Lecture 4. Donnan Potential

Transcription

1 Lecture 4 Langmuir-Blodgett films II Langmuir Blodgett films. II. Donnan Potential

2 Floating monolayers Generally, amphiphilic molecules adsorb on the liquid-air interface Insoluble amphiphiles can create a solid monolayer on a liquid surface Structure of a phospholipid p p

3 Formation of floating monolayer monolayer can be spread in 2 ways: depositing droplet of a pure substance on the surface, that spreads till equilibrium spreading pressure is reached depositing substance in a volatile solvent with positive spreading pressure surface pressure Π = γ w γ area per molecule f σ = A A/ N M Surface film balance (Langmuir trough)

4 Formation of floating monolayers Measurements on insoluble and soluble amphiphiles p

5 Principal monolayer phases S: Solid phase: low compressibility, vertical chains, rectangular (herringbone), A=0.192 nm 2/ chain. LS: Super Liquid: vertical chains, hexagonal (rotator), A=0.198 nm 2 /chain.

6 Principal monolayer phases L c : liquid condensed: linear but less steep, probably fully extended chains but tilted, rotator, A=0.198 nm 2 per chain L e : liquid expanded: chains disordered A>0.4 nm 2 per chain equation of state: Π Π ( A A ) = NkT ( ) ˆ 0 0 Amagat equation: 0 Π ( A A ) = qkt (q<0) ˆ G: gaseous phase, molecules widely separated, move independently, A~ 8 nm 2 per chain Π A = NkT

7 Surface pressure vs. area isotherms Π σ A = kt c γ Γ = RT c γ = γ bc 0 a Π + σ 2 A σ 0 = σ A Π σ A = kt ( ) kt

8 Phase transitions First order transitions appear as constant pressure regions on the isotherm G->L e, L c, S First order (often without plateau) L e->l c, L e->s Second order: L c ->S transition L e -L c viewed by Brewster microscopy

9 Monolayer collapse

10 Surface pressure isotherms 2 nd Order Transition (G) 1 st Order Transition gaseous phase (G) liquid expanded (L e or L 1 ) liquid condensed (L c or L 2 ) Solid phase (S)

11 Deposition of Langmuir-Blodgett films efficiency of deposition - transfer ratio ratio of area transferred to the solid to area decrease in the monolayer usual substrates (depends on application): mica, silicon, quartz etc. film quality depends on ph, ionic strength in subphase X-type Z-type Y-type

12 Deposition of Langmuir-Blodgett films Langmuir-Shaefer technique (works for rigid monolayers, can be also used for SAMs) hydrophobic solid

13 Study of film structure surface film balance surface potential using air electrode vibrating plate technique surface potential φ = ψ + χ m Δ E = ψ ψ Volta (outer) potential 0 Galvani (inner) potential ψ φ with monolayer clean phase surface viscosity shear viscosity surface dilational modulus dynamic modulus

14 Surface potential Surface potential vs. area per molecule for n-hexadecanoic acid

15 Surface potential measurement of surface potential

16 Study of film structure Electron diffraction Electron microscopy Ellipsometry Brewster angle microscopy Fluorescent microscopy Scanning probe microscopy Surface Plasmon Resonance transition L e -L c viewed by Brewster microscopy AFM microscopy

17 Study of film structure grazing incidence X-ray (GIXD) d = nλ /2sin θ hk low penetration depth can be achieved (5nm at ) typically angles sensitive to the electron density in the layer

18 Study of film structure neutron scattering UV-VIS spectroscopy Fourier transform infrared spectroscopy py as reflection adsorption (FTIR-RAS)

19 Mixed monolayers A B component B squeezed out of the film A+B totally miscible A+B inmiscible effect of miscibility on pressure/area isotherms

20 Interactions in a monolayer two water-insoluble components AAB = xa A A+ xa B B mixing free energy (excess) Π Π Π E G AABd xa AAd xb ABd * * * Π Π Π Δ = Π Π Π

21 Penetration of monolayers by soluble surfactants penetration of surfactant increases the surface pressure per monolayer molecule, although the amount of surfactant is difficult to determine injection of dodecylsulfonate

22 Reaction in monolayers arachidic acid on acidic and alkaline subphases

23 Temperature dependence C 14 H 29 COOH C 15 H 31 COOH Effect of temperature and chain length on the pressure/area isotherms

24 Fluorescent microscopy of LB incorporating a small fluorescent dye probe in the phospholipid monolayer

25 Brewster angle microscopy Typical structures of phospholipid and pentadecanoic acid

26 Phase diagram for LB monolayers b S b S Gibbs phase rule for LB F = ( C + C ) ( P + P ) + 3 For 1 surface phase: F=3 (Π and T=const) For 2 surface phases F=2 ((Π T=const) CH 3 (CH 2 ) 20 COOH Phase diagram for n-docosanoic acid

27 Brewster angle microscopy Interaction of gramicidin with phospholipid LB

28 Applications of LB films molecular size, shape and packing can be determined membrane modeling lung surfactant water conservation Molecular electronics Second harmonic generation Chemical and biological sensors

29 LB applications: Gas sensor response of a 45 layer copper-phtalocyanine LB film response of a 45 layer copper phtalocyanine LB film to 2 ppm of NH 3.

30 LB applications: Diode

31 LB applications: Insulator Energy band diagram for MIS structure

32 Osmosis and Donnan potential

33 Colligative properties Osmosis spontaneous passage of pure solvent into solution separated by semipermeable membrane Van t Hoff equation: Π = [ BRT ], [ B] = n / V B

34 Colligative properties Van t Hoff equation: Π = [ B] RT, [ B] = n / V B μ * * A ( p) = μa ( p+π ) + RTlnxA p+π * +Π = μ + μ * ( p ) ( p) V dp A A m p For dilute solution: RTx B = ΠV n B / n A m V / n / A van t Hoff factor for ionic solution was a proof for Arrhenius theory More generally: Π = [ BRT ] (1 + bb [ ] +...) Osmotic virial coefficients

35 Donnan membrane equilibrium semipermeable membrane V

36 Donnan potential

37 Donnan membrane equilibrium μ = μ + zfφ i i i μ μ = 0 = μ μ β α β α + + Na Na Cl Cl β c c β α RT Na RT Φ Φ = ln = ln α F c F c c c β z c = 1 2 c + Na P P α β Na Na α + + Na β + Cl α + Cl

38 Probelms 5.3: A cholesterol monolayer has an area of and 0.40 nm 2 molecule -1 at 5 and 20 mn m -1 respectively and a monolayer of dipalmitoyl phosphatidylcholine (DPPC) has areas of 0.68 and 0.43 nm 2 molecule -1 at these surface pressures. The corresponding areas for a mixed monolayer consisting of 1:3.4 cholesterol:dppc are 0.38 and 0.40 nm 2 molecule -1. Calculate the excess areas of mixing at these two surface pressures. 6.4 After equilibration the Donnan potential across a membrane separating a solution of a negatively charged polyelectrolyte and a solution of sodium chloride is found to be 45 mv (with the polyelectrolyte l l t solution at the lower potential). Calculate the ratio of sodium ion concentrations in the two solutions. (Ignore any osmotic movement). Surface Pressure, Area, Π / mn m -1 A / m 2 mg : Calculate the average molar mass and molecular area of egg albumin from the data in the table. The data refer to a monolayer of egg albumin spread on water in a film balance at 25ºC. Ch. 13 Problem