Piezochromic Effect and Orientational Order in Monolayers and LB Multilayers of Poly(p-phenylenesulfonate)− Dioctadecyldimethylammonium Bromide Complexes

Abstract

By spreading dioctadecyldimethylammonium bromide (DODA) on a subphase containing rigid-rod-like poly(p-phenylenesulfonate)s (PPPS), polyelectrolyte complexes are formed in situ at the air-water interface. These complex monolayers are investigated by recording surface pressure/area isotherms, UV/vis spectroscopy, and, after transfer onto a solid substrate, X-ray photoelectron spectroscopy (XPS) and X-ray reflectivity (XR). XPS and XR measurements indicate that although the polyelectrolytes are aggregated in aqueous solution, the complexes are stoichiometric. The isotherms show that the area per amphiphile in the complex monolayer is significantly higher than in the monolayer of pure DODA. This is due to the distance of the ionic sites along the rigid-rod-like polymer chain and results in a more tilted arrangement of the alkyl chains as confirmed by the XR measurements. UV/vis spectroscopy reveals that the A-band of the PPPS is red-shifted in the complex, and a further red shift of the A-band upon compression of the complex monolayer is observed. This two-dimensional piezochromic effect is discussed to be a result of the electronic interaction of the π-systems, which improves with decreasing distance during compression. Upon transfer to solid substrates the rigid-rod-like polyelectrolytes are oriented to be parallel to the dipping direction as evidenced by polarized UV/vis spectroscopy. The degree of order in the LB film shows a linear dependence on the axial ratio of the rigid-rod-like PPPS.

Adsorption of anionic polyelectrolytes to dioctadecyldimethylammonium bromide monolayers

“…The isotherms for the complexes of DODA with PPPS are shown in Figure 8. The area per amphiphilic molecule is increased compared to the amphiphile spread on pure water as has been discussed previously and the differences for the lowest degree of polymerization have been attributed to be due to endgroup effects [23]. From the isotherms a change of the surface pressure upon transfer of the DODA monolayer from a pure water subphase to a PPPS solutions can be deduced (Fig.…”

“…The DODA monolayers are stable and suitable for the transfer according to the Fromherz method in both regions [12,14,16]. The area requirements for DODA on pure water are given by the coulomb interactions of the ionic head groups and the packing of the alkyl chains [23].…”

“…Poly(sodium-pstyrenesulfonate) (PSS) (M w about 100 000 g/mol, Acros) was purified by dialysis. Poly(phenylene sulfonate)s PPPS (P n of 4, 36, and 54) were synthesized and characterized as described previously [21][22][23].…”

“…The area per amphiphile is larger for the DODA/CMC complex than for the pure DODA monolayer. As has been discussed before, the different area requirements can be attributed to the differences in the distance of the ionic sites along the polymer backbone of CMC and PSS, respectively [7,23]. For the adsorption experiments the monolayer was compressed to an area of A = 0.92 nm 2 per amphiphile, which corresponds to a surface pressure of Π Start = 10 mN/m.…”

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

“…Simultaneously, the surface charge density of the monolayer increases by a factor of two. The adsorbed PEs influence the interactions between the lipid molecules, which can best be investigated by exploring the LE/LC phase transition [18][19][20] PEs in salt-free solution have a much larger persistence length than neutral polymers [21]. If the contour length L K is smaller than the persistence length L P (L K  L P ) then the polymer is approximated as an elastic rod.…”