Apresentação_SDS

8/12/2019 Apresentao_SDS

1/18

1

4.4.2 Phase diagrams

ordered structures at high concentrations: lyotropic phases

interfacial curvaturemay be changed by varyingconcentrationbecause effective cross-sectional area of head group changes

normal structures:for surfactantshaving a head grouparea larger than thecross-section areaof the tail


2/18

2

micelles: large mean and Gaussian curvature

at low concentrations: L1(micelles with no long-range translational order

at high concentrations:

micelles packed in cubic structure: I1, e.g. bccor rod-like micelles in hexagonal structure: HI

bilayers in lamellar structure: L

saddle-splay surfaces in bicontinuous phasese.g. gyroid phase: V1, three-fold connection nodes

two continuous channels of water,separated by bilayer of surfactant molecules


3/18

3

surfactant concentration

L1 HI L HII L2a b c d

usual sequence of phases:

a-d: intermediate phases

a: often cubic micellar structureb: often bicontinuous cubic structure

inverse structureswhen solvent in minority phase:L2: inverse micellar solutionHII: inverse hexagonal phasec: often inverse bicontinuous phase V2

d: often inverse micellar cubic phase I2


4/18

4

Phase diagram of SDS/water system

SDS: anionic surfactant

phase boundaries vertical as in prediction

Krafft pointquite highlarge regions of hydrated crystal phases

solubilitycurve

CMC


5/18

5

Phase diagram of nonionic surfactants

phase diagrams of CmEn

constant hydrophobic chain length mincreasing EO chain length n(biphasic regions not indicated)


6/18

6

for short E chains (C12E4):

preferred mean interfacial curvature = 0lamellar (L) and inverse micellar phases (L2) with Ns 1

longer E chains (C12E6, C12E8):increasing tendency for normal (L1, H1) phases

temperature plays important rolee.g. solubility of poly(oxyethlene)decreases with increasing temperature

phase boundaries not vertical


7/18

7

4.4.3 Membranes

bilayers of surfactants formed for Ns 1e.g. for double-tailed surfactants:membranes formed right above CMCmean and Gaussian curvatures = 0

Lphase:strong thermal fluctuations at RTmay lead to sponge phase

stiffness may be controlled by charges

fluctuations entropic force, i.e. effective repulsion between bilayers

peristaltic mode:F~ d-5

spacing d

undulation mode:F~ d-3


8/18

8

Applications of membranes for DNS delivery

J.O. Rdler et al., Science 275, 810 (2001)

transfer and expression of extracellular DNAto cell nucleus to replace defective gene

use viruses or synthetic nonviral vectors

cationic liposomes attach to anionic animal cells low toxicity nonimmunogenicity easy production

synthetically based carriersof DNA vectors for gene therapymade from cationic liposomes

liposomes change tobirefringent liquid-crystalline

condensed globulesmultilamellar structure with

alternating lipid bilayerand DNA monolayers


9/18

9

Vesicles

vesicle:hollow aggregateshell: one or several bilayers

liposome:vesicle formed by lipidssimple model for cellcosmetics, drug delivery

unilamellarvesicle

MLV: multilamellar vesicleSUV: small unilamellar vesicleLUV: large unilamellar vesicle

optical micrograph


10/18

10

Preparation of vesicles

vesicles not in thermodynamic equilibriumbut often kinetically stable

sonication of dilute lamellar phases/mechanical shearlamellae break upreassemble as vesiclessmall vesicles with broad size distribution

dissolution of dry phospholipids in watermultilamellar vesicles

dispersion of lamellar phase formedat high concentration by excess of water

dispersion of surfactant in organic solventthen addition of excess of water


11/18

11

Drug delivery using vesicles

liposome formation in presence of drug

injection into bloodstream - drug is protected by vesicle

liposome binds to cell wall delivery of drug directly to cell

incorporation of membrane proteins specific targeting

cancer therapy: of liposome < 200 nm

cannot penetrateendothelial wall ofhealthy blood vesselsbut can penetratethe leaky vessels in tumors

liposomes are also of usefor oral delivery ofdietary/nutritional

supplements


12/18

12

4.4.4. Liquid foams

coarse dispersion of gas in liquid

liquid is minority phase

usually not thermodynamically stable

surfactant: foaming agent

retard drainage of liquid from foam

prevent rupturemetastable foams

in vertices (plateau borders):liquid pressure lower than in channelsliguid flowrupture

gas

content

polyhedralcells

sphericalbubbles


13/18

13

surfactants form lamellaeparallel to liquid film surface

excess of surfactant at liquid film surface

destabilization

Gibbs effect: draining

strong thinning of film

increase of surface area

decrease of surface excessconcentration of surfactant

increase of surface tension (Gibbs effect)

opposes thinning

Marangoni effect:surfactant flows to regionsof reduced surface excessto restore original (lower) surface tension

(convection of surfactant along interface)

Gibbs and Marangoni effects

Gibbs and Marangonieffects oppose thedestabilizing influence


14/18

14

4.4.5. Emulsions

two immiscible liquidsI and II

emulsion of phase IIdispersed in phase i

unstable emulsionseparates

surfactant positions itselfon interfaces betweenphase I and phase IIstabilizes emulsion

mixture of two or more

immiscible or partiallymiscible liquids

one liquid (the dispersed phase)is dispersed in the other phase(the continuous phase)

examples: vinaigrette, milk,technical fluids,


15/18

15

free energyrequired

to disperse a liquid of volume Vinto drops of radius R:

R

VG

3=

lower interfacial tension reduction in free energy

stabilization of emulsion

emulsions: thermodynamically unstable

microemulsions:

thermodynamically stable

smaller droplet size than in emulsions

slow kinetics of exchange of moleculesin/out of stabilizing film


16/18

16

Emulsions

two types: water-in-oil (w/o) oil-in-water (o/w) emulsions

milk is /w emulsion:fat droplets in aqueous phase

mayonnaise is o/w emulsion:vegetable oil invinegar or lemon juicesurfactant: lecithin

margarine is w/o emulsion

size of dispersed particles ~0.1-10 mscatter light emulsions appear cloudy


17/18

17

flocculation

due to net attractive forces between dispersed droplets

coagulation

droplets aggregate irreversibly

creaming/sedimentation

for unaggregated droplets

coalescence

droplets merge

large droplets grow at expense of small onesOstwald ripening

Breaking up of emulsions


18/18

18

usingemulsifiers, e.g surface-active agents

reduction of interfacial tension

increasing long-term kinetic stability

activity of surfactant emulsifier:

measured by hydrophile-lipophile balance (HLB)which runs from 1 (hydrophobic surfactant)

to 20 (hydrophilic surfactant)

Stabilization of emulsions

Apresentação_SDS

Documents

Transcript of Apresentação_SDS