The phase stability of crude oil is dependent on a multitude of factors, including temperature, pressure, and component fractions, especially long chain paraffin and polar asphaltene fractions. Paraffins precipitate out of the crude oil during pipeline transportation due to solubility limits, and form paraffin-oil gel deposits on the pipe walls. The presence of asphaltenes in crude oil is postulated to affect the formation of these paraffin gels. To quantify this effect, a controlled stress rheometer was used to study the gelation temperature and the yield stress of a model paraffinoil system. It was observed that the addition of asphaltenes in small proportions (∼0.1 wt %) resulted in a significant decrease both in the gelation temperature and the yield stress of the model system, indicating that the presence of asphaltenes hinders the gelation mechanism. Addition of higher amounts of asphaltenes resulted in macroscopic phase separation: a deposit consisting of asphaltenes and paraffins separated out of the liquid. The effects of operating conditions and the asphaltene polarity on the gelation process were also studied. Polarized light microscopy and nuclear magnetic resonance spectroscopy were used to obtain insights into the rheometric results.

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Fluorescence recovery after photobleaching (FRAP) is a versatile tool for determining diffusion and interaction/binding properties in biological and material sciences. An understanding of the mechanisms controlling the diffusion requires a deep understanding of structure-interaction-diffusion relationships. In cell biology, for instance, this applies to the movement of proteins and lipids in the plasma membrane, cytoplasm and nucleus. In industrial applications related to pharmaceutics, foods, textiles, hygiene products and cosmetics, the diffusion of solutes and solvent molecules contributes strongly to the properties and functionality of the final product. All these systems are heterogeneous, and accurate quantification of the mass transport processes at the local level is therefore essential to the understanding of the properties of soft (bio)materials. FRAP is a commonly used fluorescence microscopy-based technique to determine local molecular transport at the micrometer scale. A brief high-intensity laser pulse is locally applied to the sample, causing substantial photobleaching of the fluorescent molecules within the illuminated area. This causes a local concentration gradient of fluorescent molecules, leading to diffusional influx of intact fluorophores from the local surroundings into the bleached area. Quantitative information on the molecular transport can be extracted from the time evolution of the fluorescence recovery in the bleached area using a suitable model. A multitude of FRAP models has been developed over the years, each based on specific assumptions. This makes it challenging for the non-specialist to decide which model is best suited for a particular application. Furthermore, there are many subtleties in performing accurate FRAP experiments. For these reasons, this review aims to provide an extensive tutorial covering the essential theoretical and practical aspects so as to enable accurate quantitative FRAP experiments for molecular transport measurements in soft (bio)materials.

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Solutions of cellulose in a mixture of tetrabutylammonium fluoride and dimethyl sulfoxide (TBAF/DMSO) containing small and varying amounts of water were studied by nuclear magnetic resonance (NMR). By measuring the composition dependences of 19 F NMR and 1 H NMR chemical shifts and line widths, details on the dissolution and gelation mechanisms for cellulose in TBAF/DMSO were elucidated. Our results suggest that the strongly electronegative fluoride ions act as hydrogen bond acceptors to cellulose hydroxyl groups, thus dissolving the polymer by breaking the cellulose-cellulose hydrogen bonds and by rendering the chains an effective negative charge. It was found that the fluoride ions also interact strongly with water. Small amounts of water remove the fluoride ions from the cellulose chains and allow reformation of the cellulose-cellulose hydrogen bonds, which leads to formation of highly viscous solutions or gels even at low cellulose concentrations.

A series of novel zwitterionic surfactants each with two hydrophilic and two hydrophobic groups in the molecule (so-called heterogemini surfactants) has been synthesized. One of the hydrophilic groups is a phosphodiester anion and the other is a quaternary ammonium salt. Two methylene groups separate the two headgroups. The critical micelle concentration values of the surfactants were determined using du Nouy tensiometry and steady-state fluorescence and are of the order of 10 -5 M. Very low surface areas per molecule were observed suggesting that the monolayer formed is extremely tightly packed. NMR self-diffusion measurements gave information about the micelle size distribution. A broad distribution of self-diffusion coefficients was observed and indicated that the time scale of monomer-aggregate and/or aggregate-aggregate exchange is slow compared to the NMR time scale used (100 ms). A mean aggregate size of about 55 nm is obtained for one sample. Adsorption of the gemini surfactants at hydrophilic and at hydrophobized silica was studied by reflectometry. The more symmetrical gemini surfactants gave very low values of surface area per molecule on the hydrophobic surface, indicating a very tight packing of surfactant molecules. At higher surfactant concentrations all gemini surfactant gave very high adsorbed amount, most probably due to formation of aggregates at the surface.

The phase behavior of solvent-rich mixtures of glycerol monooleate and cetyltrimethylammonium bromide is reported for two solvents, water and aqueous solutions of 100 mM NaCl. The main concern is the properties of the lamellar phases and the formation of curvature defects within the bilayers. Defective lamellar phases are shown to form in both water and brine. The defective regions of lamellar phase are delineated by means of small-angle X-ray scattering (SAXS) studies. Direct visualization of the aggregate structure at high dilution is made by cryo-transmission electron microscopy (cryo-TEM). A rich micellar polymorphism is found, involving connected threadlike micelles as well as elongated micelles with noncircular cross sections. The correlation between the observed aggregate structures and the overall phase behavior is used to discuss possible defect geometries in the disrupted bilayers. We also report that the water-based lamellar phase made from the intact bilayers seems to contain a large fraction of curved multilamellar aggregates. Interestingly, the corresponding bilayers that show curvature defects appear to build lamellar phases free of such textural defects.

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