Colloid chemistry. The colloid state. Classification and...

Colloid chemistry. The colloid state. Classification and characterization

of colloid systems.

Levente NovákIstván BányaiZoltán Nagy

Department of Physical Chemistry

General information

Lectures and examinations

● 12 or 13 lectures on Tuesdays (10:00–12:00) and Wednesdays (13:00–15:00) at the Pharmacy Building, Seminar Room 1.

● Lecture slides will be available for download the day following the lecture from:

http:/t/tkolloid.unideb.hu/ten/tcolloid-aand-asurface-achemistry-afor-achemical-aengineers/t

or

http:/t/tkolloid.unideb.hu/ten/ttheory/t

● Written test in the exam period (100 minutes) of 80 to 100 points.● For “pass” (2) grade approximately 50% of the total points have to be attained

● Test questions (not yet available) will be chosen from the set downloadable from the page above.

● An example test will be found at the same site afer the end of the lectures.

● ATTENTION! Exam test questions may change slightly and numerical values may difer from those in the downloadable set → READ ALWAYS CAREFULLY THE QUESTIONS BEFORE ANSWERING THEM.

Suggested reading

Barnes, GT, Gentle, IR: Interfacial ScienceOxford UP. ISBN 0-a19-a927882-a2, 2005

Pashley, R. M.: Applied Colloid & Surface ChemistryWiley&Sons, ISBN 0-a470-a86883-aX, 2004

Cosgrove T.: Colloid scienceBlackwell Publishing ISBN:978-a14051-a2673-a1, 2005

Erbil, H. Y.: Surface ChemistryBlackwell, ISBN 1-a4051-a1968-a3, 2006

McCash, E. M.: Surface ChemistryOxford UP. ISBN 978-a0-a19-a850328-a6, 2001 (2007)

Crowe, J.: Chemistry for the BiosciencesOxford UP. ISBN 0-a19-a928097-a5, 2006

Lectures planned

1. Physical chemistry basics. Colloid systems.2. Molecular and interparticle interactions.3. Liquid-agas, solid-agas, solid-aliquid interfaces.4. Surface chemistry: L-aG, S-aG, S-aL surfaces.5. Adsorption at gas-asolid interface.6. Adsorption from solutions. Strong electrolytes.7. Electric double layers.8. Electrokinetic phenomena.9. Colloid stability: lyophobic colloids.10. Foams, emulsions.11. Macromolecules.12. Association colloids.13. Rheology and structure.

Synopsis

● Place of colloid science and notion of colloids● Physical chemical concepts● Historical elements● Classification of colloidal systems● Characterization of colloidal systems

● Size● Shape● Distribution● Interactions

Place of colloid science and notion of colloids

Importance of colloids● Everyday experiences (foods, paints, creams, cosmetics, etc.)● Wetting of solids and liquids, spreading● Solubilization of drugs● Importance of particle size

● Silicosis● Red mud (industrial accident in Hungary)● Smog (health implications → size of particles very important)● Fluorescence of nanodots is size dependent (used e.g. in diagnostics)● Nanotechnology

● Importance of particle shape● Asbestos (needle-alike microcrystals)● Color of gold sols (also influenced by size)

● Importance of microstructure● New alloys with given microstructure (implants)● Functional polymers (biological macromolecules, drug delivery)● Drug release rate● Drug inhibition● Polymorphism of materials● TiO2 catalytic activity (cosmetics)

Controversy about colloids

Colloidal silver taken as a biocide→ adverse efects (argyrosis)

Qite a bad business for the customer:

● bulk silver ≈ 0.5–0.6 EUR/tg● colloidal silver > 10 EUR/tg

1 liter: 10 EUR[Ag] = 1000 ppm = 1 g/tL

Place of Colloid Science

● Partly Physical Chemistry

● the chemical composition is secondary

● states are independent on the composition

● Partly Physics

● physical properties play a veryimportant role

● basic laws of physics are used

● Partly Biology

● biological systems are colloidal systems

● mechanism of living systems are determined by surface chemistry

Colloid Science and Physical Chemistry

● Physical chemistry of systems where beside the usual intensive variables* (p, T, c) important role is played by● size (r or d)● surface (A)● shape (e.g. l/tw or w/th)

● In the case of particles size lies between 1-a2 and 200-a1000 nm (colloidal size range).

● The surface free enthalpy change is very important during the changes describing the system.

*Intensive variables: variables not depending on sample size.

Physical chemical concepts

● Gibbs' phase rule● Relation between the number of phases (P ), the number of components

(C) and the degrees of freedom* (F ) of a system in thermodynamical equilibrium (J. W. Gibbs)

● Homogeneous and heterogeneous systems● Homogeneous → isotropic (all intensive variables are the same

everywhere) e.g. pV=nRT● Heterogeneous → anisotropic, there are phase boundaries where the

intensive variables vary abruptly

F=C−P+2

*Degree of freedom: number of independent intensive variables

Are colloid systems homogeneous or heterogeneous?

Are they continuous? Discontinuous?Or does it depend on the magnification?

Let's count the number of phases!

Multiphase

Homogeneous

systems

Heterogeneous

systems

Gold sol(colloidal gold)

Conclusion

Can not be determined from visual aspect for:bouillon, milk, beer, pudding, fog, smoke, soil, dentifrice, glue, blood, mayonnaise, egg yolk, opal, soap solution, etc.

Colloids

Historical background

● Debate over homogeneous or heterogeneous nature● Colloids and crystalloids, separable by dialysis (T. Graham)

– crystalloids difuse faster and are crystallizable– colloids difuse slower and are not crystallizable

● Solution theory and suspension theory– colloids are solutions (view of biologists)– colloids are suspensions (view of soil chemists)

● Experiments– R. A. Zsigmondy and H. Siedentopf → development of the

ultramicroscope– P. P. Weymarn → observation of analytical precipitates

The ultramicroscope

● It is a conventional light microscope with a special illumination system.

● Light shines from one side onto a cuve placed under the microscope.

● No direct light enters the objective.● Small particles scatter light (Tyndall cone) → they appear

as bright spots over a dark background.● Particles of 1 nm are visible provided light is bright enough.● Their morphology is not visible, only their location and

motion.

The ultramicroscopeMicroscope tube

Collector lens Slit Condenser lens Sample in cuve

Light source

A look through the ultramicroscope

Difraction pattern of theparticles (not the particlesthemselves!)

Dark background

● The system is heterogeneous → particles are moving (Brownian motion)● The velocities of the particles present an energy distribution in agreement

with the Boltzmann-aMaxwell equation

Crystalloids to colloids

● Precipitates appear from reactions yielding poorly soluble compounds

● Depending on the initial conditions, the size or the particles can be small or larger

● It is possible to form colloidal systems from diverse compounds → the colloid state does not depend on the composition but the dispersity and distribution

P. P. Weymarn

“Weymarn law”:

Colloidal dispersions are formed from very dilute or very concentrated solutions but not from intermediate solutions.

Definition of the colloids

● Colloids are disperse systems in which the size of the discontinuities* in at least one direction lies between 1 nm and 500 nm.

● In colloidal systems the surface is playing a determinant role.

*These discontinuities may be the particles themselves or the gaps between them.

m

nm

Atoms, small molecules, ions

macromolecules

smoke

fog, mist

colloid (submicroscopic)

micelles viruses bacteria, spores, pollens

microscopic heterogeneous

Homogeneous systemsHeterogeneous sytems

(macroscopic or microscopic multiphase)

Colloid systems

homogeneous (amicroscopic)

1010 810 610910 710 510 410 310

0.1 1 10 210 310 410 510 610

Colloid science—surface science, nanotechnology

Size dependent properties

● specific conductivity of metals ≈2 nm● transparency of ceramics ≈20 nm● color of metals ≈50 nm● hardness of metals ≈250 nm● malleability of ceramics ≈500 nm

The total surface energy increases with decreasing particle size

Molecules and particles

● If we examine the systems A—D, the heterogeneous system (A) and the homogeneous system (D) are simpler than B and C

● Their basic element is the molecule● In the systems B and C we additionally have

particles (molecule clusters)● Particle is a cluster of molecules behaving as

a kinetic entity and possessing a defined translational heat movement (e.g. sedimentation, electrophoretical mobility, etc.)

● A new type of interaction appears, the particle—particle interaction

Universality of the theorem of submicroscopic discontinuities

● Wo. Ostwald: the colloid state is independent of the chemical properties● A. Buzágh: colloid systems have submicroscopic discontinuities

Motion in force fields

● Mechanical forces and thermal motion act on colloidal systems dispersed in fluids● gravity● viscous drag (=friction)● thermal motion

Size (nm) 10 1000 100000

Velocity (m/ts) 5.45×10-a11 5.45×10-a7 5.45×10-a3

Efect of brownian motion

Size (nm) 10 1000 100000

Velocity (m/ts) 153 0.153 1.53×10-a4

F frict=6 πηr v=4 r 3

π(ρ−ρ0)g

3=F grav

Particle sedimentation (ρ =2 g/cm3

)

32 kB T =

12 m v 2=4 × 10−21 Jat 298K

Thermal motion can not beneglected for small particles

Results in collisions →

coagulation, precipitation

Motion in force fields

● Velocity due to the sedimentation:

● Velocity due to the Brownian motion:

v=2r 2

(ρ−ρ0)g9η

v=√3k B T

m

v : velocity (m/ts)r : radius of the particle (m)ρ : density of the particle (kg/tm3)ρ0 : density of the medium (kg/tm3)g : gravitational acceleration (m/ts2)η : viscosity of the dispersion (Pa·s)

v : velocity (m/ts)kB : Boltzmann constant (J/tK)T : temperature (K)m : mass of the particle (kg)

The colloidal state (Summary)

● In the colloidal state, heterogeneity or homogeneity have no physical meaning or have a diferent meaning than in classic systems

● All types of matter can be in the colloidal state● Colloidal state does not depend on the chemical composition● Colloid systems are

● composed of particles or discontinuities of 1–500 nm in size● systems where surface afects strongly the behavior of the particles● greatly influenced by the Brownian motion (thermal motion is larger

than sedimentation rate)

Classification of colloidal systems

Colloidal systems by structure

● Incoherent systems → individual particles● Dispersion colloids (phase colloids) → Lyophobic● Macromolecular colloids → Lyophilic● Association colloids → Lyophilic

● Coherent systems → crosslinks (skeleton or scafold structure)● Porodin systems (porous)● Reticular systems (network-alike)● Spongoid systems (sponge-alike)

Colloidal systems by structure

sol-agel transition

Dispersion colloids or sols

Classification

Coherent systems

Association colloids

Surfactant(soap, detergent)

Amphiphilic molecules(partly hydrophilic,partly hydrophobic)

Spherical micelle(association of individual molecules)

Macromolecular colloids

● From macromolecules (much bigger in size than small molecules)

● Importance of size and shape (as well as their distribution)

Characterization of colloidal systems

Stability of colloidal systems

● Thermodynamically colloids can be● Stable (real solutions)

– Lyophilic colloids– ΔG<0 (Gsol < Gorig)

– Association colloids and macromolecular colloids● Unstable (disperse systems)

– Lyophobic colloids– ΔG>0 (Gsol > Gorig)

– High specific surface area– Dispersion colloids (sols)

● Kinetically colloids can be– Stable → do not change their properties over the duration of the observation– Unstable → gradual change (aggregation, coarsening, precipitation)

Synthesis: dependence on the addition order

Homogeneous solutions

Colloidal dispersions


● Degree of dispersion (=size)● Size and size distribution

● Morphology (shape and internal structure)● Important as even the same size distribution can lead to very

diferent properties● Spatial distribution of the dispersed particles

● Concepts of homogeneity and inhomogeneity● Interactions between particles

● Can influence the former properties

Size determination

● Microscopy● Normal microscope: down to 0,2 µm (=200 nm)● Ultramicroscope: only detection down to 1 nm● Electron microscope: down to 1 nm

● Sieving● Generally from 25 µm to >100 mm● Wet sieve: 10 µm – 100 µm

● Size-aexclusion chromatography● Sedimentation

● Gravitational: down to 1 µm (Brownian motion!)● Centrifugal: down to 1 nm, depends on centrifugal force

● Light scattering (dynamic)● NMR difusiometry

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Degree of dispersion (size)

● Dispersity (degree of dispersion)● The higher it is, the smaller are the particles in average● Homodisperse system → all particles are of the same size

(ideal case)● Heterodisperse system (characterization: polydispersity)

● Averages → several diferent types exist, do not inform of the individual details

● Group property constituted from individual properties● Arithmetical average (of property x with weighing factor ϕ)

x̄=∑ xi ϕi

∑ ϕi

Why use diferent averages?

● Diferent averages are sensitive to diferent properties

● Colligative properties (properties related to the number of particles, e.g. osmotic pressure) → proportional to the number average

● Properties linked to the surface area or volume of particles (viscosity, difusion coeficient) → proportional to the area and volume (or weight) averages respectively

Let's consider a real example

We have 100 balls of diferent diameter. Let's calculate the

● Number-aaverage diameter → dN = 39.5

● Area-aaverage diameter → dA = 47.13

● Volume-aaverage diameter → dV = 49.03

Conclusion

the bigger balls have a more pronounced influence on dA and d

V than on d

N

Number Diameter Area Volume n×d n×A×d n×V×d n×A n×V

5 10 314.1593 523.5988 50 15707.96 26179.94 1570.796 2617.994

10 20 1256.637 4188.79 200 251327.4 837758 12566.37 41887.9

30 30 2827.433 14137.17 900 2544690 12723450 84823 424115

50 50 7853.982 65449.85 2500 19634954 1.64×108 392699.1 3272492

5 60 11309.73 113097.3 300 3392920 33929201 56548.67 565486.7

Sum 100 3950 25839600 2.11×108 548207.9 4306600

Let's consider a real example

d̄ N=∑ d i N i

∑ N i

=∑ 5×10+10 × 20+30× 30+50 ×50+5×60

∑ 5+10+30+50+5=

3950100

=39.5

d̄ A=∑ d i A i

∑ A i

=∑ 5 ×10 ×314+10 ×20 × 1256+30 ×30 ×2826+50 ×50 ×7850+5×60 × 11304

∑ 5×314+10 ×1256+30× 2826+50 × 7850+5×11304=

25826500547930

=47.13

d̄ V =∑ d i V i

∑V i

=∑ 5× 10 ×523.6+10 × 20× 4188.79+30 ×30 × 14137.17+50 × 50× 65449.85+5 ×60×113097.3

∑ 5× 523.6+10 × 4188.79+30 × 14137.17+50 × 65449.85+5× 113097=

211141205.94306599,922

=49.03

Number Diameter Area Volume n×d n×A×d n×V×d n×A n×V

5 10 314.1593 523.5988 50 15707.96 26179.94 1570.796 2617.994

10 20 1256.637 4188.79 200 251327.4 837758 12566.37 41887.9

30 30 2827.433 14137.17 900 2544690 12723450 84823 424115

50 50 7853.982 65449.85 2500 19634954 1.64×108 392699.1 3272492

5 60 11309.73 113097.3 300 3392920 33929201 56548.67 565486.7

Sum 100 3950 25839600 2.11×108 548207.9 4306600

Number-weighted diameter average:

Area-weighted diameter average:

Volume-weighted diameter average:

Polydispersity index

● Polydispersity index (PDI) is a simple but good measure of the dispersion of a property's distribution.

● It is calculated as the ratio between the weight-aaverage (or volume-aaverage) and the number-aaverage of a given property:

● PDI≥1.● If PDI=1, the sample is monodisperse.● If PDI>1, the sample is polydisperse.

● “Polydispersity index” is proposed to be replaced by the term “dispersity” (symbol: Ð) but is still widely used

PDI=x̄W

x̄N

Average, distribution, standard deviation

● Averages and PDI can characterize a system but do not tell much about the details (e.g. what is the percentage of particles with a given size).

● The normal (Gaussian) distribution ofen approximates well measured properties of a population. This gives a better description of the distribution of the individual properties within a population of particles.

● f(x) is called the probability density function of the normal distribution, x is the mean value, σ is the standard deviation and φ is the frequency

f (x )=1

σ √2πe

(x −x̄ )2

2 σ2

σ2=∑ (x−x̄ )

2 d φφ

Average and standard deviationΦ

(pro

babi

lity)

x

● The probability density function of the normal distribution is a bell-ashaped curve, the x coordinate of its apex (highest point) corresponds to x (the average value of property x).

● The cumulative distribution function (CDF) Φ(x) of the normal distribution is an “S”-ashaped curve. The average x corresponds to the x coordinate of its inflexion point (y=50% of the total height).

Cumulative distribution

curve

Diferential distribution

curve f(x)=dΦ(x)/tdx

σ σ

.84

.16

≈60% of the total height on the diferential

distribution curve

Morphology

Non-isometric morphologies

1. Prolate (short ellipsoid)2. Oblate (long ellipsoid)3. Rodlike4. Discoidal5. Statistical coil

● Morphology afects properties of the particles, e.g.:● Degrees of freedom → specific orientation gives additional degrees of

freedom to anisometric (non-aisometric) particles● Flow behavior → increasing non-anewtonian flow properties with increasing

anisometry● Toxicity → the long, needle-alike morphology of asbestos is causing its

carcinogenity (crystals punch through cell membranes)

State and shape

Corpuscularsystem

Fibrillar system

Lamellar particles

Spherical secondaryparticles

Spatial distribution

● Homogeneous● Difuse (e.g. by sedimentation or

thermal difusion)● Heterogeneous● Ordered

Nematic Smectic Tactoid

● Caused by intermolecular interactions● Specific behavior (optical birefringence, liquid crystals, biological

membranes, clay minerals)

Colloid chemistry. The colloid state. Classification and...

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