Nonstoichiometric polymer-surfactant complexes obtained in a lamellar lyotropic medium

Composition and structural transitions of polyelectrolyte-surfactant complexes in the presence of fatty acid studied by NMR and cryo-SEM

Insoluble complexes formed when a cationic polyelectrolyte is neutralized by the oppositely charged surfactant sodium dodecylethersulfate (SDES) in the presence and absence of lauric acid (LA) have been examined directly using NMR spectroscopy and cryo-SEM. Below the SDES critical micelle concentration (CMC') the insoluble complex contains about 10 times more water than just above CMC'. This is related to a structural transition of the complex, where water is contained mainly in larger compartments below CMC' and then mainly in narrower compartments above CMC'. The structure of the complex's solid matrix was monitored by recording two-dimensional T2-diffusion correlation spectra of the water proton resonance, which reveal the presence of several different water environments which correspond to different complex structures. Structural features in the micrometer range were confirmed using cryo-SEM. When LA is present, the larger water compartments seen below CMC' are to some extent present in the entire SDES concentration range, which is not the case in the absence of LA. Furthermore, the inclusion of LA into the SDES aggregates above CMC' leads to a lamellar sheetlike organization of the polyelectrolyte-stabilized surfactant phase. In the absence of LA, a stringy network of fibers is seen in cryo-SEM images, indicating a spherical or rodlike SDES phase. Consequently, the complex without LA holds about 1.7-1.9 times more water than the complex with LA above the SDES CMC'. T1 relaxation, (13)C chemical shifts, and (1)H resonance line widths of SDES in the system support the above observations. The combination of MAS NMR, T2-diffusion correlation, and cryo-SEM proved to be an effective method for studying structural transitions in the surfactant-polyelectrolyte(-LA) insoluble complexes.

Copolymer-surfactant complexes obtained in a lamellar lyotropic medium

Polymer-surfactant complexes formed between charged copolymers and oppositely charged surfactants are analyzed as a function of the charge density in the macromolecule. Copolymers of ionizable diallyldimethylammonium chloride (DADMAC) and neutral acrylamide are obtained at different comonomer ratios. When mixed with the lamellar medium formed by the anionic surfactant 1,4-bis(2-ethylhexyl)sodium sulfosuccinate (AOT) in water, they give rise to highly condensed lamellar phases in equilibrium with another lyotropic phase. The structure of these phases is studied by SAXS and optical microscopy revealing the formation of copolymer-surfactant complexes which present a lamellar structure. The composition of the phases is inaccessible to direct determination, because they do not separate macroscopically (in most of the samples). Thus, the stoichiometry is determined using a model which considers the charge density of the copolymers. This model allows, from the experimental data provided by SAXS, to calculate the composition and volume ratio of the phases. The results indicate that these complexes are nonstoichiometric, containing a lesser amount of DADMAC than surfactant units. The neutral sequences of acrylamide can be considered as bridges along the water domains remaining anchored to the AOT bilayers by the cationic DADMAC units. When the charge density diminishes, the bridges become longer, rendering structures with higher water content.