Investigation of liquid crystal systems and emulsions by means of nuclear magnetic resonance
Investigation of lipid – water systems by NMR1,2,3. The partial phase diagram and the hydration properties of the 1 – palmitoyl – 2 – oleoyl – phosphatidylethanolamine (POPE) – water system, in the absence and presence of 30 mol % cholesterol, have been investigated by solid state phosphorus NMR of the lipid and deuterium NMR of heavy water. The POPE – D2O phase diagram resembles other phosphatidylethanolamine (PE) – water systems: - below water-to-lipid ratios (Ri) of 3 the lamellar gel (Lb or Lc)-to-hexagonal type II (HII) phase sequence is observed on increasing the temperature. For Ri³3 the thermotropic sequence (Lb or Lc)-La-HII is detected. On increasing hydration from RI=3, the HII phase is detected from 40°C to 85°C whereas the gel phase is observed from 30°C to 40°C. The limiting hydrations of the gel, La and HII phases are R » 3, 17 and 20, respectively. The number of bound water molecules is ca. 8 in both La and HII phases. The presence of cholesterol stabilizes the hexagonal phase 20°C below temperature at which it is observed in its absence and reduces the limiting hydration of the fluid and hexagonal phases to RI » 9 and 14, respectively. The structure and/or dynamics of the water bound to the interface are markedly modified on going from La to the HII phase.
Investigation of lipid – surfactant - water systems by NMR3. The polymorphism of a ternary system lecithin – alkanphosphondiethylesteric acid (PAE6) – heavy water was studied by phosphorus and deuterium NMR. Three phases were identified by means of the characteristic shapes of the 31P and 2H absorption spectra. A lamellar phase exists at a low concentration of the admixture PAE6 (under 10% by weight). With the increase of the concentration of the hydrophobic admixture (20 – 30% by weight), a hexagonal phase appears. At a still higher PAE6 concentration a new phase transition from hexagonal to isotropic phase is observed. The phosphorus spectra are single lines and the sample is optical isotropic. We develop a model for explanation of such properties of the liquid crystal systems3,5. The model consider the so-called “elementary molecular complex” (EMC) which consists of one main amphiphilic molecule, water molecules and hydrophobic admixtures. The model is based on the assumption that the different phases require different shapes of the EMC. Within the framework of the model we describe three phases – lamellar liquid crystal, inverted hexagonal and inverted micellar or some cubic one. The phase transition sequence lamellar – inverted hexagonal – inverted micellar phase can be realized at increasing the temperature or decreasing the water concentration. The theoretical predictions of the phase transition temperatures are in good agreement with the experimental data3,5.
Nuclear magnetic resonance apparatus for measuring the spin-lattice relaxation time T13,6. T1 gives information about the intramolecular and supramolecular motions of the studied systems. The simplest methods ofor measurement of T1 are based on the adiabatic fast passage through resonance (method of Chiarotti et al.7). We constructed an NMR apparatus based on a autogenerator scheme for measurment of T1 by the method of Chiarotti et al. By means of this apparatus we studied the system Triton X-114 – water at high surfactant concentrations (55 and 70 weight %). The observed changes of T1 can be explained as follows:
- At lower Triton concentration the system is in normal micellar phase. The surfactant forms micelles, swimming in a continuous water medium. The observed temperature dependence of T1 can be explained with the change of the water viscosity.
- At the high (» 70%) Triton’s concentration an inverted micellar phase is formed. The hydrocarbon chains form a continuous medium. The changes of the viscosity of this hydrocarbon medium lead to the observed relaxation time changes.
Application of low resolution NMR for determination of drop-size distribution of oil-in-water emulsions. Recently developed method8 was applied to determine the oil droplets size distribution in commercial mayonaise. NMR measurements were performed on a Bruker Minispec mq-20 with “oil droplets” software application. Optical microscopy was used as a referent method. The result showed that:
- NMR mean drop sizes are ~ 20% lower that microscopy data;
- Width of distribution is about ~ 7% lower;
Typical size distribution are shown on the figure bellow.
Figure 1 Comparison of the data from NMR and optical microscopy for two mayonnaise samples (MK1 and MK2)
References:
1. R. Marinov, E. J. Dufourc, J. Chim. Phys.
92 (1995) 1727.
2. R. Marinov, E. J. Dufourc, Eur. Biophys. J. 24 (1996) 423.
3. R. Marinov, Investigation of liquid crystal systems by means of NMR, PhD
thesis, 1996, ISSP, Sofia
4. A. Zheliaskova, R. Marinov, A. Derzhanski, J. Mol. Struct. 513 (1999) 9.
5. R. Marinov, A. Zheliaskova, Derzhanski, J. Disp. Sci. Tech. 17(6) (1996)
591.
6. R. Marinov, K. Shoumarov, A. Derzhanski, Bulg. J. Phys. 25 (1998) 161.
7. G. Chiarotti, G. Cristiani, L. Giulloto, G. Lauzi, Nuovo Cim. 4 (1954) 519.
8. G. J. W. Goudappel, J. P. M. van Duynhoven, M. M. W. Mooren, J. Colloid
Interface Sci. 239 (2001) 535.