Amphiphiles. Amphiphiles are both polar and non-polar surface active. Components in commercial amphiphiles. Thomas Ederth IFM/Molecular physics TFYA47

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1 Amphiphiles Thomas Ederth IFM/Molecular physics TFYA47 Amphiphiles are both polar and non-polar surface active Synonyms: Amphiphile Surfactant Tenside Detergent Wetting agent Emulsion Polar (hydrophilic) headgroup Non-polar (hydrophobic) tail Lipids = Naturally occurring substances which are insoluble in water, and often with amphiphilic properties. Some examples: Soap ( tvål ) Soduim salts of fatty acids Soft soap ( såpa ) Potassium salts of fatty acids Phosphoglycerides Glycerol esterified with fatty acids and phosphorous acid Classes of amphiphiles, with some examples (Named primarily after the charge of the polar headgroup) Anionic Sodium dodecyl sulphate, SDS Zwitterionic (ampholytic) Dodecyl betaine Components in commercial amphiphiles Polar headgroups Non-polar tails S 3 - Na + N + C 2 - Cationic Cetyl pyridinium bromide N + Br - Non-ionic Polyoxyethylene(4)lauryl ether (C 12 E 5, Brij 30) H Gemini ( twin ) +_ +_ Block copolymers Pluronic PE x PB y PE z

2 Commercial use of amphiphiles Aggregation Typicallt n 3 Sodium laureth sulphate, SLS SLS and SDS are among the most commonly used amphiphiles in consumer hygiene products. M kton Household detergents Industrial cleaning Personal care Crop protection ilfield Paints and coatings Textile Construction Emulsion polymerisation Food Leather re/mineral Plastic additives Pulp and paper Explosives ther Total CAC/ CMC The free energy of solvation of the nonpolar parts in water is high, and aggregation occurs at the critical aggregation concentration (cac), or critical micelle concentration (cmc) Above CAC/CMC the concentration of free monomers does not increase, just the number of aggregates, and the surface is saturated with amphiphiles. Increasing amphiphile concentration Bulk conc. Monomers CMC Micelles Surface conc. Self-organization of amphiphiles The aggregation is driven by Hydrophobic interaction between hydrocarbons van der Waals-attraction between chains Hydrogen bonds and electrostatic attraction between polar groups Steric and electrostatic repulsion between polar groups work against aggregation! Phase separation instead of micelle formation is possible (particularly for non-ionic surfactants), but is unfavourable for ionic amphiphiles due to repulsion between polar heads, and a considerable loss of entropy for the counterions. Micelles (and other aggregates) are dynamic... This is not what micelles look like! Simulation of micelle structure in a primitive (coarse-grained) amphiphile. Smit, Langmuir 9, 9 (1993)

3 Factors affecting CMC Properties of some common surfactants The critical micelle concentration decreases if the hydrophobic tail is made longer is moderately affected by temperature changes can be reduced dramatically by addition of other amphiphiles decreases with increasing salinity for ionic amphiphiles is lower for non-ionic than for ionic amphiphiles decreases if counterions with higher valency are added can vary considerably for molecules with small differences in molecular structure These factors also influence the structure (shape) of the aggregates! Many measurable properties change their concentration dependence at the cmc... This is used to determine cmc experimentally! Surfactant CMC (mm) Molecules per micelle % bound counterions SDS AT CTAB Triton X CMC-dependence on n and m in C n E m - amphiphiles CMC-variation for different n CMC-variation for different m 6 C n E 9 C 6 E m C 8 E m C 10 E m C 12 E m C 14 E m Synergy effects in mixed amphiphiles N + S 3-50:50 (ideal) C 2 - Surface tension in the mixed system C 12 Betaine SDS SDS Surface composition: Calculated (from surface tension) Experiment Ideal mixture C n E 3 C n E 5 C n E 7 C 16 E m 50:50 (experiment) C 12 Betaine Circles = Experimental data Squares = Calculated data Daful, J. Phys. Chem. B 115, 3434 (2011) This system cannot be understood if it is considered as an ideal mixture!

4 At T Kr the solubility is equal to that at the cmc. Krafft-temperature The Krafft-temperature is the lowest possible temperature for micelle formation. Solubilization Detergent (cleaning) action A normally insoluble substance can be present at high concentration in surfactant solutions by being dissolved in the polar interor of micelles. This is the basic mechanism for the cleaning function of amphiphiles! Micelle Below T Kr the monomer solubulity is too low for micelle formation to be possible. Above T Kr the solubility increases quickly since monomers form (easily soluble) micelles. Nonpolar molecule Inverted micelle Dirt Polar molecule Molecular structure Critical packing parameter How does the molecular structure affect the properties of amphiphiles? The geometry of the amphiphile determines the shape of the aggregates it will form. SDS Chain length CPP v = l a max 0 Volume, v Area, a 0 Not the geometric area of the molecule, but an effective area, where repulsion between polar heads is accounted for. Aerosol-T n m o Pluronic, E n P m E o The Critical Packing Parameter (CPP) gives an idea of which aggregates will minimize the free energy for a particular amphiphile. Too coarse a model to permit detailed interpretation or predictions!

5 CPP and 3D-structure CPP and aggregate structure Lamellae, bilayers CPP ~ 1 Micelles CPP < 1/3 Lyotropic phases Phase diagrams Liquid crystalline phases Lyotropic: Liquid crystalline form where the solvent concentration is most important for the properties. Thermotropic: Temperature is the variable with greatest impact on the phase. Cylindric micelles 1/3 1/2 Bicontinuous ( sponge ) phases CPP > ~1 Vesicles 1/2-1 Inverted micelles CPP > 1 Two-dimensional systems Insoluble amphiphiles on water surfaces form 2D-systems with unique phase properties. ( like e.g. layers in smectic phases (liquid crystals), lipid bilayers and monolayers adsorbed on solid surfaces.) π Surface pressure ( yttryck ): π = γ0 γ γ Water π Isotropic solution (micelles) at low concentrations, crystalline at low temperatures, and some liquid crystalline phases (cubic, hexagonal and lamellar). The nearly vertical phase boundaries suggests a moderate temperature dependence. γ0 Surface tension of water γ Surface tension with the film Air Phase diagram for dodecyl trimethyl ammonium chloride. The decrease in surface tension caused by the surface film γ0

6 Langmuir s surface balane 2D-phases The amphiphiles must be insoluble in water, otherwise they pass under the barrier! A droplet of e.g. stearic acid dissolved in hexane is injected at the surface; the solvent evaporates and leaves a monolayer of stearic acid at the surface. The surface film pressure can be varied by moving a barrier which restricts the film. Surface pressure is measured using a hydrophilic plate immersed in the film (the Wilhelmy metod). Gas Hydrocarbon chains in contact with the sub-phase, little interaction between the molecules. Liquid Chains are upright, but disordered. Solid chains are frozen in a crystalline state, cross sectional area ~20 Å 2 per chain. Gaseous Liquid expanded Liquid condensed Solid Collapsing π Α phase diagrams Brewster angle microscopy (BAM) leic acid (unsaturated) Effects of chain length, (un)saturation, and temperature At the Brewster angle no p-polarized light is reflected from a transparent medium. tan θ = n B n θ B 2 1 Air n arctan 53 1 o θ B Surface film Reflected Reflekterad intensity intensitet from the från air-water luft-vatten boundary gränsen Myristic acid C 14 CH C 15 CH Shorter chains, unsaturation, and higher temperatures give greater chain mobility, and less propensity for crystallization! Water n 2 A thin film on the surface changes the Brewster condition, and the surface will reflect some incoming light. n θ R s B R p

7 BAM in practice Biological membranes A bilayer membrane consists of a lipid bilayer with adsorbed or trans membrane proteins, carbohydrates, glycolipids, etc. Dimyristoyl phosphatidyl ethanol amine (DMPE) on water at 22 o C. a) Clean water surface b)-e) Increasing surface pressure f) Fully covering film Purpose of the membrane: Spatial confinement of the contents Control the transport of ions, molecules, energy, information... Solvent for membrane proteins Phase behaviour in bilayers CMC/CAC by some diacyl PC-lipids Gel: Frozen phase with acyl chains packed and extended, slow diffusion in the membrane. Unsuitable for biological membranes Fluid (Liquid disordered): Rapid diffusion and rotation of lipids, disorder (and thinning) of the nonpolar region. Liquid ordered: rdered chains but still relatively rapid diffusion. For pure phospholipids there is a welldefined temperature, T m, for the transition from gel to fluid phases which can be eliminated by e.g. addition of cholesterol. CMCs for some phosphatidylcholine lipids with varying length in the nonpolar tails. Remember: CMC is also approximately the bulk concentration of free molecules Giocondi, Biochim. Biophys. Acta, 1798, 703 (2010)

8 Mesophases Between liquid and crystal Liquid crystals Translational order is lost, rientational order is retained. T Liquid crystals Formes primarily by rodlike, but not too long molecules, often with a hydrocarbon chain, whose tendency to disorder prevents crystallization as the temperature is lowered. Rather, a series of phase changes with successively increasing disorder are obtained! θ Plastic crystals Translational order is retained, rientational order is lost. T Thermotropic Temperature is the variable which primarily determines the phase state. Lyotropic Solvent concentration determines the phase state (usually the case for amphiphiles). rder parameter, S: S = 2 3cos θ 1 2 S = 1: Perfect order S = 0: Disorder, isotropic Thermotropic phases Isotropic Completely disordered structure, normal liquid. Smectic A or C rdered structure of twodimensional layers. (There are many different smectic phases with varying structure and order in the layers!) Nematic Long-range orientational, but not positional order. Crystalline (Anisotropic) Long-range orientational and positional order in all directions.

9 Skumbildning Bubblor, skum och ytfilmer Fusion av t.ex. bubblor är en spontan process totala ytarean minskar men det kan förhindras med amfifiler! Ytaktiva ämnen orsakar Air Air 1) Repulsion mellan ytorna Water R 2γ P = R P > P A F W Platta ytor vid F P = P > P A W Elektrostatisk repulsion mellan joniserade eller polära huvudgrupper, Sterisk repulsion mellan polymera amfifiler. 2) Ytelasticitet Hög ytelasticitet är nödvändigt för att få ett stabilt skum. När en skumlamell sträcks av t.ex. mekaniska vibrationer sänks surfaktantkoncentrationen i den sträckta delen. Detta ger en höjning av ytspänningen och en återställande kraft, och amfifiler som diffunderar på ytan drar även med sig lösningsmedel, vilket ytterligar stabiliserar filmen. Gibbs-Marangoni-effekten! Diffusionen av ytaktivt ämne från bulk till ytan måste vara så långsam att den ökade ytspänningen kompenseras av diffusion längs ytan. Ämnen med högt CMC ger inte stabila skum då dessa har hög koncentration av monomerer i lösningen. Ytaktiva ämnen som inte bildar miceller ger inget skum (ex. etanol i vatten). Vätskemängden avgör om vi har ett torrt skum eller ett vått skum! Bildar ett kontinuerligt nätverk! Skummets anatomi Lägre tryck i junctions där flera Plateaugränser möts skapar kapillärsugning som bidrar till att dränera skumet.

10 Diffusion mellan skumceller Jämviktsstruktur hos torrt skum Tryckskillnader driver diffusion av gas mellan celler i ett skum. I ett tredimensionellt skum möts planen vid 120 i varje Plateau-gräns (vid jämvikt), och vinkeln mellan Plateaugränser blir då 109,5! m detta inte är uppfyllt kommer dynamiken i filmen att sträva efter detta tillstånd! Ett tvådimensionellt skum består av cirkulära bågar vars krökning avgörs av tryckskillnaden mellan cellerna. Topologiska förändringar i skum: 4-vägskorsningar elimineras 3-sidiga celler försvinner Krafter som verkar på ett skum rsak Gravitation Skillnad i Laplacetryck mellan lamell och Plateaugräns Tryckskillnad i bubblor av olika storlek Överlapp mellan elektriska dubbelskikt Dessutom påverkas stabiliteten av vätskans viskositet. Makromolekyler (proteiner, kolhydrater) kan öka viskositeten och förhindra dränering Effekt Dränering till skumbas Dränering till Plateaugräns Diffusion av gas från små till stora bubblor genom filmen Ökad skumstabilitet Ökad elektrolyt ger mindre repulsion Naturliga skum Kork Spottstritens skum Ben Användning av skum Öl & champagne Mat (äggvita, vispgrädde, glass...) Skumseparation av ytaktiva ämnen ( foam fractionation ) Flotation Brandsläckning Rengöring ljeutvinning