Lamellar structures of anionic poly(amido amine) dendrimers with oppositely charged didodecyldimethylammonium bromide

The lamellar structures of anionic poly(amido amine) dendrimers possessing carboxylate terminal groups at half-generations G = n.5 (G2.5 and G4.5) with the cationic surfactant didodecyldimethylammonium bromide (DDAB) were investigated by small-angle X-ray scattering (SAXS) at 25 degreesC. The incorporation of dendrimers into the lamellar liquid crystals, at DDAB concentrations in the range of 4similar to30 wt %, results in the transition from a monophasic lamellar structure (L-alpha) to biphasic lamellar mixtures (L-alpha and L-alpha(D)), at similar 2[DDAB]/[COONa] ratios for the two dendrimers with the later and earlier generations (56 for the G4.5 dendrimer and 62 for the G2.5 dendrimer). The L-alpha structure consists of DDAB bilayers and dendrimer molecules doped into the water domains. The oppositely charged dendrimer preferably sticks to the DDAB bilayers, resulting in the decrease in the domain size of the wide spaced L-alpha structure with increasing the dendrimer concentration. In the narrow spaced L-alpha(D) structure, monolayers of collapsed dendrimers are adsorbed tightly between the DDAB bilayers. The thicknesses of the dendrimer monolayers are found to be 4.2 nm for the G4.5 dendrimer and 3.0 nm for the G2.5 dendrimer. These values are smaller than the diameters of the corresponding dendrimers found in aqueous solution. It is also deduced from Gaussian fits of SAXS patterns that the G4.5 dendrimer can form more ordered L-alpha(D) structures with domain sizes larger than those of the G2.5 dendrimer, due to the stronger electrostatic interaction between the cationic surfactants and the oppositely charged dendrimers with terminal groups located at shorter distances.