0 Copyright 1995 American Chemical Society
NOVEMBER 1995 VOLUME 11, NUMBER 11
Letters Preparing Nontoxic Microemulsions M. Kahlweit," G. Busse, B. Faulhaber, and H. Eibl Max-Planck-Institutfur Biophysikalische Chemie, Postfach 2841, 0-37018 Gottingen, Germany Received July 3, 1995. In Final Form: August 29, 199P We present a recipe for preparing nontoxic microemulsions using an unsaturated fatty acid ethyl ester as oil, a long-chain soy bean lecithin as amphiphile, and pentane-1,2-diol as cosolvent. The mixture permits the preparation of microemulsions at any prescribed temperature within the experimentalwindow, with the mixture of the two amphiphilic compounds being rather efficient emulsifiers. Furthermore, the mixture turns out to be rather insensitive to NaCl brine concentrations up to 2 wt%. The preparation of microemulsions with mineral oils and synthetic amphiphiles, either ionic or nonionic, has become a well-established practice.l Because, however, both mineral oils and synthetic amphiphiles are, in general, toxic, we studied the applicability of biological amphiphiles, namely, lecithins and alkylmonoglucosides, for emulsifying mineral 0ils~9~ and found both amphiphiles, in combination with medium-chain alkanols, to be rather efficient emulsifiers. The next step is, evidently, to substitute biological oils for mineral oils. As such we started with triglycerides, in particular, triolein (glycerol trioleate). For classifylng it with respect to its effective carbon number, we compared the position and extension of its three-phase body with those of long-chain n-alkanes, using as nonionic amphiphile. In Figure 1one can see sections through the phase prisms with the "tails" of the three-phase "fish" at a = 25 wt% where CY.
= 100 oil/(water
+ oil)
in wt%
(1)
As to the mean temperature ?1 of its three-phase body, triolein could be compared with docosane (k = 221,with the efficiency of the amphiphile, taking l l y at the tail as a measure,
* Corresponding author. E-mail:
[email protected]. Abstract Dublished inAdvalzce ACSAbstructs. October 15.1995. (1) Kahlweit, M. J . Phys. Chem. 1995,99, 1281: Busse, G.; Faulhaber, B. Langmuir 1995,11,1576. (2)Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1995,11,3382. (3)Kahlweit, M.; @
H,O- oils-C,,E,
8 0 60
t
01 0
-
"
"
20
LO
'
"
60
80
'b/wt% CloE, Figure 1. Sections through the HzO-oil-CloEr prisms for classifyingtriolein (at a = 25 wt %) and OAEE (oleic acid ethyl ester) at a = 50 w t % with respect to their effective carbon number. y
100 amphiphile/(water
+ oil + amphiphile)
in wt% (2) however, being considerably lower, which makes it doubtful whether the homogeneous mixture can be viewed as a microemulsion in the narrower sense. Proceeding to a longer-chain amphiphile CiEj improves the efficiency only little. For reducing the effective carbon number of the oil we
0743-7463/95/2411-4185$09.00100 1995 American Chemical Society
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4186 Langmuir, Vol. 11, No. 11, 1995
1
20
C
40%
-
D Amphiphile (C)
0 0
-
10
20
T/wt%
C
Figure 3. Sectionsthrough the phase tetrahedron at 40 "C at various a (represented in rectangular instead of triangular coordinates),demonstrating that the addition of propane-l,2diol enforces a 2 3 2 inversion.
--
ST= S p 4
tion4). It shows on top the unfolded phase tetrahedron a t fixed T, with the A-B-C triangle as the base and D on top. If either the A-B-D or the A-C-D triangle shows a plait point to the left of the broken center line, adding D to the A-B-C mixture will lead to a 2 3 2 inversion (provided, evidently, that the A-B-C base shows 2 a t this temperature, center left). If either the A-B-D or the B-C-D triangle shows a plait point on the right of the center line, adding D will lead to a 3 -5 inversion (provided, the base shows 2 center right). Consider first the effect of propanol (C3E0, Figure 2, bottom). C3Eo is completely miscible with both water and oil. Hence, the lower A-B-D triangle shows a plait point the position of which depends on the carbon number k of the (mineral) oil. Fork I6, it lies on the water-rich side (left), for k z 8, on the oil-rich side (right). Accordingly, adding propanol to nonionic A-BK-C mixtures with k 5 6 in the 2 state lowers their three-phase body; adding it to mixtures with k 1 8 in the 2 state raises it. Consider now propanediol. Propanediol is much more hydrophilic than propanol, which makes it incompletely miscible with oils. Consequently, the two-phase region runs from A-B to B-D of the A-B-D triangle side from which it proceeds to the upper right B-C-D triangle. Experiments tell us that nonionic C,Ej are completely miscible with propanediol. Consequently, the two-phase region with a C3diol terminatesat aplait point within the B-C-D triangle. Hence, propanediol has a similar effect as propanol (for k 2 8) in that it raises the three-phase body. Actually, when comparing the effect on the HzO-decane-CloE4 mixture with a three-phase body a t T = 32 "C, we found propane-1,2-diol to be a little more efficient than propanol for making the body rise to 40 "C. Long-chain lecithins such as Epikuron 200 are more soluble in oil than in water (2). For enforcing a 2 3 2 inversion, one, therefore, has to add either propanol or a n alkane-l,2-diol. With OAEE as oil, we found the CSdiol (pentane-1,2-diol) to be the most efficient cosolvent. In Figure 3 one can see sections through the phase tetrahedron a t 40 "C and a = 25, 50,and 70 wt %. The abcissa refers to the weight percent y of Epikuron in the ternary A-B-C mixture, the ordinate 6 to the weight percent
- -
5-.
Figure 2. Discussing the effect of additives (D)on the phase behavior of H2O (A) -oil (B)-amphiphile (C) mixtures (in field representation). Top: the unfolded isothermal phase tetrahedron. Center left: enforcing a 2 3 2 inversion. Center right enforcing a 2 3 2 inversion. Bottom: the effect of propanol (C3Eo)and ofpropane-l,2-diol(&-diol). For discussion see text.
--
--
substituted triolein by unsaturated fatty acid alkyl esters, in particular, oleic acid ethyl ester (OAEE), which was chosen because its decomposition product ethanol is less toxic than, in particular, methanol. Highly purified OAEE was synthesized in our institute by Prof. Eibl. The tail of the H20-OAEE-CloEd fish at a = 50 wt% is also shown in Figure 1. As was to be expected, lowering the effective carbon number makes the mean temperature of the fish drop and the efficiency of the amphiphile increase. As to the position and extensions ofita three-phase body, OAEE could be compared with octane (k = 81, showing that appropriately chosen biological oils can be readily emulsified by CiE) For preparing a nontoxic microemulsion, we substituted CiEj by the notoxic Epikuron 200 (L. Meyer, Hamburg, Germany) which is a n inexpensive long-chain soy bean lecithin. We recall Figure 7 in ref 2: Water and alkanes can be homogenized by Epikuron using propanol as cosolvent, while with medium- and short-chain lecithins one has to use medium-chain alkanols. Because, however, alkanols are toxic, too, we substituted them by nontoxic alkane-l,2-diols (Ci-diol, Fluka), where i denotes their carbon number. For discussing the effect of alcohols (D)on the phase behavior of nonionic H2O (A)-oil (B)-amphiphile (C) microemulsions, consider Figure 2 (in field representa-
--
6 = 100 cosolvent/(water
+ oil + amphiphile +
cosolvent) (3) of Cs-diol in the entire mixture. At fixed temperature, increasing 6 makes the amphiphile-rich middle phase move from the oil-rich to the water-rich side. The body (4) See also Figure 2 in ref 3.
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Langmuir, Vol. 11, No. 11, 1995 4187
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