocculation B. Increased repulsion between particles C. Neutralisation of surface charge leading to aggregation D. Enhanced solvation and increased viscosity

Answer 41

✅ Correct Answer: C Explanation: Electrolytes compress the electrical double layer, reducing repulsion and increasing aggregation.

Question 42

# 38 Which property of a surfactant is most crucial for forming a stable micelle in aqueous solution? A. Short alkyl chains B. Balanced hydrophilic-lipophilic ratio C. Low melting point D. High cloud point

Answer 42

✅ Correct Answer: B Explanation: A surfactant must have the correct balance between hydrophilic and lipophilic portions to form micelles.

Question 43

# 38 Which of the following best describes the relationship between the Krafft point and micelle formation? A. Micelles form below the Krafft point but only in presence of electrolytes B. The Krafft point represents the concentration at which micelles form C. Micelles can only form above the Krafft point if CMC is also exceeded D. The Krafft point is the temperature at which micelles begin to precipitate

Answer 43

✅ Correct Answer: C Explanation: Both adequate temperature (above Krafft point) and concentration (above CMC) are needed for micellisation.

Question 44

# 38 A parenteral formulation contains a lyophobic colloid stabilised with sodium citrate. Which event is most likely if a calcium salt is added to this formulation? A. Enhanced drug solubility due to ion pairing B. Improved electrostatic repulsion between particles C. Precipitation due to charge neutralisation D. No change, as calcium has low ionic strength

Answer 44

✅ Correct Answer: C Explanation: Calcium ions may neutralise negative charges (e.g. citrate-stabilised particles), leading to flocculation or precipitation.

Question 45

# 38 A patient is administered an intravenous lipid emulsion. What feature of this colloidal system prevents embolism formation? A. Low density B. High surface tension C. Steric stabilisation by phospholipids D. Complete water miscibility

Answer 45

✅ Correct Answer: C Explanation: Phospholipids provide steric stabilisation to keep lipid droplets dispersed and prevent coalescence.

Question 46

# 38 A surfactant has a CMC of 2.5 mM and the solution volume is 100 mL. How many micelles are likely to form when the total surfactant added is 10 mM? (Assume each micelle contains 100 molecules) A. 7.5 × 10²⁰ B. 7.5 × 10¹⁹ C. 2.5 × 10¹⁹ D. 5.0 × 10²⁰

Answer 46

✅ Correct Answer: B Explanation: Surfactant available for micelles = 10 mM – 2.5 mM = 7.5 mM = 7.5 × 10⁻³ mol/L In 0.1 L: 7.5 × 10⁻³ × 0.1 = 7.5 × 10⁻⁴ mol Molecules = 7.5 × 10⁻⁴ mol × 6.022 × 10²³ ≈ 4.52 × 10²⁰ molecules Micelles = 4.52 × 10²⁰ ÷ 100 ≈ 4.5 × 10¹⁸ → Closest: B

Question 47

# 38 A 1% w/v solution of surfactant X forms micelles at 25°C. Given molar mass = 300 g/mol and CMC = 5 mM, what percentage of surfactant is in micelles? A. 20% B. 40% C. 60% D. 80%

Answer 47

✅ Correct Answer: D Explanation: 1% w/v = 1 g/100 mL = 10 g/L Moles = 10 ÷ 300 = 0.0333 mol = 33.3 mM Surfactant above CMC = 33.3 – 5 = 28.3 mM % in micelles = 28.3 ÷ 33.3 × 100 ≈ 85% → Closest: D

Question 48

# 38 A suspension shows rapid sedimentation despite small particle size. Which parameter most likely explains this? A. High zeta potential B. Flocculated structure C. High viscosity D. Presence of surfactant at CMC

Answer 48

✅ Correct Answer: B Explanation: Flocculation causes larger effective particle masses to settle rapidly, even if primary particles are small.

Question 49

# 38 Which of the following distinguishes cloud point from Krafft point in non-ionic surfactant systems? A. Cloud point refers to turbidity from micelle breakdown B. Krafft point only applies to non-ionic surfactants C. Cloud point marks phase separation; Krafft point marks solubility threshold D. Krafft point is always lower than cloud point

Answer 49

✅ Correct Answer: C Explanation: Cloud point is where micelle solution turns turbid (non-ionic), Krafft is minimum temp needed for ionic surfactants to form micelles.

Question 50

# 38 What is the most likely mechanism of drug loading in micelle-based drug delivery systems? A. Drug binds to polar heads of surfactant B. Drug is trapped within aqueous core of vesicle C. Drug is solubilised within hydrophobic core of micelles D. Drug precipitates inside the shell of micelles

Answer 50

✅ Correct Answer: C Explanation: Micelles can encapsulate hydrophobic drugs in their core, improving solubility.

Question 51

# 39 What size range defines colloidal particles?

Answer 51

Less than 1 μm in diameter.

Question 52

# 39 What does size distribution in colloids refer to?

Answer 52

The range of different particle sizes within a sample; it is not always uniform.

Question 53

# 39 How do lyophilic colloids behave in a good solvent?

Answer 53

They maximise interaction with the dispersing phase, adopt an extended shape, surface tension contributes almost nil to free energy, and dispersion is spontaneous.

Question 54

# 39 How do lyophobic colloids behave in a bad solvent?

Answer 54

They minimise interactions with the dispersing phase, adopt a compact or spherical shape, surface tension contribution is high, and dispersion requires energy input.

Question 55

# 39 What is dialysis used for in colloids?

Answer 55

To separate colloidal particles from small molecules or ions using a semi-permeable membrane.

Question 56

# 39 What is Brownian motion?

Answer 56

The random movement of colloidal particles due to thermal agitation, observable up to 5 μm.

Question 57

# 39 What is sedimentation in colloids and how is it studied?

Answer 57

Downward motion under gravity; studied using ultracentrifugation (~10⁶ g).

Question 58

# 39 How does diffusion work in colloids?

Answer 58

Particles move from areas of high concentration to low concentration along a concentration gradient.

Question 59

# 39 How does viscosity vary in colloids?

Answer 59

Increases with solvation, particle shape (elongated > spherical), concentration, and molecular weight.

Question 60

# 39 How are colloidal particles visualised?

Answer 60

Using an ultramicroscope (as bright spots on a dark background) or electron microscope (gives size and shape).

Question 61

# 39 What does light scattering in colloids help determine?

Answer 61

Particle size and shape via dynamic or static light scattering.

Question 62

# 39 What is the Faraday-Tyndall effect?

Answer 62

Scattering of light by colloidal particles.

Question 63

# 39 How can colloidal particles acquire charge?

Answer 63

Ion adsorption, ion dissolution, ionisation of surface groups; charges may be permanent or pH-dependent.

Question 64

# 39 What is zeta potential?

Answer 64

The effective surface charge of a particle in a colloid; influences stability, especially in hydrophobic colloids.

Question 65

# 39 What is the Donnan membrane effect?

Answer 65

The influence of charged colloids on the diffusion of small ions across a membrane.

Question 66

# 39 What forces influence colloidal stability?

Answer 66

Electrostatic repulsion and steric attraction (linked to solvation).

Question 67

# 39 What is the DLVO theory?

Answer 67

It explains colloidal stability using total potential energy: VT = VR + VA Where Vr is repulsion (electrostatic) and Va is attraction (Van der Waals).

Question 68

# 39 What causes coagulation and flocculation in lyophobic colloids?

Answer 68

Coagulation at the primary minimum (strong forces), flocculation at the secondary minimum (weak forces); both influenced by electrolytes.

Question 69

# 39 How can lyophilic colloids be destabilised?

Answer 69

With organic solvents, low electrolyte concentrations, or high salting-out conditions.

Question 70

# 39 Name pharmaceutical uses of hydrophilic colloids.

Answer 70

Thickening agents, stabilisers (suspensions/emulsions), tablet binders, solubilising agents (micelles), drug carriers, and detoxifying agents (e.g. activated charcoal).

Question 71

# 39 What is Brownian motion in colloidal systems?

Answer 71

The random, erratic movement of colloidal particles caused by collisions with dispersion medium molecules.

Question 72

# 39 Why is Brownian motion important in colloidal dispersions?

Answer 72

It helps prevent sedimentation, maintaining particle dispersion and stability.

Question 73

# 39 What factors affect Brownian motion?

Answer 73

Particle size (smaller = more movement), viscosity (lower = more movement), and temperature (higher = more movement).

Question 74

# 39 What is the significance of sedimentation in colloidal dispersions?

Answer 74

Sedimentation determines how fast particles settle under gravity, affecting the stability of suspensions.

Question 75

# 39 What is Stokes’ Law in relation to colloids?

Answer 75

It calculates the sedimentation velocity of particles based on their radius, density difference, and the medium's viscosity.

Question 76

# 39 According to Stokes’ Law, which particles sediment faster?

Answer 76

Larger and denser particles in low-viscosity media.

Question 77

# 39 What is the Tyndall effect?

Answer 77

Light scattering by colloidal particles, making the dispersion appear turbid or cloudy.

Question 78

# 39 How is the Tyndall effect used in practice?

Answer 78

It helps distinguish colloidal dispersions from true solutions.

Question 79

# 39 Give a clinical example where light scattering is important.

Answer 79

Turbidity monitoring in IV infusions to detect contamination or aggregation.

Question 80

# 39 How does osmotic pressure of colloids compare to that of true solutions?

Answer 80

It is significantly lower due to fewer dispersed particles.

Question 81

# 39 Why is osmotic pressure important in formulating IV colloids?

Answer 81

It influences fluid movement and volume expansion in blood vessels.

Question 82

# 39 Name three colloidal systems used as plasma expanders.

Answer 82

Gelatin, dextran, and hydroxyethyl starch.

Question 83

# 39 What role do colloids play in nanomedicine?

Answer 83

They serve as carriers for targeted drug delivery, e.g., liposomes or nanoparticles.

Question 84

# 39 How can colloidal drug carriers be targeted to specific tissues?

Answer 84

By modifying their surface with ligands that bind to specific receptors.

Question 85

# 39 A colloidal system is showing high viscosity even at low concentration. What is the most likely explanation? A. The particles are spherical and non-solvated B. The particles are elongated and highly solvated C. The system has high zeta potential D. The system is undergoing Brownian motion

Answer 85

Elongated, solvated particles increase viscosity even at low concentrations.

Question 86

# 39 Which of the following would most likely destabilise a lyophobic colloid? A. Increase in zeta potential B. Addition of a polymeric stabiliser C. Addition of an electrolyte above the critical coagulation concentration D. Decrease in temperature

Answer 86

C – Electrolytes screen the repulsive forces, reducing zeta potential and promoting coagulation.

Question 87

# 39 A colloid has a very low zeta potential (~ -5 mV). What does this suggest about its stability? A. Highly stable due to steric repulsion B. Highly unstable due to insufficient repulsive force C. Neutral due to equilibrium of forces D. Stable only under acidic conditions

Answer 87

B – Low zeta potential means particles can easily aggregate.

Question 88

# 39 What distinguishes flocculation from coagulation in colloidal dispersions? A. Flocculation is reversible, coagulation is not B. Flocculation occurs at the primary minimum C. Coagulation involves electrostatic stabilisation D. Coagulation is reversible under shear

Answer 88

A – Flocculation involves weak interactions (secondary minimum), while coagulation is usually irreversible.

Question 89

# 39 A colloidal particle has a radius of 50 nm and a density of 1.2 g/cm³. If the dispersion medium is water (density = 1.0 g/cm³), what force causes sedimentation under gravity? A. Zero, due to Brownian motion B. Buoyant force exceeds gravitational force C. A net downward force due to density difference D. Cannot sediment under gravity

Answer 89

C – The particle is denser than the medium, so net force acts downward.

Question 90

# 39 If light scattering analysis shows an intensity distribution peak at 100 nm but number distribution peaks at 70 nm, what can be inferred? A. All particles are 100 nm B. Large particles are dominating light scattering C. Small particles have higher intensity D. Sample is monodisperse

Answer 90

B – Light scattering intensity is proportional to the 6th power of radius; larger particles dominate.

Question 91

# 39 In a formulation, doubling the concentration of a lyophilic colloid increases viscosity from 0.5 to 2.0 Pa·s. What does this imply about the polymer structure? A. Likely spherical micelles B. Likely rod-like or highly solvated C. Low molecular weight polymer D. Poor solvent interaction

Answer 91

B – A sharp increase in viscosity with concentration indicates extended, rod-like, or highly solvated polymers.

Question 92

# 39 A patient is administered an injectable formulation with colloidal particles. Which property is most important for avoiding capillary blockage? A. High viscosity B. Small and uniform particle size C. Strong Brownian motion D. High zeta potential

Answer 92

B – Ensures safety in IV administration and avoids embolism risk.

Question 93

# 39 Why is dialysis important in preparing colloidal drug carriers? A. To sterilise the colloid B. To remove residual surfactant C. To separate free drug or ions from the carrier D. To increase particle solubility

Answer 93

C – Dialysis separates small solutes (e.g., unencapsulated drug or ions) from the colloid.

Question 94

# 39 A pharmacist observes phase separation in a colloidal suspension. Which adjustment is most appropriate to restore stability? A. Increase temperature B. Add salt C. Add a charged stabiliser or increase zeta potential D. Reduce solute concentration

Answer 94

C – Increasing electrostatic repulsion helps maintain dispersion stability.