Langmuir 1994,10, 2528-2532
2528
Microemulsions with Mixtures of Nonionic and Ionic Amphiphiles M. Kahlweit,* B. Faulhaber, and G. Busse Max-Planck-Institut fuer Biophysikalische Chemie, Postfach 2841, 0-37018Goettingen, Germany Received March 2, 1994. In Final Form: June 2, 1994@ We present the results of the first in a series on the effect of ionic surfactants on the phase behavior of mixtures of water, oil, and nonionic amphiphiles, with the latter being the majority component in the mixture of the two surfactants. Adding small amounts of single-tailed sodium alkyl sulfates halves the amount of n-alkyl polyglycol ether required for homogenizing equal masses of water and alkanes in that the three-phase body disappears, making room for a rather wide homogeneous region at low surfactant concentrations.
Introduction In a recently published paper’ we studied the phase behavior of quinary mixtures of the type
+
+
(H,O Na,SO,)-(B, C,E,)-NaC,SO, with Bk denoting alkanes of carbon number k, C,Eo alcohols of carbon number n (>4), and NaC,S04 sodium n-alkyl sulfates of carbon number m, as it depends, in particular, on m and n. The results suggested replacing alcohols (which can be viewed as both polar ”co-oils” and hydrophobic “cosurfactants”) by medium-chain n-alkyl polyglycol ethers CIEj, because increasingj from zero should amplify the role of the nonionic amphiphile as cosurfactant. In this paper we present the results of the first in a series of studies, with CIE, as the majority, and NaC,S04 as the minority component. Similar experiments have already been performed by other authors, with emphasis on the effect of traces of ionic amphiphiles either on the stability of nonionic lyotropic mesophases2 or on the stability of o/w microemulsions with zwitterionic surfactant^.^ We start with the ternary mixture
H,O-B,-C,Ej
(H,O
-
--
-
Abstract published in Advance A C S Abstracts, J u l y 15, 1994.
(1)Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1994,10,1134. ( 2 ) Schomacker, R.;Strey, R. J.Phys. Chem. 1994,98,3908.
(3)Gradzielski, M.;Hoffmann, H. Adu. Colloid Interface Sci. 1992, 42,149. (4) Kahlweit, M.; Busse, G.; Winkler, J. J. Chem. Phys. 1993,99, 5605.
0743-7463/94/2410-2528$04.50/0
+ NaC,,SO,)-B,-C,E,
As in our previous papers, we define as composition variables
a
Bk/(H,O
+ B,)
in wt %
(1)
and C,Ej/(H,O
+ B, + C,Ej)
NaC,SO,/(H,O
+ NaC,SO,)
in wt % (2) The “fish shape of the ternary mixture HzOyoctaneC6Ez is shown in broken lines in Figure 1. At T x 8 “C, one requires y M 35 wt % C6Ez for homogenizing equal masses ofwater and oil ( a= 50 wt %). At this composition, however, the homogeneous interval is rather narrow so that for a wider interval, one has to add considerably more amphiphile. Because nonionic amphiphiles are poor solvents for ionic amphiphiles, we, for adding small amounts of ionics,prepared aqueous solutions of NaC,S04, their composition being defined by
y
the phase behavior of which is well-known. At ambient temperatures, such mixtures separate into two phases with CiEj mainly dissolved in the lower aqueous phase (2). In a well-defined temperature interval AT, they separate into three coexisting liquid phases (3), and a t elevated T into two phases again, wjth CiEj mainly dissolved in the upper oil-rich phase (2). At the mean temperature T of AT, the amount of amphiphile required for homogenizing equal masses of the two solvents reaches a distinct minimum, as does the interfacial tension between the water-rich (a) and the oil-rich phase (b). Conductivity measurements demonstrate that (stirred) 2 macroemulsions are oil in water (o/w)emulsions, whereas 2 macroemulsions are (w/o) emulsions, the (o/w) (w/o) inversion taking place near T.4 As it is, furthermore, known, mixtures with ionic amphiphiles (with salt and, if necessary, alcohol added) show the reverse phase sequence with rising T , namely 2 3 2 with a (w/o) (o/w) inversion. @
Because, with nonionics, the position and width of AT on the temperature scale depend sensitively on ( i , j )and k,5 application of such amphiphiles in industry is faced with the problem how to find the most efficient amphiphile for solubilizing a given oil in a given temperature range. In principle, this can be done by increasing the amphiphilicity (i, j ) such that AT lies in the desired temperature range. However, high efficiencies are, in general, paid for with narrow AT‘S which makes application more sensitive to temperature fluctuations. This raises the question as how to increase the efficiency of nonionic amphiphiles but keep the temperature range of high efficiency as wide as possible. In this paper we shall show that one way of achieving this goal is to add small amounts of an ionic amphiphile.
E’
E
in wt % (3)
and then mixed the aqueous solution with pure oil and CiEj. The effect of adding NaCloS04 to C6Ez at a = 50 wt % is shown in Figure 1 with 6’ as parameter. First, the fish shape contracts near its “tail” (upper left), making room for a homogeneous mixture at lower y . At E‘ = 1wt % (upper right), the fish has disappeared, leaving a rather wide homogeneous region. With further increasing 6’ (bottom),the vertical extension of the homogeneous region grows, whereas its front remains at a practically constant y . In this experiment a is kept fixed a t 50 wt %, while y is varied. Consequently, the mass fraction ofthe aqueous ( 5 ) See, e.g., Figure 21 in Kahlweit, M.; Strey, R. Angew. Chem.,Int. Ed. Engl. 1985,24,654.
0 1994 American Chemical Society
Langmuir, Vol. 10, No. 8, 1994 2529
Microemulswns with Nonionic and Ionic Amphiphiles IH,O* NOC,oSO,)- B,-C,E,
H,O- B, -C6E2
0:50wI%
6 ::r-l o E’=o,5wt%
E‘=l,Ow 1%
LO
0
20
0
60
LO
V/Wt%
20
60
LO
Y/wt%
/,A/ +-.-20
LO
60
80
\‘ 0 loo
-a
(H,O + NaC,,SO,)
0
20
LO
0
60
V/Wt%
20
+Be-C,E,
60
LO
Y/Wt%
Figure 1. Effect of NaCloSO4 on the three-phase body of the HzO-octane-CcEz mixture at equal masses of H2O and oil (a = 50 wt %). The “fish” shape in broken lines is that of the ternary mixture. As the concentration (e’) of NaCloSO4 in the aqueous phase is increased, the three-phasebody disappears, making room for a homogeneous ‘‘cusp”.
/
- BB - CbE2
’ o ’ (H20+ NaCloSOJ y = 20wtYo
6 = 2.OwtYo
T/I O C
-a
Figure 3. Isothermgl sectionthrough the pseudoternaryphase prism at 12.5 “C > T x 8 “C. Adding E‘ = 0.5 wt % NaCloSOc shifts the three-phase triangle of the truly ternary mixture (top) toward the oil-rich side (bottom).
50-/-
/ - 0
0‘ 0
‘
1
I
50
100
-
amphiphile required for obtaining a homogeneous channel of comparable width.
a/wt%
Figure 2. Section through the pseudoternary phase prism at fixed amphiphile concentration y = 20 wt %, and fixed ionid nonionicratio 6 = 2 wt %, showing a wide homogeneouschannel ascending slightly from the water-rich (a = 0) to the oil-rich side (a = 100 wt %). solution decreases with increasing y . Because fxed, the ratio
6
= NaC,S041C,Ej
in wt %
6’ is
kept
(4) too, decreases with increasing y and, hence, the effect of the ionic. This makes the homogeneous “cusp” drop, and the phase behavior approach that of the ionic-freeternary mixture with increasing y. Because the same is true if the water content is decreased by increasing a at fxed y , we measured the dependence of the extension of the homogeneous region on a by varying 6‘ such that 6 remained fixed. Figure 2 shows the result a t fxed 6 = 2 wt %, i.e., fxed molar ratio = 1/67,and fixed y = 20 wt %, that is, in a section parallel to the HzO-BB-T plane of the prism in which the homogeneous regions in Figure 1show their largest vertical extension. One finds a rather wide homogeneous channel, ascending slightly from the water-rich to the oil-rich side. The addition of the ionic amphiphile thus halves the amount of the nonionic
A Phenomenologic Interpretation As is well known, adding ionic amphiphiles raises the lower critical point of the upper “loop”in the binary HzOCiEj mixture as if it makes the nonionic amphiphile effectively more hydrophilic. This effect is presumably due to the electric repulsion between the (oil-free)micelles which lowers their tendency to phase separate with rising temperature. Evidently, this stabilizing effect should also act on the olw dispersions. The three-phase body within the prism emerges from the overlapping of that (upper) loop with the opposite (lower)Bk-CiEj miscibility gap (and the HzO-BI gap). Because-in the absence of salt-the solubility of ionic amphiphiles in the Bk-CiEj mixture is very low, adding an ionic raises the loop but leaves the oppositegap essentially unaffected which makes the threephase body shrink, and eventually disappear. This interpretation is supported by Figure 3 which shows on top a n isothermal section through the phase prism of the ternary mixture H Z O - B ~ ! ~and E ~on bottom a section through the pseudoternary prism with E‘ = 0.5 wt %2 both at 12.5 “C. Because this temperature lies above T,the three-phase triangle of the ternary mixture (top) has passed its isosceles shape so that the middle phase lies on the oil-rich side. The two-phase region adjacent to the
2530 Langmuir, Vol. 10, No. 8,1994
(H,O + NaC,,SO,)
Kahlweit et al.
- 6, - C,E,
( H 2 0 +NaC,SO,)-e,-CiEj a :50wt%
c’=1 w t %
80
60
LO
20
80
0
20
0 60
LO
0
80
Figure 4. Temperature dependenceof the electric conductivity K vs a (measured in the section shown in Figure 2. K drops by orders of magnitudes as the water content exceeds %60 wt %. a-c side of the triangle terminates a t the H20-C& loop on the A-C side of the Gibbs triangle. Adding the ionic surfactant (bottom) raises the loop which makes H2O and C&2 completely miscible at this temperature. Because the electric repulsion stabilizes the o/w dispersion, the (lower) critical endpoint of the three-phase body moves further to the oil-rich side which makes the three-phase triangle shrink until it eventually disappears with further increasing E’. One may argue that one should be able to achieve a similar effect by increasing the hydrophilicity of CiEj by increasingj at fured i . This, however, raises the lower critical point of the loop as well as the upper critical point of the opposite Bk-CiEj miscibility gap and, hence, the three-phase body as a whole, with the efficiency of the amphiphile actually decreasing.
Conductivity Measurements For obtaining further information, we measured the electric conductivityK vs Tin the stirred mixtures of Figure 2, with a as parameter. The result is shown in Figure 4. At low a, that is, high water content, the high conductivity in the lower two-phase macroemulsion indicates an o/w emulsion as one finds them in mixtures with nonionic amphiphiles below T. As one enters the homogeneous channel, K drops as if the microemulsion starts to invert into a w/o emulsion. However, with W h e r increasing T,this trend is reversed so that in the upper two-phase macroemulsion the mixture has apparently returned to an o/w emulsion as ?ne finds them in mixtures with ionic amphiphiles above T. In the range 60 < a < 65 wt %, the shape of the K( T )curves changes dramatically. The conductivity in both two-phase macroemulsionsdrop by orders of magnitude indicating a rather sudden inversion from o/w to w/o emulsions, whereas the conductivity dip within the homogeneous region deepens only slightly, indicating that the structural changes of the stable microemulsion are not as strong as those in the two macroemulsions. In other words, for a < 60 wt %, the K(T)curves indicate a nonionic ionic inversion of the phase behavior, whereas for a > 65 w t %, they indicate an ionic -nonionic inversion. Thus, the type of dispersion appears to depend sensitively on a,that is, the water content and temperature. Since 6 is kept fured in this experiment, the concentration E’of the ionic amphiphile in the aqueous phase increases with decreasing water content, that is, increasing a. Whether the increase of the ionic strength in the aqueous solution causes the rather sudden inversion of the phase behavior must be left open
-
20
LO
60
y/wt%
Y/Wt%
-----I
1
60 T/OC
t
,
Lo.
20
’
0
20
LO
Y/Wt
60 %
0
20
LO
60
y/wt%
Figure 5. Dependence of the homogeneous cusp on CiEj, measured at a = 50 wt %, and fmed NaCsS04 concentration 6’ =Id%. a t this point. We add that the K(T)curves were exactly reversible with respect to raising or lowering T.
Dependence on CiEj Next, we studied the dependence of the effect of increasing amphiphilicity of CiEj by proceding from (ij) to (i+2j+l). Because experience shows that NaC,S04 becomes strongly surface active for m 2 8, these experiments were performed with NaC8S04 in order to decrease the tendency to form lyotropic mesophases. As in the experiments shown in Figure 1 , ~ was ’ kept fixed at 1wt %, which makes 6 decrease with increasing y. Figure 5 shows the results, with the broken lines again indicating the “fish”of the ternary mixtures. The extensions of the homogeneous regions are only little affected,their position on the temperature scale, however, rising somewhat with increasing ( i j ) . In addition, a Laregion appears that grows with increasing ( i j ) . With CsE3, it lies with the “cusp”of the homogeneous region, with C10E4, it forms a narrow strip below the cusp, with C12E5, it extends from the lower boundary of the cusp downward with a solid precipitate following. Figure 6 shows the corresponding “channels” measured again at fured y = 20 wt %, and varying E’ such that 6 = 2.0 wt % is kept fured. As one can see, the widths of the channels change only little with increasing ( i j ) , except for the fact that the L a region starts intruding into the channel already with C8E3, whereas in the ionic-free mixtures, this occurs only with the longer-chain amphiphiles which demonstrates that adding an ionic does not only stabilize the lyotropic mesophases but even makes them appear. Effect of Added Salt Adding salt lowers the lower critical point of the upper HaO-CiEj loop and, hence, counteracts the effect of the
Microemulsions with Nonionic and Ionic Amphiphiles (H,O+ NaC8S0, y
1001
Langmuir, Vol. 10,No. 8, 1994 2531
-
I- B, CiE,
( H,O+ No C,,SO, 1 B, -C,E a =50wt O h
6 :2,Owt%
:20 w t %
I
€‘=I w t Yo
60 ,
l 50
0
-
100
50
o
O
D
50
0
100
a/wt%
a/wt%
- 0 0
20
LO
60
Y/Wt%
50
0
,
0 0
100
I.c.~solidl
50
1 100
a/wt%
a/wt%
Figure 6. Homogeneous channels (see Figure 2) for the mixtures studied in Figure 5.
u
(H,O+ NoC,S 0,+ N 02S0, )- Be-C,E a
E.OWt%
60
0 0
20
I
LO
.__.--
50 w t %
60
0
o
20 10 60 -v/wt% Figure 8. Dependence of the homogeneous cusp on carbon number k of the alkane, demonstrated on the mixture (HzO NaCloSO4)-dodecane-CsEz. To achieve a cusp comparable to that with octane (Figure l),E‘ has to be increased t o 3 wt %.
+
the mixture
(H20 f NaC8S04-k Na,SO,)-B,-C,E, 0
20
LO
60
LO
60
-Y/wt%
y/wt%
