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- Ekholm, P., et al.
(författare)
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A quartz crystal microbalance study of the adsorption of asphaltenes and resins onto a hydrophilic surface
- 2002
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Ingår i: Journal of Colloid and Interface Science. - : Elsevier BV. - 0021-9797 .- 1095-7103. ; 247:2, s. 342-350
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of extracted and purified samples of asphaltenes and resins onto gold surfaces has been studied as a function of bulk concentration using a quarts crystal microbalance with dissipation measurements (QCM-D). With this device, which works equally well in transparent, opaque, and nontransparent samples, the adsorbed amount is measured through a change in resonant frequency of the quartz oscillator. The measured change in dissipation reports on changes in layer viscoelasticity and slip of the solvent at the surface. The results show that the adsorbed amount for resins from heptane corresponds to a rigidly attached monolayer. The adsorbed amount decreases with increasing amount of toluene in the solvent and is virtually zero in pure toluene. Asphaltenes, on the other hand, adsorb in large quantities and the mass and dissipation data demonstrate the presence of aggregates on the surface. The aggregates are firmly attached and cannot be removed by addition of resins. On the other hand, resins and asphaltenes associate in bulk liquid and the adsorption from mixtures containing both resins and asphaltenes is markedly different from that obtained from the pure components. Hence, we conclude that preformed resin aggregates adsorb to the surface. These results are compared and discussed in relation to adsorption from crude oil diluted in heptane/toluene mixtures.
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- Ese, M-H, et al.
(författare)
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An atomic force microscopy study of asphaltenes on mica surfaces. Influence of added resins and demulsifiers
- 2000
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Ingår i: Colloid and Polymer Science. - 0303-402X .- 1435-1536. ; 278, s. 532-538
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Tidskriftsartikel (refereegranskat)abstract
- Monolayers of asphaltene and resins on the water surface have been transferred at a surface pressure of 10 mN/m onto mica substrates using the Langmuir-Blodgett technique. Atomic force microscopy (AFM) has been used to examine the topography of these layers. Monolayers consisting of pure asphaltene fraction provide a rigid film with a close packed structure, while the resins build up a continuous open network. Mixed films of these two fractions, show that a gradually increase of resin concentration leads to an opening of the rigid asphaltene structure towards a more resin like configuration. Increased aggregation when the two heavy fractions are present in one film, is seen as larger individual units in the AFM-pictures. Addition of high molecular weight demulsifiers/inhibitors resulted in the same kind of influence on the asphaltene film as seen with the resins.
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- Sjöblom, Markus, 1973-
(författare)
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The Duodenal Mucosal Bicarbonate Secretion : Role of Melatonin in Neurohumoral Control and Cellular Signaling
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The duodenal lumen is exposed to aggressive factors with a high potential to cause damage to the mucosa. Bicarbonate secretion by the duodenal mucosa is accepted as the primary important defense mechanism against the hydrochloric acid intermittently expelled from the stomach.The present thesis concerns the influence of the central nervous system and the effects of the hormone melatonin on bicarbonate secretion in anesthetized rats in vivo. Effects of melatonin on intracellular calcium signaling by duodenal enterocyte in vitro were examined in tissues of both human and rat origin. The main findings were as follows:Melatonin is a potent stimulant of duodenal mucosal bicarbonate secretion and also seems to be involved in the acid-induced stimulation of the secretion. Stimulation elicited in the central nervous system by the α1-adrenoceptor agonist phenylephrine induced release of melatonin from the intestinal mucosa and a four-fold increase in alkaline secretion. The melatonin antagonist luzindole abolished the duodenal secretory response to administered melatonin and to central nervous phenylephrine but did not influence the release of intestinal melatonin. Central nervous stimulation was also abolished by synchronous ligation of the vagal trunks and the sympathetic chains at the sub-laryngeal level. Melatonin induced release of calcium from intracellular stores and also influx of extracellular calcium in isolated duodenal enterocytes. Enterocytes in clusters functioned as a syncytium.Overnight fasting rapidly and profoundly down-regulated the responses to the duodenal secretagogues orexin-A and bethanechol but not those to melatonin or vasoactive intestinal polypeptide.In conclusion, the results strongly suggest that intestinal melatonin plays an important role in central nervous elicited stimulation of duodenal mucosal bicarbonate secretion. Sensitivity of this alkaline secretion to some peripheral stimulators markedly depends on the feeding status.
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