chapter 4 - self-assembly

8/3/2019 Chapter 4 - Self-Assembly

1/57


2/57

surfactants


3/57

..

solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent,sequestering the hydrophobic tail regions in the micelle centre. This type of micelle is known as a normalphase micelle (oil-in-water micelle). Micelles are approximately spherical in shape. The shape and size of am ce e s a unct on o t e mo ecu ar geometry o ts sur actant mo ecu es an so ut on con t ons suc as

surfactant concentration, temperature, pH, and ionic strength. The process of forming micellae is known asmicellisation and forms part of the phase behaviour of many lipids according to their polymorphism.


4/57

Inspiration:

cell membrane

Fluid mosaic modelSinger and Nicholson, Science 175, 720 (1972)


5/57

Biomembrane is com osed Micelle is

of phospholipids simple model

O

O

O-

O

OO P

O

ON

Micelles (Lat. Micella = rain)Phospholipid assemble spontaneously from amphiphilic

(amphiphatic) molecules in water


6/57

micelle formingamphiphiles

(surfactants)


7/57

Physical properties of solutions ofamphiphiles are stronglyconcentration de endent


8/57


9/57


10/57

an ory s a ce mo e

PNAS 78, 676 (1981)


11/57

-Counter ions bound to the charged head groups of the surfactants explains drop in conductance-Interior properties of liquid carbons solubilize hydrophobic molecules


12/57

Critical micelle concentrations

--why? Balance attractive/repulsive interactions Average number of surfactant

molecules per micelle


13/57


14/57

Micelle formation is a strongly


15/57

Gm = 2RTlnCMC

How is G com osed?

Gm= G(Hydrophobic interaction + G(interactions between headgroups) +.

,

amphiphiles from water to the inside of the micelle.Consider reverse process: dissolution of an apolarmolecule, e.g. benzene, in water


16/57

Solubility of benzene in water displays unusual behavior


17/57

Compare dissolution of argon in wateran n y raz ne.

H2O: m.p. O0C

.

H2NNH2: m.p. 1.7 oCviscosity 0.89 CP


18/57

from n-hexane to hydrazine

G = +11.9 kJ/mol = + . moS = -7.9 J/K.mol

Thermod namic data for transfer ofargon from n-hexane to water

G = +10.4 kJ/mol - .S = -72 J/K.mol

For a better analysis consider a 2 step process


19/57

Arexane

G = +11.9 kJ/mol =

Step 1:creation of hole

S = -7.9 J/K.moland insertionof argon ArHexane

G = -1.5 kJ/mol

Step 2:y raz ne

Hexane

= - .S = -64 J/K.mol

for water

G = +10.4 kJ/molTotal:

Respons ofH = -11.2 kJ/molS = -72 J/K.mol

so vent s e

F bl


20/57

Favourable

H HFrom an enthalpic

point of view dissolutiono argon n wa er sfavourable!

Process is unfavourablefrom an entropicpoint of viewTangential

MD calculations HH

an neu ron sca er ng

OArAr O H

HLess favourable orientations


21/57

E t d i ti


22/57

Entropy-driven aggregation


23/57

Manifestations of thehydrophobic interaction

and the hydrophobic effect.These include(a) the low solubility of

hydrophobic solutes (e.g., oil)in water and vice versa

(b) the strong adhesion betweensolid hydrophobic surfaces

(c) the dewetting phenomena

leading to a large contactang e

(d) hydrophobic contaminants or

water interface

(f) protein folding

PNAS, 2006, 103, 15739


24/57

Chan es in micelle sha e with res ect to chan es in surfactant concentration


25/57

.

amphiphiles encasing an aqueous inner region. Artificial phospholipid vesicles (liposomes) are used totransport vaccines, drugs, enzymes or other substances to target cells or organs.

Li i l


26/57

Li osomes vesicles

Model systems for cell membranes

Phospholipid


27/57

Different typesof hos holi ids


28/57

Formation of vesicles


29/57

Other procedure: injection method


30/57

Shape controle:


31/57

ea group(area=a)

p=v/a.l

a(volume =vlength =l)

1/3


32/57

-structure

concept

J.N. Israelachvili et al.,Q. Rev. Biophys.,13, 121 (1980)

Transitions between assemblies


33/57

Transitions between assemblies

P < 1/

Freeze-fracture

1/3 < P

chapter 4 - self-assembly

Documents