SURFACTANTS

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. Hydro=Water Lipo=Oil Philic=Friendly Phobic=Scared + Hydrophilic Hydrophobic Lipophilic Lipophobic Lyo=Dissolve Philic=Friendly Phobic=Scared + Lyophilic Lyophobic

BASIC TERMINOLOGY Hydrophobic Lipophilic Lyophilic in oil Lyophobic in water Hydrophilic Lipophobic Lyophobic in oil Lyophilic in water

INTRODUCTION 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.

SURFACTANT STRUCTURE Surfactants have amphipathic structure Tail 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. Tail head

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Anionic (~ 60% of industrial surfactants) SURFACTANT CLASSES Carboxylic acids and their salts including various fatty acids tall oil acids, and hydrolyzed proteins: Sulfonic acids and their salts, including hydrocarbon backbones of alkylbenzene, benzene, naphthalene, toluene, phenolm lingin, olefins, diphenyloxide, petroleum cuts, succinate esters etc. Sulfuric acid or salts including sulfated primary alcohols, sulfated polyxyalkylenated alcohols etc. R-O Alkyl xanthic acids: Alkyl or aryl dithiophosporic acids: Polymeric anionics involving repeated groups containing carboxyl acid functionality: Anionic (~ 60% of industrial surfactants)

SURFACTANT CLASSES Anionic (~ 60% of industrial surfactants)

SURFACTANT CLASSES (contd.) Long chain amines derived from animal and vegetable acids, tall oil and synthetic amines: Diamines and polyamines including ether amines and imidazolines: Quaternary ammonium salts including tertiary mines and imidazolines: Quaternized and unquartenized polyoxyalkylenated long chain amines: Amine oxides derived from tertiary amines oxidized with hydrogen peroxide: Cationic (~ 10% of industrial surfactants)

SURFACTANT CLASSES (contd.) Polyoxyethylenated alcohols, alkyl phenols, alcohol ethoxylates including derivatives from nonyl phenol, coconut oil, tallow, and synthetic alcohols: Polyoxyethylenated glycols: Polyoxypropylenated glycols: Esters of carboxylic acids and alkyene oxides: Alkanolamine condensates with carboxylic acids: Polyoxyalkylenated mercaptans: Non-ionic (~ 25% of industrial surfactants)

SURFACTANT CLASSES (contd.) Acrylic acid derivatives with amine functionality: Subsituted alkylamides: n-Alkyl betaines: n-Alkyl suffobetaine: Thio alkyl amines and amides: Amphoteric or zwitterionic: (~ 10% of industrial surfactants). Generally expensive “specialty chemicals”.

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).

What is HLB of a surfactant? The Hydrophilic-lipophilic balance [HLB]of a surfactant is a measure of the degree to which it is hydrophilic or lipophilic, determined by calculating values for the different regions of the molecule.

HYDROPHILIC-LIPOPHILIC BALANCE -- Effect of Structure -- oil water Coil Cwater C6H13COO- C8H17COO- C10H21COO- HLB decreases

Water in oil emulsifier Oil in water Emulsifiers HLB value – significance HLB Value 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Use Water in oil emulsifier Oil in water Emulsifiers   Wetting Agents Detergents Solubilizer

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

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

MICELLES If concentration is sufficiently high, surfactants can form aggregates in aqueous solution  micelles. Typically spheroidal particles of 2.5-6 nm diameter. + - Hydrocarbon Layer Water Layer Hartley Spherical Micelle McBain Lamellar Micelle + oil H2O Oil in Water Micelle Water in Oil Micelle Surfactant Micelle (Klimpel, Intro to ChemicalsUsed in Particle Systems,p. 29, 1997, Fig 21)

MICELLES Onset of micellization observed by sudden change in --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)

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.

MICELLES Hydrophobic groups can perturb solvent structure and --Driving Force-- Hydrophobic groups can perturb solvent structure and increase free energy of system. Surfactant will  concentrate at S/G interface to remove hydrophobic groups from solution and lower DGo. AIR WATER

MICELLES DGo can also be decreased by aggregation into micelles --Driving Force-- DGo can also be decreased by aggregation into micelles such that hydrophobic groups are directed into interior of structure and hydrophilic groups face solvent. Decrease in DGo for removal of hydrophobic groups from solvent contact by micellization may be opposed by: (i) loss in entropy of surfactant (ii) electrostatic repulsion for charged headgroups Micellization is  a balance between various forces which can be influenced by certain phenomena (Mukerjee and Mysels, 1971).

--Example: Mayonnaise-- MICELLES --Example: Mayonnaise-- Oil + + lecithin Water matrix containing fat droplets. The surfactant (emulsifier) is lecithin. It can contain up to 12 g of fat in 15 ml + Water matrix 2 μm http://wilfred.berkeley.edu/~gordon/BLOG-images/mayo15.jpg

--Headgroup and Chain Length-- MICELLES --Headgroup and Chain Length-- Klevens (1953): surfactants with linear alkyl chains, CMC is related to number of carbons by; log10CMC = b0 - b1mc Where: mc is number of carbons in chain b0 and b1 are constants (Hunter, Foundations of Colloid Science, p. 569, 1993, Fig 10.2.1)

--Headgroup and Chain Length-- MICELLES --Headgroup and Chain Length-- 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. (Hunter, Foundations of Colloid Science, p. 569, 1993, Fig 10.2.1)

--Temperature and Pressure-- 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. (Klimpel, Intro to Chemicals Used in Particle Systems, p. 30, 1997, Fig 22)

What is cloud point & pour point? The Cloud point of a fluid is the temperature at which dissolved solids are no longer completely soluble, precipitating as a second phase giving the fluid a cloudy appearance. The pour point of a liquid is the lowest temperature at which it will pour or flow under prescribed conditions.

What is cloud point & pour point?

--Temperature and Pressure-- MICELLES --Temperature and Pressure-- TK surfactant crystals Temperature 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.

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.

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+

MICELLES --Electrolyte-- For ionic surfactants; log10CMC = b4 + b5log10Cs where b4 and b5 are constants for a specific ionic head group at a particular temperature. Depression of CMC with increasing salt due to double layer compression around charged head group and charge screening effect between head groups in micelle. Different salts vary in their effectiveness, e.g. for sodium laurate, CMC depression follows: PO42- > B4O72- > OH- > CO32- > SO42- > Cl-

MICELLES --Electrolyte-- The effect of added salt on the CMC of SDS and dodecylamine hydrochloride (DHC). (From Stigter 1975a,with permission) (Hunter, Foundations of Colloid Science, p. 572, 1993, Fig 10.2.2)

--Organic Molecules-- MICELLES --Organic Molecules-- Small amounts of organic molecules can affect the CMC, e.g. in aqueous solution of SDS, dodecanol (hydrolysis product of SDS) causes minimum in surface tension measurement. Solubilization of impurity in micelles causes rise in surface tension. Very important for detergency, stabilization and dispersion. go Surface Tension CMC Surfactant Concentration

--Organic Molecules-- MICELLES --Organic Molecules-- Solubilization characterized by large increase in solubility of lipophilic (hydrophobic) organic species above surfactant CMC. Lipophilic organics can aid or oppose micelle formation. Two classes based on mode of action. Group A (or Type I): Adsorb within micelle and reduce CMC. Typically polar molecules, e.g. alcohols and amides. Effective at low concentrations. Short chain members adsorb near micelle-water interface. Longer chain members adsorb in core  can influence micelle shape.

--Organic Molecules-- MICELLES --Organic Molecules-- Free energy of micellization lowered by screening repulsion between charged head groups (ionic surfactants) and/or reducing steric hindrance (non-ionic surfactants). CMC depression greatest for linear species  maximum when chain length approaches that of surfactant. Group B (or Type II): Modify bulk water structure around surfactant or micelle, usually at higher concentrations than Group A molecules. Structure breakers disrupt water structure about hydrophobic tails and increase entropy. Entropy increase upon micelle formation  reduced  CMC is increased.

--Organic Molecules-- MICELLES --Organic Molecules-- Examples of structure breakers are urea, formamide and guanidinium salts. Most effective on non-ionic surfactants of PEO type. Structure makers promote structuring of water, e.g. xylose and fructose. Conversely, CMC is reduced due to enhanced entropy increase upon micellization. At high bulk concentrations, species such as dioxane, esters, short-chain alcohols and ethylene glycol can increase solubility of monomeric surfactant, thus opposing micellization and raising CMC.

--Aggregation Number-- MICELLES --Aggregation Number-- Formation of micelles from association of n surfactant monomers can be described by; nS Mn where n is number of surfactant monomers needed to form a micelle  aggregation number k1 and k-1 are rate constants for forward and reverse reactions Equilibrium constant, K, can be expressed as: k1 k-1

--Aggregation Number-- MICELLES --Aggregation Number-- If Cs and Cm are concentrations of surfactant monomer and micelle, respectively; Variation of dCm/dCT with total surfactant concentration for different values of aggregation number, n. C0 is the critical micellization concentration and Cm the concentration of micelles. (Hunter, Foundations of Colloid Science, p. 572, 1993, Fig 10.3.1)

--Aggregation Number-- MICELLES --Aggregation Number-- t1 t2 Micelle size distribution. Mn is the number of aggregates of size n. The aggregates on the left side of the minimum (L) are called submicellar, those on the right-hand side (proper) micelles with mean size of n, and the width of their size distribution is given as σ. (Hunter, Foundations of Colloid Science, p. 572, 1993, Fig 10.7.1)

--Aggregation Number Trends-- MICELLES --Aggregation Number Trends-- For same polar group, n increases with increasing chain length. (2) For constant alkyl chain length, n decreases with increasing number of ethylene oxide groups in surfactant molecule. (3) Oils or long chain alcohols increase n. (4) For ethoxylated non-ionic surfactants, n drastically increases with temperature. (5) For anionic surfactants, n increases when NaCl is replaced with MgCl2 or CaCl2. (6) In aqueous solution, n ranges from 50-5,000, while in organic solvents, n usually < 10. (Hunter, Foundations of Colloid Science, p. 572, 1993, Fig 10.7.1)