Structural behavior and interactions of dendrimer within lyotropic liquid crystals, monitored by EPR spectroscopy and rheology

Micro- and macrostructural behaviors of three different lyotropic liquid crystals (LLCs) loaded with a dendrimer, namely second generation poly(propylene imine) (PPI-G2), were studied by means of rheology and electron paramagnetic resonance (EPR). The three mesophases were L_α, Q²²⁴, and H_II composed of glycerol monooleate (GMO) and water-PPI-G2 solution (and d-α-tocopherol (vitamin E) in the case of H_II). We characterized the impact of PPI-G2 interactions with the components of the host mesophases on their structural characteristics on different length scales. The incorporation of PPI-G2 within the L_α and H_II systems induced the formation of more elastic hexagonal systems with a "solidlike" behavior, while in the Q²²⁴ system a different trend with a "liquidlike" behavior was observed. As a result, the dendrimer induced a remarkable change in both the structural and viscoelastic properties of the systems. Hence, the microenvironment in the interface region within the systems was monitored by computer-aided EPR using 5-doxylstearic acid (5-DSA) as a pH-dependent probe. The microviscosity (τ) and order (S) of systems were found to be sensitive to the PPI-G2 presence: when PPI-G2 concentration increased, τ and S increased in both the L_α and Q²²⁴ systems. In the H_II systems two trends were observed, reflecting a decrease in τ and S up to 10 wt % PPI-G2 and subsequently their increase at higher dendrimer concentrations. It was assessed that PPI-G2 interacted strongly with the GMO hydroxyl groups in the L _α phase, with the water molecules in the Q²²⁴ systems. In the H_II mesophase strong interactions with both the water and GMO hydroxyl molecules were detected.