Mobile interface, pressure drop

The mass spectrometer is a very sensitive and selective instrument. However, the introduction of the eluent into the vacuum chamber and the resulting significant pressure drop reduces the sensitivity. The gas exhaust power of a normal vacuum pump is some 10 ml min-1 so high capacity or turbo vacuum pumps are usually needed. The gas-phase volume corresponding to 1 ml of liquid is 176 ml for -hexane, 384 ml for ethanol, 429 ml for acetonitrile, 554 ml for methanol, and 1245 ml for water under standard conditions (0°C, 1 atmosphere). The elimination of the mobile phase solvent is therefore important, otherwise the expanding eluent will destroy the vacuum in the detector. Several methods to accomplish this have been developed. The commercialized interfaces are thermo-spray, moving-belt, electrospray ionization, ion-spray, and atmospheric pressure ionization. The influence of the eluent is very complex, and the modification of eluent components and the selection of an interface are therefore important. Micro-liquid chromatography is suitable for this detector, due to its very small flow rate (usually only 10 p min - ). 

Since SFC is in its infancy, the same is true of the hyphenated techniques that involve it. In general, it would be expected that SCF/MS should use interfaces like GC/MS since the supercritical fluid mobile phases become gases when reduced to atmospheric pressure, but the conditions are more severe because of the higher (critical) pressure. OT columns, because of their low pressure drops, are favored. The two interfaces that have been used are a direct fluid injection (DFI) and a molecular beam apparatus. DFI has been used with packed columns26 and with OT columns,27 using both chemical ionization and electron impact ionization. For a more complete discussion of both interfaces, see the chapter on SFC in the ACS Symposium Series edited by Ahuja28 the recent review on LC/MS25 also contains considerable information about SFC/MS. 

Of direct interest in the study here is the pressure drop across the bubble in the liquid phase. For the mobile interface case this is given in dimensionless form by... 

Equation 16 indicates that for bubble trains with mobile interfaces, the pressure drop is related to the number of lamellae in the train (or number of bubbles per unit length). This important observation, first noted by Hirasaki and Lawson (1),... 

Viscous Pressure Drop. For continuous bubble trains with perfectly mobile interfaces moving through a given Dm channel, the dynamic pressure drop in the gas (foam) phase over a single tube segment D follows from Equation 16 ... 

An oil/brine/surfactant/alcohol system often forms a middle phase microemulsion in an appropriate salinity range. The salinity at which the middle phase microemulsion contains an equal volume of oil and brine is defined as the optimal salinity (9). At the optimal salinity, the interfacial tension is in the millidynes/cm range at both oil/microemulsion and microemulsion/brine interfaces, and the oil recovery is maximum (6,9). Moreover, we have shown (10) that at optimal salinity, the coalescence time or phase-separation time is minimum for oil/brine/surfactant/alcohol systems. When these systems are pumped through porous media, a minimum pressure drop or apparent viscosity is observed at the optimal salinity (10). All these phenomena occurring at optimal salinity are summarized in Figure 11. In a recent study, we have also found that the surfactant loss in porous media is minimum at the optimal salinity. Therefore, besides ultralow interfacial tension, a favorable coalescence process for mobilized oil ganglia and the minimum apparent viscosity (or minimum AP) of the oil bank and the minimum surfactant loss are the other factors contributing towards the maximum oil recovery at the optimal salinity. 

Surface tension variations affect the mobility of the fluid-fluid interface and cause Marangoni flow instabilities. Surfactant-laden flows exhibit surface tension variations at the gas-liquid or liquid-liquid contact line due to surfactant accumulation close to stagnation points [2,53]. For gas-liquid systems, these Marangoni effects can often be accounted for by assuming hardening of the gas bubble, i.e. by replacing the no-shear boundary condition that is normally associated with a gas-liquid (free) boundary with a no-slip boundary condition. It should be noted that such effects can drastically alter pressure drop in microfluidic networks and theoretical predictions based on no-shear at free interfaces must be used with care in practical applications [54]. 

In order to measure the surface tension of solutions containing surfactants, the maximum bubble pressure, pendant drop and Wilhelmy plate (immersed at a constant depth) methods are suitable capillary rise, ring, mobile Wilhelmy plate, sessile drop and drop weight methods are not very suitable. These methods are not recommended because surfactants preferably adsorb onto the solid surfaces of capillaries, substrates, rings, or plates used during the measurement. In a liquid-liquid system, if an interfacially active surfactant is present, the freshly created interface is not generally in equilibrium with the two immiscible liquids it separates. This interface will achieve its equilibrium state after the redistribution of solute molecules in both phases. Only then can dynamic methods be applied to measure the interfacial tension of these freshly created interfaces. 

The substitution of the above equation into the generalized Laplace equation (with the dependence of capillary pressure on the z coordinate accounted for) yields the Laplace equation in the differential form, the numerical integration of which leads to the exact mathematical description of the drop or bubble surface shape in the gravitational field [6,14]. The exact description of the equilibrium surface shape is of importance in the evaluation of surface tension from the experimental data at interfaces with high mobility, such as liquid-gas and liquid-liquid ones (See Chapter 1,4). 

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