Polyelectrolytes adsorbed onto oppositely charged lipid monolayers

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Polyelectrolytes adsorbed onto oppositely charged lipid monolayers
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Thomas Ortmann , Heiko Ahrens , Andreas Gröning , Frank Lawrenz , Andre Laschewsky and
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Christiane A. Helm
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We study the adsorption of linear charged polymers (polyelectrolytes) onto an oppositely charged
surface. A lipid monolayer at the air/water interface is used to define the surface; by varying the
area per molecule the surface charge is adjusted. To maximize the electrostatic force, salt-free
solution is used. Due to the electrostatic interchain repulsion, the persistence length of the
polyelectrolytes is increased. For low polymer concentrations in the subphase, we observe a flatly
adsorbed polyelectrolyte layer, the chains are aligned in a two-dimensional lamellar phase in
agreement with theoretical predictions [1,2]. On increasing the surface charge by monolayer
compression, we found that the ratio between lipid and monomer is almost constant [2]. However,
we do not know if the surface coverage of the polyelectrolyte is an equilibrium value, or if the once
adsorbed chains are immobilized on the surface. Furthermore, we do not know the role of nonelectrostatic interactions. To explore how general this two-dimensional ordered phase is, we vary
two different parameters: (i) the degree of chain charge by using random copolymers and (ii) the
surface charge density by using mixed monolayers.
The lipids are the positively charged DODA (Dioctadecyldimethyleammoniumbromide) and the
zwitterionic DPPC (Dipalmitoylphosphatidylcholine), the polyelectrolytes PSS (Polystyrenesulfonate, Mw=76 kDa) and P-TrisAAx-rand-AMPS1-x (0.01 mM with respect to monomer
concentration). The latter is made by "diluting" the negatively charged monomer sodium 2acrylamido-2-methylpropanesulfonate (AMPS) with the neutral hydrophilic monomer NTrismethylolmethylacrylamide (TrisAA) via random copoymerization. The lateral structure is
studied with X-ray GID (Grazing Incidence Diffraction) at BW1, in the liquid surfaces set-up [3].
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DODA0.75DPPC0.25PSS Mw75.6kDa 10 monoM
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DODA0.9DPPC0.1 PSS M w75.6kDa 10 monoM
Figure 1: Small angle GID measurements of mixed monolayers on a 10-5 monoMol/l PSS solution at the
surface pressure indicated in the isotherm. The lipid DODA is charged, DPPC not. The surface charge is
larger for DODA0.9DPPC0.1 (bottom row) than for DODA0.75DPPC0.25 (top row).
Fig. 1 shows diffraction peaks obtained from PSS chains in the two-dimensional lamellar phase.
Lipid monolayers with different surface charge are used. On monolayer compression, the chain
separation decreases. The two-dimensional lamellar phase is observed for DODA0.9DPPC0.1, and
for DODA0.75DPPC0.25, but not for DODA0.5DPPC0.5. For all lipid mixtures, wide angle diffraction
peaks show a homogeneous ordered phase of the alkyl chains. With decreasing surface charge, the
separation between the polymer chains increases (cf. Fig. 2). However, the ratio between charged
monomers and charged lipids is fairly constant (i.e. the chain separation increases from 25 to 31 Å,
when the area per charge of the monolayer surface increases from 48 to 59 Å2). Concluding, the
electrostatic interaction between PSS and the various monolayers determines the chain separation
between aligned PSS chains, provided the surface charge is large enough.
In the next step, highly charged DODA monolayers were used, and the line charge density of the
polyelectrolytes is varied. The two-dimensional lamellar phase is observed for the highly charged
PSS [2] and for P-TrisAA0.1-rand-AMPS0.9 with the same chain separations, both beneath lipid
monolayers in the ordered and in the fluid phase. With P-TrisAA0.5-rand-AMPS0.5 the line charge
density is decreased further. Then the two-dimensional lamellar phase can only be observed beneath
lipids in the fluid phase and the chain separation is decreased (the chain separation decreases from
47 to 31 Å, when the average distance between charges is increased from 2.5 to 5.1 Å). When the
lipids are in the solid phase, X-ray reflectivity shows a flatly adsorbed polyelectrolyte layer with a
large surface coverage. Probably the polymer chains touch and short-ranged interactions dominate.
The ratio between charged monomers and charged lipids is largest for PSS and decreases the lower
the line charge is. Apparently, non-electrostatic interactions also determine the surface coverage.
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DODA0.75DPPC0.25
DODA0.9DPPC0.1
DODA
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PSS
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Area per Lipid molecule [Å ]
Figure 2: Top: The separation between polyelectrolyte chains, dPE, deduced from the GID measurements, as
function of the lipid molecular area determined from the isotherms; centre: the ratio between monomers to
lipid molecules, nPE/nLipid; bottom: the ratio between charged monomers and charged lipids, chPE/chLipid. Left:
PSS adsorbed to mixed monolayers with different surface charge. Right: PSS and statistical copolymers
with different line charge density adsorbed to DODA.
Concluding, we observed the two-dimensional lamellar phase of negatively charged
polyelectrolytes adsorbed to positively charged surfaces. We find that a strong electrostatic force is
a necessary condition for the two-dimensional lamellar phase, but non-electrostatic forces are also
important. The chain separation can be either increased (by decreasing the surface charge) or
decreased (by decreasing the line charge density).
References
[1] R.R. Netz and J.F. Joanny, Macromolecules 32, 9013 (1999)
[2] J.-U. Günther, H. Ahrens, C.A. Helm, Langmuir 25, 1500 (2009)
[3] G. Brezesinski, H. Möhwald, Adv. in Coll. and Int. Sci 100-102, 563 (2003).

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