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Apparent molal volumes in microemulsions: An insight into the structures of these systems
Apparent ALAIN
Molal Volumes in Microemulsions: An Insight the Structures of These Systems
H. ROUX,
Laboratoire
GENEVIl?VE ROUX-DESGRANGES, GROLIER, AND ANDRfi VIALLARD
de Thermodynamique B.P.
et Cinetique Chimique, 45, 63170 AubGre, France
Universite
int:,
JEAN-PIERRE de Ciermont
E.
II,
Received December 2, 1980; accepted March 24, 1981 We investigated the “microemulsion” formed by water + sodium dodecylsulfate (NaDS) + nbutanol (n-BuOH) + toluene through high-precision density measurements at 298.15”K. The measurements were carried out along dilution lines by toluene in the pseudoternary system water + emulsifier mixture, E.M., (mass ratio n-BuOHiNaDS = 2) + toluene. The analysis of the variation of the apparent molal volume of toluene &, as a function of concentrations of both the emulsifier mixture and toluene, reveals changes in the structure of the single-phase microemulsion. Large changes are observed in the aqueous medium while in the organic medium changes are more gradual. Different zones of structures in the single-phase microemulsion are ascribed therefrom, which correspond with some zones of structures proposed by other authors, particularly along the demixtion line. INTRODUCTION
ture (E.M.);2 surfactant + cosurfactant. If the detailed structure of such systems is still ill defined-for comprehensive reviews and discussions, see, for example, Ref. (4) -some streaking features are well established; such as the existence of direct micelles-oil in water micelles (O/W)and of reverse micelles-water in oil micelles (W/O)-depending upon the relative concentrations of oil and water. However, these two types of structures are most probably not the only ones which can be found in the whole single-phase microemulsion medium. Investigations for a better knowledge and description of structures in microemulsions are currently undertaken using the classical chemical-physics methods such as nuclear magnetic resonance (5-S),
Micellar and “microemulsion” systems are presently subject to numerous investigations. This is due to their importance in many fields of practical interest like biology, medicine, environment, and more specifically, as far as we are concerned, in tertiary oil recovery processes (1, 2). The aim of all the related research carried out in connection with this process is to find out, or even predict, the best conditions of maximum solubilization of hydrocarbons in ternary systems generally formed with water, a surfactant, and a cosurfactant. The quaternary systems then obtained are known as “microemulsions. ” Microemulsions can be regarded as stable systems, formed by dispersions of “microdroplets” of one phase in another phase (3). They generally are obtained from a two-phase water-oil system in which are added small quantities of an emulsifier mixr To whom correspondence
2 The expressions “active mixture” or “active blend” are also used to characterize the mixture surfactant + cosurfactant. Throughout this paper we will use “emulsifier mixture” and “active mixture” with the same meaning.
should be sent. 250
0021-9797/81/110250-13$02.00/O Copyright 0 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface
Science, Vol. 84, No. 1, November
1981
IONIC
SURFACTANT
light-scattering (9- 1I), or relaxation techniques (12- 14). Usually these methods yield a qualitative description of the singlephase regions, and the general phase diagrams for a number of quaternary systems are now rather well known. Although a quantitative study of the stability of microemulsions can be based on a thermodynamic treatment (15- 17) little attention has been paid experimentally to the thermodynamic behavior of such systems. Desnoyers et al. (18-20) have recently shown that apparent molal properties of a given species in aqueous organic systems and in surfactant solutions are very sensitive to the structural changes occurring in these systems. Then we have started a systematic study of densities and heat capacities of microemulsions and of their associated binary and ternary systems. Here we report the results obtained from the density measurements; results from heat capacity measurements will be given in a forthcoming paper. Using the hydrocarbon as a molecular probe in the quaternary system we have studied the variations of its apparent molal volume versus its concentration. Any change in the structural environment of a molecule of hydrocarbon will be reflected by a change of its apparent molal volume, and comparison with the value for the pure solvent will bring quantitative information concerning the interactions and/or the structures in which the hydrocarbon molecules are involved. The information will be particularly useful for a better understanding of hydrocarbon solubilization in micellar systems. We have studied the microemulsion formed as follows: water (1) + sodium dodecylsulfate (NaDS) (2) + n-butanol (nBuOH) (3) f toluene (4). EXPERIMENTAL
Materials. Toluene, Fluka “puriss” reagent grade of stated purity >99.5 mole% and n-butanol, Merck “pro analysis” reagent grade of stated purity ~99.5 mole%
251
MICROEMULSIONS
were used as received. Prior to actual measurements, they were carefully dried with molecular sieves (Union Carbide Type 4 A, 8 x 12 mesh beads, from Fluka). Sodium dodecylsulfate from Touzart et Matignon, of stated purity 99 mole%, was dried under vacuum for at least 24 hr before use. The solutions were prepared by weight with deionized distilled water. Density measurements. In order to obtain apparent molal volumes with a good precision we need high-precision density measurements. For this we have used a Sodev (Model 02D) flow digital densimeter from Picker et al. (21). In this densimeter the density p of a liquid solution contained in a stainless-steel tube is related to the natural vibration period T of the tube: p = a + pr2,
111
where B and p are constants. In practice we have determined differences in density PI
P-Po=P(T2-t31
where p,, and 7,, are those of a reference solvent. Water and vacuum were used as standards to fix the value of the constant /3. The densimeter was thermostated to within ~5 mK with a Setaram flow temperature regulator which is a commercial version of one previously described (22). The period of the filled tube was measured with a high-resolution digital frequencymeter (Hewlett-Packard Model 5358 A) whose output was fed directly into a printer (Hewlett-Packard Model 5150 A). The absolute temperature was determined with a calibrated Hewlett-Packard Model 2801-A quartz thermometer, and was estimated to be accurate to within 10 mK, with a long-term stability of about t3 mK. All the measurements were carried out at 298.15”K. The solutions were circulated in the densimeter by gravity at a flow rate of approximately 0.2 cm3 min-‘. In these conditions differences in densities, p - po, can be measured to +5 x lop3 kg m-” with a sample of 5 cm3 in about 10 min. Journal of Colloid and
Interface
Science,
Vol. 84, No. 1, November
1981
252
ROUX ET AL. RESULTS
The apparent molal volumes & (in m3 mole-‘) of solute I’ are calculated according to
& = Mi - PO P
)
[31
worni
where Mi mi
p.
is the molar mass (in kg mole-‘) of solute i. is the molality (in mole kg-‘) of solute i in solution having the density p (in kg m-“). (in kg me3) is the density of the “reference solvent.”
In a series of dilutions the “reference solvent” is the starting or the initial liquid to which the following solutions refer. This “reference solvent” can be a pure solvent or a binary or ternary mixture. In the present study of the quaternary system water (1) + NaDS (2) + n-BuOH (3) + toluene (4) we have investigated the microemulsion obtained by diluting initial ternary solutions water (1) + NaDS (2) + n-BuOH (3) by toluene (4). Thus the solute is toluene (i = 4) and p. is the density of the corresponding ternary solution (p being the density of the quaternary solution). A pseudoternary system phase diagram is used to represent the above quaternary system as shown in Fig. 1. This representation is made possible since the mass ratio n-BuOHINaDS (i.e., of the “emulsifier mixture”) was kept constant, equal to 2. The densities of solutions were measured along dilution lines by toluene, in the singlephase region. The straight lines MN, in Fig. 1, represent these dilution lines: point M corresponds to the “reference solvent” in a given dilution. The densities of these “reference solvents” were measured against a primary reference solvent such as water or n-butanol. At least 12 solutions were used for each of the dilution lines investigated, which correspond respectively to the ternary mixJournal
of Colloid and Znterface Science, Vol. 84, No. 1, November
1981
WATER
TOLUENE
1. Pseudoternary phase diagram showing the single-phase region at 298°K of the microemulsion (nonhatched zone) in the quaternary system water (1) + NaDS (2) + n-BuOH (3) + toluene (4). The straight lines T represent the dilution lines by toluene. The straight line W represents a dilution line by water. U is the upper demixtion line, L is the lower demixtion line. Concentrations are in mass percentages. FIG.
tures (point M) containing approximately 5.4, 9.9, 14.1, 20.0, 30.0, 45.8, 50.0, 53.0, 59.8, 80.0, and 84.3 mass percentages of emulsifier mixture-the exact E.M.% are given in Table I. Values of the densities p of the solutions, and of the apparent molal volumes &, for toluene are listed in Table I along with the molalities m4 of toluene in the microemulsion. The curves, &, =f(mJ, representing the variation of &,, as a function of molality are shown in Fig. 2 for the water-rich mixtures and in Fig. 3 for the other mixtures. These different curves were used to graphically extrapolate, for m4 -+ 0, both the apparent molal volumes at infinite dilution, @, , and the initial slopes S&. The uncertainty on the extrapolated c#&values is estimated to be less than 0.10 cm3 mole-l for E.M. percentages higher than 20% and, below, the uncertainty is much larger but still acceptable since very large variations are observed. As for the St, values the uncertainty is within a few percent above 20% of E.M. and larger below. Then the variation of SE4 can be considered as characteristic of changes in the structure of the microemulsion as discussed further on. The variations of c#& and of Sb, versus the mass percentage of the emulsifier mixture are plotted in Figs. 4 and 5; the corresponding values of c#& are listed along with the
IONIC
SUKFACTANT
E.M.% in Table II. An interesting quantity is the value of &, on the lower demixtion curve (@a at point N, in Fig. 1); a plot of this quantity against the E.M.% in the initial ternary system is given with the estimated uncertainty, in Fig. 6. For E.M. percentages higher than 20% the uncertainty on @, is less than 0.10 cm3 mole-l and decreases continuously when E.M.% increases. Using our results, the curves &,, =f(m,), we have interpolated the values of &, on selected dilution lines by water (see Fig. 1) and therefrom we have calculated the corresponding values of &, by means of Eq. [3]. However, since the initial ternary mixture (NaDS + nBuOH + toluene) corresponding to these dilution lines is heterogeneous, we had to calculate the apparent molal volume of water using as reference an “initial” homogeneous quaternary mixture (containing the smallest amount of water). Practically we choose this “reference solvent” along the dilution line by toluene corresponding to an initial water percentage of 15.7% (i.e., the nearest dilution line by toluene to the upper demixtion curve). Then the values of +,, are relative quantities which depend on the initial composition of the reference; but their variation remains significant of the apparent behavior of water along these dilution lines-see the curves & =f (water %) in Fig. 7. DISCUSSION
General Description
of the Phase Diagram
The microemulsion formed by the quaternary system water (1) + NaDS (2) + nBuOH (3) + toluene (4) has been extensively investigated by Lemanceau and colleagues (6, 8, 12, 23, 24), Graciaa et al. (25-27), and Zana and colleagues (38). From their studies a general description of the phase diagram of this system is possible, as shown by the pseudoternary phase diagram in Fig. 1; it is characterized by a large single-phase region where different zones of structures can exist; among these structures, O/W
253
MICROEMULSIONS
and W/O micellar structures are likely the best known. Figures 2 and 3 show the great dependante of &, with both the concentration of toluene and the initial amount of emulsifier mixture, along the different dilution lines MN (see Fig. 1). These variations are characteristic of the “apparent” behavior of a molecule of toluene depending on its environment in the microemulsion or, what is in some way equivalent, of the “apparent” structure “seen” by a molecule of toluene. The values of both @, and 4:: versus the initial percentage of emulsifier mixture -Figs. 4 and 6-go through a marked maximum for small E.M.% then diminish and tend toward respectively the value at infinite dilution of toluene in n-butanol (&) and the value for pure toluene (&). Beyond the observed maximum, although the variations of $Ed and &a appear to be almost continuous, it is possible to distinguish different trends which also correspond to different variation trends of s&---Fig. 5. These different trends allow the definition of different zones of structures which correspond to zones proposed by other authors in the whole single-phase (6, 8, 12) and along the demixtion curve (34, 35): i.e., water-rich zones (A and B), active mixture-rich zone (G), oil (toluene)rich zone (D), and intermediate zones CC, E, F). The border lines for these zones are drawn through the values of &:, or &, interpolated on dilution lines by water as shown in Fig. 8. The ends of these border lines are respectively given on Figs. 4 (or 5) and 6, as a, b, e, f (concentration zero in toluene) and as a’, b’, c’ (on the lower demixtion line). Then the border lines aa’, bb’, ec’ constitute the supposed limits between the main regions of structures. We will now conduct a discussion of our results, looking at the evolution of +,,, 4!49 $E, S!,, and +,, in the above-defined zones. The A, B, C, and D zones, along the lower demixtion line are particularly interesting in the sense they represent the reJournal ofCol/oid
and Inierfoce
Sciencr,
Vol. 84, No. 1, November
1981
254
ROUX
ET AL.
TABLE Densities Microemulsion of Emulsifier
p of Solutions and Apparent Molal Volumes versus Molality rn4, along the Dilution Lines Mixture (E.M.%)
m4 (mole kg-‘)
P
5.367 0 0.02275 0.02697 0.04100 0.04843 0.06749
P
4%
(kg m-9
9.925
108.35 108.33 108.34 108.38 108.21 108.27 108.08 108.00 107.95
0 0.2379 0.4388 0.6416 0.8746 1.0925 1.6916 2.3677 2.9093 3.7256 4.4213 5.874 6.532 7.455 8.946 10.330
107.80 107.79 107.77 107.73 107.69 107.68 107.65 107.59 107.54 107.50 107.44 107.39 107.37
Science, Vol. 84, No. 1, November
1981
E.M.% 109.49 109.14 108.95 108.93 108.77 108.66 108.68 E.M.%
953.721 951.490 948.849 946.638 943.109 939.968 936.330 932.529 926.927 921.683 918.894 915.663 53.010
E.M.%
108.07 107.97 108.31 108.20 108.37 108.41
980.757 978.591 977.210 975.826 974.311 972.846 971.312 971.194 45.793
0 0.2208 0.4970 0.7408 1.1590 1.5652 2.0804 2.6774 3.6868 4.8117 5.5036 6.3894
( 10e6 ms mole-‘)
E.M.%
990.032 989.754 989.248 988.319 988.055 987.209 986.521 20.015
0 0.1472 0.2492 0.3547 0.4698 0.5900 0.7184 0.7270
E.M.%
949.089 946.661 945.165 942.281 939.302 937.386 932.926 929.030 924.388 920.600 917.874 913.113 909.018 907.161
Journal of Colfoid and Inrerface
105.65 106.16 106.17 106.22 106.82
0 0.01895 0.05398 0.11613 0.13527 0.19214 0.23944
108.91 109.16 109.01 109.02 108.92 108.92 108.85
970.476 967.870 964.576 961.943 959.746 957.011 954.252 952.166 949.792 948.292
49.999 0 0.2583 0.4248 0.7628 1.1392 1.3999 2.0481 2.6855 3.5588 4.3779 5.0352 6.3567 7.7061 8.3975
(mole kg-‘)
E.M.%
986.471 985.779 985.339 984.576 983.633 983.033 982.084 981.295 30.008
0 0.2110 0.4921 0.7284 0.9308 1.2149 1.4961 1.7519 2.0420 2.2354
(lo-’ m3 mole-‘)
E.M.%
993.482 993.193 993.126 992.941 992.841 992.551
14.091 0 0.04610 0.07443 0.12647 0.19062 0.23338 0.2999 0.3576
+ “a of Toluene at 298.15”K, in the Single-Phase MN (Fig. 1) Characterized by the Initial Percentage
m4
$“4
(kg m”)
I
945.813 943.661 941.920 940.248 938.410 936.769 932.609 928.427 925.399 921.354 918.293 912.845 910.716 907.993 904.163 901.192
107.97 107.94 107.93 107.88 107.83 107.77 107.71 107.63 107.55 107.50 107.45 E.M.% 107.78 107.78 107.75 107.73 107.70 107.65 107.61 107.59 107.54 107.51 107.44 107.41 107.39 107.35 107.31
IONIC
SURFACTANT TABLE
(mol~b-’
)
(kg& 59.798
0
0.2648 0.5151 0.8528 1.1543 1.7999 2.6349 3.8887 5.3452 7.524 9.694 12.814 16.345
84.289 0
0.2879 0.7039 0.9160 1.1809 1.8973 2.7552 3.7446 4.725 7.236 13.339 23.968 34.450, 51.747
I-Continued
6.
m,
P
4”.
(10% m3 mole-‘)
(mole kg-‘)
0% m-3)
( 1O-6 m3 m01e-~)
E.M.%
938.540 936.396 934.510 932.100 930.081 926.097 921.578 915.868 910.459 904.137 899.299 894.002 889.584
255
MICROEMULSIONS
79.974 0
107.61 107.52 107.49 107.46 107.45 107.43 107.39 107.35 107.30 107.26 107.21 107.17
0.2177 0.4603 0.6426 0.8898 1.1164 1.6565 2.1233 2.7570 4.7680 8.945 16.356 25.546 32.731 43.092
915.286 914.113 912.886 911.995 910.833 909.817 907.546 905.739 903.439 897.557 889.391 881.407 876.216 873.737 871.375
E.M.%
107.24 107.18 107.17 107.16 107.16 107.15 107.16 107.19 107.18 107.16 107.11 107.07 107.04 107.02
E.M.%
910.078 908.723 906.869 906.005 905.552 902.280 899.466 8%.647 894.232 889.322 881.888 875.427 872.131 869.172
107.09 107.13 107.09 106.40 107.13 107.15 107.16 107.16 107.15 107.12 107.07 107.04 107.01
gions of maximum of solubilization of toluene in the microemulsion, with a minimum of emulsifier mixture and at a given constant ratio E.M./H20. Examination of the Different Zones of Structures of the Microemulsion Zone A: water-very-rich region (Fig. 8). Although the relative concentration of emulsifier mixture in rather small (<20%), its mass ratio (n-butanol/NaDS) being equal to 2, we have in fact, direct mixed micelles of n-butanol and surfactant (39) in which the alcohol enhances the solubilization of
toluene in such micelles. However, the volume of mice&r phase is rather small and only small quantities of toiuene can be dissolved. The variations of c/$, and & are parallel (Figs. 4 and 6); they increase rapidly, with the concentration of active mixture, from the value at infinite dilution of toluene in water-$& (toluene/water) = 97.7 x lop6 m3 mole-’ (28)-toward 109.5 x lop6 m3 mole-’ for c#& and 109 x 10e6 m3 mole-l for c/&. These apparent molal values are much larger than the actual molar volume of pure toluene (29), V, = 106.87 x lop6 m3 mole-l. Jourd
of Colloid and Interface
Science, Vol. 84, No. 1, November
1981
256
ROUX ET AL.
105
0
0.4
0.2
0.6
m4(toluone)
0.8 mol
kg’
/ FIG. 2. Water-rich region of the microemulsion formed by the quaternary system water (1) + NaDS (2) + n-BuOH (3) + toluene(4). Apparent molal volume &, of toluene versus the molality m4 of toluene, for different initial concentrations, in mass percentages, of the emulsifier mixture (E.M.).
At infinite dilution #ta, which reflects the volumic contributions of toluene in both the aqueous phase and the miceilar phase, goes up rapidly with the concentration of the active mixture (Fig. 4), as well as c#$ along the demixtion line (Fig. 6). For a given concentration in active mixture the variation of &, with the concentration of toluene depends upon the initial concentration of the ternary mixture. The initial slopes of curves in Fig. 2 show a significant change from positive, values to negative values toward a minimum (see Fig. 5). The minimum of S& (Fig. 5) which corresponds to a maximum for both the curves c#$, =f (E.M.%), Fig. 4, and
107 0
10
20
30
40
50
m4(toluene)
mol kg-’ /
FIG. 3. Emulsifier mixture-rich region of the microemulsion formed by the quaternary system water (1) + NaDS (2) + nBuOH (3) + toluene (4). Apparent molal volume 4, of toluene versus the molality m4 of toluene, for different initial concentrations, in mass percentages, of the emulsifier mixture (E.M.). Journal
of Cc&id
and Interface
Science, Vol. 84, No. 1, November
1981
IONIC SURFACTANT
257
MICROEMULSIONS
bilized in an aliphatic medium and this would correspond to a maximum of structuration of the mixed micelles, This observation would be corroborated by the hypothesis of molecular orientations in long-chain n-alkanes formulated by Bothorel (31, 32) to explain light-scattering measurements in hydrocarbons and used by Patterson et al. (33) to explain the excess enthalpies of binary mixtures of simple quasi-globular molecules in n-alkanes. So, apparently, when the concentration of active mixture increases, the micelles in which toluene is solubilized are more and more structured and the maximum of organization “seen” by toluene is reached around 20%. At this point it should be mentioned that similar trends were observed in “detergentless
I
I
1
I
1
I
1
I
YI II II tob3ne ‘:
2 --!,
1
I
t”‘“e”x7 I
I
414 Q /I
-1
t
i
A
‘Bj
I
-2 0
F
I
I 20
110
IG;
40
60
I
80
100
% E.M.
FIG. 5. Initial slope S&, of the curves & =f(m,), plotted versus the initial percentage of the emulsifier mixture (%E.M.). Limits of zones A, B, E, F, and G.
demixtion
1
FIG. 4. Apparent molal volume of toluene at infinite dilution d& versus the initial percentage of the emulsifier mixture (%E.M.). Limits of zones A, B, E, F, and G. The average experimental uncertainties, as indicated in the text, are shown on the diagram.
systems” by Desnoyers et al. (36) in the ternary system water + isopropanol + benzene; the apparent molal volume of transfer of benzene from water to water + isopropan I4”v, W + W + P) 1 exhibits a pronounced maximum in the water-rich region which is attributed by these authors to a maximum of stabilization. This latter remark would confirm the important role played by the alcohol. Zone B: water-rich-region (Fig. 8). In this region there are still O/W direct micelles; their number increases as well as their volume when the percentage of active mixture is increased (34, 35). The initial slopes S& (Fig. 5) are less negative and tend toward zero at about 30% of active mixture. The observed diminution of +&, $t4 could be due to the fact that the micelles, being larger, are less structured. This interpretation is in accord with the existence of large micelles in this region,
Journal of Co/bid
ond Inferface
Science, Vol. 84, No. 1, November
1981
258
ROUX ET AL.
110
I
1
I
1
I
1
I
L
0
20
40
60
60
100
% E.M.
FIG. 6. Apparent molal volume of toluene d$, along the demixtion line plotted against the initial percentage of the emulsifier mixture .(%E.M.). Limits of zones A, B, C, and D. The average experimental uncertainties, as indicated in the text, are shown on the diagram.
along the saturation line, reported by Goset (34) and Graciaa (35): effectively, using their hypothesis, between 0 and 30% of active mixture, the amount of n-butanol, in the aqueous part, is small -l%, as well as that of NaDS, -O.I%, and practically all the n-butanol molecules contribute to the “mantle” of the micelles with NaDS. We can then estimate, on an additivity basis, the volume of the aqueous part, and from our density measurements the volume of the microemulsion. The volume fraction of the micellar part, calculated therefrom increases from 0.051 (for 5.4% of E.M.) to 0.444 (for 30% of E.M.) in good agreement with the values deduced from Goset and Graciaa’s work. Zone D: oil-rich region (Fig. 8). In this Journdl
of
Colloid
ami'lnterfwe
Science. Vol. 84, No. 1, November
1981
region, which is defined from the variation of C#J&= f(E.M.%) along the demixtion line (Fig. 6), the microemulsion, concentrated in toluene and poor in water and in active mixture, is mainly formed with reverse micelles W/O (34, 35). When the initial E.M. percentage increases, between 60 and g5%, & slightly decreases as compared to what one observes in other zones. The high precision of our measurements (kO.03 cm3 mole-l in this zone) allows us to consider this rather smooth change as significant. Particularly, extrapolation of c#$, beyond the solubility limit, toward 100% of active mixture, yields a value very close to the molar volume of pure toluene. This means that adding toluene does not change significantly its properties-toluene practically “sees” only toluene. Thus toluene is not a suitable probe for this region. In contrast, water would be an appropriate one since the microemulsionW/O reverse micelles -is composed of a large organic continuous part, mainly toluene, with water trapped in the micellar part (35). Intermediate zones (Fig. 8). These zones are defined either along the water + active mixture-rich side of the microemulsion TABLE
II
Apparent Molal Volumes of Toluene at 298.15”K: +I& at Infinite Dilution in the Microemulsion and & at the Demixtion, along the Percentage of Emulsifier Mixture (E.M.) Percentage E.M.
5.4 9.9 14.1 20.0 30.0 45.8 50.0 53.0 59.8 80.0 84.3
(IO”
b: ma mole-‘)
(lo+
4G ma mole-‘)
105.0
106.8
108.0 109.1 109.5 108.5 108.0 107.8 107.8 107.7 107.3 107.1
108.4 108.9 108.7
108.0 107.5 107.4 107.3 107.2 107.0 107.0
IONIC
'i
17.9
2 cl E
17.8
SURFACTANT
259
MICROEMULSIONS
298 K
0
20
40
60
80
100
96 Hz0 FIG. 7. Apparent molal volume concentrations, in mass percentages,
&., of water of toluene.
(zones E, F, G) or along the lower demixtion line (zone C). In these intermediate zones the “apparent” property of toluene changes gradually from its value in one type of structure to that in the other type. The exact limits (dashed curves in Fig. 8) of these zones cannot be established very accurately since the apparent volumic property of toluene (or of water) does not exhibit sharp changes on the border lines. Zone C: inversion of micelles region (Fig. 8). This zone is most interesting because it is intermediate between two regions of well-defined structures (zones B and D). If this zone is not accurately defined in the microemulsion its boundaries, along the saturation line, are well established: between 30 and 60% of active mixture in the initial ternary system- see & = f(%E.M.) in Fig. 6. The variation of +i4 shows a smooth variation from zone B to zone D, due to the different toluene contributions to apparent molal volume, depending on whether toluene is trapped in the micelles or present in the continuous phase. This indicates a progressive change between the two types of structures as already pointed out by other authors (8). The picture of the microstructures in this region is not well established,
versus
the
water
percentage
for
different
initial
however, Schriven (37) has recently proposed an interesting model of “bicontinuous structure” to explain the gradual passage from one type of structure to the other one.
TOLUENE FIG. 8. Definition of the main zones of structures in the quaternary system water (1) + NaDS (2) + nBuOH + toluene (4), by means of the apparent volumes I$,, and &,. The curves 4, =f (E.M.%) and +,, =f (E.M.%) correspond to a dilution line by water obtained for an “initial” percentage of toluene of 9%. Journal of Colloid and Interface
Science, Vol. 84, No. I, November
1981
260
ROUX
ET
Zone G: emulsifier mixture-rich zone (Fig. 8). In this region the microemulsion is concentrated in active mixture but contains very little water and toluene. Its structure, proposed by Lemanceau and colleagues (8), would be of “oillike” type. Probably the continuous phase would mainly be a mixture of alcohol, surfactant, and toluene with lamellar structures. We notice a change in the variation of c/& at around 80% in active mixture (Fig. 4) and it seems that in absence of demixtion c#& would extrapolate toward the value of the apparent molal volume of toluene in nbutanol: r#$, (tolueneln-BuOH) = 106.3 x lO-‘j m3 mole-l (unpublished work from our laboratory). Furthermore the initial slopes of the curves c#+,,= f(mJ increase and even become positive (see Figs. 3 and 5), for initial concentration of active mixture higher than 80%. Zones E and F: the other regions (Fig. 8). These regions which cover the rest of the single phase are defined, at infinite dilution of toluene, from the curves c#& =f(E.M.%) and S& =f (E.M.%)-Figs. 4 and 5, respectively. Border line ec’ is obtained from the variations of both &,, and 4, with the E.M.% along dilution lines by water, border line f”c’ from variations of $,, only. Presently we cannot give an explanation as for the existence of these lines. But, they could be related to the limit of existence of spherical micelles as predicted by Bothorel et al. (40); limit which has been experimentally confirmed for this particular quaternary system by Bellocq and Fourche (4 1). The line ec’ which is unambiguously established by the variation of &,-Fig. 7-seems to be the separation line between a predominantly aqueous region and a predominantly organic region. According to Lemanceau and colleagues (8) we would again have structures of different types coexisting together, with a progressive change from spherical micelles to lamellar micelles when increasing the percentage of active mixture. Journal of CoNoid and Inrerface
Science, Vol. 84, No. 1, November
1981
AL.
Apparent
Molal Volume of Water in the Single Phase
It is obvious that water, due to its specific properties, plays a major role in the organization of the different structures which appear in the whole microemulsion. It was interesting to look at the variation of &, along dilution lines by water (see Fig. 7 and 8) since they cross some of the borders between the main zones of structures. The variations of c#+,,along these dilution lines are rather small. The reason is that in the particular pseudoternary phase diagram studied here the microemulsion contains relatively large amounts of water and any added molecule of water “sees” mainly water molecules. But though small, the variations of &, clearly show two characteristic changes: one corresponding to the border line aa’ and the other corresponding to the border line ec’. It would be interesting to look at the apparent molal volume of water in reverse micelles (in zone D). Our present experiments do not yield precise values of &, in this region and such a study could be envisaged in a system exhibiting a broader zone of reverse micelles. Most probably the apparent molal properties of water at infinite dilution in such systems would show spectacular changes reflecting the behavior of water in micelles as observed by Desnoyers et al. (36) in the ternary system benzenewater-2-propanol (when water is at infinite dilution in the mixture benzene + 2-propanol in benzene-rich solutions). CONCLUSION
Toluene has been used as a “molecular probe” of the microemulsion water + NaDS + n-BuOH + toluene in the sense that its apparent molal volume &, is very sensitive to the environment “seen” by toluene. From the variation of &,, versus both the concentration of the emulsifier mixture and
IONIC SURFACTANT
the concentration of toluene we confirm the partition of the single-phase “microemulsion’ ’ in main regions of structures, as proposed by other authors using quite different techniques. The sensitivity of our density measurements allows us to determine more precisely some of these regions, especially along the lower demixtion line: namely, zones A and B (water-rich region), where O/W micelles predominate, zone D (oil-rich region), where W/O micelles predominate, and the transition zone C where inversion between these two types of structures takes place. These measurements give access to the apparent volume of the different species in the system and then render possible a more quantitative treatment of the microemulsion when other thermodynamic data are at hand. ACKNOWLEDGMENTS We thank Professors Bothorel and Lemanceau for helpful discussions and Professor Desnoyers for stimulating discussions and communication prior to publication, of some of his results. Financial support from the Delegation Gentrale a la Recherche Scientifique et Technique (DGRST), particularly from the Committee “Recuperation Assistee du P&role” is gratefully acknowledged. REFERENCES 1. Desnoyers, J. E., Beaudoin, R., Perron, G., and Roux, G., in ‘Chemistry for Energy” (M. Tomlinson, Ed.), p. 35. A.C.S. Symposium Series No. 90, Amer. Chem. Sot., Washington, D. C., 1979. 2. Berg, R. L., Noll, L. A., and Good, W. O., in “Chemistry of Oil Recovery” (R. T. Johansen and R. L. Berg, Eds.), p. 81. A.C.S. Symposium Series No. 91, Washington, D. C., 1979. 3. Overbeek, J. Th. G., in “Colloid Stability,” Faraday Discuss. Chem. Sot. 65, 7 (1978). 4(a). Mittal, K. L. (Ed.), “Micellization, Solubilization and Microemulsions,” Vols. 1 and 2. Plenum, New York, 1977. 4(b). Mittal, K. L. (Ed.), “Solution Chemistry of Surfactants,” Vols. 1 and 2. Plenum, New York, 1979. 4(c). Prince, L. M. (Ed.), “Microemulsions. Theory and Practice.” Academic Press, New York, 1977.
261
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5. Hansen, J. R., .I. Phys. Chem. 78, 256 (1974). 6. Biais, J., Clin, B., Lalanne, P., and Lemanceau, B., J. Chim. Phys. 74, 1197 (1977). 7. Staples, E. J., and Tiddy, J. T., J. Chern. Sot. Faraday
Z 74, 2530 (1978).
8. Bellocq, A.-M., Biais, J., Clin, B., Lalanne, P., and Lemanceau, B., J. Calloid Interface Sci. 70, 524 (1979). 9. Herrman, C.-U., Wiirz, U., and Kahlweit, M., Ber.
Bunsenges.
Phys.
Chem.
82, 560 (1978).
10. Sjoblom, E., and Friberg, S., J. Colloid Interface Sci. 67, 16 (1978). 11. Cabazat, A. M., Langevin, D., and Pouchelon, A. J. Culloid Interface Sci. 73, 1 (1980). 12. Lalanne, P., Biais, J., Clin, B., Bellocq, A.-M., and Lemanceau, B., J. Chim. Phys. 75, 236 (1978). 13. Teubner, M., Diekmann, S., and Kahlweit, M., Bet-. Bunsenges. Phys. Chem. 82, 1278 (1978). 14. Baumgardt, K., Klar, G., and Strey, R., Ber. Bunsenges. Phys. Chem. 83, 1222 (1979). 15. Ruckenstein, E., and Chi, J. C., J. Chem. Sot. Faraday
ZZ 72, 1690 (1975).
16. Tanford, C., in “Micellization Solubilization and Microemulsions” (K. L. Mittal, Ed.), Vol. 1. Plenum, New York, 1977. 17. Ruckenstein, E., and Nagarajan, R., in “Micellization, Solubilization and Microemulsions” (K. L. Mittal, Ed.), Vol. 1. Plenum, New York, 1977. 18. Roux, G., Perron, G., and Desnoyers, J. E., J. Phys.
Chem.
82, 966 (1978).
19. Desnoyers, J. E., De Lisi, R., Ostiguy, C., and Perron, G. in “Solution Chemistry of Surfactants” (K. L. Mittal, Ed.), Vol. 1. Plenum, New York, 1979. 20. Desnoyers, J. E., -De Lisi, R., and Perron, G., Pure Appl. Chem. 52, 433 (1980). 21. Picker, P., Tremblay, E., and Jolicoeur, C., J. Solution Chem. 3, 377 (1974). 22. Picker, P., Jolicoeur, C., and Desnoyers, J. E., J. Chem. Educ. 45, 614 (1968). 23. Biais, J., Mercier, M., Lalanne, P., Clin, B., Bellocq, A.-M., and Lemanceau, B., C.R. Acad. Sci. Ser. C 285, 213 (1977). 24. Lalanne, P., Biais, J., Clin, B., Bellocq, A.-M., and Lemanceau, B., C.R. Acad. Sci. Ser. C 286, 55 (1978). 25. Graciaa, A., Lachaise, J., Martinez, A., Bourrel, M., and Chambu, C., C.R. Acad. Sci. Ser. B 282, 547 (1976). 26. Graciaa, A., Lachaise, J., Martinez, A., and Rousset, A., C.R. Acad. Sci. Ser. B 284, 343 (1977).
27. Graciaa, A., Lachaise, J., Chabrat, P., Letamendia, Journal of
Cc&id
and Interface
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1981
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28. 29. 30. 31. 32. 33. 34.
ROUX ET AL.
L., Rouch, J., Vaucamps, C., Bourrel, M., and Chambu, C., 1. Phys. Left. 38, 253 (1977). Desnoyers, J. E., and Ichhaporia, F. M., Canad. J. Chem. 47, 4639 (1969). Fortier, J. L., and Benson, G. C., J. Chem. Thermodyn. 8, 411 (1976). Grolier, J.-P. E., and Faradjzadeh, A., Int. Data Ser. Select. Data Mixtures Ser. A. 2, 131 (1979). Bothorel, P., J. Colloid Sci. 27, 529 (1968). Bothorel, P., Such, C., and Clement, C., J. Chim. Phys. 69, 1453 (1972). Lam, V. T., Picker, P., Patterson, D., and Tancrede, P., J. Chem. Sot. Faraday II 70, 1465 (1974). Goset, G., These de Specialite (3bme cycle), Univ. Pau (France), 1978.
Journol
of Colloid and Interface
Science, Vol. 84, No. 1, November
1981
35. Graciaa, A., These Doctorat d’Etat, Univ. Pau (France), 1978. 36. Lara, J., Perron, G., and Desnoyers, J. E., J. Phys. Chem. 85, 1600 (1981). 37. Schriven, L. E., in “Micellization, Solubilization and Microemulsions” (K. L. Mittal, Ed.), Vol. 2, p. 877. Plenum, New York, 1977. 38. Lang, J., Djavanbakht, A. and Zana, R., J. Phys. Chem. 84, 1541 (1980). 39. Roux-Desgranges, G., Roux, A. H., Grolier, J. P., Viallard, A., J. Colloid Interface Sci., in press. 40. Bothorel, P., Biais, J., Clin, B., Lalanne, P., and Maelstaf, P., C.R. Acad. Sci. Ser. C 289, 409 (1979). 41. Bellocq, A. M., and Fourche, G., J. Colloid Interface Sci. 78, 275 (1980).
Molal Volumes in Microemulsions: An Insight the Structures of These Systems
H. ROUX,
Laboratoire
GENEVIl?VE ROUX-DESGRANGES, GROLIER, AND ANDRfi VIALLARD
de Thermodynamique B.P.
et Cinetique Chimique, 45, 63170 AubGre, France
Universite
int:,
JEAN-PIERRE de Ciermont
E.
II,
Received December 2, 1980; accepted March 24, 1981 We investigated the “microemulsion” formed by water + sodium dodecylsulfate (NaDS) + nbutanol (n-BuOH) + toluene through high-precision density measurements at 298.15”K. The measurements were carried out along dilution lines by toluene in the pseudoternary system water + emulsifier mixture, E.M., (mass ratio n-BuOHiNaDS = 2) + toluene. The analysis of the variation of the apparent molal volume of toluene &, as a function of concentrations of both the emulsifier mixture and toluene, reveals changes in the structure of the single-phase microemulsion. Large changes are observed in the aqueous medium while in the organic medium changes are more gradual. Different zones of structures in the single-phase microemulsion are ascribed therefrom, which correspond with some zones of structures proposed by other authors, particularly along the demixtion line. INTRODUCTION
ture (E.M.);2 surfactant + cosurfactant. If the detailed structure of such systems is still ill defined-for comprehensive reviews and discussions, see, for example, Ref. (4) -some streaking features are well established; such as the existence of direct micelles-oil in water micelles (O/W)and of reverse micelles-water in oil micelles (W/O)-depending upon the relative concentrations of oil and water. However, these two types of structures are most probably not the only ones which can be found in the whole single-phase microemulsion medium. Investigations for a better knowledge and description of structures in microemulsions are currently undertaken using the classical chemical-physics methods such as nuclear magnetic resonance (5-S),
Micellar and “microemulsion” systems are presently subject to numerous investigations. This is due to their importance in many fields of practical interest like biology, medicine, environment, and more specifically, as far as we are concerned, in tertiary oil recovery processes (1, 2). The aim of all the related research carried out in connection with this process is to find out, or even predict, the best conditions of maximum solubilization of hydrocarbons in ternary systems generally formed with water, a surfactant, and a cosurfactant. The quaternary systems then obtained are known as “microemulsions. ” Microemulsions can be regarded as stable systems, formed by dispersions of “microdroplets” of one phase in another phase (3). They generally are obtained from a two-phase water-oil system in which are added small quantities of an emulsifier mixr To whom correspondence
2 The expressions “active mixture” or “active blend” are also used to characterize the mixture surfactant + cosurfactant. Throughout this paper we will use “emulsifier mixture” and “active mixture” with the same meaning.
should be sent. 250
0021-9797/81/110250-13$02.00/O Copyright 0 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface
Science, Vol. 84, No. 1, November
1981
IONIC
SURFACTANT
light-scattering (9- 1I), or relaxation techniques (12- 14). Usually these methods yield a qualitative description of the singlephase regions, and the general phase diagrams for a number of quaternary systems are now rather well known. Although a quantitative study of the stability of microemulsions can be based on a thermodynamic treatment (15- 17) little attention has been paid experimentally to the thermodynamic behavior of such systems. Desnoyers et al. (18-20) have recently shown that apparent molal properties of a given species in aqueous organic systems and in surfactant solutions are very sensitive to the structural changes occurring in these systems. Then we have started a systematic study of densities and heat capacities of microemulsions and of their associated binary and ternary systems. Here we report the results obtained from the density measurements; results from heat capacity measurements will be given in a forthcoming paper. Using the hydrocarbon as a molecular probe in the quaternary system we have studied the variations of its apparent molal volume versus its concentration. Any change in the structural environment of a molecule of hydrocarbon will be reflected by a change of its apparent molal volume, and comparison with the value for the pure solvent will bring quantitative information concerning the interactions and/or the structures in which the hydrocarbon molecules are involved. The information will be particularly useful for a better understanding of hydrocarbon solubilization in micellar systems. We have studied the microemulsion formed as follows: water (1) + sodium dodecylsulfate (NaDS) (2) + n-butanol (nBuOH) (3) f toluene (4). EXPERIMENTAL
Materials. Toluene, Fluka “puriss” reagent grade of stated purity >99.5 mole% and n-butanol, Merck “pro analysis” reagent grade of stated purity ~99.5 mole%
251
MICROEMULSIONS
were used as received. Prior to actual measurements, they were carefully dried with molecular sieves (Union Carbide Type 4 A, 8 x 12 mesh beads, from Fluka). Sodium dodecylsulfate from Touzart et Matignon, of stated purity 99 mole%, was dried under vacuum for at least 24 hr before use. The solutions were prepared by weight with deionized distilled water. Density measurements. In order to obtain apparent molal volumes with a good precision we need high-precision density measurements. For this we have used a Sodev (Model 02D) flow digital densimeter from Picker et al. (21). In this densimeter the density p of a liquid solution contained in a stainless-steel tube is related to the natural vibration period T of the tube: p = a + pr2,
111
where B and p are constants. In practice we have determined differences in density PI
P-Po=P(T2-t31
where p,, and 7,, are those of a reference solvent. Water and vacuum were used as standards to fix the value of the constant /3. The densimeter was thermostated to within ~5 mK with a Setaram flow temperature regulator which is a commercial version of one previously described (22). The period of the filled tube was measured with a high-resolution digital frequencymeter (Hewlett-Packard Model 5358 A) whose output was fed directly into a printer (Hewlett-Packard Model 5150 A). The absolute temperature was determined with a calibrated Hewlett-Packard Model 2801-A quartz thermometer, and was estimated to be accurate to within 10 mK, with a long-term stability of about t3 mK. All the measurements were carried out at 298.15”K. The solutions were circulated in the densimeter by gravity at a flow rate of approximately 0.2 cm3 min-‘. In these conditions differences in densities, p - po, can be measured to +5 x lop3 kg m-” with a sample of 5 cm3 in about 10 min. Journal of Colloid and
Interface
Science,
Vol. 84, No. 1, November
1981
252
ROUX ET AL. RESULTS
The apparent molal volumes & (in m3 mole-‘) of solute I’ are calculated according to
& = Mi - PO P
)
[31
worni
where Mi mi
p.
is the molar mass (in kg mole-‘) of solute i. is the molality (in mole kg-‘) of solute i in solution having the density p (in kg m-“). (in kg me3) is the density of the “reference solvent.”
In a series of dilutions the “reference solvent” is the starting or the initial liquid to which the following solutions refer. This “reference solvent” can be a pure solvent or a binary or ternary mixture. In the present study of the quaternary system water (1) + NaDS (2) + n-BuOH (3) + toluene (4) we have investigated the microemulsion obtained by diluting initial ternary solutions water (1) + NaDS (2) + n-BuOH (3) by toluene (4). Thus the solute is toluene (i = 4) and p. is the density of the corresponding ternary solution (p being the density of the quaternary solution). A pseudoternary system phase diagram is used to represent the above quaternary system as shown in Fig. 1. This representation is made possible since the mass ratio n-BuOHINaDS (i.e., of the “emulsifier mixture”) was kept constant, equal to 2. The densities of solutions were measured along dilution lines by toluene, in the singlephase region. The straight lines MN, in Fig. 1, represent these dilution lines: point M corresponds to the “reference solvent” in a given dilution. The densities of these “reference solvents” were measured against a primary reference solvent such as water or n-butanol. At least 12 solutions were used for each of the dilution lines investigated, which correspond respectively to the ternary mixJournal
of Colloid and Znterface Science, Vol. 84, No. 1, November
1981
WATER
TOLUENE
1. Pseudoternary phase diagram showing the single-phase region at 298°K of the microemulsion (nonhatched zone) in the quaternary system water (1) + NaDS (2) + n-BuOH (3) + toluene (4). The straight lines T represent the dilution lines by toluene. The straight line W represents a dilution line by water. U is the upper demixtion line, L is the lower demixtion line. Concentrations are in mass percentages. FIG.
tures (point M) containing approximately 5.4, 9.9, 14.1, 20.0, 30.0, 45.8, 50.0, 53.0, 59.8, 80.0, and 84.3 mass percentages of emulsifier mixture-the exact E.M.% are given in Table I. Values of the densities p of the solutions, and of the apparent molal volumes &, for toluene are listed in Table I along with the molalities m4 of toluene in the microemulsion. The curves, &, =f(mJ, representing the variation of &,, as a function of molality are shown in Fig. 2 for the water-rich mixtures and in Fig. 3 for the other mixtures. These different curves were used to graphically extrapolate, for m4 -+ 0, both the apparent molal volumes at infinite dilution, @, , and the initial slopes S&. The uncertainty on the extrapolated c#&values is estimated to be less than 0.10 cm3 mole-l for E.M. percentages higher than 20% and, below, the uncertainty is much larger but still acceptable since very large variations are observed. As for the St, values the uncertainty is within a few percent above 20% of E.M. and larger below. Then the variation of SE4 can be considered as characteristic of changes in the structure of the microemulsion as discussed further on. The variations of c#& and of Sb, versus the mass percentage of the emulsifier mixture are plotted in Figs. 4 and 5; the corresponding values of c#& are listed along with the
IONIC
SUKFACTANT
E.M.% in Table II. An interesting quantity is the value of &, on the lower demixtion curve (@a at point N, in Fig. 1); a plot of this quantity against the E.M.% in the initial ternary system is given with the estimated uncertainty, in Fig. 6. For E.M. percentages higher than 20% the uncertainty on @, is less than 0.10 cm3 mole-l and decreases continuously when E.M.% increases. Using our results, the curves &,, =f(m,), we have interpolated the values of &, on selected dilution lines by water (see Fig. 1) and therefrom we have calculated the corresponding values of &, by means of Eq. [3]. However, since the initial ternary mixture (NaDS + nBuOH + toluene) corresponding to these dilution lines is heterogeneous, we had to calculate the apparent molal volume of water using as reference an “initial” homogeneous quaternary mixture (containing the smallest amount of water). Practically we choose this “reference solvent” along the dilution line by toluene corresponding to an initial water percentage of 15.7% (i.e., the nearest dilution line by toluene to the upper demixtion curve). Then the values of +,, are relative quantities which depend on the initial composition of the reference; but their variation remains significant of the apparent behavior of water along these dilution lines-see the curves & =f (water %) in Fig. 7. DISCUSSION
General Description
of the Phase Diagram
The microemulsion formed by the quaternary system water (1) + NaDS (2) + nBuOH (3) + toluene (4) has been extensively investigated by Lemanceau and colleagues (6, 8, 12, 23, 24), Graciaa et al. (25-27), and Zana and colleagues (38). From their studies a general description of the phase diagram of this system is possible, as shown by the pseudoternary phase diagram in Fig. 1; it is characterized by a large single-phase region where different zones of structures can exist; among these structures, O/W
253
MICROEMULSIONS
and W/O micellar structures are likely the best known. Figures 2 and 3 show the great dependante of &, with both the concentration of toluene and the initial amount of emulsifier mixture, along the different dilution lines MN (see Fig. 1). These variations are characteristic of the “apparent” behavior of a molecule of toluene depending on its environment in the microemulsion or, what is in some way equivalent, of the “apparent” structure “seen” by a molecule of toluene. The values of both @, and 4:: versus the initial percentage of emulsifier mixture -Figs. 4 and 6-go through a marked maximum for small E.M.% then diminish and tend toward respectively the value at infinite dilution of toluene in n-butanol (&) and the value for pure toluene (&). Beyond the observed maximum, although the variations of $Ed and &a appear to be almost continuous, it is possible to distinguish different trends which also correspond to different variation trends of s&---Fig. 5. These different trends allow the definition of different zones of structures which correspond to zones proposed by other authors in the whole single-phase (6, 8, 12) and along the demixtion curve (34, 35): i.e., water-rich zones (A and B), active mixture-rich zone (G), oil (toluene)rich zone (D), and intermediate zones CC, E, F). The border lines for these zones are drawn through the values of &:, or &, interpolated on dilution lines by water as shown in Fig. 8. The ends of these border lines are respectively given on Figs. 4 (or 5) and 6, as a, b, e, f (concentration zero in toluene) and as a’, b’, c’ (on the lower demixtion line). Then the border lines aa’, bb’, ec’ constitute the supposed limits between the main regions of structures. We will now conduct a discussion of our results, looking at the evolution of +,,, 4!49 $E, S!,, and +,, in the above-defined zones. The A, B, C, and D zones, along the lower demixtion line are particularly interesting in the sense they represent the reJournal ofCol/oid
and Inierfoce
Sciencr,
Vol. 84, No. 1, November
1981
254
ROUX
ET AL.
TABLE Densities Microemulsion of Emulsifier
p of Solutions and Apparent Molal Volumes versus Molality rn4, along the Dilution Lines Mixture (E.M.%)
m4 (mole kg-‘)
P
5.367 0 0.02275 0.02697 0.04100 0.04843 0.06749
P
4%
(kg m-9
9.925
108.35 108.33 108.34 108.38 108.21 108.27 108.08 108.00 107.95
0 0.2379 0.4388 0.6416 0.8746 1.0925 1.6916 2.3677 2.9093 3.7256 4.4213 5.874 6.532 7.455 8.946 10.330
107.80 107.79 107.77 107.73 107.69 107.68 107.65 107.59 107.54 107.50 107.44 107.39 107.37
Science, Vol. 84, No. 1, November
1981
E.M.% 109.49 109.14 108.95 108.93 108.77 108.66 108.68 E.M.%
953.721 951.490 948.849 946.638 943.109 939.968 936.330 932.529 926.927 921.683 918.894 915.663 53.010
E.M.%
108.07 107.97 108.31 108.20 108.37 108.41
980.757 978.591 977.210 975.826 974.311 972.846 971.312 971.194 45.793
0 0.2208 0.4970 0.7408 1.1590 1.5652 2.0804 2.6774 3.6868 4.8117 5.5036 6.3894
( 10e6 ms mole-‘)
E.M.%
990.032 989.754 989.248 988.319 988.055 987.209 986.521 20.015
0 0.1472 0.2492 0.3547 0.4698 0.5900 0.7184 0.7270
E.M.%
949.089 946.661 945.165 942.281 939.302 937.386 932.926 929.030 924.388 920.600 917.874 913.113 909.018 907.161
Journal of Colfoid and Inrerface
105.65 106.16 106.17 106.22 106.82
0 0.01895 0.05398 0.11613 0.13527 0.19214 0.23944
108.91 109.16 109.01 109.02 108.92 108.92 108.85
970.476 967.870 964.576 961.943 959.746 957.011 954.252 952.166 949.792 948.292
49.999 0 0.2583 0.4248 0.7628 1.1392 1.3999 2.0481 2.6855 3.5588 4.3779 5.0352 6.3567 7.7061 8.3975
(mole kg-‘)
E.M.%
986.471 985.779 985.339 984.576 983.633 983.033 982.084 981.295 30.008
0 0.2110 0.4921 0.7284 0.9308 1.2149 1.4961 1.7519 2.0420 2.2354
(lo-’ m3 mole-‘)
E.M.%
993.482 993.193 993.126 992.941 992.841 992.551
14.091 0 0.04610 0.07443 0.12647 0.19062 0.23338 0.2999 0.3576
+ “a of Toluene at 298.15”K, in the Single-Phase MN (Fig. 1) Characterized by the Initial Percentage
m4
$“4
(kg m”)
I
945.813 943.661 941.920 940.248 938.410 936.769 932.609 928.427 925.399 921.354 918.293 912.845 910.716 907.993 904.163 901.192
107.97 107.94 107.93 107.88 107.83 107.77 107.71 107.63 107.55 107.50 107.45 E.M.% 107.78 107.78 107.75 107.73 107.70 107.65 107.61 107.59 107.54 107.51 107.44 107.41 107.39 107.35 107.31
IONIC
SURFACTANT TABLE
(mol~b-’
)
(kg& 59.798
0
0.2648 0.5151 0.8528 1.1543 1.7999 2.6349 3.8887 5.3452 7.524 9.694 12.814 16.345
84.289 0
0.2879 0.7039 0.9160 1.1809 1.8973 2.7552 3.7446 4.725 7.236 13.339 23.968 34.450, 51.747
I-Continued
6.
m,
P
4”.
(10% m3 mole-‘)
(mole kg-‘)
0% m-3)
( 1O-6 m3 m01e-~)
E.M.%
938.540 936.396 934.510 932.100 930.081 926.097 921.578 915.868 910.459 904.137 899.299 894.002 889.584
255
MICROEMULSIONS
79.974 0
107.61 107.52 107.49 107.46 107.45 107.43 107.39 107.35 107.30 107.26 107.21 107.17
0.2177 0.4603 0.6426 0.8898 1.1164 1.6565 2.1233 2.7570 4.7680 8.945 16.356 25.546 32.731 43.092
915.286 914.113 912.886 911.995 910.833 909.817 907.546 905.739 903.439 897.557 889.391 881.407 876.216 873.737 871.375
E.M.%
107.24 107.18 107.17 107.16 107.16 107.15 107.16 107.19 107.18 107.16 107.11 107.07 107.04 107.02
E.M.%
910.078 908.723 906.869 906.005 905.552 902.280 899.466 8%.647 894.232 889.322 881.888 875.427 872.131 869.172
107.09 107.13 107.09 106.40 107.13 107.15 107.16 107.16 107.15 107.12 107.07 107.04 107.01
gions of maximum of solubilization of toluene in the microemulsion, with a minimum of emulsifier mixture and at a given constant ratio E.M./H20. Examination of the Different Zones of Structures of the Microemulsion Zone A: water-very-rich region (Fig. 8). Although the relative concentration of emulsifier mixture in rather small (<20%), its mass ratio (n-butanol/NaDS) being equal to 2, we have in fact, direct mixed micelles of n-butanol and surfactant (39) in which the alcohol enhances the solubilization of
toluene in such micelles. However, the volume of mice&r phase is rather small and only small quantities of toiuene can be dissolved. The variations of c/$, and & are parallel (Figs. 4 and 6); they increase rapidly, with the concentration of active mixture, from the value at infinite dilution of toluene in water-$& (toluene/water) = 97.7 x lop6 m3 mole-’ (28)-toward 109.5 x lop6 m3 mole-’ for c#& and 109 x 10e6 m3 mole-l for c/&. These apparent molal values are much larger than the actual molar volume of pure toluene (29), V, = 106.87 x lop6 m3 mole-l. Jourd
of Colloid and Interface
Science, Vol. 84, No. 1, November
1981
256
ROUX ET AL.
105
0
0.4
0.2
0.6
m4(toluone)
0.8 mol
kg’
/ FIG. 2. Water-rich region of the microemulsion formed by the quaternary system water (1) + NaDS (2) + n-BuOH (3) + toluene(4). Apparent molal volume &, of toluene versus the molality m4 of toluene, for different initial concentrations, in mass percentages, of the emulsifier mixture (E.M.).
At infinite dilution #ta, which reflects the volumic contributions of toluene in both the aqueous phase and the miceilar phase, goes up rapidly with the concentration of the active mixture (Fig. 4), as well as c#$ along the demixtion line (Fig. 6). For a given concentration in active mixture the variation of &, with the concentration of toluene depends upon the initial concentration of the ternary mixture. The initial slopes of curves in Fig. 2 show a significant change from positive, values to negative values toward a minimum (see Fig. 5). The minimum of S& (Fig. 5) which corresponds to a maximum for both the curves c#$, =f (E.M.%), Fig. 4, and
107 0
10
20
30
40
50
m4(toluene)
mol kg-’ /
FIG. 3. Emulsifier mixture-rich region of the microemulsion formed by the quaternary system water (1) + NaDS (2) + nBuOH (3) + toluene (4). Apparent molal volume 4, of toluene versus the molality m4 of toluene, for different initial concentrations, in mass percentages, of the emulsifier mixture (E.M.). Journal
of Cc&id
and Interface
Science, Vol. 84, No. 1, November
1981
IONIC SURFACTANT
257
MICROEMULSIONS
bilized in an aliphatic medium and this would correspond to a maximum of structuration of the mixed micelles, This observation would be corroborated by the hypothesis of molecular orientations in long-chain n-alkanes formulated by Bothorel (31, 32) to explain light-scattering measurements in hydrocarbons and used by Patterson et al. (33) to explain the excess enthalpies of binary mixtures of simple quasi-globular molecules in n-alkanes. So, apparently, when the concentration of active mixture increases, the micelles in which toluene is solubilized are more and more structured and the maximum of organization “seen” by toluene is reached around 20%. At this point it should be mentioned that similar trends were observed in “detergentless
I
I
1
I
1
I
1
I
YI II II tob3ne ‘:
2 --!,
1
I
t”‘“e”x7 I
I
414 Q /I
-1
t
i
A
‘Bj
I
-2 0
F
I
I 20
110
IG;
40
60
I
80
100
% E.M.
FIG. 5. Initial slope S&, of the curves & =f(m,), plotted versus the initial percentage of the emulsifier mixture (%E.M.). Limits of zones A, B, E, F, and G.
demixtion
1
FIG. 4. Apparent molal volume of toluene at infinite dilution d& versus the initial percentage of the emulsifier mixture (%E.M.). Limits of zones A, B, E, F, and G. The average experimental uncertainties, as indicated in the text, are shown on the diagram.
systems” by Desnoyers et al. (36) in the ternary system water + isopropanol + benzene; the apparent molal volume of transfer of benzene from water to water + isopropan I4”v, W + W + P) 1 exhibits a pronounced maximum in the water-rich region which is attributed by these authors to a maximum of stabilization. This latter remark would confirm the important role played by the alcohol. Zone B: water-rich-region (Fig. 8). In this region there are still O/W direct micelles; their number increases as well as their volume when the percentage of active mixture is increased (34, 35). The initial slopes S& (Fig. 5) are less negative and tend toward zero at about 30% of active mixture. The observed diminution of +&, $t4 could be due to the fact that the micelles, being larger, are less structured. This interpretation is in accord with the existence of large micelles in this region,
Journal of Co/bid
ond Inferface
Science, Vol. 84, No. 1, November
1981
258
ROUX ET AL.
110
I
1
I
1
I
1
I
L
0
20
40
60
60
100
% E.M.
FIG. 6. Apparent molal volume of toluene d$, along the demixtion line plotted against the initial percentage of the emulsifier mixture .(%E.M.). Limits of zones A, B, C, and D. The average experimental uncertainties, as indicated in the text, are shown on the diagram.
along the saturation line, reported by Goset (34) and Graciaa (35): effectively, using their hypothesis, between 0 and 30% of active mixture, the amount of n-butanol, in the aqueous part, is small -l%, as well as that of NaDS, -O.I%, and practically all the n-butanol molecules contribute to the “mantle” of the micelles with NaDS. We can then estimate, on an additivity basis, the volume of the aqueous part, and from our density measurements the volume of the microemulsion. The volume fraction of the micellar part, calculated therefrom increases from 0.051 (for 5.4% of E.M.) to 0.444 (for 30% of E.M.) in good agreement with the values deduced from Goset and Graciaa’s work. Zone D: oil-rich region (Fig. 8). In this Journdl
of
Colloid
ami'lnterfwe
Science. Vol. 84, No. 1, November
1981
region, which is defined from the variation of C#J&= f(E.M.%) along the demixtion line (Fig. 6), the microemulsion, concentrated in toluene and poor in water and in active mixture, is mainly formed with reverse micelles W/O (34, 35). When the initial E.M. percentage increases, between 60 and g5%, & slightly decreases as compared to what one observes in other zones. The high precision of our measurements (kO.03 cm3 mole-l in this zone) allows us to consider this rather smooth change as significant. Particularly, extrapolation of c#$, beyond the solubility limit, toward 100% of active mixture, yields a value very close to the molar volume of pure toluene. This means that adding toluene does not change significantly its properties-toluene practically “sees” only toluene. Thus toluene is not a suitable probe for this region. In contrast, water would be an appropriate one since the microemulsionW/O reverse micelles -is composed of a large organic continuous part, mainly toluene, with water trapped in the micellar part (35). Intermediate zones (Fig. 8). These zones are defined either along the water + active mixture-rich side of the microemulsion TABLE
II
Apparent Molal Volumes of Toluene at 298.15”K: +I& at Infinite Dilution in the Microemulsion and & at the Demixtion, along the Percentage of Emulsifier Mixture (E.M.) Percentage E.M.
5.4 9.9 14.1 20.0 30.0 45.8 50.0 53.0 59.8 80.0 84.3
(IO”
b: ma mole-‘)
(lo+
4G ma mole-‘)
105.0
106.8
108.0 109.1 109.5 108.5 108.0 107.8 107.8 107.7 107.3 107.1
108.4 108.9 108.7
108.0 107.5 107.4 107.3 107.2 107.0 107.0
IONIC
'i
17.9
2 cl E
17.8
SURFACTANT
259
MICROEMULSIONS
298 K
0
20
40
60
80
100
96 Hz0 FIG. 7. Apparent molal volume concentrations, in mass percentages,
&., of water of toluene.
(zones E, F, G) or along the lower demixtion line (zone C). In these intermediate zones the “apparent” property of toluene changes gradually from its value in one type of structure to that in the other type. The exact limits (dashed curves in Fig. 8) of these zones cannot be established very accurately since the apparent volumic property of toluene (or of water) does not exhibit sharp changes on the border lines. Zone C: inversion of micelles region (Fig. 8). This zone is most interesting because it is intermediate between two regions of well-defined structures (zones B and D). If this zone is not accurately defined in the microemulsion its boundaries, along the saturation line, are well established: between 30 and 60% of active mixture in the initial ternary system- see & = f(%E.M.) in Fig. 6. The variation of +i4 shows a smooth variation from zone B to zone D, due to the different toluene contributions to apparent molal volume, depending on whether toluene is trapped in the micelles or present in the continuous phase. This indicates a progressive change between the two types of structures as already pointed out by other authors (8). The picture of the microstructures in this region is not well established,
versus
the
water
percentage
for
different
initial
however, Schriven (37) has recently proposed an interesting model of “bicontinuous structure” to explain the gradual passage from one type of structure to the other one.
TOLUENE FIG. 8. Definition of the main zones of structures in the quaternary system water (1) + NaDS (2) + nBuOH + toluene (4), by means of the apparent volumes I$,, and &,. The curves 4, =f (E.M.%) and +,, =f (E.M.%) correspond to a dilution line by water obtained for an “initial” percentage of toluene of 9%. Journal of Colloid and Interface
Science, Vol. 84, No. I, November
1981
260
ROUX
ET
Zone G: emulsifier mixture-rich zone (Fig. 8). In this region the microemulsion is concentrated in active mixture but contains very little water and toluene. Its structure, proposed by Lemanceau and colleagues (8), would be of “oillike” type. Probably the continuous phase would mainly be a mixture of alcohol, surfactant, and toluene with lamellar structures. We notice a change in the variation of c/& at around 80% in active mixture (Fig. 4) and it seems that in absence of demixtion c#& would extrapolate toward the value of the apparent molal volume of toluene in nbutanol: r#$, (tolueneln-BuOH) = 106.3 x lO-‘j m3 mole-l (unpublished work from our laboratory). Furthermore the initial slopes of the curves c#+,,= f(mJ increase and even become positive (see Figs. 3 and 5), for initial concentration of active mixture higher than 80%. Zones E and F: the other regions (Fig. 8). These regions which cover the rest of the single phase are defined, at infinite dilution of toluene, from the curves c#& =f(E.M.%) and S& =f (E.M.%)-Figs. 4 and 5, respectively. Border line ec’ is obtained from the variations of both &,, and 4, with the E.M.% along dilution lines by water, border line f”c’ from variations of $,, only. Presently we cannot give an explanation as for the existence of these lines. But, they could be related to the limit of existence of spherical micelles as predicted by Bothorel et al. (40); limit which has been experimentally confirmed for this particular quaternary system by Bellocq and Fourche (4 1). The line ec’ which is unambiguously established by the variation of &,-Fig. 7-seems to be the separation line between a predominantly aqueous region and a predominantly organic region. According to Lemanceau and colleagues (8) we would again have structures of different types coexisting together, with a progressive change from spherical micelles to lamellar micelles when increasing the percentage of active mixture. Journal of CoNoid and Inrerface
Science, Vol. 84, No. 1, November
1981
AL.
Apparent
Molal Volume of Water in the Single Phase
It is obvious that water, due to its specific properties, plays a major role in the organization of the different structures which appear in the whole microemulsion. It was interesting to look at the variation of &, along dilution lines by water (see Fig. 7 and 8) since they cross some of the borders between the main zones of structures. The variations of c#+,,along these dilution lines are rather small. The reason is that in the particular pseudoternary phase diagram studied here the microemulsion contains relatively large amounts of water and any added molecule of water “sees” mainly water molecules. But though small, the variations of &, clearly show two characteristic changes: one corresponding to the border line aa’ and the other corresponding to the border line ec’. It would be interesting to look at the apparent molal volume of water in reverse micelles (in zone D). Our present experiments do not yield precise values of &, in this region and such a study could be envisaged in a system exhibiting a broader zone of reverse micelles. Most probably the apparent molal properties of water at infinite dilution in such systems would show spectacular changes reflecting the behavior of water in micelles as observed by Desnoyers et al. (36) in the ternary system benzenewater-2-propanol (when water is at infinite dilution in the mixture benzene + 2-propanol in benzene-rich solutions). CONCLUSION
Toluene has been used as a “molecular probe” of the microemulsion water + NaDS + n-BuOH + toluene in the sense that its apparent molal volume &, is very sensitive to the environment “seen” by toluene. From the variation of &,, versus both the concentration of the emulsifier mixture and
IONIC SURFACTANT
the concentration of toluene we confirm the partition of the single-phase “microemulsion’ ’ in main regions of structures, as proposed by other authors using quite different techniques. The sensitivity of our density measurements allows us to determine more precisely some of these regions, especially along the lower demixtion line: namely, zones A and B (water-rich region), where O/W micelles predominate, zone D (oil-rich region), where W/O micelles predominate, and the transition zone C where inversion between these two types of structures takes place. These measurements give access to the apparent volume of the different species in the system and then render possible a more quantitative treatment of the microemulsion when other thermodynamic data are at hand. ACKNOWLEDGMENTS We thank Professors Bothorel and Lemanceau for helpful discussions and Professor Desnoyers for stimulating discussions and communication prior to publication, of some of his results. Financial support from the Delegation Gentrale a la Recherche Scientifique et Technique (DGRST), particularly from the Committee “Recuperation Assistee du P&role” is gratefully acknowledged. REFERENCES 1. Desnoyers, J. E., Beaudoin, R., Perron, G., and Roux, G., in ‘Chemistry for Energy” (M. Tomlinson, Ed.), p. 35. A.C.S. Symposium Series No. 90, Amer. Chem. Sot., Washington, D. C., 1979. 2. Berg, R. L., Noll, L. A., and Good, W. O., in “Chemistry of Oil Recovery” (R. T. Johansen and R. L. Berg, Eds.), p. 81. A.C.S. Symposium Series No. 91, Washington, D. C., 1979. 3. Overbeek, J. Th. G., in “Colloid Stability,” Faraday Discuss. Chem. Sot. 65, 7 (1978). 4(a). Mittal, K. L. (Ed.), “Micellization, Solubilization and Microemulsions,” Vols. 1 and 2. Plenum, New York, 1977. 4(b). Mittal, K. L. (Ed.), “Solution Chemistry of Surfactants,” Vols. 1 and 2. Plenum, New York, 1979. 4(c). Prince, L. M. (Ed.), “Microemulsions. Theory and Practice.” Academic Press, New York, 1977.
261
MICROEMULSIONS
5. Hansen, J. R., .I. Phys. Chem. 78, 256 (1974). 6. Biais, J., Clin, B., Lalanne, P., and Lemanceau, B., J. Chim. Phys. 74, 1197 (1977). 7. Staples, E. J., and Tiddy, J. T., J. Chern. Sot. Faraday
Z 74, 2530 (1978).
8. Bellocq, A.-M., Biais, J., Clin, B., Lalanne, P., and Lemanceau, B., J. Calloid Interface Sci. 70, 524 (1979). 9. Herrman, C.-U., Wiirz, U., and Kahlweit, M., Ber.
Bunsenges.
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82, 560 (1978).
10. Sjoblom, E., and Friberg, S., J. Colloid Interface Sci. 67, 16 (1978). 11. Cabazat, A. M., Langevin, D., and Pouchelon, A. J. Culloid Interface Sci. 73, 1 (1980). 12. Lalanne, P., Biais, J., Clin, B., Bellocq, A.-M., and Lemanceau, B., J. Chim. Phys. 75, 236 (1978). 13. Teubner, M., Diekmann, S., and Kahlweit, M., Bet-. Bunsenges. Phys. Chem. 82, 1278 (1978). 14. Baumgardt, K., Klar, G., and Strey, R., Ber. Bunsenges. Phys. Chem. 83, 1222 (1979). 15. Ruckenstein, E., and Chi, J. C., J. Chem. Sot. Faraday
ZZ 72, 1690 (1975).
16. Tanford, C., in “Micellization Solubilization and Microemulsions” (K. L. Mittal, Ed.), Vol. 1. Plenum, New York, 1977. 17. Ruckenstein, E., and Nagarajan, R., in “Micellization, Solubilization and Microemulsions” (K. L. Mittal, Ed.), Vol. 1. Plenum, New York, 1977. 18. Roux, G., Perron, G., and Desnoyers, J. E., J. Phys.
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82, 966 (1978).
19. Desnoyers, J. E., De Lisi, R., Ostiguy, C., and Perron, G. in “Solution Chemistry of Surfactants” (K. L. Mittal, Ed.), Vol. 1. Plenum, New York, 1979. 20. Desnoyers, J. E., -De Lisi, R., and Perron, G., Pure Appl. Chem. 52, 433 (1980). 21. Picker, P., Tremblay, E., and Jolicoeur, C., J. Solution Chem. 3, 377 (1974). 22. Picker, P., Jolicoeur, C., and Desnoyers, J. E., J. Chem. Educ. 45, 614 (1968). 23. Biais, J., Mercier, M., Lalanne, P., Clin, B., Bellocq, A.-M., and Lemanceau, B., C.R. Acad. Sci. Ser. C 285, 213 (1977). 24. Lalanne, P., Biais, J., Clin, B., Bellocq, A.-M., and Lemanceau, B., C.R. Acad. Sci. Ser. C 286, 55 (1978). 25. Graciaa, A., Lachaise, J., Martinez, A., Bourrel, M., and Chambu, C., C.R. Acad. Sci. Ser. B 282, 547 (1976). 26. Graciaa, A., Lachaise, J., Martinez, A., and Rousset, A., C.R. Acad. Sci. Ser. B 284, 343 (1977).
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L., Rouch, J., Vaucamps, C., Bourrel, M., and Chambu, C., 1. Phys. Left. 38, 253 (1977). Desnoyers, J. E., and Ichhaporia, F. M., Canad. J. Chem. 47, 4639 (1969). Fortier, J. L., and Benson, G. C., J. Chem. Thermodyn. 8, 411 (1976). Grolier, J.-P. E., and Faradjzadeh, A., Int. Data Ser. Select. Data Mixtures Ser. A. 2, 131 (1979). Bothorel, P., J. Colloid Sci. 27, 529 (1968). Bothorel, P., Such, C., and Clement, C., J. Chim. Phys. 69, 1453 (1972). Lam, V. T., Picker, P., Patterson, D., and Tancrede, P., J. Chem. Sot. Faraday II 70, 1465 (1974). Goset, G., These de Specialite (3bme cycle), Univ. Pau (France), 1978.
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of Colloid and Interface
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1981
35. Graciaa, A., These Doctorat d’Etat, Univ. Pau (France), 1978. 36. Lara, J., Perron, G., and Desnoyers, J. E., J. Phys. Chem. 85, 1600 (1981). 37. Schriven, L. E., in “Micellization, Solubilization and Microemulsions” (K. L. Mittal, Ed.), Vol. 2, p. 877. Plenum, New York, 1977. 38. Lang, J., Djavanbakht, A. and Zana, R., J. Phys. Chem. 84, 1541 (1980). 39. Roux-Desgranges, G., Roux, A. H., Grolier, J. P., Viallard, A., J. Colloid Interface Sci., in press. 40. Bothorel, P., Biais, J., Clin, B., Lalanne, P., and Maelstaf, P., C.R. Acad. Sci. Ser. C 289, 409 (1979). 41. Bellocq, A. M., and Fourche, G., J. Colloid Interface Sci. 78, 275 (1980).