Modified data handling for rapid low‐field nuclear magnetic resonance characterization of lyotropic liquid crystal composites

Abstract

A refined nuclear magnetic resonance (NMR) technique was developed to determine the relative volumetric proportions of various phases present in lyotropic liquid crystal (LLC) composites such as soap/detergent bars. Conventionally, the FID (free induction decay, referred to hereafter as Method 1) technique is used for analysis of these systems. This technique is suitable only for composites containing a high concentration of solids and liquid crystals. Method 1 gives erroneous results for composites with a high proportion (>10%) of isotropic liquid phase (L 1 ). The procedure currently practiced (Method 2) for analyzing systems containing >10% L 1 entails a considerable amount of experimental and analysis time and involves subjectivity in data analysis. Typically, the phase characterization of the composite using conventional low-field NMR techniques takes more than 1 h. Furthermore, the hardware and data acquisition features of currently available conventional low-field NMR spectrometers are inadequate for accurate estimation of relative phase volumes in LLC composites. We developed a modified data-handling technique (FID-Carr-PurcellMeiboom-Gill technique, or FIDCPMG technique) which enabled rapid phase characterization of LLC composites and minimized subjectivity while analyzing the data. The standard design of a conventional low-field NMR spectrometer was upgraded by incorporating a high-power transmitter and a fast digitizer. The phase composition of four model LLC composites (with L 1 percentages varying from 7 to 90%) was determined using FIDCPMG technique and was compared with results from conventional techniques. Phase composition of the LLC composite could be determined in less than 5 min.A lyotropic liquid crystal (LLC) composite, such as a soap, comprises several phases such as solid, isotropic liquid (L 1 ), and LLC, e.g., lamellar (L α ) and hexagonal (H 1 ) phases. The relative volumes of various phases in LLC composites depend on chemical composition and processing history. Knowledge of the phase composition is of utmost importance in understanding the structure-property correlations in LLC composites.A considerable amount of work has been reported on applying nuclear magnetic resonance (NMR) principles to understand the structure and relaxation behavior of pure LLC phases (also referred to as mesophases) formed by surfactant-water mixtures (1-6). Low-field (or wideline) NMR techniques have been developed for quick estimation of solid-liquid ratios (7-10) and for quantitative observation of protons in different chemical environments (11).Typical spin-spin relaxation time constants (T 2 ) for most common phases encountered in surfactant systems include the following (2,6,11), where each hydrocarbon chain phase is paired with the T 2 range (in µs): solid crystal, 12-15; gel (bilayer), 20-30; L α (lamellar), 50-250; H 1 (hexagonal), 170-300; cubic, 1000; and water, ~10 5 . Thus, the time constant is in the range 12-15 µs for solids, a few hundred milliseconds for L 1 , and some intermediate value for...

Determination of MG and TG phase composition by time‐domain NMR

“…In the detergent area, several applications have been described that show how the phase behavior of surfactants can be assessed by curve fitting of the time-domain data (9). So far, only a few examples are known where the phase behavior of lipids has been assessed (10,11).…”

“…Commonly, one observes a signal in which the Lorentzian signal is multiplied by Gaussian field inhomogeneity functions (10,11). The field inhomogeneity can be removed, however, by application of a Carr-Purcell-Meiboom-Gill pulse train, thus producing pure Lorentzian decays again (9). Gaussian functions are characteristic for solid phases that still have molecular mobility, such as semisolid ("amorphous") phases, liquid crystals, or crystal phases with high degrees of mobility (e.g., α-polymorphs).…”