Lipid Monolayers and Adsorbed Polyelectrolytes with Different Degrees of Polymerization

Abstract

Polystyrene sulfonate (PSS) of different molecular weight M(w) is adsorbed to oppositely charged DODAB monolayers from dilute solutions (0.01 mmol/L). PSS adsorbs flatly in a lamellar manner, as is shown by X-ray reflectivity and grazing incidence diffraction (exception: PSS with M(w) below 7 kDa adsorbs flatly disordered to the liquid expanded phase). The surface coverage and the separation of the PSS chains are independent of PSS M(w). On monolayer compression, the surface charge density increases by a factor of 2, and the separation of the PSS chains decreases by the same factor. Isotherms show that on increase of PSS M(w) the transition pressure of the LE/LC (liquid expanded/liquid condensed) phase transition decreases. When the contour length exceeds the persistence length (21 nm), the transition pressure is low and constant. For low-M(w) PSS (<7 kDa) the LE/LC transition of the lipids and the disordered/ordered transition of adsorbed PSS occur simultaneously, leading to a maximum in the contour length dependence of the transition enthalpy. These findings show that lipid monolayers at the air/water interface are a suitable model substrate with adjustable surface charge density to study the equilibrium conformation of adsorbed polyelectrolytes as well as their interactions with a model membrane.

Lipid Monolayers with Adsorbed Oppositely Charged Polyelectrolytes: Influence of Reduced Charge Densities

“…On monolayer compression the low-angle peak shifts to larger Q xy values (from 0.12 to 0.16 Å −1 ), indicating a decrease of the interchain separation d PE . In the coexistence region, the low-angle peak observed in the fluid phase exhibits a constant position [14]. Simultaneously, at larger Q xy -values a peak with an unusual structured rod in Q z -direction appears (two peaks at the same Q xy and different Q z positions).…”

“…The lipids were dioctadecyldimethylammonium (DODA) with a positively charged head group [14]. PSS is a strong polyanion, therefore, in these experiments, the electrostatic force is very large [5,17,18].…”

“…π c decreases on increase of the molecular weight, until the contour length corresponds to the persistence length, then π c is independent of the molecular weight. For polystyrene sulfonate (PSS) a persistence length of L P = 210 Å was found (molecular weight 16 kDa) [14] which is an order of magnitude larger than the persistence length of neutral polymers (L P,steric = 30 Å) [23]. It suggests an effective ion concentration at the monolayer/polyelectrolyte interface of 10 mmol/L, even though the ion concentration in solution is two to three orders of magnitude smaller (≈0.01 mmol/L).…”

“…We use PEs with large molecular weight, to actually adsorb a flexible polymer, and not an elastic rod. The PE concentration is low and leads to PE adsorption in a lamellar manner, 0.01 mmol/L with respect to the monomer concentration [12][13][14]. We use GID to measure the surface coverage of the PEs [24] due to its high resolution.…”

“…We think the surface coverage is correct within a few percent, since we see that film-to-film variation and film history (first or second compression, expansion, etc.) influence the position of the Bragg peaks [14], but we never explored these effects in detail.…”