Surfactant Aggregation

Transcription

1 Surfactant Aggregation Background What Is A Surfactant? S u r f a c t a n t Surface active agent... A chemical that, when dissolved in water, moves toward "surfaces" What Does A Surfactant Do?... Cleans by helping dissolve things that aren't water soluble... Lathers by "solubilizing air" in water... Many biological processes (membranes, digestion, etc.) How Do They Do It?... TEAMWRK... Individual surfactant molecules do nothing, they spontaneously group together to work "SelfAggregation"

2 What Is SelfAggregation? Surfactant Aggregation Background... Aggregate gathered together into a mass so as to constitute a whole... Selfaggregation forces exist that direct this process What are these forces? Why Do Surfactants SelfAggregate?... There are only two "rules of the game": Hydrophobic Interactions Electrostatic Effects il + Repulsion + il Attraction Attraction H 2 Repulsion

3 Surfactant Aggregation Background... Molecular inhomogeneity leads to molecular recognition and organization. Why Do Surfactants SelfAggregate (continued)?... Surfactant Structure: S3 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2 "Tail" Group Hydrophobic Water Hating Lipophilic il Loving H C N+R3 "Head" Group Hydrophilic Water Loving Lipophobic il Hating "Amphiphilic"

4 Behavior of Surfactants in Water The rigin of the Surface Tension of Water Water Molecules HydrogenBond to Each ther: H + H + H + H + H + H + H + H + HydrogenBonding Forces are UnBalanced at the Surface: Unbalanced Intermolecular Forces Result in Surface Tension Surface Water Molecules Bulk Water Molecules Surfactant Molecules Adsorbing at the Surface Reduce Surface Tension

5 SURFACTANT AGGREGATIN Monolayer Formation Air AirWater Interface H 2 Addition of Surfactant to Water Hydrophobic Forces Repel Water Tail group in air Head group in water H 2 Surfactant Moves to Air Water Interface ( surface active ) Monolayer at AirWater

6 SURFACTANT A GGREGATIN Micelle Formation Monolayer at AirWater Interface Saturated Additional Surfactant Dispersed in Bulk Water CMC = Critical Micelle Concentration = Concentration at which micelles begin to form Enough surfactant to pass CMC Balance of hydrophobic and electrostatic forces Environmentdependent Surfactant Aggregates in Spherical Micelles A Surfactant Micelle Head groups on outside Hydrophobic pocket

7 How Small is a Micelle? If hair (80 ) was as wide as a football field A micelle (0.005 ) would be 6mm in diameter!!

8 MICELLE M + M (n1) M n Many equilibrium steps, so we approximate nm M n

9 Behavior of Surfactants in Water Surface Tension Reduction: Measurement of CMC's Surface Tension (mn/m or dyn/cm 2 ) Critical Micelle Concentration (CMC) Surfactant at Surface Surface Saturated Micelles Formed Air Surface

10 Factors Affecting the Critical Micelle Concentration (CMC) lower CMC more stable micelles Factors Affecting HeadtoHead Repulsion Head Group Charge Ionic > Nonionic = Anionic 10 3 M = Nonionic 10 4 M ph can influence Head Group Position Nonterminal position increases CMC relative to terminal position Ethoxylation [R(CH 2 CH 2 ) n R'] Nonionics: increases CMC Anionics: decreases CMC Counterion of Ionic Surfactants Anionics = CMC deceases in order: Li + > Na + > K + > Cs + > N(CH 3 ) 4 + > Ca 2+ Cationics = CMC deceases in order: F > Cl > Br > I Factors Affecting Chain/Chain Attraction Hydrocarbon Chain Length (n C ) CMC decreases logarith. as n C increases Empirical relationship: log 10 CMC = ABn C = For anionics, A 1.7, B 0.3 = For nonionics, A 3.5, B 0.05 Hydrocarbon Structure Branching = Carbon atoms off of a main chain contribute 1/2 that of main chain carbons. Unsaturation = Increases CMC = Cis isomer increases CMC more than trans ther Main Chain Moities Benzene Ring = Accounts for 3.5 carbon atoms Propoxylation = Each propylene oxide 0.4 methylene carbons Electrolyte Decreases CMC (more for ionics than nonionics) Effectiveness: = 1/2 S 2 4 > F > Cl > Br > N 3 > I = NH + 4 > K + > Na + > Li + > 1/2 Ca 2+ Secondary Polar Functions C's between 2 polar group and 1 polar group 1/2 as effective as those w/o substitution rganic Additives Low Water Solubility: Slight decrease in CMC High Water Solubility: Minimal effects (below "cosolvent" levels) Partially Soluble Materials: Decreases CMC

11 Surfactant Aggregation verview of Aggregate Types (The "Surfactant Menagerie") Dilute Solution Concentrated Solution "Particles" Liquid Crystal Phases Crystal Phases Monolayer at AirWater Interface Cylindrical Micelles (random) Hexagonal Phase (Middle) Anhydrous Crystal Unorganized Monomers Vesicles Spherical Micelles Planar BiLayers Increasing Surfactant Concentration in Water Lamellar Phase (Neat) Crystal Hydrate

12 Surfactant Aggregation verview of Aggregate Types (The "Surfactant Menagerie") Dilute Solution Concentrated Solution "Particles" Liquid Crystal Phases Crystal Phases Monolayer at AirWater Interface Cylindrical Micelles (random) Hexagonal Phase (Middle) Anhydrous Crystal Unorganized Monomers Vesicles Spherical Micelles Planar BiLayers Increasing Surfactant Concentration in Water Lamellar Phase (Neat) Crystal Hydrate

13 Surfactant Aggregation verview of Aggregate Types (The "Surfactant Menagerie") Dilute Solution Concentrated Solution "Particles" Liquid Crystal Phases Crystal Phases Monolayer at AirWater Interface Cylindrical Micelles (random) Hexagonal Phase (Middle) Anhydrous Crystal Unorganized Monomers Vesicles Spherical Micelles Planar BiLayers Increasing Surfactant Concentration in Water Lamellar Phase (Neat) Crystal Hydrate

14 Surfactant Aggregation verview of Aggregate Types (The "Surfactant Menagerie") Dilute Solution Concentrated Solution "Particles" Liquid Crystal Phases Crystal Phases Monolayer at AirWater Interface Cylindrical Micelles (random) Hexagonal Phase (Middle) Anhydrous Crystal Unorganized Monomers Vesicles Spherical Micelles Planar BiLayers Increasing Surfactant Concentration in Water Lamellar Phase (Neat) Crystal Hydrate

15 Surfactant Aggregation verview of Aggregate Types (The "Surfactant Menagerie") Dilute Solution Concentrated Solution "Particles" Liquid Crystal Phases Crystal Phases Monolayer at AirWater Interface Cylindrical Micelles (random) Hexagonal Phase (Middle) Anhydrous Crystal Unorganized Monomers Vesicles Spherical Micelles Planar BiLayers Increasing Surfactant Concentration in Water Lamellar Phase (Neat) Crystal Hydrate

16 Surfactant Aggregation verview of Aggregate Types (The "Surfactant Menagerie") Dilute Solution Concentrated Solution "Particles" Liquid Crystal Phases Crystal Phases Monolayer at AirWater Interface Cylindrical Micelles (random) Hexagonal Phase (Middle) Anhydrous Crystal Unorganized Monomers Vesicles Spherical Micelles Planar BiLayers Increasing Surfactant Concentration in Water Lamellar Phase (Neat) Crystal Hydrate

17 Carboxylate (Soap): Proportion Present Sulfate or Sulfonate: Proportion Present How Does ph Affect Surfactants in Solution? pka RS RSH RS RSH or pka ph a Amphoteric (amphoacetate): 1 ph or RC RCH The protonated form of the surfactant does not really behave as a "surfactant" (unless there is more than one polar group) Micellar surfactants can have pk a s ~4 units higher than their monomer counterparts (unless electric effects dictate otherwise) Proportion Present H + H + RNCH RNCH 2 C Na + 2 CH RNCH 2 C Na + R R R pka pka ph

18 SURFACTANT FUNCTINALITY Wetting Water is not attracted on an oily/greasy surface. However, the presence of surfactants facilitate the wetting process Similar as in emulsification

19 SURFACTANT FUNCTINALITY SLUBILIZATIN Micelles act as solubilizers Hydrophobic molecules are taken into the lipophilic core of the micelle An important application is to make a solvent such as hydrocarbon dissolve in water and surfactant + oil micelle solubilized oil

20 SURFACTANT FUNCTINALITY EMULSIFICATIN Adsorption at the water oil interface results in dispersion of one phase into the other depending on the properties of the system Emulsions water / oil or oil / water

21 SURFACTANT FUNCTINALITY DETERGENCY It involves wetting, emulsification, solubilization and micellization

22 Basic Surfactant Chemistry What is a surfactant Behavior of surfactants in water Surfactant Type verview Structural variables Surfactant synthesis and range of materials available SurfactantInfluenced Performance Properties Lather Mildness Rinse feel Deposition of Actives Cleaning SurfactantInfluenced Product Physical Properties Product Rheology Stabilization of Dispersed Phases Surfactant Information Resources

23 What is Lather? The Structure of Lather Air

24 Air Gravity Drainage Air AirWater Interface Liquid Lamella Air

25 Air Air Impeded Drainage Air

26 Salt (Electrolyte) Level Electrostatic Repulsions Impact of Electrolyte Level on Lather Volume Closer packing of head groups increases hydrodynamic friction => slower lamellar drainage. Minimization of electrostatic repulsion across the barrier => constriction and eventual rupture. s Lather Volume (Stability)

27 Impact of CoSurfactants on Lather Electrostatic Repulsions * Also lowers CMC, see mildness impact. + NonIonic Surfactant +Amphoteric Surfactant Reduced drainage (steric blockage) 2. Tighter interfacial packing* (reduced electrostatic repulsion) 1. Tighter interfacial packing* (reduced electrostatic repulsion) 2. Reduced drainage (steric blockage)

28 Effects of "Soils" on Lather Soils Can Be Part of the Product Formula or Part of the Surface Being Cleaned Depleted Level of "Free" Surfactant Plus Soil Emulsified Soil Particle Solubilized Soil Examples of Soils: Fatty Acids Fatty Alcohols Triglycerides Important Variables: Length of hydrocarbon chain Particle Size Melting Poin t

29 Impact of Particulates on Lather µ + Small Particles +Large Particles * Particles can be ingredients or form in situ (e.g., rxn. with Ca 2+) Drainage impeded by particle "dam" => better lather stability Important factors ptimum Size Particle shape Concentration Surface hydrophilicity Particle spanning lamella ruptures bubbles => decreased lather stability Important factors ptimum size Surface hydrophilicity

30 Surfactant Influences in Active Deposition Types of Actives Soluble Materials Insoluble Particles Dispersed Liquids Cl Cl Cl H 3 C Cl Cl H Triclosan (TCS) Cationic Polymers CH 2 H CH H CH 2 H Glycerin NH C NH Cl Trichlocarban (TCC) N S Zinc Pyrithione (ZPT) 2 Zn 2+ H 3 C Si CH 3 CH 2 R H 3 C Si CH 3 CH R n CH 2 H 3 C Si CH 3 R CH 3 Petrolatum ils Silicones

31 Most actives (except watersoluble nonpolymers) prior to deposition: Water Soluble Polymers: Cationic: Insoluble Solid Particles: Relatively Hydrophilic: Nonionic: associate with surfactants Hydrophobic: Polymer Hydrophobic Domains Dispersed Liquid Droplet :

32 Why Does Surfactant Adsorption Affect Deposition of Actives? Exposure (adsorption) : Keratinous Surface Poor adsorption if... Species charge same as surface charge. Species surface hydrophilic Rinse (retention): Wat er + + Keratinous Surface + If adsorbed species hydrophilic, will be carried away with rinse water. Rinse water ions (Ca 2+ ) may alter. Phase separation upon dilution (complex coacerva.)

33 Typical conditions: Huge surfactant excess over active, and Surfactant mostly anionic To maximize deposition: Minimize surfactant level, Minimize surfactant interaction with actives Particle size Dissimilar surfaces Have surfactant charge be opposite that of surface, Use Ca 2+ from rinse water Employ complex coacervation

34 SURFACTANT INFLUENCED PRDUCT PHYSICAL PRPERTIES VISCSITY STABILIZATIN F DISPERSED PHASES

35 Viscosity of Surfactant Solutions 1000 Na C 1214 E 2 S 100 Viscosity (P) 10 1 Spherical Micelles Cylindrical Micelles Hexagonal Hexagonal + Viscous Isotropic Viscous Isotropic Lamellar Phase

36 Effects of Additives on Surfactant Rheology Factors that decrease surf ace charge d ensity of micelle allow closer packing of surfactants and shift phase sequence to lower concentrations: Viscosity (P) + Electrolyte Examples of Additives: Electrolytes (salt) Cosurfactants = Alkanolamides Concentration (%)

37 Role of Surfactants in Stabilization of Dispersed Phases For small particles ( < 1µ), molecular stabilization can occur: Electrostatic stabilization by ionic surfactants: Steric stabilization by nonionic surfactants:

38 For large particles, rheological stabilization relevant: Structured Liquid

39 SURFACTANTS IN PERSNAL CARE PRDUCTS PRDUCT TYPE LEVEL IN PRDUCT IN PRDUCT FUNCTINS IN USE LEVEL IN USE Personal Cleansing Bars ~ 60% product structure lather Body Wash ~ 15% viscosity control cleaning stabilize dispersed rinse feel phases active deposition Hair Care mildness Shampoos ~ 20% viscosity control stabilize disperse phases 510% 510% 23% Dentrifice 25% solubilize flavor /oils foaming / cleaning 12% Moisturizers 25% emulsifiers 17%

40 SURFACTANTS IN FABRIC and HME CARE PRDUCTS PRDUCT TYPE Fabric Care LEVEL IN PRDUCT IN PRDUCT FUNCTINS Granules 1225% cleaning IN USE LEVEL IN USE 0.02% Liquids 1630% viscosity control sudsing 0.04% Dish Care Liquids ~30% viscosity control cleaning, sudsing 0.06% Automatic nil Hard Surface Floor Cleaners 222% cleaning 0.04% Gel Cleansers 210% gel agent cleaning 210% Abrasive cleansers 13% cleaning ~0.5%

41 SURFACTANT CLASSIFICATIN ANINIC Are negatively ionised NNINIC Do not contain ionisable groups R S 3 R H

42 SURFACTANT CLASSIFICATIN Cationic Are positively ionised R N Amphoteric Have a ph dependent ionisation profile. e.g. Anionic at high ph, cationic at low ph and neutral somewhere in between. Zwitterionic surfactants are charged but have a net neutral charge over most ph s. High ph negative Low ph positive R N H C 2 Middle ph net neutral R N H 2 C 2 H R C N 2 H 2

43 SLUBILIZING GRUPS Anionic Sulfonate Sulfate Carboxylate R S Na R S Na Nonionic Poly(ethylene oxide) Sugar derived (e.g. glucose) R Na Cationic R Quaternary ammonium n R H H H H H Amphoteric R N R ' Cl Betaine Sulfobetaine R N C 2 R N S 3

44 HYDRPHBHIC GRUPS The hydrophobic group has very little interaction with the solvent. Most common solvent is water, therefore the lyophobic part receive the name of hydrophobic For example for detergents usually the hydrophobic part contains between 820 atoms of carbon

45 HYDRPHBIC GRUPS 1. ALIPHATIC HYDRCARBN Straight Chain Trivial names are given to chain length mixtures derived from natural sources. Coco, Tallow, Palm. Branched Chain Introducing branching can deliver unique properties Watch out is rate of biodegradation

46 HYDRPHBIC GRUPS 2. ALKYLBENZENES Generally present as a mixture of isomers R

47 Basic Surfactant Chemistry What is a surfactant Behavior of surfactants in water Surfactant Type verview Structural variables Surfactant synthesis and range of materials available SurfactantInfluenced Performance Properties Lather Mildness Rinse feel Deposition of Actives Cleaning SurfactantInfluenced Product Physical Properties Product Rheology Stabilization of Dispersed Phases Surfactant Information Resources