Hydrophilic capillaries

In layers bound by the hydrophilic surfaces, the situation changes. Near such a surface, the water dipoles are oriented normal to the surface. This geometry results in an increase in the density of water and a decrease in the tangential mobility of water molecules within layers that are several nanometers thick. From a macroscopic point of view, there should be an increase in the viscosity of the boundary layers of water. With a decrease in the radius of the quartz hydrophilic capillary, the average viscosity of water increases. 

Capillary Force Valves, Fig. 3 Capillary forces in hydrophilic capillaries result in increased pressure in trapped air bubbles. Capillary forces in hydrophobic capillaries prevent liquid from entering the capillary and can be overcome by larger pressure... 

Figure 13 shows an integrated sedimentation structure based on hydrophobic valving for the initial, overflow-based metering, a radially flat and narrow transfer channel to throttle the flow such that the suspension has sufficient time to sediment in the separation chamber before pure supernatant is passed on to the plasma chamber. From there, the plasma can, for instance, be forwarded downstream structures by a hydrophilic capillary while halting the rotation. 

There are numerous ways in which surface tension can be measured. The technique that is perhaps the most suitable for electrolyte solutions is the Maximum Bubble Pressure technique, as the continuous generation of fresh interface assists in acquiring data and is not influenced by low levels of contamination. Typically a bubble is produced from a fine capillary every five to ten seconds and as it grows the pressure is monitored. When the pressure reaches a maximum, the radius of the bubble is equivalent to the inside radius of the hydrophilic capillary tip. At this time the radius of the bubble and the internal pressure are known, therefore the surface tension can easily be calculated using the Laplace equation. This bubble then detaches and a new bubble is created, allowing for easy repeat measurements of surface tension at a freshly generated interface. 

A characteristic time scale of the equilibration of the surfactant concentration in a cross section of the capillary is x RVD 10" sec, if we use for estimations R 1 pm and D 10 cmVsec. A characteristic time scale of the spontaneous capillary rise into partially hydrophilic capillaries is around 10 sec (see Figure 5.15), which is much bigger than 10 sec. Hence, the surfactant concentration is constant in any cross section of the capillary and depends only on the position, x (Figure 5.12), that is, C = C(t, x). 

A drop of an aqueous solution of the mixture to be separated is now placed near the bottom of the paper strip and allowed to evaporate in the air. The strip is now again suspended in the closed cylinder, but with the bottom of the strip just immersed in the solvent. The capillary action of the paper will cause the solvent to rise steadily up the strip, and during this process the solvent, which now contains the mixture in solution, is continuously extracted by the retained water molecules in the paper. A highly hydrophobic (water-repellent) solute will move up closely behind the solvent-front, whereas a highly hydrophilic solute will barely leave the original point where the drop of the mixed solutes in solution has been dried. In an intermediate case,... 

Figure 24 shows that the values of AX derived from Figs. 14 and 15 are consistent with the values of C measured by Valette.390 On the other hand, the same values of C cannot fit Fig. 24 if the values of AX estimated by Valette389 are used. The same is the case for the values of C as reported by Popov et alm for single-crystal faces grown in a Teflon capillary. These authors observed the opposite sequence, i.e., C(111) > C(100), thus concluding that the (111) face is more hydrophilic than the (100) face. However, as pointed out earlier, they measured the same Eam0 as the other... 

Most electrode materials are hydrophilic and readily wetted by aqueous solutions. Two methods are used to create and maintain an optimum gas/solution ratio in the electrode. The first method employs a certain excess gas pressure in the gas space. This causes the liquid to be displaced from the wider pores in finer pores the liquid continues to be retained by capillary forces. The second method employs partial wetproofing of tfie electrode by the introduction of hydrophobic materials (e.g., fine PTFE particles). Tfien the electrolyte will penetrate only those pores in the hydrophilic electrode material where the concentration of hydrophobic particles is low. 

In order to separate neutral compounds, Terabe et al. [13] added surfactants to the buffer electrolyte. Above their critical micellar concentration (cmc), these surfactants form micelles in the aqueous solution of the buffer electrolyte. The technique is then called Micellar electrokinetic capillary chromatography, abbreviated as MECC or MEKC. Micelles are dynamic structures consisting of aggregates of surfactant molecules. They are highly hydrophobic in their inner structure and hydrophilic at the outer part. The micelles are usually... 

CEC capillary columns filled with hydrophilic polymer gels mimic those used for capillary gel electrophoresis [91]. Typically, the capillary is filled with an aqueous polymerization mixture that contains monovinyl and divinyl (crosslinking) acrylamide-based monomers as well as a redox free radical initiating system, such as ammonium peroxodisulfate and tetramethylethylenediamine (TEMED). Since initiation of the polymerization process begins immediately upon mixing all of the components at room temperature, the reaction mixture must be used immediately. It should be noted, that these gels are very loose, highly swollen materials that usually contain no more than 5% solid polymer. 

Replacement of the hydrophilic acrylamide by the more hydrophobic N-iso-propylacrylamide, in combination with the pre-functionalization of the capillary with (3-methacryloyloxypropyl) trimethoxysilane, afforded a monolithic gel covalently attached to the capillary wall. A substantial improvement in the separations of aromatic ketones and steroids was observed using these fritless hydrogel columns, as seen by the column efficiencies of 160,000 found for hydrocortisone and testosterone [92]. The separations exhibited many of the attributes typical of reversed-phase chromatography and led to the conclusion that, in contrast to the original polyacrylamide-based gels, size-exclusion mechanism was no longer the primary mechanism of separation. 

R. Loos, M.C. Alonso and D. Barcelo, Solid-phase extraction of polar hydrophilic aromatic sulfonates followed by capillary zone electrophoresis-UV absorbance detection and ion-pair liquid chromatography-diode array UV detection and electrospray mass spectrometry. J. Chromatogr.A 890 (2000) 225-237. 

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