Chee2940 Lecture 13 - Surf Act Ants & Polymers

CHEE2940: Particle Processing Lecture 13: Surfactants and Polymers This Lecture Covers Introduction to surfactants and polymers Self-assembly and adsorption Industrial applications Chee 2940: Surfactants and Polyers

13.1

TECHNOLOGICAL IMPORTANCE

Surfactants: • Flotation of minerals and coal • Enhanced oil recovery • Stabilization of suspension, emulsion • Paints. .

Polymers: • Flocculation • Adhesion • Stabilization. Chee 3920: Surfactants and Polyers

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13.2

WHAT IS A SURFACTANT?

Surfactant = Surface Active Agent. Surfactants are organic compounds with • hydrophilic (water loving) groups (their "heads") • hydrophobic (water hating) groups (their "tails"). tail

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head

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Tail group (hydrocarbon & fluorocarbon - apolar): Weak affinity for bulk solvent Strong affinity for air and oil Adsorption at air (oil)-water interface. Head groups: Strong affinity for bulk solvent (water) Weak affinity for air and oil Charged (polar) or neutral Adsorption at mineral-water interface. – Examples: sulphate, (SO3) + +2 +3 amines: H2N ,HN , N . Chee 3920: Surfactants and Polyers

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Figure 13.1 Structure of an anionic (sodium oleate, top) and cationic (dodecylammonium chloride, bottom) surfactants. (Wills, 1998) Chee 3920: Surfactants and Polyers

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13.3 SOME COMMON SURFACTANTS Anionic surfactants (negatively charged) (~60% of industrial surfactants)

- Sulphates

Example: Sodium Dodecyl Sulphate C12H25O(SO3)Na Chee 3920: Surfactants and Polyers

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- Sulphonates

Example: Sodium dodecylbenzene sulphate. - Fatty acids and soaps Example: Sodium oleate. CH3(CH2)7=CH(CH2)7COONa Chee 3920: Surfactants and Polyers

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- Xanthates Example: Sodium ethyl xanthate. CH3CH2OCS2Na - Dithiophosphates

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Cationic surfactants (positively charged) (~10% of industrial surfactants)

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Primary amines Secondary amines Tertiary amines

Example: Cetyl trimethyl ammonium bromide (CTAB) — C16H33N(CH3)3Br Chee 3920: Surfactants and Polyers

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Nonionic surfactants (~25% of industrial surfactants) Alcohols: ethanol C2H5OH, hexanol C6H13OH Polyethylene oxide C10H21-(OCH2CH2)6-OH or simpler (C10E6) Biological surfactants Lecithin (in eggs) - phospholipids

Many bio-surfactants have double chains and two polar heads (zwitterionic, one + and one -). Chee 3920: Surfactants and Polyers

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13.4

SOLUBILITY AND SELF-ASSEMBLY (MICELLISATION) OF SURFACTANTS

The longer the hydrophobic tails, the lower the solubility in water, and the greater the surface affinity. Dilute concentration => complete solubility. -4

At slightly higher concentration (~10 mol/L), the low soluble tails associate to form micelles in order to keep the tails out of the water. Chee 3920: Surfactants and Polyers

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C.M.C. = critical micelle concentration A representation of an anionic micelle:

Figure 13.2. Schematic representation of an anionic micelle Chee 3920: Surfactants and Polyers

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CMC values for some common oxyhydryl collectors (Leja, 1980).

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Three structures of micelles (Figure 13.3) are now widely accepted. These include: spherical micelles which occurs at the lowest CMC cylindrical or rod-shaped micelles which occurs at higher CMC, and disk-shaped micelles at the highest CMC.

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Figure 13.3. Micelles in water: spherical, cylindrical, and bimolecular (Leja, 1980) Chee 3920: Surfactants and Polyers

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Figure 13.4. CMC’s of sodium oleate solutions as measured by conductance (Miller 1986). Chee 3920: Surfactants and Polyers

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(Israelachvili, 1992) Chee 3920: Surfactants and Polyers

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Biological membrance

Figure 13.5. Association of surfactants and biosurfactants - lipids (Israelachvili, 1992). Chee 3920: Surfactants and Polyers

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13.5

SURFACTANT ADSORPTION AT WATERFLUID SURFACE/INTERFACE - Adsorption is the build up of a molecule at a surface (such as an oil/water interface).

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Adsorption generally occurs because different parts of a molecule having an affinity for the two different phase on either side of the interface. - Adsorption is not to be confused with absorption. - Absorption is the build up of a molecule into the

interior of a single phase. Example: Dissolution of air or carbon dioxide into water is an absorption process. Chee 3920: Surfactants and Polyers

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Surfactants adsorb at water-fluid interface in order to keep the tails out of water. Surface energy & surface tension are reduced

Interfacial energy & interfacial tension are reduced

Surface pressure, π, tends to push the surfactants apart (due to crowding) and Chee 3920: Surfactants and Polyers

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counteracts the surface tension => reducing the surface energy.

γ = γ0 −π Surface tension with surfactant

Depends on surfactant type and concentration Surface tension without surfactant (solvent)

γ can be << γ0

Adsorbed surfactants stabilise emulsions Chee 3920: Surfactants and Polyers

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Repulsion between polar heads stablise the emulsion system.

Large interfacial area => lots of surface energy; unstable system.

Large interfacial area but small interfacial energy due to surfactant adsorption => stable emulsion

Adsorbed surfactants reduce surface tension Chee 3920: Surfactants and Polyers

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Measurement of surface pressure of monolayer Chee 3920: Surfactants and Polyers

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of insoluble surfactants – Langmuir trough To measure pressure

To change available

Air

Water

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A

From Shaw &Hunter

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As A (available area/molecule) decreases, π Surface excess (surface pressure) increases. Γ=−

C ∂γ RT ∂C

Concentration Gibbs adsorption isotherm: Simple balance: Γ = 1/ ( AN ) ; R = N k (Gas constant, Avogadro No.) Henry adsorption: γ = γ − mC (dilute concentration) A

A B

0

Proportionality

Surface pressure isotherm (low conc.): Boltzmann constant

π A = k BT

Absolute temperature

(analogous to PV = nRT for perfect gases) Chee 3920: Surfactants and Polyers

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13.6

SURFACTANT ADSORPTION AT WATERSOLID INTERFACE The adsorption also depends on the solid properties: wettability, surface charge, and surface reactions. At hydrophobic surface (graphite): Adsorption occurs with the tails – physical adsorption. At hydrophilic surface (quartz): Adsorption occurs with the polar heads – electrical &/or chemical adsorption. Chee 3920: Surfactants and Polyers

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Figure 13.6 (previous page) – surface micelles Images (200 nm × 200 nm) of surface surfactant structures obtained by the soft contact Atomic Force Microscopy technique (Nguyen et al., 2004): A. Highly ordered linear hemi-cylindrical structures of cetyl trimethyl ammonium bromide (CTAB) at a hydrophobic graphite surface. B. & C. Spherical structures of sodium dodecyl sulphate (SDS) at a positively charged fused alumina surface; and of CTAB at a negatively charged silica surface, respectively. D. Short worm-like cylindrical structures of CTAB at a hydrophilic negatively charged mica surface. Chee 3920: Surfactants and Polyers

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13.7

POLYMER ADSORPTION AT WATER-SOLID INTERFACE

Polymer is a chemical compound with high molecular weight (MW) consisting of a number of structural units (monomer) linked together by covalent bonds. Monomer – chemical species from which a polymer is made. MW of typical monomer, M0 = 50 – 100. Chee 3920: Surfactants and Polyers

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MW of polymer, M = nM0 = 1,000 – 1 x 10 or more where n is number of repeating units. Example: Monomer (propylene)

Polymer (polypropylene)

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polyethylene oxide, PEO polypropylene oxide, PPO polyvinyl alcohol, PVA

polyacrylic acid, PAA

polymethylacrylic acid, PMAA

polyacrylamide, PAM

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Polymer play important role in a range of technologies: flocculation, adhesion, and stabilisation. The addition of polymers to a suspension can have various effects on the interaction between surfaces, which include (Figure 13. 7): • Polymer bridging (attraction) – low conc., • Steric effects (repulsion) – medium high conc. • Depletion interaction (attraction) – high conc., • Electrical charge patch (attraction) – high conc. Chee 3920: Surfactants and Polyers

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Attraction

Repulsion

Bridging flocculation

Steric stabilization

(Low concentration – ppm)

(Medium concentration)

Adsorbing polymer

Nonadsorbing polymer Depletion flocculation

Depletion stabilization

(Medium concentration)

(High concentration)

Figure 13.7 Surface forces mediated by macromolecules.

For particles fully covered polymer, repulsion may occur => steric stabilisation. Utilised in paints and coatings, food, drugs. Chee 3920: Surfactants and Polyers

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When particles are fully covered and polymer chain is long enough, adsorption may occur on two or more particles => bridging flocculation. Basis of separation process in water (drinking and wastewater) treatment, mineral processing, paper manufacturing. Natural polymers (eg. Protein, DNA) are critical to life and biological processes. Applications include medicine and food industry. Chee 3920: Surfactants and Polyers

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13.8

EXAMPLES OF SURFACTANT USE

Detergents

(From Hunter)

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• Detergency is used to remove dirt & oil from fabrics and other solid surfaces. • Typical surfactants used: soaps (salts of fatty +2 acid) which form scum in hard water (Mg and +2 Ca ). • New detergents (synthetic): SDS & similar surfactants (polyethylene oxides). • Adsorbed detergents lower the interfacial energy, allowing the removal of dirt and oils. Chee 3920: Surfactants and Polyers

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• Fundamentals of detergency

- For solid dirt, the work of adhesion, W:

W = ( γ DW + γ SW ) − γ SD

Interfacial energy after dirt removal

Interfacial energy before dirt removal

Detergents lower γDW and γSW so that W become negative and dirt lifts off. Chee 3920: Surfactants and Polyers

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- For oil, the work of adhesion, W:

W = γ OW + γ SW − γ SO   ⇒ W = γ OW (1 − cos θ ) γ SW = γ SO − γ OW cosθ  Young equation

Detergents lower γOW and γSW so that contact angle, θ, is reduced, increasing dirt removal. Chee 3920: Surfactants and Polyers

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Flotation of Minerals and Coal Air

Skimmer

Particle

Froth

Valuable minerals stick to bubbles and go up to be removed (hydrophobic particles) Waste material stays in suspension (hydrophilic particles) to be discharged

Pulp

Entrained Particles

Bubble Baffle Rotor

Stator

Fig 13.8. Schematic of froth flotation in a mechanical cell Chee 3920: Surfactants and Polyers

Hydrophobic = water repellent, non-wetted by water. Hydrophilic = water attractive, wetted by water.

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Air

Particle Water

To float a particle, surface tension must be more than balance gravity.

θ = 0 -> wetting, no flotation θ = 180o -> hydrophobic and flotation.

Use of surfactants: - Collectors – adsorb to mineral surface and increase hydrophobicity. Eg. Na ethyl

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- Frothers - adsorb to air bubbles to stabilise froth (“soap suds”). Eg. MIBC – methyl isobutyl carbinol.

Figure 21. Influence of frothers on the stability of bubbles during their coalescence: a) A bubble before coalescence covered by frother molecules; b) Frother molecules are stretched on the deformed surface to increase the surface tension against coalescence; and c) The bubble after the surface deformation. (Leja, 1980) Chee 3920: Surfactants and Polyers

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Figure 13.9. Draining froth formed over the pulp surface (left), producing to dry foam on the top and wet foam at the bottom (right). Waste particles drop back to the pulp with the drained water (left). Nguyen, 2004. Chee 3920: Surfactants and Polyers

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Emulsions and Foams • Oil-in-water emulsions: oil droplets in water. Examples: salad dressings, milk, and mayonnaise • Water-in-oil emulsions: water droplets in oil. Examples: margarine, explosive emulsions (Orica) Chee 3920: Surfactants and Polyers

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• Foams: air-in-water Examples: whipped cream, shave foam, fire fighting foams *** Emulsions are difficult to foam due to high oilwater surface tension => Surfactants (called emulsifiers) are used to reduce the surface tension so that emulsions can be created. Similarly, surfactants (called frothers) are used in creating foams and froths. *** Chee 3920: Surfactants and Polyers

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Examples of emulsifiers • Natural emulsifiers: Lecithin (from eggs or soybeans) • Synthetic emulsifiers: - SPAN – orbitan esters - TWEEN – polyethylene oxide sorbitan esters

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Practical consideration: How do I know whether I get an OW or a WO emulsion? *** A) Larger volume phase is usually the continuous phase and the smaller volume phase is usually the disperse phase …but… B) The continuous phase is the phase in which the surfactant is more soluble!

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Solubility of surfactants depends on the ratio of hydrophilic (lyophobic) to hydrophobic (lyophilic) parts of the molecule. A surfactant with mainly hydrophobic parts is more soluble in oil, less soluble in water. A surfactant with mainly hydrophilic parts is more soluble in water, less soluble in oil.

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Hydrophile-Lipophile Balance – HLB - Is the ratio of hydrophobic to hydrophilic bits of a surfactant (on the scale of 1 to 40). - Is used to predict the surfactant hydrophobicity. - HLB = 7.0 for 50%÷50% surfactants - HLB < 7.0 for hydrophobic surfactants (water-in-oil emulsifiers). - HLB > 7.0 for hydrophilic surfactants (oil-in-water emulsifiers). [Lipophile = attraction for oil by a surface of a material or a molecule]. Chee 3920: Surfactants and Polyers

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Proteins and biomacromolecules • Some proteins are surfactants having hydrophobic and hydrophilic parts, and are soluble in water => can be used for food emulsifiers and foaming agents • Other proteins are insoluble and act as “particles” in dispersions, such as casein micelles. • Other biopolymers may also act as “particles” rather than soluble species, such as starch granules. Chee 3920: Surfactants and Polyers

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Biomolecules can self-assemble to form biostructures such as cell walls and membrances (also see Figure 13.5)

(From Stryer, biochemistry) Chee 3920: Surfactants and Polyers

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