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Electrorheological Phenomena in Lecithin–Decane–Water Mixtures
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
179, 201–206 (1996)
0203
Electrorheological Phenomena in Lecithin–Decane–Water Mixtures YU . A. SHCHIPUNOV * ,1
AND
P. SCHMIEDEL†
*Institute of Chemistry, Far East Department, Russian Academy of Sciences, 690022 Vladivostok, Russia; and †Lehrstuhl fu¨r Physikalische Chemie I, Universita¨t Bayreuth, D-95470, Bayreuth, Germany Received June 3, 1994; accepted August 22, 1995
Preliminary results are presented on the phase behavior and the effects of a weak external electric field on ternary mixtures composed of soybean lecithin, n-decane, and small amounts of water. At a water content in the range 1 to 2.1 water molecules per lecithin molecule, a highly-viscous homogeneous organogel, previously described by Luisi et al., has been obtained. The voltage applied has induced a phase separation and a significant decrease in the solution viscosity. These effects are possibly due to an orientation of tubular reverse micelles in respect to the electric field lines, decreasing their entanglement coupling. At a water content exceeding 2 molecules per lecithin molecule, two-phase systems have been formed which consisted of a low-viscous solution and compact gel or solid precipitate. In the latter case the electric field first has brought about the typical electrorheological response, i.e., increased viscosity. On further processing, a sharp drop in the viscosity has been observed. Optical examination has shown that under the action of an external electric field the solid particles assembled into columns have experienced phase transformations initially into transparent birefringent mesophase and finally into optically isotropic mass. q 1996 Academic Press, Inc. Key Words: lecithin; organogel; electrorheological phenomena; birefringence.
INTRODUCTION
It has been recently found by Luisi and collaborators (1– 4) that the addition of trace amounts of water in nonaqueous solution of soybean lecithin has induced the formation of jelly-like phases (organogels). The thickening effect is caused by transformation of spherical reverse micelles into tubular ones. The latter, sometimes called ‘‘living’’ or ‘‘equilibrium’’ polymers, form an entangled network which shows similarities with that of polymers in the semidilute regime in good solvents. In particular, the rheological properties of the lecithin organogel and polymer solutions resemble each other. The formation of jelly-like phases has been also observed (5–8) in circumstances where an external electric field has been applied to a two-phase system composed of water and 1
To whom correspondence should be addressed.
nonaqueous phospholipid solution. The field has induced a set of interfacial phenomena. They all have been brought about by transfer of water in organic phase. This suggests that the jelly-like phases produced with the help of an external electric field should bear a resemblance to the organogels prepared by dissolving certain quantities of water in the nonaqueous solution, as made by Luisi with collaborators. The noteworthy feature of the jelly-like phases generated by voltage applied is that they can be regulated by an external electric field (5–8). If the gel thus obtained actually resembles the organogel ascertained by Luisi et al., in such a situation, the latter should be sensitive to the action of an applied voltage too. So far the possibility of this sensitivity has not been explored. It therefore seems worthwhile to examine the effects of an electric field on the lecithin organogels produced by dissolving trace amounts of water in the lecithin solutions. Here we consider the behavior of ternary mixtures composed of soybean lecithin, n-decane, and water under the action of an external field. Seeing that the added water has first of all brought about drastic changes in the viscoelastic properties of nonaqueous phospholipid solutions (1–4), mainly rheological measurements have been performed. Attempts will be made to determine the phospholipid structures exhibiting a response to the applied voltage and to understand the mechanisms of the sensitivity to external influences. EXPERIMENTAL
Materials Soybean lecithin from Sigma was used without any additional purification. n-Decane was of purum quality (Fluka). Doubly distilled water was obtained in the common manner. Methods Gel preparation. Initially, a weighed sample of lecithin, needed for a 10 g/liter concentration, was dissolved in ndecane at ambient temperature. This usually proceeded over some hours. Then a calculated quantity of water was added. To reach equilibrium, the solutions, stirred from time to
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time, were allowed to stay at ambient temperature for 3 to 7 days. They were kept in bottles of dark glass and, after adding each component, dried inert gas was bubbled through the solutions to prevent or at least moderate the oxidation processes. Rheological measurements. Oscillatory shear measurements were carried out in the frequency range 0.001–5 Hz with the Bohlin CS Rheometer. Two types of cells were used. For the measurements without the electric field, a double-gap system with concentric cylinders was employed. The effects of an external field were studied in a cell of in-house design. It was equipped with an oscillating horizontal disc immersed in an cylinder. The gap between the bottom of the latter and the disc could be regulated from 0 to 5 mm with a precision {0.1 mm. A voltage from an external source was applied to the disc and the bottom of the cylinder. Optical studies. They were made by using a rectangular spectrophotometer cell equipped with two horizontally positioned platinum wire electrodes. Light from a halogen lamp was directed through a water heat filter and two crossed polarizers placed on opposite sides of the cell. RESULTS AND DISCUSSION
Water-Induced Phase Transitions in a Lecithin–Oil– Water System After dissolving water in a 10 g/liter phospholipid solution, a significant change in its viscoelastic properties has been observed. n-Decane alone and n-decane with added lecithin, without dissolved water, have exhibited Newtonian rheology with low viscosity (1.3 mPas, at 257C). The gradual addition of water has led to a transformation of the low viscous fluid to a viscoelastic one, the rheological behavior of which has become similar to that found for entangled polymers in the semidilute regime (Maxwell behavior) (1– 4, 9). The dynamic viscoelastic functions—the storage modulus and the loss modulus—as well as the zero-shear viscosity have been enhanced ca. 10 5 times. This is seen from the initial ascending portion of the curve in Fig. 1, representing a relationship between the logarithm of zeroshear viscosity and the number of molecules of added water per molecule of lecithin.2 As the water content increased to 2.1 H2O molecules, its further dissolution made the zero-shear viscosity decrease. The effect is caused by separating the homogeneous jellylike phases into low viscous fluid and highly viscoelastic 2 Specimens of the naturally occurring lecithin always contain some quantity of tightly bound water that is severely difficult to remove. Usually, a water content in phospholipid samples, prepared by the same procedure, is almost invariable. The soybean lecithin holds from 1 to 2 H2O molecules per phospholipid molecule (10). Since the perfect amount of intrinsic water is unknown, we indicate throughout the article only the added water.
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FIG. 1. Zero shear viscosity h0 of ternary mixtures composed of soybean lecithin, n-decane, and water versus the number of added water molecules per phospholipid molecule. The vertical lines indicate approximate boundaries between phases: (1) one-phase low viscous solution with Newtonian rheology; (2) one-phase homogeneous fluid (jelly-like phase) with viscoelastic rheological behavior; (3) two-phase mixture consisted of low viscous solution and highly viscous gel; (4) two-phase mixture composed of low viscous solution and solid precipitate. The full circles within the region 3 show measurements at low oscillation frequencies ( õ10 mHz); empty circles show measurements at high oscillation frequencies over a range of 10–100 mHz. The observed differences in viscosities are caused by an influence on the measured parameters of the separated gel that mixes quite easily with the upper less-viscous solution in the course of measurements.
compact mass settling to the bottom. Continuing with the further addition of water, we have observed a solidification of the gel and its transformation into a nontransparent solid precipitate. An approximate boundary between the corresponding two-phase regions 3 and 4 is shown with a vertical line on the right in Fig. 1. Thus, it is to be inferred from the foregoing consideration that the water induces a sequence of structural reorganizations of phospholipid aggregates in organic media. Before water dissolved, the initial nonaqueous solution contains spherical reverse micelles (region 1 in Fig. 1). With its gradual addition, they undergo, as ascertained by Schurtenberger et al. (3, 11, 12), by means of small-angle neutron scattering and quasielastic light scattering, a one-dimensional elongation and a rearrangement from spheres into the flexible rods. The micelle shape change, consistent with the pseudophase transition, leads to entanglement coupling of the rod-shaped aggregates and the formation of an entangled temporary three-dimensional network, responsible for the extremely large zero-shear viscosities (region 2, Fig. 1). The lecithin organogel thus obtained contains a considerable amount of hydrocarbon (ca. 85% by weight) entrapped in the space between the entangled worm-like reverse micelles. The expanding of the micelles along their major axis continues up to 2.1 water molecules per lecithin molecule. The further addition of water has caused a collapse of the network structure. This has been manifested in the decrease in zeroshear viscosity and in the gel separation as a second phase.
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FIG. 2. Zero-shear viscosity in the electric field h 0E relative to the zeroshear viscosity without a voltage-applied h0 as a function of the electric field strength E. The water content in the mixture is 2.1 H2O molecules per lecithin molecule.
FIG. 3. Temporal changes of viscosity h measured at a constant oscillation frequency of 17 Hz at 257C. The water content is 4.5 H2O molecules per lecithin molecule. The arrow marks the time t when a 80 kV/m electric field has been applied.
The latter also implies the issue of the organic solvent entrapped in the network structure. The process has come to a close when the condensed gel has transformed into a solid state. The nature of the last phase transformations has not been revealed yet.
though all the observed optical effects are caused by an ordering arrangement of the phospholipid aggregates, the processes in each of the particular cases ought to be differentiated. In the homogeneous jelly-like phase ( region 2, Fig. 1 ) , which is composed of rod-shaped reverse micelles ( 12, 13 ) , the electric field makes the rods polarized and oriented in respect to the electric field lines. The ordering of cylindrical micellar aggregates reduces their entanglement coupling and perturbs the transient network structure, being responsible for the viscoelastic properties of the organogel. As a consequence, the zero-shear viscosity tends to be diminished ( Fig. 2 ) . There is another concomitant effect as well. The ordering arrangement under the action of an external electric field is accompanied by a tightening up of the gel into a more compact mass in the interelectrode space. This results in the organic solvent—entrapped in the space between entangled cylindrical micelles—being released. With the voltage switched off, the gel thus separated has settled on the bottom
Effects of External Electric Field The electric field has brought about changes in the rheological properties of both the jelly-like phases and the twophase mixtures but effects have diverged considerably. When a voltage has been applied, the zero shear viscosity and the dynamic viscoelastic functions have undergone a dramatic decrease or increase that has depended on the type of ternary mixture. Viscosity decrease has occurred in the homogeneous jelly-like phases and in the two-phase system with separated gel (regions 2 and 3 in Fig. 1, respectively). The effect has been particular pronounced in the former case. This is illustrated by Fig. 2, which shows how the zeroshear viscosity drops with successive rising electric field strength. The opposite effect, namely, the increased zeroshear viscosity and dynamic viscoelastic functions, has taken place in the two-phase mixtures with solid particles (region 4, Fig. 1). The viscosity, changing with time after the application of a 80 kV/m electric field (indicated by the arrow) at a fixed shear rate, can be seen in Fig. 3. Furthermore, the initial low viscous mixture, exhibiting Newtonian behavior, has taken on, as evidenced by Fig. 4, viscoelastic properties. They are similar to those observed in homogeneous jellylike phases. The above-considered electrorheological phenomena are accounted for by the rearrangement of phospholipid structures under the action of an external electric field. Evidence can be obtained if one follows the behavior of a ternary mixture placed between two platinum electrodes in a spectrophotometer cell. As seen in Fig. 5, an applied voltage has made optically isotropic mixtures strongly birefringent. Al-
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FIG. 4. The complex viscosity h ( D ), storage modulus G * ( s ) and loss modulus G 9 ( l ) as a function of the oscillation frequency for ternary mixture with 4.5 water molecules per molecule of soybean lecithin. The system is under the action of a 80 kV/m electric field.
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FIG. 5. Photographs of anisotropic mesophases produced in the spacing between two platinum electrodes (horizontal dark bands on the top and the bottom) under an external electric field. The electrode gap is 6 mm. The water content is (left) 1.3 and (right) 4.5 H2O molecules per lecithin molecule. The electric field intensity is (left) 125 and (right) 50 kV/m. The photographs were taken in transmitted light with crossed polarizers within 5 min after a voltage was switched on.
of the cell. Visually, this jelly-like mass has borne a similarity to the gel spontaneously precipitating at a water content above 2.1 H2O molecules per lecithin molecule without the help of an external electric field (region 3, Fig. 1). It should be mentioned 3 that the rearrangement of the jelly-like network in the external electric field happens over minutes. The relatively low rate of ordering is explained by the hindrance of micelles in their rotation owing to the overlap of micellar chains. According to the current views (see, e.g., Refs. (14, 15)), the dynamic properties of transient structures of ‘‘living’’ or ‘‘equilibrium’’ polymers are dictated by two processes. One of them is the motion of flexible micellar chains responsible for the reptation, the other consists of the reversible breaking and recombination of micelles. It is apparent that the rearrangement rate of lecithin organogel under the action of an external electric field is determined by a structural relaxation time ( t ). The time t may be equated to t É ( trepttb ) 1 / 2 ,
where trept and tb are the relaxation times for reptation process and for the reversible breaking of micelles, respectively. For the discussed case, the experimental estimate for t has been made in (16). The structural relaxation time for a jellylike network of lecithin occurs within a few seconds. This makes it clear why the viscosity drop (Fig. 2) and the development of birefringence (Fig. 5) take a substantial amount of time. As regards two-phase mixtures with solid precipitate ( region 4, Fig. 1 ) , their response to an applied voltage has resembled in some respects that of the common elec3 The authors are grateful to one of the referees who has called our attention to this matter.
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trorheological fluids, which consist of nonpolar organic solvent and colloid suspensions of various materials ( 13, 17 ) . In both of these juxtaposed systems, the suspended particles, subjected to an electric field, assemble into chains and columns in the space between the electrodes. The fibrinated structures thus obtained are clearly seen on the right in Fig. 5. Such a fibril formation gives rise to zero-shear viscosity by a few orders of magnitude ( the ascending part of the curve in Fig. 3 ) . At the same time, there are noticeable differences in behavior between phospholipid-containing systems and the typical electrorheological fluids. In the ternary mixtures with lecithin the electrorheological response, that is, the increased viscosity, is significantly slower: the transition from one state to another occurs on the minute timescale (Fig. 3) whereas the fibrillation in the electrorheological fluids usually proceeds within milliseconds (13, 17). Moreover, the phenomena take place at various intensities of the electric field. The typical field strengths, reported for a variety of colloid suspensions (see, for example, Ref. 17), are in the range of 500–2,000 kV/m. The oil-based media with dispersed lecithin structures have exhibited the transformation into a highly structured mixture even at 10 kV/m. The high sensitivity of self-organized lecithin assemblies to the applied voltage is in accord with a conclusion drawn in (5–8) that this is the characteristic property of the neutral phospholipids in the immiscible liquids systems. The common electrorheological phenomena are reversible (13, 17). A sequence of the increased/decreased viscosities could also be observed in the phospholipid-containing systems by applying the voltage in an on/off regime. At the same time, the electric field treatment leads to gradual latent alterations in the phospholipid structures within quite a long time, resulting in a spontaneous fast-proceeding phase transformation of them afterwards. This conclusion can be ex-
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FIG. 6. Photographs of the phospholipid mesophase formed in the ternary mixture in the space between two platinum electrodes under an external electric field. The photographs were taken in succession (left) 1 and (right) 5 min after the electric field strength had been increased from 100 to 150 kV/m.
tracted from the temporal dependence of viscosity in Fig. 3. It is seen that at first the voltage applied gives rise to viscosity, then the curve shows a weaker slope. The increase continues for about 20 min. After that, there is a sharp drop in the viscosity. Such a dependence appears to be a consequence of a phase transformation of the phospholipid structures under the action of an external electric field. This inference is confirmed by examining the ternary mixture in transmitted light directed through crossed polarizers. The photographs in Fig. 6 show an anisotropic mesophase (left) obtained from columns of solid precipitate (Fig. 5) as a result of their exposure to field. With further increases of the time of the field treatment, the mesophase has undergone a transformation into isotropic mass (right). Upon removal of the field, a transparent jelly-like phase has been found instead of the initial solid precipitate in the cell. The structure of this gel, as well as the nature of the field-induced transformations has not been revealed yet. Particular emphasis should be placed on the facts of phase transformations in the investigated ternary mixtures with soybean lecithin, governed by weak electric fields. If one takes into account that the jelly-like phases are widely used in biotechnology and pharmacology for the solubilization of drugs and enzymes, cells, and cell particles (see, e.g., (18– 20)), the regulation of the phase state in such systems with the help of voltage could permit, in some cases, a heightening of the effectiveness of processes. CONCLUSIONS
The preliminary study of ternary mixtures of soybean lecithin with n-decane and small amounts of water has showed the phospholipid to be prone to form electrically active structures. The field-induced effects have depended strongly on the type of phospholipid aggregates. At present, the absence of sufficient information does not permit us make this matter
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clear in minute detail, but our experimental results in combination with the known literature data enable us to draw some rather definite conclusions. At a water content of 1 to 2.1 H2O molecules per lecithin molecule in n-decane, at which there is a homogeneous viscous jelly-like phase composed of entangled cylindrical reverse micelles, the application of voltage has resulted in decreasing zero-shear viscosity and dynamic viscoelastic functions. The loss in viscoelastic properties is accompanied by a tightening up of the gel and the release of organic solvent. The perturbation of the entangled network structure, because of the orientation of the rod-shaped phospholipid aggregates in respect to the electric field lines, appears to be responsible for the observed effects. Mixtures of soybean lecithin at a higher water content ( ú4 H2O molecules per phospholipid molecule), at which there is a solid precipitate, have exhibited electrorheological phenomena: the voltage applied has led, as in the case of the common electrorheological fluids with colloid suspensions, to viscoelastic rheological behavior due to the formation of fibrinated structures by the solid particles in the interelectrode gap. However, there are some discrepancies. The electrorheological response in phospholipid-containing mixtures has been observed at significantly lower electric field strengths than occur in typical electrorheological fluids. What is more, a weak electric field has induced a phase transformation of phospholipid structures, manifested in the loss in birefringence and the sharp decrease in viscosity within some period of time. The nature and the mechanisms of the field-induced phase transfer are not clear at present and they are still to be clarified. ACKNOWLEDGMENT Financial support from the Sonderforschungsbereich 213 ‘‘Topomac’’ for the stay of one of us (Y.A.S.) at Bayreuth University is gratefully acknowledged.
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REFERENCES 1. Scartazzini, R., and Luisi, P. L., J. Phys. Chem. 92, 829 (1988). 2. Schurtenberger, P., Scartazzini, R., and Luisi, P. L., Rheol. Acta 28, 372 (1989). 3. Luisi, P. L., Scartazzini, R., Haering, G., and Schurtenberger, P., Colloid Polym. Sci. 268, 356 (1990). 4. Walde, P., Giuliani, A. M., Boicelli, C. A., and Luisi, P. L., Chem. Phys. Lipids 53, 265 (1990). 5. Shchipunov, Yu. A., and Kolpakov, A. F., Kolloidn. Zh. 46, 1204 (1984). 6. Shchipunov, Yu. A., and Kolpakov, A. F., J. Dispersion Sci. Technol. 9, 258 (1988). 7. Shchipunov, Yu. A., and Kolpakov, A. F., Adv. Colloid Interface Sci. 35, 31 (1991). 8. Shchipunov, Yu. A., and Kolpakov, A. F., Colloids Surf. 76, 15 (1993). 9. Hoffmann, H., Lobl, M., and Wunderlich, I., Tenside Deterg. 22, 290 (1985).
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10. Bergenstahl, B., and Fontell, K., Prog. Colloid Polym. Sci. 68, 48 (1983). 11. Schurtenberger, P., Scartazzini, R., Magid, L. J., Leser, M. E., and Luisi, P. L., J. Phys. Chem. 94, 3695 (1990). 12. Schurtenberger, P., Magid, L. J., King, S. M., and Lindaer, P., J. Phys. Chem. 95, 4174 (1991). 13. Block, H., and Kelly, J. P., J. Phys. D 21, 1661 (1988). 14. Cates, M. E., Macromolecules 20, 2289 (1987). 15. Cates, M. E., and Candau, S. J., J. Phys. Condensed Matter 2, 6869 (1990). 16. Shchipunov, Yu. A., Kolloidn. Zh. 57, 591 (1995). 17. Gast, A. P., and Zukoski, C. F., Adv. Colloid Interface Sci. 30, 153 (1989). 18. Fendler, J. H., ‘‘Membrane Mimetic Chemistry.’’ Wiley, New York, 1982. 19. Kadam, K. L., Enzyme Microbiol. Technol. 8, 266 (1986). 20. Leser, M. F., and Luisi, P. L., Chimia 44, 270 (1990).
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0203
Electrorheological Phenomena in Lecithin–Decane–Water Mixtures YU . A. SHCHIPUNOV * ,1
AND
P. SCHMIEDEL†
*Institute of Chemistry, Far East Department, Russian Academy of Sciences, 690022 Vladivostok, Russia; and †Lehrstuhl fu¨r Physikalische Chemie I, Universita¨t Bayreuth, D-95470, Bayreuth, Germany Received June 3, 1994; accepted August 22, 1995
Preliminary results are presented on the phase behavior and the effects of a weak external electric field on ternary mixtures composed of soybean lecithin, n-decane, and small amounts of water. At a water content in the range 1 to 2.1 water molecules per lecithin molecule, a highly-viscous homogeneous organogel, previously described by Luisi et al., has been obtained. The voltage applied has induced a phase separation and a significant decrease in the solution viscosity. These effects are possibly due to an orientation of tubular reverse micelles in respect to the electric field lines, decreasing their entanglement coupling. At a water content exceeding 2 molecules per lecithin molecule, two-phase systems have been formed which consisted of a low-viscous solution and compact gel or solid precipitate. In the latter case the electric field first has brought about the typical electrorheological response, i.e., increased viscosity. On further processing, a sharp drop in the viscosity has been observed. Optical examination has shown that under the action of an external electric field the solid particles assembled into columns have experienced phase transformations initially into transparent birefringent mesophase and finally into optically isotropic mass. q 1996 Academic Press, Inc. Key Words: lecithin; organogel; electrorheological phenomena; birefringence.
INTRODUCTION
It has been recently found by Luisi and collaborators (1– 4) that the addition of trace amounts of water in nonaqueous solution of soybean lecithin has induced the formation of jelly-like phases (organogels). The thickening effect is caused by transformation of spherical reverse micelles into tubular ones. The latter, sometimes called ‘‘living’’ or ‘‘equilibrium’’ polymers, form an entangled network which shows similarities with that of polymers in the semidilute regime in good solvents. In particular, the rheological properties of the lecithin organogel and polymer solutions resemble each other. The formation of jelly-like phases has been also observed (5–8) in circumstances where an external electric field has been applied to a two-phase system composed of water and 1
To whom correspondence should be addressed.
nonaqueous phospholipid solution. The field has induced a set of interfacial phenomena. They all have been brought about by transfer of water in organic phase. This suggests that the jelly-like phases produced with the help of an external electric field should bear a resemblance to the organogels prepared by dissolving certain quantities of water in the nonaqueous solution, as made by Luisi with collaborators. The noteworthy feature of the jelly-like phases generated by voltage applied is that they can be regulated by an external electric field (5–8). If the gel thus obtained actually resembles the organogel ascertained by Luisi et al., in such a situation, the latter should be sensitive to the action of an applied voltage too. So far the possibility of this sensitivity has not been explored. It therefore seems worthwhile to examine the effects of an electric field on the lecithin organogels produced by dissolving trace amounts of water in the lecithin solutions. Here we consider the behavior of ternary mixtures composed of soybean lecithin, n-decane, and water under the action of an external field. Seeing that the added water has first of all brought about drastic changes in the viscoelastic properties of nonaqueous phospholipid solutions (1–4), mainly rheological measurements have been performed. Attempts will be made to determine the phospholipid structures exhibiting a response to the applied voltage and to understand the mechanisms of the sensitivity to external influences. EXPERIMENTAL
Materials Soybean lecithin from Sigma was used without any additional purification. n-Decane was of purum quality (Fluka). Doubly distilled water was obtained in the common manner. Methods Gel preparation. Initially, a weighed sample of lecithin, needed for a 10 g/liter concentration, was dissolved in ndecane at ambient temperature. This usually proceeded over some hours. Then a calculated quantity of water was added. To reach equilibrium, the solutions, stirred from time to
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time, were allowed to stay at ambient temperature for 3 to 7 days. They were kept in bottles of dark glass and, after adding each component, dried inert gas was bubbled through the solutions to prevent or at least moderate the oxidation processes. Rheological measurements. Oscillatory shear measurements were carried out in the frequency range 0.001–5 Hz with the Bohlin CS Rheometer. Two types of cells were used. For the measurements without the electric field, a double-gap system with concentric cylinders was employed. The effects of an external field were studied in a cell of in-house design. It was equipped with an oscillating horizontal disc immersed in an cylinder. The gap between the bottom of the latter and the disc could be regulated from 0 to 5 mm with a precision {0.1 mm. A voltage from an external source was applied to the disc and the bottom of the cylinder. Optical studies. They were made by using a rectangular spectrophotometer cell equipped with two horizontally positioned platinum wire electrodes. Light from a halogen lamp was directed through a water heat filter and two crossed polarizers placed on opposite sides of the cell. RESULTS AND DISCUSSION
Water-Induced Phase Transitions in a Lecithin–Oil– Water System After dissolving water in a 10 g/liter phospholipid solution, a significant change in its viscoelastic properties has been observed. n-Decane alone and n-decane with added lecithin, without dissolved water, have exhibited Newtonian rheology with low viscosity (1.3 mPas, at 257C). The gradual addition of water has led to a transformation of the low viscous fluid to a viscoelastic one, the rheological behavior of which has become similar to that found for entangled polymers in the semidilute regime (Maxwell behavior) (1– 4, 9). The dynamic viscoelastic functions—the storage modulus and the loss modulus—as well as the zero-shear viscosity have been enhanced ca. 10 5 times. This is seen from the initial ascending portion of the curve in Fig. 1, representing a relationship between the logarithm of zeroshear viscosity and the number of molecules of added water per molecule of lecithin.2 As the water content increased to 2.1 H2O molecules, its further dissolution made the zero-shear viscosity decrease. The effect is caused by separating the homogeneous jellylike phases into low viscous fluid and highly viscoelastic 2 Specimens of the naturally occurring lecithin always contain some quantity of tightly bound water that is severely difficult to remove. Usually, a water content in phospholipid samples, prepared by the same procedure, is almost invariable. The soybean lecithin holds from 1 to 2 H2O molecules per phospholipid molecule (10). Since the perfect amount of intrinsic water is unknown, we indicate throughout the article only the added water.
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FIG. 1. Zero shear viscosity h0 of ternary mixtures composed of soybean lecithin, n-decane, and water versus the number of added water molecules per phospholipid molecule. The vertical lines indicate approximate boundaries between phases: (1) one-phase low viscous solution with Newtonian rheology; (2) one-phase homogeneous fluid (jelly-like phase) with viscoelastic rheological behavior; (3) two-phase mixture consisted of low viscous solution and highly viscous gel; (4) two-phase mixture composed of low viscous solution and solid precipitate. The full circles within the region 3 show measurements at low oscillation frequencies ( õ10 mHz); empty circles show measurements at high oscillation frequencies over a range of 10–100 mHz. The observed differences in viscosities are caused by an influence on the measured parameters of the separated gel that mixes quite easily with the upper less-viscous solution in the course of measurements.
compact mass settling to the bottom. Continuing with the further addition of water, we have observed a solidification of the gel and its transformation into a nontransparent solid precipitate. An approximate boundary between the corresponding two-phase regions 3 and 4 is shown with a vertical line on the right in Fig. 1. Thus, it is to be inferred from the foregoing consideration that the water induces a sequence of structural reorganizations of phospholipid aggregates in organic media. Before water dissolved, the initial nonaqueous solution contains spherical reverse micelles (region 1 in Fig. 1). With its gradual addition, they undergo, as ascertained by Schurtenberger et al. (3, 11, 12), by means of small-angle neutron scattering and quasielastic light scattering, a one-dimensional elongation and a rearrangement from spheres into the flexible rods. The micelle shape change, consistent with the pseudophase transition, leads to entanglement coupling of the rod-shaped aggregates and the formation of an entangled temporary three-dimensional network, responsible for the extremely large zero-shear viscosities (region 2, Fig. 1). The lecithin organogel thus obtained contains a considerable amount of hydrocarbon (ca. 85% by weight) entrapped in the space between the entangled worm-like reverse micelles. The expanding of the micelles along their major axis continues up to 2.1 water molecules per lecithin molecule. The further addition of water has caused a collapse of the network structure. This has been manifested in the decrease in zeroshear viscosity and in the gel separation as a second phase.
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FIG. 2. Zero-shear viscosity in the electric field h 0E relative to the zeroshear viscosity without a voltage-applied h0 as a function of the electric field strength E. The water content in the mixture is 2.1 H2O molecules per lecithin molecule.
FIG. 3. Temporal changes of viscosity h measured at a constant oscillation frequency of 17 Hz at 257C. The water content is 4.5 H2O molecules per lecithin molecule. The arrow marks the time t when a 80 kV/m electric field has been applied.
The latter also implies the issue of the organic solvent entrapped in the network structure. The process has come to a close when the condensed gel has transformed into a solid state. The nature of the last phase transformations has not been revealed yet.
though all the observed optical effects are caused by an ordering arrangement of the phospholipid aggregates, the processes in each of the particular cases ought to be differentiated. In the homogeneous jelly-like phase ( region 2, Fig. 1 ) , which is composed of rod-shaped reverse micelles ( 12, 13 ) , the electric field makes the rods polarized and oriented in respect to the electric field lines. The ordering of cylindrical micellar aggregates reduces their entanglement coupling and perturbs the transient network structure, being responsible for the viscoelastic properties of the organogel. As a consequence, the zero-shear viscosity tends to be diminished ( Fig. 2 ) . There is another concomitant effect as well. The ordering arrangement under the action of an external electric field is accompanied by a tightening up of the gel into a more compact mass in the interelectrode space. This results in the organic solvent—entrapped in the space between entangled cylindrical micelles—being released. With the voltage switched off, the gel thus separated has settled on the bottom
Effects of External Electric Field The electric field has brought about changes in the rheological properties of both the jelly-like phases and the twophase mixtures but effects have diverged considerably. When a voltage has been applied, the zero shear viscosity and the dynamic viscoelastic functions have undergone a dramatic decrease or increase that has depended on the type of ternary mixture. Viscosity decrease has occurred in the homogeneous jelly-like phases and in the two-phase system with separated gel (regions 2 and 3 in Fig. 1, respectively). The effect has been particular pronounced in the former case. This is illustrated by Fig. 2, which shows how the zeroshear viscosity drops with successive rising electric field strength. The opposite effect, namely, the increased zeroshear viscosity and dynamic viscoelastic functions, has taken place in the two-phase mixtures with solid particles (region 4, Fig. 1). The viscosity, changing with time after the application of a 80 kV/m electric field (indicated by the arrow) at a fixed shear rate, can be seen in Fig. 3. Furthermore, the initial low viscous mixture, exhibiting Newtonian behavior, has taken on, as evidenced by Fig. 4, viscoelastic properties. They are similar to those observed in homogeneous jellylike phases. The above-considered electrorheological phenomena are accounted for by the rearrangement of phospholipid structures under the action of an external electric field. Evidence can be obtained if one follows the behavior of a ternary mixture placed between two platinum electrodes in a spectrophotometer cell. As seen in Fig. 5, an applied voltage has made optically isotropic mixtures strongly birefringent. Al-
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FIG. 4. The complex viscosity h ( D ), storage modulus G * ( s ) and loss modulus G 9 ( l ) as a function of the oscillation frequency for ternary mixture with 4.5 water molecules per molecule of soybean lecithin. The system is under the action of a 80 kV/m electric field.
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FIG. 5. Photographs of anisotropic mesophases produced in the spacing between two platinum electrodes (horizontal dark bands on the top and the bottom) under an external electric field. The electrode gap is 6 mm. The water content is (left) 1.3 and (right) 4.5 H2O molecules per lecithin molecule. The electric field intensity is (left) 125 and (right) 50 kV/m. The photographs were taken in transmitted light with crossed polarizers within 5 min after a voltage was switched on.
of the cell. Visually, this jelly-like mass has borne a similarity to the gel spontaneously precipitating at a water content above 2.1 H2O molecules per lecithin molecule without the help of an external electric field (region 3, Fig. 1). It should be mentioned 3 that the rearrangement of the jelly-like network in the external electric field happens over minutes. The relatively low rate of ordering is explained by the hindrance of micelles in their rotation owing to the overlap of micellar chains. According to the current views (see, e.g., Refs. (14, 15)), the dynamic properties of transient structures of ‘‘living’’ or ‘‘equilibrium’’ polymers are dictated by two processes. One of them is the motion of flexible micellar chains responsible for the reptation, the other consists of the reversible breaking and recombination of micelles. It is apparent that the rearrangement rate of lecithin organogel under the action of an external electric field is determined by a structural relaxation time ( t ). The time t may be equated to t É ( trepttb ) 1 / 2 ,
where trept and tb are the relaxation times for reptation process and for the reversible breaking of micelles, respectively. For the discussed case, the experimental estimate for t has been made in (16). The structural relaxation time for a jellylike network of lecithin occurs within a few seconds. This makes it clear why the viscosity drop (Fig. 2) and the development of birefringence (Fig. 5) take a substantial amount of time. As regards two-phase mixtures with solid precipitate ( region 4, Fig. 1 ) , their response to an applied voltage has resembled in some respects that of the common elec3 The authors are grateful to one of the referees who has called our attention to this matter.
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trorheological fluids, which consist of nonpolar organic solvent and colloid suspensions of various materials ( 13, 17 ) . In both of these juxtaposed systems, the suspended particles, subjected to an electric field, assemble into chains and columns in the space between the electrodes. The fibrinated structures thus obtained are clearly seen on the right in Fig. 5. Such a fibril formation gives rise to zero-shear viscosity by a few orders of magnitude ( the ascending part of the curve in Fig. 3 ) . At the same time, there are noticeable differences in behavior between phospholipid-containing systems and the typical electrorheological fluids. In the ternary mixtures with lecithin the electrorheological response, that is, the increased viscosity, is significantly slower: the transition from one state to another occurs on the minute timescale (Fig. 3) whereas the fibrillation in the electrorheological fluids usually proceeds within milliseconds (13, 17). Moreover, the phenomena take place at various intensities of the electric field. The typical field strengths, reported for a variety of colloid suspensions (see, for example, Ref. 17), are in the range of 500–2,000 kV/m. The oil-based media with dispersed lecithin structures have exhibited the transformation into a highly structured mixture even at 10 kV/m. The high sensitivity of self-organized lecithin assemblies to the applied voltage is in accord with a conclusion drawn in (5–8) that this is the characteristic property of the neutral phospholipids in the immiscible liquids systems. The common electrorheological phenomena are reversible (13, 17). A sequence of the increased/decreased viscosities could also be observed in the phospholipid-containing systems by applying the voltage in an on/off regime. At the same time, the electric field treatment leads to gradual latent alterations in the phospholipid structures within quite a long time, resulting in a spontaneous fast-proceeding phase transformation of them afterwards. This conclusion can be ex-
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FIG. 6. Photographs of the phospholipid mesophase formed in the ternary mixture in the space between two platinum electrodes under an external electric field. The photographs were taken in succession (left) 1 and (right) 5 min after the electric field strength had been increased from 100 to 150 kV/m.
tracted from the temporal dependence of viscosity in Fig. 3. It is seen that at first the voltage applied gives rise to viscosity, then the curve shows a weaker slope. The increase continues for about 20 min. After that, there is a sharp drop in the viscosity. Such a dependence appears to be a consequence of a phase transformation of the phospholipid structures under the action of an external electric field. This inference is confirmed by examining the ternary mixture in transmitted light directed through crossed polarizers. The photographs in Fig. 6 show an anisotropic mesophase (left) obtained from columns of solid precipitate (Fig. 5) as a result of their exposure to field. With further increases of the time of the field treatment, the mesophase has undergone a transformation into isotropic mass (right). Upon removal of the field, a transparent jelly-like phase has been found instead of the initial solid precipitate in the cell. The structure of this gel, as well as the nature of the field-induced transformations has not been revealed yet. Particular emphasis should be placed on the facts of phase transformations in the investigated ternary mixtures with soybean lecithin, governed by weak electric fields. If one takes into account that the jelly-like phases are widely used in biotechnology and pharmacology for the solubilization of drugs and enzymes, cells, and cell particles (see, e.g., (18– 20)), the regulation of the phase state in such systems with the help of voltage could permit, in some cases, a heightening of the effectiveness of processes. CONCLUSIONS
The preliminary study of ternary mixtures of soybean lecithin with n-decane and small amounts of water has showed the phospholipid to be prone to form electrically active structures. The field-induced effects have depended strongly on the type of phospholipid aggregates. At present, the absence of sufficient information does not permit us make this matter
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clear in minute detail, but our experimental results in combination with the known literature data enable us to draw some rather definite conclusions. At a water content of 1 to 2.1 H2O molecules per lecithin molecule in n-decane, at which there is a homogeneous viscous jelly-like phase composed of entangled cylindrical reverse micelles, the application of voltage has resulted in decreasing zero-shear viscosity and dynamic viscoelastic functions. The loss in viscoelastic properties is accompanied by a tightening up of the gel and the release of organic solvent. The perturbation of the entangled network structure, because of the orientation of the rod-shaped phospholipid aggregates in respect to the electric field lines, appears to be responsible for the observed effects. Mixtures of soybean lecithin at a higher water content ( ú4 H2O molecules per phospholipid molecule), at which there is a solid precipitate, have exhibited electrorheological phenomena: the voltage applied has led, as in the case of the common electrorheological fluids with colloid suspensions, to viscoelastic rheological behavior due to the formation of fibrinated structures by the solid particles in the interelectrode gap. However, there are some discrepancies. The electrorheological response in phospholipid-containing mixtures has been observed at significantly lower electric field strengths than occur in typical electrorheological fluids. What is more, a weak electric field has induced a phase transformation of phospholipid structures, manifested in the loss in birefringence and the sharp decrease in viscosity within some period of time. The nature and the mechanisms of the field-induced phase transfer are not clear at present and they are still to be clarified. ACKNOWLEDGMENT Financial support from the Sonderforschungsbereich 213 ‘‘Topomac’’ for the stay of one of us (Y.A.S.) at Bayreuth University is gratefully acknowledged.
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10. Bergenstahl, B., and Fontell, K., Prog. Colloid Polym. Sci. 68, 48 (1983). 11. Schurtenberger, P., Scartazzini, R., Magid, L. J., Leser, M. E., and Luisi, P. L., J. Phys. Chem. 94, 3695 (1990). 12. Schurtenberger, P., Magid, L. J., King, S. M., and Lindaer, P., J. Phys. Chem. 95, 4174 (1991). 13. Block, H., and Kelly, J. P., J. Phys. D 21, 1661 (1988). 14. Cates, M. E., Macromolecules 20, 2289 (1987). 15. Cates, M. E., and Candau, S. J., J. Phys. Condensed Matter 2, 6869 (1990). 16. Shchipunov, Yu. A., Kolloidn. Zh. 57, 591 (1995). 17. Gast, A. P., and Zukoski, C. F., Adv. Colloid Interface Sci. 30, 153 (1989). 18. Fendler, J. H., ‘‘Membrane Mimetic Chemistry.’’ Wiley, New York, 1982. 19. Kadam, K. L., Enzyme Microbiol. Technol. 8, 266 (1986). 20. Leser, M. F., and Luisi, P. L., Chimia 44, 270 (1990).
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