Association colloids

Levente NovákIstván BányaiZoltán Nagy

Department of Physical Chemistry

Types of colloidson the basis of structure (appearance)

Porodin

colloids

Incoherent (fluid-like) Coherent (solid-like) = gel

ColloidalDispersions (sols)

Macromolecular solutions

Association Colloids

Colloidal solutions

(porous)Reticular Spongoid

corpuscular fibrillar lamellardispersion macromolecular association lyophobic lyophilic lyophilic

Forms from the following particle types:

Association colloids

Particles formed from amphipatic molecules in thermodynamically stable structures.

Etymology

Amphiphilic (=amphipatic) molecules

● Molecules which have hydrophilic and hydrophobic parts.

● Hydrophobic part generally consists of a hydrocarbon chain.

● If the hydrophilic part is large enough, the molecule is water soluble.

● If the hydrophobic part predominates, the apolar moiety is completely expelled from the aqueous phase and the molecules accumulate at the water-air interface.

● Between these border cases there is a continuous transition depending on the hydrophilic/hydrophobic ratio.

Carboxybetaines

PolyalcoholsPolyethers

Polar amides

12. lecture

E 322

Sorbitan monostearate (Span 60)

Tween 20-80(sorbitan ether-esters)

Span 20-120(sorbitanesters)common unit: sorbitan

Tween 20 or polyoxyethylene (20) sorbitan monolaurate The number 20 following the polyoxyethylene part does not refer to the total number of oxyethylene groups found in the molecule. It is just a notation loosely related to the composition of the molecule.

Sorbitan monostearate is used in the manufacture of food and healthcare products, and is a non-ionic surfactant with emulsifying, dispersing, and weting properties (E491)

Nonionic surfactants

McBain (1913):conductivity change of sodium palmitate → formation of micelles

Micelles are the simplest self-assembling systems

Amphiphilic compounds are surface active substances → surfactants (soaps, detergents)

Amphiphilic molecules and micelles

Spherical micelle

12. lecture

„Inverse” micelles (in oil)

Particles in thermodynamically stable state formed from the association of amphiphilic molecules.

Recommended sites:

● htp://physchem.ox.ac.uk/~rkt/lectures/amphi.html

● htp://www.ualberta.ca/~csps/:PPS8(2)/C.Rangel-Y.agui/solubili/ation.htm

Monomer

Association colloids

Generally 50-100 monomers form a micelle (but can be

higher or lower)

lc

a0P : packing parameter (no units)lc : length of the tail (m)a0 : equilibrium area of the polar head (m2)V0 : volume of the tail (m3)R : radius of the micelle (m)

P =V0/(lca0)

P = 0 – 1/3 P = 1/3 – 1/2 P = 1/2 -1 P >1

The shape of micelles

Spherical Cylindrical Lamellar Inverse micelle

Self-assembly of binary amphiphilic fluids into bicontinous cubic phases

htp://www.touchbriefings.com/pdf/1133/Tiberg.pdf

Surfactant structures

x

Two opposing forces control the self-association process: hydrocarbon-water interactions that favor aggregation (i.e. pulling surfactant molecules out of the aqueous environment: hydrophobic interaction), and head group interactions that favor hydration (desaggregation).

ΔGmicelles = ΔG1 + ΔG2 + ΔG3

Interactions:● water-water interaction (hydrophilic-hydrophilic): ΔG1

● surfactant-water interaction (hydrophilic-hydrophilic, hydrophilic-hydrophobic): ΔG2

● tail-tail interaction (hydrophobic-hydophobic): ΔG3

● head-head interaction negligible (heads are not in contact with each other): ΔG4=0

ΔGmicelles = ΔG1 + ΔG2 + ΔG3

Entropy efect: breaking the order of water molecules around micelles (favors aggregation).

Thermodynamics of micelle formation

When the hydration energy does not compensate the absent energy of the water-water interaction then surfactants molecules associate so less water molecules are needed for their hydration. If the hydration energy is higher, that is the solubility of molecule is beter, then micelle forms at higher concentrations.

hydration

water-water interaction

Thermodynamics of micelle formation

As the surface becomes crowded with surfactant more molecules will arrange into micelles. At some concentration the surface becomes completely loaded with surfactant and any further additions must arrange as micelles. This concentration is called the Critical Micelle Concentration (cmc).

Above a certain critical concentration the surface tension becomes independent of concentration, leading to a clear change in slope of the plot of surface tension against concentration, and is particularly marked in the logarithmic plot.

Critical micelle concentration

Critical micelle concentration (IUPAC definition):● There is a relatively small range of concentrations separating the

limit below which virtually no micelles are detected and the limit above which virtually all additional surfactant molecules form micelles.

● Many properties of surfactant solutions, if ploted against the concentration, appear to change at a diferent rate above and below this range.

● By extrapolating the loci of such a property above and below this range until they intersect, a value may be obtained known as the critical micelli/ation concentration (critical micelle concentration), symbol cM, abbreviation cmc (or c.m.c.).

● As values obtained using diferent properties are not quite identical, the method by which the cmc is determined should be clearly stated.

Critical micelle concentration

Π : osmotic pressure (Pa)κ : conductivity (S·m-1)γs : surface tension (N·m)γi : interfacial tension (N·m)τ : turbidity (without unit)δ : NMR chemical shif (ppm)Λ : molar conductivity (S·m2·mol-1)

cmc c (mol·dm-3)

Prop

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Π

κ

γs

γi

Λ

τ

δ

Efect of the apolar chain length on the cmc

nC (number of carbons)

8 10 12 14 16 18

cmc (mol·m-3) 140 33 8.6 2.2 0.58 0.23

Explanation: ΔG3 (tail-taik) decreases (becomes more negative) with increasing chain length. However cmc reaches a limit afer n=18, probably because of the nonlinearity of the chains (→ coiling, hydrophobic-hydrophobic interactions do not increase further).

The length of the hydrocarbon chain is a major factor in determining the cmc.

For a homologous series of linear single-chain surfactants the cmc decreases logarithmically with carbon number nC:

log10(cmc) = A−B∙nC.

A and B are constants for a given homologous series and temperature, and nC is the number of carbon atoms in the chain CnH2n+1. Generally each increase of length by 1 carbon approximately halves the cmc value.

cNaCl (mol dm-3) 0 0.01 0.03 0.1 0.3

cmc (mol·m-3) 8.1 5.6 3.1 1.5 0.7

Efect of electrolytes on the cmc

Explanation: the repulsion between head groups is shielded by counter ions (in this case Na+ and Cl-)

cmc of Na dodecylsulfate as a function of NaCl concentration:

Na+

The principal efect of the salts is to partially screen the electrostatic repulsion between the head groups and lower the cmc. At the same time, decreasing the efective charge makes the surfactant be less soluble.

Salt efect

● Efect of the hydrophilic group. For surfactants with the same hydrocarbon chain, varying the hydrophilic nature (i.e. from ionic to non-ionic) has an important efect on the cmc values. For instance, for a C12 hydrocarbon the cmc with an ionic head group lies in the range of 1×10-3 mol·dm-3 , while a C12 non-ionic material exhibits a CMC in the range of 1×10-4 mol·dm-3

● The efect of added salt. The presence of an indiferent electrolyte causes a decrease in the cmc of most surfactants. The greatest efect is found for ionic materials. The principal efect of the salts is to partially screen the electrostatic repulsion between the head groups and so lower the cmc. Non-ionic and /witerionic surfactants display a much smaller efect.

● Counter ion efects. An increase in the degree of ion binding (screening) will decrease the cmc. The decrease depends on the hydrated radius and the valence of the ion (icreasing radius → increasing cmc; increasing valence → decreasing cmc).

● For a given hydrophobic tail and anionic head group, the cmc decreases as Li+>Na+>K+>Cs+>N(CH2CH3)4

+ > Ca2+~Mg2+ .● For cationic series such as alkyltrimethylammonium surfactants, the cmc

decreases in the order F-→Cl-→Br-→I- (anions are less hydrated than cations).

Hydrophilic efets on the cmc

Surfactants in the food industry

Kraft point (IUPAC)The temperature (more precisely, narrow temperature range) above which the solubility of a surfactant rises sharply. At this temperature the solubility of the surfactant becomes equal to the critical micelle concentration. It is best determined by locating the abrupt change in slope of a graph of the logarithm of the solubility against T or 1/T.

Kraft point

Surfactants in medicine

Atelectasis is defined as the lack of gas exchange within alveoli, due to alveolar collapse or fluid consolidation

● Lung surfactant is the natural detergent in the lungs that reduces alveolar surface tension and prevents atelectasis at the end of expiration.

● Pulmonary surfactant is predominantly dipalmitoyl phosphatidyl choline (DPPC).

● Respiratory Distress Syndrome (preterm newborns lack the lung surfactants: infant respiratory distress syndrome, IRDS)

or dipalmitoyl phosphatidylcholine (DPP)

There is a thin film of liquid inside lung alveoli. Alveolar walls are highly curved as individual alveoli are very small. In consequence alveoli have a tendency to collapse due to the high surface tension.A phospholipid-rich film maintains the surface tension at very low levels (DPPC: dipalmitoyl phosphatidylcholine).

Δ p=2 γ

r

Lung surfactant

● Removal of hydrophobic particles from solid surface

1) Weting (of both the hydrophilic fabric and the hydrophobic dirt)

2) Dissolution (by making the weted dirt to roll up and lif of)

3) Solubili/ation (incorporation of the oily dirt into the hydrophobic core of the micelles)

Detergency (washing efect)

htp://www.funsci.com/fun3_een/exper2/exper2.htm

Detergency efect