356
J Phys. Chem. 1990, 94, 356-361
Appticatbn of the Pseudophase Ion-Exchange Modei to Kinetics in MicroemuMms of Anionic Detergents Ricardo Da Rocha Pereira, Dino Zanette,* and Faruk Nome* Departamento de Quimica, Universidade Federal de Santa Catarina, 88049 FlorianGpolis, SC, Brazil (Received: January 18, 1989: In Final Form: May 15, 1989)
The acid-catalyzed hydrolysis of 2-(p-methoxyphenyl)-1,3-dioxolanehas been studied in microemulsions of 1-butanol, sodium dodecyl sulfate (SDS), water, and toluene. The kinetic data was interpreted with the pseudophase ion-exchange (PPIE) formalism considering the microemulsions as a system formed by three phases (oil, water, and interphase), the degree of dissociation of the detergent being considered variable. The rate constant at the interphase is about the same as in water (k2,w = 11.6 M-'s-') and about 3.5 times larger than in micelles, indicating that similar to micellar systems the catalytic power of microemulsions consists of concentrating the reactive partners at the charged interphase.
Introduction It is well-known that aqueous charged interphases play an important role in the enhancement of the rate of chemical reactions.ls2 In bimolecular reactions between an organic substrate and an univalent ion of charge opposite to that of the interphase, the pseudophase ion-exchange (PPIE) formalism has been applied to aqueous ionic micelles," synthetic vesicles,'*9 reversed micelles,I0 and microemulsions.'1J3 Originally developed for the treatment of ratesurfactant concentration profile~,3*~ the PPIE model ignores details of the micellar structure and by considering the micelle as a bulk phase allows treatment of the distribution of species in terms of partitioning (organic substrate) and ion exchange (univalent ion). The apparent failure of the PPIE observed in reactive counterion surfactants or at high detergent or salt concentration and in the presence of an excess of very hydrophilic counterions (such as OH- and F) has been demonstrated to be related to the assumption of a constant degree of counterion dissociation (a). Indeed, when the variation of (Y under the reaction conditions is taken into account, the PPIE model can be applied to reactive counterion surfactants quite successfully. Microemulsionsare ideal systems to test the validity of the PPIE model with cy as a variable. Indeed, having a very high interfacial ( I ) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975. (2) Fendler, J . H. Membrane Mimetic Chemistry; Wiley: New York,
1982. (3) Quina. F. H.; Chaimovich, H. J . Phys. Chem. 1979, 83, 1844. (4) Romsted, L. S. Ph.D. Thesis, Indiana University, Bloomington, IN, 1975. (5) Berezin, 1. V.; Martinek, K.; Yatsimirski, A. K. Russ. Chem. Reo. (Engl. Transl.) 1913, 43, 487. (6) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; p 1015. (7) Fendler, J. H.; Hinze, W. L. J . Am. Chem. Soc. 1981, 103, 5439. (8) Cuccovia, I. M.: Quina, F. H.; Chaimovich, H. Tetrahedron 1982,38, 917. (9) Chaimovich, H.; Bonilha, J . B. S.; Zanette, D.; Cuccovia, I . M. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; p 1121. (10) El Seoud, 0. A.; Chinelatto, A. M. J . Colloid Interface Sci. 1983, 95, 163. ( 1 1 ) Mackay, R. A. J . Phys. Chem. 1982, 86, 4756. (12) Bunton, C. A.; de Buzzaccarini, F.; Hamed, F. H. J . Org. Chem. 1983, 48, 246 1 . (13) Athanassakis, V.; Bunton, C. A.; McKenzie, C. D. J . Phys. Chem. 1986, 90, 5858. ( I 4) Silva, I. S.;Zanette, D.; Nome, F. Arual. Fis.-Quim. Org., Cont 1985, 123. (15) Nome, F.; Rubira, A. F.; Franco, C.; Ionescu, L. G. J . Phys. Chem. 1982, 86, I88 1 . (16) Stadler, E.; Zanette, D.; Rezende, M. C.; Nome, F. J . Phys. Chem. 1984,88, 1892. (17) Bunton, C. A.; Romsted, L. S . In Solution BehaviorofSurfactants: Theoretical and Applied Aspects; Mittal, K. L., Fendler, E. J., Eds.; Plenum: New York, 1982; p 975.
TABLE I: Values of Partial Molar Volumes (v),Density ( d ) , and Molecular Mass (&f)Used To Calculate the Volume Fraction of the Components in the Microemulsion' V, cm3/mL 1 -butanol water
SDS toluene
91.60 17.73 253.95 107.40
d , g/cm3
h?, g/mol
0.8092 1.0250 0.9840 0.8580
74.12 18.02 288.38 92.15
Values taken from ref 22.
area ( IO9 cm2/L), they can effectively catalyze chemical reactions occurring at the interfacial boundaries of the s y ~ t e m . l ~ ~The ~~J~*'~ microemulsion considered in this study is formed by water, 1 butanol, toluene, and sodium dodecyl sulfate, and the reaction under study is the acid-catalyzed hydrolysis of 2-(p-methoxyphenyl)- 1,3-dioxolane. First-order rate constants for this reaction were obtained over the whole phase diagram. It is demonstrated that a single rate constant can explain our results under a wide range of experimental conditions, including normal and reverse micelles for the water- and oil-rich regions and bicontinuous for middle domains. Experimental Section Sodium dodecyl sulfate, SDS (Merck), was recrystallized three times from absolute ethanol and vacuum dried. The surface tensionsurfactant concentration profile showed no minimum, and the critical micelle concentration, cmc = 8.0 X low3M at 25 OC, agreed with that reported in the literature.zo All solutions were prepared with distilled, deionized water that was boiled and cooled under nitrogen to remove carbon dioxide and kept under a nitrogen atmosphere. Toluene and 1-butanol (Merck) were purified by distillation, and acetonitrile (Merck, Uvasol) was used without further purification. The substrate 2-@-methoxyphenyl)-l,3-dioxolane @-MFD) was prepared according to Fife and Jao.21a Phase Diagram. The phase diagram of the system was determined by mixing appropriate weight fractions of the three components 0.003 M HCI solutions (W), toluene (T), and 1butanol/SDS (E). The weight ratio of 1-butanol/SDS used was 2.82. The phase boundary was determined by titration and is represented by solid (18) (a) Hermansky, C.; Mackay, R. A. In Solution Chemistry ofSurfactants; Mittal, K L., Ed.; Plenum: New York, 1979; p 723. (b) Mackay, R. A,; Hermansky, C . J. J . Phys. Chem. 1981, 85, 739.
(19) Burnside, B. A,; Knier, B. L.; Mackay, R. A,; Durst, H. D.; Longo, F. R. J . Phys. Chem. 1988, 92, 4505. (20) Mukerjee, P.; Mysels, K. J . Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1971. (21) (a) Fife, T. H. J . Am. Chem. Soc. 1967, 89, 3228. (b) Wentworh, W . E. J . Chem. Educ. 1965, 42, 96.
0022-3654/90/2094-0356$02.50/00 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. I , 1990 357
Kinetics in Microemulsions of Anionic Detergents BUT -. 2B2
TABLE 11: First-Order Rate Constants as a Function of Concentration of HCI for the Hydrolysis Reaction of p-MFD at 25 OC"
SDS
A
lO2kob,,,,
103[HC1], M
s-l
0.8 1 I .72
0.82" 2.73" 6.83" 13.7" 20.5" 27.3" 0.64c 2.10' 5.30' 10.6c 15.9' 21.3'
5.15 10.9 15.3 21.0 0.35 1.91 4.43 8.82 13.1 15.6
I03[HCI], M
1O2kOb,,,, s-'
2.40b 6.OOb 12.0b 18.0b 24.0b
2.43 5.50 11.5 17.3 22.9
0.29d 0.9Sd 2.44d 4.8Sd 7.32d 9.76d
0.14 0.75 1.95 2.58 6.78 7.60
"The initial compositions of toluene, HCl/water solution and emulsifier are, respectively, as follows: (a) 6.5%, 28.0%, 65.5%; (b) 19.25%, 24.0%, 56.75%; (c) 27.25%, 21.75%, 51.00%; (d) 65.0%, 10.0%, 25.0%. WATER
20
40
60
TOLUENE
Figure I . Phase diagram for the system formed by SDS, I-butanol, toluene, and 0.003 M aqueous solution of HCI. The solid lines represent the phase boundaries of the microemulsion domain. Dashed lines and points X,-X, are indicative of the compositions studied.
lines in Figure 1 . All solutions with compositions inside the boundary domain are clear and optically transparent. The phase diagram obtained with the use of water instead of 0.003 M aqueous HCI was identical with that described in Figure 1. Kinetics. The formation of the p-methoxybenzaldehyde was followed spectrophotometrically at 290 nm by a Shimadzu Model UV 2 10-A spectrophotometer equipped with a thermostated water-jacketed cell compartment, at 25 "C. This wavelength was selected due to the absorption of toluene below 290 nm. The first-order rate constants were calculated from a generalized least-squares method2Ib and were linear up to at least four half-lives, with correlation coefficients greater than 0.99. For the kinetic measurements, the microemulsions were prepared by mixing appropriate weights of the three components under the conditions indicated by the dashed lines in Figure 1. The volume fractions of the components of the microemulsion were calculated from data in Table I according to the method of Bahri and Letellier.zz
Results The pseudoternary phase diagram of the system I-butanol/ SDS/water/toluene, is shown in Figure 1. The microemulsion domain is similar to that reported elsewhere.22*uThe dashed lines in Figure 1 represent the compositions used in the kinetic and conductivity experiments. Kinetic Results. The second-order rate constant in water (k2,w) for the acid hydrolysis of p-MFD at 25 OC was calculated from k2,w= k,,,/[HCI], where kobs,wis the first-order rate constant in water. We found a value of k2,w= 11.6 M-' s-I, which is similar to that found by Fife and Jao (k2,, = 13.61 M-' s-I2') in aqueous dioxane. The first-order rate constants for the hydrolysis of p-MFD in microemulsions (kobs,J at compositions represented by XI, X,, X,and X4 in Figure 1 are given in Table 11. A linear increase of khtm was observed between 1 X IO-, M and 25 X M HCI, indicating that under all conditions the reaction is first order in HCI. The results obtained for the hydrolysis of p-MFD in micellar solutions of S D S in the presence of 0.004 M HCI at various detergent concentrations are depicted in Figure 2. The ratesurfactant concentration profile is similar to many other bimolecular reaction profiles described in the literat~re.~-~-" The curve (22) Bahri, H.; Letellier, P. J . Chim. Phys. Phys.-Chim. Eiol. 1985, 82, 803. (23) Van Nieuwkoop, J.; Snoei, G . J . Colloid Interface Sci. 1985, 103,417.
TABLE 111: First-Order Rate Constants in the Microemulsion as a Function of Phase Volume of Water for the Hydrolysis Reaction of p-MFD with the Initial Composition 68.25% 1-Butanol/SDS and 14.25%Toluene at 25 "C
4w 103kobs,me.S-I
0.1222 I .30
0.1961 2.60
0.2867 7.20
0.3388 9.10 I
0.5
t
10' [ SDS] ,M Figure 2. Plot of the observed first-order rate constant for the acidcatalyzed hydrolysis of p-MFD (kob,,) in aqueous solutions of SDS, in M HCI a t 25.0 "C. the presence of 4.0 X
has a kobs,,,maximum at 0.030 M SDS, the maximum rate constant in the micelle being about 10-fold larger than in water. The kinetic data were treated using the PPIE model3 in order to calculate the second-order rate constant in the micellar phase from the dependence of the first-order rate constant, kobs.,,,on the surfactant and total hydrogen ion concentrations [H+IT(eq I ) , where P is the effective volume per mole of micellized de[H+lT[(k2,m/ Y)KsKH/Na([Na+lm/[Na+lw) + k2,wl kobs,m
=
( I -k KscD)(I
+ KH/Na[Na+lm/[Na+lw) (1)
tergent, 0.25 m L / m ~ l K, ; ~ represents ~ the binding constant of the substrate; KHiNais the ion-exchange constant; C, is the concentration of micellized detergent; the subscripts m and w denote micelle and water, respectively; and the brackets indicated stoi(24) (a) Bunton, c . A,; Mhala, M. M.; Moffatt, J . R.; Monarres, D.; Savelli, G. J . Org. Chem. 1984, 49, 426. (b) Bunton, C. B.; Wolfe, B. J . Am. Chem. SOC.1973, 95, 3742. (c) Cuccovia, 1. M.; Schroter, E. H.; Monteiro, P. M.; Chaimovich, H . J . Org. Chem. 1978, 43, 2248. (d) Nome, F.; Schwingel, E. W.; Ionescu, L. G. J . Org. Chem. 1980, 45, 705. (e) Gonsalves, M.; Probst, S.; Rezende, M. C.; Nome, F.; Zucco, C.; Zanette, D. J . Phys. Chem. 1985, 89, 1127. (25) (a) Shinoda, K.; Soda, T. J . Phys. Chem. 1963, 6 7 , 2072. (b) Mukerkee, P. J . Phys. Chem. 1962, 66, 1733.
358
The Journal of Physical Chemistry, Vol. 94, No. I , I990
Da Rocha Pereira et al.
TABLE IV: First-Order Rate Constants in Microemulsions as a Function of the Phase Volume of Oil for the Hydrolysis Reaction of p-MFD with the Initial Microemulsion Composition of 70.25% 1-Butanol/SDS and 29.75% HCI Aqueous Solution at 25 'C
4" 0.0000 0.1520 0.2067 0.2579 0.3088 0.3595
103kOh,,,, 9.27 8.00 6.08 5.73 3.75 3.23
S-1
@J" 0.4100 0.5 104 0.6099 0.6594 0.7085 0.7479
I 0 3 k ~ ~S-I. ~ ~ , 3.05 2.38 2.00 1.37 1.32 1.15
TABLE V: First-Order Rate Constants in the Microemulsion as a Function of the Phase Volume of Oil for the Hydrolysis Reaction of p-MFD with the Initial Microemulsion Composition of 30.25% l-Butanol/SDS and 69.75% HCI Aqueous Solution, Maintaining the Total Amount of 1-Butanol/SDS Constant
0.1 157 0. I693 0.2220 0.2737 0.3245
272.0 176.0 174.0 112.0 99.8
0.4236 0.47 19 0.5793 0.5660 0.61 19
22.8 17.8 12.3 20.7 12.6 9 0
chiometric molar concentrations. The solid line in Figure 2 corresponds to the fitting of the experimental data with eq 1 that allowed us to calculate values of K , = I O and k2,, = 4.77 f 0.91 M-' s-' with use of an ionexchange constant KHINa= The acid hydrolysis of p-MFD was then studied in different regions of the phase diagram (lines 1, 3, and 7 in Figure I). For the experiments along line I , the total amount of toluene was kept constant and the ratio of the other components changed. The results are given in Table 111; the values of kobs,meincreased as the volume fraction of water (&) increases. Table IV contains the first-order rate constants obtained when an initial mixture of 70.25% I-butanol/SDS and 29.75% aqueous HCI solution was titrated with toluene up to the boundary of the phase diagram (line 3, Figure 1). In this experiment, the ratio emulsifier/aqueous HCI is kept constant, while the volume fraction of toluene & increases. The experimentally determined rate constant decreases rapidly as a function of added oil. The dependency is not linear; at the highest volume fraction of oil studied, the rate is about 1 order of magnitude lower than in its absence. The effect of microemulsion composition (line 7, Figure 1) on the rate of the hydrolysis of p-MFD keeping the emulsifier constant is shown in Table V. The dependency of the rate constants on 4oagain is not linear with the kobs,mevalues, decreasing as the volume fraction of toluene increases. Conductiuity Results. The effect of toluene addition on the specific conductivity of emulsifier/aqueous HCI mixtures is presented in Figure 3A-E. The initial emulsifier/aqueous HCI ratios were 80/20, 70/30, 60/40, 50/50, and 40/60 by weight and correspond to lines 2-6, respectively, in Figure 1. The Bruggeman e q ~ a t i o n ~was ~ t *used ~ to treat the conductivity results (eq 2) where K , and K represent the specific conductivity at &, = 0 and at 4o> 0 , respectively. The Bruggeman equation is a classical semiempirical treatment developed for coarse emulsions and relates the conductivity with the volume fraction of the dispersed phase. The constants (3 and m are adjustable parameters that best fit the experimental data. Parts A-E of Figure 3 show the application of eq 2 to the experimental data, with a single value of p = 1 and different m values ( m = 4.98,4.40, 3.08, 1.91, and 1.59 for A-E, respectively). Mackay et al.27found (3 and m values around 1.2 and 2.1-2.5, (26) Bunton, C. A.; Ohmenzetter, K.; Sepulveda, L. J . Phys. Chem. 1977,
8 I , 2000.
(27) Mackay. R. A.; Agarwal, R. J . Colloid Interface Sci. 1978.65. 225.
Figure 3. Plot of the ratio of conductivity in the presence and absence of toluene ( K / K , ) as a function of the volume fraction of toluene (bo)for weight ratios emulsifier/water of (A) 80/20, (B) 70/30, (C) 60/40, (D) 50/50, and (E) 40/60.
~
MFDo
+ H:
PROD.
MFD,
+ H i
PROD.
~
I
Figure 4. Schematic representation of the proposed kinetic model in microemulsion systems.
respectively, for microemulsion of tween 60/ I-pentanol/water/ hexadecane (or heavy mineral oil, nujol) with 0.1 M aqueous NaCI. They suggested that B could be interpreted as the inverse of the maximum phase volume for monodispersed hard spheres (&, = 0.74) and m could be related to the polydispersity of the microemulsion. Van Nieuwkoop and SnoeiZ3studying a similar system, SDS/ 1 -butanol/heptane/water, also found that the Bruggeman equation fitted the conductivity data using (3 = 1 and different m values. It is important to remark that it is difficult to explain the results in terms of physical models since the microdroplet structure changes from O/W to W/O, through a bicontinuous region where oil and water form continuous interpenetrating domain^.^^,^^ Discussion A number of quantitative models have been developed over the past several decades for the interpretation of kinetic results for bimolecular reactions in charged interphase^.'^^ The pseudophase ion-exchange model for micelles assumes that the reaction takes (28) (a) De Gennes, P. G.;Taupin, C. J . Phys. Chem. 1982,86,2294. (b) Dvolaitzky, M.; Laguh, M.; Le Pesant, J. P.; Ober, R.; Sauterey, C.; Taupin, C. J . Phys. Chem. 1980,84, 1532. (29) Lagues, M.; Sauterey, C. J . Phys. Chem. 1980, 84, 3503. (30) Bunton, C. A,; Romsted, L. S . In The Chemistry of the Functional Groups. Supplement 8: The Chemistry of Acid Deriuatiues; Patai, S . , Ed.; Wiley-Interscience: London, 1979; Part 2, p 945.
Kinetics in Microemulsions of Anionic Detergents
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 359
place in the two pseudophases. Micelles act as a separate phase, and the surface behaves like an ion-exchange resin, the distribution of the ionic species being described by an ion-exchange constant. The theoretical treatment of kinetic data in our microemulsion is developed in light of the pseudophase ion-exchange model, considering the microemulsion as a pseudophase system formed by three different components, namely, an oil phase, aqueous phase, and interphase (Figure 4). The following assumptions have been made: (a) Surfactant and cosurfactant are located preferentially in the interphase. (b) The reaction can take place in any of the three pseudophases, the relative contributions of each phase being given by the distribution of the reactants @-MFD and H+). Thus, the rate of acid hydrolysis of p-MFD is given by -d[p-MFD]/dt = ~~,,[H+],[~-MFD],I$, + k2,o[H+lobMFDIo4o
+ k2,i [H+Ii b - M F D I i4i (3)
where k2,w,k2,,, and k2,iare the second-order rate constants in water (w), oil (o), and the interphase (i). (c) Considering that p-MFD is not located in the aqueous phase due both to its low solubility in water and the high surfactant concentration ([SDS] > 0.2 M in all the experiments) and that the hydrogen ion concentration in the oil phase is negligible, eq 3 can be simplified to
(d) The distribution of p-MFD between the oil phase and the interphase (Figure 4) is governed by the partition coefficient (Po& being expressed by eq 5.
Po,i = [p-MFD]i/ [p-MFD],
(5)
20
0
0
20
IO3 [ HCI] ,M
Figure 5. Plot of the observed first-order rate constant for the acidcatalyzed hydrolysis of p-MFD (/C,,~,,J in the presence of microemulsion compositions corresponding to points (A) X , , (B) X,,(C) X,,and (D) X , in Figure 1.
where CT is the total amount of surfactant. With eq 11-13 being combined with eq 10, it is possible to obtain the following, where A = [H+Ii&: ( I - KH/Na)A2 + [aCT + KH/Na[H+lT+ KH/Na(l - a)CTIA KH/Na(l - @)CTIH+lT= 0 (I4)
Since for the SDS surfactant KH/Na is approximately equal to unity,26 eq 14 becomes
With eq 5 being combined with the mass balance equation for the total concentration of p-MFD (eq 6), an expression for the [ ~ - M F D ] T= [p-MFD]@i
+ [p-MFD]&,
(6)
substrate concentration in the interphase is obtained (eq 7).
bMFD1i
= b M F D 1 T / ( 4 i + 40/po/i)
(7)
Replacing eq 7 in eq 4 and integrating, one obtains the general expression for the value of the observed rate constant in microemulsions (eq 8). kobs.mc
= k2,i[H+]ih/('#)i + @o/Po/i)
KHW + Na+i V H+i + Na+,
(9)
the ion-exchange constant being given by
Using the mass balance to estimate the total sodium ion concentration, [Na+ITand sodium concentrations in the water [Na'], and in the interphase [Na+Ii, one obtains [Na'IT = [Na+Ii4i + [ N a + l , ~
can be further simplified to
[H+li$i = (1 - ~ ) [ H + I T
(16)
Finally, replacing eq 16 into eq 8, one obtains a rather simple expression for the observed rate constant in microemulsion (kobs,,J (eq 17).
(8)
(e) In order to use eq 8, we need to express [H+Ii in terms of the experimentally known total concentration of acid ([H+IT). For a system without any other counterion, the concentration of hydronium ion in the interphase and in water is governed by KiIw (Figure 4). Indeed, since the interphase behaves like an ion-exchange resin, and sodium ion is also present in solution, the process is slightly more complicated than described in Figure 4 and the concentration of acid is a function of the ion-exchange constant (KHINa)between sodium ion and H+ H+,
which since CT >> [H'],,
(11)
According to eq 17, kobs,medepends on the degree of dissociation of the surfactant ( a ) . Several kinetic models have been suggested in the past,11-13J8J9but none took into account changes in the degree of dissociation ( a )for anionic or cationic detergents, despite the fact that it is well-known that additives like alcohols may change d r a ~ t i c a l l y .In ~ ~aqueous micelles, the current models assume constant a, even though there are strong variations of composition, counterions, and surfactant concentration.2 4 8 15-17,24 However, it has been demonstrated that the apparent failures observed with reactive counterion surfactants such as CTAOH and CTAF are directly related to variable a values that were estimated directly from conductivity data.31 Using a similar approach, we have estimated a values from eq 18, where a, corresponds to the degree of dissociation of the 9
K/K,
= a/a,
9
I
(18)
surfactant in the absence of toluene. The observed variation of K / K , (Figure 3A-E), which according to eq 18 reflects CY changes, is not unexpected. In our microemulsions, we have a high concentration of 1-butanol and therefore may expect big changes in the degree of d i s s o ~ i a t i o n . ~ ~ ~ ~ ' ~ ~ ~ (31) Neves, M. de F. S.; Zanette, D.; Quina, F.; Moretti, M. T.; Nome,
F.J . Phys. Chem. 1989, 93, 1502.
(32) Bunton, C. A.; de Buzzaccarini, F. J . Phys. Chem. 1982.86, 5010.
360 The Journal of Physical Chemistry, Vol. 94, No. 1. 1990 TABLE VI: Second-Order Rate Constants in Microemulsions Obtained from Fitting of the Experimental Data" phase diagram phase diagram location k2,,, M" s-I location kz,,, M-l SKI point XI 12.36 f 1.20 line 1 14.13 f 5.67 point X , 16.53 f 0.54 line 3 15.70 f 1.47 point X 3 14.96 f 1.62 line 7 13.21 f 4.53 point X , 20.43 f 4.31
.
s-'
0
E.
t
d
x M
OValues of Po,, = 0.25,
4.77 f 0.91 k4-I
Da Rocha Pereira et al.
a.
= 0.52, kz,w = 11.6 M-'
s-l,
and k2,m =
0
were used.
c
IO t
Y
0
Xi
0
l
5
1.0
40
Figure 7. Plot of the observed first-order rate constant for the acidcatalyzed hydrolysis of p-MFD (kobs,m)in the presence of microemulsion compositions corresponding to line 3 in Figure 1.
u t
M
0.5
0
6 50 0/-
0
-
40
i
Figure 6. Plot of the observed first-order rate constant for the acidcatalyzed hydrolysis of p-MFD (kots,me)in the presence of microemulsion compositions corresponding to line I in Figure 1.
Conductivity behaviors of emulsions and microemulsions have been studied with various models,23,27J3-36 since conductivity measurements are an important tool to identify the type of microemulsion droplets. When the water concentration is low, the microemulsion can be oil-continuous and the conductivity goes to low values. The water droplets behave as the electrically conducting constituent in the system and are isolated by an oil continuous phase. If the volume fraction of water increases, the conductivity increases and the conductivity droplets begin to contact each other and form conductive elements. Variations on the anionic content reflect on the Conductivity. Basically, we related the conductivity changes with the degree of dissociation of the surfactant that contributes with ionic components (sodium ions from SDS) to the water phase. The degree of dissociation changes because the concentration of I-butanol in the interphase decreases as a result of the solubilization of I-butanol in the oil phase, which is increasing in volume fraction. The a values obtained with this approach are consistent with values reported in the literature for similar systems.4~20~23,32 The a. value used in the simulations ( a , = 0.52) yields a values varying from 0.52 (h = 0) to 0.05 ( c $ ~= 0.75). These numbers are not far from those reported in the literature. Bunton and B ~ z z a c c a r i n found i~~ that the addition of I-butanol to aqueous cetyltrimethylammonium bromide (CTAB) increases a. Nieuwkoop and SnoeiZ3investigated changes in the mixture SDS/ I-butanol/water in various concentrations of alcohol and observed that a increased considerably with increasing amounts of 1-butanol, the detergent being almost completely dissociated with 3 vol % alcohol. Thus, with the a values calculated by means of eq 18, it is possible to fit the kinetic data according to eq 17 and accordingly calculate POiiand k2,i values. Figure 5 contains the fitting of the experimental data obtained under the conditions described by X I , X 2 , A',, and X4 in Figure I . In all cases, there is an excellent correlation between the ( 3 3 ) Lagourette, B.; Peyrelasse, J.; Boned, C.; Clausse, M. Nature 1979,
281. 60.
(34) Lagues, M.; Sauterey, C. J . Phys. Chem. 1980.84, 3503. (35) Dvolaitzky, M.; Lagues, M.; Le Pesant, J. P.: Ober, R.; Sauterey, C.; Taupin, C . J . Phys. Chem. 1980, 84, 1532. (36) Fang, J.; Venable, R. L. J . Colloid Inferface Sci. 1987, 116, 269.
$0
Figure 8. Plot of the oserved first-order rate constant for the acid-catalyzed hydrolysis of p-MFD (kobo,mc)in the presence of microemulsion compositions corresponding to line 7 in Figure 1.
calculated line and the experimental points (data taken from Table 11). Table VI contains the best values obtained from the fitting of the experimental data. The Po , value (0.25) shows that the substrate p-MFD has a slight prekrence for the oil phase over the interphase, a reasonable result since p-MFD is a fairly hydrophobic substrate. The important fact is that the same value of Poiifitted all the kinetic data over a large interval of oil in the phase diagram (+, varies from 0 to 0.75). Figures 6-8 contain the fits of the experimental data of Tables 111-V from eq 17. As can be seen, reasonable fits are obtained for all cases. Indeed, the data fitted in Figures 6-8 represent lines 1, 3, and 7 in Figure 1. Clearly, eq 17, though a rather simple expression, can fit experimental data under a wide variety of conditions over the entire phase diagram. Indeed, Table VI shows that, despite the complexity of the system, within limits of experimental error, single values of Poiiand k2jcan be used to explain the reactivity of p-MFD in our system. Basically, the value of k2,iis about 3.5 times larger than in micelles and about equal to that obtained in water. In conclusion, with use of the pseudophase ion-exchange model, it is possible to explain the results obtained for the acid hydrolysis of p-MFD over a wide range of compositions for the 1-butanol/SDS/water/toluene microemulsion system. Not surprisingly, our results are consistent with the normal finding in micellar catalysis, that most of the experimentallyobserved catalytic activity corresponds to an increase in local concentration of the reactive partners at the charged interphase.
J . Phys. Chem. 1990, 94, 361-370
We are currently investigating cationic microemulsion systems in order to extend the application of the pseudophase ion-exchange model and the concept of variable CY.
Acknowledgment. W e are grateful to CNPq (Conselho Na-
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cional de Desenvolvimento Cientifico e Tecnologico, Brazil) and to F I N E P (Financiadora de Estudos e Projetos) for financial support of this work. Registry No. 2-@-MethoxyphenyI)-1,3-dioxolane, 2403-50-1.
Thermodynamic and Modelistic Study of Surface Solutions: Aqueous Solutions Containing 2-Butanol P. Nikitas* and A. Pappa-Louisi Laboratory of Physical Chemistry, Department of Chemistry, University of Thessaloniki, 54006 Thessaloniki, Greece (Received: February 10, 1989; I n Final Form: May 1 1 , 1989)
The surface properties of dilute aqueous solutions containing 2-butanol have been studied by means of surface tension measurements in the temperature range 10-35 OC. The experimental data have been analyzed both at a macroscopic level by means of thermodynamic arguments and at a molecular level on the basis of a statistical model proposed. The thermodynamic treatment was directed to the determination of the size ratio parameter n, which was found to affect decisively the results. The method presented allows the experimental determination of n with acceptable accuracy provided that the surface activity coefficient of the adsorbate is equal to unity at high surface coverages. For aqueous solutions of 2-butano1, we found the value n = 1 h 0.5, which indicates that the water molecules in the surface solution are associated in clusters. The system exhibits small positive deviations from Raoult's law. The excess free energy of mixing increases with increasing temperature, leading to small negative values of the excess entropy of mixing. The surface properties were also analyzed in terms of competition for adsorption between solute and solvent molecules. Thus, the observed decrease of 2-butanol adsorption at the mercury/solution interface is attributed to the increase of the water adsorbability at this interface due to image forces. In addition, strict thermodynamic relationships for the determination of standard free energies of adsorption are proposed. Finally, a simple statistical mechanical model, which is based on lattice statistics and takes into account both normal and in-plane dipole interactions, is presented. It was found that it represents adequately the experimental behavior only when n = 1. The deficiencies of the model and its reliability are discussed.
1. Introduction The properties of surface solutions, either electrified or not, in the presence of organic substances have been the object of a great number of studies. However, this subject is still of current interest, since certain problems concerning the composition of the surface solution, its behavior as a bulk one, and the interpretation of the equilibrium between bulk and surface solution in terms of microscopic models remain open. Recently, a thermodynamic method that allows the experimental determination of the size ratio parameter, Le., the ratio of the molar surface volumes of the constituents of a surface, has been proposed.' The size ratio parameter is necessary for the calculation of the surface composition in terms of molar fraction. In addition, it gives useful information for the surface behavior of the solvent, whether its molecules are associated in clusters or not. Thus, an experimental determination of the size ratio parameter allows the rigorous thermodynamic interpretation of the experimental data and can reduce the number of a priori assumptions, which are necessary for the microscopic modeling of surface solutions. For the latter, significant developments have been reported during the past decade for the case of charged interfaces2-10 ( I ) Nikitas, P. J . Electroanal. Chem. 1988, 263, 147. (2) Sangaranarayanan, M. V.; Rangarajan, S. K. J . Elecrroanal. Chem. 1984, 176, I . ( 3 ) Sangaranarayanan, M. V.; Rangarajan, S. K. J . Electroanal. Chem. 1984. 176, 29. (4) Sangaranarayanan, M. V.; Rangaraian, S. K. J . Electroanal. Chem. . 1984, 176,45. ( 5 ) Guidelli, R. J . Electroanal. Chem. 1980, 110, 205. (6) Guidelli, R. J . Electroanal. Chem. 1981, 123. 59. (7) Guidelli, R.; Foresti, M. L. J . Electroanal. Chem. 1986, 197, 103. (8) Nikitas, P. J . Chem. Soc., Faraday Trans. I 1986, 82, 977 (9) Nikitas, P. J . Phys. Chem. 1987, 91, 101. (IO) Nikitas, P. Electrochim. Acra 1987, 32, 205.
0022-3654/90/2094-036 1$02.50/0
TABLE I: Surface Tensions of Water (mN Temperatures
m-l)
at Different
t , "C
ref present results 17
25 30 35 71.18 70.35 74.33 73.53 72.76 72.02 74.22 73.49 72.75 71.97 71.18 70.38 IO
15
20
In the present paper, we attempt to study the properties of aqueous solutions in the presence of 2-butano1, taking into account the above developments. This system was chosen because the corresponding charged interface formed between a mercury electrode and electrolyte aqueous solutions of Na2S04containing 2-butanol has been thoroughly studied by means of very precise electrocapillary measurements.11,12 Therefore, we can compare the surface behavior of water and 2-butanol in the two interfaces. In addition, Parsons13 has announced that Mohilner had made precise measurements of the adsorption of 2-butanol on the mercury electrode as a function of temperature. Unfortunately, these measurements have not been published yet. However, we know from Parsons13 that the interpretation of these measurements by treating the interface as a two-dimensional binary solution leads to positive deviations from Raoult's law associated with a strongly negative excess entropy. On the basis of these findings, Parsons concluded that the random mixing assumption, adopted in a number of microscopic models of surface solutions, is untendable for this system and that it is likely that the same behavior may be valid for other similar systems. These interesting results were motivation for us to study the surface behavior of the aqueous (1 1) Nakadomari, H.; Mohilner, D. M.; Mohilner, P. R. J . Phys. Chem. 1976, 80, 1761. (12) Mohilner, D. M.; Nakadomari, H.; Mohilner, P. R. J . Phys. Chem. 1977, 81, 244. (13) Parsons. Electrochim. Acta 1984, 29, 1563.
0 1990 American Chemical Society
