Colloid chemistry

Lectures 1 & 2: Colloidal systems.History,classifications and examples.

Examples of colloidal systems from daily life

CosmeticsCosmetics

DetergentsDetergents

1

Lecture 1 INTRODUCTION TO COLLOIDS

1. Definition of Colloids

Colloids Disperse systems

Micro heterogeneous systems

Colloid = κολλα (Greek word = glue) Basic Characteristics of Colloids: (i) One of the phases is dispersed into the other one. (ii) The material inside the dispersed phase retains some of its bulk

properties.

Phase 1

Phase 2

Phase 1

Phase 2

Phase 1 – continuous phase (disperse media) Phase 2 – discontinuous phase (disperse phase) Typical size of the colloidal particles: 5 5nm Diameter m≤ ≤ µ Networked colloids (e.g. Phase 1: Phase 2 = 1:1)

Bicontinuous microemulsion Porous rock Polyurethane foam

1. partly physical chemistry- it is not the chemical composition which is important- the state is independent of the composition

2 partly physics- the physical properties are of great importance- basic law of physics can be applied

3 partly biology- biological materials are colloids- the mechanisms of living systems are related to colloid- and interfacial chemistry

Colloid science is interdisciplinary

size range of discontinuity:

1 nm to 500 nm (1000 nm)

1 nm = 10 Å = 10-7 cm = 10-9 m

- small particle size and small pore size implylarge interfacial area and theinterfacial properties are therefore important !

The colloidal domain

distance x distance x

dens

ity ρ

(x)

dens

ity ρ

(x)

colloidal dispersions(incoherent systems)

porous materials; gels(coherent systems)

W. Ostwald: the colloidal state is independent of the chemical compositionA. Buzágh: colloids → systems with submicroscopic discontinuities (1-500 nm)

Colloidal discontinuities

Classification of colloidson the basis of structure

incoherent systems coherent systems (gels)

colloidal macromolecular associationdispersions solutions colloids

liophobic liophilic liophilic

colloids

porodin reticular spongoid

corpuscular fibrillar lamellar

TEM

HRTEM

4 ± 25 % nm cubooctahedral Pd particles224 ± 21% nmLDH particles

TEM

198 ± 17% nm SiO2 particles

TEM

SEM22 ± 20% nm O / Wmicroemulsion particles

optmicr

cryoTEM

Incoherent systems: (colloidal) dispersions

4 ± 31% nmPd particles

TEM

3.2 ± 41% µm O / Wemulsion particles

Surface matterslamella

fibrilla

corpuscula

3

3. Surface Area and Surface Free Energy of Colloids Very large area of the interfaces in the colloidal systems!!! φ - volume fraction of particles (disperse phase) Number of particles per 1cm3 of colloid

343p

NV Rφ φ

= =π

Total interfacial area per 1cm3 of colloid:

22

343

4 34pRNA N R

RRπ φ φ

= π = =π

Example: for 0.4 (40%)φ = and 20R nm=

161.2 10 12 000 000 000 000 000N = × = particles per 1cm3 of colloid Total interfacial area = NAp= 60m2 !!!

4

Surface Free Energy – Surface Tension (σ)

Facts About Surface and Interface

• 1. Increase in Surface and Energy with Decrease in Size.

12.6111

1015120.1253

50.3640.252

25.180.5 1

Area (cm2)No.R (cm)Cut No

Facts

• 2. The existence of matter in the colloidal state may be:

• desirable, or

• undesirable.

• Therefore, it is important to know both:

• how to make, and

• how to destroy colloidal systems.

• 3. Colloid science is an interdisciplinary subject.

• 4. Colloid science can be understood at both

descriptive and theoretical levels.

• 5. The factors which contribute most to the overall

nature of a colloidal system are:

– Particle size.

– Particle shape and flexibility

– Surface (including electrical) properties

– Particle-particle interactions

– Particle-solvent interactions

Facts

Change of surface free energywith particle size

when the particle size decreases: the specific surface area increasesthe degree of dispersion increases

Size-dependent pecific surface area: S/V(surface to volume ratio)

S / V

S / V

Specific surface area: S/V(surface to volume ratio)

colloid

Stability of liophilic and liophobic colloids

- liophilic (solvent loving)- liophobic (solvent hating)- hydrophilic- hydrophobic- lipophilic- lipophobic

colloidal dispersions: liophobic colloids - thermodynamically not stable; kinetically may be stable

macromolecular solutions: liophilic colloidssurfactant solutions: liophilic colloids- both thermodynamically and kinetically stable

Dispersion ColloidsHydrophobic Particles

structure of a polypeptide molecule in aqueous solution

Non-particulate incoherent systems:macromolecular solutions

some possible comformations ofproteins in water

Non-particulate incoherent systems:association colloids (surfactants)

chemical structure of a single surfactant molecule: sodium dodecyl sulfate

Surfactant micellessurfactant molecule

hydrophobicalkyl chain

hydrophilichead group

self-assembling

spherical micelle

hydrophilic shellhydrophobic core

cationic surfactantanionic surfactantnonionic surfactant

orientation → energy minimumHardy-Harkins principle

30-100 moleculesd-3-5 nm(association)

Shapes of surfactant aggregates

Surfactants as biocolloids

plasma membranes are primarily lipid bilayers with associated proteins and glycolipids(cholesterol is also a major component of plasma membranes)

Surfactants as biocolloids

Surfactants as biocolloids

Gel: it is a solid or semisolid system of at least two constituents,consisting of a condensed mass and interpenetrated by a fluid (liquid or gas)(liogel; aerogel). Network without distinct boundaries. No sedimentation.

Coherent systems: gels

2) Macromolecules bound by strong van der Waals forces or cross-linkedby chemical bonds:

1) Floccules of small particles bound by strong van der Waals forces:

/ / surfactant molecules + liquidsurfactant molecules + liquid

/ ”SOAP” GEL/ ”SOAP” GEL

Formation of liogels

/

/

Coherent systems: xerogels(porous MCM-type materials)

Xerogels: porous materials

coherent system: gelatin (hydrogel)

Coherent systems: liogels(hydrogels and organogels)

LiogelsLiogels show a variety of flow (rheological) behaviours:

T= 15 0C T= 20 0C T= 25 0C T= 30 0C T= 35 0C T= 400C T= 450C

Liogels

Hydrogels may show distinct temperature and pH dependent behaviour:

Classification of disperse systems by size

Classification of dispersed systems

dispersed systems

amicroscopic

“true” solution

submicroscopic systems

colloids

coarse systems

micro heterogeneous

1 nm 500 nm(1000 nm)

homogeneous colloids

homogenous or heterogeneous?

heterogeneous

• true solutions (“molecular dispersions”)• (molecules, ions) in gas, liquid (solutions) • < 1 nm, diffuse easily, pass through paper filters

• fine dispersions (colloidal dispersions )• sols (”lyophobic colloidal solutions”); • microemulsions, micelles, polymers

(”lyophilic colloidal solutions”); • smoke, films & foams• 1 to 1000 nm, diffuse slowly, separated by ultrafiltration

• coarse dispersions• most pharmaceutical suspensions and emulsions, dust,

powder, cells, sands• >1µm, do not diffuse, separated by filtration

Classification of disperse systemsby size

Solutions

♦ Have small particles

(ions or molecules)

♦ Are transparent

♦ Do not separate

♦ Cannot be filtered

♦ Do not scatter light

Colloids♦ Have medium size particles

♦ Cannot be filtered

♦ Separated with semipermeable membranes

♦ Scatter light (Tyndall effect)

Suspensios

♦ Have very large particles

♦ Settle out

♦ Can be filtered

♦ Must stir to stay suspended

Classification of disperse systemsby size

systemssystems

micellesmicelles

Colloid systems

fog

Classification of colloidal dispersionsby shape

1. prolate(a>b) 2. oblate (a<b) 3 rod 4. plate 5. coil

Classification of colloidal dispersionsin terms of the physical states of the

internal and external phases

L/G: fog, mist, spray(liquid aerosols)

S/G: smoke, loose soot (powders)(solid aerosols)

G/L: sparkling water, foam,whipped cream

(liquid gas dispersions)

L/L: milk; mayonnaize; crude oil((micro)emulsions)

S/L: paint, ink, toothpaste(sols/suspensions)

G/S: polysterene foam,silica gel

(aerogels, xerogels)

L/S: opal, pearl(solid emulsions)

S/S: pigmented plastics(solid suspensions)

Classification of colloidal dispersionsin terms of the physical states of the

internal and external phases

Some tidbits from thehistory of colloids

motion.

Brownian motion

Dynamics of colloidal particles

Brownian motion

The Faraday-Tyndall effect.Dark-field microscopy: the ultramicroscope.

Zsigmondy, 1903

Ultramicroscopic images

blood red cells

Ag nanoparticles

The Faraday-Tyndall effect

The Faraday-Tyndall effect

Dialysis

Kidney and dialysis

Artificial kidney

Water and small solute particles

pass through a semipermeable

membrane, large particles are

Retained inside.

Hemodialysis is used medically

(artificial kidney) to remove

waste particles such as

urea from blood.

A dialysis unit

Principle ofdialysis

Osmotic pressure of the blood

Osmotic Pressure of the Blood♦ Cell walls are semipermeable membranes

♦ The osmotic pressure of blood cells cannot change or damage occurs

♦ The flow of water between a red blood cell and its surrounding environment must be equal

isotonic solutions♦ Exert the same osmotic pressure as red blood cells. ♦ Medically 5% glucose and 0.9% NaCl are used their solute concentrations

provide an osmotic pressure equal to that of red blood cells

H2O

hypotonicsolutions

♦ Lower osmotic pressure than red blood cells

♦ Lower concentration of particles than RBCs

♦ In a hypotonic solution, water flows into the RBC

♦ The RBC undergoes hemolysis;

it swells and may burst

H2O

hypertonicsolutions

♦ Has higher osmotic pressure than RBC♦ Has a higher particle concentration ♦ In hypertonic solutions, water flows out of the RBC♦ The RBC shrinks in size (crenation)

H2O

Stability of colloidal dispersions