4. Amphiphiles. 4.1 Types of amphiphiles. 4.2 Surface activity Surface tension Interface tension

Transcription

1 4. Amhihile 4.1 Tye of amhihile 4.2 Surface activity Surface tenion Interface tenion 4.3 Micellization and the critical micelle concentration Surface tenion and the CMC Gibb adortion equation The Krafft temerature The cloed aociation model The hydrohobic effect Molecular tructure and interfacial curvature 1

2 4.3 Micellization and the critical micelle concentration Surface tenion and the CMC Surface tenion [mn m -1 ] 72 I II III ~35 urface tenion of water decreae with increaing urfactant concentration u to the critical micelle concentration (CMC) lnc ionic urfactant: CMC ~ 10-2 M nonionic urfactant: CMC ~ 10-4 M 2

3 Croing the CMC regime I regime II regime III CMC urfactant concentration uon addition of urfactant above CMC: formation of micelle in ubhae 3

4 How many urfactant molecule are at the urface? adortion concern only a tiny fraction of the molecule, till ignificant effect on urface tenion tyical CMC: 2x10-3 M number of molecule (molecular area 25 Å 2 ) adorbed onto 100 cm 2 : N 4x10 16 in olution: 1.2x10 21 molecule/liter < 1 molecule out of 10 4 adorbed C. Tauin, in Soft Matter Phyic, M. Daoud, C.W. William (Ed.), Sringer Berlin

5 4.3.2 The Gibb adortion equation urface tenion concentration of urfactant? exce of comonent j at interface σ: σ j j ( α β n n ) n n + j j urface exce in equilibrium: Γ j σ n j A σ α β µ j µ j µ j A: urface area µ: chemical otential conider urfactant at air-water interface: d γ Γ µ d : urfactant chemical otential of urfactant: dµ RTd ln c c: concentration of urfactant in water Γ 1 dγ RT d ln c T 5

6 regime I: linear decreae of urface tenion with urfactant concentration: γ γ 0 kc urface reure π γ γ 0 kc Γ RT πa n RT in dilute olution: urfactant behave like ga regime II: γ ln c Γ cont. γ γ 0 m ln c Γ n A m RT number of urfactant molecule er urface area i contant i.e. urface exce aturate already below CMC reaon: total conc. of urfactant bulk conc. + urface exce bulk conc. increae lightly with lnc, even though urface aturated increae in acking denity in urface layer u to CMC 6

7 regime III: urface tenion nearly contant becaue chemical otential deend only weakly on urfactant concentration chemical otential: below CMC: c c i.e. total urfactant concentration unimer concentration far above CMC: c c Kc c c 1/ ( K) 1/ : aociation number, i.e. average number of molecule er micelle, tyically K: equilibrium contant of micellization chemical otential µ RT ln c RT RT chemical otential deend weakly on urfactant concentration µ θ + ln c ln 7 [ K]

8 Influence of head grou on CMC for ionic urfactant, CMC higher than for nonionic urfactant reaon: electrotatic reulion between head grou mut be overcome to form micelle CMC increae with head grou charge Influence of alt on CMC for ionic urfactant, addition of alt decreae the CMC becaue it reduce the reulion between charged head grou. 8

9 Influence of hydrohobic grou on CMC hydrohobicity: the more hydrohobic the chain i, the lower the CMC (e.g. fluorinated urfactant) log(cmc) number of carbon atom length of hydrohobic chain: the longer the hydrohobic chain, the lower the CMC: log(cmc) A B n C A, B: contant n c : number of carbon atom ionic urfactant : CMC decreae by a factor of ~2 er CH 2 added nonionic urfactant: CMC decreae by a factor of ~3 er CH 2 added 9

10 4.3.3 The Krafft temerature olubility curve CMC olubility of ionic urfactant deend trongly on temerature: increae raidly in narrow temerature range Krafft temerature: temerature at which the olubility curve meet the CMC curve Krafft temerature deend on: length of alkyl chain crytal tructure interaction between head grou alt content interlay between T-deendent CMC curve and T-deendent olubility of the urfactant oition of the Krafft oint 10

11 4.3.4 The cloed aociation model dynamic equilibrium between unimer and micelle containing unimer S S dierity in i ~20-30% equilibrium contant: K c c c : concentration of micelle of aociation number, [mol/l] c : concentration of unimer Gibb energy change of micellization er mole of micelle: mic G θ RT ln K RT ln c c er mole of micelle: mic G θ RT RT ln K ln c + RT ln c für large and at the CMC: mic G θ RT ln c CMC 11

12 for ionic urfactant, include counterion C: S x + ) y α α n/ degree of diociation of the urfactant ( n C S x, y: charge of urfactant/counterion G n RT 2 ln θ mic c CMC above CMC: added molecule go into micelle concentration of ecie CMC unimer micelle urfactant concentration 12

13 unimer + CMC urfactant concentration aggregation number (number of unimer er micelle) indeendent of urfactant concentration number of micelle increae with urfactant concentration 13

14 for nonionic urfactant: θ mic H R d ln c d(1/ T ) CMC θ ln c mich CMC + cont. RT lot lnc CMC v. 1/T to determine enthaly of micellization driving force for micelle formation: increae in entroy of ytem of molecule in micelle comared to unaociated molecule reaon: unaociated molecule ordering of urrounding water gain in entroy of water uon micelle formation >> lo of configurational entroy 14

15 Hydrohobic effect liquid water molecule ha 4 H-bond in a tetrahedral geometry hydrogen bond life time ~1. 3D hydrogen bonding network hydrohobic interaction: entroic hydrohobic unit induce ome order in the urrounding water. D. Chandler, Nature437 (2005) water molecule urrounding a mall molecule (left) or a cluter of mall molecule (right) 15

16 4.3.6 Molecular tructure and interfacial curvature rediction of hae of micelle (here, cylinder, bicontinuou, lamellar) two model: 1. curvature model elatic energy related to curved urface 2. hae of urfactant molecule acking of molecule C. Tauin, in Soft Matter Phyic, M. Daoud, C.W. William (Ed.), Sringer Berlin

17 Micellar hae and ize: General conideration 1t condition: ace requirement of olar head and hydrohobic chain define ontaneou curvature c 0 of the interface minimization of elatic energy in abence of contraint 2nd condition: ize of micelle maller than or equal to the tretched length of hydrohobic tail area er headgrou: geometric electrotatic for uncharged membrane: F elatic < k B T membrane highly flexible G. Porte, in Soft Matter Phyic, M. Daoud, C.W. William (Ed.), Sringer Berlin

18 Model 1: Curvature model calculation of morhology baed on the curvature of a continuou urfactant film differential geometry of urface in each oint P: mean curvature and Gauian curvature R 1, R 2 : radii of curvature addle urface in bicontinuou tructure c 1 < 0, c 2 > 0 1 c 1, c2 R1 c 1 + c H 2 2 K c c R mean curvature 2 Gauian curvature 18

19 elatic free energy of a curved urface: F el κ,κ : F mean + F Gau 1 κ ( c1 + c2 c0 ) κc1c 2 elatic moduli for mean and Gauian curvature can be meaured uing dynamic light cattering related to membrane flexibility κ : κ : bending modulu, imortant for 1D deformation addle lay modulu, imortant for addle where c 1 -c 2 19

20 Effect of coolute on curvature ure urfactant monolayer inertion of hort amhihile oil in water emulion comenation by welling with oil molecule C. Tauin, in Soft Matter Phyic, M. Daoud, C.W. William (Ed.), Sringer Berlin

21 Packing model 1/3 1/2 1 N micelle cylinder veicle bilayer invere micelle definition of acking arameter N V al V: urfactant volume a: area of head grou l: tail length 21

22 Packing arameter aociation number 3 4πRmic / 3 V volume 4πR a 2 mic urface area V armic V: volume er molecule R mic : micellar radiu a: head grou area l: tail length 1 3 V al 1 3 urfactant acking arameter: N V al variation of N oible mainly by altering a by variation of olvent 22

23 Packing arameter for urfactant with alkyl chain OSO3 - SDS: dodecylulfate length of alkyl chain: l / nm n C nm: C-C bond length nm: rojection of C-C bond length onto chain axi volume of alkyl chain: ( ) 3 V / nm n C + n Me n Me 1 or 2: number of methyl grou SDS: l 1.551nm V a 0.324nm OSO nm 2 N 0.33 aociation number herical micelle 4πl 2 49 a 23