248
J. Phys. Chem. 1993, 97, 248-254
Microemulsions with Formamide as Polar Solvent K. V. Schubert, G. Busse, R. Strey, and M. Kahlweit’ Max- Planck-Instirut fur biophysikalische Chemie, Postfach 2841, 0-3400 Gdttingen, Germany Received: July 14, 1992; In Final Form: October 9, 1992
Microemulsions are, in general, prepared by mixing water, a nonpolar “oil”, and a n amphiphile, either nonionic or ionic. In this paper we study the effect of replacing water by another polar protic solvent, namely, formamide (FA). Because hydrocarbons are slightly more soluble in F A than in H 2 0 , the repulsive hydrophobic interaction between the hydrocarbon tails of the amphiphiles and F A is weaker than in H2O. As a consequence, both the mutual solubility and the cmc increase considerably upon replacing H2O by FA. This can be compensated by increasing the carbon number of the tails of the amphiphiles. With the nonionic CiEj one has to increase the carbon number i by about five, whereas with ionic amphiphiles one has to proceed from single-tailed to doubletailed amphiphiles as, e.g., (Cm)2DABr. If one does so, one may prepare microemulsions with FA having essentially the same properties as with H20 as polar solvent. Again one finds a reverse phase behavior for nonionic and ionic amphiphiles and comparable solubilization capacities, as well as the gradual evolution of a correlation peak in small-angle neutron scattering curves and a complete wetting partial nonwetting transition as one proceeds from weakly to strongly structured mixtures.
-
I. Introduction In the literature,l properties of aqueous solutions of amphiphiles are interpreted as being the consequence of competing “opposing forces”, namely, the repulsive hydrophobic interaction between their hydrocarbon tails and water and the attractive hydrophilic interaction between their head groups and water. This has raised the question as to the interaction between amphiphiles and polar protic solvents other than water. Quite a number of authors have, therefore, studied micelle formation of both nonionic and ionic amphiphiles in such solvents,2 whereas fewer studies have been published on the effect of substituting other solvents for H2O in ternary mixtures of H2O (A), oils (B), and amphiphiles. Rico, Lattes, and co-workers3 used formamide with a cationic amphiphile, Bergenstahl et ale4used various solvents including formamide with an anionic amphiphile, Warnheim and SjabergS used formamide with a nonionic amphiphile, Martino and Kaler6 used mixtures of propylene glycol and glycerol with a nonionic amphiphile, and Friberg et al. used various solvents including ammonia’ with an anionic amphiphile. Except for refs 3,5,and 6, none of the mixtures separated into three coexisting liquid phases. After having recently published studies on where to find three-phase bodies in mixtures with formamide and nonionic a m p h i p h i l e ~ ,we ~ , ~shall in this paper present results of more detailed experiments, using both nonionic (C) and ionic (D) amphiphiles. The essential features of binary A-C (or D) mixtures as well as those of ternary A-B-C (or D) mixtures are experimentally well studied,I0 though by no means theoretically fully understood. Consider first the oil-free binary mixtures (see Figure 3 in ref 10). Disregarding at this point lyotropic mesophases, both the phase diagrams of A-C and A-D mixtures show miscibility gaps. There is, however, an important difference between the two: at ambient temperatures, nonionic amphiphiles are completely miscible with HzO. Because the attractive interaction between the head groups and H20 decreases with rising temperature, the repulsive interaction between their tails and H20 eventually overcomes the attractive interaction which leads to a phase separation at a lower critical point (cps). Ionic amphiphiles, on the other hand, are only partially miscible with water at low temperatures. Because the dissociation of their head groups increases with rising T,the attractive interaction eventually overcomes the repulsive interaction which leads to complete miscibility at an upper critical point (cpa). As it turns out, this 0022-3654/5S/2091-0248%04.0~/0
reverse temperature dependence of the interaction between the head groups and H20 reflects itself also in the reverse phase behavior of H20-oil-amphiphile mixtures. HzO-oil-amphiphile mixtures separate into three coexisting liquid phases within a well-defined temperature interval A T = Tu - T I . At the mean temperature 7of AT one finds a maximum of the efficiency of the amphiphile in homogenizing water and oil and a minimum of the interfacial tension Crab between the water-rich and the oil-rich phase, both properties which are of importance for applying amphiphiles in research and industry. However, due to the reverse temperature dependence of the solubility of nonionic and ionic amphiphiles in water, the evolution of the three-phasebody is reverse, too. With nonionicamphiphiles, the three-phase body appears a t TI by separation of the (lower) aqueous phase into a water-rich and an amphiphile-rich phase (Figure 1, left). With further rising T,oil becomes an increasingly better solvent for the amphiphile than water. Consequently, the amphiphile-rich phase moves clockwise to the oil-rich side of the phase prism where it merges with the (upper) oil-rich phase at Tu.With ionic amphiphiles, on the other hand, it is the oil-rich phase that separates at TI(Figure 1, right). With further rising T,water becomes an increasingly better solvent for the amphiphile than oil, so that the amphiphile-rich phase moves counterclockwise to the water-rich side where it merges with the water-rich phase at Tu.At fixed mean composition one thus finds with rising T for nonionic amphiphiles the phase sequence 2 3 2, but for ionics 2 3 2. The separation into three phases arises from the interplay between the three binary miscibility gaps: H & o i l , H20amphiphile, and oil-amphiphile. Accordingly, the mean temperature T of the three-phase body is correlated with the temperatures of the critical points of the Hz0-amphiphile gaps (Toand Ta, respectively) and that (TJ of the (lower) miscibility gap between oils and amphiphiles. As a consequence, the dependence of Ton the carbon number of the oil, or the strength of the tails and heads of the amphiphiles, as well as on additives can be predicted qualitatively by studying the effect on the corresponding critical temperatures. Consider first increasing the carbon number k of the oil at fixed amphiphile (Figure 2 , top). Increasing k will make T, rise. With nonionics this decreases their tendency to change from H2O to oil, that is, makes T rise, whereas with ionics it increases their tendency to change from oil to HzO, that is, makes Tdrop. Consider now increasing
--
0 1993 American Chemical Society
--
The Journal of Physical Chemistry, Vol. 97, No. 1, 1993 249
Microemulsions with Formamide as Polar Solvent
ionic amphiphile
nonionic amphiphile (C)
(Dl
M
2
T
I
3
2
Figure 1. Evolution of the three-phase triangles with temperature for nonionic (left) and ionic amphiphiles (right). For discussion see text.
the carbon number i of the tail of the amphiphile at fixed head group and fixed oil (Figure 2, center). With nonionics, this makes To drop which makes T drop, too. With ionics, on the other hand, it makes T6 rise which makes Frise. Consider, finally, the effect of a lyotropic salt as, e.g., NaCl (Figure 2, bottom). Because lyotropic salts "salt out" amphiphiles, adding a salt makes TB drop but 7'6 rise. With nonionics, T thus drops with increasing salt concentration, whereas with ionics it rises. Adding a lyotropic salt is thus equivalent to making amphiphiles less hydrophilic. On the basis of these considerations, one may now discuss what to expect if water is replaced by another polar protic solvent. Because the properties of the binary oil-amphiphile mixture will, evidently, not be affected, it suffices to study the effect on the polar solvent-oil and, in particular, the polar solvent-amphiphile mixture. Replacing H2O by another Solvent will, evidently, affect the interactions between the tails as well as the heads of the amphiphiles and thesolvent. Theeffect on the interaction between the tails and the solvent can be predicted qualitatively by comparing the solubility of hydrocarbons in the solvent with that in H20, whereas for predicting the effect on the interaction between the heads and the solvent one may take its dielectric number c as crude measure for the strength of the hydrogen bonds. Higher solubility of hydrocarbons decreases the repulsive hydrophobic interaction; weaker hydrogen bonds decrease the attractive hydrophilic interaction,and viceversa. The two effects may thus either act in the same or opposite directions. Consider now formamide (FA; c = 109.5), and N-methylformamide (NMF; e = 182.4) as substitutesfor H2O. As measure for the solubility of hydrocarbons in these solvents, we took their miscibility with hexanol ( c 6 b ) . While H2@C& mixtures are immiscible up to the boiling temperature, mixtures become miscible above about 40 OC, whereas NMF-C& and even N M F - C s b mixtures are completely miscible in the entire
experimental window. From this we deduce that although the solubility of hydrocarbons in FA is somewhat higher than in H20, it may still be sufficiently low for maintaining a repulsive hydrophobic interaction between the tails and FA. The solubility of hydrocarbons in NMF, on the other hand, appears to be too high for that purpose. Formamide p.A. was purchased from Baker and used without further purification. As electric conductivity at 25 OC we measured KFA = 48.8 mS m-I. At that temperature, it remained constant over 24 h. At 25 OC, the surface tension a of H20-FA mixtures against air can be described almost perfectly by the simple equation"
+
exp(-a/a) = (1 - x ) exp(-ao/a) x exp(-a,/a) (1) with a = 5-47 mJ m-2, where is the mole fraction of FA, a. E = 58.1 mJ m-2. = 72.1 mJ m-2, and 11. Effect of FA on H@-Amphiphile
Mixtures
The higher solubility of oils in FA, compared with that in H20, reduces the repulsive interaction between the hydrocarbon tails of the amphiphiles and FA such that the amphiphiles become considerably less 'hydrophobic" in comparison with H2O as solvent. This has large effects on the properties of the A-C (or D) mixtures. Because the mutual solubilities between the polar solvent and amphiphiles as well as the cmc's are essentially determined by the competition between the opposing forces, one expects that decreasing the repulsive hydrophobic interaction between the hydrocarbon tails and the polar solvent will make both the solubilities and the cmc's increase. This has been confirmed by experiment.12 Consider first mixtures with nonionic amphiphiles, namely, n-alkyl polyglycol ethers (CiE,). Figure 3 (top) shows the effect
Schubert et al.
250 The Journal of Physical Chemistry, Vol. 97, No. 1, 1993 (HZO FA 1 - CloEL
I lgiven amphiphilel 70 T / OC
--
t
50
-
LO
L,
carbon number of oil LO -
I=
ionic
30
I
60
60 -
20
-j
-
20 r
i
-
nonionic
1
0,l
0,Ol
hydrophobicity ot amphiphile
I lgiven
1
7
10 CjoEL / w t YO
100
55 amphiphile and oil] 50
ionic
T
d/mNm-' L5
t
-
I
nonionic
salt concentration Figure 2. Dependence of the mean temperature t of the three-phase body on the carbon number of the oil (top), the carbon number of the
tailof theamphiphile (center),and theconcentration of anadded lyotropic salt. The full lines represent the dependence for nonionic and the dashed lines that for ionic amphiphiles (schematic).
+
LO 35
30
25L
T=25"C
of FA on the upper loop of the ( H 2 0 FA)-CloE4mixture, with 0,001
0.01
1
0.1 CloEL /
in weight percent as parameter. As H20 is replaced gradually by FA, the loop shrinks which makes Tp rise. On the bottom of the figure one can see the effect on the cmc a t 25 OC, determined by surface tension measurements. The log cmc increases linearly with increasing $. At low +, the limiting slope (-&/a log c)~,,, at the cmc remains practically constant but decreases somewhat as one approaches pure FA. In pure FA, the cmc is about 2 ordersof magnitude higher than in pure H20. Becauseexperiment shows that in H2O decreasing the carbon number i in CiE, by 2 makes the cmc increase by about 1 order of magnitude, one deduces as rule of thumb that the cmc of CjEj in pure FA is about that of Cj-4Ej in pure H20. The phase diagrams of HZO-ionic amphiphile mixtures are more complex because here the miscibility gaps are, in general, hidden behind lyotropic mesophases. The most transparent mixtures are apparently those with dialkyldimethylammonium salts, (Cm)2DAX, with X standing for the counterion. The diagrams published by Kunieda and Shinodal3 (with X = Cl-) and by Evans and co-workers14 (with X = B r ) show lamellar mesophases in equilibrium with isotropic micellar solutions. Although these L, regions resemble the shapeof lower miscibility gaps, their (upper) apexes should not be mistaken for critical points. With decreasing carbon number m of the chains the lyotropic regions shrink, until for (C&DABr they have disappeared, leaving only the upper boundary of the true miscibility gap with a critical point at Ta = 20 OC.15 As a consequence, one is unable to measure Ta for the longer-chain (C,)2DABr. Presumably, however, it drops with decreasing m, though not
10
Wt%
Figure 3. Effect of FA on the upper loop of the HZO-CIOE~ mixture (top), and on the cmc at 25 OC, with $ as parameter.
necessarily in the same manner as the apexes of the L, regions. Because adding FA makes amphiphiles less hydrophobic, one expects both the La regions and the miscibility gaps to shrink upon addition of FA, that is, the apex temperature TL.of the La regions to drop. Figure 4 shows TL, for (H2O + FA)-(C,)2DABr mixtures vs with m as parameter. The curve for m = 8 refers to Ta. Also shown are the Krafft temperatures (dashed curve) below which the amphiphiles precipitate as crystals. In HzO, the cmc of an ionic amphiphile is most conveniently determined by measuring the concentration dependence of the electric conductivity and identifying the break of slope with the cmc. In FA, however, this procedure meets with difficulties because the conductivity of electrolytes in FA is much lower than in H20 so that the break a t the cmc becomes much less distinct.i6 Figure 5 shows as an example the concentration dependence of the electric conductivity K of (NH3)4NBr and ( 1/2)Na2SO4, in both H2O (empty points) and FA (full points, with K F A = 48.8 mS m-I being subtracted). As one can see, K is much lower in FA than in HzO, due to the higher viscosity of FA (3.3 CPa t 25 OC,compared with 0.9 CP of H2O) as well as possibly to the strong cation-0- and anion-N+ interaction in FAi7which should decrease the mobility of the solvated ions further. This makes surface tension measurements the more reliable method for determining the cmc in FA. Another difficulty arises from the fact that the temperature of the Krafft point of ionic amphiphiles
+,
The Journal of Physical Chemistry, Vol. 97, No. 1, 1993 251
Microemulsions with Formamide as Polar Solvent
lH20+ FA 1 - ( C m I 2DABr
m =12
0' 0
'
I
20
'
' LO
'
-
'
'
60 Jl/wt%
'
'
1
80
100
Figure 4. Effect of FA on the apex temperature TL. of the L, regions in H20-(Cm)2DABr mixtures, with m as parameter. The curve for m = 8 holds for the critical temperature Ta of the miscibility gap.
120,
A
I
0
20
-
LO 60 Jl/wt%
80
100
Figure 6. Effect of FA on the cmc of (Cm)2DABr at 35 OC, with m as parameter.
I
FA
Y
20 -
n 0
2
L
6
8
1 10
c / mmoll-' Figure 5. Electric conductivity K of (1/2)NazSOs and (CH9)dNBr in H 2 0 (full points) and FA (empty points), the latter representing ( K - K F A ) , with K F A denoting the electric conductivity of pure FA.
lies considerably higher in FA than in Hz0,I8so that single-tailed ionic amphiphiles, in particular, are less suited for preparing microemulsions a t ambient temperatures. Because one, furthermore, expects the above-mentioned ruleof thumb for nonionics also to hold for ionics in a similar manner, we chose the doubletailed (C,)*DABr for our experiments. The substances were purchased from Sogo, Tokyo, and used without further purification. Figure 6 shows the effect of FA on the cmc of (C,)*DABr at 35 OC with m = 12, 14, and 16. At low J., the results for m = 16 showed a strong scatter for which reason they were omitted in the figure. Contrary to the nonionic CiEj, log cmc increases stronger than linearly with J. to reach a value in pure FA that lies about 3 orders of magnitude higher than in pure HzO which may explain why with standard single-tailed ionic amphiphiles the cmc in pure FA is difficult to detect. 111. Effect of FA on H20-Oil-Amphiphile Mixtures
The above results demonstrate that adding FA makes amphiphiles effectively less hydrophobic and, hence, less surface active. Accordingly, one expects in view of Figure 2 (center) the mean temperature 7of the three-phase bodies in mixtures with
nonionic amphiphiles to rise upon addition of FA but in those with ionics to drop. One, furthermore, expects the efficiency of th_e amphiphiles to decrease, that is, the amount of amphiphile ( 7 ) required for homogenizing equal volumes (or masses) of the polar solvent and oil to increase. With respect to the effect on AT and g a b , however, one has to distinguish between weakly and strongly structured mixtures. For illustrating this point we recall the evolution of the threephase body in mixtures from a tricritical point (tcp) as amphiphilicity is increased gradually. Now, the amphiphilicity of nonionic amphiphiles can be readily increased stepwise by simultaneously increasing i and] in CiEj, whereas with ionics the strength of the head group can only be changed by exchanging the entire head group or the counterion. For this reason, the evolution of the three-phase bodies from a tcp is easier to demonstrate experimentally by using nonionics (see section IV in ref 19). Mixtures with weak short-chain nonionic amphiphiles separate into two phases only, with the (connected) critical line changing from the oil-rich to the water-rich side with rising T . As the amphiphilicity is increased gradually, this critical line will eventually break at a tricritical point. At this particular plait point the homogeneous mixture separates into three coexisting phases instead of two. The characteristic properties of a threephase body, namely, its three-phase temperature interval AT, the height h of the isosceles three-phase triangle (at and the interfacial tension u,b( T= between the water-rich and the oil-rich phase rise from zero to increase rapidly. As one passes through the range of medium-chain amphiphiles, however, these three properties pass through maxima to decrease again upon further increasing amphiphilicity. The maxima coincide with the transition from complete wetting partial nonwetting of the water/oil interface by the amphiphile-rich middle phase. This suggests considering mixtures in which the above-mentioned properties of the three-phase body increase with increasing amphiphilicity as near-tricritical weakly structured mixtures and mixtures for which they decrease with increasing amphiphilicity as strongly structured mixtures. This distinction was recently supported by small-angle neutron scattering (SANS).*O Weakly structured mixtures differ from strongly structured mixtures by the nature of the correlation function: in the first case the
n,
n,
-
Schubert et al.
252 The Journal of Physical Chemistry, Vol. 97, No. 1, 1993
- n-octane
FA
LO
-CiEj
I
t
-20
0
I
I
I
,
10
20
30
40
0
,
I
10
20
30
40
,
1
,
I
20 . .T, _V .K.
I 50
T = 54.5 O C
0
1
-20 L
0
- 1u-*
I
I
20
10
30
LO
50
(T-f 11 K
0 20
(T-PVK
20
10
30
I
LO
50
f = 19,o OC
I
t o -717 --
-2 0
0
10
0
10
20
I
-
20
30
LO
I
I
30
LO
1
O C
C8E3
I
ClaE6
50
t o -20
- n-octane -
l
1
C6E2
f = 61,l
(T-f)/K
FA
z
l
50
I 50
y/wt%
Figure 7. Transition from weakly to strongly structured FA-octaneCiE, mixtures by stepwise increasing the amphiphilicity of CjE, from C6E2 (top) to C18E6 (bottom). The diagram for C6E2 was determined in a HzO-FA mixture with = 88 wt %. For discussion see text. $J
correlation function decays monotonically, whereas in the second it shows damped oscillations. The transition from the first to the secondcaselies in that rangein which AT, h, and 6,bpass through their maxima. In H20-oil-amphiphile mixtures with nonionic amphiphiles the transition from weakly to strongly structured mixtures lies near C6Ep In view of the rule of thumb CiE, in H20 = Ci+4Ej in FA,oneexpects this transition tomove tonear CloEjin mixtures with pure FA. This, too, is confirmed by experiment. Figure 7 showsvertical sections through the phase prisms of FA-n-octaneCiE, mixtures at equal volumes of FA and oil, with C6E2, CsE3, C&, C12E4, C&, and C1&, to be compared with Figure 7 in ref 19. The mixture with C6E2 (with $ = 88 wt %) separates intc; two phases only, whereas that with CsE3 separates into three phases from which it follows that the tcp must lie between the
2
J 10
I
-
8
6
L
CiEj / w t % Figure 8. Vertical section through a H20-0ctane-Cl2E4 (dashed lines) and a FA-~ctane-C& prism (full lines), demonstrating that the effect of FA can be compensated by increasing the carbon number i of CiEj by about 5.
TABLE I: FA-+Octane-C& CjEj TI Tu,“C f C6E2 65.0 64.5 61.0 57.6 C8E3 54.5 61.1 C I O E ~ 48.0 31.0 26.2 C12E4 17.1 19.0 21.0 C16E5 17.0 32.4 30.8 C18E6 29.2
AT
0 6.9 13.1 13.9 4.0 3.2
yc
;,wt %
AT
14.0 5.0 2.1 0.6 0.3
47.6 37.3 27.4 18.7 5.3 2.9
0 23.3 22.4 16.6 4.7 2.6
two. With further increasingamphiphilicity, the‘fish” first grows, passes through a maximum near C I O E and ~ , shrinks again. The corresponding data are summarized in Table I, with ycdecoting the amphiphile concentratjon at the “head” of the fish and y that a t its “tail”, so that A y E y - yccan be taken as measure for the height h of the isosceles three-phase triangle at 7. Consequently, if FA is added to H20-oil-CiEj mixtures with i < 10, one expects the three-phase body to shrink as it moves closer to a tcp. With i > 10, on the other hand, one expects the three-phase body first to grow as it moves closer to the maximum before it eventually starts shrinking again. The efficiency, however, will decrease in both cases. Consider first mixtures with i < 10. Figure 3 in ref 8 shows vertical sections through the phase prisms for the three ternary mixtures H@-C6H12-C&, FA-C~HI~-C&,and FA-CbHlzC 1 2 E at ~ equal masses of the polar solvent and oil. The “fish” with pure H2O (dashed line) lies at T = 22 OC, with y = 15 wt %. As one replaces H 2 0 by FA (empty points), T rises, as predicted, to about 49 OC, and y increases to about 25 wt %. According to the thumb rule one should then retain the original values for and y by replacing CsE4 with C I ~ E 4 .Actually, experiment (full points) shows that one, in addition, has to increase the number j of ethylene oxide groups by one. Accordingly, the rule of thumb reads for weakly structured mixtures with nonionic amphiphiles: CiE, in H 2 0= Ci+4E,+~in FA. One, furthermore, deduces from this result that (H2O FA)-oil-CiEj mixtures with i < 10 are well suited for studying near-tricritical behavior (see section VI in ref 21), including the gradual evolution of a correlation peak in SANS curves and a complete wetting partial nonwetting transition as one proceeds from weakly to strongly structured mixtures.20 Consider now mixtures with i > 10. Figure 8 shows vertical sections through the phase prisms for H20-n-octane-C12E4 (dashed lines), and FA-n-octane-cl& (full lines), the latter
+
+
The Journal of Physical Chemistry, Vol. 97, No. I , 1993 253
Microemulsions with Formamide as Polar Solvent taken from Figure 7. The fish with pure H20 lies a t T = 13 OC, with i. 3 wt %. Upon replacing H2O by pure FA, T rises to about 26 OC and 7 increases to about 19 wt % (see Figure 7). For restoring the original efficiency of the amphiphile, onededuces from Figure 8 that the carbon number i of the amphiphile has to be increased by about 5 , so that the rule of thumb becomes for strongly structured mixtures: C,E, in H2O = C,+SE,+Iin FA. Qualitatively, our results are supported by those found by Martino and Kaler? who studied (propylene glycol glycerol)alkane-C,E, mixtures a t 22.5 OC. The dielectric number of the two polar solvents read c = 32.0 and e = 42.5, respectively, from which one deduces that both solvents are only weakly hydrogen bonding. Hydrocarbons, however, are much more soluble in propylene glycol than in glycerol: propylene glycol-C~&,mixtures are completely miscible in the experimental window, whereas glycerol-CsEo are immiscible up to about 74 OC. Accordingly, adding propylene glycol makes the H20-CIE, loops shrink (thus T rise), whereas glycerol makes them grow (thus T drop). For lowering the three-phase bodies in propylene glycol-alkaneC,E, mixtures to room temperature, Martino and Kaler, therefore, added glycerol. Alternatively, they could have increased the carbon number i of C,E, (as they did in their Figure 4). Consider now mixtures with cationic amphiphiles of the type (Cm)2DABr. Partial phase diagrams of ternary HzO-n-alkane(C12)2DABrmixtures at 25 OC have been published by Evans, Ninham, and co-workers.22 They show rather small homogeneous regions surrounded by two-phase regions and a variety of lyotropic mesophases. This, however, was to be expected because the apex of the L, region in the binary A-D diagram lies close to 150 O C (see Figure 2 in ref 14). Accordingly, one expects Ta as well as the three-phase bodies-if they should exist at all-to lie above the boiling point. Because adding FA makes Tadrop, this suggests lowering the three-phase bodies into the experimental window by gradually adding FA. As is the case with most ionic amphiphiles, one, furthermore, has to add a lyotropic salt for actually enforcing a separation into three phases. For demonstrating the effect of FA, we chose the quinary mixture (H20 + FA NaBr)-nhexane-(Cl2)2DABr. The salt was added to the (HzO FA) mixture, its concentration expressed in
8C
7c
+
+
c
[NaBr]/([H,O]
+ [FA] + [NaBr])
6C T / OC
t
$=LO
5c LO
30
20
d
= 50
0 0
,
,
2
L
,
6
,
,
8
10
Y /wt%
Figure 9. Vertical sections through the phase prisms of (H20+ FA + NaBr)-hexan-(C&DABr mixtures a t equal mass fractions of the polar solvent and oil, and fixed salt concentration c, with )I as parameter.
(FAtNaBr) -
90 T/
in weight percent. Figure 9 shows the ‘tails” of the fishes at equal volumes of the polar solvent and oil, and fixed c = 0.3 wt %, with J, as parameter. Because the phase behavior of this quinary mixture is represented in a pseudoternary prism, the three-phase bodies do not lie parallel to the base for which reason the mean temperature Tof the three-phasebody loses its meaning. Instead, one may define the position of the three-phase body on the temperature scale by the temperature ? at its tail a t equal volumes of the polar solvent and oil. As expected, pdrops with increasing J, to fall below the melting point for J, = 50 wt 5%. Within the experimental window, however, the efficiency of the amphiphiledoes not decrease, which is in accord with the finding in section 11, namely, that at low J, the surface activity of ionic amphiphiles is only little affected. Figure 2 (center) tells that, for raising the three-phase bodies in pure FA into the experimental window, one has to increase the tail length of the ionic amphiphile. Accordingly, we increased m from 12 to 16. Figure 10 shows the fish for FA-n-alkane(Bk)-(C16)2DABr mixtures, with fixed c = 0.6 wt %, and the carbon number k of the oil as parameter. The fish for decane (k = 10) is limited by the Krafft temperature (=31 O C in the presence of the oil). Figure 2 (top) then tells that for making ? rise further one has to decrease k. Indeed, as decane is replaced by nonane (k = 9), octane (k = 8), or heptane (k = 7), T rises, with the efficiency of the amphiphile decreasing slightly.
,&, -
10
+
(3)
/
B, - (C,,),DABr
0 = 50 VOI%
OC
70
-J?--
0
2
-- K r af f t - t e m perat ur e
L
-
6
0
10
y / w t Yo Figure 10. Vertical sections through the phase prisms of (FA + NaBr)alkane(Bk)-(C&DABr mixtures at equal volume fractions of FA and oil, and fixed salt concentration c, with the carbon number k of the oil as parameter.
IV. Interfacial Tension 0.b
In H20-oil-amphiphile mixtures, the minimum of the interfacial tension b.& between the water-rich and the oil-rich phase at themean temperature pof the three-phase body is an inevitable consequence of the fact that the body is bounded by a lower and an upper critical tie line.1° The minimum of bab a t Tis, hence, not a particular property of strongly structured mixtures but can also be found in weakly structured mixtures with a similar phase behavior. The difference between mixtures with weak and strong
254
The Journal of Physical Chemistry, Vol. 97, No. 1, 1993
amphiphiles lies in the absolute magnitude of c a b ( T = n . With weak amphiphiles c a b is of the order of 1 mJ m-2, whereas with strong amphiphiles it may drop to
