Christian Totland

1H NMR relaxation of water: a probe for surfactant adsorption on kaolin.

In this study, (1)H NMR is used to investigate properties of sodium dodecyl sulfate (SDS), tetradecyl trimethyl ammonium bromide (TTAB), and dodecyl trimethyl ammonium bromide (DTAB) adsorbed on kaolin by NMR T(1) and T(2) measurements of the water proton resonance. The results show that adsorbed surfactants form a barrier between sample water and the paramagnetic species present on the clay surface, thus significantly increasing the proton T(1) values of water. This effect is attributed to the amount of adsorbed surfactants and the arrangement of the surfactant aggregates. The total surface area covered by the cationic (DTAB and TTAB) and anionic (SDS) surfactants could be estimated from the water T(1) data and found to correspond to the fractions of negatively and positively charged surface area, respectively. For selected samples, the amount of paramagnetic species on the clay surface was reduced by treatment with hydrofluoric (HF) acid. For these samples, T(1) and T(2) measurements were taken in the temperature range 278-338 K, revealing detailed information on molecular mobility and nuclear exchange for the sample water that is related to surfactant behavior both on the surface and in the aqueous phase.

Thermotropic Behavior of a Cationic Surfactant in the Adsorbed and Micellar State: An NMR Study

Knowledge about how temperature affects the internal structure and dynamics of surfactant aggregates can lead to a better understanding of their behavior in complex environments and processes. (13)C chemical shifts of the cationic surfactant tetradecytrimethylammonium bromide (TTAB) in micellar solution and when adsorbed on silica particles are recorded in the temperature range 8-78 °C, and give information on the conformation of the alkyl chain carbons. Adsorbed TTAB has conformational disorder similar to free TTAB at about 70 °C, with an increase in the rate of conformational changes occurring above 50 °C. Furthermore, no significant change in TTAB adsorption density was observed in the temperature range studied, and the results indicate a bilayer arrangement of the adsorbed surfactants. The number of gauche conformers increases linearly with temperature for the alkyl chain carbons in TTAB micelles. However, the total increase in gauche conformers is significantly smaller for micellar than for adsorbed TTAB within the temperature range studied. The fraction of micellized TTAB molecules in solution is found to increase with temperature.

The nature of alcohol co-adsorption sites and their effects on surfactant aggregate structure on silica investigated by 2H and 13C NMR

The effects of medium- and long-chain alcohols on surfactant aggregation in solution are well understood; however, our molecular level understanding of alcohol interaction with adsorbed surfactant aggregates is comparatively scarce. Here we use 2H and 13C NMR spectroscopy to study the system tetradecyltrimethylammonium bromide (TTAB) adsorbed on silica at the plateau level, where co-adsorption of 1-butanol and 1-heptanol is investigated. It is found that 13C and 2H NMR spectra of samples where the alcohol is labelled with deuterium in the α-segment give clear indications about the nature of the alcohol co-adsorption sites. 1-Butanol and 1-heptanol co-adsorb at a common co-adsorption site. However, 1-heptanol co-adsorbs at a second site as well. The common co-adsorption site is interpreted as being the alcohol in the contact area between the hydrophobic TTAB tail and the bulk solution. For higher concentrations of 1-heptanol, co-adsorption at the second site is interpreted as being 1-heptanol molecules intercalated between adsorbed TTAB molecules, presumably with the hydroxyl group positioned between TTAB head groups. NMR results, adsorption densities and zeta potentials further show that the differences in 1-butanol and 1-heptanol co-adsorption are related to structural changes of the adsorbed TTAB aggregate.

Long-range surface-induced water structures and the effect of 1-butanol studied by 1H nuclear magnetic resonance.

Thin films of water between glass plates were investigated in this study with regard to water structure and dynamics in the temperature range of 278-313 K. We further investigated how addition of 1-butanol (0.05 and 0.5 M) affects the range and properties of the surface-induced water structures. From the observation of two (1)H nuclear magnetic resonance (NMR) water resonances and two relaxation components, it was found that the interfacial water exists in a two-state mixture in dynamic equilibrium, with the respective structures interpreted as being high-density water (HDW) and low-density water (LDW). In the absence of 1-butanol, the LDW state is more pronounced, with a further shift in equilibrium toward the LDW state with an increase in temperature. However, in water film samples containing 1-butanol, the HDW state dominates at low temperatures while the LDW state becomes more visible at higher temperatures. Furthermore, the addition of 1-butanol significantly increased the extent of the surface-induced water structures. NMR relaxation shows that the dynamics of water in the HDW state is significantly affected by the presence of 1-butanol and further indicates that the distribution of values for the enthalpy of activation associated with translational motion of water molecules in the HDW state is narrower in the 0.05 M 1-butanol sample than in the 0.5 M 1-butanol sample.

Investigating mobility of crude oil adsorbates on mineral surfaces by NMR

We have applied diffusion and relaxation Nuclear Magnetic Resonance experiments to investigate the translational and rotational mobility of adsorbents on quartz and calcite mineral surfaces. On both surfaces it was found that water is the dominant molecule. On the quartz surface the majority of water molecules have a relatively high degree of both rotational and translational mobility, while a minor fraction of water molecules, and all hydrocarbon molecules, have a significantly lower mobility. On the calcite surface the translational mobility is very low for all the adsorbed molecules, while there is a large diversity in rotational mobility, indicating that the hydrocarbon molecules are strongly attached to the surface, but that some part of each molecule still have a large degree of rotational mobility. Diffusion and relaxation experiments give a detailed description of both the molecular mobility of adsorbed species on these mineral surfaces, which leads to new insight with respect to aging processes on a molecular level.

Analysis of wild and farmed salmon using 13C solid-state NMR and MRI directly on fillet tissue

Developments in feed composition and technologies cause a continuous change in farmed salmon fatty acid composition and quality. Hence, fast and direct techniques for the analysis of salmon are desirable. Here, it is demonstrated that 13C Magic Angle Spinning (MAS) NMR spectroscopy can quantify the most relevant fatty acids in salmon fat within 40 minutes without use of chemical extractions. The method is optimized with regard to parameters affecting the total spectral acquisition time and accuracy. Farmed and wild Atlantic salmons are compared to illustrate the effects of a natural and synthetic diet on their quality and fat composition. 13C T1 relaxation reveals that the fatty acid composition in farmed salmon fat results in significantly different lipid packing compared to that in wild salmon, with possible negative impacts on the quality. Further, micro-MRI is applied to obtain complementary data on fillet quality and fat distribution.

Effects and location of coplanar and noncoplanar PCB in a lipid bilayer: a solid-state NMR study

Coplanar and noncoplanar polychlorinated biphenyls (PCBs) are known to have different routes and degree of toxicity. Here, the effects of noncoplanar PCB 52 and coplanar PCB 77 present at 2 mol % in a model system consisting of POPC liposomes (50% hydrated) are investigated by solid-state (13)C and (31)P NMR at 298 K. Both PCBs intercalate horizontally in the outer part of the bilayer, near the segments of the acyl chain close to the glycerol group. Despite similar membrane locations, the coplanar PCB 77 shows little effect on the bilayer properties overall, except for the four nearest neighboring lipids, while the effect of PCB 52 is more dramatic. The first ∼2 layers of lipids around each PCB 52 in the bilayer form a high fluidity lamellar phase, whereas lipids beyond these layers form a lamellar phase with a slight increase in fluidity compared to a bilayer without PCB 52. Further, a third high mobility domain is observed. The explanation for this is the interference of several high fluidity lamellar phases caused by interactions of PCB 52 molecules in different leaflets of the model bilayer. This causes formation of high curvature toroidal region in the bilayer and might induce formation of channels.