Interaction of nitrates with pluronic micelles and their role in the phase formation of mesoporous materials

The addition of NaNO3 to the reaction mixture of hexagonal SBA-15 changes the product to a Ia3d cubic phase. To understand the specific action of the salt, electron-spin echo envelope modulation (ESEEM) spectroscopy was applied. It allowed us to follow the local concentration of the individual ions within the micelles through their dipolar interaction with spin probes introduced into different regions of the Pluronic P123 (PEO20PPO70PEO20) micelles. The N-15 and Na-23 modulation depth probed the local concentration profile of the anions and cations, and the water content in these regions was followed through the modulation depth of H-2 in D2O solutions. The ions were found to penetrate the hydrophilic corona region (the polyethylene oxide, PEO) of the micelles, concomitant with penetration of water molecules, reaching saturation at a bulk NaNO3 concentration of 0.2 M. In acidic micellar solutions, protons repel the Na+ ions from the corona and for the same total [NO3-] the acidity increases the NO3- capacity of the corona. However, a general decrease is noted in the nitrate and the water content of the corona with an increasing salt concentration in the bulk solution. This effective dehydration of the EO groups decreases the curvature of the micellar assembly of the Pluronic, and leads to the formation of the final cubic phase rather than the hexagonal phase. Time-resolved freeze quench ESEEM measurements, carried out on the reaction mixtures of the hexagonal and cubic phases, show that in both cases the nitrate concentration within the corona is reduced with time due to depletion of the protons through exchange with positively charged silica precursors. These results show that ESEEM measurements on Pluronic-like spin probes offer a new molecular level, quantitative method to observe the variations in location and amounts of the cations and anions within micelles.