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Langmuir 1995,11, 1576-1583
Preparing Microemulsions with Lecithins M. Kahlweit,” G. Busse, and B. Faulhaber Max-Planck-Institut fur Biophysikalische Chemie, Postfach 2841, 0-37018 Gottingen, Germany Received December 22, 1994. I n Final Form: February 22, 1995@ We present the results of the first in a series of studies on the microemulsificationof hydrocarbons by biological amphiphiles. As such we use lecithins (C,)zPC of well-defined carbon number m from 8 to 16 and n-alkanes of carbon number K from 8 to 16. For preparing microemulsionswith short-chain lecithins, one has to add butanol as cosolvent,with long-chainlecithins propanol is added as cosolvent. With mediumchain lecithins, one has to add a mixture of the two alcohols. The role of the alcohols is discussed phenomenologically on the basis of isothermal phase tetrahedra. The effect of temperature on the phase behavior is weaker than with nonionic n-alkyl polyglycol ethers (CiEj), or anionic AOT, which makes lecithin-based microemulsions less sensitive to temperature fluctuations. The effect of added chloride salts is weak in lecithin-butanol mixtures, but strong in lecithin-propanol mixtures. We, finally, demonstrate that adding inexpensive soy bean lecithin increases the efficiency of CiEj in microemulsifying alkanes considerably which might be of interest for industrial applications.
Introduction We start by defining what we mean by “preparing a microemulsion”: By varying either a field variable (such as temperature), andor an appropriately chosen density variable (such as the concentration of a fourth component), one can make mixtures of water, oil, and a n amphiphile separate into three coexisting stable liquid phases, namely, an amphiphile-rich phase in equilibrium with a waterrich, and an oil-rich excess phase. With increasing strength of the amphiphile, the amphiphile-rich phase becomes increasingly more structured until it can be viewed as a macroscopically homogeneous, isotropic colloidal dispersion of water and oil domains, separated by saturated interfacial layers of the amphiphile, with the fluctuating domains having a mean size such that the dispersed solvents exhibit the properties of bulk phases. It is this dispersion that we call a “microemulsion” in the narrower sense. For preparing a single-phase microemulsion, one may then vary the mass fractions of the components a t fixed chemical potentials such that the volume fraction of the amphiphile-rich phase grows a t the expense of the two excess phases until water and oil become completely homogenized as microemulsion. It is this procedure that we have in mind when speaking of “preparing a microemulsion”. Microemulsifjmg mineral oils by synthetic amphiphiles, either nonionic or ionic, has become a well-established practice.l Less well studied is the use of biological amphiphiles although they should be of interest because of their biological degradability, as well as for preparing nontoxic microemulsions for application in pharmacy. In this paper we present the results of the first in a series of studies on the microemulsification of mineral oils by biologic amphiphiles. As such we use lecithins (1,2dialkanoyl-sn-glycero-3-phosphocholine), denoted by (C,)ZPC, where m is the carbon number of each of their two hydrocarbon chains. Although lecithins are zwitterionic, one expects them to behave-in principle-like rather lipophilic nonionic n-alkyl polyglycol ethers, CiEj. A lecithin-based microemulsion was-to our knowledge-first studied by Shinoda and Kaneko2in 1988.They used soy bean lecithin (Epikuron 200, being a mixture of Abstract published in Advance A C S Abstracts, April 15,1995. (1)Kahlweit, M. J. Phys. Chem. 1995,99,1281. (2)(a) Shinoda, K.; Kaneko, T. J.Dispersion Sci. Technol. 1988,9, 555. (b) Shinoda, K.; Araki, M.; Sadaghiani, A,; Khan, A,; Lindman, B. J.Phys. Chem. 1991,95,989.(c) Shinoda, K.; Shibata,Y.; Lindman, B. Langmuir 1993,9,1254. @
long-chain lecithins), and hexadecane ( B d as oil. At 25 “C,the ternary mixture showed 2 (w/o),which, if lecithins behaved like nonionic amphiphiles, would indicate that the three-phase body lies below the experimental temperature. For raising it to 25 “C, the authors, therefore, added propanol (C3Eo)which is known to raise the threephase body in nonionic microemulsions (with alkanes of carbon number K I 8). Upon increasing the mass fraction of C3E0in the (HzO C3Eo)mixture, the three-phase body indeed traversed the experimental temperature until, a t high alcohol concentrations, the mixture showed 2 (o/w) (Figure 1in ref 2a). When we repeated the experiment with Epikuron 200 purchased from the same source (Lucas Meyer, Germany), we found that we had to increase the mass fraction of C3Eo to about 17 wt % (instead of 13 wt % required by Shinoda and Kaneko) to reach the “balanced state”. We suspect that this is due to variations between different samples of Epikuron with respect to the distribution of chain lengths, impurities, and water content. This is supported by the fact that Shinoda and Kaneko reported that their Epikuron sample contained 1.3 wt % HzO which resulted in a phase separation in the Epikuron-Bl6 mixture a t low lecithin concentrations (Figure 2 in ref 2a), while we found-much to our surprise-1 wt % of our sample to be completely soluble in hexadecane, though it took about 1h to get the substance in solution. Only if we added a drop of water did it separate into two phases. This makes lecithin mixtures such as Epikuron less well suited for systematic studies, which is why we performed the following experiments with well-defined, though expensive lecithins purchased from Bachem and Sigma.
+
Some Experiments For comparing (C,)ZPC with CiEj, we first studied the effect of the carbon number m of (C,)zPC on the phase behavior of the mixture HzO-octane (B&-(C,)zPC-C,Eo a t 40 “C, with C,Eo denoting an alcohol of carbon number n. As in our previous papers, we define the composition variables in the alcohol-free ternary mixture by
+ oil) amphiphile/(water + oil + amphiphile) a
y
oil/(water
both in wt %. The experiments were performed as follows: We prepared mixtures a t a = 50 wt %, and various y , and then titrated the mixtures with alcohol, thus
0743-746319512411-1576$09.00/00 1995 American Chemical Society
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erecting vertical sections through the phase tetrahedra of the quaternary mixtures a t fixed T,and fixed a. For reasons of convenience, the results are plotted in rectangular instead of triangular coordinates. With m = 14, we found that upon adding propanol (C~EQ) we traversed the three-phase body with the sequence 2 3 2 (Le., Winsor I1 I11 I; Figure 1, top) from which we deduced that the three-phase body of the alcohol-free ternary mixture lies below 40 "C. Because lowering i in CiEj makes the three-phase body rise,l we expected that with m = 8 we would have to add less C3Eo than with m = 14 for raising the three-phase body to 40 "C. The experiment failed from which we deduced that the three-phase body of the ternary mixture lies above 40 "C. Because butanol (C4Eo)is known to lower the three-phase body in nonionic microemulsions (irrespective of the carbon number of the oil), we, accordingly, replaced C3Eo by C4Eo. Indeed, upon adding C4E0, we traversed t_hethree-phase body with the reverse sequence 2 3 2 (Figure 1, center). Insofar, lecithins did seem to behave like CiEj. However, when we prepared mixtures with mean compositions within the three-phase body, and varied temperature, we found both three-phase bodies to exist over almost the entire experimental window whereas with CiEjthe three-phase bodies are limited to rather narrow temperature intervals. In the bottom of Figure 1 one can see a vertical section through the pseudoternary phase prism along the axis of the "fish" for m = 8, that is, at mean compositions along the broken line in the center of Figure 1. This made us suspect that the three-phase body may not exist in the ternary mixtures but shows up only upon adding alcohol. After a sufficient amount of alcohol has been added, varying temperature has only a small effect because of the weak temperature dependence of the distribution of alcohols between water and hydrocarbons. The effect of temperature is thus similar to that in the classic anionic microemulsion (HzO NaC1)-toluene-SDS-butanol in which the three-phase body, too, appears only upon adding butanol, then, however, extending over the entire experimental window.
- -
- -
z
4OoC
0' 0
10
5
15
I
-Y/wt%
--
+
k=8
m =14
L 0
4OoC
5 -Y/wt
10
1
15 %
1001
The Three Binary Mixtures The three-phase body in mixtures with nonionic CiEj evolves from the overlapping of the upper HzO-CiEj loop, and the lower oil-CiEj gap with the central HzO-oil gap. Of these, the latter gap is evidently not affected if CiEj is replaced by (C,)ZPC. 1. H20-(Cm)2PC. First, we tried to clarify whether HzO-(C,)zPC mixtures, too, show an upper loop. Unfortunately, rather little is known about the water-rich side of the phase diagrams of water-lecithin mixtures because most scientists working with phospholipids concentrate on the amphiphile-rich side for studying vesicles as model for biological membranes. The only diagram we know of is that of HzO-(C&PC, first determined by Tausk, Oudshoorn, and Overbeek3in 1974. They found a narrow lower miscibility gap with an (upper) critical point a t Tp x 46 "C, and yp 1.5 wt % lecithin. In 1990, Huang, Thurston, Blankschtein, and Benedek4 found Tp x 47 "C, and yp x 2.3 wt %. With the sample from Sigma we found Tp 44 "C, and yp x 1.4 wt % which we-in view of the different sources of the compoundconsidered to be in sufficient agreement for the purpose of our studies. While the other authors studied the effect of added salts, we added (C&PC in order to study the (3) Tausk, R. J. M.; Oudshoorn, C.; Overbeek, J. Th. G. Biophys. Chem. 1974,2, 53. (4) Huang, Y.-X.; Thurston, G. M.; Blankschtein, D.; Benedek, G. B. J.Chem.Phys. 1990,92,1956. See also: Phys.Reu.Lett.1985,54,955.
3
U
0
5 -V/wt%
10
15
Figure 1. Vertical sections through phase tetrahedra of HzOoctane-(C,)zPC-C,Eo mixtures at 40 "C and a = 50 wt %. With m = 14 (top) one has to add propanol (n = 3), with m = 8 (center), butanol (n = 4) for preparing a microemulsion.
Bottom: Vertical section through pseudoternary phase prism at mean compositions along the axis of the "fish" with m = 8 (broken line in the central figure), demonstrating the weak effect of temperature.
effect of m on the phase diagram. The result is shown in Figure 2. The miscibility gaps grows rapidly with increasing mass fraction 6 (in wt % of (Cl&PC in the
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1578 Langmuir, Vol. 11, No. 5, 1995
T
t
30 20 10 6= 0
2ot
- P
I I
Figure 3. Section through the “dome” of water-nonionic
0
01
-‘b/wt%
1
10
100
(C,I,PC
Figure 2. Phase diagrams of H20-(Cm)2PC mixtures with increasing mass fraction 6 in wt % of (Cd2PC in [(CshPC + (C&PC], demonstrating the strong effect of m on the upper
critical point.
mixture of the two amphiphiles from which we deduce that the longer-chain lecithins (m > 8)show rather narrow lower miscibility gaps on the water-rich side with their upper critical temperature Tp lying well above the boiling temperature, followed by a sequence of lyotropic mesophases at higher amphiphile concentration^.^ The nature of these mesophases is less relevant for the purpose of our studies except that they may screen the miscibility gaps. The upper loop in H20-CiEj mixtures represents a n isobaric section through a “dome” of the two-phase body in T-p-composition space (see Figure 1in ref 6). As to whether HzO-(C,)ZPC mixtures show such a dome, there are three possibilities: (i) They do not. (ii) They do, but the dome (and, hence, the loop) lies above the boiling temperature. (iii) They do, but the attractive interaction between the head groups and water is too weak to overcome the tendency of water to expel the hydrocarbon chains, so that the lower gap and the upper loop remain connected a t atmospheric pressure. This is illustrated schematically in Figure 3 which shows a vertical section through the dome a t fixed composition. At atmospheric pressure, indicated by the vertical broken line, the dome of mixture a lies on the left of the broken line so that the mixture is completely miscible (example: HzO-propanol in which the loop can be “salted out” by adding, e.g., KC19 In mixture b the loop and the lower gap are separated (example: H20-CiCJ), whereas in mixture c the two are still connected, only to separate a t higher pressures (example: HzO-see-butyl alcohol). However, whether case a, b, or c applies to HzO-(C,)zPC mixtures, is less relevant for the purpose of our studies. What matters is that HzO-(C,)zPC mixtures with m L 8 show a rather narrow lower miscibility gap a t low lecithin concentrations the upper critical temperature Tp of which rises steeply with increasing m. Hence, (C,)ZPC differs from nonionic C,Ej in that increasing chain length m makes the upper ( 5 ) Jiirgens, E.; Hohne, H.; Sackmann, E. Ber.Bunsenges.Phys. Chem. 1983,87,95. See also the schematic diagram Figure 7 in Seddon, J . M. Biochim. Biophys. Acta 1990, 1031, 1. (6) Firman, P.; Haase, D.; Jen, J.; Kahlweit, M.; Strey, R. Langmuir 1986,1, 718.
amphiphile mixtures in 2’-p-composition space at fixed composition, with broken line indicating atmosphericpressure. critical point of their lower gap rise, whereas with CiEj increasing i makes the lower critical point of their upper loop drop. Consider now the effect of alcohols (C,Eo) on waterlecithin mixtures. Adding alcohol makes water less polar, hence, increasing the solubility of lecithin and repressing the mesophases. As to the solubility of lecithins in pure alcohols, we found small amounts of lecithin to be completelysoluble in all alcohols down ton = 1(methanol). Only if we added about 25 wt % water to methanol did the miscibility gaps appear. From this we deduce that the solubility of lecithins in short- and medium-chain alcohols increases with increasing n. For studying the effect, we titrated 2 wt % (C12)zPC-H20 mixtures with alcohols of carbon number n = 2 , 3 , and 4. The upper temperature of the miscibility gap along this particular path (not being that of the critical point) drops steeply upon adding alcohol, the effect increasing with increasing n which explains why Shinoda and Kaneko had to add more ethanol than propanol in their experiment for reaching the balanced state. With n = 4,the mixture separates again a t higher butanol concentrations which is due to the fact that butanol and water are only partially miscible. 2. Alkane (Bk)-(C,hPC. We emphasize that the following experiments were performed with lecithin samples as supplied. Because we found, in particular, the shorter-chain lecithins to be rather hygroscopic, we cannot exclude that our samples contained traces ofwater which falsified the results. For physical reasons one expects the upper critical temperature T, of the (lower) Bk-(C,)ZPC gap to rise with increasing carbon number W of the oil (at fixed m) but to drop with increasing carbon number m of the tails (at fixed k ) . Actually, we found our charges of (C,)2PC to be insoluble in alkanes a t room temperature, irrespective of k and m. Upon raising T above 50 “C,the lecithins dissolved, leaving a transparent, isotropic solution. The dissolution temperature rises slightly with increasing W (at fixed m) and drops slightly with increasingm (at fured k ) . It is practically independent of concentration down to very low amphiphile concentrations a t which the precipitate becomes difficult to detect. The dissolution temperatures show a strong hysteresis, indicating that the homogeneous solutions can be considerably supercooled. Furthermore, the temperatures varied between different samples by about 10 deg, presumably due to differences in the water content. Consider now the effect of alcohols on oil-lecithin mixtures. Adding alcohol makes alkanes less nonpolar,
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Preparing Microemulsions with Lecithins
H2°
1-1 1
T
Bk
hence, increasing the solubility of lecithins. For studying the effect, we titrated 1wt % (C&PC-Bk mixtures with C,Eo (n = 3,4).The lecithin dissolves rapidly upon adding alcohol, with butanol being more efficient than propanol, and the amount of alcohol required to dissolve the lecithin increasing, evidently, with increasing k. If a drop ofwater is added to the homogeneous solution, the mixture separates into two transparent isotropic phases that become miscible upon raising T. The fact that butanol is more efficient than propanol in increasing the solubility of lecithin in alkanes made us suspect that the effect may not be due to the lowering of the effective carbon number of the oil, but rather to a more specific effect such as an association between alcohols and lecithins, comparable to the adsorption of alkanes by lipid bilayer^.^ However, in view of the problem addressed in this paper, we postponed more detailed studies of the effect of alcohols on the solubility of lecithins in both water and oils. For the following discussion we shall, therefore, assume that Bk-(C,)ZPC mixtures do show a lower miscibility gap upon adding a little alcohol and a trace of water. For physical reasons we shall, furthermore, assume that its upper critical temperature T, rises with increasing k but drops with increasingm and that the critical line ascends steeply upon adding water.
The H20-Bk-(Cm)2PC Prism On the basis of these results we may now construct a schematic (pseudoternary) phase prism for the wateroil-lecithin mixture (Figure 41, disregarding mesophases as well as precipitates. Each side of the prism shows a lower miscibility gap, namely, a wide water-oil gap, a narrow water-lecithin gap a t low lecithin concentrations, and a n oil-lecithin gap (in the presence of a little alcohol and a trace of water) the position and extension of which is not clear a t present. The (upper) critical temperature ofthe HZO-Bk gap lies well above the boiling temperature and plays no role in the further discussion. The (upper) critical temperature Tp of the HZO-(C,)zPC gap depends on m only, rising steeply with increasing m. T, of the Bk-(C,)zPC depends on both k and m, rising with increasing k (at fixed m) and dropping with increasing m (at fixed k). Consequently, a t fixed k, increasingm makes Tp rise, but T,drop, and vice versa. The (pseudo) ternary (7) See, e.g., Gruen, D.W. R.; Haydon, D.A. PureAppl. Chem. 1980, 52, 1229.
mixtures must, therefore, show a three-phase body a t low temperatures (disregarding the solidification of the solvents). The upper critical tie-line of this three-phase body appears to lie below the experimental window, its position on the temperature scale being determined by the lower one of the endpoints (cep) of the two critical lines that enter the phase prism at cp,, and cpp, respectively. At fixed oil, one expects in mixtures with high m-that is, high Tp but low T,-cep, to be the lower one, whereas with low m-that is high T, but low Tg-cepp to the lower one. Consider now the effect of increasing the carbon number k of the oil a t fixed m. Because Tp depends on m only, the major effect of increasing k is to raise T,, and thereby tepa. Hence, if for some m and k, cep, < cepp, increasing k may eventually reverse the position of the two endpoints such that cep, > cepe. As we have shown in ref 1,the separation of a wateroil-amphiphile mixture into three coexistingliquid phases is the consequence of a rapid distribution inversion of the micelles upon varying a n appropriate parameter. Hence, for driving an amphiphile from solvent A into solvent B, or vice versa, one must change the properties of the two solvents such that the micelles become more soluble in (B) than in (A),or vice versa. With nonionic amphiphiles raising temperature makes water expel the micelles which manifests itselfin the upper loop. Ionic micelles, however, become more soluble in water upon raising T so that they must be salted out. Because the miscibility gap between water and medium- and long-chain lecithins extends well above the boiling temperature, raising Tor adding salt is of little help. That is where the alcohols come in. They distribute between water and oil, making water less polar and oil less nonpolar. This increases the mutual solubility between lecithin and water a s well as oil, repressing precipitation as well as the mesophases.
The Effect of Alcohol The distribution of alcohols (C,Eo) between oils (Bk)and water depends sensitively on both n and k. Increasing n increases their solubility in oil, whereas increasing k has the reverse effect. This can be readily demonstrated by mixing equal volumes of water, oil, and alcohol, and then observing the volume fractions of the two phases. If the volume fraction of the lower aqueous phase is larger than the alcohol is more that of the upper oil-rich phcse soluble in water, whereas if (2), it is more soluble in the oil. One finds that ethanol (n = 2) is more soluble in water than in oil, irrespective ofk. Propanol (n = 3) shows a distribution inversion a s k is increased. With alkanes, in particular, it is more soluble in oil for k = 6 (hexane) but more soluble in water fork I 8 (octane). All alcohols with n I4 are much more soluble in oil than in water, being only partially miscible with water. Experiments, furthermore, tell that the distribution depends only weakly on temperature. The effect of k and n on the position of the plait points in H20-Bk-(Cm)~PC-C,Eo mixtures with kI 8, and m 2 8 is illustrated schematically in Figure 5 which shows the unfolded phase tetrahedra of the quaternary mixtures a t a fixed intermediate temperature, with the HzO-Bk-(C,)zPC triangle a s the base, and C,Eo on top, disregarding precipitates and mesophases. Consider first the lower HZO-Bk-C,Eo triangles. Adding alcohol increases the mutual solubility between HzO and Bk until, a t rather high alcohol concentrations, the mixture becomes homogeneous. For pure propanol, the plait point (Y)lies on the oil-rich side of the binodal moving toward the Bk corner with increasing k (Figure 5 , top). If propanol is replaced by a mixture of propanol and butanol, increasing fraction of C4Eo makes plait point ( Y ) move along the binodal toward the H20 corner (indicated by the
(z),
1580 Langmuir, Vol. 11, No. 5, 1995
Kahlweit et al.
add
Figure 6. Evolution of the three-phase triangle upon adding alcohol at fEed temperature. It appears at the endpoint of critical line ( v ) on the oil-rich side with propanol (C3Eo)(left) and on the water-rich side with butanol (&Eo) (right).
Figiwe 5. Unfolded phase tetrahedron at fmed temperature, with water-oil-lecithin triangle as the base, and alcohol on top. Plait point ( Y ) lies on the oil-rich side of the binodal in the lower triangle with propanol (C3Eo)(top) and on the water-rich side of the binodal in the upper left triangle with butanol (CdEo) (bottom).
that is, high T,, one has to place it in the upper left triangle by adding butanol for lowering T,. Accordingly, one expects that in mixtures with medium k , and medium m, one may have to place it somewhere in between, that is, may have to add a mixture of propanol and butanol. As we shall see further below, this is indeed the case. As to the phase diagrams of the two other ternary mixtures we are a t present not in the position to draw them even schematically. All we can say is that (Cm)2PC is insoluble in pure oil (Bk) a t room temperature but dissolves upon either raising T or adding alcohol.
arrow), gradually approaching the H20-CnEo side of the triangle until it eventually touches that side. Further increasing butanol fraction makes plait point (Y)cross the H20-CnEo side and enter the upper left H20-(Cm)2PCC,Eo triangle where it leads to a miscibility gap along the H20-CnEo side of that triangle which grows into the triangle-with plait point (Y)lying on the water-rich side of the binodal-until pure butanol is reached (Figure 5, bottom). The final extension of that gap depends on the effect of (Cm)2PCin the opposite corner on the mutual solubility between water and butanol, that is, on m. The H20-(Cm)2PC side of the upper left triangle, too, shows a miscibility gap the extension of which along that side depends on m only. Its extension into the triangle, however, depends on the effect ofthe alcohol on the mutual solubility between water and lecithin, that is, on n. Its plait point (PI, too, lies on the water-rich side of the binodal, thus facing plait point (v). Hence, with pure propanol, each of the two triangles shows a plait point, whereas with pure butanol, both plait points lie in the upper left triangle. From this we deduce that the major effect of adding propanol to the ternary H20-oil-lecithin mixture is to increase the mutual solubility between water and lecithin which lowers Tp as if m was decreased. The major effect of adding butanol, on the other hand, is to increase the mutual solubility between oil and lecithin which lowers T, as if m was increased. Plait point (Y)in Figure 5 can thus be viewed as a lever that can be shifted between its position with pure propanol on the oil-rich side of the lower triangle, and its position with pure butanol in the upper left triangle. With high m and low k , that is, high Tp,one has to place it on the oil-rich side of the lower triangle by adding propanol for lowering Tp. With low m and high k ,
The Three-phase Body On the basis of the above considerations we shall now propose a phenomenological model for the evolution of the three-phase triangles in the quaternary mixtures. Consider first mixtures with high m, that is, high Tp,but low T,. In this case one has to add propanol (upper tetrahedron in Figure 5 ) . As a little propanol is added, it dissolves the lecithin in the (Cm)2PCcorner of the tetrahedron, and lowers T, further. Because in this case it is the critical line cl, that terminates a t the upper critical tie-line of the lower three-phase body of the ternary mixture (see above), T, plays no role so that we have to consider only the effect of propanol on Tp. Adding some propanol makes the H2O-(Cm)2PC gap in the upper left triangle disappear, and thereby the central gap disconnect from the water-amphiphile side, so that it may look as drawn schematically in the upper left of Figure 6, with plait point (p)lying on the water-rich side of the binodal (2). Upon addition of more propanol, the end point of the critical line that enters the tetrahedron a t plait point (v) in the lower triangle eventually touches the central gap on its oil-rich side which makes the oil-rich phase separate into an oil-rich (b)and an amphiphile-rich phase (c)(Figure 6, center left). Adding still more propanol makes the amphiphile gradually become more soluble in the aqueous phase, hence, making phase c move toward the waterrich side (indicated by the arrow) where it eventually merges with the aqueous phase (a) at the endpoint of the critical line clp (Figure 6, bottom left). From then on the central phase diagram shows only one plait point on the oil-rich side (2). Consider next a mixture with low m, that is, low Tp,but high T,. In this case one has to add butanol (lower tetrahedron in Figure 5). As a little butanol is added, this
C4EO
Langmuir, Vol. 11, No. 5, 1995 1581
Preparing Microemulsions with Lecithins
’I
\
‘ hiah m
Figure 7. Recipe for preparing lecithin-basedmicroemulsions (schematic). With short-chain(low k ) alkanes and long-chain (high m ) lecithins add propanol (upper left) and with shortchain (low m ) lecithins add butanol (lower left). Increasing k at fixed m is like lowering m at fixed k (full diagonals). With medium k and m, add mixtures of propanol and butanol (horizontalbroken line). The star on the upper right indicates the position of the experiment described in ref 2s.
dissolves the precipitate, and lowers TBfurther. Because ~ terminates a t the in this case it is the critical line c l that upper critical tie-line of the lower three-phase body, Tp plays no role so that we have to consider only the effect of butanol on T,. Adding some butanol lowers T, so that the central gap may look as drawn schematically in the upper right of Figure 6, with plait point ( a )lying on the oil-rich side of the binodal (2). Upon addition of more butanol, the endpoint of the critical line that enters the tetrahedron a t plait point (v)in the upper left triangle eventually touches the central gap on its water-rich side which makes the water-rich phase separate into a waterrich (a) and a n amphiphile-rich phase (c)(Figure 6 center right). Adding still more butanol makes the amphiphile gradually become more soluble in the oil-richphase, hence, making phase c move to the oil-rich side (indicated by the arrow) where it eventually merges with the oil-rich phase (b)a t the endpoint of the critical line cl, (Figure 6, bottom right). From then on, the centr_algap shows only one plait point on the water-rich side (2). Clearly, the planes of all diagrams in Figure 6 are tilted with respect to the base of the tetrahedron due to the uneven distribution of the alcohols between water and oil.
Recipe for Preparing Microemulsions Whether this model is a n approach to reality, only further studies can show. At our present of knowledge, however, it is capable of explaining qualitatively the phase behavior of water-oil-lecithin-alcohol mixtures. The recipe for preparing a microemulsion is shown schematically in Figure 7. With long-chain lecithins (high m ) , propanol (C3Eo) has to be added, the amount of alcohol required increasing with increasing m (at fixed k ) . With short-chain lecithins (low m ) , butanol (C4Eo) has to be added, the amount of alcohol increasing with decreasing m (at fured k ) . Increasing the carbon number k of the oil a t fixed m is equivalent to lowering m a t fixed k . It lowers the amount of propanol required but raises that of butanol required as indicated by the (not necessarily parallel) diagonals, so that with medium m, one has to replace propanol by butanol as one increases k a t fixed m. The star on the right of Figure 7 indicates the position of the experiment performed by Shinoda and Kaneko2 with hexadecane and Epikuron. Indeed, for emulsifying octane with Epikuron, we had to add more propanol than required with hexadecane because we moved along the corresponding diagonal to the upper left of Figure 7. In the
Figure 8. Illustrating the effect of temperature: 50 wt % sections through the t.hree-phasebodies in T- y’-6 (alcoholin lecithin + alcohol) space, shaping an upright tube. Horizontal section yields isothermal “fish, vertical section through axis of the fish yields diagram similar to that on bottom of Figure 1.
transition range between propanol and butanol (indicated by the horizontal broken line), one has to add mixtures of the two alcohols. We emphasize that this recipe holds only for alkanes of carbon number k L 8. As one lowers k to 6, or even proceeds to aromatic oils, propanol becomes more soluble in the oil and, hence, has a similar though weaker effect as butanol. As we shall show in a forthcoming paper, propanol must then be replaced by a more hydrophilic short-chain amphiphile such as C3E1or CzEo. As one would expect, the more alcohol has to be added for reaching the three-phase body, the more lecithin is required for completely homogenizing water and oil. In other words: the closer the three-phase body to the border line between propanol and butanol in Figure 7, the less alcohol and lecithin is required for preparing a microemulsion.
Effect of Temperature As to the effect of temperature, the following should hold. Because the distribution of the alcohols depends only weakly on T , the shape of the three-phase triangles should change only little with rising T , with the middle phase (c) slowly taking up oil for high m , but water for low m (see Figure 1 bottom). As a consequence, the temperature range in which a homogeneous mixture of fixed composition remains homogeneous is wider than in microemulsions with either nonionic CiEj, or anionic AOT which both show a strong effect of temperature. This makes lecithin-based microemulsions less sensitive to temperature variations. For illustrating the effect of temperature, consider Figure 8 in which we plotted T vs the mass fraction y’ of [(C,)2PC C,Eol, and the mass fraction
+
6
C,Ed[(C,),PC
+ C,E,I
(both in wt %) a t fixed k , and fixed a = 50 wt % in rectangular coordinates. The T- y’ plane in front is blank because of our lack of knowledge concerning the details of the phase prism of the alcohol-free water-oil-lecithin mixture. The T- y’ plane in the rear shows the a = 50 wt % section through the phase prism of the lecithin-free water-oil-alcohol mixture (bottom triangles in Figure
Kahlweit et al.
1582 Langmuir, Vol. 11, No. 5, 1995 5). Adding a sufficiently high amount of alcohol makes the water-oil mixture become homogeneous, with the amount of alcohol required decreasing slightly with rising T. The two-phase region shows 2 with propanol, but 2 with butanol (in brackets). Imagine now increasing 6 from zero a t fixed experimental temperature and various y’ (horizontal plane), in the first experiment with high m (i.e., adding propanol) and in the second experiment with low m (i.e., adding butanol, in-brackets). At low Q, the mixture with propanol shows 2, that with butanol 2.As 6 is increased, both mixtures traverse the three-phase body, sho-wing the sequence 2 3 2 with propanol, but 2 3 2 with butanol. At 6 = 100 wt %, both mixtures terminate a t the rear T-y’ plane. If one now erects a vertical section through the axis of the horizontal “fish”, one obtains a diagram looking similar to that shown in Figure 1 bottom. In this particular representation of the phase behavior, the a = 50 wt % sections through the three-phase bodies shape a n almost vertical tube extending practically over the entire experimental window. Clearly, isothermal sections through the tube differ from those shown in Figures 1and 9 because in Figure 8 we treated (lecithin alcohol) as a “pseudocomponent”.
--
--
C,E,/C,E,=l/l
s 4-
3 a ‘ W C 0
t
40-
*O: I
I
0 0
IO
Effect of Added NaCl As to the effect of added salts, both Tausk et al.3 and Huang et al.4found the effect of added chloride salts such as NaCl on the binary (C&PC-HzO gap (Figure 2) to be rather weak up to 1M solutions (see Figure 1in ref 3 and Figure 3 in ref 4). Accordingly, one expects the effect of NaCl on the phase behavior of HzO-Bk-(Cm)~PC-C,Eo mixtures to be mainly that of “salting out” the alcohols out ofthe aqueous phase into the oil-rich phase. Evidently, the effect should be small in lecithin-butanol mixtures in which the alcohol is already mainly dissolved in the oil-rich phase but strong in lecithin-propanol mixtures in which the alcohol is mainly dissolved in the aqueous phase so that the salting out of C3Eo must be compensated
20
-v/wt%
+
Some Supporting Experiments First, we wish to demonstrate that with mixtures lying near an intersection between a diagonal and the broken horizontal line in Figure 7 one has to add mixtures of C3Eo and C4Eo for preparing a microemulsion. For this purpose we chose the mixture HZO-B~Z-(CI~)ZPC-C,EO. Attempts to microemulsifying this mixture a t 40 “C with either pure C3Eo or pure C4Eo failed, for which reason we chose a mixture of equal masses of the two alcohols. The result is shown a t the top of Figure 9. Secondly, we wish to support the scheme shown in Figure 7 by increasing k from 8 to 14 a t fixed m = 10 in H Z O - B ~ - ( C ~ ~ ) Z P C - C that ~ E O is, , by moving along the lower one of the diagonals in Figure 7 a t 40 “C. The result is shown in the center of Figure 9. As predicted by Figure 7, the amount of C4Eo required for preparing a microemulsion increases with increasing k , a s does the amount of lecithin. The large amount of C ~ E O required for preparing a microemulsion with B14 suggests an increase in the carbon number of the alcohol, hence, pushing plait point (v) in Figure 5 deeper into the upper left triangle. Indeed, if one replaces pure C4Eo by a mixture of equal masses of C4Eo and CbEo, the amount of alcohol required drops by almost a factor of 4 (Figure 9, bottom). Clearly, one may now vary both m and the mixing ratio of various alcohols for optimizing the procedure. We, finally, note that the phases separate rather slowly a t low alcohol concentrations, but rapidly a t mean concentrations within the fish.
(C,L PC- C , E,
H,O-B,,-
5
10
15 ’d/wt%
20
25
.
0 5
10
15
20
25
’d / w t %
Figure 9. (top)For preparing a microemulsion with (C1&PC and Blz, one has to add mixtures of CsEo, and C4Eo. (center) For preparing microemulsions with (C&PC and Bk,the amount of C4Eo required increases with increasing k . (bottom) For increasingthe efficiencyofthe alcohol in (C&PC-B14 mixtures, one may add mixtures of C4EO and CsEo.
by increasing the alcohol concentration. For studying the
Preparing Microemulsions with Lecithins
Langmuir, Vol. 11, No. 5, 1995 1583
effect, we chose the mixture with k = 10, m = 10, n = 4 (lower diagonal in Figure 7) and k = 8, m = 14, n = 3 (upper diagonal in Figure 71, and increased the NaCl concentration in HzO up to E = 3 wt % (%0.5 M). In the first mixture, we found the amount of C4Eo required for preparing the microemulsion to decrease slightly with increasing E (with the efficiency of the amphiphile increasing), but in the latter mixture that of CSEo to increase strongly with increasing E (with the efficiency decreasing). This makes short-chain lecithin-butanol mixtures better suited for experiments with varying brine concentration.
Amplifying the Efficiency of CiEj Because in most of the above experiments alcohol is the major component compared with lecithin, the fish in Figure 8 lie, in general, on the alcohol-rich side. Alternatively, one may, therefore, view the experiments as attempts to prepare microemulsions by adding lecithin as "cosurfactant" to short-chain alcohols which suggests studying the effect of lecithin on the emulsification of hydrocarbons by short- and medium-chain CiEj. For this purpose lecithin mixtures such as Epikuron suffice which makes the procedure rather inexpensive. As example, Figure 10 shows the effect of replacing pure C8C3 by a mixture of (C8E3 Epikuron) in H z O - B ~ ~ - ( C ~ E ~ Epikuron) represented in a pseudoternary phase prism a t a = 50 wt %. Upon increasing the mass fraction 6 of Epikuron in the mixture of the two amphiphiles, the amount of amphiphile required for homogenizing water and hexadecane drops from y w 40 wt % for pure CsE3 to y x 10 wt % for 6 = 40 wt %, that is, by a factor of about 4,while the mean temperature of the fish drops only slightly. In this case, the vertical plane in the rear of Figure 8 shows the fish for pure C8E3. Upon addition of Epikuron, the a = 50 wt % sections through the three-phase bodies shape a (slightly descending) horizontal tube8that shrinks until it eventually gets screened by precipitation or mesophases. This indicates that adding lecithin lowers the interfacial tension between the water and oil domains dramatically, in accord with ref 2c in which Shinoda et al. report Dab = 3x m J m-2 for their experiment with propanol instead
+
H,O- B,6- [C,E3+Epikuron1
8or----l t o 60
0
t 20 a.50 w t 0 h
+
~
~
(8) See for comparison Figure 3 in Kahlweit, M.; Strey, R. J.Phys. C h e n . 1987,91, 1553.
of C8E3. The increase of efficiency, however, has to be paid for by the appearance of a lamellar mesophase. At fixed k, the origin of the tube on the rear plane in Figure 8 is, evidently, determined by the position of the fish for the pure CiEj. By appropriately chosing CiEj, one should, therefore, be able to prepare microemulsions with (CiEi lecithin) mixtures a t any prescribed temperature. In a forthcoming paper we shall present the results of studies on the applicability of alkyl polyglucosides for preparing microemulsions.
+
Acknowledgment. We are indebted to Mrs. S. Grzeszik for assistance with the experiment and to Mrs. J . DaCorte for drawing the figures. L4941035D
