NMR self-diffusion measurements

Micelles are the simplest organised form of the self-assembly produced by amphiphilic molecules due to the so-called hydrophobic effect , firstly recognized by Tanford.NMR parameters experience dramatic effects as a result of the strong intermolecular interactions among the amphiphiles. In the case of isotropic liquid systems, NMR experiments can be easily performed and modelled, since many advances have been produced in the last two decades.Hence, information on critical micelle concentration (c.m.c.), molecular conformations and interactions, counterion binding, hydration can be obtained from chemical shifts, relaxation, and self-diffusion NMR measurements, also in mixed systems. 

Since it was proposed in the early 1980s [6, 7], spin-relaxation has been extensively used to determine the surface-to-volume ratio of porous materials [8-10]. Pore structure has been probed by the effect on the diffusion coefficient [11, 12] and the diffusion propagator [13,14], Self-diffusion coefficient measurements as a function of diffusion time provide surface-to-volume ratio information for the early times, and tortuosity for the long times. Recent techniques of two-dimensional NMR of relaxation and diffusion [15-21] have proven particularly interesting for several applications. The development of portable NMR sensors (e.g., NMR logging devices [22] and NMR-MOUSE [23]) and novel concepts for ex situ NMR [24, 25] demonstrate the potential to extend the NMR technology to a broad application of field material testing. 

The self-diffusion coefficient measurements were made using the pulsed-gradient spin-echo (PGSE) method which has recently been reviewed (JLL). For a nucleus in an isotropic solution, the PGSE-NMR normalized signal intensity, 1/1q is given by ... 

The relationship between the transport diffusivity (D), as measured under non-equilibrium conditions in an uptake experiment and the tracer self diffusivity (Ds), measured under equilibrium conditions in an NMR experiment, has been discussed by Ash and Barrer(30) and Karger(31,32)t who show that... 

The NMR self-diffusion coefficient measurements also provide a convenient tool for the study of the micellization process. They are based on the large... 

Figure 8 shows the self-diffusion coefficients, Db, of methane, ethane and propane as a function of soibate loading at various temperatures. These corrected diffusion coefficients were calculated from Equation 12 using diffusion coefficients obtained fi m both sorption and desorption half-cycles. These diffusion coefficients were identical. The corrected, Db, diffusion coefficients in Figure 8 are <5 smaller than the intracrystalline self-diffusion coefficients measured directly by NMR which are also included in this figure and are in close agreement with the corrected diffusion coefficients obriuned by the full FR method. 

Figure 8 Corrected diffiisivUies of methane, ethane and propane at various temp tures and sorbate concentrations as determined by single-step FR method (open symbols) and self-diffusion coefficients measured by the NMR pulsed-field gradient technique (filled symbols).

Most books related to analysis and characterization are divided into chapters on different techniques, such as Fluorescence or Self-diffusion NMR , i.e. the division is by method. By contrast, the division here is by problem. As an example, when the reader wants to find out how to best measure micelle size he (or she) does not need to know from the beginning which methods to consider. The reader can go directly to Chapter 38, Measuring Micelle Shape and Size, where the relevant information is collected. 

Figure 7.1.4 shows the effect of concentration of polystyrene on mutual and self-diffusion coefficients measured by pulsed-gradient spin-echo NMR. The data show that the two coefficients approach each other at high concentrations of polymer as predicted by theory. ... 

Self-diffusion measurements using the NMR technique have also elucidated the matter of water structure in the pool up to 10-15 water molecules per — 03 group (when AOT is the surfactant) were shown to be structurally perturbed [42]. From self-diffusion NMR studies of AOT-induced w/o microemulsions, Maitra proposed several regions of bound water in the water pool domain [43]. The state of water in the pool for cationic surfactant-containing microemulsions (water/dodecyidimethyl ammonium bromide or didodecyldimethyl ammonium bromide/chloroform or cyclohexane) was examined by NMR self-diffusion studies at different [Fl20]/[amphiphile] ratios or rvalues. Three types of water molecules— bound, aggregated, and free—in various proportions have been envisaged [44]. 

In addition to combining self-diffusion coefficient measurements with impedance analysis to determine ionicity as describe above, a novel method has been developed to determine the fraction ofions in ILs exclusively from self-diffusion coefficients obtained using PFG-NMR [18]. EquiHbrium constants of the ionization process from measured ion self-diffusion coefficients were calculated using this method. Enthalpy and entropy changes of ionization and ion self-diffusion processes have been obtained for a series of ionic liquids using this method. 

Unlike most other characterization techniques introduced in this chapter that only allow conclusions to be drawn on the bulk level, self-diffusion NMR gives detailed information on colloidal systems by focussing on the molecular level. This complex technique will therefore be introduced in detail. NMR spectroscopy, based on physical properties of the molecular spin, is a very powerful method for the measurement of self-diffusion of small molecules in complex solution [67] with direct insight into general aspects of the solution structure [68]. 

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