internal structure and charge compensation of polyelectrolyte multilayers department of chemical...
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Internal Structure and Charge Compensation of
Polyelectrolyte Multilayers
Department of Chemical & Biological Engineering
Colorado State University
David (Qiang) Wang
PE are important materials
• Can be soluble in water
• Can be adsorbed onto charged surfaces
PE are difficult to study
PE are charged polymers
• Both long-range (Coulomb) and short-range (excluded volume) interactions present in the system
Decher, Science, 277, 1232 (1997)
PE Layer-by-Layer (LbL) Assembly
Polyelectrolytes (PE)
“Fuzzy Nanoassemblies:Toward Layered Polymeric Multicomposites”
Decher, Science, 277, 1232 (1997)
Black curve: Concentration profile of each layer.
Blue (Red) dots: Total concentration profile of anionic (cationic) groups from all layers.
Green dots: Concentration profile of a labeling group applied to every fourth layer.
Model System for PE Adsorption
0 x
P = A,1,2++++++
l
A,b
cs,bA
b0
solvent molecule (S)cation (+) anion ()
• Monovalent, 1D system• Ions from salt counterions from PE and
substrate• Ions have no volume and short-rang
interactions• Polymer segments have the same density 0 as
solvent molecules• All polymer segments have the same statistical
segment length a• No short-range interactions between polymers
Parameters in the model:
SF substrate charge density;
vP charge valency of PE;
pP degree of ionization of PE
(Smeared or Annealed);
PS Flory-Huggins parameter for
solvent quality;
A,b bulk polymer concentration;
cs,bA bulk salt concentration;
80 dielectric constant.Quantities to be solved:
(x) electrostatic potential (in units of kBT/e);
A(x) polymer
segmental density.
xw(1)
Layer Profiles – Symmetric, Smeared PESF0.1 (2.61mC/m2), v1v2, p1p20.5, 1S2S1,
cs,b1cs,b20.05 (0.667M), A,b7.5×104 (10mM) (with a0.5nm and 0a3)
2
for layer A
for eve l
1 odd
ay r n e
i
i
SF0.1 (2.61mC/m2), v1v2, p1p20.5, 1S2S1,
cs,b1cs,b20.05 (0.667M), A,b7.5×104 (10mM) (with a0.5nm and 0a3)
2
for layer A
for eve l
1 odd
ay r n e
i
i
Layer Profiles – Symmetric, Smeared PE
SF0.1 (2.61mC/m2), v1v2, p1p20.5, 1S2S1,
cs,b1cs,b20.05 (0.667M), A,b7.5×104 (10mM) (with a0.5nm and 0a3)
Layer Profiles – Symmetric, Smeared PE
Three-Zone Structure – Symmetric, Smeared PESF0.1 (2.61mC/m2), v1v2, p1p20.5, 1S2S1,
cs,b1cs,b20.05 (0.667M), A,b7.5×104 (10mM) (with a0.5nm and 0a3)
Charge Compensation – Smeared PE
( )( )SF P P
1
( )
th
: amount of PE adsorbed
in the deposition.
At steady state,
iji
j
j
v p
j
( )
( )
0 for odd
0 for even
i
i
i
i
SF0.1 (2.61mC/m2), v1v2, A,b7.5×104 (10mM),
cs,b1cs,b20.05 (0.667M), 1S2S1 (with a0.5nm and 0a3)
( )( ) ( 1) ( 1)P P 2
ii i iv p
( ) ( 1)( ) ( 2)P P P P
i ii i v p v p
( )( )SF P P
1
( ) ( 1)( ) ( 2)P P P P
At steady state,
iji
j
i ii i
v p
v p v p
Charge Compensation – Asymmetric, Smeared PESF0.1 (2.61mC/m2), v1v2, A,b7.5×104 (10mM),
p1p20.5 (with a0.5nm and 0a3)
Charge Density Profiles – Asymmetric, Smeared PESF0.1 (2.61mC/m2), v1v2, p1p20.5, 1S1, 2S0.6,
cs,b1cs,b20.05 (0.667M), A,b7.5×104 (10mM) (with a0.5nm and 0a3)
1S2S1
Annealed vs. Smeared PE – 1st LayerSF0.1 (2.61mC/m2), v1, p10.5, 1S1, cs,b10.05 (0.667M),
A,b7.5×104 (10mM) (with a0.5nm and 0a3)
Charge Fractions in Multilayer – Symmetric, Annealed PESF0.1 (2.61mC/m2), v1v2, p1p20.5, 1S2S1,
cs,b1cs,b20.05 (0.667M), A,b7.5×104 (10mM) (with a0.5nm and 0a3)
Each depositionchanges the charges carried by the PE in a few previously deposited layers, of which the density profiles are fixed in our modeling. Thus,
(i): charges carried by PE adsorbed in the ith deposition.
(i): amount of PE adsorbed in the ith deposition.
( ) ( 1) ( )+ .i i i
SF0.1 (2.61mC/m2), v1v2, p1p20.5, 1S2S1,
cs,b1cs,b20.05 (0.667M), A,b7.5×104 (10mM) (with a0.5nm and 0a3)
Annealed vs. Smeared PE – Polymer Density in Zone II
Smeared PEM
AnnealedPEM
0.805 0.004
0.816 0.010
SF0.1 (2.61mC/m2), v1v2, p1p20.5, 1S2S0.5,
cs,b1cs,b20.05 (0.667M), 1,b2,b7.5×104 (10mM) (with a0.5nm and 0a3)
Non-Equilibrium & Solvent Effects – Symmetric, Smeared PE
Multilayer does not form in or good solvent.
• We have used a self-consistent field theory to model the layer-by-layer assembly process of flexible polyelectrolytes (PE) on flat surfaces as a series of kinetically trapped states.
• Our modeling, particularly for asymmetric PE having different charge fractions, bulk salt concentrations, or solvent qualities, reveals the internal structure and charge compensation of PE multilayers. We have also compared multilayers formed by strongly and weakly dissociating PE.
• Our results qualitatively agree with most experimental findings.
Summary
Q. Wang, Soft Matter, in press