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BASIC TERMINOLOGY Hydrophilic: A liquid/surface that has a high affinity to water. Hydrophobic: A liquid/surface that has very low affinity to water Lipophilic: A liquid/surface that has a high affinity to oil. Lipophobic: A liquid/surface that has a very low affinity to oil.

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Surfactants : 

Surfactants A molecule that contains a polar portion and a non polar portion. A surfactant can interact with both polar and non polar molecules. A surfactant increases the solubility of the otherwise insoluble substances. In water, surfactant molecules tend to cluster into a spherical geometry non polar ends on the inside of the sphere polar ends on the outside These clusters are called micelles

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Surfactants are molecules that preferentially adsorb at an interface, i.e. solid/liquid (froth flotation), liquid/gas (foams), liquid/liquid (emulsions). Significantly alter interfacial free energy (work needed to create or expand interface/unit area). Surface free energy of interface minimized by reducing interfacial area. INTRODUCTION

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Surfactants have amphipathic structure Tail or hydrophobic group Little affinity for bulk solvent. Usually hydrocarbon (alkyl/aryl) chain in aqueous solvents. Can be linear or branched. Head or hydrophilic group Strong affinity for bulk solvent. Can be neutral or charged. SURFACTANT STRUCTURE

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SURFACTANT CLASSES Anionic (~ 60% of industrial surfactants)

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SURFACTANT CLASSES (contd.) Cationic (~ 10% of industrial surfactants)

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Non-ionic (~ 25% of industrial surfactants) SURFACTANT CLASSES (contd.)

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SURFACTANT CLASSES (contd.) Amphoteric or zwitterionic (~ 10% of industrial surfactants). Generally expensive “specialty chemicals”.

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HYDROPHILIC-LIPOPHILIC BALANCE Griffin (1949): the hydrophilic-lipophilic balance (HLB) of a surfactant reflects its partitioning behavior between a polar (water) and non-polar (oil) medium. HLB number, ranging from 0-40, can be assigned to a surfactant, based on emulsification data. Semi-empirical only. Strongly hydrophilic surfactant, HLB ? 40 Strongly lipophilic surfactant, HLB ? 1 HLB dependent upon characteristics of polar and non-polar groups, e.g. alkyl chain length, headgroup structure (charge, polarity, pKa).

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HYDROPHILIC-LIPOPHILIC BALANCE A value of 10 represents a “mid-point” of HLB.

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0 2 6 8 10 12 14 16 18 4 No dispersibility in water poor dispersibility in water Water in oil emulsifier Wetting agent Milky dispersion; unstable Translucent to clear solution Clear solution Detergent Solubilizer Oil-in-water emulsifier HYDROPHILIC-LIPOPHILIC BALANCE triglycerol monooleate: Cream and ointment stabilizers Polysorbate 20 Insecticidal sprays HLB

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MICELLES If concentration is sufficiently high, surfactants can form aggregates in aqueous solution ? micelles. Typically spheroidal particles of 2.5-6 nm diameter. (Klimpel, Intro to ChemicalsUsed in Particle Systems,p. 29, 1997, Fig 21) Hartley Spherical Micelle

Micelle Structure of a Surfactant : 

Micelle Structure of a Surfactant

Amphiphiles : 

Hydrophilic head Hydrophobic tail Amphiphiles Coarse-grained Model beads connected by an anharmonic spring. Interactions between every two beads are governed by a Lennard-Jones (LJ) and FENE potentials.

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MICELLES --CMC-- Onset of micellization observed by sudden change in measured properties of solution at characteristic surfactant concentration ? critical micelle concentration (CMC). (Klimpel, Intro to ChemicalsUsed in Particle Systems, p. 29, 1997, Fig 20)

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MICELLES --CMC Trends-- For the same head group, CMC decreases with increasing alkyl chain length. (2) CMC of neutral surfactants lower than ionic (2) CMC of ionic surfactants decreases with increasing salt concentration. (3) For the same head group and alkyl chain length, CMC increases with increase in number of ethylene oxide groups. (4) For mixed anionic-cationic surfactants, CMC much lower compared to those of pure components.

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MICELLES --Example: Mayonnaise-- Water matrix containing fat droplets. The surfactant (emulsifier) is lecithin. It can contain up to 12 g of fat in 15 ml Water matrix

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MICELLES --Headgroup and Chain Length-- (Hunter, Foundations of Colloid Science, p. 569, 1993, Fig 10.2.1) Branching or addition of double bonds or polar groups to alkyl chain generally increases CMC. Addition of benzene ring equivalent to addition of ~ 3.5 carbons (methylene groups). Replacement of hydrogens in alkyl chain with fluorine initially increases CMC, followed by marked decrease as fluorine substitution goes to saturation.

Cloud point versus Krafft point surfactants : 

Cloud point versus Krafft point surfactants The Krafft point is defined as the temperature at which the solubility of the surfactant equals the critical micellization concentration (cmc) = melting point of hydrated surfactant The Cloud Point is the LCST MW PEO = 5 × 106 [CmH2m+1NHCO(CH2)nOSO3Me, abbreviated as m-n-Me (Me=Na, 0.5 Ca)]

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MICELLES --Temperature and Pressure-- For ionic surfactants there exists a critical temperature above which solubility rapidly increases (equals CMC) and micelles form ? Kraft point or Kraft temperature (TK), Below TK solubility is low and no micelles are present.

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MICELLES --Temperature and Pressure-- surfactant crystals TK Surfactants much less effective below Kraft point, e.g. detergents. For non-ionic surfactants, increase in temperature may result in clear solution turning cloudy due to phase separation. This critical temperature is the cloud point. Cloud point transition is generally less sharp than that of Krafft point.

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MICELLES --Electrolyte-- Addition of electrolyte significantly affects CMC, particularly for ionic surfactants. For non-ionic and zwitterionic surfactants; log10CMC = b2 + b3Cs where Cs is salt concentration (M) b2 and b3 are constants for specific surfactant, salt and temperature. Change in CMC attributed to “salting in” or “salting out” effects. Energy required to create volume to accommodate hydrophobic solute is changed in electrolyte solution due to water-ion interactions ? change in activity coefficient.

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MICELLES --Electrolyte-- If energy required is increased by electrolyte, activity coefficient of solute is increased and salting out occurs ? micellization is favored and CMC decreases. Conversely, for salting in, CMC increases. Effects of electrolyte depend on radii of hydrated anions and cations and is greater for smaller hydrated ions, i.e. follow lyotropic series. CMC depression follows order: F- > BrO3- > Cl- > Br- > NO3- > I- > CNS- and NH4+ > K+ > Na+ > Li+

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In a typical surfactant system, ? bulk concentration, ? surface concentration -- until cmc is reached. cmc (critical micelle concentration) – surfactant conc. where micellization occurs. A. Review: Micelles CMC Surface excess

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Forces driving micelle formation: a) hydrophobic force b) entropy Forces opposing micelle formation: a) concentration gradient b) thermal (Brownian) motion c) charge repulsion between ionic polar heads Note: cmc’s of nonionic surfactants are much lower than those of ionic surfactants. Why?

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B. Micellar Kinetics Micelles are NOT static structures. Micelles are unstable structures with two characteristic relaxation times – fast relaxation time (t1) and slow relaxation time (t2)

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C. Techniques Used to Measure Micellar Kinetics Pressure-Jump (conductivity or optical detection) Temperature-Jump (optical detection) Stopped-Flow (conductivity, optical detection and fluorescence) Ultrasonic Absorption Fluorescence Shock-Tube

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D. Effect of Surfactant Conc. on Micelle Lifetime It has been shown that micelle ‘slow’ relaxation time, t2, is a function of surfactant concentration For all surfactants that form micelles, t2 increases to a certain maximum value For ionic surfactants, t2 begins to decrease from the maximum value

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For nonionic surfactants, t2 remains constant at a maximum value. Remember: nonionic surfactants have much longer micellar relaxation times (t2) than ionic surfactants – on the order of seconds to minutes!

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E. Technological Processes Foaming (foamability & foam stability) Fabric Wetting Solubilization Emulsification Importance of Micellar Kinetics in Technological Processes

The Importance of Micelle Break-up in Foaming : 

The Importance of Micelle Break-up in Foaming Thin Liquid Film Air Air Air Surfactant solution

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The Importance of Micelle Break-up in Emulsification More stable micelles Less monomer flux Higher interfacial tension Larger droplet size

Geometrical Effects in aggregation : 

Geometrical Effects in aggregation Packing Parameter J. N. Israelachvili, Intermolecular and surface forces, Academic, New York (1985).

Critical Micelle Concentration (CMC) : 

Critical Micelle Concentration (CMC) Definition of the CMC Brownian Dynamics Results for amphiphiles with different sizes of the hydrophilic group

Effect of the large head on micelle size distribution : 

Effect of the large head on micelle size distribution The geometry of amphiphile effects the Cluster Distribution Bigger head group molecules form smaller clusters with narrower distribution

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