Interactions of PAMAM Dendrimers with SDS at the Solid–Liquid Interface

This work addresses structural and nonequilibrium effects of the interactions between well-defined cationic poly­(amidoamine) PAMAM dendrimers of generations 4 and 8 and the anionic surfactant sodium dodecyl sulfate (SDS) at the hydrophilic silica–water interface. Neutron reflectometry and quartz crystal microbalance with dissipation monitoring were used to reveal the adsorption from premixed dendrimer/surfactant solutions as well as sequential addition of the surfactant to preadsorbed layers of dendrimers. PAMAM dendrimers of both generations adsorb to hydrophilic silica as a compact monolayer, and the adsorption is irreversible upon rinsing with salt solution. SDS adsorbs on the dendrimer layer and at low bulk concentrations causes the expansion of the dendrimer layers on the surface. When the bulk concentration of SDS is increased, the surfactant layer consists of aggregates or bilayer-like structures. The adsorption of surfactant is reversible upon rinsing, but slight changes of the structure of the preadsorbed PAMAM monolayer were observed. The adsorption from premixed solutions close to charge neutrality results in thick multilayers, but the surface excess is lower when the bulk complexes have a net negative charge. A critical examination of the pathway of adsorption for the interactions of SDS with preadsorbed PAMAM monolayers and premixed PAMAM/SDS solutions with hydrophilic silica revealed that nonequilibrium effects are important only in the latter case, and the application of a thermodynamic model to such experimental data would be inappropriate

Multilayers at Interfaces of an Oppositely Charged Polyelectrolyte/Surfactant System Resulting from the Transport of Bulk Aggregates under Gravity

We show conclusively that multilayers at interfaces of an oppositely charged polyelectrolyte/surfactant system can result from the transport under gravity of bulk aggregates with internal molecular structure. This process was demonstrated by measurements of poly­(diallyldimethylammonium chloride)/sodium dodecyl sulfate solutions at the air/liquid and solid/liquid interfaces using neutron reflectometry. In the latter case a novel approach involving the comparison of reflection up versus down measurements provided key evidence. Interfacial multilayers indicated by a strong Bragg peak and clear off-specular scattering are exhibited under three conditions: (1) only for samples in the phase separation region, (2) only for fresh samples where a suspension of bulk aggregates remains in solution, and (3) only when the creaming or sedimentation process occurs in the direction of the interface under examination. This bulk transport mechanism is an alternative route of formation of interfacial multilayers to surface induced self-assembly. The two processes evidently give rise to interfaces with very different structural and rheological properties. Such directionality effects in the formation of nanostructured liquid interfaces may have implications for a broad range of soft matter and biophysical systems containing macromolecules such as synthetic polymers, proteins, or DNA

New Perspective on the Cliff Edge Peak in the Surface Tension of Oppositely Charged Polyelectrolyte/Surfactant Mixtures

We present how dramatically the nonequilibrium nature of an oppositely charged polyelectrolyte/surfactant mixture can affect the interfacial properties. We show for the first time that the cliff edge peak in the surface tension of the poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate system is produced as a direct result of depletion of surface-active material from the bulk solution due to a slow precipitation process in the phase separation region. Simple illustrations are given of how to control the production of the peak, to eliminate the feature for equivalent aged solutions through the use of different sample handling methods, and even to change its characteristics at short surface ages. The potential to tune nonequilibrium, steady-state interfacial properties for such strongly associating systems is clearly demonstrated. We propose that our findings in general may be applicable to a broad range of mixtures containing surfactants and oppositely charged macromolecules such as polymers, proteins, and DNA

Interactions of PAMAM Dendrimers with Negatively Charged Model Biomembranes

We have investigated the interactions between cationic poly­(amidoamine) (PAMAM) dendrimers of generation 4 (G4), a potential gene transfection vector, with net-anionic model biomembranes composed of different ratios of zwitterionic phosphocholine (PC) and anionic phospho-l-serine (PS) phospholipids. Two types of model membranes were used: solid-supported bilayers, prepared with lipids carrying palmitoyl-oleoyl (PO) and diphytanoyl (DPh) acyl chains, and free-standing bilayers, formed at the interface between two aqueous droplets in oil (droplet interface bilayers, DIBs) using the DPh-based lipids. G4 dendrimers were found to translocate through POPC:POPS bilayers deposited on silica surfaces. The charge density of the bilayer affects translocation, which is reduced when the ionic strength increases. This shows that the dendrimer–bilayer interactions are largely controlled by their electrostatic attraction. The structure of the solid-supported bilayers remains intact upon translocation of the dendrimer. However, the amount of lipids in the bilayer decreases and dendrimer/lipid aggregates are formed in bulk solution, which can be deposited on the interfacial layers upon dilution of the system with dendrimer-free solvent. Electrophysiology measurements on DIBs confirm that G4 dendrimers cross the lipid membranes containing PS, which then become more permeable to ions. The obtained results have implications for PAMAM dendrimers as delivery vehicles to cells