Intermolecular associations between a cationic lipid and two model polymers were evaluated from preparation and characterization of hybrid thin films cast on silicon wafers. The novel materials were prepared by spin-coating of a chloroformic solution of lipid and polymer on silicon wafer. Polymers tested for miscibility with the cationic lipid dioctadecyldimethylammonium bromide (DODAB) were polystyrene (PS) and poly(methyl methacrylate) (PMMA). The films thus obtained were characterized by ellipsometry, wettability, optical and atomic force microscopy, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and activity against Escherichia coli. Whereas intermolecular ion-dipole interactions were available for the PMMA-DODAB interacting pair producing smooth PMMA-DODAB films, the absence of such interactions for PS-DODAB films caused lipid segregation, poor film stability (detachment from the silicon wafer) and large rugosity. In addition, the well-established but still remarkable antimicrobial DODAB properties were transferred to the novel hybrid PMMA/DODAB coating, which is demonstrated to be highly effective against E. coli.

Hybrid films from poly (methylmethacrylate) (PMMA) and dioctadecyldimethylammonium bromide (DODAB), cetyltrimethylammonium bromide (CTAB), or tetrapropylammonium bromide (TPAB) were characterized by determination of wettability, ellipsometry, atomic force microscopy, active compounds diffusion to water, X-ray photoelectron spectroscopy (XPS) with determination of atomic composition on the films surface, and biocidal activity against Pseudomonas aeruginosa or Staphylococcus aureus. QAC mobility in the films increased from DODAB to CTAB to TPAB. Diffusion and optimal hydrophobic-hydrophilic balance imparted the highest bioactivity to CTAB. DODAB sustained immobilization at the film surface killed bacteria upon contact. TPAB ability to diffuse was useless because of its unfavorable hydrophobic-hydrophilic balance for bioactivity.

The interaction between dioctadecyldimethylammonium bromide (DODAB), chloride (DODAC), or acetate (DODAAc) cationic bilayer fragments (BF) and polystyrene sulfate (PSS) particles (2 × 1011 particles/mL) was evaluated at and above equivalence of total surface areas for bilayers and particles (A b/A p) over a range of low NaCl and lipid concentrations, by means of zeta-potential, ζ, and particle diameter, D z , analysis. In pure water, lipid addition at and above A b/A p = 1 rapidly increased D z and ζ, which reached stable plateau values above those expected at bilayer coverage. By addition of small NaCl concentrations (0.05−5.00 mM), D z decreased with ζ still positive. Therefore, the effect of low salt concentration on the particle coverage with lipid bilayer fragments was surface rearrangement of bilayer fragments on the surface from uneven coverage with some bilayer fragments adsorbed vertically, to perfect bilayer coverage upon addition of small amounts of NaCl. This occurred over a broad range of lipid concentration (0.04−1.0 mM DODA), possibly due to NaCl-induced sealing of adjacent bilayer patches adsorbed on particles. At 0.2 mM lipid, the effect of salt, counterion, and bilayer type (BF or large vesicles, LV) on surface tension at the air−water interface (γ) complemented the evaluation of bilayer behavior at a hydrophobic interface. The γ decay rate and total extent increased with counterion size, NaCl concentration, and frequency of hydrophobic defects in the lipid bilayer. Upon addition of 2 mM NaCl to 0.2 mM DODAB BF or LV, γ decay was much faster for BF, showing the importance of hydrophobic defects in the bilayer to induce its fusion to the air−water interface.

Determinations of surface tension (γ) at the air−water interface, contact angles (θ), and in and ex situ ellipsometric mean thickness (d) were used to study the interaction between dioctadecyldimethylammonium bromide (DODAB) small vesicles and spin-coated polystyrene sulfate (PSS) films on silicon wafers. Upon the addition of NaCl (50 mM final concentration) to a 0.2 mM DODAB dispersion, adsorption from vesicles on PSS films immediatly yielded a DODAB layer 6.0 nm thick which remained stable as a function of time. However, in water, in situ DODAB adsorption monotonically increased reaching at most 1.6−1.8 nm as a function of time (from 15 min of interaction), which were not values consistent with bilayer deposition. At early stages in pure water, DODAB adsorption linearly increased with the square root of time, indicating a vesicle diffusion controlled process with ca. 1.0 × 10-11 m2 s-1 as the vesicle diffusion coeficient (D) in nice agreement with reported D for similar vesicles. In contrast, adding 50 mM salt, resulted in a very fast adsorption kinetics determined by the hydrophobic attraction between salt-induced defects on the bilayer and the film surface. Ex situ measurements of DODAB adsorption were difficult because wetting/drying cycles of the PSS film increased its mean thickness. From 0.1 up to 1.0 mM DODAB, the adsorbed DODAB film in air was more hydrophobic (advancing contact angle, θa, = 84 ±3°) than the bare PSS film (θa = 71 ± 4). Air−water surface tension for a DODAB dispersion in pure water rapidly decreased upon salt addition (7−50 mM NaCl), suggesting salt-induced vesicle fusion with the air−water interface occurred, in nice agreement with salt-induced vesicle fusion at the hydrophobic polymer−water surface.

The effect of monovalent salt nature and concentration over a range of low ionic strengths (0-10 mM LiCl, NaCl, KCl, or CsCl) and at two different pH values (6.3 and 10.0) on adsorption of dioctadecyldimethylammonium bromide (DODAB) bilayer fragments (BF) onto flat SiO(2) surfaces was systematically evaluated by means of in situ ellipsometry. High-affinity adsorption isotherms fitted by the Langmuir model indicated that adsorption maxima were consistent with bilayer deposition only around 10 mM monovalent salt at both pH values. In pure water, the mean thickness of the DODAB adsorbed layer was close to zero with bilayer deposition taking place only around 10 mM ionic strength. In the presence of 10 mM CsCl or LiCl, the highest and the lowest affinity constants for DODAB adsorption onto SiO(2) were, respectively, obtained consistently with the expected facility of cation exchange at the surface required for DODAB adsorption. The cation more tightly bound to the solid surface should be Li(+), which would present the largest resistance to displacement by the DODAB cation, whereas the less tightly bound cation should be Cs(+) due to its largest ionic radius and lowest charge density. In other words, DODAB adsorption proceeds in accordance with charge density on the solid surface, which depends on the nature and concentration of bound counterions as well as DODAB cation ability to displace them. AFM images show a very smooth DODAB film adsorbed onto the surface in situ with a large frequency of BF auto-association from their edges. The present results for flat surfaces entirely agree with previous data from our group for DODAB adsorption onto silica particles.