4212
J. Phys. Chem. 1990, 94, 4212-4216
The diameter of these close-packed clusters is estimated to be about 6 A, which well agrees with the value obtained by the method of Greegor and Lytle,j5 described above. The coordination number of Cu atoms in the metal clusters, on the other hand, remains unchanged through the CO-oxygen redox cycle, as recognized in Table I. It might be considered, therefore, that the number of Cu atoms in the species X is substantially retained during the interconversion between Cu metal and CuO clusters at the low temperature examined: Le., the CuO clusters also consist of I O Cu atoms. From the points of view described so far, the species X is plausibly small CuO clusters in the zeolites. Upon reaction with
CO, these clusters are directly reduced into metal clusters at a low temperature, while the original Cu2+species in the zeolite are reduced to Cu+ at a higher temperature. The CuO clusters, furthermore, function as the catalytic centers in CO oxidation at low temperatures, via the reversible redox mechanism.
Acknowledgment. We express our gratitude to Dr. Y. Udagawa, Dr. K. Tohji, and Mr. T. Mizushima (The Institute for Molecular Science, Okazaki, Japan) for their helpful support in EXAFS experiments and analysis of the spectra. Registry No. CO, 630-08-0; Cu,7440-50-8
Conductivity Study of Microemulsions. Evaluation of Hydration of Oil/Water Microemulsions Applying Bruggeman Equation Satyaranjan Bisal, Pranab Kumar Bhattacharya, and Satya Priya Moulik* Department of Chemistry, Jadavpur University, Calcutta 700 032, India (Received: July 10, 1989; In Final Form: December 5, 1989)
For hydrated dispersions, the appropriate extent of water binding is required to test the validity of the effective medium theory (EMT) equation of Bruggeman. The extents of hydration of five poly(ethy1ene g1ycol)s (PEG 200, 300, 400, 600, and 1000) have been determined from viscosity, and, through the measurement of conductance of a 0.01 mol dm-3 NaCl in PEG solution, the validity of the Bruggeman equation for them has been confirmed. The Bruggeman equation has been applied to evaluate the extents of hydration of 16 o/w microemulsion systems, derived from both aliphatic and aromatic oils and stabilized by nonionic surfactants (TX 100 an Tween 20) and cosurfactnts (I-butanol, I-hexanol, and n-hexylamine). Except Tween 20 hexylamine stabilized xylene and toluene systems, the hydration of microemulsions has been found to be lower than that of TX 100 and Tween 20 micelles. It depends on the type of surfactant and the surfactant/cosurfactant ratio as well as the type of oil. Xylene- and toluene-derived microemulsions exhibited greater hydration than those derived from hexane, heptane, and decane.
+
Introduction Microemulsions are likely to exhibit special conductance behavior. Water-in-oil type systems often show percolation tendency, where, after a threshold concentration of water,'-I0 the conductance increases very sharply. At lower water level and reasonable water/surfactant mole ratio, there can be also a percolation transition with respect to t e m p e r a t ~ r e . ~ . ' . 'Percolation ~~~ studies of microemulsions have been gaining importance in recent years. This can bring out potential information about the internal structures of the water solubilized in the continuous oil phase. Very recently, the effective medium theory (EMT) of conductance
has been applied to derive structural information about w/o m i c r o e m ~ l s i o n s . ~ ~The - ~ ~droplet size, their number, and the fraction of surfactant and cosurfactant forming the interphase have been evaluated from the water-induced percolation behavior. We have applied and extended the study" to several systems with oils (hexane, heptane, octane, and xylene), surfactants (SDS, CTAB, and AOT), and cosurfactants (1-butanol and n-hexylamine) at different temperatures. The effective medium theories of conductance have been applied in the past to dispersed systems including proteins and Although conductance behaviors of w/o systems are accumulative in the literature, the reverse o/w types are only rarely studied. Mackay et aLZ2have applied the EMT theory of B r ~ g g e m a nto~describe ~ the properties of nonionic surfactant stabilized dispersed oil in a dilute aqueous electrolyte solution. The deviations have been accounted for due to the solvation effect of the microdroplets. The Bruggeman equation has also been tested by Burger-Guerrisi and Tondrez4
( I ) Lagues, M. J. Phys. Lett. 1979, 40, L-331. (2) Grest, G . C.: Webman, I.; Safran, S. A,; Bug, A. L. R. Phys. Rea. A 1986. 33. 2842. (3) Bhattacharya, S.; Stokes, J. P.; Kim, M. W.; Huang, J. S. Phys. Reo. Lett. 1985, 55, 1884. (4) Eicke, H.-F.; Hilfiker. R.; Thomas, H. Chem. Phys. Lett. 1986, 125, 295. (5) Bug, A. L. R.; Gefen, Y. Phys. Rea. A 1987, 35, 1301. (6) Eicke, H.-F.; Kubik, R.; Hasse, R.; Zschokke, I. In Surfactants in Soluiion; Mittal, K . L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 3. (7) Moha-Ouchane, M.; Peyrelasse, J.; Boned, C. Phys. Reo. A 1987, 35, 3027. (8) Peyrelasse, J.: Moha-Ouchane. M.; Boned, C. Phys. Reo. A 1988, 38, 904. (9) Peyrelasse, J.; Moha-Ouchane, M.; Boned, C. Phys. Reo. A 1988, 38, 4155. (IO) Borkovec, M.; Eicke, H.-F.; Hammerich, H.; Dasgupta, B. J . Phys. Chem. 1988, 92, 206. ( I I ) Eicke, H.-F.; Geiger, S.; Hilfiker, R.; Sauer, F. A,; Thomas, H. In Time Dependent Effects in Disordered Materials; Pynn, R., Riste, T., Eds.; Plenum: New York, 1987. ( I 2) Geiger, S.;Eicke, H.-F.; Spielmann, D. Z . Phys. E-Condens. Matter 1987, 68, 175. (13) Mathew. C.;Patanjali, P. K.; Nabi, A,; Maitra, A. N. Colloids SurJ 1988, 30, 253.
(14) Lagourette, B.; Peyrelasse, J.: Boned, C.:Clausse, M. Nature 1979,
281, 60.
(15) Lagues, M.; Ober, R.; Taupin, C. J . Phys. (Paris) Lett. 1978, 39, L-487. (16) Venable, R. L.; Fang, J. J . Colloid Interface Sci. 1987, 116, 269. ( 1 7) Bisal, S. R.; Bhattacharya, P. K.; Moulik, S. P. J . Phys. Chem. 1990, 94, 350. (18) Moulik, S. P. Electrochim. Acta 1973, 18, 981. (19) Bull, H. B.; Breese, K. J . Colloid Interface Sci. 1969, 29, 492. (20) Mandal, A. B.; Ray, S.; Biswas, A. M.; Moulik, S. P. J. Phys. Chem. 1980, 84, 856. (21) Mandal, A . B.; Gupta, S.; Moulik, S. P. Indian J . Chem. 1985, 24, 670. (22) Mackay, R. A.; Hermansky, C.: Aganval, R. In Colloid and Interface Sci.; Vol. It Aerosol, Emulsions and Surfactants; Kerker, M., Ed.; Academic: New York, 1976; pp 289-303. (23) Bruggeman, D. A. G . Ann. Phys. 1935, 24, 636. (24) Burger-Guerrisi, C.; Tondre, C. Progr. Colloid Polym. Sci. 1987, 73, 30.
, 0 1990 American Chemical Society 0022-365419012094-4212$02.50/0 ,
I
1
Conductivity Study of Microemulsions on an o/w microemulsion system stabilized by a nonionic surfactant. In the present paper, we have attempted to study the conductance behavior of a good number of o/w microemulsions using five oils (hexane, heptane, decane, xylene, and toluene), two surfactants (Tween 20 and Triton X IOO), and three cosurfactants ( 1 -butanol, I -hexanol, and n-hexylamine). We have anticipated deviations from the Bruggeman equation owing to the solvation (hydration) of the surfactant-stabilized oil droplets and analyzed the results to derive their hydration properties with reference to several calibrating solutes, the poly(ethy1ene glycol)s. These compounds have been observed to nicely obey the Bruggeman equation, if appropriate hydration values obtained from viscosity20 are introduced to correct their volume fractions in solution. The results of five PEGS, two nonionic surfactants, TX 100 and Tween 20, and 16 o/w microemulsion systems are presented in what follows. Experimental Section Materials. The chemical characteristics of the oils (hexane (HX), heptane (HP), decane (DC), xylene (XU)), the surfactants (Triton X 100 (Tx) and Tween 20 (Tw)), and the cosurfactants (I-butanol (BL), 1-hexanol (HL), and n-hexylamine (HA)) have been described in previous The Tween 20 was purified and both Tween and Triton X were analytically assessed following the procedure described in ref 27. The cmc values determined by surface tension method were 0.05 mM (lit. 0.05 mM) for Tween and 0.22 mM (lit. 0.20 mM) for Triton at 298 K. The toluene (TI) used was of BDH AnalaR quality. The poly(ethy1ene glycol)s (PEG 400 and 600) were the products of BDH, England, used earlier.27 The PEG 200, 300, and 1000 were synthesis grade products of E. Merck, Germany. A. R. BDH NaCl was used as the electrolyte for the preparation of a 0.01 mol d m 3 standard solution in double-distilled conductivity water of specific conductance 2-3 WScm-’ at 303 K. Method. All measurements were taken at 303 K in a temperature-controlled water bath accurate within f0.05 “C. The conductance measurements were taken at a frequency of 1 kHz in a Jenway, England, conductometer in a temperaturecompensated cell of cell constant 1 X 10 cm-I. The multiplier was set at a level of maximum display in the apparatus. The error limits of conductance were within & I % . The viscosity of the PEG solutions was measured in a calibrated20 Ostwald viscometer with a flow time of 120 s for water at 303 K. A calibratedz0 long-necked pycnometer was used for density measurements. All measurements were duplicated and the results presented are based on the averages of these duplicate runs. The maximum error limits in the density and viscosity were f0.006% and *0.7%, respectively. In an actual conductance experiment, an o/w microemulsion was prepared by mixing the ingredients in the required proportion in 0.01 mol d m 3 aqueous NaCl medium. The maximum water tolerance level was known from the reported phase diagram^^^,^^ or separate experiments done on individual systems. The conductances of the isotropic clear microemulsion solutions were successively measured at 303 K in the water bath by titrating the parent solution with 0.01 mol dm-j NaCl solution, with thorough mixing at each addition, allowing sufficient time to attain equilibrium. In the cases of PEG’s, TX 100, and Tween 20, solutions of known concentrations of the solutes in 0.01 mol dm-3 NaCl were titrated with the same aqueous electrolyte solution measuring the conductance at each addition taking necessary precautions. The volume fractions of the dispersed phase were calculated from the compositions and the densities of the materials. For microemulsions the surfactant and the cosurfactant molecules were considered to reside entirely in the interphase between the oil droplet and water. (25) Bisal, S. R.; Bhattacharya, P. K . ; Moulik, S. P. J . Surf. Sci. Technol. 1988, 4 , 121,
(26) Bisal, S . R.; Bhattacharya, P. K.; Moulik, S. P. Indian J . Chem. 1989,
28, 550.
(27) Moulik, S. P.; Cupta, S.; Das, A. R. Can. J . Chem. 1989, 67, 356.
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4213 Results and Discussion Hydration of the Calibrating Compounds, the PEG’s. The intrinsic viscosities of the PEG’s were graphically evaluated by plotting vSp against c taking [v] = limA vspic, where [ T I , T ~ ~ , and c are the intrinsic viscosity, specific viscosity, and the concentration of the solute, g mL-’, respectively. The intrinsic viscosities of PEG’s were evaluated to be 2.46, 2.86, 3.13, 3.44, and 4.39 mL g-’ for PEG 200, 300, 400, 600, and 1000, respectively. The water binding extents of the PEG’s were evaluated from the intrinsic viscosities in terms of the equation2’ [VI = V[OPEG + 6Dwl
(1)
where V, DPEG, 6, and if, represent the shape factor of the solute, the partial specific volume of PEG, the extent of bound water (g/g) to the solute, and the partial specific volume of water, respectively. The low molecular weight PEG’s assume spherical shape in aqueous medium,28so the shape factor v = 2.5. The partial specific volume, DPEG, was obtained from the partial molar volume (OPECo), found from density measurementsz9 by the relation DPEG = B p E c o / M p E G , where M p E G is the molecular weight. Since the solutions were fairly dilute, we assumed fiWale,= 1 /pwater.The 6 values obtained by the above procedure were 0.37,0.42,0.46,0.57, and 1.02 (g/g) for PEG 200, 300,400,600, and 1000, respectively, at 303 K. It is seen that PEG’s of higher molecular masses show increased hydration. Earlier report^^^,^^ evidenced that each oxygen center in PEG (and ethylene oxide residues in nonionic surfactants) binds 2-4 water molecules. The present values are lower. Bahri et aLZ8have recently reported that 0.16-3.20 water molecules may bind a single oxygen center for PEG’s of molecular weights in the range 200-1000. Conductivity of NaCl in PEG, TX 100, and Tween 20 Solutions. Increased concentration of PEG, TX 100, and Tween 20 in a solution demonstrated lowering of conductance of a 0.01 mol d d NaCl solution. This is in line with earlier conductance studies in nonelectrolyte solutions, micelles, and proteins.18%20The transporting ions (Na’ and CI-) hardly undergo complexation interactionz4 with the nonelectrolytic solutes. The results can therefore be straightforwardly analyzed in light of the effective medium theory (EMT) of conductance proposed by BruggemanZ3 and by Maxwell.31 The Bruggeman equation has been found to hold successfully for nonconducting d i s p e r ~ i o n s ~ ~
where k and ko are the conductances of the dispersion and of the disperse medium, respectively, and a is a constant dependent on the shape of the disperse phase particle whose volume fraction is 4. For spherical dispersions, a = 3/2, and eq 2 takes the form
k / k o = (1 - 4)3/2
(3)
The nature of the variation of conductance with I#J is exemplified in curve 1 of Figure 1. Curve 2 in this figure is according to the conductance equation of Maxwell to be presented later. For a nonhdrated (nonsolvated) dispersion, I#J can be estimated from the concentration and the densities of both the phases in the mixture. For a solvated dispersion, the real value of 4 is greater than the calculated value, so that I#Jreal = f4app,where f is a factor which takes care of the solvation effect. It is implied that the minimum value off = 1. Equation 3 for spherical dispersions then becomes (4) (28) Bahri, H.; Guveli, D. E. Colloid Polym. Sci. 1988, 266, 141. (29) Dasgupta, P.; Moulik, S . P. J . Phys. Chem. 1987, 91, 5826. (30) Nilsson, P. G.; Lindman, B. J . Phys. Chem. 1983, 87, 4756. (31) Maxwell, J. C. Electricify and Magnetism, 3rd ed.;Oxford University: London, 1892; Vol. 1, p 314. (32) For conducting dispersion in nonconducting medium, e.g., ionic surfactant stabilized water/oil microemulsions, the EMT theory of Bottcher (Bottcher, C. J. F.; Recl. Trau. Chim. 1945, 64, 47) and Granqvist and Hunderi (Granqvist, C. G.; Hunderi, 0. Phys. Reu. B 1978, 18, 1554) are applicable. Curve 3 of Figure 1 is according to these theories for spherical dispersion.
4214
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
Bisal et al.
TX/HA/HX
f
/
i
I
0
I
10
4
i I
32
G r o n q vBios t t & c hHe ur r d e r i \ (for s p h e w o ! d i s p e r s i o n ) \ (cucve3) I
06
0 4
@
\ I
08
1
08
OL
nhigm/gmi
,
10
I
10
Figure 3. Interrelation between nh (g/g) and f at 303 K for the five PEG’s 200, 300, 400, 600, and 1000.
+
Figure 1 . Conductivity ratio ( k / k o ) vs volume fraction of the disperse phase (4) plots for PEG 400 and PEG 600 at 303 K. (A)nonhydrated PEG 400; (A’)hydrated PEG 400; (0) nonhydrated PEG 600; (0’) hydrated PEG 600. Inset of Figure 1 represents ( k / k o ) vs 4 plot of a microemulsion system (TX/HA/Hx/water). Curves 1, 2, and 3 are according to Bruggeman, Maxwell, and Bottcher as well as Granqvist and Hunderi for spherical dispersions.
mental data with the theoretical curve, considering the hydration effect, is also presented in Figure 1 for PEG 400 and 600. Good correlation is observed up to 4 = 0.4 and thereafter the results deviate. We have therefore restricted treatment of the data to 4 = 0.35 or less. Fitting of the conductance data with eq 3 with a constant value of hydration (realized from viscosity) demonstrates minor variations of water binding capacities of the PEG’s so long I$ d 0.3. At higher volume fractions, interparticle interactions and the possibility of bicontinuous structure formation in the case of microemulsions will make the conductance equation inappropriate. In Figure 3, we have demonstrated the excellent correlation (coefficient = 0.999) between the extent of hydration, nh, and f for all five PEG’s. The line is therefore taken as the calibration line for nh and$ A knowledge of either one of the two evaluates the other. nh and f are related by the equation f = 1 1.12nh (5)
+
which simplifies the matter a great deal. nh can be directly determined from a knowledge offobtained through conductance measurements. Combining eq 4 and 5 we have
( k / k 0 ) 2 ’ 3= 1 - (1
01 0
I
0 25
e-
I
0 50
)
Figure 2. ( k / k o ) 2 /vs 3 @ plots of PEG 200 (e), PEG 400 (B),PEG 600 ( X ) , and PEG I000 ( 0 )solutions at 303 K. Broken lines represent plots of ( k / k o ) 2 / 3against hydrated volume fraction ( $ J ~ , ) A . is the shift in the ordinate scale unit to avoid overlap.
A plot of ( k / k 0 ) 2 / against 3 dappought to be linear, yieldingffrom the slope. Such representative plots for PEG 200, 400, 600, and 1000 are presented in Figure 2. The lines are nicely linear with high degree of correlation. The ( / ~ / k , )versus ~/~ (estimated from the above-reported hydration in terms of eq 1) plots are represented in this figure as broken lines. Allowing for experimental uncertainties, the slopes are unity. This demonstrates that for PEG’s hydration is solely responsible for the deviation of the experimental conductance from eq 3. The fitting of the experi-
+ 1.12nh)d
(6)
which is valid for water-soluble low molecular weight poly(ethy1ene oxide) polymers that assume a spherical shape in solution. Assuming a pseudophase micellar model, the hydrations of TX 100 and Tween 20 micelles were evaluated from conductance measurements by applying eq 6. The results are given in Table I . They are important for a comparative understanding of the hydration behaviors of the free surfactants and when they are in combination as in microemulsions. Combination of eq 1 and 6 with necessary manipulation can correlate the two transport properties, conductance and intrinsic viscosity, in terms of the equation 1.12([ q ] - 2.5D,l‘wMs
2.5uWMw The Microemulsion Systems. Formation Characteristics. The microemulsions herein studied were stable up to a fair degree of water addition. This was known from the phase diagrams studied ~ e p a r a t e l y . ~The ~ . ~dependence ~ of the higher limits of o/w mole ratios on the mole ratios of S / C S (surfactant/cosurfactant) to sustain the microemulsion state are exemplified in Figure 4. It is seen that the o/w ratio bears an inverse relation with the S/CS; proportionately more cosurfactant is required for the dispersion of increased quantity of oil. Such a property is more or less independent of the type of the components. For several systems,
Conductivity Study of Microemulsions TABLE I: Characteristics of the PEG and O/W Microemulsion Systems at 303 K
no. I 2 3 4
5 6 7 8 9
IO II 12 13 14 15
16 17 18 19 20 21 22 23
systema PEG 200 PEG 300 PEG 400 PEG 600 PEG 1000 T W 20 TX 100 TX/BL/HP ( 1 6/62/22) TX/BL/Hp ( 1 61671 17) TX/BL/DC ( 1 71661 17) TX/HA/Hx (23139138) TX/HA/Hx (37132131) TX/HA/Dc (27143130) TW/HL/Hx (9/47/44) TW/HL/Hx ( I 4/43/43) TW/HL/Hx (23139138) TW/HL/Dc (16/50/34) T W / H A/ Hx (7147146) TW/H A/Hx ( I 3/44/43) TW/HA/Hx (23139138) T W / H A/ Dc (9/54/37) TW/HA/Xy ( I 3/42/45) T W / H A/TI ( I 2/39/49)
1.8
hydration mol/mol glgb of oxygen 0.36 (0.37) 0.79 0.43 (0.42j 0.95 0.46 (0.46) 1.07 0.54 (0.57) 1.21 1.03 (1.02) 2.45 0.3 1 0.82 0.18 0.66 0.09 0.57
1.401 1.467 1.514 1.610 2.150 1.325 1.192 1.104
correln coeff of eq 3 0.999 0.999 0.999 0.999 0.999 0.999 0.998 0.999
0.13
0.80
1.147
0.999
0.07
0.44
1.08 1
0.999
f
0.998
0.999 1.ooo
0.999
0.03
0.12
1.033
0.999
0.15
0.49
1.163
0.999
0.04
0.17
1.048
0.999
0.09
0.47
1.098
0.999
0.05
0.21
1.059
0.999
0.14
0.46
1.156
0.999
0.07
0.36
1.074
0.999
0.24
1.oo
1.266
0.999
0.20
0.84
1.224
0.999
I
oo281
AiL
Oo2&
? I 0
1.5
t
"2
(k/ko? 09
06
0.997
"Numbers in parentheses refer to mole percents of the respective components. bValues in parentheses for PEG's refer to hydration obtained from viscosity.
t
0 012
Oo8I
0 O 004
I
1
I
02
04
06
.
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4215
1
08
S/CS(mole/mole)
-
1
10
, 12
Figure 4. Dependence of maximum oil/water mole ratio with surfactant/cosurfactant mole ratio (SICS) of different microemulsion systems at 303 K. Numbers correspond to systems presented in Table I .
we have used the minimum amount of the surfactant required for stabilization. We have n o t processed conductance data beyond 4 = 0.3. In this condition the microdroplets behaved as isolated
0.3
0
0
I
2 , A = 0.2 3 , A 0.L & , A = 0.6 5 , A = 0.8
0.25
0.50
CP-
Figure 5. ( k l k , ) vs 9 plots for five microemulsion systems at 303 K. Lines, I , T W / H A / H x (system 18); 2, T W / H A / X y (system 22); 3, T W / H L / D c (system 17); 4, T X / B L / H p (system 8); 5 , T X / H A / D c (system 13). A is the shift in the ordinate scale unit to avoid overlap. The broken lines refer to hydrated volume fractions (@rml). System 13 (line 5 ) has practically zero hydration, so that bap = @rml. System 17 (line 3) has low hydration. The close overlaping &roken line referring ,, is not shown. to its &
spheres in the electrolyte medium. The Conductance Behauior. The microemulsion systems investigated were of oil-in-water type which were stabilized by the nonionic surfactants Tween 20 or Triton X 100 along with the cosurfactants 1-butanol, 1-hexanol, and n-hexylamine with the oils used being hexane, heptane, decane, xylene, and toluene. The droplets were nonconducting and dispersed in a medium of 0.01 mol dm-3 aqueous NaC1. Equation 4 is applicable to them if the roplets are spherical. Results of electron microscopy, viscosity, ultracentrifugation, and light scattering studies have demon~ t r a t e dthat ~ ~the oil dispersions stabilized by nonionic surfactants are spherical in shape and behave like hard spheres. The Bruggeman equation (4)is therefore applicable, which is depicted in Figure 5 for five different systems. We have studied 16 microemulsion systems, although all are not shown, to save space. The fvalues are obtained from the slopes and the extent of hydration nh evaluated from eq 5. It has been confirmed for the PEG's that f is constant and it can essentially take care of the extent of hydration. We assume that the microemulsions also have similar property, and the contributions of other factors (geometry, local environments, etc.), if any, are minor. The extent of hydration found per unit mass of microemulsion as well as per mole of the oxygen center are presented in Table I. The results show that weight/weight hydration of free surfactant is more than when it is in the microemulsion. This is quite likely because the microemulsion has more nonpolar mass compared to the surfactant. But even on the mole/mole of oxygen basis, the difference is still significant except for systems 9, 22, and 23. In the micelles, the polar head groups of TX 100 and Tween 20 are close and can encage water.*' They are distantly placed on the surface of the microemulsion droplets so the opportunity to trap water molecules is less. Foster et al.34 have shown that the degree of hydration (33) Hermansky, C.; Mackay, R. A. J . Colloid Interface Sci. 1980, 73, 324 and references cited therein.
4216
The Journal of Physical Chemistry, Vol. 94, No. IO, 1990
Bisal et al. TABLE II: Radius and Number of Droplets per Milliliter of Microemulsions Formed with Minimum Surfactant at 303 K
\
system
I.
TX/BL/Hp (16162122) TX/HA/Hx (29139138)
I
I I
1 20
)
15
R.
A
nd/mL
X
lo-’’
44
11.1-9.7
53
6.3-3.5
system TW/HL/Hx (9147144) TW/HA/Hx (7/47/46)
ndmL
R , A x LO-’? 67
3.3-3.1
66
3.3-2.5
A representative case (system 11 in Table I) has been exemplified in the insert of Figure 1. Interrelation among nh, (S -t cs),and oil. The nature of interrelation among nh moles of (S + CS) and the moles of oil is presented in Figure 6. In this figure the base points are linked for showing that the results appear in different planes. The f t h values take sharp turns both a t lower and higher (S CS). Increased oil has moderate effect on nh. In this comparison, the nonaliphatic oils xylene and toluene are not considered, as they belong to another class. The significant difference of hydration of aliphatic oils from aromatic oils speaks in favor of the vital role of their class in controlling the nature of the interphase. The points representing xylene and toluene are, however, shown in the figure for a ready comparison. Model Estimation of the Droplet Size and Number. An estimate of the droplet size and its number in unit volume in the range of validity of the Bruggeman equation has been made assuming that the S and CS molecules remain entirely in the interphase. Using sodium oleate and I-pentanol for the dispersion of liquid paraffin in water, Attwood et al.3s had shown a 3:1 mole ratio distribution of the C S and S in the interphase of oil and water. We do not consider a similar distribution since, unlike sodium oleate, the surfactants used have been nonionic. For simplified calculation, Hermansky and M a ~ k a also y ~ considered ~ the entire existence of S and CS in the interphase. The crosssectional areas of the components, 1 -butanol,36 I - h e ~ a n o ln-, ~ ~ hexylamine,16 Triton X and Tween 20,36have been taken to be 20, 20, 25, 55, and 80 A2, respectively, for calculation. The results are presented in Table 11. Four systems are chosen; two are TX 100 derived and two are Tween 20 derived categories at lower values of the surfactants to closely comply with the requirement that the entire amount of S and C S is present in the interpha~e.~’Both the radius and the number of droplets per unit volume are of the order of magnitude obtained for microemulsions in general. It is observed that TX 100 produces lower size of micro oil droplets than Tween 20.
+
t ( 5 t C S )
Figure 6. Three-dimensional profile of the interrelation among hydration (nh). SICS mole ratio, and oil (mole). Symbols as in Figure 4.
of o/w microemulsions vary with the water content (i.e., composition). This has been observed at 4 > 0.35 in the present study. Like the PEG’S, TX 100, and Tween 20, in the case of each microemulsion, a constant value o f f is required to correlate conductance with eq 5 so long as 4 < 0.3. Individual Hydration Properties. The TW-HA-Hx as well as TX-BL-Hp systems have shown increased hydration with increased S/CS ratios; this is expected since the surfactant head groups are water binding centers. Comparatively the hydration of the TX 100 stabilized systems is more than the Tween 20 stabilized systems. A straightforward accounting of this effect is difficult a t this stage. System 14 of Table I is an exception in whichf = 1 and nh = 0. It has minimum T W compared to similar systems, 15 and 16, which show increased nh with increasing T W content. Although there are minor differences in hydration among the hydrocarbons (a decrease has been observed for higher homologues), the xylene- and toluene-derived systems exhibit higher hydration. The TX-HA stabilized cases are an exception, where the conductance equation of Bruggeman does not hold, and thefvalues obtained are lower than 1. These systems more or less obey the equation of Maxwell without modification
k / k o = 2(1 - 4)(2
+ $)-I
(8)
Acknowledgment. We thank CSIR, Government of India, for financial assistance. Registry No. H X , 110-54-3; HP, 142-82-5; DC, 124-18-5; XY, 1330-20-7; Tx, 9002-93-1; Tw, 9005-64-5; BL, 71-36-3; H L , 11 1-27-3: H A , I 1 1-26-2; PEG, 25322-68-3; NaCI, 7647-14-5.
(35) Attwood, D.; Currie, L. R. J.; Elworthy, P. H. J . Colloid Interface Sei. 1914, 46, 255. (36)Schick, M. J. Nonionic Surfactants;Dekker: New York, 1966; Vol. 1
(34) Foster, K . R.; Cheever, E.; Leonard, J. B.; Blum, F. D.; Mackay, R. A. ACS Symp. Ser. 1985,272 (Macro-Microemulsions), 275. Foster, K. R.; Epstein, B. R.; Jenin, P. C.; Mackay. R.A . J . Colloid Interface Sci. 1982, 88,233.
(37) Being oil-soluble, the cosurfactant may entirely remain in the interphase. If the minimum amount of surfactant required for stable microemulsion formation is used, there is high probability that practically the whole amount of it will also reside in the interphase.
