J. Phys. Chem. 1988, 92, 3939-3943
3939
Ternary Water-in-Oil Microemulsions Consisting of Cationic Surfactants and Aromatic Solvents Ronald E. Verrall? Stefania Milioto,l and Raoul Zana* Institut Charles Sadron (CRM-EAHP), 6 rue Boussingault, 67000 Strasbourg, France (Received: September 22, 1987; In Final Form: January 29, 1988)
The solubility of water in solutions of dodecylalkyldimethylammonium bromide surfactants (referred to as N 12-n-1-1-Br, where n is the number of carbon atoms of the variable alkyl chain) and aromatic solvents has been measured as a function of n, temperature, surfactant concentration, and nature of the solvents. Very large water solubility maxima were observed with the N12-4- 1-1-Br homologue in chlorobenzene,bromobenzene, and 1,2-dichlorobenzene. Maxima of solubility of lower magnitude also were found in other aromatic solvents for different values of n. The minimum surface areas, A , per head group at the air-water interface were determined for the various surfactants. The A vs n curve and the log cmc vs n curve both show a clear change of slope at or around n = 4. These results are discussed in terms of surfactant packing at the interface, oil penetration between the surfactant chains, and electrostatic repulsion between the head groups.
Introduction Since microemulsions were first observed,' there have been numerous studies regarding their physicochemical, rheological, and structural properties. Water-in-oil microemulsions in particular have been the focus of increasing attention in recent years. The amount of published work is extensive and an exhaustive referencing of the work is not feasible. However, conference proceedings2-8 and a number of recent papers"2 and references therein serve to illustrate the present state of understanding of microstructures in these systems. Notwithstanding the many studies, formulation of a consistent theory of microstructure and phase behavior has remained elusive. Recently, a study') of structural characteristics of water-in-oil microemulsions stabilized by n-dodecyl-n-butyldimethylammonium bromide revealed that large concentrations of water can be solubilized in chlorobenzene in the presence of this surfactant. N o reasons were given for this result, but it seemed to us important to explore the wider implications of the role of the second chain and thereby the role of the surfactant structure and packing at the interface in the solubilization process. With this object in mind we undertook a systematic solubility study of the family of dodecylalkyldimethylammonium bromide surfactants with varying length and structure of the second alkyl chain. The role of the aromatic oil also was more widely investigated by using several other benzene derivatives. We report the results of the water solubilization of several aromatic oils in the presence of a series of quaternary ammonium bromide surfactants of the general formula C,?H,,(C,H2n+I)N+(CH3)2Brreferred to as N12-n-1-1-Br where 1 6 n < 12. For the compound N12-4-1-1-Br the solubility of water also was studied as a function of temperature and surfactant concentration. Critical micelle concentration (cmc) data obtained from conductivity measurements in aqueous solutions in aqueous solutions and surface tension data at the air-water interface are reported for a number of homologues of this series. A qualitative rationalization of the results is given in terms of chemical and physical properties of the oil, of surfactant packing, and of electrical interactions between head groups in the surfactant layer. l4 Experimental Section Preparation and Purification of the Surfactants. The preparation and purification of the N12-n-1-1-Br surfactants with n = 1, 2, 3, 4, 6 , 8, 10, and 12 was previously reported.', For the purpose of the present investigation we have also prepared the isobutyl ( n = 4'), isopentyl (n = 5'), and pentyl (n = 5) surfactants, 'Permanent address: Department of Chemistry, University of Saskatchewan. Saskatoon. Canada S7N OWO. *Permanent address: Universita Degli Studi di Palermo, Istituto di Chimica-Fisica, Palermo, Italy.
0022-3654/88/2092-3939$01.50/0
using the method previously reported. The isobutyl and isopentyl surfactants were purified by recrystallization in ether. The pentyl surfactant was purified by chromatography over silica gel. The impure surfactant, from which the unreacted reagents had been removed as much as possible by evaporation under reduced pressure (1 mm), was dissolved in ethyl acetate and then adsorbed over silica gel. The surfactant was recovered by eluting with ethyl acetate-ethanol mixtures of increasing ethanol content, evaporating the solvent mixture and drying at 50 "C under high vacuum, in the presence of CaCl,. The purity of the surfactants was determined by elemental analysis. Both the pentyl and isopentyl surfactants are highly hygroscopic. Surface Tension. All measurements were made with a Lauda tensiometer using the ring method. Triply distilled water of low conductivity was used to prepare the solutions. Techniques were followed to ensure that the ring and glassware used in the measurements and preparation of the solutions were scrupulously clean. Measurements were made at 25 f 0.1 OC in cells thermostated by means of a constant temperature bath. Sets of measurements were taken at 5-10-min intervals until measured values were constant. These values then were corrected by using the tables of Harkins and Jordan.I6 The surface tension of water was measured regularly in order to provide values for the pure solvent and to check that the technique was being properly carried out. Conductivity. Critical micelle concentrations were measured (1) Hoar, T. P.; Schulman, J. H. Nature (London) 1942, 152, 102. (2) Mittal, K. L., Ed.; Micellization, Solubilization and Microemulsions; Plenum: New York, 1977; Vol. 1 and 2. (3) Mittal, K. L., Ed.; Solution Chemistry of Surfactants; Plenum: New York, 1979; Vol. 1 and 2. (4) Mittal, K. L., Fendler, E., Eds.; Solution Behavior of Surfactants: Theoretical and Applied Aspects; Plenum: New York, 1982; Vol. 1 and 2. ( 5 ) Mittal, K. L., Lindman, B., Eds.; Surfactants in Solutions, Plenum: New York, 1984; Vol. 1-3. (6) Mittal, K. L., Bothorel, P., Eds.; Surfactants in Solutions; Plenum: New York, 1986; Vol. 4-6. (7) Robb, I. D., Ed.; Microemulsions; Plenum: New York, 1982. (8) Shah, D. O., Ed.; Macro- and Microemulsions: Theory and Applications; ACS Symposium Series 272; American Chemiral Society: Washington, DC, 1985. (9) Kaler, E. W.; Davis, H. T.; Scriven, L. E. J. Chem. Phys. 1983, 79, 5685. (10) Kotlarchyk, M.; Chen, S.H.; Huang, J. S.; Kim, M. W . Phys. Reu. Lert. 1984, 53, 941. (1 1) Auvray, L.; Cotton, J. P.; Ober, R.; Taupin, C. J . Phys. Chem. 1984, 88, 4586. (12) Chen, S. J.; Evans, D. F.; Ninham, B. W.; Mitchell, D. J.; Blum, F. D.; Pickup, S.J . Phys. Chem. 1986, 90, 842. (13) Kubota, K.; Tominaga, Y.; Kon-no, K.; Kitahara, A. Rep. Prog. Polym. Phys. Jpn. 1985, 28, 453. (14) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1 1981, 77, 601. (15) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (16) Harkins, W. D.; Jordan, H. F. J . Am. Chem. SOC.1930, 52, 1751.
0 1988 American Chemical Society
3940
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 W
I
I
R
Figure 1. Solubility of water at 30 O C in 0.1 M solutions of N12-n-1-1-Br surfactants in aromatic solvents, expressed as molar concentration ratio w of water and surfactant: (0)chlorobenzene: (A) 1,2-dichIorobenzene; ( X ) bromobenzene: (0) nitrobenzene; (V)toluene; (e) m-nitrotoluene.
conductometrically by using a Wayne-Kerr automatic precision bridge B 905. All cmc values were obtained from specific conductivity vs concentration plots. The apparent degree of micelle ionization was taken as the ratio of the slopes of the K vs C plot above and below the cmc.I5 Solubility. Water solubilities in oil-surfactant solutions of known concentrations were determined by dropwise addition of H 2 0from a buret into a closed thermostated cell containing the oilsurfactant solution. The solubility of water was taken as that point where the first visual evidence of phase separation occurred. This was usually within one or two drops after the solution became very cloudy. The solubility was expressed as the molar concentration ratio w = [ H 2 0 ] / [surfactant]. Reproducibility of the solubility is estimated to be f3 based on replicate determinations for several of the systems. For the systems having large magnitudes of w ,phase separation gave an upper phase (translucent water-in-oil microemulsion) of ca. 30 vol % and a transparent lower phase of nearly pure halobenzene oil. It is interesting to note that because the oil density is greater than that of water, the water-in-oil microemulsion is the upper phase in these Winsor I systems,17 contrary to classical Winsor I systems involving alkanes.
Results and Discussion Solubility, Critical Micelle Concentrations, Maximum Surface Excess Concentrations,and Minimum Surface Area per Molecule. Figure 1 shows the variation of water solubility at 30 "C vs the number of carbon atoms n of the variable chain in the series of N 12-n-1-I-Br surfactants dissolved in different oils. The isobutyl and isopentyl homologues were considered to behave as surfactants with a linear alkyl chain with n = 3.5 and 4.5, respectively (see below). It is evident that the solubilities of water in solutions of the N 12-4-1-1-Br homologue in chlorobenzene, bromobenzene and 1,2-dichlorobenzene are much larger than those for the other homologues in this series. However, water solubilities in toluene, m-nitrotoluene, and nitrobenzene also show maxima for other homologues, although of much smaller magnitude. (In the following the value of w at the maximum is referred to as wM.) Figure 2 shows that the magnitude of w increases with both decreasing temperature and surfactant concentration, for N124- 1- 1-Br in chlorobenzene. A typical p!ot of surface tension (y) at 25.0 O C of aqueous solutions of N12-IO-1-1-Br vs log C (bulk phase concentration) is shown in Figure 3. The surface tension data were used to calculate maximum surface excess concentrations (I?) of the surfactants at the aqueous solution-air interface according to eq I . I * 7r is the surface pressure (n = yo - y, where yo is the surface r = [dr/d(log C + log y+)]/4.61RT (1) (17) Winsor, P.A. Solvent Properties of Amphiphilic Compounds; Butterworths: London, 1954. (18) Rosen, M. J.; Dahanayake, M.; Cohen, A. W. Colloids Surf. 1982, 5 , 159.
Verrall et al.
1
-C
63
a0
IS
sc
103
Figure 2. Variation of the solubility of water in chlorobenzene solutions of N12-4-1-1-Brsurfactant with the temperature T ( X , C = 0.1 M) and the surfactant concentration C (0, T = 30 " C ) .
20
L
laq
-10
-125
C (M/O
~
-3 5
-3 75
Figure 3. Variation of the surface tension of aqueous solutions of N1210-1-1-Bras a function of concentration at 25 & 0.1 O C . TABLE I: Air-Water Surface Properties of Surfactants at 25 O C surfactant (n) io6r,molm-2 1 3.14 2 2.74 3 2.55 4 2.28 4' 2.22 5 2.21 5' 2.17 6 2.09
8
1.93
10
1.78
N12-n-1-1-Br 1o2A, nm2 52.8 (45.7,O 5 S b ) 60.6
65.1 72.9 74.7 75.2 76.6
79.3 86.2 93.1
"From ref 19. bFrom ref 20.
tension of water at 25 "C), and y+ is the mean activity coefficient evaluated from the Debye-Hiickel limiting law equation valid for 1:l electrolytes at 25 OC
where I is the ionic strength. Values of the mean activity coefficient obtained from eq 2 will be slightly lower than those obtained by using an extended form of the Debye-Huckel equation but, in the absence of accurate values of the mean distance of approach of the ions and given the rather low concentrations studied for most surfactants, it was felt that nonideal behavior could be adequately accounted for by eq 2. The slope of the linear portion of the curve of P vs (log C + log y+) below the cmc was determined by the method of least mean squares. The minimum surface area per surfactant head group at the aqueous solution-air interface, A (nm2/molecule), is then calculated from A = 10l8/NAr
(3)
where N A is the Avogadro number. Values of r and A measured in this work and previously rep ~ r t e d ' are ~ , ~presented ~ in Table I. The A values are plotted (19) Lucassen-Reynders, E.
Prog. Surf. Membr. Sci. 1976, 3, 253.
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3941
Ternary Water-in-Oil Microemulsions
TABLE 111: Values of the Molar Volume, Solubility in Water, and Dielectric Constant of the Investigated Oils, and Values of the Maximum Water Solubility and Variable Alkyl Chain Length toluene bromobenzene chlorobenzene o-dichlorobenzene m-nitrotoluene nitrobenzene
106.0 104.7 101.7 112.7 117.8 102.6
0.4716 0.4530 0.4930 0.14525 0.50'O 2*O
2.4 5.4 5.7 9.9 23 34.8
23 65 78 70 20 22
5 4 4 4 3
2
"Values from the Handbook of Chemistry and Physics, 44th ed.; Chemical Rubber Co.: Boca Raton, FL, 1963. *Values in cmg.mol-'. Values in grams per kilogram; the superscript refers to the temperature. dValues at 25 OC.
Figure 4. Variation of the minimum surface area per head group A at the air-water interface ( O ) , of the cmc in water (a);and of the packing ratio V / A (X) as a function of the number n of C atoms of the short chain for the N12-n-1-1-Br surfactants. (+) Cmc values of the corresponding N12-n-1-1-CI surfactants.
vm,
TABLE I 1 Thermodynamic and Micellar Parameters for N12-n- 1- 1-Br Surfactants
n
M
Yacmc
1 2 3 4 4' 5 5' 6 8 10
14.9 14.3 10.9 7.65 9.7 5.6 6.9 3.05 1.08 0.637
0.867 0.869 0.885 0.903 0.891 0.916 0.907 0.937 0.962 0.978
Jmol-' 12.93 12.43 9.647 6.908 8.643 5.130 6.258 2.859 1.039 0.366
0.26" 0.28" 0.32" 0.37" 0.31 0.36 0.36 0.50" 0.62" 0.74"
oils studied. This could provide an assessment of the variation of water solubility w with 6 on the oil side of the interface, something that was not possible when alkanes are used since they have similar, low values of E, Le., below 2.3. Consequently, we examined the data for possible correlation of the unusually high water solubilities for some of the homologues with some physical solubility in water S, properties of the oil: molar volume cohesive energy density, and dielectric constant t . The values of Tmand S listed in Table I11 do not correlate with the solubilities wM determined in the present work. It has been known for some timeZ2that strong oil penetration between surfactant tails in the interfacial layer, as opposed to head group repulsions and electrostatic double-layer effects on the aqueous side of the interfacial region,23is largely responsible for establishing microstructure in water-in-oil microemulsion systems. Thermodynamic studiesZ4of liquid mixtures of flexible alkanes and rigid, flat molecules such as benzene and its derivatives have been tentatively discussed in terms of ordering effects in the mixtures relative to the pure liquid linear alkanes. But it is by no means clear to what extent similar effects would prevail in the case of double-tailed alkane chains and aromatic molecules. An attempt to correlate the relative order of solubilities with nonchemical interactions between the oil and the variable chain by means of cohesive energy densitiesZSwas unsuccessful. It would appear that neither nonchemical interactions between the oil and this chain nor ordering effects of the long alkyl chain induced by the aromatic oils are the primary reasons for the superior solubility exhibited by the solution of the n = 4 homologue in the three halobenzene derivatives. However, Table I11 shows that, as the dielectric constant of the oil increases, the water solubility maxima are shifted from homologues with a longer second chain to those with a shorter second chain. It has been reported26that rather small amounts of benzene, carbon tetrachloride, and nitrobenzene considerably increase the water solubilization capacity of an AOT/isooctane solution. This was interpreted as indicating that increasing polarizability or polarity of the fourth component leads to interaction with the aqueous pseudophase. Again, from a general perspective, our results with the ternary systems are consistent with this observation. Nevertheless, the three halobenzenes are not the most polar oils which were used, yet they show the greatest solubilization of water. In an attempt to account for the experimental solubilities we turn our attention to consideration of the trend in the head group surface area at the air-aqueous solution interface with n (Figure 4). Recall that the corresponding values at the interfacial region of these water-in-oil microemulsions will not be the same; indeed they are expected to be smaller. This is borne out by comparing the extrapolated value, 101 A2 for the N-12-12-1-1-Br homologue at the air-aqueous solution interface (Figure 4) with the value
-1 8 920 -18710 -19330 -20 160 -19 900 -21 435 -20 625 -21 780 -23 500 -24715
"From ref 15.
in Figure 4 as a function of the number n of carbon atoms in the variable chain. The variation of the cmc of the surfactant with n is also represented on a semilogarithmic scale in Figure 4. It can be seen that the points for the isobutyl and isopentyl homologues fall on the smooth cmc curve drawn through the points of the linear chain homologues when these surfactants are assigned values of n N 3.5 and 4.5, respectively. The area per head group is seen to increase with n. However, two important observations are to be noted. First, the overall positive trend shows a break a t ca. n = 4 with two well-defined s l o p occurring above and below this point. The cmc curve also shows a distinct change of slope at n > 3. Second, the data points for the homologues having branched chain structures lie above the A vs n curve for linear chain homologues and appear to show a distinct dependence. This can only be substantiated by carrying out studies of other isoalkyl members of the series. Gibbs energies of micellization were calculated by using the relation2' AGOmic= (2 - a ) R T In atcmc
(4)
where a+cmc is the surfactant mean activity at the cmc and a is the degree of micelle ionization. Values of parameters used in this calculation and the results obtained are shown in Table TI. An approximate linear decrease in AGOmicis observed with increasing values of n, as is to be expected with increasing hydrophobic character. It is important to recall that the oils used in this study have two characteristics that distinguish them from the alkane oils used in many previous studies. First, the aromatic moelcules are more rigid, in contrast to the flexible linear alkyl chains. Second, there is a wide variation in the dielectric constant t among the aromatic (20) Menger, F.; Wrenn, S . J . Phys. Chem. 1974, 78, 1387. (21) Mukerjee, P. Adu. Colloid Interface Sei. 1967, 1 , 241.
(22) Clowes, G. H. A. J . Phys. Chem. 1916, 20, 407. ( 2 3 ) Allen, M.; Evans, D.F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1987, 91, 2320. (24) Wilhelm, E.; Lainez, A,; Roux, A. H.; Grolier, J.-P.E. Thermochim. Acta 1986, 105, 101. (25) Bristow, G. M.; Watson, W. F. Trans. Faraday SOC.1958,54, 1731. (26) Eicke, H.-F. J . Colloid Interface Sci. 1979, 68, 440.
3942 The Journal of Physical Chemistry, Vol. 92, No. 13, 1988
Verrall et al.
3
B
Figure 5. Schematic representation of the N12-n-1-1-Brsurfactants at
the air-water interface. For the sake of illustration the long-chain axis has been set perpendicular to the oil-water interface, for the homologues with n = 1 (A), 2 (B), and 3 (C), while the trace of the plane of symmetry (dotted line) of the nitrogen tetrahedron is not perpendicular to this interface. When n 3 5 (D), the short chain transfers from the water side to the oil side and the whole molecule tilts about the trace of the plane of symmetry which becomes perpendicular to the interface of 68 A2 reported for a lamellar phase with water.23 Notwithstanding the expected difference in absolute values, the airaqueous solution head group areas illustrate an important feature; i.e., the increase in area per additional methylene group in the variable chain begins to diminish beyond n = 4. This leads to consideration of the question: what is the location of the second chain with respect to the oil/water interface? Two responses to this question are provided by the following results. First, the change of slope seen in the log cmc vs n plot of Figure 4 already has been interpreted as indicating that for n > 4 the second chain is directed toward the micelle hydrophobic (oil-like) core, whereas for n 4 3 it is part of the head group and, thus, resides in water.15 Second, the partition coefficient of alcohols between oil and water increases exponentially with the alcohol chain length.27~28As a result, whereas 1-propanol is preferentially located in water, 1-pentanol is preferentially located in oil. Even though these two results refer to rather different phenomena, they indicate that a drastic change of behavior takes place for the variable alkyl chain with four carbon atoms. The surface area data of Figure 4 reveal that, most likely, the propyl group ( n = 3) is still part of the head group and contributes fully to the head group surface area, whereas the pentyl group ( n = 5) is part of the hydrophobic moiety, and thus resides essentially on the oil side of the interface. As for the butyl group ( n = 4) it is probably partitioned between the oil side and the water side of the interface. Figure 5 attempts to represent schematically the various homologues at the oil-water interface, taken as planar for the sake of clarity. The figure suggests that, as is indeed observed, the surface area per head group should increase less when the short chain is on the oil side. The butyl homologue has not been represented but is expected to show behavior combining that of both the propyl and pentyl homologues. This would introduce additional molecular disorder at the water-oil interface. This situation is reminiscent of that in four-component microemulsion systems where the major role of the cosurfactant, which is to increase the solubility of water in oil or conversely, has been attributed to the increase of molecular disorder29in, or flexibility30 of, the interfacial layer of surfactant and cosurfactant separating oil and water. In order to check this data we have measured the solubility of water in chlorobenzene solutions of mixtures of the N12-3-1-1-Br and N12-5-1-1-Br homologues. The results shown in Figure 6 illustrate that the water solubility does exhibit a very sharp maximum at a mole fraction of N12-5-1-1-Br close to 0.47, that is, for an average n slightly below 4. This is virtually the same value of n as that obtained from the water solubility data for pure surfactants (see Figure 1). Notice also that the limiting value of w measured for the mixtures is very close to that for N 12-4-1- 1-Br. We have calculated the values of the packing ratio V/Al3I (where Vis the volume of the hydrophobic moiety, A is the surface ~~
~~
~~~
(27) Hayase, K.; Hayano, S . Bull. Chem. SOC.Jpn. 1977, 50, 83. (28) De Lisi, R.; Milioto, S.; Turco Liveri, V. J . Colloid Interface Sri.
1987, 117, 64.
(29) Bansal, V.; Chinnaswamy, K.; Ramachandran, C.; Shah, D. J . Colloid Interface Sci. 1979, 72, 524. Bansal, V.; Shah, D.; OConnell, J. P. J . Colloid Interface Sri. 1980, 75, 462. Lemaire, B.; Bothorel, P.; Roux, D. J . Phys. Chem. 1983, 87, 1023. (30) De Gennes, P. G.; Taupin, C. J . Phys. Chem. 1982,86, 2294. (31) Israelachvili, J.; Mitchell, J.; Ninham, B. J . Chem. Soc., Faraday Trans. 2 1976, 7 2 , 1525.
3
025
05
L-75
'0
Figure 6. Solubility of water at 30 O C in a solution of a mixture of N12-3-1-1-Br and N12-5-1-1-Br surfactants in chlorobenzene as a function of the mole fraction of N12-5-1-1-Br in the mixture at 8. total surfactant concentration -0.1 M. The symbols X and 0 correspond to two sets of experiments.
area per head group, and 1 is the length of the hydrophobic moiety). In the present case, 1 corresponds to the length of the longer of the two alkyl chains, that is, the dodecyl group. It is thus constant and only the V/A ratio is considered below. The values of A used are those in Table 11. The volumes V have been calculated by using Tanford's equation,32on the assumption that, for the butyl, pentyl, hexyl, octyl, and decyl homologues, the number of methylene groups contributing to the volume of the hydrophobic moiety are 13, 16, 18, 20, and 22, respectively, whereas for the methyl, ethyl, and propyl homologues this number is taken as 12. On this assumption V/A goes through a minimum for 3 n < 4 (Figure 4). We cannot assess at the present time the importance of this fact in explaining our results. Let us now discuss the parameters responsible for the large values of w observed with some of the systems investigated. Figure 1 shows that, with the exception of nitrobenzene, the maxima in solubility are clustered about the N12-4- 1-1-Br homologue. Also the largest values of the maximum of solubility ( w M )are obtained for those solvents with intermediate values of 6 , say between n = 5 and 10. These results emphasize the importance of the surfactant packing at the interface and the nature of the solvent. The effect of the latter occurs likely through its penetration between the surfactant alkyl chains which affects packing, and its dielectric constant which influences the electrostatic interactions between head groups. This last parameter appears to be of primary importance in determining the values of wM and n. For the investigated surfactants both the micelle ionization degree CY in water and the surface area per head group A increase with n. However, the calculations using the values listed in Table I and I1 show that the ratio C Y / Awhich is proportional to the surface charge density increases with n and so do the repulsions between head groups. The same situation probably prevails in the water-in-oil microemulsions studied here. The high solubilization capacity appears to correspond to a low a / A and thus to a smaller electrostatic contribution to the energetics of the system. The electrostatic term may then be of comparable magnitude to those relating to the solvent penetration between surfactant alkyl chains and to surfactant packing. Under these conditions a rather small change in the length of the second chain may cause significant energy stabilization of the interface and lead to a large increase in water solubility. Shortening the surfactant second chain would reduce water solubility via an increase of surfactant packing (see Figure 1) whereas using a surfactant with a longer second chain would reduce solubility via increased repulsion between head groups. (Note that increases of either surfactant packing or head group repulsions result in an increased rigidity of the surfactant film separating oil and water thus favoring bilayers over droplet a change of solvent nature would affect s t r ~ c t u r e s . ~ ~Likewise, .~*) (32) Tanford, C. J. Phys. Chem. 1972, 76, 3020.
J. Phys. Chem. 1988, 92, 3943-3952 wM and the value of n at the maximum solubility via solvent penetration (packing) as well as solvent polarity (electrostatic term). The above reasoning has been checked by studying the effect of substituting the bromide counterions by chloride counterions on the solubility. Preliminary cmc and solubility determinations have been performed with the N12-3-1-1, N12-4-1-1, N12-5-1-1, N12-6-1-1, and N12-8-1-1 chloride surfactants in chlorobenzene. The values of wM were found to be 4, 4, 16, 16, and 4 for n = 3, 4, 5, 6, and 8, r e ~ p e c t i v e l y . ~An ~ additional experiment performed with an equimolar mixture of the n = 5 and n = 6 chloride surfactants, i.e., = 5.5, yielded W M = 37. Thus W M goes through a maximum for a larger n value for the chloride than for the bromide surfactants. However, the packing effect in chloride surfactants must be somewhat similar to that for the bromide surfactants since the cmc vs n curve also shows a change of slope at about n = 4 (see Figure 4). On the other hand, at a given n the a-values for the chlorides were found to be larger than for the bromides, in agrement with reported results.34 This is probably true also for the surfactant film in our ternary water-in-oil-microemulsions. The larger electrostatic contribution in chloride relative to bromide surfactants must therefore be responsible for the differences observed between the two types of systems. (33) Note that the phase behavior of the N12-6-I-1-CI surfactant in chlorobenzenesolution upon addition of water was found to be more complex than for all other surfactants and is being further investigated. Indeed the phase which separates at wM is very different from that observed with the N12-6-1-1-Br homologue for instance. Phase separation was observed to take place at wM = 68. This difficulty, however, does not affect our conclusions. (34) Fabre, H.; Kamenka, N.; Khan, A,; Lindblom, G.; Lindman, B.; Tiddy, G. J. J. Phys. Chem. 1980, 84, 3428.
3943
Conclusions The systems waterlN12-n-1-1-Br surfactant/aromatic oils show a maximum of water solubility, at a value of n around 4. The study of this effect as a function of the nature of the solvent and n has permitted us to show the importance of the surfactant packing at the oil-water interface (largely determined by the value of n and the nature of the solvent) and of the electrostatic interactions between head groups in determining the formation of water-in-oil microemulsions of high water content. Note Added in Proof. Since this paper was submitted for publication Leung and Shah35and Hou and Shah36have reported the existence of maxima of water solubilization in water-in-oil microemulsions when varying other parameters than in our study and given a semiquantitative interpretation to these maxima.
Acknowledgment. We thank the PIRSEM under AIP No. 1887. R.E.V. thanks the CNRS for financial support during his stay in Strasbourg. This collaboration between the University of Palermo and the Institut C. Sadron of Strasbourg has greatly benefited from the help of Pr. R. DeLisi (Palermo). Registry No. N 12-1-1-1-Br, 1 119-94-4; N 12-2-1-1-Br, 68207-00- 1; N12-3- 1-1-Br, 2948 1-57-0; N12-4- 1-1-Br, 2948 1-60-5; N 12-6-1- 1-Br, 84524-39-0; N 12-8- 1- 1-Br, 42436-34-0; N 12- 10- 1- 1-Br, 76476-02-3; N12- 12-1-1-Br, 3282-73-3; Nl2-4’-l-l-Br, 114532-79-5; N12-5’- I-I-Br, 114532-80-8; N12-5-1-1-Br, 114532-81-9; toluene, 108-88-3; bromobenzene, 108-86-1; chlorobenzene, 108-90-7; o-dichlorobenzene, 95-50-1; nz-nitrotoluene, 99-08- 1; nitrobenzene, 98-95-3.
(35) Leung, R.; Shah, D. J. Colloid Interface Sci. 1987, 120, 330. (36) Hou, M.; Shah, D. Langmuir 1987, 6, 1986.
Solvation of Halide Ions wlth H,O and CH,CN in the Gas Phase Kenzo Hiraoka,* Susumu Mizuse, Faculty of Engineering, Yamanashi University, Takeda-4, Kofu 400, Japan
and Shinichi Yamabe* Department of Chemistry, Nara University of Education, Takabatake-cho, Nara 630, Japan (Received: October 13, 1987) The gas-phase solvation reactions X-(S),, + S = X-(S),, for S = HzO and CH3CN and X- = F, C1-, Br-, and I-, were studied by using a pulsed electron beam high-pressure mass spectrometer. For hydration reactions, the values of -AGo,l,n are in the order F > C1- > Br- > I- for all n. However, the -AHo,,,, values for C1- decrease more rapidly than those for Br-, and a crossover is observed between n = 3 and 4. For the solvation reactions with CH,CN, the -AHoo,l, value for F was found to be much larger than those for other halide ions, indicating the participation of covalent bonding in the cluster F-H3CCN. The covalent bond formation leads to the rapid decrease in the -WWl,, values, and -AHoWl,nfor F becomes smaller than that for C1- at n = 7. Much more gradual decreases in -AHoWl,,values for C1-, Br-, and I- are observed up to the highest values of n measured; Le., the completion of the first solvation shell does not seem to occur up to n = 8. Due to crowding in the inner shell, the -ASo,,,, values increase with n. The difference between the cumulative enthalpies and does not approach the differential bulk solvation enthalpies and entropies, respectively. entropies (Le., -AHoo,, and -ASoO,n)
Introduction Cluster ions represent an aggregated state of matter, having properties midway between the gas phase and the condensed phase. Investigations into the formation and the properties of increasingly larger clusters offer a deep insight into the mode of the molecular interactions and a way of studying the molecular details of the course of change between the gaseous phase and the condensed
(1) Castleman, A. W., Jr.; Tang, I. N. J. Chem. Phys. 1972, 57, 3629.
0022-3654/88/2092-3943$01 .50/0
Equilibria for the ion-solvent molecule clustering reactions involving positive and negative ions can be measured in the gas phase. The van’t Hoff plots of the equilibrium constants lead to the determination of thermochemical data AGO, AHo, and ASo for the stepwise addition of solvent molecules to the ion. In our (2) Magnera, T. F.; Caldwell, G.; Sunner, J.; Ikuta, S.; Kebarle, P. J. Am. Chem. SOC.1984, 106, 6140. (3) Hiraoka, K.; Takimoto, H.; Morise, K. J. Am. Chem. SOC.1986, 108, 5683. (4) Hiraoka, K. Bull. Chem. Soc. Jpn. 1986, 59, 2571.
0 1988 American Chemical Society
