Influence of Different Solutions on Electrically Conductive Films Composed of Carbon Nanotubes and Polydimethyldiallylammonium

For engineering and biomedical applications, nanometer-thin films with high electrical conductivity in aqueous solutions are desirable. Multilayers of polydimethyldiallylammonium chloride (PDADMA) and oxidized carbon nanotubes (CNTs) were built using the layer-by-layer technique. CNTs with a low linear charge density were used. The surface coverage of the CNTs was monitored with optical absorption. The film thickness and the surface coverage of the CNTs increased linearly with the number of CNT/PDADMA bilayers deposited. On immersion into aqueous solutions, the film thickness decreased or remained constant. This finding is attributed to the hydrophobic character of the CNTs and the backbone of PDADMA. The films showed ohmic behavior, both in air and in solutions. The electrical conductivity was 0.95 × 104 S/m in air and increased to 1.36 × 104 S/m in solution, provided the thickness of the CNT/PDADMA bilayers was as low as 1.9 nm. We suggest that high electrical conductivity can be achieved by flat adsorption of the CNTs

Lipid Monolayers and Adsorbed Polyelectrolytes with Different Degrees of Polymerization

Polystyrene sulfonate (PSS) of different molecular weight <i>M</i>_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 <i>M</i>_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 <i>M</i>_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 <i>M</i>_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-<i>M</i>_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