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Thermodynamics of Formation of Biological Microemulsion (with Cinnamic Alcohol, Aerosol OT, Tween 20, and Water) and Kinetics of Alkaline Fading of Crystal Violet in Them
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
186, 1–8 (1997)
CS964554
Thermodynamics of Formation of Biological Microemulsion (with Cinnamic Alcohol, Aerosol OT, Tween 20, and Water) and Kinetics of Alkaline Fading of Crystal Violet in Them LANA MUKHOPADHYAY, NITA MITRA, PRANAB K. BHATTACHARYA,
AND
SATYA P. MOULIK1
Centre for Surface Science, Department of Chemistry, Jadavpur University, Calcutta 700032, India Received April 10, 1995; accepted August 19, 1996
measured the enthalpies of solution of water in chloroform as well as chloroform in water aided by cationic, anionic, and nonionic surfactants. Thermal inversion for both water and oil dispersions near the phase separation point for certain compositions has been observed. Chemical and biochemical reactions in nonbiological microemulsions have been studied moderately (13–17). Inversion of sucrose and hydrolysis of p-nitrophenyl phosphate with alkaline phosphatase in such microemulsion media have been reported from our laboratory (18, 19). Alkaline hydrolysis of crystal violet in microemulsion media has been studied on a number of occasions (20–25); a recent study in aqueous and nonaqueous microemulsion media has been reported by us (26). Kinetic studies in biological microemulsion medium are rare in literature. Moberger and Larsson (5) have reported the kinetics of oxidation of fats in biological microemulsion containing vegetable oil. In this presentation, we report the energetics of the formation of biological microemulsions (of both w/o and o/w types of cinnamic alcohol/AOT/water and cinnamic alcohol/TW20/water combinations) along with their measured specific heats. Cinnamic alcohol is a material of plant origin and the mutual solubility with water is negligible. It is, therefore, considered an oil (water immiscible organic liquid) following the same rationale of considering water-insoluble alcohols (carbon numbers 6–10) as oils in microemulsion study (27). The kinetics of the alkaline fading of the dye crystal violet in both types of microemulsion also has been reported together with the activation parameters for the rate process and the salt effect on it. Since the systems studied are a class by themselves, we present a composite account of their phase behaviors, thermodynamics of formation, and behaviors as reaction media.
The ternary phase diagrams for the formation of biological microemulsions of the combinations CA/AOT/water and CA/Tween 20/water have been presented. The thermodynamics of solution of water in AOT/CA as well as in Tween 20/CA forming w/o microemulsions and solution of oil in Tween 20/water forming o/w microemulsions have been calorimetrically studied. The solution processes are essentially exothermic (with a few exceptions) and have yielded negative entropies, i.e., producing an ordering effect. This has been supported by the measured specific heats of the resulting solution. The pseudo-first-order rate constants (k1) for the alkaline fading of crystal violet in w/o microemulsion medium for the CA/AOT/water and CA/TW 20/water have been found to depend on [water]/[AOT] mole ratio (v) with maxima at v Å 10. For o/w preparations of CA/TW-20/water, k1 has shown maximum at [CA]/[TW-20] mole ratio v* Å 2, whereas the k2 value has increased with v*. The salt effect on the reaction has shown significant deviation from expectation in w/o microemulsion. The free energies of activation for the w/o and o/w systems are of the same order, whereas both DH ‡ and DS ‡ values differ significantly. q 1997 Academic Press Key Words: biological microemulsion; thermodynamics of formation; kinetics of CV/ 0 OH0 reaction; AOT; Tween 20.
INTRODUCTION
Formation of biological microemulsions containing vegetable oil has been reported recently (1–5). To understand the formation and stability of biological microemulsion, energetic information is necessary. Thermodynamic studies on microemulsion formation are not plentiful in literature (6, 7). The heat change due to dissolution of water in AOT/ heptane mixture has been determined calorimetrically (8); the enthalpy of solution of water has been found to be endothermic. Endothermic heat of dissolution of water in AOT/oil mixture has been found to be strikingly dependent on the water/ AOT mole ratio. Recently (12), we have calorimetrically 1
EXPERIMENTAL
Materials The surfactant AOT [Na salt of sulfosuccinic acid bis] (2-ethylhexyl ester) and TW-20 (polyoxyethylene sorbitan
To whom correspondence should be addressed. 1
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0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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FIG. 2 Thermograms of microemulsion systems at 303K. 1, CA/AOT/ Water (W/O), v Å 16.8; 2, CA/TW-20/Water (O/W), v* Å 2.3; 3 and 4, CA/ TW-20/Water (W/O), v Å 119 and 35.1 respectively. Scale: Ordinate 2.55 cm. Å 1 mV; Abcissa, 1 cm. Å 1 min. S: Start of a run, E: end of a run. FIG. 1 Ternary phase diagrams of CA/AOT/Water and CA/TW-20/ Water at 303K. 1f: monophasic region; 2f: biophasic region. Full line indicates CA/AOT/Water system; broken line indicates CA/AOT/Water in presence of cholesterol and butanol.
monolaurate) were the same as those samples (Sigma) used earlier (28). The oil cinnamic alcohol (CA) having a refractive index 1.5819 and b.p. 2577C was from Fluka (Switzerland). The dye crystal violet (CV), sodium chloride, and sodium hydroxide were Excellar grade BDH products. The bile salt, sodium cholate (NaC), was a 98% pure Sigma product. Doubly distilled conductivity water was used throughout the study. Method Calorimetric measurements. Thermometric measurements were taken at 298 K in a Tronac Model 458 automatic titration calorimeter. The calibration method used was as described earlier (28). The instrument was accurate to the extent of 0.5% on 2 calories. In the actual experiment, 20 ml of titrant mixture was taken in the reaction vessel and either water or oil was taken in the burette. The whole system was immersed in the 60liter capacity water maintained at temperature 298 { 0.0002 K. After the attainment of temperature equilibrium the titre was added to the titrant mixture at the rate of 0.316 ml/min under constant stirring until the limit of phase separation was reached. The heat change of electrical equivalents was recorded with time with a Houston Instruments (Omniscribe
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D5000) strip chart recorder and processed in the usual way (28). The observed heat change per mole of water or CA was considered as the enthalpy of the solubilization process. For kinetic measurements, CA/AOT/water (w/o) and CA/ TW-20/water (both w/o and o/w) microemulsion (mE) were used as the media for the kinetic study. A general method TABLE 1 Thermodynamic Parameters for Water Dissolution in CA/AOT and CA/TW-20 Media as a Function of Their Composition and H2O/Amphiphile Mole Ratio (v) at 303 K Composition (wt%) oil/ amphiphile/H2O
DGs7/kJ mol01
v
DH s7/kJ mol01
DS 7/JK01 mol01
Cp /J/gm/7C obs./calc.
00.9 01.9 01.5 02.3
3.5/2.7 2.7/2.7 2.8/2.7 2.2/2.7
04.46 04.89 04.96 02.11 02.07 05.21 05.96
3.2/2.9 2.9/2.8 2.7/2.9 3.1/2.8 3.0/2.8 2.8/2.8 2.6/2.7
CA/AOT/water (w/o) 82/9/9 63/22/15 89/5/6 76/13/11
16.8 20.9 24.2 29.6
1.22 1.71 2.08 2.78
0.94 1.14 1.60 2.09
CA/TW-20/water (w/o) 50/33/17 55/30/15 62/26/12 67/22/11 71/18/11 77/13/10 89/4/7
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35.1 34.0 31.4 34.0 41.6 52.4 119
0.89 1.05 1.37 1.54 1.60 1.81 2.51
00.464 00.426 00.130 /0.904 /0.973 /0.231 /0.704
FORMATION OF BIOLOGICAL MICROEMULSION
TABLE 2 Thermodynamic Parameters for Oil Dissolution in Water/TW20 Media as a Function of Their Composition and CA/TW-20 Mole Ratio (v*) at 303 K Composition (wt%)
v*
DGs7/ kJmol01
DH s7/ kJmol01
DS s7/ JK01 mol01
Cp /J/gm/7C obs./calc.
CAT/TW-20/water (o/w) 5/19/76 6/23/71 8/28/64 9/32/59 13/39/48
2.73 2.3 2.6 2.6 2.9
11.9 11.3 10.3 9.8 8.4
4.8 5.8 5.6 6.7 6.4
023.4 018.3 015.7 010.2 06.8
4.4/3.6 3.8/3.4 4.2/4.0 6.1/3.4 5.9/3.3
of preparation of mE was reported earlier (19). Sodium hydroxide solution (1 mol dm03) was prepared in aqueous medium. CV has a very high solubility in CA. It was observed that after vigorous shaking in the presence of CA and water, the dye remains in the CA medium. The dye solution (1 m mol dm03) was, therefore, prepared in CA. The overall concentration of CV was 15 m mol dm03 and that of NaOH was 20 m mol dm03 in all the mE media. For the study of salt effects, both NaCl and NaC were prepared in aqueous medium. Reaction kinetics were studied by spectrophotometric method in a Shimadzu (Japan) 160A UV–Vis spectrophotometer, used as described earlier (26). Temperature measurements were taken at 298–312 K, maintained by circulating water from a thermostated water bath around the cells with temperature uncertainties of {0.017. Eight ternary mixtures of CA/AOT/water (w/o), seven and nine ternary mixtures of CA/TW-20/water of o/w and w/o, respectively, were used as the microemulsion reaction media. For the detailed study of temperature and salt effects, CA/TW-20/water combinations in weight percents 5/30/65 and 65/30/5 were used. The spectral measurements were
3
taken at lmax of CV at 590 and 600 nm for aqueous and mE media, respectively. RESULTS AND DISCUSSION
Phase Diagram The ternary phase diagrams of the systems CA/AOT/water and CA/TW-20/water are presented in Fig. 1. The phase boundaries are on the whole symmetric. No triphasic zone has been observed. A greater single-phase mE zone has been obtained with AOT as amphiphile compared to TW-20. The plait point P corresponds to 30% (w/w) AOT in the mixture which is 50% for TW-20. The additives cholesterol and butanol (at 1:1 weight ratio with AOT) have reduced the mE zone and raised the plait point to the 40% level. AOT alone is a better microemulsifier than in combination with butanol. Realization of appreciable single phase mE zone advocates potential use of CA in the preparation of microheterogeneous dispersions. Calorimetric Observations and Energetics Typical thermograms for CA/AOT/water and CA/TW-20/ water systems are presented in Fig. 2. The enthalpy and heat capacities are shown in Tables 1 and 2. The w/o microemulsion formation at different v values have been found to be all exothermic in nature. The enthalpy of solution (DHs) of water in CA / AOT combinations has increased with v and passed through a maximum at v á 18 (Fig. 3A). The initial increase is rapid compared to the fall beyond the maximum. The addition of water in the CA / amphiphile mixture initially forms reverse micelles of very small water core that grow in size. Species called microemulsion is formed. The solubilization process yielding w/o microemulsion continues up to the phase transition point. The measured enthalpy thus represents the overall associated heat
FIG. 3 (A) Enthalpy of dissolution of water (DH s7) as a function of v in the CA/TW-20/water (w/o) system. (B) Variation of DH s7 with v in CA/ TW-20/H2O (w/o) system. (C) Dependence of DH s7 on v in CA/TW-20/H2O (o/w) system.
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FIG. 4 ln A0 /At vs time plot in different microenvironments at 298K: (l) CA/TW-20/H2O (w/o), v Å 11.4; (s) CA/AOT/H2O (w/o), v Å 4.1; (n) CA/TW-20/H2O (o/w), v* Å 1.5. Scales are indicated on the graphs.
up to the phase separation point per mol of water addition and stands for the enthalpy of solution of water in CA / amphiphile medium. For CA solubilization in water / amphiphile mixture producing o/w microemulsion, the enthalpy is expressed per mol of CA addition. This aspect will be further discussed at the end of the section in relation to the thermodynamic analysis. The physicochemical state of the microwater pool for AOT and other amphiphile-aided microemulsions is considered to be different (29) beyond v § 10. The observed maximum at v Å 18 in this report is appreciably greater than 10, the pool-size-dependent water state does not have a direct bearing on the observed maximum in DHs. The CA/TW-20/water systems have been studied with reference to the formation of both w/o and o/w microemulsion. The v values for the studied w/o cases are considerably higher; lower values are restricted by the phase diagram (the studied end points are shown on the diagram as full circles). The solution process of water in CA/TW-20 medium has been exothermic up to v Å 32, beyond which it is endothermic (Fig. 3B). The changed sign of the thermometric event is a striking manifestation of the ternary system. The dispersion of water in the binary mixtures of CA and TW-20 have shown conspicuous energetic manifestations at considerably higher levels of water addition.
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The o/w dispersions of the CA/TW-20/water ternary system have manifested endothermicity with a maximum at v* Å 2 (Fig. 3C). The changed thermodynamic activity at low CA/amphiphile mole ratio is again a striking phenomenon seldom manifested in the field of microemulsion study (6– 12). Thermometric studies on microemulsification of oil in water are very rare in literature (26). The results herein presented are therefore important; the use of CA as oil has made the study special. The exothermicity and the maximum in the formation of both w/o and o/w mE for CA/AOT/water and CA/TW-20/ water systems, respectively, show parallel manifestations. Identification of the types of interaction leading to the final results is not easy. Semiempirical attempts have been made in the past for deciphering and quantification (27, 28, 30). Since the energetics of pairwise interactions among the three component systems herein studied are not known, we refrain from attempting to elaborate the inner mechanism for the enthalpic observations but only add a comment. At the interface between oil and water, organization of amphiphiles contributes a major share to the overall heat making the process exothermic. Its final balance with other interactions ends up in maximum DH 7s at a particular v. At higher v, the exothermicity becomes prominent, which is also observed in w/o mE formation for the ternary CA/TW-20/water system, although the solubilization process is endothermic at lower v. Following the procedure described in a previous study (3), a model-independent thermodynamic analysis of the solubilization processes herein studied has been presented as follows. At the turbidity or phase separation point the dispersed phase (in mole fraction (Xs)) is at the state of its maximum solubility. Considering the dispersed phase (water or CA) as the solute, and the solution is ideal, the standard free energy of solution (DG7s ) to reach the turbid or phase separation point is given by the relation (3) DG7s Å 0RT ln Xs,
[1]
where Xs is the mole fraction concentration of the dispersed phase and R and T have their usual significance. The standard enthalpy of the solubilization process has been calorimetrically obtained; the standard entropy of solution DS 7s then follows from the relation, DS 7s Å (DH 7s 0 DG7s )/T.
[2]
All the thermodynamic parameters obtained for the w/o and o/w systems studied are given in Tables 1 and 2. They can be recognized as the energetic parameters for microemulsion formation. These include all sorts of physical–chemical interactions for the dispersion and stabilization of the dispersing solvent. Like hydroxylic amphiphiles, CA can undergo
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FIG. 5 (A) Variation of first-order rate constant (k1) with v in CA/AOT/H2O (w/o) system at 298 K. (B) Dependence of k1 on v in CA/TW-20/H2O system (w/o) at 298 K. (C) k1 vs v* plot for CA/TW-20/H2O (o/w) system at 298 K.
both dipolar and hydrophobic interactions with water and TW-20 during the solubilization process, thereby contributing a fair share to the energetics. The thermodynamic parameters for the CA/AOT/water and CA/TW-20/water systems given in Table 1 are all significantly low. They have a regular trend with v, the entropies are all negative. The results are on the whole comparable with the reported alkanol/AOT/water system (30). The enthalpies of solution of water in heptane/AOT binary mixtures (8) have been found to be two- or three-fold greater than the present results. Several w/o combinations of CA/TW20/water system on the lower side of v have shown endothermicity, the rest are exothermic. Exothermicity of solubilization of water in a Triton X-100/butanol/heptane ternary mixture has been reported in literature (28). The thermodynamic parameters for CA solubilization in water/TW-20 mixtures are all higher; the DS 7s values are again negative during solubilization, the system is initially disrupted and finally stabilized to end up in a more ordered state, making the entropy values negative. The solubilization of oil and the final ordering is akin to ‘‘iceberg’’ formation according to Frank’s proposition (31) that produces DS 7s Å 0(20–25) J mol01 K01. The DS 7s values given in Table 2 head toward the above limit of DS 7s at low v. The calculated specific heats, on the basis of ideal mixing for all the studied systems, are lower than the experimental values. They are strikingly different at the studied higher [CA]/[TW-20] mole ratios for the o/w systems. No system-
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atic trends in Cp values with v for w/o systems of heptane/ AOT/water have been observed (8). Lower calculated Cp values than those observed also have been found for w/o a microemulsion system of heptane/TX-100/butanol/water (28). Cp values higher than the ideal mixing advocate organization of the components resulting in ordering of the microheterogeneous state of the studied ternary systems. This corroborates the findings of negative entropy of solution of both water in oil or oil in water in the presence of both AOT and TW-20. Why it is prominent for the o/w dispersions at higher v than in w/o dispersions needs close examination. Rate Constants of CV / 0 OH 0 Kinetics in mE Medium The pseudo-first-order rate constants for the alkaline fading of crystal violet in both the ionic and nonionic surfactantaided microemulsion media have been determined. The rate constant (k1) has been calculated from the slope of the straight line plots between ln A0 /At and time (t) in accordance with the previous report (26) (Fig. 4). The rate constants have been found to vary nonlinearly with v. A dome-shaped dependence has been obtained in the case of w/o microemulsion containing AOT (Fig. 5A) with a maximum at v Å 10. A sharp rise also at v Å 10 has been observed for the CA/ TW-20/water w/o microemulsion system (Fig. 5B). For the o/w microemulsion containing TW-20, the rate constants remain almost invariant for v* above 2 (Fig. 5C). The results are presented in Table 3. The k1 values on the studied w/o
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TABLE 3 The Pseudo- First-Order and Second-Order Rate Constants of CV/-OH0 Reaction in Microemulsion Media at 298 K v
[NaOH] pool/mol dm03
k1 1 102/s01
k2 1 102/s01 mol01 dm3
CA/AOT/water (w/o) 3.5 4.1 7.0 8.2 10.5 12.4 14.1 17.6
2.1 2.6 2.8 2.8 2.9 2.5 2.1 0.7
4.5 6.8 9.1 9.7 11.4 13.9 15.9 17.0 22.8
0.50 0.7 1.6 5.6 3.2 3.0 2.7 2.8 1.8
0.352 0.352 0.177 0.178 0.118 0.148 0.088 0.07
5.9 7.4 15.8 15.7 24.6 16.9 23.8 9.5
CA/TW-20/water (w/o) 0.356 0.336 0.336 0.356 0.356 0.356 0.356 0.356 0.356
1.5 1.9 4.5 15.7 8.9 8.4 7.6 7.8 5.1
CA/TW-20/water (o/w) v* 4.6 2.3 1.9 1.5 1.3 1.1 1.02
3.6 3.7 4.0 3.4 2.7 2.2 2.0
0.023 0.024 0.028 0.030 0.032 0.035 0.038
156 154 142 114 84 63 53
and o/w media have been found to be comparable. v Å 10 closely corresponds to the maximum solvation of AOT anion and the cation in the microwater pool (8), the physical state of water manifests distinct departure beyond this composition. Different physicochemical processes have been reported (19, 22) to undergo appreciable changes at v ú 10, where the dispersed water tends to assume the properties of bulk water. However, the observed maximization of k1 is a rare event in compartmentalized liquids. The second-order rate constants and local concentration of OH0 in the water pool of different microemulsion compositions of the ternary CA/AOT/water and CA/TW-20/water systems are also given in Table 3. According to the principle of chemical kinetics, k1 and k2 for the CV/ 0 OH0 reaction should be independent of [OH0]. But in the studied mE media, the k1 and k2 values have been found to depend on both v and [OH0] in the microaqueous pool. Since the variation of [OH0] is not drastic in the CA/AOT/water system, and [OH0] is kept constant
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in the CA/TW-20/water system, the dimension of the aqueous pool in mE is considered to control the physical–chemical state of water (29) and to guide the kinetic process (18, 19, 32). We are not in favor of seeking an apparent correlation between the rate constant and [OH0]. In o/w mE medium of the CA/TW-20/water system, although OH0 ions are not in the oil pool, both k1 and k2 have decreased with [OH0]. This dependence originates from the influence on the reaction kinetics by the water/oil interface where CV/ preferentially resides. The second-order rate constants for the CA/AOT/water (w/o) system are higher than those for CA/TW-20/water (w/o). The k2 values in o/w medium are higher than those in w/o medium. The interior environment of the microwater pool with AOT has lower activation barrier than with TW20. On a molecular basis, AOT0 and its Na/ ion can be solvated by 12 (6 for AOT0 and 6 for Na/) water molecules (8). The TW-20 molecule with its 20 ethylene oxide containing head groups can attach 40 water molecules (2 for each oxygen center in the ethylene oxide moiety (32)). Therefore, at equal v, the hydration of TW-20 is 3.3 times more than that of AOT, and the w/o interface is more rigid with TW-20 as compared to AOT. The barrier for the formation of the activated complex is thus greater with TW-20 than that with AOT. Further, the local polarity at the interface with AOT (being an electrolyte) is different from that with TW-20. This also helps to lower the activation barrier for the ionic reaction between CV/ and OH0 in AOT-aided compartmentalized environments. Activation Parameters The energetics of activation process for the fading kinetics have been estimated by processing the temperature effect on the rate constant according to the Arrhenius and Eyring’s hypothesis (Fig. 6). In the narrow range of temperature variation, the composition of m E has been assumed to remain unchanged. The parameters for w/o and o/w environments produced by CA/TW-20/water microemulsions are given in Table 4. The enthalpies and free energies of activation are positive for both systems. The enthalpy of activation for the o/w system is higher than that for the w/o system, while the free energy of activation is almost the same. The entropy of activation is negative and is large for the w/o system. The higher negative entropy has supported more stabilized activation complex in the water pool of the microemulsion. The enthalpy – entropy compensation phenomenon (26) has been envisaged. The DG ‡ values support comparable spontaneity of the formation of the activation complex for both w/o and o/w systems, whereas weightage of entropy is more for the latter. The Salt Effect The effect of ionic strength (m) on the rate constant (k1) of the CV–NaOH reaction in the mE media has been analyzed
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q
FIG. 6 ln k vs T01 plot in CA/TW-20/H2O system: (s) w/o and (n) o/w system at v Å 11.4 and v* Å 1.5, respectively.
(neglecting salt-induced composition change) and is presented in Fig. 7, where good linear dependence is observed. The Bro q ¨ nsted–Bjerrum equation, log k1 Å log(k0)1 / 1.018 ZAZB m (where k0 is the hypothetical rate constant at zero ionic strength and ZA, ZB are the charges on the reacting species CV/ and OH0 respectively) is, therefore, obeyed (Fig. 7). The variation of the rate constant with ionic strength for NaCl and NaC and the experimental slopes for the CA/ TW-20/water (w/o and o/w) systems are given in Table 5. For w/o and o/w systems, the slopes obtained are, respectively, 0.4 and 0.18; they are lower than the theoretical value of 1.018. Taking the experimentally determined (measured with a Delik impedence bridge, Japan) dielectric constant of CA Å 15, the theoretical slope of B–B plot becomes 11.86, which is much higher than the experimental value. The CV/ 0 OH0 reaction essentially takes place at the water–CA interface of the microdispersions (26). The lowering of the slope thus cannot be essentially accounted for by the bulk dielectric constant of CA. The effect of NaC, at different concentrations on the rate constant, is given in Table 6. In TABLE 4 Activation Parameters for the CV/-OH0 Reaction in w/o and o/w CA/TW-20/Water Microenvironments at 298 K Composition CA/TW-20/H2O
DG‡/kJ mol01
DH‡/kJ mol01
DS‡/kJ01 mol01
5/30/65 (v Å 1.5) o/w 65/30/5 (v* Å 11.4) w/o
81.0 { 4 81.4 { 4
38.8 { 2 7.2 { 0.4
0142 { 7 0249 { 12
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FIG. 7 Dependence of log k on m in CA/TW-20/H2O medium at 298 K. (s): v Å 11.4 and (n): v* Å 1.5.
aqueous medium, although the rate decreases in presence of NaC, it remains more or less independent at higher concentrations of NaC. A similar type of variation has been observed in o/w microemulsion medium. The observed magnitudes of decrease have been 4- and 2.6-fold for w/o and o/w media, respectively. With the variation in the concentrations of NaC from 0.05 to 0.3 mol dm03, the rate constant TABLE 5 Effect of NaCl on the Kinetics of CV/-OH0 Reaction in (w/o and o/w) CA/TW-20/Water Microemulsion Mediuma,b at 298 K [NaCl]/mol dm03
System
log k1
w/o system (m)pool 0 0.1 0.2 0.3 0.5 1.0
0.597 0.675 0.745 0.809 0.925 1.16
01.49 01.62 01.64 01.66 01.70 01.82
o/w system (m)Aq 0 0.1 0.2 0.3 0.5 1.0
0.17 0.36 0.48 0.57 0.73 1.01
01.42 01.44 01.47 01.48 01.52 01.58
a Percent composition of microemulsion: 65/30/5 as CA/TW-20/water (v Å 11.4); [NaOH] pool Å 0.36 mol dm03. b Percent composition of microemulsion: 5/30/65 as CA/TW-20/water (v* Å 1.5); [NaOH] Å 0.03 mol dm03.
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TABLE 6 The Effect of NaC on k1 in Different Environments at 298 K k1 1 103/s01 Microemulsion [NaC]/mol dm03
Aq.
w/oa
o/w b
0 0.01 0.02 0.05 0.10 0.20 0.30
3.22 — — 1.4 1.26 1.24 1.23
3.2 — — 8.9 8.1 1.45 1.12
8.00 3.70 3.70 3.00 3.00 — —
a b
Percent composition of microemulsion: 65/30/5 as CA/TW-20/water. Percent composition of microemulsion: 5/30/65 as CA/TW-20/water.
has decreased from 0.009 to 0.001 s01 in w/o microemulsion medium. Being surface active, NaC affects the interfacial properties of the microdroplets in the mE. It offers rigidity to the interface (33); the contact of reacting species becomes less efficient. This is of course a simplified explanation of the observed fact, which needs substantiation by further studies. Bearing of the Energetics of Solution on Activation Parameters The energetics of formation of w/o and o/w mE and the measured Cp values of the resulting systems have supported ordering or structuring of the dispersion environment (Tables 1 and 2). The ordering follows the sequence CA/AOT/water (w/o) õ CA/TW-20/Water (w/o) õ CA/TW-20/water (o/w). The stabilization of the CV/ at the water/CA (oil) interface in thus more in the o/w system than in the w/o system. This has a bearing on energetics of the CV/ 0 OH0 reaction kinetics. There the free energies of activation (DG‡) in CA/ AOT/water (w/o) and CA/TW-20/water (o/w) mE’s are the same (Table 4) whereas the entropies of activation are different (0249 and 0142 J mol01 K01, respectively). The more organized water/CA interfacial environment of the latter (Table 2) has provided additional stability to the activation complex to produce DG‡ of equal magnitude. ACKNOWLEDGMENT This work was financially supported by the Department of Science and Technology, Government of India.
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REFERENCES 1. Das, M. L., Bhattacharya, P. K., and Moulik, S. P., Indian J. Biochem. Biophys. 26, 24 (1989). 2. Das, M. L., Bhattacharya, P. K., and Moulik, S. P., Langmuir 7, 636 (1991). 3. Paul, B. K., Das, M. L., Mukherjee, D. C., and Moulik, S. P., Indian J. Chem. A 30, 328 (1991). 4. Paul, B. K., and Moulik, S. P., Indian J. Biochem. Biophys. 28, 174 (1991). 5. Moberger, L., and Larsson, K., J. Dispersion Sci. Tech. 8, 207 (1987). 6. Kertes, A. S., Chaston, S., and Lai, W. C., J. Colloid Interface Sci. 73, 94 (1980). 7. Lan, W. C., and Kertes, A. S., Colloids Surf. 4, 379 (1982). 8. Ray, S., Bisal, S. R., and Moulik, S. P., Langmuir 10, 2507 (1994). 9. Goto, A., Harada, S., Fujita, T., Nicra, Y., Yioshioka, H., and Kishimoto, H., Langmuir 9, 86 (1993). 10. Goto, A., Yioshioka, H., Kishimoto, H., and Fujita, T., Langmuir 8, 441 (1992). 11. D’Aprano, A., Lizzo, A., and TurcoLiveri, V. J., J. Phys. Chem. 91, 4749 (1987). 12. Mukherjee, K., Mukherjee, D. C., and Moulik, S. P., J. Colloid Interface Sci., submitted for publication. 13. Lopez, P., Rodriguez, A., Herrera, C. G., and Sanchez, F., J. Chem. Soc. Faraday Trans. 88, 2701 (1992). 14. Mishra, B. K., Valaulikar, B. S., Kanjappu, J. R., and Manohar, C., J. Colloid Interface Sci. 127, 373 (1989). 15. Fletcher, P. D. I., Rees, G. D., Robinson, B. H., and Freedman, R. B., Biochem. Biophys. Acta 832, 204 (1985). 16. Ruckenstein, E., and Karpe, P., J. Colloid Interface Sci. 139, 408 (1990). 17. Fletcher, P. D. I., Freedman, R. B., Mead, J., Old Field, C., and Robinson, B. H., Colloids Surf. 10, 193 (1984). 18. Das, M. L., Bhattacharya, P. K., and Moulik, S. P., Langmuir 6, 1591 (1990). 19. Gupta, S., Mukhopadhyay, L., and Moulik, S. P., Colloids Surf. B. 3, 191 (1994). 20. Valiente, M., and Rodenas, E., J. Colloid Interface Sci. 127, 522 (1989). 21. Valiente, M., and Rodenas, E., J. Phys. Chem. 95, 3368 (1991). 22. Leis, J., Mejoto, J. C., and Pena, M. E., Langmuir 9, 889 (1993). 23. Datta, J., Bhattacharya, A., and Kundu, K. K., Indian J. Chem. A 27, 115 (1988). 24. Valiente, M., and Rodenas, E., Langmuir 6, 775 (1990). 25. Valiente, M., and Rodenas, E., J. Colloid Interface Sci. 138, 299 (1990). 26. Mukherjee, L., Mitra, N., Bhattacharya, P. K., and Moulik, S. P., Langmuir 11, 2866 (1995). 27. Ray, S., and Moulik, S. P., Langmuir 10, 2511 (1994). 28. Moulik, S. P., Das, M. L., and Bhattacharya, P. K., Langmuir 8, 2135 (1992). 29. Moulik, S. P., Paul, B. K., and Mukherjee, D. C., J. Colloid Interface Sci. 161, 72 (1993) and references therein. 30. Ray, S., and Moulik, S. P., J. Colloid Interface Sci. 173, 28 (1995). 31. Frank, H. S., and Evans, M. W., J. Chem. Phys. 13, 507 (1945). 32. Moulik, S. P., Gupta, S., and Das, A. R., Die Macromol. Chem. 67, 556 (1988). 33. Mukhopadhyay, L., Bhattacharya, P. K., and Moulik, S. P., Indian J. Chem. A 32, 485 (1993).
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CS964554
Thermodynamics of Formation of Biological Microemulsion (with Cinnamic Alcohol, Aerosol OT, Tween 20, and Water) and Kinetics of Alkaline Fading of Crystal Violet in Them LANA MUKHOPADHYAY, NITA MITRA, PRANAB K. BHATTACHARYA,
AND
SATYA P. MOULIK1
Centre for Surface Science, Department of Chemistry, Jadavpur University, Calcutta 700032, India Received April 10, 1995; accepted August 19, 1996
measured the enthalpies of solution of water in chloroform as well as chloroform in water aided by cationic, anionic, and nonionic surfactants. Thermal inversion for both water and oil dispersions near the phase separation point for certain compositions has been observed. Chemical and biochemical reactions in nonbiological microemulsions have been studied moderately (13–17). Inversion of sucrose and hydrolysis of p-nitrophenyl phosphate with alkaline phosphatase in such microemulsion media have been reported from our laboratory (18, 19). Alkaline hydrolysis of crystal violet in microemulsion media has been studied on a number of occasions (20–25); a recent study in aqueous and nonaqueous microemulsion media has been reported by us (26). Kinetic studies in biological microemulsion medium are rare in literature. Moberger and Larsson (5) have reported the kinetics of oxidation of fats in biological microemulsion containing vegetable oil. In this presentation, we report the energetics of the formation of biological microemulsions (of both w/o and o/w types of cinnamic alcohol/AOT/water and cinnamic alcohol/TW20/water combinations) along with their measured specific heats. Cinnamic alcohol is a material of plant origin and the mutual solubility with water is negligible. It is, therefore, considered an oil (water immiscible organic liquid) following the same rationale of considering water-insoluble alcohols (carbon numbers 6–10) as oils in microemulsion study (27). The kinetics of the alkaline fading of the dye crystal violet in both types of microemulsion also has been reported together with the activation parameters for the rate process and the salt effect on it. Since the systems studied are a class by themselves, we present a composite account of their phase behaviors, thermodynamics of formation, and behaviors as reaction media.
The ternary phase diagrams for the formation of biological microemulsions of the combinations CA/AOT/water and CA/Tween 20/water have been presented. The thermodynamics of solution of water in AOT/CA as well as in Tween 20/CA forming w/o microemulsions and solution of oil in Tween 20/water forming o/w microemulsions have been calorimetrically studied. The solution processes are essentially exothermic (with a few exceptions) and have yielded negative entropies, i.e., producing an ordering effect. This has been supported by the measured specific heats of the resulting solution. The pseudo-first-order rate constants (k1) for the alkaline fading of crystal violet in w/o microemulsion medium for the CA/AOT/water and CA/TW 20/water have been found to depend on [water]/[AOT] mole ratio (v) with maxima at v Å 10. For o/w preparations of CA/TW-20/water, k1 has shown maximum at [CA]/[TW-20] mole ratio v* Å 2, whereas the k2 value has increased with v*. The salt effect on the reaction has shown significant deviation from expectation in w/o microemulsion. The free energies of activation for the w/o and o/w systems are of the same order, whereas both DH ‡ and DS ‡ values differ significantly. q 1997 Academic Press Key Words: biological microemulsion; thermodynamics of formation; kinetics of CV/ 0 OH0 reaction; AOT; Tween 20.
INTRODUCTION
Formation of biological microemulsions containing vegetable oil has been reported recently (1–5). To understand the formation and stability of biological microemulsion, energetic information is necessary. Thermodynamic studies on microemulsion formation are not plentiful in literature (6, 7). The heat change due to dissolution of water in AOT/ heptane mixture has been determined calorimetrically (8); the enthalpy of solution of water has been found to be endothermic. Endothermic heat of dissolution of water in AOT/oil mixture has been found to be strikingly dependent on the water/ AOT mole ratio. Recently (12), we have calorimetrically 1
EXPERIMENTAL
Materials The surfactant AOT [Na salt of sulfosuccinic acid bis] (2-ethylhexyl ester) and TW-20 (polyoxyethylene sorbitan
To whom correspondence should be addressed. 1
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0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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FIG. 2 Thermograms of microemulsion systems at 303K. 1, CA/AOT/ Water (W/O), v Å 16.8; 2, CA/TW-20/Water (O/W), v* Å 2.3; 3 and 4, CA/ TW-20/Water (W/O), v Å 119 and 35.1 respectively. Scale: Ordinate 2.55 cm. Å 1 mV; Abcissa, 1 cm. Å 1 min. S: Start of a run, E: end of a run. FIG. 1 Ternary phase diagrams of CA/AOT/Water and CA/TW-20/ Water at 303K. 1f: monophasic region; 2f: biophasic region. Full line indicates CA/AOT/Water system; broken line indicates CA/AOT/Water in presence of cholesterol and butanol.
monolaurate) were the same as those samples (Sigma) used earlier (28). The oil cinnamic alcohol (CA) having a refractive index 1.5819 and b.p. 2577C was from Fluka (Switzerland). The dye crystal violet (CV), sodium chloride, and sodium hydroxide were Excellar grade BDH products. The bile salt, sodium cholate (NaC), was a 98% pure Sigma product. Doubly distilled conductivity water was used throughout the study. Method Calorimetric measurements. Thermometric measurements were taken at 298 K in a Tronac Model 458 automatic titration calorimeter. The calibration method used was as described earlier (28). The instrument was accurate to the extent of 0.5% on 2 calories. In the actual experiment, 20 ml of titrant mixture was taken in the reaction vessel and either water or oil was taken in the burette. The whole system was immersed in the 60liter capacity water maintained at temperature 298 { 0.0002 K. After the attainment of temperature equilibrium the titre was added to the titrant mixture at the rate of 0.316 ml/min under constant stirring until the limit of phase separation was reached. The heat change of electrical equivalents was recorded with time with a Houston Instruments (Omniscribe
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D5000) strip chart recorder and processed in the usual way (28). The observed heat change per mole of water or CA was considered as the enthalpy of the solubilization process. For kinetic measurements, CA/AOT/water (w/o) and CA/ TW-20/water (both w/o and o/w) microemulsion (mE) were used as the media for the kinetic study. A general method TABLE 1 Thermodynamic Parameters for Water Dissolution in CA/AOT and CA/TW-20 Media as a Function of Their Composition and H2O/Amphiphile Mole Ratio (v) at 303 K Composition (wt%) oil/ amphiphile/H2O
DGs7/kJ mol01
v
DH s7/kJ mol01
DS 7/JK01 mol01
Cp /J/gm/7C obs./calc.
00.9 01.9 01.5 02.3
3.5/2.7 2.7/2.7 2.8/2.7 2.2/2.7
04.46 04.89 04.96 02.11 02.07 05.21 05.96
3.2/2.9 2.9/2.8 2.7/2.9 3.1/2.8 3.0/2.8 2.8/2.8 2.6/2.7
CA/AOT/water (w/o) 82/9/9 63/22/15 89/5/6 76/13/11
16.8 20.9 24.2 29.6
1.22 1.71 2.08 2.78
0.94 1.14 1.60 2.09
CA/TW-20/water (w/o) 50/33/17 55/30/15 62/26/12 67/22/11 71/18/11 77/13/10 89/4/7
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35.1 34.0 31.4 34.0 41.6 52.4 119
0.89 1.05 1.37 1.54 1.60 1.81 2.51
00.464 00.426 00.130 /0.904 /0.973 /0.231 /0.704
FORMATION OF BIOLOGICAL MICROEMULSION
TABLE 2 Thermodynamic Parameters for Oil Dissolution in Water/TW20 Media as a Function of Their Composition and CA/TW-20 Mole Ratio (v*) at 303 K Composition (wt%)
v*
DGs7/ kJmol01
DH s7/ kJmol01
DS s7/ JK01 mol01
Cp /J/gm/7C obs./calc.
CAT/TW-20/water (o/w) 5/19/76 6/23/71 8/28/64 9/32/59 13/39/48
2.73 2.3 2.6 2.6 2.9
11.9 11.3 10.3 9.8 8.4
4.8 5.8 5.6 6.7 6.4
023.4 018.3 015.7 010.2 06.8
4.4/3.6 3.8/3.4 4.2/4.0 6.1/3.4 5.9/3.3
of preparation of mE was reported earlier (19). Sodium hydroxide solution (1 mol dm03) was prepared in aqueous medium. CV has a very high solubility in CA. It was observed that after vigorous shaking in the presence of CA and water, the dye remains in the CA medium. The dye solution (1 m mol dm03) was, therefore, prepared in CA. The overall concentration of CV was 15 m mol dm03 and that of NaOH was 20 m mol dm03 in all the mE media. For the study of salt effects, both NaCl and NaC were prepared in aqueous medium. Reaction kinetics were studied by spectrophotometric method in a Shimadzu (Japan) 160A UV–Vis spectrophotometer, used as described earlier (26). Temperature measurements were taken at 298–312 K, maintained by circulating water from a thermostated water bath around the cells with temperature uncertainties of {0.017. Eight ternary mixtures of CA/AOT/water (w/o), seven and nine ternary mixtures of CA/TW-20/water of o/w and w/o, respectively, were used as the microemulsion reaction media. For the detailed study of temperature and salt effects, CA/TW-20/water combinations in weight percents 5/30/65 and 65/30/5 were used. The spectral measurements were
3
taken at lmax of CV at 590 and 600 nm for aqueous and mE media, respectively. RESULTS AND DISCUSSION
Phase Diagram The ternary phase diagrams of the systems CA/AOT/water and CA/TW-20/water are presented in Fig. 1. The phase boundaries are on the whole symmetric. No triphasic zone has been observed. A greater single-phase mE zone has been obtained with AOT as amphiphile compared to TW-20. The plait point P corresponds to 30% (w/w) AOT in the mixture which is 50% for TW-20. The additives cholesterol and butanol (at 1:1 weight ratio with AOT) have reduced the mE zone and raised the plait point to the 40% level. AOT alone is a better microemulsifier than in combination with butanol. Realization of appreciable single phase mE zone advocates potential use of CA in the preparation of microheterogeneous dispersions. Calorimetric Observations and Energetics Typical thermograms for CA/AOT/water and CA/TW-20/ water systems are presented in Fig. 2. The enthalpy and heat capacities are shown in Tables 1 and 2. The w/o microemulsion formation at different v values have been found to be all exothermic in nature. The enthalpy of solution (DHs) of water in CA / AOT combinations has increased with v and passed through a maximum at v á 18 (Fig. 3A). The initial increase is rapid compared to the fall beyond the maximum. The addition of water in the CA / amphiphile mixture initially forms reverse micelles of very small water core that grow in size. Species called microemulsion is formed. The solubilization process yielding w/o microemulsion continues up to the phase transition point. The measured enthalpy thus represents the overall associated heat
FIG. 3 (A) Enthalpy of dissolution of water (DH s7) as a function of v in the CA/TW-20/water (w/o) system. (B) Variation of DH s7 with v in CA/ TW-20/H2O (w/o) system. (C) Dependence of DH s7 on v in CA/TW-20/H2O (o/w) system.
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FIG. 4 ln A0 /At vs time plot in different microenvironments at 298K: (l) CA/TW-20/H2O (w/o), v Å 11.4; (s) CA/AOT/H2O (w/o), v Å 4.1; (n) CA/TW-20/H2O (o/w), v* Å 1.5. Scales are indicated on the graphs.
up to the phase separation point per mol of water addition and stands for the enthalpy of solution of water in CA / amphiphile medium. For CA solubilization in water / amphiphile mixture producing o/w microemulsion, the enthalpy is expressed per mol of CA addition. This aspect will be further discussed at the end of the section in relation to the thermodynamic analysis. The physicochemical state of the microwater pool for AOT and other amphiphile-aided microemulsions is considered to be different (29) beyond v § 10. The observed maximum at v Å 18 in this report is appreciably greater than 10, the pool-size-dependent water state does not have a direct bearing on the observed maximum in DHs. The CA/TW-20/water systems have been studied with reference to the formation of both w/o and o/w microemulsion. The v values for the studied w/o cases are considerably higher; lower values are restricted by the phase diagram (the studied end points are shown on the diagram as full circles). The solution process of water in CA/TW-20 medium has been exothermic up to v Å 32, beyond which it is endothermic (Fig. 3B). The changed sign of the thermometric event is a striking manifestation of the ternary system. The dispersion of water in the binary mixtures of CA and TW-20 have shown conspicuous energetic manifestations at considerably higher levels of water addition.
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The o/w dispersions of the CA/TW-20/water ternary system have manifested endothermicity with a maximum at v* Å 2 (Fig. 3C). The changed thermodynamic activity at low CA/amphiphile mole ratio is again a striking phenomenon seldom manifested in the field of microemulsion study (6– 12). Thermometric studies on microemulsification of oil in water are very rare in literature (26). The results herein presented are therefore important; the use of CA as oil has made the study special. The exothermicity and the maximum in the formation of both w/o and o/w mE for CA/AOT/water and CA/TW-20/ water systems, respectively, show parallel manifestations. Identification of the types of interaction leading to the final results is not easy. Semiempirical attempts have been made in the past for deciphering and quantification (27, 28, 30). Since the energetics of pairwise interactions among the three component systems herein studied are not known, we refrain from attempting to elaborate the inner mechanism for the enthalpic observations but only add a comment. At the interface between oil and water, organization of amphiphiles contributes a major share to the overall heat making the process exothermic. Its final balance with other interactions ends up in maximum DH 7s at a particular v. At higher v, the exothermicity becomes prominent, which is also observed in w/o mE formation for the ternary CA/TW-20/water system, although the solubilization process is endothermic at lower v. Following the procedure described in a previous study (3), a model-independent thermodynamic analysis of the solubilization processes herein studied has been presented as follows. At the turbidity or phase separation point the dispersed phase (in mole fraction (Xs)) is at the state of its maximum solubility. Considering the dispersed phase (water or CA) as the solute, and the solution is ideal, the standard free energy of solution (DG7s ) to reach the turbid or phase separation point is given by the relation (3) DG7s Å 0RT ln Xs,
[1]
where Xs is the mole fraction concentration of the dispersed phase and R and T have their usual significance. The standard enthalpy of the solubilization process has been calorimetrically obtained; the standard entropy of solution DS 7s then follows from the relation, DS 7s Å (DH 7s 0 DG7s )/T.
[2]
All the thermodynamic parameters obtained for the w/o and o/w systems studied are given in Tables 1 and 2. They can be recognized as the energetic parameters for microemulsion formation. These include all sorts of physical–chemical interactions for the dispersion and stabilization of the dispersing solvent. Like hydroxylic amphiphiles, CA can undergo
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FIG. 5 (A) Variation of first-order rate constant (k1) with v in CA/AOT/H2O (w/o) system at 298 K. (B) Dependence of k1 on v in CA/TW-20/H2O system (w/o) at 298 K. (C) k1 vs v* plot for CA/TW-20/H2O (o/w) system at 298 K.
both dipolar and hydrophobic interactions with water and TW-20 during the solubilization process, thereby contributing a fair share to the energetics. The thermodynamic parameters for the CA/AOT/water and CA/TW-20/water systems given in Table 1 are all significantly low. They have a regular trend with v, the entropies are all negative. The results are on the whole comparable with the reported alkanol/AOT/water system (30). The enthalpies of solution of water in heptane/AOT binary mixtures (8) have been found to be two- or three-fold greater than the present results. Several w/o combinations of CA/TW20/water system on the lower side of v have shown endothermicity, the rest are exothermic. Exothermicity of solubilization of water in a Triton X-100/butanol/heptane ternary mixture has been reported in literature (28). The thermodynamic parameters for CA solubilization in water/TW-20 mixtures are all higher; the DS 7s values are again negative during solubilization, the system is initially disrupted and finally stabilized to end up in a more ordered state, making the entropy values negative. The solubilization of oil and the final ordering is akin to ‘‘iceberg’’ formation according to Frank’s proposition (31) that produces DS 7s Å 0(20–25) J mol01 K01. The DS 7s values given in Table 2 head toward the above limit of DS 7s at low v. The calculated specific heats, on the basis of ideal mixing for all the studied systems, are lower than the experimental values. They are strikingly different at the studied higher [CA]/[TW-20] mole ratios for the o/w systems. No system-
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atic trends in Cp values with v for w/o systems of heptane/ AOT/water have been observed (8). Lower calculated Cp values than those observed also have been found for w/o a microemulsion system of heptane/TX-100/butanol/water (28). Cp values higher than the ideal mixing advocate organization of the components resulting in ordering of the microheterogeneous state of the studied ternary systems. This corroborates the findings of negative entropy of solution of both water in oil or oil in water in the presence of both AOT and TW-20. Why it is prominent for the o/w dispersions at higher v than in w/o dispersions needs close examination. Rate Constants of CV / 0 OH 0 Kinetics in mE Medium The pseudo-first-order rate constants for the alkaline fading of crystal violet in both the ionic and nonionic surfactantaided microemulsion media have been determined. The rate constant (k1) has been calculated from the slope of the straight line plots between ln A0 /At and time (t) in accordance with the previous report (26) (Fig. 4). The rate constants have been found to vary nonlinearly with v. A dome-shaped dependence has been obtained in the case of w/o microemulsion containing AOT (Fig. 5A) with a maximum at v Å 10. A sharp rise also at v Å 10 has been observed for the CA/ TW-20/water w/o microemulsion system (Fig. 5B). For the o/w microemulsion containing TW-20, the rate constants remain almost invariant for v* above 2 (Fig. 5C). The results are presented in Table 3. The k1 values on the studied w/o
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TABLE 3 The Pseudo- First-Order and Second-Order Rate Constants of CV/-OH0 Reaction in Microemulsion Media at 298 K v
[NaOH] pool/mol dm03
k1 1 102/s01
k2 1 102/s01 mol01 dm3
CA/AOT/water (w/o) 3.5 4.1 7.0 8.2 10.5 12.4 14.1 17.6
2.1 2.6 2.8 2.8 2.9 2.5 2.1 0.7
4.5 6.8 9.1 9.7 11.4 13.9 15.9 17.0 22.8
0.50 0.7 1.6 5.6 3.2 3.0 2.7 2.8 1.8
0.352 0.352 0.177 0.178 0.118 0.148 0.088 0.07
5.9 7.4 15.8 15.7 24.6 16.9 23.8 9.5
CA/TW-20/water (w/o) 0.356 0.336 0.336 0.356 0.356 0.356 0.356 0.356 0.356
1.5 1.9 4.5 15.7 8.9 8.4 7.6 7.8 5.1
CA/TW-20/water (o/w) v* 4.6 2.3 1.9 1.5 1.3 1.1 1.02
3.6 3.7 4.0 3.4 2.7 2.2 2.0
0.023 0.024 0.028 0.030 0.032 0.035 0.038
156 154 142 114 84 63 53
and o/w media have been found to be comparable. v Å 10 closely corresponds to the maximum solvation of AOT anion and the cation in the microwater pool (8), the physical state of water manifests distinct departure beyond this composition. Different physicochemical processes have been reported (19, 22) to undergo appreciable changes at v ú 10, where the dispersed water tends to assume the properties of bulk water. However, the observed maximization of k1 is a rare event in compartmentalized liquids. The second-order rate constants and local concentration of OH0 in the water pool of different microemulsion compositions of the ternary CA/AOT/water and CA/TW-20/water systems are also given in Table 3. According to the principle of chemical kinetics, k1 and k2 for the CV/ 0 OH0 reaction should be independent of [OH0]. But in the studied mE media, the k1 and k2 values have been found to depend on both v and [OH0] in the microaqueous pool. Since the variation of [OH0] is not drastic in the CA/AOT/water system, and [OH0] is kept constant
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in the CA/TW-20/water system, the dimension of the aqueous pool in mE is considered to control the physical–chemical state of water (29) and to guide the kinetic process (18, 19, 32). We are not in favor of seeking an apparent correlation between the rate constant and [OH0]. In o/w mE medium of the CA/TW-20/water system, although OH0 ions are not in the oil pool, both k1 and k2 have decreased with [OH0]. This dependence originates from the influence on the reaction kinetics by the water/oil interface where CV/ preferentially resides. The second-order rate constants for the CA/AOT/water (w/o) system are higher than those for CA/TW-20/water (w/o). The k2 values in o/w medium are higher than those in w/o medium. The interior environment of the microwater pool with AOT has lower activation barrier than with TW20. On a molecular basis, AOT0 and its Na/ ion can be solvated by 12 (6 for AOT0 and 6 for Na/) water molecules (8). The TW-20 molecule with its 20 ethylene oxide containing head groups can attach 40 water molecules (2 for each oxygen center in the ethylene oxide moiety (32)). Therefore, at equal v, the hydration of TW-20 is 3.3 times more than that of AOT, and the w/o interface is more rigid with TW-20 as compared to AOT. The barrier for the formation of the activated complex is thus greater with TW-20 than that with AOT. Further, the local polarity at the interface with AOT (being an electrolyte) is different from that with TW-20. This also helps to lower the activation barrier for the ionic reaction between CV/ and OH0 in AOT-aided compartmentalized environments. Activation Parameters The energetics of activation process for the fading kinetics have been estimated by processing the temperature effect on the rate constant according to the Arrhenius and Eyring’s hypothesis (Fig. 6). In the narrow range of temperature variation, the composition of m E has been assumed to remain unchanged. The parameters for w/o and o/w environments produced by CA/TW-20/water microemulsions are given in Table 4. The enthalpies and free energies of activation are positive for both systems. The enthalpy of activation for the o/w system is higher than that for the w/o system, while the free energy of activation is almost the same. The entropy of activation is negative and is large for the w/o system. The higher negative entropy has supported more stabilized activation complex in the water pool of the microemulsion. The enthalpy – entropy compensation phenomenon (26) has been envisaged. The DG ‡ values support comparable spontaneity of the formation of the activation complex for both w/o and o/w systems, whereas weightage of entropy is more for the latter. The Salt Effect The effect of ionic strength (m) on the rate constant (k1) of the CV–NaOH reaction in the mE media has been analyzed
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q
FIG. 6 ln k vs T01 plot in CA/TW-20/H2O system: (s) w/o and (n) o/w system at v Å 11.4 and v* Å 1.5, respectively.
(neglecting salt-induced composition change) and is presented in Fig. 7, where good linear dependence is observed. The Bro q ¨ nsted–Bjerrum equation, log k1 Å log(k0)1 / 1.018 ZAZB m (where k0 is the hypothetical rate constant at zero ionic strength and ZA, ZB are the charges on the reacting species CV/ and OH0 respectively) is, therefore, obeyed (Fig. 7). The variation of the rate constant with ionic strength for NaCl and NaC and the experimental slopes for the CA/ TW-20/water (w/o and o/w) systems are given in Table 5. For w/o and o/w systems, the slopes obtained are, respectively, 0.4 and 0.18; they are lower than the theoretical value of 1.018. Taking the experimentally determined (measured with a Delik impedence bridge, Japan) dielectric constant of CA Å 15, the theoretical slope of B–B plot becomes 11.86, which is much higher than the experimental value. The CV/ 0 OH0 reaction essentially takes place at the water–CA interface of the microdispersions (26). The lowering of the slope thus cannot be essentially accounted for by the bulk dielectric constant of CA. The effect of NaC, at different concentrations on the rate constant, is given in Table 6. In TABLE 4 Activation Parameters for the CV/-OH0 Reaction in w/o and o/w CA/TW-20/Water Microenvironments at 298 K Composition CA/TW-20/H2O
DG‡/kJ mol01
DH‡/kJ mol01
DS‡/kJ01 mol01
5/30/65 (v Å 1.5) o/w 65/30/5 (v* Å 11.4) w/o
81.0 { 4 81.4 { 4
38.8 { 2 7.2 { 0.4
0142 { 7 0249 { 12
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FIG. 7 Dependence of log k on m in CA/TW-20/H2O medium at 298 K. (s): v Å 11.4 and (n): v* Å 1.5.
aqueous medium, although the rate decreases in presence of NaC, it remains more or less independent at higher concentrations of NaC. A similar type of variation has been observed in o/w microemulsion medium. The observed magnitudes of decrease have been 4- and 2.6-fold for w/o and o/w media, respectively. With the variation in the concentrations of NaC from 0.05 to 0.3 mol dm03, the rate constant TABLE 5 Effect of NaCl on the Kinetics of CV/-OH0 Reaction in (w/o and o/w) CA/TW-20/Water Microemulsion Mediuma,b at 298 K [NaCl]/mol dm03
System
log k1
w/o system (m)pool 0 0.1 0.2 0.3 0.5 1.0
0.597 0.675 0.745 0.809 0.925 1.16
01.49 01.62 01.64 01.66 01.70 01.82
o/w system (m)Aq 0 0.1 0.2 0.3 0.5 1.0
0.17 0.36 0.48 0.57 0.73 1.01
01.42 01.44 01.47 01.48 01.52 01.58
a Percent composition of microemulsion: 65/30/5 as CA/TW-20/water (v Å 11.4); [NaOH] pool Å 0.36 mol dm03. b Percent composition of microemulsion: 5/30/65 as CA/TW-20/water (v* Å 1.5); [NaOH] Å 0.03 mol dm03.
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MUKHOPADHYAY ET AL.
TABLE 6 The Effect of NaC on k1 in Different Environments at 298 K k1 1 103/s01 Microemulsion [NaC]/mol dm03
Aq.
w/oa
o/w b
0 0.01 0.02 0.05 0.10 0.20 0.30
3.22 — — 1.4 1.26 1.24 1.23
3.2 — — 8.9 8.1 1.45 1.12
8.00 3.70 3.70 3.00 3.00 — —
a b
Percent composition of microemulsion: 65/30/5 as CA/TW-20/water. Percent composition of microemulsion: 5/30/65 as CA/TW-20/water.
has decreased from 0.009 to 0.001 s01 in w/o microemulsion medium. Being surface active, NaC affects the interfacial properties of the microdroplets in the mE. It offers rigidity to the interface (33); the contact of reacting species becomes less efficient. This is of course a simplified explanation of the observed fact, which needs substantiation by further studies. Bearing of the Energetics of Solution on Activation Parameters The energetics of formation of w/o and o/w mE and the measured Cp values of the resulting systems have supported ordering or structuring of the dispersion environment (Tables 1 and 2). The ordering follows the sequence CA/AOT/water (w/o) õ CA/TW-20/Water (w/o) õ CA/TW-20/water (o/w). The stabilization of the CV/ at the water/CA (oil) interface in thus more in the o/w system than in the w/o system. This has a bearing on energetics of the CV/ 0 OH0 reaction kinetics. There the free energies of activation (DG‡) in CA/ AOT/water (w/o) and CA/TW-20/water (o/w) mE’s are the same (Table 4) whereas the entropies of activation are different (0249 and 0142 J mol01 K01, respectively). The more organized water/CA interfacial environment of the latter (Table 2) has provided additional stability to the activation complex to produce DG‡ of equal magnitude. ACKNOWLEDGMENT This work was financially supported by the Department of Science and Technology, Government of India.
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