Nuclear magnetic spin-lattice relaxation experiments have been performed in partially filled porous glasses with wetting and nonwetting fluids. The frequency dependence of the spin-lattice relaxation rate in Vycor (4 nm pores) and VitraPOR #5 (1 μm pores) silica glasses was studied as a function of the filling degree with the aid of field-cycling NMR relaxometry. The species of primary interest were water (“polar”) and cyclohexane (“nonpolar”). Spin-lattice relaxation was examined in the frequency range from 1 kHz to 400 MHz with the aid of a field-cycling NMR relaxometer and an ordinary 400 MHz NMR spectrometer. Three different mobility states of the fluid molecules are distinguished: The adsorbed state at the pore walls, the bulklike liquid phase, and the vapor phase. The adsorbate spin-lattice relaxation rate is dominated by the “reorientation mediated by translational displacements” (RMTD) mechanism taking place at the adsorbate/matrix interface at frequencies low enough to neglect rotational diffusion of the molecules. The experimental data are analyzed in terms of molecular exchange between the different mobility states. Judged from the dependence of the spin-lattice relaxation rates on the filling degree, limits for slow and fast exchange (relative to the RMTD time scale) can be distinguished and identified. It is concluded that water always shows the features of slow exchange irrespective of the investigated pore sizes and filling degrees. This is in contrast to cyclohexane which is subject to slow exchange in micrometer pores, whereas fast exchange occurs in nanoscopic pores. The latter case implies that the vapor phase contributes to molecular dynamics in this case at low filling degrees while it is negligible otherwise.

Geopolymers are zeolites like structures based on hydrated aluminosilicates units of SiO₄ and AlO₄. These units, known as poly(sialate), poly(sialate)-siloxo or poly(sialate)-disiloxo are chemically balanced by the group I cations of K⁺, Li⁺, or Na⁺. Simultaneously, the chemical reaction of formation, known as geopolymerization, governs the orientation of the unit, generating mesoporous structures. Multiple methods can be used for pore structure and porosity characterization. Among them, nuclear magnetic resonance (NMR) relaxometry allows the detection of the porous structure in a completely nonperturbative manner. NMR relaxometry may be used to monitor the relaxation of protons belonging to the liquid molecules confined inside the porous structure and, thus, to get access to the pore size distribution. This monitoring can take place even during the polymerization process. The present study implements transverse relaxation measurements to monitor the influence introduced by the curing time on the residual liquid phase of geopolymers prepared with two different types of reinforcing particles. According to our results, the obtained geopolymers contain three types of pores formed by the arrangement of the OH⁻ and Si groups (Si-OH), Si-O-Si groups, Si-O-Al groups, and Si-O rings. After 48 days, the samples cured for 8 h show a high percentage of all three types of pores, however, by increasing the curing time and the percentage of reinforcing particle, the percent of pores decrease, especially, the gel pores.

Multiple spin echoes arise after “nonlinear” evolution of coherences in the presence of modulated demagnetizing fields. Such modulations can be prepared, for example, with the aid of a sequence of two 90° radio-frequency pulses in the presence of pulsed or steady field gradients. The echo amplitudes are sensitively attenuated by translational diffusion so that diffusivities can be determined on this basis. Homo- and heteronuclear variants of multiple-echo pulse sequences are considered here. A formalism based on the Bloch/Torrey equations is presented that describes the features displayed by the experimental data. The resulting attenuation formula for the homonuclear case generally accounts for all radio-frequency and field gradient pulse intervals occurring in the frame of this “pulsed gradient nonlinear spin echo” technique. Furthermore, an analogous formalism is reported for the heteronuclear case where the two nuclear species may populate different molecules with different diffusivities. It is shown that, apart from the conventional attenuation mechanism due to incomplete refocusing of the coherences, there are three additional processes contributing to homo- and heteronuclear multiple-echo attenuation by diffusion: Leveling of the magnetization helix and hence of the z magnetization grid formed by the second radiofrequency pulse, further leveling of that z magnetization grid by displacements of the dipoles producing the grid, and molecular displacements relative to the spatially modulated demagnetizing field.

Spin echo signal attenuation by diffusion is examined for coherence evolution in the course of ordinary pulsed gradient spin echoes and for nonlinear evolution in the presence of a spatially modulated demagnetizing field. It is shown, that, for given field gradient pulse widths (or equivalently for a given gradient strength), echo attenuation by diffusion is much more efficient for nonlinear echoes than for Hahn echoes. Remarkably, in the case of nonlinear echoes the refocusing process is spoiled by diffusion not only during the gradient intervals but also thereafter. The effect of displacements occurring in the gradient intervals is enhanced according to the order of the nonlinear echo the pulse sequence is adjusted for. A second attenuation mechanism takes place after the gradient pulses due to displacements in the presence of the spatially modulated demagnetizing field. This effect even occurs when the gradient intervals are too short to contribute. A complete formalism is presented describing all features of the test experiments. It is shown that nuclear magnetic resonance diffusometry based on nonlinear echo signals permits one to measure small diffusion coefficients with moderate field gradients. Nonlinear echo experiments demonstrate that the coherence pathway dominating by far is of a purely single-quantum nature.