A fluid membrane is described, termed the electroconvective liquid crystal membrane (ECLCM), comprised of a sandwich-like configuration in which a fluid layer is contained within a structure. The structure containing the fluid layer separates distinct regions having different concentrations of one or more diffusant species and is capable of being accessed by the diffusant species. The fluid layer is comprised of any fluid in which an electrohydrodynamic (EHD) flow can be induced, including liquid crystals and liquid crystal-like fluids. The ECLCM includes means for applying an electric field to the fluid layer such that an EHD flow is induced within the fluid layer. The EHD flow alters the passage of the diffusant species across the ECLCM. The fluid layer may be modified by the addition of other components which act as carriers to the passage of selected diffusants. These additional components move in the EHD flow and can be chemically bonded to the fluid layer, added as dopants into the fluid layer, or may be separate solid or liquid phases of other materials not soluble in the fluid layer. The membrane of the present invention can function in an electrochemically modulated complexation (EMC) process wherein a complexing agent is added to the fluid layer and electrolyzed to high and low affinity redox states for separation of different molecular species. The present invention includes a method for separating and purifying molecules.

FIELD OF THE INVENTION 
This invention relates to membranes containing fluids in which an 
electrohydrodynamic (EHD) flow is induced, including liquid crystal (LC) 
and LC-like fluids. A fluid capable of exhibiting EHD flow is contained in 
a sandwich-like membrane configuration, termed an ElectroConvective Liquid 
Crystal Membrane (ECLCM). The fluid layer is contained within a structure 
which is accessible to diffusant molecules. The structure contains means 
for applying an electric field to the membrane, inducing an EHD flow 
within the fluid layer and modifying the passage of diffusant molecules 
across the membrane. The permeation rate of CH.sub.4 and N.sub.2 through a 
N-(4-methoxybenzylidene)-4-butylaniline (MBBA) membrane is increased by a 
factor of fifty upon application of an alternating electric field of 100 
V/mm to the fluid. The fluid layer can be modified by the addition of 
other components to enhance the solubility and/or diffusion of selected 
diffusant molecules. 
BACKGROUND OF THE INVENTION 
A membrane can be viewed as a semi-permeable barrier between two phases of 
liquid, gas, or liquid/gas. The membrane acts to prevent contact between 
the two phases and the semi-permeable nature of the membrane allows 
restricted diffusion of specific molecules such that a separation takes 
place. The movement of molecules across the barrier can be restricted in a 
very specific manner. 
An immiscible liquid can serve as a membrane between two liquid, gas, or 
liquid/gas phases. Different diffusant molecules will have different 
solubilities and diffusion coefficients in a liquid, and therefore, yield 
selective permeabilities to achieve a separation (Noble and Way (1987) in 
Liquid Membranes: Theory and Applications, American Chemical Society 
347:1-26). The use of a liquid membrane can result in a larger flux due to 
the higher diffusion coefficients of diffusant molecules in liquids than 
in solids. When an AC electric field is applied across a non-conducting 
liquid membrane, the diffusion flux across the membrane can increase 
sharply. This increased flux is not caused by an increased current, but by 
electrohydrodynamic (EHD) mixing (Hoburg and Malihi (1978) Phys. Fluids 
21:2118-2119), which is a disordering of the fluid resulting in an 
alteration of mass transport across the fluid region. In other words, the 
increase results from a type of convection driven by electrical forces 
rather than phase changes or by heating of the fluid layer. Convection 
driven by electrical forces differs from other types of convection, such 
as free convection driven by gravity, or forced convection driven by 
mechanical forces (e.g., pressure) (Plonski et al. (1979) J. Membrane Sci. 
5:371-374). Full disordering or turbulence of the liquid is obtained with 
a low AC field strength of 100 V/mm. 
Liquid crystals (LCs) or LC-like fluids are mesomorphic phase materials 
exhibiting characteristics intermediate between crystalline solids and 
true amorphous liquids. LCs are usually composed of strongly elongated 
molecules with a tendency toward ordering and alignment of the molecules 
characteristic of solid crystals but retaining relative motion and flow 
between the crystals. LCs or LC-like fluids retain their mesomorphic phase 
characteristics up to a transition temperature at which the fluid 
undergoes a transition to a normal liquid phase. LCs are classified in 
three categories according to their general symmetry, as nematic, 
cholesteric, or smectic. Below the transition temperature, the LC fluid 
exhibits dielectric anisotropy and electric conductivity anisotropy. These 
anisotropic physical properties can be modified with various physical or 
chemical agents locally or throughout with great facility, giving rise to 
numerous technological applications. 
Hwakek and Carr (1987) Heat Transfer Eng. 8:36-69, and U.S. Pat. No. 
4,515,206, issued May 7, 1985 to E. F. Carr, entitled: Active Regulation 
of Heat Transfer, used electroconvection to regulate the passage of heat 
flux through a Nematic Liquid Crystal (NLC), demonstrating a field induced 
enhancement of effective thermal conduction by a factor of 25. The NLCs 
are liquids characterized by long range ordering of the long axes of their 
rod shaped molecules. NLCs, by virtue of their fluidity and intrinsic 
anisotropy, exhibit dramatic EHD effects at low applied electric fields 
(de Gennes (1974) Liquid Crystals Cambridge University Press, Cambridge; 
Chandrasekhar (1977) Liquid Crystals Cambridge University Press, 
Cambridge; Orsay Group on Liquid Crystals (1971) Mol. Cryst. Liq. Cryst. 
12:251). A few hundred volts/mm can generate fully developed turbulent 
convection in appropriately designated NLCs. 
N-(4-methoxybenzylidene)-4-butylaniline (MBBA) is a typical liquid crystal 
having negative dielectric anisotropy. In the absence of an applied field, 
it is at rest and gas transport across the membrane is limited by 
molecular diffusion in the liquid crystal. When an electric field is 
applied, charge accumulates at the walls (defects), which are 
perpendicular to the electrodes. Forces due to the interaction of the 
electric field with the space charge at the wall tend to shear the sample. 
When the direction of the electric field is alternating, the walls are 
always charged in the alternating direction of the director. An AC field 
of approximately 100 V/mm produces fully turbulent flow. This chaotic flow 
disorders the NLC, generating disclinations in the molecular orientation 
field which strongly scatter light, producing the so-called "dynamic 
scattering" LC electro-optic effect (Berne and Pecora (1976) Dynamic Light 
Scattering, Wiley, New York). The disclination lines generated by EHD flow 
can be observed optically. The lines form parallel to the flow velocity, 
indicating the flow of the NLC perpendicular to the electrodes and back 
and forth between them. As applied, EHD flow mixes the LC layer, 
convecting dissolved species across the LC layer and forming an eddy 
diffusion process, thereby enhancing its apparent permeability. 
There have been several previous studies of electric field effects on gas 
permeation through liquid layers. Kajiyama and co-workers (Kajiyama et al. 
(1982) J. Membrane Sci. 11:39-53; Washizu et al. (1984) Polym. J. 
16:307-316; Kajiyama et al. (1985) J. Memb. Sci. 24:73-81; Shinkai et al. 
(1986) J. Chem. Soc., Chem. Commun., p. 933; Kajiyama et al. (1988) J. 
Membrane Sci. 36:243-255; Kajiyama (1988) J. Macromol., Sci. Chem. 
A25(5-7):583-600; Qiao and Wang (1987) Membrane Sci. & Tech. (Ch.) 7:1-7) 
have demonstrated permeation control in NLCs confined in polymer composite 
structures, using applied electric fields to orient molecules of the NLC 
and exploiting the anisotropy of the diffusion coefficients. The use of 
EHD stirring to facilitate mass transfer across a fluid membrane has been 
demonstrated by Plonski et al. (1979) supra. In those experiments, the ion 
flux through a nonconducting (octanol) film separating aqueous ionic 
solutions was controlled by a factor of 10 by an applied electrical field. 
However, the large field required to alter ion flux made the films 
unstable. 
The modification of LC structure has been used in the controlled release of 
drugs [U.S. Pat. No. 4,513,034 issued Apr. 23, 1985, to R. V. Sparer, 
entitled: Variable Permeability Liquid Crystalline Membrane; U.S. Pat. No. 
4,968,539, issued Nov. 6, 1990, to Aoyagi et al., entitled: Liquid Crystal 
Membranes]. Sparer describes a LC layer contained in a porous structure 
which provides access to the LC layer to different molecules. The flow of 
solutes or permeants through the membrane is regulated through application 
of electric, thermal, or magnetic fields, which serve to alter the phase 
of the LC. For example, an electric field with a strength of 300-500 volts 
per centimeter causes the liquid crystal layer to change from the 
cholesteric to the nematic phase at room temperature. 
The membrane of Aoyagi et al. is comprised of a hydrophobic polymer 
membrane upon which is immobilized a liquid crystal-forming compound which 
has a transition temperature between 25.degree.-45.degree. C. A heating 
member applies an electric field to the LC layer, heating the LC layer 
above the gel/LC transition temperature, resulting in diffusion of a drug 
out of a drug reservoir layer. 
In one configuration of a liquid membrane, a liquid is impregnated in the 
pores of a porous solid for mechanical support. This form is commonly 
known as an immobilized liquid membrane (ILM) (Noble and Way (1987) 
supra). The ILM has been recognized as an effective technology to simplify 
the process of creating an interface between two phases and recovering the 
products of separation. Selective transport across the ILM can be 
facilitated by carriers. However, there are two primary problems 
associated with the use of ILMs. Solvent loss can occur through 
evaporation, dissolution, or large pressure differences forcing solvent 
out of the pore support structure. Further, carrier loss can occur due to 
irreversible side reactions or solvent condensation on one side of the 
membrane. Pressure differences can force the liquid to flow through the 
pore structure and leach out the carrier (Noble et al. (1989) Chem. Eng. 
Prog. 85:58-70). These problems decrease the ILM's lifetime and have 
limited its successful commercialization. 
As stated above, the use of a liquid phase can enhance the solute flux due 
to the higher coefficients in liquids than in solids. Further enhancement 
can be accomplished by using a nonvolatile carrier in the liquid (King 
(1987) Chapter 15 in Handbook of Separation Process Technology (R. W. 
Rousseau, ed.), Wiley-Interscience Publishing Co. This carrier molecule 
can selectively and reversibly react with the solute. This reversible 
reaction provides a means of enhancing the solute flux and improving the 
selectivity at the same time. By combining the advantages of high 
diffusion coefficients in liquids with the added carrying capacity of the 
carrier, larger fluxes can be obtained in liquid membranes than in polymer 
membranes. The selective nature of the carrier provides much better 
separations than those obtainable solely on relative solubility and 
diffusion. 
Electrochemical processes have been used for chemical separations (Newman 
(1973) Electrochemical Systems. Prentice-Hall, Englewood Cliffs, N.J.). 
The most general applications are electroplating of metals in the 
processing of ores and the formation of metal coatings. In cases where a 
redox process altered the thermodynamics of a reversible complexation 
reaction, electrochemical cycles have been devised that result in 
separation for different species. Koval et al. (1988) Separat. Sci. 
Technol. 23:1389-1399, devised a mechanism that combines electrical energy 
and reversible complexation for the removal of sulfur and nitrogen 
compounds from a feed organic phase and subsequently concentrates them in 
a receiving organic phase using an equilibrium stage process. The core of 
their separation process is the reversible reaction between complexing 
agents (or carriers) and the sulfur and nitrogen compounds. The process 
which uses electrochemistry to modulate the complexation reaction is 
termed Electrochemically Modulated Complexation (EMC). 
In an EMC process, a complexing agent, dissolved in the contacting 
(aqueous) phase, is electrolyzed to its high solute affinity redox state. 
The solute is extracted from a feed phase by partitioning into the 
contacting phase via reaction with the complexing agent. The complexing 
agent is then electrolyzed to its low solute affinity redox state and the 
solute partitions into the receiving phase upon contact with the aqueous 
phase. The contacting phase is then recycled. 
In the EMC process, the complexing agent must meet four requirements: (1) 
it must be soluble only in the contacting (aqueous) phase in order to 
prevent any loss; (2) it must have a solute binding site and it must 
undergo a chemically reversible redox cycle in the presence and absence of 
the solute; (3) a considerable difference must exist in the affinity of 
the solute for the complexing agent in its two oxidation states; and (4) 
the kinetics of the solute-complexing agent reaction should be 
sufficiently rapid with respect to interfacial mass transfer. Complexing 
agents which meet these requirements include metal chelates which 
reversibly bind gases like CO.sub.2, CO, or H.sub.2 S. These metal 
chelates contain iron, copper, or cobalt (e.g., primary transition 
metals). Suitable complexing agents include iron or copper porphyrins 
which are soluble in an organic phase and contain a metal center. 
BRIEF SUMMARY OF THE INVENTION 
The fluid membrane described herein, termed the electroconvective liquid 
crystal membrane (ECLCM), is comprised of a sandwich-like configuration in 
which a structure contains a fluid layer comprised of a fluid in which EHD 
flow can be induced. The structure containing the fluid layer separates 
distinct regions having different concentrations of one or more diffusant 
species and is capable of being accessed by the diffusant species. The 
ECLCM further includes means for applying an electric field to the fluid 
layer such that an EHD flow may be induced within the fluid layer. The EHD 
flow alters the passage of the diffusant species across the ECLCM. 
The fluid layer can be modified by the addition of other components which 
act as carriers to the passage of selected diffusants. These additional 
components move in the EHD flow and can be chemically bonded to the fluid 
layer, added as dopants into the fluid layer or may be separate solid or 
liquid phases of other materials not soluble in the fluid layer. 
The membrane of the present invention can function in an EMC process 
wherein a complexing agent is added to the fluid layer and electrolyzed to 
high and low affinity redox states for separation of different molecular 
species. 
The invention includes a method for separating molecules. Further, the 
invention includes a method for controlling the transport of compounds 
across a membrane by the addition of specific dopant species to the ECLCM.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A novel immobilized fluid membrane is described herein, termed the 
ElectroConvective Liquid Crystal Membrane (ECLCM). The ECLCM is a 
sandwich-like membrane configuration in which a fluid layer comprised of a 
fluid capable of exhibiting EHD flow is contained in a structure to which 
diffusant molecules have access. An electric field is applied to the fluid 
layer to induce an EHD fluid flow, altering and enhancing the diffusion 
and permeability characteristics of specific molecules across the 
membrane. 
A preferred embodiment of the present invention encompasses a fluid 
membrane in which certain flow characteristics can be switched on or off. 
Potentially useful liquids which may comprise the fluid membrane include 
liquid crystal and nonliquid crystal liquids, including such liquids as 
N-(4-methoxybenzylidene)-4-butylaniline (MBBA), 2-ethyl hexanol (2EH), and 
1-octanol. A potentially useful liquid is herein defined as any liquid in 
which an EHD flow may be induced. In one embodiment of the present 
invention, the liquid layer of the ECLCM is comprised of 
N-(4-methoxybenzylidene)-4-butylaniline (MBBA). Further included in this 
invention is application of an AC field to the ECLCM to enhance 
permeability of specific diffusants across the fluid membrane. 
The switchable convective fluid membrane concept of this invention makes 
possible a variety of separation capabilities not available in the current 
art. The present invention makes possible an electrically controllable 
filter with a much larger ratio of electric field on to off permeability 
than is currently available. Such a filter has applications, for example, 
in dynamic sampling, where the application of voltage is timed with 
respect to some diffusant producing event. 
This invention includes a method for controlling the transport of compounds 
across a membrane by inducing an EHD flow in a fluid layer contained 
within a structure wherein the EHD flow alters the passage of diffusant 
molecules across the fluid membrane. In one embodiment of the invention, a 
chemical agent is added to the fluid layer to enhance diffusion and 
separation of specific molecules. 
This invention further includes a method for separating molecules by 
inducing an EHD flow in a fluid layer contained within a structure, the 
EHD flow altering the passage of specific molecules across the fluid 
layer. 
The configuration of the ECLCM basically consists of two gas permeable 
electrodes and a fluid layer. This design is very important and separates 
the present invention from prior art non-gas permeable electrodes, for 
example, those used in devices to regulate heat flux. The electrodes must 
be gas permeable and guard against the leakage of the fluid into the pores 
of the electrodes. The fluid layer is contained between two electrodes 
forming a sandwich-like membrane structure, which contains the fluid but 
allows the passage of gas through the membrane. The electrodes enable 
application of an electric field to the fluid, producing EHD turbulence 
and mixing of the fluid layer, facilitating mass transport through the 
membranes. 
For this permeability enhancement mechanism to be effective, the membrane 
structure requires that the gas permeability of the electrodes be large 
compared to that of the liquid crystal layer at rest. Thus, the barrier 
layers on them are ideally either thin compared to the fluid layer or 
materials of higher permeability or both. In one embodiment, this membrane 
may be used as a "switch" to rapidly transport solutes to a sensing 
element while protecting the sensor from the gas environment. The signal 
(solute concentration) to the sensor can be further enhanced by the 
addition of a complexing agent in the membrane which selectively and 
reversibly binds with the solute of interest (facilitated transport). 
The important advantages of the present invention are achieved by the use 
of the EHD flow rather than through the use of a change of order or phase. 
The present invention differs from membranes of the prior art in that it 
uses an electric field to induce a flow to disorder the fluid which 
results in the alteration of mass transport across the fluid region, 
whereas the prior art applied the electric field to enhance the order in a 
liquid crystal layer. 
In contrast to the Sparer membrane (supra), the present invention does not 
involve either a phase change or heating of a fluid layer. Because the 
present invention makes use of fluids capable of exhibiting EHD flow, much 
less energy input is required and there is no resulting heat induction of 
the fluid membrane. Additionally, the present invention is directed to a 
method of separating molecules, not to a method of controlling drug 
delivery as taught by the Sparer patent. In contrast to the Aoyagi 
invention (supra), the present invention uses the application of an 
electric field to induce EHD flow in a fluid capable of exhibiting EHD 
flow, rather than using electricity to generate heat. The permeability 
characteristics of the present invention are not the result of either a 
phase change or heating of the fluid layer. Further, the present invention 
teaches the enhancement of the passage of specific diffusants by addition 
of other components to a fluid layer capable of exhibiting EHD flow, 
including complexing agents with high affinities for specific molecules 
for selective enhancement and separation of such molecules. 
Select fluids can exhibit a variety of EHD effects, in which an applied 
electric field induces flow of the fluid coupled with molecular 
reorientation and electrical current. The EHD flow does not result in a 
phase change in the fluid. In certain fluid materials, like the 
thermotropic fluid nematics, the EHD effects can be quite pronounced, with 
electric fields of a few volts/micron producing fully turbulent flow. This 
turbulence is well known in the art to make the fluid highly scattering to 
visible light, leading to the concept of the "dynamic scattering" mode of 
liquid crystal electro-optics. EHD effects have been widely studied in 
nematics and are discussed in the standard texts on liquid crystals (P. G. 
deGennes (1974) Liquid Crystals, Oxford Press, London); B. S. 
Chandrasekhar (1977) Liquid Crystals, Cambridge University Press, 
Cambridge). 
EHD flow appears at a well-defined electric field threshold as a periodic 
pattern of cells of vortex-like steady-state fluid motion. As the field 
strength is increased, a sequence of additional instabilities leads first 
to oscillatory flow which is periodic in time, then, at higher fields, 
quasi-periodic in time, and then, at the highest fields, chaotic fully 
developed turbulent flow, having a local fluid velocity that changes on 
length and time scales which decrease as the field strength is increased. 
A variety of factors are known to enhance the EHD effect. For example, 
fluid material with negative dielectric anisotropy generally favors a low 
threshold for initial instability. Additionally, some doping of the fluid 
with ions is beneficial. There are several commercially available liquid 
mixtures developed for dynamic scattering applications in which EHD 
turbulence can be generated with electric fields of about 1 volt/micron. 
EHD flow should also be achievable in many of the lyotropic liquid crystal 
phases in which rod or lamellar shaped micellar aggregates orientationally 
order. 
To illustrate the advantages of using EHD-induced flow to facilitate 
transport, EHD-induced flow can be compared to gradient-driven diffusive 
transport. In gradient-driven diffusive transport, a concentration 
difference sets up a diffusant concentration gradient in the fluid and 
thereby a gradient-driven diffusant flux. If c.sub.1 and c.sub.2 are the 
concentrations of a diffusant molecule in the adjacent phase on either 
boundary of a fluid layer of thickness t, D is the diffusion constant, and 
S is the partition coefficient between the fluid phase and the adjacent 
phase, the steady-state flux from the higher to the lower concentration is 
described by the equation: 
EQU J.sub.t =SD(c.sub.2 -c.sub.1)/t 
The effect of EHD-induced flow is most easily visualized by considering 
fully developed turbulent flow. In this situation, the fluid motion 
homogenizes the concentration in the center of the fluid, described by the 
following equation: 
EQU c.sub.m =S(c.sub.1 +c.sub.2)/2 
Near the surfaces of the fluid layer, the flow will be suppressed by the 
boundary condition at the substrates, leaving a boundary layer across 
which the concentration gradients will appear. At sufficiently high 
electric field strength, the boundary layer will be of a thickness that is 
small compared to the fluid thickness t, in which case the gradients will 
be large compared to that noted above in the absence of flow. The 
diffusive flux will now be given by the equation: 
EQU J.sub.b =SD(c.sub.2 -c.sub.1)/2b=J.sub.t t/2b 
which is larger than J.sub.t by the factor t/2b. With typical nematic 
liquid crystal materials developed from EHD flow, b comparable to 1 micron 
should be achievable. Thus, for a 50 micron thick fluid layer, flux ratios 
of J.sub.b /J.sub.t of about 25 can be achieved. As the applied field is 
lowered, the flux ratio will decrease, approaching 1 as the threshold of 
the first instability to steady flow is approached. 
EHD-induced flow can be achieved not only in nematic liquid crystal media, 
but also in such media in which other monomolecular, polymeric, or 
particulate components are mixed. This leads to a variety of additional 
means of employing EHD flow in fluid membranes. For example, chemically 
specific agents can be incorporated in the fluids which exhibit high 
affinity for a particular diffusant. In an EHD-induced flow, the particles 
will be convected back and forth across the fluid layer, picking up 
diffusant on one side and depositing it on the other. This method offers 
the significant advantage of flexibility of chemically specific agent 
selection for particular diffusant application. 
A second important class of applications of the present invention exploits 
the differences in the partition coefficient of various diffusant 
molecules in the fluid region. A principal means known in the art of 
separating one molecular species from another in a mixture is to contact 
the mixture with a second phase in which one of the species is much more 
soluble. Higher solubility (S) means larger concentrations in the fluid 
and thereby larger flux (J), using the above arguments. The use of EHD 
induced flow can markedly enhance the performance of such selective 
membranes by employing the flow to increase the effective diffusion 
coefficient (D), as discussed above. Thus, the separation factor (ratio of 
SD for two components) becomes the ratio of S for the two components as D 
approaches the same value for each component. The product SD is normally 
termed permeability. The solubility in the fluid medium will provide 
selectivity while the EHD flow maximizes flux through the membranes. 
In a non-limiting example, a complexing agent (carrier) is added to the 
fluid phase with particular affinity for some selected diffusant. Since 
either an aliphatic-aromatic or partially aqueous lyotropic liquid crystal 
solvent can be chosen, a wide variety of complexing agents can be 
incorporated into the fluid region. For example, the particles can be 
catalytic. Thus, a reaction and a separation can take place within the 
structure. One specific example of such particles would be zeolites. 
The complexing agent can be added to the fluid phase in several ways. These 
include dissolution in the fluid and chemical attachment to the fluid 
molecules. Also, the complexing agent can be attached to solid particles 
or dissolved in fluid droplets with are dispersed in the fluid phase. 
The incorporation of a complexing agent has two distinct advantages. The 
total solute concentration in the fluid region c.sub.m is increased. This, 
in turn, increases the solute flux across the fluid membrane. Also, the 
specificity or separation factor is increased since the solubility of the 
other components in the feed phase is not increased. The incorporation of 
a complexing agent in static liquid films is known to increase 
permeability by producing flux enhancements up to several hundred fold. 
Typically, the flux enhancement increases as the solute feed concentration 
decreases. This is due to the large solute transport due to the carrier at 
low solute driving forces. Mass transport in these systems is 
diffusion-limited under conditions of large flux enhancement. Thus, the 
use of EHD flow should provide even larger enhancements. 
An additional distinct advantage of the incorporation of a complexing agent 
in this process is the ability to separate and concentrate solutes. In an 
EMC process, complexing agents have two oxidation states with large 
changes in solute binding between oxidation states. The electric field 
which is used to induce EHD can also be used to perform redox reactions at 
each electrode. In this manner, the solute can be bound at the feed side 
of the membrane in one oxidation state of the complexing agent and 
released on the opposite side (permeate side) in the second oxidation 
state. Due to change in the oxidation state, solute can be released at the 
permeate side at a higher concentration than in the feed phase. 
The fluid membrane of the present invention is described in Example 1. 
Example 2 describes the determination of permeation flux differences for 
two individual gases, CH.sub.4 and N.sub.2. Example 3 describes the 
permeability of CH.sub.4 and N.sub.2 through the ECLCM. Example 4 
describes the permeability and selectivity of solvents other than MBBA for 
N.sub.2, H.sub.2, CH.sub.4, and CO.sub.2. 
FIG. 1 shows the setup used to observe the convective motion of the liquid 
crystal or fluid which exhibits EHD flow in an electric field. Glass 
plates 2 and 2' on each side of the fluid layer 1 are used to observe the 
convective motion. Brass electrodes 3 and 3' are used to impose the 
electric field. FIG. 3 shows the configuration of the ECLCM cell for 
differential pressure operation. The fluid layer 1 exhibits EHD flow. An 
electrode 2 is on each side of the fluid 1 and is connected to the AC 
field generator 7. The electrodes are porous to allow rapid gas permeation 
through that portion of the device. The applied field is located in the 
space between the electrodes 2. A barrier layer 3 prevents the fluid 1 
from entering the pores of the electrodes 2. A plastic screen 4 and SS 
screen 5 function as mechanical supports which allow a pressure drop to be 
applied across the device without deflecting the electrodes or attached 
materials. The PTFE ring 6 is a mechanical support to enclose the fluid 
layer 1 and is also filled with fluid through an opening on top of the 
ring. The zero differential pressure system of FIG. 4 has a liquid layer 1 
which exhibits EHD flow. Two chambers 2 and 3 are filled with different 
individual gases at the same volume and pressure. A liquid piston 5 in a 
pipette was used to measure the volume change in the two chambers. Two 
electrodes 6 on either side of the fluid layer 1 are connected to the AC 
field generator 4. The permeation system for an individual gas of FIG. 5 
has a liquid layer 1 between two electrodes connected to an AC field 
generator 2. A small chamber 8 was evacuated with a vacuum 5 and the 
pressure in it monitored by a low-pressure gauge 3. A constant gas 
pressure was maintained in large chamber 9 and monitored by a pressure 
gauge 4. The gas pressure was controlled by a surge tank 7 to which was 
connected a gas inlet 6. 
EXAMPLE 1 
The Electro-Convective Liquid Crystal Membrane (ECLCM) 
The liquid crystal MBBA was employed because the EHD results on MBBA cells 
are available (Hwalek and Carr (1987) supra; Winkle et al. (1990) Mat. 
Res. Soc. Symp. Proc. 177:311-316). The ambient temperature is within the 
temperature range of its nematic phase. It was also found that its 
electroconvective flow rate increases with increasing frequency, at 
constant voltage, to a maximum enhancement at about 40 Hz at room 
temperature, and the frequency between 25 and 90 Hz is desirable for the 
enhancement (Winkle et al. (1990) supra). The frequency of the power 
supply (60 Hz) was chosen in this work, and all the experiments are 
conducted at ambient temperature, approximately 22.degree. C. 
The cell geometry used to observe the convective motion of liquid crystal 
in an electric field is shown in FIG. 1(a). A thin layer of a nematic 
liquid crystal, MBBA, was contained between two parallel nonporous 
electrodes spaced 1.7 mm apart. The thickness of the liquid layer was 
about 75 microns. The setup (FIG. 1(b)) consists of an optical microscope 
with a polarizer and an analyzer, a CCD camera, a video recorder and a 
television monitor. A series of photomicrographs at 4 different voltages 
taken directly from the television monitor are shown in FIG. 2. In the 
off-state (i.e., no voltage on the cell), the LC molecules were in a state 
of relative rest and some crystal defects (walls) could be clearly 
observed (FIG. 2a). When an AC electric field was applied to the LC layer, 
a distortion and reorientation of the LC molecules alignment was observed. 
Related with this distortion was a slow circulating cellular flow of the 
LC (FIG. 2b). Along with the increase of the AC voltage, the cellular flow 
developed into a convective motion (FIG. 2c) and then a turbulent 
convective flow was formed (FIG. 2d). 
The ECLCM cell is shown in FIG. 3. The nematic liquid crystal is the 
dielectric in a porous capacitor formed by a pair of porous silver filters 
coated by an approximately 50 micron thick silicon rubber film on the LC 
side by a Celgard 2400. For experiments in which a differential pressure 
is applied across the membrane, a piece of stainless steel screen and 
another piece of sieved plastic plate were used as the backing of the two 
electrodes, respectively. The thickness of the fluid layer is about 9.63 
mm and that of the sieved plastic place is 4.67 mm. The dimensions of the 
fluid layer were chosen to insure convection and do not imply an optimal 
length. The permeation cell could work under more than 930 torr of 
differential pressure. The silicon rubber film serves as a gas permeable 
fluid-impermeable barrier which keeps the fluid from filling the pores of 
the silver electrode. If the fluid were to fill the silver pores, the 
electrode permeability would be drastically lowered and little 
electroconvective enhancement achieved. 
EXAMPLE 2 
Determination of permeation flux difference of two gases using a liquid 
piston. 
The initial gas permeation experiments were conducted without any total 
pressure difference using the system shown in FIG. 4. The thickness of the 
fluid layer between two silicon rubber coated porous silver electrodes was 
about 3.5 mm, the diameter of the membrane was 41.5 mm and there was no 
stainless steel or plastic screen backing for the electrodes. There were 
two chambers filled with different individual gases, N.sub.2 and CH.sub.4 
respectively, at the same volume and pressure. A liquid piston in a 
pipette was used to monitor the volume change of the gases in the two 
chambers. The necessary pressure to drive the liquid piston was less than 
0.1 torr. The effect of temperature on the experimental results could be 
omitted since the chambers were at the same temperature. There was no 
pressure difference between the two chambers during the experiment. 
The permeation flux difference of two individual gases in the two chambers, 
which is monitored using the liquid piston, can be estimated based on the 
equation suggested by Henis and Tripodi (1981) J. Membrane Sci. 8:233-246: 
EQU F.sub.i =Q.sub.i A.DELTA.P.sub.i /L (1) 
The .DELTA.P.sub.i refers to the partial pressure difference of gas i. It 
is approximately a constant .DELTA.P.sub.o during the experiment. The gas 
volume difference observed by the movement of the liquid piston can be 
estimated by: 
EQU dV/dt=1/2(F.sub.1 -F.sub.2) (2) 
EQU dV/dt=1/2(Q.sub.1 -Q.sub.2)A.DELTA.P.sub.0 /L (3) 
The gas permeation enhancement can be observed by monitoring the movement 
of the liquid piston directly. But in fact, the gas permeation resistance 
of the two electrodes should not be omitted here. It is inconvenient to 
estimate the permeability of the fluid for individual gases in the 
electric field based on the results obtained through the zero differential 
pressure system. 
The smaller chamber shown in FIG. 4 was evacuated before starting the 
experiment, and the gas pressure in it was monitored by a pressure gauge. 
The gas pressure in the larger chamber was constant (P.sub.0) and 
controlled by a surge tank. The volume of the smaller chamber was a 
constant V.sub.0, and the individual gas pressure in it, P.sub.i, which is 
much smaller than that in the larger chamber, P.sub.0, can be estimated 
using the equations: 
EQU V.sub.0 dP.sub.i /dt=F.sub.i P.sub.s (4) 
Substituting for F.sub.i : 
EQU dP.sub.i /dt=Q.sub.i AP.sub.0 P.sub.s /LV.sub.0 (5) 
When the convection of the fluid is induced, the mass transfer resistance 
of the fluid layer is extremely reduced. In this case, the resistance of 
the electrodes has to be considered. A multilayer composite membrane model 
is effective here. The permeation flux can be estimated according to the 
total resistance of the membrane which is the sum of the resistance of all 
the layers (Henis and Tripodi (1981) supra): 
EQU F.sub.i =-.DELTA.P.sub.i /R.sub.c +R.sub.b (6) 
The R.sub.b is determined from gas permeation measurements without an fluid 
layer and the resistance of the fluid layer is given by: 
EQU R.sub.c =L/Q.sub.ci A (7) 
The R.sub.c will be reduced as convection is induced in the fluid layer and 
the Q.sub.ci, the gas permeability in the fluid layer, can be estimated 
by: 
EQU Q.sub.ci =L/R.sub.c A (8) 
The system was operated with a constant feed pressure and vacuum on the 
downstream side of the membrane. The system was allowed to go to 
steady-state before any data was collected. Therefore, there was no 
initial time lag in pressure due to the gas accumulation in the membrane. 
The results obtained using the zero differential pressure system of FIG. 4 
are shown in FIG. 6. The CH.sub.4 permeated preferentially to the N.sub.2. 
When the alternating current voltage reached up to 1300 volts, the 
permeation flux difference of the two gases increased up to 7 times that 
measured without an electric field. The total mass transfer resistance of 
the composite membrane almost reached the value obtained with only the two 
electrodes alone, that is, at high field the permeation rate of the gases 
was limited by the resistance of the two porous silver electrodes and its 
coating of silicone 
EXAMPLE 3 
CH.sub.4 and N.sub.2 Permeability 
The permeability of CH.sub.4 and N.sub.2 through the ECLCM under pressure 
was determined. The results are shown in FIG. 7. The effective 
permeability of gases through the ECLCM can be enhanced by an alternating 
current electric field, and that the maximum permeation flux is limited by 
the electrodes as in the pressure-free case. The gas permeation of the two 
electrodes without a fluid layer was determined using the same system in 
same condition. From the results resistance of the two electrodes was 
calculated to be 5,8.times.10.sup.5 sec cm Hg/cm.sup.3 for CH.sub.4 and 
18.6.times.10.sup.5 for N.sub.2. The permeability of the fluid for 
CH.sub.4 and N.sub.2 has been estimated according to equations 4-8. The 
results are shown in FIG. 8. When the sandwich membrane was in the off 
state, the gas permeation through the fluid layer was controlled by the 
molecular diffusion process. The permeability of the fluid layer was about 
2.3.times.10.sup.-8 cm.sup.3 (STP) cm/sec cm.sup.2 cm Hg for CH.sub.4 and 
0.9.times.10.sup.-8 for N.sub.2. When the AC voltage was increased to 3000 
V, the permeability of the fluid layer was increased by a factor of more 
than 50, and the resistance of the fluid layer was less than one fourth of 
that of two electrodes. For this experiment, a defect-free coating is very 
important for the electrodes. If the fluid soaked into the silicone rubber 
coating and partly into the silver films, the permeation flux could be 
reduced. 
It can be seen from the results shown in FIG. 8 that the permeability of 
the fluid layer for CH.sub.4 determined under varying pressure differences 
are the same, and no obvious variation of the selectivity for CH.sub.4 
from N.sub.2 was found at different AC fields. On the other hand, the 
apparent permeation flow rate (FIG. 7) is directly proportional to the 
pressure difference and the selectivity of this sandwich membrane is kept 
between 2.4 and 3.2 which are that of MBBA and silicone rubber, 
respectively. The total electric current is much less than 0.1 mA. The 
selectivity of such a sandwich membrane depends on the mass transfer 
resistance of both the fluid layer and the electrodes: 
EQU .alpha..sub.ij =L.sub.b R.sub.bj +L.sub.c R.sub.cj /L.sub.b R.sub.bi 
+L.sub.c R.sub.ci (9) 
When the convective effect in the fluid layer is maximized, the 
permeability of gases in the fluid layer just depend on the gas 
solubility. The Henry's Law Coefficient of some gases and solvent vapors 
are shown in Table 1. When the permeability of gases in the fluid is very 
high, the total selectivity may be controlled by the electrodes. 
TABLE 1 
______________________________________ 
Henry's Law Coefficient of Gases in MBBA 
Gas H [cm.sup.3 (STP)/ cm.sup.3 cm Hg] 
______________________________________ 
N.sub.2 7.1 E-4 
O.sub.2 1.7 E-3 
CH.sub.4 2.6 E-3 
CO.sub.2 1.3 E-2 
CH.sub.3 CH.sub.2 CH.sub.3 
4.6 E-2 
CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.3 
1.1 E-1 
CH.sub.3 CH.sub.2 OH 
1.8 E-1 
CH.sub.3 OH 3.7 E-2 
ClCH.dbd.CCl.sub.2 
1.6 E-2 
HCOOH 2.1 E-2 
CH.sub.3 COOH 2.6 E-3 
______________________________________ 
EXAMPLE 4 
Permeability and Selectivity of Other Select Solvents for N.sub.2, H.sub.2, 
CH.sub.4, and CO.sub.2. 
The permeation flux of individual gases through liquid membranes supported 
by Celgard 2400, under a gas pressure of 405 torr and in the absence of an 
electric field were determined. The permeability of 2-ethylhexanol (2EH) 
and 1-octanol was estimated by comparing their permeation flux (Q.sub.s) 
with that of MBBA(Q.sub.MBBA): P.sub.s =P.sub.MBBAQ s/Q.sub.MBBA (Table 
2): 
TABLE 2 
______________________________________ 
PERMEABILITY AND SELECTIVITY OF SOME SOL- 
VENTS FOR N.sub.2, H.sub.2, CH.sub.4, AND CO.sub.2 / 
Solvent 
P.sub.N2 
P.sub.H2 
P.sub.CH4 
P.sub.CO2 
.alpha..sub.CO2/H2 
.alpha..sub.CO2/CH4 
.alpha..sub.CO2/N2 
______________________________________ 
MBBA 0.9 4.0 2.3 19.2 4.78 8.33 21.28 
2EH 6.29 14.04 17.92 
58.56 
4.17 3.36 9.31 
1- 3.39 17.4 20.8 70.7 4.06 3.4 20.86 
octanol 
______________________________________ 
Permeability (P.sub.2) is expressed as .times.10.sup.-8 cm.sup.3 (STP) 
cm/sec cm.sup.2 cm Hg. .alpha..sub.A/B is the selectivity of gas A from 
gas B (P.sub.A /P.sub.B). The permeation rate was dramatically increased 
for MBBA, 2EH, and 1-octanol when an EHD flow is induced (Table 2 and FIG. 
8).