Langmuir 1994,10, 1134-1139
1134
Weakly to Strongly Structured Mixtures. 2. Ionic Amphiphiles M. Kahlweit,’ G. Busse, and B. Faulhaber Max-Planck-Institut fiir Biophysikalische Chemie, Postfach 2841,D-37018Gbttingen, Germany Received November 9,1993. I n Final Form: February 4,1994” We present a study of the phase behavior of quinary mixtures of water, salt, oils, alcohols, and singletailed sodium n-alkyl sulfates, as it depends, in particular, on the carbon number of the alcohol, and that of the ionic amphiphile. We confirm that adding too much alcohol makes strongly structured mixtures become weakly structured so that the three-phase body may even disappear at a tricritical point. As with nonionic amphiphiles, this process is accompanied by a nonwetting wetting transition at the water/oil interface by the middle phase, and a gradualdecrease of the conductivity change near the mean temperature of the three-phase body. A tricritical point may also-at least in principle-be approached by decreasing the carbon number of the ionic amphiphile, although an experimental verification is difficult due to the fact that the three-phase bodies drop out of the experimental window. The consequences of the results with respect to further research are briefly discussed. -+
Introduction we studied the In three previously published transition from weakly to strongly structured mixtures in ternary mixtures of water, alkanes (Bk),and nonionic alkyl poly(glyco1ethers) (CiEj). The results can be summarized as follows: Starting with weak short-chain amphiphiles, the three-phase bodies evolve from tricritical points (tcp) upon increasing either the carbon number k of Bk at fixed amphiphilicity (i, j ) of CiEj, or upon increasing i and j at fixed k. In the first case, the three-phase bodies grow monotonically with increasing distance from the tcp, the mixtures, however, remaining weakly structured, whereas in the second case, the three-phase bodies first grow, then pass a maximum in the range of medium-chain amphiphiles, and shrink again as one proceeds to long-chain amphiphiles. In that range in which the three-phase bodies pass their maximum, one observes a gradual evolution of properties that distinguish weakly structured from strongly structured mixtures, such as the evolution of cmc surfaces, nonwetting of the waterloil (wlo) interface by the amphiphile-rich middle phase, an olw wlo dispersion inversion near the mean temperature of the three-phase body, and a correlation peak in the I ( q ) curves of SANS, etc. This raises the question as to whether one can find an equivalent transition in mixtures with ionic amphiphiles. The answer meets with some difficulties because with the nonionic CiEj one can change both the hydrophobicity i of the tails and the hydrophilicity j of the head groups stepwise. With ionics, however, one can only change the carbon number of their tails stepwise,whereas for changing the hydrophilicity of their head groups one, in general, has to exchange the entire group. Mixtures with ionics require the addition of salt for enforcing a separation into three liquid phases, so that one actually deals with (at least) quaternary mixtures, the phase behavior of which cannot be represented exactly in three dimensions. In addition, both the amphiphile and the salt introduce electric charges which may complicate the interpretation. Of the ionic amphiphiles of which the carbon number m
-
Abstract published in Advance ACS Abstracts, March 15,1994. (1) Kahlweit, M.; Strey,R.; Aratono, M.; Busse, G.;Jen, J.; Schubert, K. V. J . Chem. Phys. 1991,95, 2842. (2) Kahlweit, M.; Strey, R.; Busse, G. Phys. Rev. E 1993,47, 4197. (3) Kahlweit, M.; Busse, G.; Winkler, J. Chem. Phys. 1993, 99, 5605. 0
of their tails can be readily changed, we chose the singletailed sodium n-alkyl sulfates, denoted by NaC,S04. As is well known,the phase behavior of mixtures with ionic amphiphiles is the reverse of that with nonionics. (i) Withnonionics, the mean temperature Tof the threephase bodies drops with decreasing carbon number of the oil, and increasing carbon number of the tail of the amphiphile. With ionics one finds the reverse. Experience tells that the carbon number of standard single-tailed ionic amphiphiles is too low to enforce a threephase body within the experimental window, that is, between the melting and boiling temperatures, even with oils of low effective carbon number such as,e.g., aromatic oils. For raising the three-phase bodies into the experimental window one, therefore, has to either increase the hydrophobicity of the amphiphiles by using double-tailed amphiphiles or decrease the hydrophobicity of the oil by adding a medium-chain alcohol, C,Eo (n 1 4). Thus, for studying the phase behavior of mixtures with NaC,S04, one has to use quinary mixtures of the type
H,O
+ salt + oil + C,Eo + NaC,SO,
Disregarding gravity, and a t fixed pressure, such a mixture has five thermodynamic variables, namely, temperature T and four composition variables. For representing its phase behavior in three dimensions one must, therefore, reduce the number of parameters by two. This can be done either by keeping T fixed, treating one pair of components as a “pseudocomponent” and representing the phase behavior in a pseudoquaternary phase tetrahedron, or by treating two pairs of components as pseudocomponents and representing the phase behavior in a pseudoternary phase prism with T as the ordinate. The first representation is more exact, having the disadvantage that, for studying the temperature dependence of the phase behavior, the tetrahedron must be determined at various temperatures which is rather time consuming. The second representation is less exact but easier to perform. It should be emphasized, however, that these pseudocomponents may not be (and are, in general, not) pseudocomponents in the narrower sense. Because inorganic salts are much more soluble in water than in oils or alcohols, the most appropriate choice is to
Ql43-7463/94/241Q-1134$Q4.5Q/Q 0 1994 American Chemical Society
Weakly to Strongly Structured Mixtures
Langmuir, Vol. 10, No. 4,1994 1136
treat H2O + salt as pseudocomponent “brine”, its composition being defined by ef
[saltI/[H,O
+ salt]
(wt 7%)
(1)
The major effect of salts is that of decreasing the hydrophilic interaction between Hz0 and the head groups of the amphiphiles. This lowers the mutual solubility between HzO and amphiphiles. Salts thus “salt out” both nonionic and ionic amphiphiles out of the aqueous phase into the oil-rich phase. For ionics, this effect has been demonstrated by Granet et aL4who prepared mixtures of equal volumes of alkanes and aqueous solutions of NaCl and Na(C,)&HSOs, the latter denoting branched sodium alkanesulfonates with m between 7 and 11, and an amphiphile concentration 5 times the cmc (being a function of e), and then measured the amphiphile concentration in the aqueous phase vs e at 45 OC. At low e, they found the amphiphiles almost completely dissolved in the aqueous phase. Upon increasing e, however, they observed a distinct inversion of the distribution. As was to be expected (see their Figure 5 ) , the following apply: (ii) The higher the carbon number k of the alkane, the more salt that is required to drive the amphiphile into the oil-rich phase. (iii)The higher the carbon number m of the amphiphile, the less salt that is required. As a consequence, one finds the following: (iv)Adding a salt makes Tdrop with nonionics, but rise with ionics. Alcohols C,Eo (n 1 4), on the other hand, are much more soluble in oils than in water. As was recently demonstrated by Kegel et al.,6the distribution coefficient K = X , ~ X H ~increases O strongly with increasing n. Furthermore, alcohols, too, are salted out of the aqueous phase by NaC1, the effect decreasing with increasing n. Alcohols can be viewed as polar “co-oils” as well as hydrophobic nonionic “cosurfactants”. As such they distribute between the oil-rich bulk phase and the interfacial layers which makes the oil effectively less hydrophobic, but the amphiphile effectively more hydrophobic. Because both effects act in the same direction (see rule i), one finds the following: (v) Alcohols act in the same direction as salts. Adding C,Eo ( n 1 4) thus makes T drop with nonionics, but rise with ionics. Accordingly, it is a matter of convenience whether one treats oil + C&, or amphiphile + C& as the pseudocomponent. In the following the mass fraction of the “oil” in the oil + brine mixture is defined by CY
= [oill/[oil+ brine]
(2)
If oil + C,Eo is treated as the pseudocomponent, its composition is defined by @
[C,Eol/[oil+ C,Eol
(3)
The mass fraction of the amphiphile in the entire mixture is defined by y
[amphiphilel/[oil + brine + amphiphile]
(4)
6 = [C,EoI/ [amphiphile + C,Eol
(5)
All of the above parameters are in weight percent. In the literature one finds both kinds of representation. Treating amphiphile + C,& as the pseudocomponent was, in particular, preferred by French groups. As an example, consider the mixture (H20+ NaC1) + toluene + (NaCI2SO4+ C4Eo) studied by Cazabat et al.s at 20 OC. They chose the mean composition [HzO + NaC11 = 46.8 wt 5% , [toluene] = 47.2 wt % (Le., a = 50 w t %), NaC1zS04 + C4& with 6 = 67 wt % ,and y = 6 wt % . Upon increasing e, they found the phase sequence 2 3 2 with the three-phase (salt) interval between e = 5.35 and e = 7.38 wt % , In 1985, we7 represented the corresponding three-phase body in a pseudoquaternary phase tetrahedron with the triangle H2O + toluene + (NaClzSO4+ C&; 6 = 67 wt %) as the base, and the mass fraction of NaCl (in the entire mixture) on top. Figure 1, redrawn from Figure 11 in ref 7, shows an a = 50 wt % section through the tetrahedron at 25 OC. It shows the “fish”with the “tail” at y = 18 wt % . The arrow through the fish at y = 6 wt 7% represents the path taken in ref 6. Note that because a = 50 w t % ,the mass fraction of NaCl in the entire mixture is about e/2. In accord with rule v one finds that the higher y (at fixed 6), i.e., the higher the (mean) C& concentration, the less salt that is required to enforce a separation into three phases, and the lower the C& concentration, the more salt that is required. Another important conclusion can be drawn from an experiment by Kunieda8who searched for a tricriticalpoint in the mixture
--
(H20 + NaC1) + (isooctane + C,Eo) + NaC,S04 Kunieda treated oil + CnEo as the pseudocomponent. Because alkanes are more hydrophobicthan aromatic oils, he had to add more alcohol than required with toluene for raising the three-phase body into the experimental window. Keeping CY = 50 wt % and e = 1.25 wt 7% fixed, he found in accord with rule v the three-phase body to rise with increasing 8. More important, however, is the result that the three-phase body shrinks and retreats to higher y upon increasing @ until it disappears near @ = 72 wt % at a tricritical point (see his Figure 2). From this it follows that adding too much alcohol decreases the effective amphiphilicity of ionic amphiphiles, thereby weakening the stability of the structures. These experiments inspired us to study the phase behavior of mixtures with n-alkanes (Bk) as oils and-for reducing the types of ions-Na2S04 as salt (HZO + N%S04) + (Bk + C,EO)
+ NaC,S04
treating brine and the Bk + C,& mixture as pseudocomponents. The parameters varied were k, m, n, 8, and c. The results are only little affected if NaCl is used at about 2/3 of c of Na2SO4. The surfactants were purchased from Henkel (m= 6,10,12,14) and from Merck (m= 8). The organic solvents were purchased from Merck (“for synthesis”) and used as supplied.
If amphiphile + C,Eo is treated as the pseudocomponent, its composition is defined by
Effect of the Alcohol Concentration @ First, we repeated Kunieda’ss experiment with (HzO + Na2SO4) + (BE+ C&) + NaCsS04 at fixed a = 50 wt %
(4) Granet, R.; Khadirian, R. D.; Piekarski, 5.Colloid Surf. 1990,49, 199. (6)Kegel, W. K.; van Aken, G. A.; Bouts, M. N.; Lekkerkerker, H. N. W.;Overbeek, J. Th.; de B N ~ P., L. Langmuir 1993, 9, 262.
(6)Cazabat, A. & Langevin, D.; Meunier, J.; Pouchelon, P. J. Phys. (Paris),Lett. 1982,43, I.-89. (7) Kahlweit, M.; Strey, R.; Haase, D.J. Phys. Chem. 1986,89, 163. ( 8 ) Kunieda, H.J. Colloid Interface Sci. 1988,122, 138.
Kahlweit et al.
1136 Langmuir, Vol. 10, No. 4, 1994 H20 -Toluene - (NaC,2SOL+ CLEO) - NoCl
(H20+ Na,SO,)
E
- (b+%E,,) - NaGSO,
= 1.5 w t %
NaC12S0,/CLEo = 1/2
-
0
10
20 (NaC,,SO,
30 +
C,Ed/wt%
Figure 1. Section through the three-phase body of the HzO + toluene (NaC12SOI + C&) NaCl mixture a t 25 "C, represented in a phase tetrahedron. The arrow represents the path taken by Cazabat et alae
+
+
(H20+ Na,SOL)
- CB,
+ &E,,)
- Na&SO,
100
-
80 T/ O C
t
x/Srn-'
Figure 3. Electric conductivity K vs T for the mixtures shown in Figure 2, demonstrating the loss of structure with increasing
8. 6o
increase of K. The steepness of the conductivity change near T can be taken as a qualitative measure for the stability of the dispersions. As one approaches a tcp with nonionics by decreasing i and j , the change of K gradually disappears (see Figure 8 in ref 3). Accordingly, one expects the same in mixtures with ionics upon approaching the tcp by increasing 8. This is indeed the case. The 0' ' ' ' ' ' ' ' J conductivity was measured in a self-registrating bridge 0 2 L 6 a described previ~usly.~ Figure 3 shows K vs T i n the above y/wt% mixtures, measured a t a y just behind the tails of the fish. As expected, the change of K decreases with increasing 8, Figure 2. KuniedaV experiment, demonstrating the evolution of a three-phase body from a tricritical point upon decreasing with the low K at low T increasing, which indicates agradual the alcohol concentration 0 in the (HzO + NaSOd) + (Ba+ C ~ O ) weakening of the stability of the insulating interfacial + NaC&3Oc mixture. layers in the w/o dispersion, whereas the high K in the o/w dispersion at elevated T is evidently less affected by the and t = 1.5 wt %. Our result, shown in Figure 2, is loss of stability. Note, furthermore, that the lower points essentially identical with that found by Kunieda. Upon of maximum curvature of the ~ ( 7 7curves lie somewhat increasing 6, the three-phase body rises, retreats to higher below the "head" of the fish, whereas the higher points lie y, and shrinks until it apparently disappears at a tricritical within the tail, a fact which is presumably due to the point near p = 75 wt % . We then studied both the wetting pseudoternary representation. From these two additional behavior and the electric conductivity of the mixture upon experiments we deduce that one is indeed approaching a increasing p. We recall that, as one approaches a tricritical tricritical point, with the added alcohol gradually weakpoint with the nonionic CiEj by decreasing its amphiphiening the structure. licity ( i , j ) at fixed lz, one observes a nonwetting- wetting transition at the HzO/oil interface by the amphiphile-rich Effect of the Salt Concentration c middle phase. In the above experiment the tcp is In 1975, Lang and Widomgsearched for a tricritical point approached by decreasing both the effective carbon in the quaternary mixture number of octane and the effective amphiphilicity Of NaCeSO4. Accordingly, one expects a nonwetting wetting transition upon increasing p. This is indeed the case. H 2 0 4- C6H6(benzene) 4- CzEo4- (NH4),S04 Choosing a mean composition within the three-phase bodies, the volume of the middle phase was reduced until Because HzO and CZ&are completely miscible, they salted a drop of it was left. Its wetting behavior was then observed out the alcohol by adding (NHd)zS04 which leads to a at a temperature on the symmetry axis of the fish in a separation of the mixture into three coexisting liquid l-cm quarz cuvette, with the drop having no contact with phases at a tricritical point at an elevated temperature. the wall. For 0I30 wt 5% , the drops contracted to a lens; Upon increasing the salt concentration, the three-phase for 1 40 wt % ,they spread. We have, however, refrained body drops and grows, with the mixture remaining weakly from studying the wetting behavior near the critical end structured. With ionic amphiphiles the effect of salt is points. reversed, that is, adding salt makes the three-phase body Microemulsions exhibit a steep change of electric rise as it grows. This is demonstrated in Figure 4 which conductivity K as one traverses the three-phase body either shows the dependence of the phase behavior of by raising T a t fixed mean composition or by adding salt at fixed 5". As we have shown in ref 3, this change of K is caused by a dispersion inversion. Nonionics show an o/w w/o inversion with rising T, and thus a decrease of K , (9)Lang,J. C.; Widom, B.Physica A 1978,81, 190. whereas ionics show a w/o o/w inversion, and thus an LO
t
-
1
-
-
-
Weakly to Strongly Structured Mixtures (H20+ Na,SO,)
Langmuir, Vol. 10, No. 4, 1994 1137
- (6, + C6EJ - NaC&OL
(H$+Na2SOLl-l~+C,Ed - NaC,,SO,
7
loo
80
Lot 01 0
'
' L
-
"
2
"
'
1 8
6
a p ==L50o w t % %
0
2
y/wt%
Figure 4. Effect of brine concentration t on the phase behavior NaC&304 mixture at of the (H2O + NazSO4) + (BE C&) fixed j3, showing a pseudo-near-tricritical behavior.
+
+
-
6
L
8
y/wt%
Figure 6. Effect of the carbon number n of the alcohol on the phase behavior of the (H2O + Na2SO4) + (BE+ C,&) + NaCloSO4mixture at fixed j3 and t.
(H$+ Na2SOL)-(Be+ C6Ed - NaCBSO,
(H20+NazS0,)-IB,+C,E,)-NoC,S0,
60 a=50wt%
1
P=50wt%
i An
I
n.8, v = 2 5 w t %
~
04..v1-
y: 3.5wt Yo E: 1.2 w t %
0 lo4
103
-
10-2
IO-'
1
'
n=6. ~
~
3
5
IO
n/Sm
Figure 5. Electric conductivity K vs T for two of the mixtures shown in Figure 4, demonstrating that the phase behavior is not near-tricritical.
on the salt concentration t at a = 50 wt % and 0 = 50 wt 9%. At this a,the three-phase body appears between e = 1.2 and t = 1.3 wt %, rising and growing with further increasing e. So far, the phase behavior resembles that of a near-tricritical mixture. If so, one would expect the electric conductivity to exhibit a behavior similar to that shown in Figure 3 upon increasing 0.Figure 5 shows K vs T for e = 1.2 w t % and y = 3.5 w t %, in comparison with the curve for t = 1.5 wt % taken from Figure 3. As one can see, the conductivity change is even steeper in the mixture without a three-phase body which indicates that the three-phase body does not evolve from a tricritical point but is salted out of an already strongly structured two-phase mixture. Effect of II in C,Eo With nonionics, adding a medium-chain alcohol makes the three-phase body drop, the effect increasing with increasing n.I0 For studying the effect of n on mixtures with ionic amphiphiles, we chose NaCloS04 as the amphiphile and determined the three-phase bodies at a = 50 wt %, = 40 wt %, and t = 1.5 wt %, with n as the parameter. The result is shown in Figure 6. With ionics, adding alcohol makes the three-phase body rise, the effect, too, increasing with increasing n. While with toluene as the oil, adding a little C4Eo suffices to raise the threephase body into the experimental window, the more hydrophobic alkanes require considerably more alcohol. (10)See Figure 5 in ref 2.
I
a =50 wt %
p=LOwt% 1.5 w t %
E=
"1c5 IO-,
-
IO-* 10"
I
1
IO
x /Sm-'
Figure 7. Electric conductivity K vs T for the mixtures shown in Figure 6, demonstrating the increase of structure with increasing n.
Even with 40 w t % C4Eo in BE,the three-phase body still lies below the melting temperature, to appear just above it with C5Eo. With further increasing n, the three-phase body rises and shrinks. However, in contrast to Figure 2, it moves toward low y which indicates that the structure becomes stronger upon increasing n. To check this presumption, we again measured the electric conductivity K vs T, with n as the parameter. Figure 7 shows the result. Upon increasing n, the conductivity change becomes increasingly steeper. Again, the high conductivity in the o/w dispersions is only little affected, whereas in the w/o dispersions it drops by orders of magnitude with increasing stability of the interfacial layers. Effect of m in NaC,,,S04 The effect of the carbon number m in NaC,SOdis shown in Figure 8, measured at a 50 wt % and t = 1.5 wt % (except for m = 6). In accord with rule i the three-phase body rises with increasing m. The effect is rather strong, so that, for keeping the bodies within the experimental window, one has to decrease /3 as m is increased. We recall that, with the double-tailed ionic AOT, one may skip the alcohol altogether, though salt is still required for enforcing a separation into three phases.
Kahlweit et al.
1138 Langmuir, Vol. 10,No. 4, 1994 (H20+Na&)-(BB-
C&)-
the phase behavior of the mixture
NaC,SO,
(H20 + NaCl + NaCI2SO4)+ (C&2 (cyclohexane) + C,Eo)
80 T/
T
1.)
-
LO
=3.0
E
0
"
'
I
"
2
0
'
L
6
a
y/wt%
--L
Figure 8. Effect of the carbon number m of the amphiphile on the phase behavior of the (HzO + NaaSOd + (Be+ C&) + NaC,S04 mixture at fixed e. Note that /3 has to be lowered as m is increased for keeping the three-phase bodies within the experimental window. (HP+ NaSO,) - (9,+ %Eo) - NaC&lL
'L p=50wt%
0
E
0
= 1.5wt% 2
L
6
8
y/wt%
Figure 9. Effect of the carbon number k of the oil on the phase behavior of the (HzO + NaSO4) + (Bk + c&) + NaCaSO, mixture. Note that the difference between the effective carbon numbers is decreased by the presence of the alcohol.
As one would expect, the conductivity change decreases with decreasing m,which indicates that, as with nonionic amphiphiles, one may-at least in principle-reach a tcp by decreasing the amphiphilicity of ionic amphiphiles at fixed k,although the experimental verification is difficult because the transition to near-tricriticalmixtures lies below the melting temperature. Effect of k in B h The effect of the carbon number k of the alkanes on the phase behavior at fixed a = 50 wt %, 6 = 50 w t %, and e = 1.5 w t %, with C& as the alcohol and NaCaSOr as the amphiphile, is shown in Figure 9, to be compared with that in mixtures with the nonionic C&4 shown in Figure 2 in ref 11. In accord with rule i, increasing k makes the three-phase bodies rise with nonionics, but drop with ionics. With the two amphiphiles having the same carbon number, the effect is considerably smaller with the ionic because the addition of the alcohol makes the actual differences between the effective carbon numbers of the oils smaller than in the mixtures with C& and pure alkanes. Comparison with the Literature In a recently published paper, Kegel and Lekkerkerker12 studied the effect of the carbon number n of alcohols on (11) Kahlweit, M.;Strey, R.; Haaee, D.; Firman, P. Langmuir 1988,4, 785.
-
at 25 OC, with n = 5-7, and CY 40 w t % (corresponding to about equal volume fractions of the aqueous and the oil-rich phases). The aqueous phase was composed of 0.2 M NaCl (e 1.2 w t %) and various amounts of NaC12S04 (wt 7%). They then studied the phase sequence at fixed (initial)NaClzSO4 concentration upon graduallyincreasing 6 (see their Figure 2). While we, in determining our phase diagrams, kept 6 fixed and raised T at various y, Kegel and Lekkerkerker kept T fixed and increased 6at various y, thereby raising the three-phase bodies from below the melting temperature to 25 "C like an elevator. In accord with our Figure 6,the higher n, the less alcohol that is required. With n = 6 and 7, in particular, they observed the mixtures to show the phase sequence 2 3 4 3 2 upon increasing /3 at fixed low y. The separation into four coexisting liquid phases requires at least four components. It results from the interplay of four threephase bodies of which the authors traversed two. For locating the four-phase body, and clarifying the origin of the three-phase bodies, the representation of the phase behavior in a phase tetrahedron is the convenient one. Each of the four ternary phase diagrams representing the sides of the tetrahedron may or may not show a threephase triangle. If not, the mixtures must separate into three phases upon addition of the fourth component. If these four three-phase bodies overlap, this results in a four-phase tetrahedron within the phase tetrahedron. In a forthcoming paper we shall demonstrate the evolution of such a four-phase body in the more transparent quaternary nonionic mixture
-
---
H20 + alkane + perfluoroalkane + C,Ei which arises from the fact that alkanes and perfluoroalkanes are not completely miscible. In the mixture studied by Kegel and Lekkerkerker the four-phase body arises apparently from the intrusion of the lamellar L, phase into the original three-phase body which leads to the formation of four three-phase triangles among the water-rich, the amphiphile-rich, the oil-rich, and the L, phases, respectively. The authors discussed this process in terms of the effect of the alcohol chain length on the elastic moduli associated with the mean and the Gaussian curvatures. We do not intend to question their interpretation. All we wish to emphasize is that the effect of alcohols depends on their concentration, and thereby on temperature. For nonionics this was demonstrated by Jonstriimer and Strey13who traced the La phase in the oil-free H20 + mixture at fixed amphiphile concentration upon addition of C& (see their Figure 1). In the alcohol-free mixture, the L, temperature interval lies at about 60 "C, to drop below the melting temperature after addition of about 2 wt % C& only. Accordingly, if the alcohol is added at fixed temperature, say 40 "C, it appears as if it causes the formation of a lyotropic mesophase. Actually, however, the L, phase is already present in the alcohol-freemixture at a higher temperature, and is just lowered to 40 OC by the alcohol. In view of the fact that the phase behavior of mixtures with ionics is the reverse of that with nonionics, one expects in mixtures (12) Kegel, W.K.;Lekkerkerker, H. N. W. Colloids Surf.A 1993,76, 241. (13) Jonstr6mer, M.;Strey, R. J . Phye. Chem. 1992,96, 5993.
Langmuir, Vol. 10,No. 4,1994 1139
Weakly to Strongly Structured Mixtures (YO+NaCI) - (C,H,
E
+C,E,)-NaC+O,
(H$+Na,SOJ-
t
- NaCnSOL
= 1.2wt% 80
80 T/
(B,+C,Ej)
T/
OC
OC
6o :15
LO
20
p.12 \p.lowt%
0
c
0
L
2
c
c
"
LO 20 -
1
6
L
8
y/wt%
Figure 10. Effect of alcohol concentration 8 on the phase behavior of the (HzO + NaCl) + (C&Z + C&) + NaClzSO, mixture studied by Kegel and Lekkerkerker12at 25 "C, demonstrating the loes of stability of the La meaophase with increasing
8. with ionic amphiphiles the lyotropic mesophases to rise upon addition of alcohol. If, however, too much alcohol is added, their stability decreases. This is demonstrated in Figure 10which shows the phase behavior of the mixture (H20+ NaC1) + (C&2
0' 0
'
+ C$,) + NaC12S0,
at a = 50 wt %,ande = 1.2 wt %,with8 as the parameter. As expected, the three-phase body rises with increasing 8, with the L, phase gradually retreating from the threephase body. At these elevated temperatures, the phases separate rather rapidly. Note that Kegel and Lekkerkerker found the four-phase body with 8 8 wt 5% at 25 O C .
-
Conclusion Hitherto, microemulsionswith pure single-tailedanionic amphiphiles were mostly prepared at ambient temperatures with aromatic oils, NaClzSO4, and C&. In this paper we have demonstrated how to prepare homogeneous microemulsions with alkanes at any temperature within the experimental window by appropriately varying the
2 '
"
2
' L
-
p=LOwt% ~:15wt%"
6
'
J
8
y/wt%
+ + (Bk + c&) + NaC&3Ol mixture at fiied 8 and e.
Figure 11. Effect of j on the phase behavior of the (Ha0 NazSO,)
carbon number of the amphiphile and that of the alcohol. For industrial application these recipes are presumably of less interest because of the large amount of alcohol required. However, the results suggest replacing alcohols by alkyl poly(glyco1 ethers) CiEb While alcohols (j = 0) can be viewed as being both co-oils and cosurfactants, increasing j from 0 will amplify the role of the additive as cosurfactant which permits monitoring the effective amphiphilicity of the mixed interfacial layer by varying i, j , m, and the mixing ratio of the pseudocomponent (NaC,S04 + CiEj). A preliminary result is shown in Figure 11, in which we replaced C& in Figure 6 by C& and CsE2 at the same 8. As one can see, C& makes the threephase body rise above that with C&, while C & &makes it drop, from which we deduce that j = 1 makes the interfacial layer more hydrophobic, while j = 2 makes it more hydrophilic. The strong inclination of the two fish is due to the fact that the nonionic amphiphiles are inappropriately treated as co-oils. A study of the phase behavior with mixtures of single-tailedionic and nonionic amphiphiles is in progress.l4 (14) For a similar study with the double-tailed AOT,see: Kahlweit, M.;Strey, R.J. Phys. Chem. 1988,92,1557.
