Kinetic influence of siliceous reactions on structure formation of mesoporous silica formed via the co-structure directing agent route

We investigate the mechanism responsible for the formation of mesoporous silica formed with the so-called costructure directing agent (CSDA) route. The synthesis relies on the interaction between silica source (tetraethylorthosilicate), cationic surfactant (C18H37N+(CH3)2(CH2)3N+(CH3)3Br2), and CSDA (carboxyethylsilanetriol), which results in a material functionalized with carboxylic groups. Depending on the concentration of HCl in the synthesis, the structure is defined by Fm3¯m (at high pH) and by Fd3¯m (at low pH), with a gradual transition in the intermediate pH range. Here, we aim at finding the origin for the structural change triggered by pH and investigate the effects of the hydrolysis of the silica source on the overall kinetics of the synthesis. A fast process results in Fm3¯m, regardless of pH, and a slow process results in Fd3¯m. The hydrolysis step is the important structural control parameter. We studied the cross-linking of silica and CSDA using 29Si NMR. The cross-linking is similar for the two structures, and possibly the Fd3¯m structure contains slightly more CSDA. 13C PT ssNMR was used to investigate the surfactant mobility/rigidity during the synthesis. The rigidity of the Fm3¯m is established much faster than that of the Fd3¯m

Shape fluctuations and water diffusion in microemulsion droplets : A nuclear spin relaxation study

Water 2H and 17O spin relaxation data for the microemulsion phase in the AOT/D 2O/isooctane system are reported. The difference between the transverse (R 2) and the longitudinal (R 1) relaxation rates has been measured as function of droplet size, droplet volume fraction, temperature, and resonance frequency. The 2H longitudinal relaxation rate dispersion has been measured over an extensive frequency range, using the field-cycling technique. The focus in the study is on the contribution from slow molecular processes to the quadrupolar relaxation behavior. For the first time in any system, the theoretically predicted relation between the 17O/ 2H ratios of quadrupolar line splittings and of R 2 - R 1 is verified. The extensive experimental data are used to discriminate among three different dynamic models. It is found that water diffusion within the microemulsion droplet cannot account for the experimental data. Instead, a substantial shape polydispersity seems to be required, where at any instant a large fraction of the droplets are nonspherical. However, it is not necessary that the equilibrium shape is nonspherical

The state of water in non-ionic surfactant solutions and lyotropic phases : Oxygen-17 magnetic relaxation study

Water 17O longitudinal and transverse relaxation rates have been measured in aqueous solutions and mesophases of the non-ionic alkyl oligo(ethylene oxide) surfactants C12En (n = 4, 5, 8) over wide ranges of temperature and concentration. In addition, two reference systems have been investigated: high molecular weight poly(ethylene oxide) and the dimer 1,2-dimethoxy ethane. The relaxation data are consistent with the following picture of the state of water in the headgroup shell of C12En aggregates. The shell is compact, containing less than 5 and possibly as little as 2-3 water molecules per EO group. The shell exhibits substantial structural integrity; its water content is essentially invariant with respect to changes in concentration. The headgroup shell is highly dynamic, the rate of water rotation in the shell being reduced by at most a factor of 5 (at room temperature) as compared to bulk water

Nuclear spin quenching a new probe of exchange kinetics and droplet size in disperse systems

A new experimental technique, nuclear spin quenching (NSQ), is introduced, which can be used to measure droplet size and inter-droplet exchange rates of various intrinsic molecular species in disperse systems. The physical basis of the method is the quenching of the fine structure in spin-coupled N.M.R. spectra by intermolecular proton exchange, the rate of which is controlled by droplet exchange. The theoretical framework needed to interpret NSQ experiments is presented as well as an experimental case study on a water-in-oil microemulsion system. Being non-perturbing and having a large dynamic range, the NSQ technique should become a valuable complement to existing techniques for the study of structure and dynamics in disperse fluids

Water Dynamics in Microemulsion Droplets. A Nuclear Spin Relaxation Study

The state of water in aqueous microemulsion droplets in the system AOT/D2O/isooctane has been investigated by 2H and 17O NMR. Longitudinal relaxation rates are reported as a function of droplet size, droplet volume fraction, temperature, and resonance frequency. We conclude that the surface-induced perturbation of water rotation is of short range (limited to the primary hydration of the AOT head groups) and of modest magnitude (less than a factor of 10 slower rotation than in bulk water)

Hydration of ionic surfactant micelles from water oxygen-17 magnetic relaxation

Water oxygen-17 relaxation rates have been measured for aqueous solutions of micelles composed of ionic surfactants of varying alkyl chain length, head group, and counterion. The concentration of surfactant and salt was also varied. From these measurements, supplemented with relaxation data for short-chain molecules and with quadrupolar splittings from anisotropic mesophases, we derive structural and dynamic information about the molecular details of the water-micelle interaction. Water molecules at the micelle surface reorient anisotropically, typically 2-3 times slower than in pure water. The average lifetime for water molecules associated with sodium dodecyl sulfate micelles is 6-37 ns. The water-hydrocarbon contact in the micellar solutions is equivalent to less than two fully exposed methylene groups per amphiphile. Small head groups and small counterions produce the largest effects on the 17O relaxation rate, as expected from geometrical and electrostatic considerations

Rapid measurement of heteronuclear transverse relaxation rates using non-uniformly sampled R1ρ accordion experiments

Multidimensional, heteronuclear NMR relaxation methods are used extensively to characterize the dynamics of biological macromolecules. Acquisition of relaxation datasets on proteins typically requires significant measurement time, often several days. Accordion spectroscopy offers a powerful means to shorten relaxation rate measurements by encoding the “relaxation dimension” into the indirect evolution period in multidimensional experiments. Time savings can also be achieved by non-uniform sampling (NUS) of multidimensional NMR data, which is used increasingly to improve spectral resolution or increase sensitivity per unit time. However, NUS is not commonly implemented in relaxation experiments, because most reconstruction algorithms are inherently nonlinear, leading to problems when estimating signal intensities, relaxation rate constants and their error bounds. We have previously shown how to avoid these shortcomings by combining accordion spectroscopy with NUS, followed by data reconstruction using sparse exponential mode analysis, thereby achieving a dramatic decrease in the total length of longitudinal relaxation experiments. Here, we present the corresponding transverse relaxation experiment, taking into account the special considerations required for its successful implementation in the framework of the accordion-NUS approach. We attain the highest possible precision in the relaxation rate constants by optimizing the NUS scheme with respect to the Cramér-Rao lower bound of the variance of the estimated parameter, given the total number of sampling points and the spectrum-specific signal characteristics. The resulting accordion-NUS R1ρ relaxation experiment achieves comparable precision in the parameter estimates compared to conventional CPMG (Carr-Purcell-Meiboom-Gill) R2 or spin-lock R1ρ experiments while saving an order of magnitude in experiment time

Nuclear magnetic resonance relaxation in micelles : Deuterium relaxation at three field strengths of three positions on the alkyl chain of sodium dodecyl sulphate

A 2H n.m.r. relaxation study of sodium dodecyl sulphate (SDS) in the micellar phase is presented. The surfactant is specifically deuterated in the α-, γ- and ω-positions. Relaxation data are obtained at two temperatures and at three field strengths, and show a dependence on the field strength. The data are analysed with the 'two-step' model of relaxation, and the fast correlation times as well as the order parameters for the three deuterated positions on the hydrocarbon chain are presented. The order parameters obtained are very similar in magnitude to those found in the SDS/water hexagonal phase. In particular, the order parameter is very low (ca. 0.03) for the ω-methyl deuterons. In addition, a common slow correlation time is obtained from the data. The slow correlation time is discussed in terms of the micellar rotational tumbling process and surfactant lateral diffusion along the micellar surface, and a value for the diffusion coefficient of the latter process is obtained