Langmuir 1985,1, 718-724
718
philic, up to the moving triple-phase boundary a t the meniscus. Capillarity is thus responsible for the meniscus rise around the vertical wire. Gravity on the other hand is responsible for the liquid fall when the adhesion forces between iron, the corrosion products, and liquid are not sufficient to hold the liquid column. The surface tension forces play an important role in the corrosion process for they are responsible for liquid ascent
and fall, leading to the renewal of the exposed corroding surface. We believe these findings should help to establish the interplay of electrochemical and surface tension factors in metal corrosion. Registry No. Fe, 7439-89-6;FeSO,, 7720-78-7; HzS04,766493-9.
On the Effect of Electrolytes on the Mutual Solubility between H 2 0 and Nonionic Amphiphiles P. Firman, D. Haase, J. Jen, M. Kahlweit,* and R. Strey Max-Planck-Institut fuer Biophysikalische Chemie, 0-3400 Goettingen, West Germany Received May 3, 1985. I n Final Form: July 9, 1985 In this paper we report on an attempt to quantlfy the effect of electrolytes on the mutual solubility between H20 and simple nonionic amphiphiles. On the basis of the studies by Schneider and co-workers,we have determined the minimum amount of lyotropic salts necessary to enforce phase separation between H20 and 1-propanol (C,E,),as well as the amount of hydrotropic salts necessary to make the upper loop of the system H20-ethyleneglycol monobutyl ether (C4E1)vanish. The results of the first series of experiments are compared with the Hofmeister series. The latter series of experiments demonstrate that the effect of a hydrotropic electrolyte, whether surface active or not, is quite similar to that of a hydrophilic nonionic amphiphile or that of pressure. The significance of these effects for the discussion of thb phase behavior of multicomponent liquid mixtures including oil (microemulsions)is noted.
I. Introduction The effect of an added electrolyte on the mutual solubility between H 2 0 and organic compounds has been studied systematically first by Lewith’ and Hofmeister2 about a century ago. They chose the system H20-2 w t % globulin and determined the amount of electrolyte necessary to salt out the globulin. With respect to the efficiency of sodium salts in salting out the globulin, Hofmeister established the following order (taken from the table on p 256 in ref 2): 1/2NazS04(1.60) > 1/2NazCr04 (2.61) > NaCl(3.63) > NaNO, (5.42) > NaC103 (5.52). The figures represent the minimum amount of salt in mole/liter to salt out the globulin. The less salt needed, the more efficient the electrolyte. For the cations he found Li+ > K+ > Na+ > NH4+> Mg2+. These series are since quoted either as “Hofmeister” or as “lyotropic” series. In 1916 Neuberg3 found that electrolytes with an inorganic and an organic ion increase the mutual solubility between H 2 0 and organic compounds and called such salts “hydrotropic”. Since then it has turned out that the results of these authors also hold for mixtures of H20 and nonionic amphiphiles. Accordingly, we suggest calling all electrolytes that decrease the mutual solubility between H 2 0 and nonionic amphiphiles “lyotropic” and those that increase the &nutualsolubility “hydrotropic”. Since the studies of Hofmeister a number of attempts have been made to quantify the effect of the electrolytes. Of these, two should be mentioned. In the thirties, Buechner and co-workers4studied the salting out of agar (1) Lewith, S. Arch. Ezp. Pathol. Pharmaeol. 1888,24, 1.
(2)Hofmeister, F.Arch. E r p . Pathol. Pharmakol. 1888,24, 247. (3) Neuberg, C. Biochem. 2. 1916,76, 107. (4)See, e.g.: Buechner, E. H. Kolloid-2. 1936,7 5 , l . Voet, A. Chem. Reu. 1937,20, 169.
0743-7463/85/2401-0718$01.50/0
and gelatin by different sodium salts. Their results were correlated by Bruins5 by an empirical equation which permits a “lyotropic number” N to be ascribed to each ion, some of which are listed. (the N values for S042-and C1were arbitrarily chosen). The lower N the more efficient the corresponding anion. S042-( N = 2) > C1- ( N = 10) > C 1 0 ~ ( N = 10.7) > Br- ( N = 11.3) > NO3- ( N = 11.6) More recently, Schott and co-workers6 attempted to quantify the efficiency of inorganic anions by measuring the lowering of the cloud point of mixtures of H 2 0 and commercial detergents such as Triton and Brij by salts. For this purpose they prepared a H,O-detergent mixture with a composition close to the lower critical point of the upper loop of that mixture, then added a certain amount of different salts, and measured the difference AT between the cloud point of the binary and that of the ternary system, taking A T as a measure for the lyotropy of the corresponding salt. Some of their results taken from ref 6b are shown. They correspond to 1 M solutions, except for Na2S04(0.5 M). The larger AT, the more efficient the corresponding salt. Na2S04( A T = -38) > NaCl ( A T = -16.5) > NaBr ( A T = -6) = NaNO, (AT = -6) Similar measurements have been reported already by Luck7 in 1964. This procedure has three essential disadvantages: (i) The commercial detergents are ill defined, since they show (5) Bruins, E. M. Proc. A c Q ~Amsterdam . 1932,35, 107. See also: McBain, J. W. “Colloid Science”; Heath Boston, 1950; p 131 ff. (6) (a) Schott, H.; Royce, A. E.; Han, S. K. J. Colloid Interface Sci. 1984,98,196. (b) Schott, H. Colloids Surf. 1984,11, 51. (7) Luck, W . A. P.Fortschr. Chem. Forsch. 1964,4,693.
0 1985 American Chemical Society
Langmuir, Vol. 1, No. 6, 1985 719
Effect of Electrolytes on Solubility
T
T
t
t
-P
Figure 1. Upper A-C loop as a vertical section at constant p through the nose in c-T-p space (schematical).
an unknown distribution of ethoxy groups. (ii) For a given electrolyte, the lowering of the cloud point depends on the amphiphilicity of the detergent. (iii) For a given electrolyte, the lowering of the cloud point is, in general, a nonlinear function of the salt concentration. For these reasons the results obtained by these authors have only semiquantitative significance. The same holds for the experiments by Buechner and c o - ~ o r k e r s . ~
11. Determination of Lyotropic and Hydrotropic
Numbers In order to find a more quantitative method, we suggest a procedure that is based on the studies of Schneider and co-workerss on the effect of pressure and electrolytes on the upper loop between H 2 0 and nonionic amphiphiles. Schneider and his group applied, among other substances, well-defined short-chain n-alkyl polyglycol ethers of the type CiEj, where Ciis the carbon number of the hydrophobic alkyl chain and j the number of the hydrophilic ethoxy groups. This includes short-chain alcohols = 0).
The binary systems H20-CiE, shows an upper closed loop at 1bar which shrinks with decreasing amphiphilicity of the CiE.to completely disappear for the very hydrophilic ones. As dchneider et al. have shown, this can be visualized by considering the loop as a vertical section through a “nose” in composition-T-p space, as demonstrated schematically in Figure 1. The more amphiphilic the CiEj, i.e., the larger both i and j , the longer the nose, and vice versa. For the very hydrophilic amphiphiles the nose becomes too short to show up at 1bar, but many still “lurk” behind the c-T plane at that pressure. Indeed, if one adds an appropriate amount of a lyotropic salt, the nose cuts through the c-T plane (at 1bar) with its tip at a certain composition and a certain temperature. With further increasing salt concentration the loop widens. Since-for a given amphiphile-the amount of salt needed to make the tip of the nose show up changes from salt to salt, we suggest taking this amount as a measure for the lyotropy of the corresponding electrolyte. In order to develop a well-defined experimental procedure, let us consider the phase diagram of the ternary system H 2 0 (A)-nonionic amphiphile (C)-electrolyte (E). A t constant pressure, the phase behavior of a ternary system is conveniently represented in an upright prism with the Gibbs triangle A-C-E as basis and temperature as ordinate (Figure 2a). (8) See, e.g.: Schneider, G. M. Ber. Bunsenges. Phys. Chem. 1972, 76, 325.
V
H,O(A)
@
Figure 2. (a) Phase prism of A-C-E system with lyotropic salt as E (schematical);(b)vertical section through phase prism erected on the broken line in (a), seen from the E edge of the prism
(schematical).
The binary A-C system is assumed to show no upper loop between its melting and boiling point. At low temperatures, the phase diagram, i.e., a horizontal section through the prism, shows a homogeneous region along the A-C side of the triangle and a two-phase region between the homogeneous region and the salt corner with solid E as second phase. At a somewhat higher temperature, the (lyotropic) salt starts to enforce a phase separation, the nose begins to emerge from a lower critical end point. At a little higher temperature, the phase diagram shows three two-phase regions: the two regions along the A-E side and the C-E side with solid E as the second phase and the horizontal section through the nose with two liquid phases. Between this section and the E corner one finds a threephase triangle with solid E as the third phase. The plait point of the two-phase region ascends on the surface of the nose, forming a convex critical line which does not necessarily lie on a vertical plane. With further rising temperature, the critical line retreats from the A-C-T plane to terminate a t its upper critical end point: The nose has disappeared. The plait point will, in general, not lie on the apex of the two-phase region, since the distribution coefficient of the salt which determines the declination of the tie lines, will, in general, differ from unity. If one now draws a vertical section through the prism erected on the broken line on the bottom of Figure 2a, i.e., at a constant A/C ratio, and looks at this section from the C edge of the prism, one can see a section through the nose as shown schematically on Figure 2b. This section does, in general, neither coincide with the section along the critical line, nor does it necessarily cut through the tip of the nose. Since its profile can be described by an analytical function, one expects that its shape, at least close to its apex, can be represented by a parabola
(T - T,)’
+
(Y(X
- x,) = 0,
(Y
; hydrotropic
T
T
TP
t
2@
I
'a
-
salt
Figure 5. (Top) Profile of the lyotropic nose; (bottom)profile of the hydrotropic nose (schematical). salt(E)
C i E, IC)
C /A
E
Figure 4. (a) Phase prism of the A-C-E system with hydrotropic salt as E (schematical);(b) vertical section through phase prism erected on the broken lime in (a),seen from the E edge of the prism
(schematical).
This suggests performing X lo5 K2, and x, = 2.07 equivalent experiments with different electrolytes in H20-C3Eo mixtures with a constant H20/C3Eoratio. If the profiles, a t least near their apex, can be described in sufficient approximation by eq 1,one may take the number x, as a measure for the lyotropy of the corresponding electrolyte. A similar procedure can be applied to determine the hydrotropic numbers of salts which increase the mutual solubility between H20 and nonionic amphiphiles. Again we refer to the studies of Schneider and his group, in particular, to ref 10. In this paper, Schneider studied the effect of pressure on the upper loop of the binary system H20-n-C4E1 which shrinks with increasing pressure to disappear at about 800 bars and 30 wt % C4E1. Since the addition of a hydrotropic salt has a similar effect, we suggest determining the hydrotropic numbers of such salts by measuring the amount of salt needed to make the loop vanish (at 1 bar). The phase diagram is shown schematically on Figure 4. In this case the binary system is assumed to show a loop with a lower and an upper critical point (Figure 4a). As one adds a hydrotropic salt, the mutual solubility between the two liquids increases until the two phases merge a t the plait points, which form a critical line ascending from the lower to the upper critical point of the loop. If one draws a vertical section at a (10) Schneider, G. M. 2.Phys. Chem. (Munich) 1963, 37, 333.
constant A/C ratio, one fiids the profiie of the nose at that particular A/C ratio, as shown schematically in Figure 4b. For the same reasons as above one again expects the profiie of the nose to be represented, at least close to its apex, by a parabola, eq 1,now, however, with CY > 0. One may thus again determine the profile of the nose at a constant A/C ratio for different hydrotropic salts and take the number x , as a measure for their hydrotropy. The experimental procedure is summarized schematically in Figure 5 which shows on the upper part the profile of the nose for a system which appears after addition of a lyotropic salt and on the lower part that of a nose which vanishes after addition of a hydrotropic salt. If both profiies can be represented in sufficient approximation by eq 1, the sign of CY indicates whether the salt is lyotropic or hydrotropic. If CY > 0, we shall call the salt hydotropic, if CY C 0, lyotropic. The values of x, can be taken as a (relative) measure for the efficiency of the salt within the corresponding series. 111. Experimental Results A. Materials. C4E1 was purchased from Merck (Darmstadt) and redestilled. C5E2,C6E3,and C8E4were purchased from Bachem (Basel) and C12E6 from Nikko (Tokyo) and used as supplied. The salts (p.A.) were purchased either from Merck or from Fluka (Buchs); the ionic detergents were supplied by Henkel (Duesseldorf). B. Lyotropic Salts. First we shall show that the lowering of the cloud point of a H20-CiE,.mixture depends indeed on the amphiphilicity of the surfactant. For this purpose we studied the effect of NaCl on the lowering of the LCST of five CiE.with increasing i and j . All these surfactants have a HhB value of about 12, their LCST (without salt) lying between 35 and 50 "C. The results are shown on Figure 6, the difference AT = T,- T being taken at the lowest point of the loop with salt. As one can see, the lowering is smaller the more amphiphilic the surfactant. This can be readily understood by considering the nose on Figure 1: The more amphiphilic the surfactant,
Langmuir, Vol. 1, No. 6,1985 721
Effect of Electrolytes on Solubility 0
Table 11. Effect of HydrotroDic Electrolytes: NaCjSOAa xa
364 363 362 362 361 362 361 361
c4
-10
c 6
AT [ K l
C8
ClO Clz (SDS)
-20
c 1 4
c16 Cl8
"Parameters
-30
ff
x 103
T,/K
Ci
x IO+/K~ 1.03 2.67 3.68 4.38 4.76 4.28 4.27 4.71
1.58 0.615 0.444 0.374 0.347 0.384 0.362 0.330
x
r io2
1.23 1.25 1.25 1.25 1.26 1.26 1.19 1.18
of eq 1 determined at H20/C4E1 = 7/3 (wt %).
Table 111. Effect of Hydrotropic Electrolytesa I
-40
0
02
Xa
-
salt 06
01,
08
10
NaCl in HO , [mol ?I
NaSCN (C4Hg)dNN03
Figure 6. Lowering of the cloud point of H20-C,E, mixtures by NaC1.
Table I. Effect of Lyotropic Electrolytesa
1LO
so42-
chloride salts Li+
NH4+ Na+ K+g Ca2+ Mg2+ Sr2+ Ba2+
297 332 315 320 318 315
54.5 42.8 40.9 21.3 4.1 4.0
-0.116 -0.156 -0.243 -0,321 -3.05 -3.80
-7.13 -6.05 -10.0 -6.69 -12.4 -15.2
327 329 320 323 339 351 333 332
56.0 43.2 21.3 20.7 25.3 24.4 13.3 12.6
-0.100 -0.225 -0.321 -0.396 -0.374 -0.347 -0.778 -0.802
-5.24 -8.96 -6.69 -7.87 -8.26 -6.87 -9.34 -9.15
"Parameters of eq 1 determined at H20/C3Eo= 3/2 (wt W). the longer the nose, and, consequently, the flatter the slope of its profile a t zero salt concentration. Next we have determined the lyotropic numbers of different electrolytes, choosing the system H20-n-C3E,-,. From the phase diagram of the system H20-C3Eo-NaC1 a t 25 "C (Figure 13 in ref 11)one can see that the plait point of the two-phase region lies close to 40 w t % C3Eo. We, accordingly, took this composition as a footprint of the vertical section through the nose. Table I summarizes our results. The ions are listed in the order of increasing efficiency, Le., decreasing x,. The significance of the quantity r will be discussed below. All the profiles are sufficiently well described by eq 1. When comparing our results with those found by the authors cited above, one notes some discrepancies, in particular with respect to the Hofmeister series for the cations which is almost inverted in Table I. We further note that RbCl and CsCl reach their solubility limit in this particular mixture before phase separation. With respect to NaC104, we confirmed that this salt, a t least a t low concentrations, acts as hydrotropic electrolyte, for which reason it will be considered separately in section V. C. Hydrotropic Salts. In view of the fact that the composition of the LCP of the upper loop of the H20-nC4E1system lies close to 30 wt % C4El, we took this com(11) Kahlweit, M.; Leasner, E.; Strey, R. J.Phys. Chem. 1984,88,1937.
x 103
375 356 357 358 361
7.91 1.29 1.08 0.862 0.664
ff
x I O ~ / K ~x 0.180 0.952 1.19 1.72 2.45
(C4Hd4NBr (CdH9)4NHS04 (CsHd4PC1 "Parameters of eq 1 determined at H20/C4El= 7/3
sodium salts
C103BrNOSc1CO2-
TJK
,
H,O
- C,E,
r io2
1.01 0.97 1.02 1.16 1.25 (wt %).
- NaC,H,,+, SO,
80
60
Figure 7. Profiles of noses for the system H20-C4E1-NaCiSO4 (sodium n-alkylsulfates),determined at H20/C4E1= 7 / 3 (wt %). For reasons of clarity the profiles for i = 16 and z = 18 were omitted.
position as a footprint of the vertical section through the nose. As first example for hydrotropic salts we chose the surface-active sodium n-alkyl sulfates, denoted as NaCiS04 with i = 4,6,8, 10,12, 14, 16, and 18. Table I1 shows the results. The corresponding parabolas are shown on Figure 7. In evaluating the parameters in Table 11, the lower (48 "C) and upper critical solution temperature (129 "C) were not used. But as one can see from T , in Table I1 and from Figure 7, the parabola describes the profile almost perfectly. This suggests rewriting eq 1 in a scaled form:
[ ( T- T,)/T,]'
+ I'(x - x , ) / x ,
=0
(2)
with
r = .x,/T: where r should be the same for all electrolytes which is indeed the case for the salts listed in Table 11. In the case of the lyotropic salts listed in Table I, the scatter is wider (r = -8.7 x 10-2). One further notes on Table I1 that the efficiency of the sodium alkylsulfates increases gradually with increasing length of the alkyl chain to reach a plateau for about i = 8, so that a clear distinction between a "simple" hydro-
722 Langmuir, Vol. 1, No. 6, 1985 1LO 1
Firman et al. 1LO t
2 .
5
0
-
10
15
x,,,, 110.~1 -\
1LO
1
H,O
- C,E, - IC,H,,+.),NBr
‘0
2
1
’
6
-
8 /.130 Xgo1t
132 Ilo-31
13L
136
’
Figure 9. Profiles of noses for the system H20-C4E1-NaC104, determined at H20/C4El= 7 / 3 (wt %).
SPC!
Table IV. Effect of NaCIOla x,
hydrotropic nose lyotropic nose 0
1
2
3
L x,
5
6
t110-31
Profiles of noses for the system H20determined at H20/C4E1= 7 / 3 (wt %). C4E1-(CIH21+1)4NBr,
Figure 8.
tropic salt and a “typical” ionic detergent is difficult. This has already been pointed out by Lawrence and co-workers for C,TAB.12 Next we studied the efficiency of hydrotropic salts which are not surface-active. As examples we chose NaSCN, (C4Hg),NX, and (C6H5),PC1. The results are listed in Table 111. As one can see, these salts are almost as efficient as the short-chain sodium alkyl sulfates (Table 11). With respect to the order of the anions, the HSO; ions are again the most effective, now, however, in salting in the nonionic amphiphile. This suggests considering the effect of a salt as a combination of the effects of the two ions. If one applies the HSAB principle13one could summarize the results as follows: The combination hard acid-hard base is lyotropic, whereas the combinations soft acid-hard base and hard acid-soft base are hydrotropic. Accordingly, salts like NaCl, CaCl,, or Na2S04decrease the mutual solubility between H 2 0 and nonionic amphiphiles, whereas salts like NaSCN, (C6H5),PCl, and the sodium alkyl sulfates increase the mutual solubility. One major exception is the C104-ion which, although classified as hard base, increases the mutual solubility when combined with hard acids (NaClO,), whereas, if combined with a soft acid (C4Hg),NC1O4,it has almost no effect on the mutual solubility between HzO and C4E1. The fact that the effect of the combination hard-soft depends somehow on the “balance” between the ions can be demonstrated by changing the nature of the cations in the series (C,H2,+J4NBr. Figure 8 shows the profiles of the noses for i = 1-6, 8, and 12. While tetramethylammonium bromide decreases the mutual solubility between HzO and C4E1, one finds “hydrotropic” noses for i = 2-5. With tetrapentylammonium bromide, however, a ~~
(12) Lawrence, A. S. C.; Boffey, B.; Bingham, A.; Talbot, K. Chem., Phys. Appl. Surf. Act. Subst., Proc. Int. Congr., 4th, 1964, 1967,2, 673. (13) Pearson, R. G. S u m B o g . Chem. 1969, 5, 1.
T,/K
x 103
374 318
7.79 131
(Y
x IO~/K* 0.159 -0.252
x
r 102
0.886 -32.6
“Parameters of eq 1 determined at H20/C4El= 7/3 (wt %).
second “lyotropic” nose appears at high concentrations. This salt thus acts hydrotropic at low concentrations but lyotropic at high concentrations. If one proceeds to i = 6, the two noses merge. With further increasing i, this effect becomes even stronger. The above system resembles all the characteristic features that were discussed by Schneider (Figure 1 in ref 8) with respect to the influence of pressure, while we changed the chemical potential of the salt a t constant pressure.
IV. Effect of NaC10, As we mentioned in section 111, NaC104 acts as hydrotropic salt, but only a t low concentrations. At high concentrations, it too enforces a phase separation, as shown on Figure 7 in ref 11. Accordingly, if one measures its effect on the mutual solubility between HzO and C4E1, one finds a “hydrotropic” nose at low concentrations, whereas at high concentrations a “lyotropic” nose appears, similar to that with (C5H11),NBr,as shown on Figure 9 to be compared with Figure 8. Table IV gives the parameters for both the hydrotropic and the lyotropic nose, in the case of the hydrotropic nose only considering the experimental points close to its apex. One notes that the lyotropic nose appears at almost the same temperature as the nose of the H,O/C,E, system (Table I). The fact that NaC10, acts as hydrotropic salt a t low concentrations, but as lyotropic salt at high concentrations, has interesting consequences for the phase behavior of the quaternary system HzO (A)-oil (B)-nonionic amphiphile (C)-electrolyte (E) with NaClO, as E. As it was shown in ref 14,the addition of hydrotropic salt to a ternary system A-B-C with a three-phase interval raises and narrow this interval as shown on Figure 1 in ref 14 for the system H20-octane-C4E1-NaC104.Accordingly,if the three-phase interval of the ternary system is sufficiently narrow, one may reach a tricritical point at which the three-phase triangle disappears. This can be readily achieved by applying the rather efficient SDS as hydrotropic salt as (14) Kahlweit, M.; Strey, R.; Haase, D. J . Phys. Chem. 1985,89, 163.
Langmuir, Vol. 1, No. 6,1985 723
Effect of Electrolytes on Solubility 90
H20- n-0ctme
A
- C'E, - NoCIO,
80 70 SI'CI
1: LO
30 20 10 0
10
-
°
20
30 L
NoCIOI IwlXI
H,O-Octane-C,E,[AI
I81
NaCIO, IC1
(El
Figure 11. Phase behavior of the system H,O-n-octane C,E1-NaCI04 with increasing salt concentrationat 25 O C . The phase diagraas in (1)were experimentally determined," the others are schematical.
0
-
salt
Figure 12. Three-phase cusps (shaded) for a quatemary system with a three-phase (temperature)interval at zero salt concentration (schematical)
of the central gap, cp. in the A-C-E triangle at a high salt concentration, and cp.' in the BX-E triangle at a low salt concentration. The situation is thus reverse to that with the lyotropic LiCl (also shown on that figure), for which cp, in the A-C-E triangle is located a t a low salt concentration. Accordingly, one finds with NaC10, a reversed sequence of the interaction between cp. and cp,' with the central gap (Figure 11,to be compared with Figure 19 in ref 11): A t a constant temperature above Tu,the first three-phase (salt) interval is caused hy the interaction between cp,' and the central gap (11.1). The three-phase triangle appears as critical tie line Rs (Figure 11 in ref 11) hy separation of the upper phase, and disappears as critical tie line PQ by merging of the two lower phases (11.2). The second three-phase (salt) interval is caused by the interaction between cp. and the central gap (11.3). It appears
124 Langmuir, Vol. 1, No. 6,1985 1LO
1
H,O - C,El - C i E l
0
i
1GO
A
80 -0
LO
2
L
6
d
,
1
I
10 12
11,
16
18
%I.
1.n
I
-0
0
Table V. Effect of Hydrophilic Nonionic Amphiphiles“
C3E1
C*El
TJK 361 360
x 103 20.1 8.30
a
x i0*/~2 0.082 0.194
p [MPal
60
80
100
,
Figure 13. Profiles of the noses for the system H20-(C4El+ CiEJ for C3E1and CzEl,determined at H20/(C,El + CiEJ = 7/3 (wt
C~E,
LO
22
I
xa
-
20
Firman et al.
x
r io2
1.25 1.25
uParameters of eq 1 determined at H20/C4E,= 7/3 (wt %).
as critical tie line PQ (11.4) and extends to the solubility limit of NaC10, in the mixture (11.5). We thus have to complete Figure 6 in ref 14 by adding a second cusp for those hydrotropic electrolytes which act as lyotropic salts a t high concentrations. Figure 12 shows the cusps schematically. The lower cusp is that for lyotropic electrolytes with a tricritical point a t a ”negative” salt concentration, the two upper ones are those for hydrotropic salts like NaC1O4 To observe a coalescence between the two upper cusps in a quaternary system a t constant pressure would again be a “lucky accident”. In general, one will find either two separated cusps or a connected one as shown on Figure 10. In order to study the coalescence of the two cusps (at constant p ) , one has to proceed to a quinary system by mixing two oils, one of which shows a connected cusp, the other one two separated cusps. V. Mixtures of Nonionic Amphiphiles What can be achieved by adding a hydrotropic electrolyte can, of course, also be achieved by adding a more hydrophilic nonionic amphiphile, in the case of C4E1 by adding either C3E1 or C2El. These two amphiphiles show no upper loop with HzO. Accordingly, if one adds either one of these amphiphiles to the H20-C4Elsystem, the loop disappears a t a certain concentration of the added amphiphile, the profile of the nose showing the same shape. This is demonstrated in Figure 13 which shows the noses Table V gives the corresponding for both C3El and C2El. parameters of eq 1. We note, however, that in order to obtain a symmetrical nose one has to apply a more hydrophilic amphiphile with the same number j of ethoxy groups. If one adds an amphiphile with a lower or higher number j , e.g., either C3Eo or C4E2, both more hydrophilic than C4E1,the noses become unsymmetrical in that the symmetry axis of the parabola points either down or upward, in the case of C3Eotoward 50 OC (see Figure 3) and
-
wt O h NaC,,SO,
(0)
wto/o (C,Htj),PCI(o)
-
w t O h C,E,
OS
1
-
(A)
5
Figure 14. Profiles of noses determined at H20/C4El= 7/3 (wt %). Broken line: Effect of pressure (after ref 10). Full line: Effect of a more hydrophilic amphiphile (C3E1), a hydrotropic not surface-activesalt (C6H5),PC1,and an ionic detergent (SDS).
the case of C4E2 toward a temperature above 100 “C. It thus appears as if the position of the center of the loop, whether present or lurking, is determined mainly by the number j of ethoxy groups: the higher j , the higher the center of the loop, at least with these “simple”amphiphiles.
VI. Summary Figure 14 summarizes the effect of hydrotropic electrolytes and more hydrophilic nonionic amphiphiles on the upper loop of H20-C4E1. Irrespective of whether the electrolyte is surface active (SDS)or not ((C6H5)4PC1), or whether the third component is a nonionic more hydrophilic amphiphile (C3E1), the effect on the mutual solubility between H 2 0 and C4E1 is very similar to that of pressure (broken line, ref 10). The profiles of the noses are almost identical, if one chooses the concentration units on the abcissa such that the tips of the noses coincide. This result may be of value for the interpretation of the effect of electrolytes on the mutual solubility between H 2 0 and nonionic amphiphiles on the molecular basis16 and, furthermore, for the discussion of the phase behavior of multicomponent liquid mixtures including H20, oil, and amphiphiles (microemulsions).
Acknowledgment. We are indebted to B. Faulhaber and T. Lieu for their assistance with the experiments, to D. Luckman for drawing the figures, and to the “MaxBuchner-Stiftung” for financial support. W s t m NO. C3&, 71-23-8; C4E1, 111-76-2; C5E2, 18912-81-7; C6E3,25961-89-1;C8E4,19327-39-0; C12E6, 3055-96-7; NaC4S04, 1000-67-5;NaC6SO4,2207-98-9; NaCBSO4,142-31-4; NaCloS04, 142-87-0; NaC12S04,151-21-3;NaC14S04,1191-50-0; NaCl6SO4, 1120-01-0;NaC18S04,1120-04-3;NaSCN, 540-72-7; (C4Hg),NN03, 1941-27-1; (C4H9),NBr,1643-19-2; (C4Hg),NHSO4, 32503-27-8; (C,H5),PC1, 2001-45-8; NaClO,, 7601-89-0. (16) See, e.g.: Luck, W. A.
P.h o g . Colloid Polym. Sci. 1978, 65, 6.
