Water electro-osmotic flow

Reversed-phase separations currently dominate in CEC. As a result, the vast majority of the mobile phases are mixtures of water and an organic solvent, typically acetonitrile or methanol. In addition to the modulation of the retention, the mobile phase in CEC also conducts electricity and must contain mobile ions. This is achieved by using aqueous mixtures of salts instead of pure water. The discussion in Sect. 2 of this chapter indicated that the electro osmotic flow is created by ionized functionalities. The extent of ionization of these functionalities that directly affects the flow rate depends on the pH value of the mobile phase. Therefore, the mobile phase must be buffered to a pH that is desired to achieve the optimal flow velocity. Obviously there are at least three parameters of the mobile phase that have to be controlled (i) percentage of the organic solvent, (ii) the ionic strength of the aqueous component, and (iii) its pH value. 

In desalting seawater the losses can amount to 20 to 30% of the volume of the treated water. The electro-osmotic flow per unit of current is ... 

In the Nemst-Planck equations used the activity coefficients were neglected a term accounting for the electro-osmosis, however, is present. Calculated and measured concentration profiles could be made to inter-correspond by adapting the term for water transport. The values indirectly determined by electro-osmotic flow were now found to agree with those measured directly. 

Calculate the rate of electro-osmotic flow of water at 25°C through a glass capillary tube 10 cm long and 1 mm diameter when the potential difference between the ends is 200 V. The zeta potential for the glass-water interface is -40 mV. 

Magnuson, M.L., Creed, J.T. and Brockhoffl C.A. (1997) Speciation of arsenic compounds in drinking water by capillary electrophoresis with hydrodynamically modified electro-osmotic flow detected through hydride generation inductively coupled plasma mass spectrometry with a membrane gas-liquid separator./. Anal. At. Spectrom., 12, 689-695. 

Pikal, M. and Shah, S. Transport mechanisms in iontophoresis III. An experimental study of the contributions of electro-osmotic flow and permeability change in transport of low and high MW solutes. Pharm. Res. 7 222, 1990. Gangerosa, L. P., Park, N.-H., Wiggins, C. A., and Hill, J. M. Increased penetration of non-electrolytes into mouse skin during iontophoretic water transport (lontohydrokinesis). J. Pharm. Exp. Ther. 212 311, 1980. 

Assuming the zeta-potential to be the same as in Problem 1, calculate the rate of electro-osmotic flow of water through a glass capillary tube of 0.05 cm. radius under a potential gradient of 1 volt per cm. 

In this situation, the solid clay particles are the immobile phase and the electro-osmotic flow causes the water to move as a plug, the entire velocity gradient being concentrated at the solid surface in a layer that is the same order of thickness as that of the diffuse double layer (Fig. 3). In concentrated solutions, the thickness of the diffuse double layer is quite small (<1 nm) whereas in very dilute solutions (which are indeed represented by the water in the clays), the diffuse double layer can assume much... 

Electro-osmotic flow - the osmotically driven mass flow of water resulting from the movement of ions in an electrophoretic system. 

There are several primary factors that affect the efficiency of separation through CE the buffer system, the pH of the electrophoresis electrolyte, the voltages applied (higher voltages improve separation), and the interface between CE and ICP-MS, which reduces the ratio of forced flow to electro-osmotic flow. The relative standard deviation values (RSD < 9%) obtained through the ICP-MS technique confirm the suitability for arsenic speciation in saline waters. In terms of reliability, one can say that the ICP-MS method is more reliable than HG-AAS for arsenic speciation in water. 

It is well known that the EK process relies on several interacting mechanisms, including (1) advection resulting from electro-osmotic flow and externally applied hydraulic gradients, (2) diffusion of the acid front to the cathode, and (3) the migration of cations and anions toward the respective electrode. The electrolysis of water is the dominant and most important electron transfer reaction that occurs at the electrodes during the EK process. 

Most oxidants are anions. The electrokinetic transport accelerates the distribution of anionic oxidants toward the anode. At the same time, the electro-osmotic flow carries water to the cathode. Depending on the soil parameters, the electrokinetic transport can be faster than the electro-osmotic flow. In very dense ground with a high amount of clay, the electro-osmotic flow can be even faster than the electrokinetic transport. In any case, it is important to test the velocity of transport in both directions, to find the optimal distances for dosing the oxidant, and to plan the arrangement of the electrodes depending on the transport velocity and the... 

To examine the transport-mode-transition region in more detail, simulations were run at different current densities [71]. The resultant membrane water profiles are shown in Figure 5.11 where a vapor-equilibrated membrane at unit activity has a water content of L = 8.8 as calculated by the modified chemical model (see Section 5.5.1). The profiles in the figure demonstrate that the higher the current density the sharper the transition from the liquid-equilibrated to the vapor-equilibrated mode as well as the lower the value of the water content at the anode GDL/membrane interface. The reason why the transition occurs at the same point in the membrane is that the electro-osmotic flow and the water-gradient flow are both proportional to the current... 

Savinell and coworkers [410,411] have measured the drag coefficient of water/ methanol in PBI at 150 °C and 180 °C and found that it is essentially zero over a wide range of composition. The same result was reported for the drag coefficient of water in PBI at 80 °C and 150 °C by Li et al. [412]. The absence of electro-osmotic flow of water or water/methanol through PBI was interpreted as protmi conductimi mechanism dominated by Grotthuss protmi hopping. 

AC Electro-Osmotic Flow, Fig. 6 (a) Comparison of ACEO pumping of water around a microfluidic loop by planar and (non-optimal) 3D electrode arrays with similar... 

AC Electro-Osmotic Flow, Fig. 7 (a) Collection of E. coli bacteria in tap water along the stagnation lines of ACEO flow on An microelectrodes at low frequency (100 Hz) and moderate voltage (1 V). (b) Preferential... 

The schematic representation of a CE apparatus is shown in Fig. 1. The mechanism of separation of water pollutants in CE is based on the electro-osmotic flow (EOF) and electrophoretic mobilities of the pollutants. The EOF propels all pollutants (cationic, neutral, and anionic) toward the detector and, ultimately, separation occurs due to the differences in the electrophoretic migration of the individual pollutants. Under the CE conditions, the migration of the pollutant is controlled by the sum of the intrinsic electrophoretic mobility (//ep) and the electro-osmotic mobility (/r o), due to the action of EOF. The observed mobility (Mobs)of the pollutants is related to and p p by the following equation ... 

Electro-osmosis is another electrokinetic phenomenon-in which an electric field is applied across a charged porous membrane or a slit of two charged nonporous membranes (see figure IV - 31). Due to the applied potential difference an electric current will flow and water molecules will flow with the ions (electro-osmotic flow) generating a pressure difference. As can be derived from nonequilibrium thermodynamics (sec chapter V) the following equation can be obtained indicating that both phenomena, electro-osmose and streaming potential, are similar... 

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