11314
J. Phys. Chem. B 2006, 110, 11314-11326
Physicochemical Studies on Cetylammonium Bromide and Its Modified (Mono-, Di-, and Trihydroxyethylated) Head Group Analogues. Their Micellization Characteristics in Water and Thermodynamic and Structural Aspects of Water-in-Oil Microemulsions Formed with Them along with n-Hexanol and Isooctane Debolina Mitra, Indranil Chakraborty, Subhash C. Bhattacharya, and Satya P. Moulik* Centre for Surface Science, Department of Chemistry, JadaVpur UniVersity, Kolkata 700 032, India
Sangita Roy, Debapratim Das, and Prasanta K. Das* Department of Biological Chemistry, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed: October 7, 2005; In Final Form: April 13, 2006
The micellization behavior of cetylammonium bromide and its mono-, di-, and trihydroxyethylated head group analogues and water/oil (w/o) microemulsion formation with them have been studied with detailed thermodynamic and structural considerations. The critical micellar concentration, micellar aggregation number, and behavior of the surfactants at the air/solution interface have been studied in detail. The results have been analyzed and discussed. The formation of the w/o microemulsion stabilized by the aforesaid surfactants in conjunction with the cosurfactant n-hexanol in isooctane has been investigated by the dilution method. The energetics of the transfer of cosurfactant from oil to the interface has been estimated. The structural parameters, namely, droplet dimension, droplet number, and population of surfactant and cosurfactant on the droplet surface, have also been estimated. The efficacy of the surfactants in respect to water dispersion in oil and cosurfactant concentration level at the oil/water interface has been worked out. Such microemulsions are prospective compartmentalized systems to assist enzyme activities. In this respect, the trihydroxyethylated head group analogue in the above series has been found to be a better performer for the preparation and stabilization of microemulsions that has correlated well with its performance than the others in the hydrolysis of p-nitrophenyl-n-hexanoate by the enzyme Chromobacterium Viscosum lipase.
Introduction Enzymology in water-in-oil microemulsions (or reverse micelles) is an area of increasing interest owing to their compartmentalizing ability and their ability to solubilize numerous biomolecules including proteins, enzymes, and nucleic acids without loss of their biological activities.1-4 The activity of surface active enzymes, for example, lipase, in cationic water/ oil (w/o) microemulsions2,5-9 with cetyltrimethylammonium bromide (CTAB) has been found to be significantly lower than that of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) based formulations.2,5,10-13 Recently, we14-16 have shown that cationic surfactants containing hydroxyethyl moieties in their head groups of varied types, geometry, and size17 as well as with varying chain lengths can significantly enhance lipase catalytic efficacy.10,11,14,17,20 We have used n-hexanol as cosurfactant in the preparation of microemulsions to study the hydrolysis of p-nitrophenyl-n-hexanoate. The catalytic efficiencies of the encapsulated enzymes have been considered to be influenced by the microstuctural parameters such as the local molar concentration of water, bromide ions, and n-hexanol14,15,20 and space in the vicinity of the enzymes. Thus, characterization of these cationic w/o microemulsions has been considered quite essential to have a better understanding of the structure-function relationship of the enzymes in the state of compartmentalization. * To whom correspondence should be addressed. E-mail: spmcss@ yahoo.com (S.P.M.);
[email protected] (P.K.D.).
Since the hydrolytic enzyme lipase essentially resides at the interface of the microemulsion droplet between water and oil,2,5,7-10,21 it is logical that the reaction should become more efficient with decreasing proportion of surfactant3,6,22 and cosurfactant14,20 in the system. Thus, the number of surfactant and cosurfactant molecules at the interface should have a correlation with lipase efficacy. For a given amount of water, the number and size of the droplets and the composition of the interface are thus considered to be vital parameters for the enzymatic process which can be assessed from the method of dilution23 used by several workers on different systems.24-27 We have studied dilution experiments on w/o microemulsions forming systems28-30 to evaluate the thermodynamics of the transfer of cosurfactant from the oil phase to the interface as well as to estimate the droplet number, size, concentration, and interfacial aggregation extents of both surfactant and cosurfactant under varied conditions of concentration and temperature. By this method, the changing behavior of the interface with changing head group types of the surfactant can be ascertained.28-30 Since the surfactants used in our enzymatic studies14-16 are of a special type synthesized by us,14,15 we have become interested in elaborately exploring their self-aggregation in aqueous media and understanding the structural and thermodynamic aspects of the formation of w/o microemulsions in their presence along with cosurfactant in an oil continuum. Herein,
10.1021/jp055720c CCC: $33.50 © 2006 American Chemical Society Published on Web 05/18/2006
CTAB and Its Modified Head Group Analogues
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11315
TABLE 1: Nomenclature and Abbreviations C0 C1 C2 C3 C1cmc1 and C1cmc2 T γ πcmc Γmax C x Amin NA κ (I1/I3) I0, IQ n ∆G°m, ∆H°m, ∆S°m, and ∆C°pm Xcmc ∆G°ads, ∆H°ads, and ∆S°ads ω noa no Xia Xoa Kd k nta nwa ns nw I and S ∆G°t, ∆H°t, ∆S°t, and ∆C°pt Vd Ad Re Rw Nd Vw, Vs, and Via Ma and Fa As and Aa ˜a N ˜ s and N Vhs and Vha
N-hexadecylammonium bromide (cetylammonium bromide), MW 322 N-hexadecyl-N-(2-hydroxyethyl)ammonium bromide, MW 367 N-hexadecyl-N,N-bis(2-hydroxyethyl)ammonium bromide, MW 410 N-hexadecyl-N,N,N-tris(2-hydroxyethyl)ammonium bromide, MW 455 cmc1 of C1 and cmc2 of C1 absolute temperature in kelvins surface tension at the air/solution interface surface pressure at the cmc maximum surface excess at the air/solution interface at the cmc molar concentration of the surfactant solution number of species produced by dissociation of a surfactant molecule in solution minimum area per molecule at the air/solution interface at the cmc Avogadro’s number specific conductance intensity ratio between the first and third vibration peaks of pyrene fluorescence at 372 and 383 nm, respectively fluorescence intensities without and with quencher Q (here CPC), respectively aggregation number of a surfactant micelle standard Gibbs free energy, enthalpy, entropy, and specific heat of micellization, respectively cmc expressed in mole fraction unit standard Gibbs free energy, enthalpy and entropy of adsorption, respectively water-to-surfactant mole ratio number of moles of alkanol (a) in oil (o) total number of moles of oil mole fraction of alkanol at the interface (i) mole fraction of alkanol in oil distribution constant of alkanol between the interface and oil number of moles of alkanol per mole of oil total number of alkanol in the system number of mole of alkanol in water (w) moles of surfactant (s) used in the experiment number of moles of water intercept and slope of the plot between nta/ns and no/ns, respectively standard Gibbs free energy, enthalpy, entropy, and specific heat of transfer of n-hexanol from oil to the interface, respectively total volume of droplets per milliliter of solution total surface area of droplets per milliliter of solution effective radius of droplet radius of water core of droplets in a w/o microemulsion total number of droplets per milliliter of solution volumes of water, surfactant, and alkanol at the interface, respectively molar mass and density of the alkanol, respectively head group area of surfactant and alkanol, respectively average aggregation number of surfactant and alkanol on the droplet surface, respectively head group volume of surfactant and alkanol, respectively
we report (1) the interfacial adsorption and thermodynamics of micellization of cetylammonium bromide and three of its hydroxyethylated analogues studied by tensiometry, conductometry, and fluorimetry and (2) w/o microemulsion formation features employing them along with the cosurfactant n-hexanol and the hydrocarbon oil isooctane. The results are expected to enrich the fundamental understanding of self-aggregation phenomena of amphiphiles and their activities at the droplet interface in w/o microemulsions with reference to the catalytic activity of interfacially located hydrolytic enzymes such as lipase. The abbreviations and symbols used in this article are presented in Table 1. Experimental Section Materials. Isooctane, cetylpyridinium chloride (CPC), and n-hexanol were of AR grade and produced from Aldrich, Sigma (U.S.A.), and SRL (India), respectively. They were used as received. Pyrene (Aldrich) was obtained as a gift from the Polymer Science Laboratory of IACS, Kolkata, India. It was purified by vacuum sublimation and then crystallized twice from
SCHEME 1
an ethanol/water mixture. All solutions were prepared in doubly distilled deionized water. The cationic amphiphiles used were C0, C1, C2, and C3; their structural formulas are presented in Scheme 1. The compound C0 was a purified gift sample by courtesy of R. Palepu of St. Francis Xavier University, Nova Scotia, Canada. The rest were synthesized following the protocal mentioned earlier.14,15 In short, 1-bromohexadecane and the corresponding amine (ethanolamine for C1, N,N-diethanolamine for C2) were taken in the molar ratio 1.2:1 in a 30% methanol/acetonitrile mixture and refluxed for 24 h. The solvent was evaporated on
11316 J. Phys. Chem. B, Vol. 110, No. 23, 2006 a rotary evaporator. Pure C1 was obtained by column chromatography of the reaction mixture on a 60-120 mesh silica gel column using 5% methanol in chloroform as the eluting solvent. C2 was obtained by crystallization of the reaction mixture from methanol/ethyl acetate mixed solvent. For the preparation of C3, 2.7 M NaOH was added dropwise to a mixture of 2-bromoethanol (0.081 mol) and n-hexadecylamine (0.027 mol) under refluxing conditions. After 24 h of refluxing, the reaction mixture was extracted with chloroform and the organic layer was dried over anhydrous sodium sulfate. Chloroform was removed on a rotary evaporator followed by drying under vacuum. The white residue was then crystallized from methanol/ ethyl acetate mixed solvent and filtered. The dried solid was purified by column chromatography on a 230-400 mesh silica gel column using a methanol/chloroform mixture as eluting solvent. Pure product was obtained with a 7% methanol/ chloroform mixture with 40% yield. All of the surfactants were desiccated before use. Their spectroscopic, elemental analysis, and high resolution mass data are presented below, which were also reported earlier.14,15 C0. 1H NMR (300 MHz, CDCl3): δ/ppm ) 0.88 (t, 3H), 1.18-1.32 (br, m, 26H), 1.74-1.82 (m, 2H) 3.02 (t, 2H), 7.99 (br, 3H). Anal. Calcd for C16H36BrN: C, 59.61; H, 11.26; N, 4.34. Found: C, 59.66 H, 10.65; N, 4.16. MS (ESI) m/z: calcd, 242.28; found, 242.2443. C1. 1H NMR (300 MHz, CDCl3): δ/ppm ) 0.88 (t, 3H), 1.18-1.32 (br, m, 26H), 1.88-2.00 (br, 2H) 3.01 (br, 2H), 3.17 (br, 2H), 4.02 (br, 2H), 8.93 (br, 2H). Anal. Calcd for C18H40BrNO: C, 59.00; H, 11.00; N, 3.82. Found: C, 58.78; H, 10.81; N, 4.08. MS (EI): calcd (for C18H40NO the 4° ammonium ion, 100%), 286; found, 254 [(M - CH2OH - H)+]. C2. 1H NMR (300 MHz, CDCl3): δ/ppm ) 0.88 (t, 3H), 1.18-1.35 (br, m, 26H), 1.81 (br, 2H), 3.54 (br, 2H), 3.67 (br, 4H), 4.03 (br, 4H), 8.77 (br, s). Anal. Calcd for C20H44BrNO2: C, 58.52; H, 10.80; N, 3.41. Found: C, 58.66; H, 11.03; N, 3.30. MS (LSIMS) m/z: calcd (for C20H44NO2 the 4° ammonium ion, 100%), 330; found, 330 (M+). C3. 1H NMR (300 MHz, CDCl3): δ/ppm ) 0.88 (t, J ) 6.9 Hz, 3H), 1.25-1.34 (m, 26H), 1.83 (br, 2H), 3.18-3.23 (br, 2H), 3.35-3.49 (br, 6H), 4.08 (br, 6H). Anal. Calcd for C22H48BrNO3: C, 58.15; H, 10.57; N, 3.08. Found: C, 58.13; H, 10.64; N, 3.08. MS (ESI) m/z: calcd, 374.62; found, 374.2458. The high purity of the surfactants (C0, C1, C2, and C3) used in the study was evident from the above results. Methods. NMR, Mass Spectra, and Elemental Analysis. The 1H NMR spectra of all of the surfactants were recorded on an AVANCE 300 MHz (BRUKER) spectrometer. Chemical shifts are reported in parts per million, using tetramethylsilane (TMS) for 1H NMR as internal standard. Mass spectrometric (MS) data were acquired by liquid secondary ion mass spectrometry (LSIMS) and electron impact (EI) techniques for C2 and C1. For C0 and C4, data were acquired by electron spray ionization (ESI) techniques using 25-70 eV in a Q-tof Micro-Quadruple mass spectrophotometer (Micromass, U.K.). Elemental analysis was carried out using a Perkin-Elmer CHN analyzer. Conductometry. The conductivity measurements were performed with a Jenway (U.K.) conductometer using a cell of unit cell constant. A concentrated surfactant solution (∼15-20 times the critical micelle concentration (cmc)) was progressively added using a Hamilton microsyringe to 10 mL of water placed in a thermostated container (with a temperature accuracy of (0.01 K). On ensuring thorough mixing and temperature equilibration after each addition, the κ value of the solution was measured. The accuracy of the measurements was within (1%.
Mitra et al. Tensiometry. Tensiometric measurements were taken with a calibrated du Nou¨y tensiometer (Kru¨ss, Germany) by the ring detachment technique. A concentrated surfactant solution (∼20 times the cmc) was progressively added to 10 mL of water taken in a jacketed glass container through which water at constant temperature was circulated. After thorough mixing and temperature equilibration after each addition, the γ value was measured. The measured γ values were corrected following the procedure of Harkins and Jordan.31 They were accurate within (0.1 mN m-1. Fluorimetry. Fluorescence measurements were taken in a Perkin-Elmer LS55 luminescence spectrometer. The excitation wavelength of pyrene was 337 nm, and the emission spectra were recorded in the range 350-600 nm. The aggregation numbers of the surfactants were determined from the steadystate fluorescence quenching (at 383 nm) of pyrene (used as probe, concentration 0.1 µM, no excimer formation) by CPC (0.0005-0.03 mM, varied according to the cmcs of the surfactants) as the quencher to a total [surfactant] of 5-400 times the cmc. The fluorescence intensity was measured at each stage of addition of CPC in the surfactant solution. In the determination of the cmc, the pyrene concentration was also taken to be 0.1 µM. As above, a concentrated surfactant solution was added to water (containing equal [pyrene] ) 0.1 µM) in multisteps and the fluorescence intensity was measured at each step after confirming thorough mixing. Dilution. In the dilution experiment, a calculated amount of surfactant was taken in a dry stoppered test tube. Fixed amounts of oil (isooctane) and water were added to it. The solution turned thick and turbid. It was placed in a thermostated water bath (accuracy (0.01 K) under constant stirring conditions using a magnetic stirrer. The solution was then titrated with the cosurfactant n-hexanol from the buret until it became just clear at the temperature of the experiment. The volume of the cosurfactant required was noted. The clear solution thus obtained was then titrated with oil: for the C2 and C3 containing systems, phase separation was observed, but the C0 and C1 containing systems became turbid. The volume of oil required to destabilize the system was noted. Titrating with cosurfactant again restabilized it. Such a process was repeated several times, each time noting the volume of oil or alkanol required to destabilize/ stabilize the system. The entire procedure was repeated at 303, 308, 313, 318, and 323 K keeping ω constant at 8 for C0 and C1 and at 52 for C2 and C3. We used such ω values that had been used by us in our earlier studies on lipase activity in w/o microemulsions with n-hexanol and isooctane.14,15 The surfactants C0 and C1 favored microemulsion formation at higher temperatures. Each experiment was repeated twice, and the mean value was used for data processing and analysis. Results and Discussion In our enzyme catalysis work,14,15 buffer solutions of pH 6-7 were used; of the three surfactants (C1, C2, and C3), the first two remained fully in the protonated form and the quartenized salt C3 was nonprotonable. In the present study, experiments were performed in doubly distilled water of pH ∼ 6; consequently, all of the ammonium compounds (C0, C1, and C2) retained their protonated structures intact in solution. They would lose protons at pH > 10. The structure of C3 was pH independent. A simple calculation, taking the pK of a base that might arise from the hydrolysis of the surfactants (salts of weak base strong acid) to be 5 (pK of base like ammonia), shows that the pH of a solution with a concentration of 0.2 mM (maximum concentration used in the experiments) is 7.0 which
CTAB and Its Modified Head Group Analogues
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11317
Figure 1. Specific conductance (κ) vs [surfactant] plots at different temperature: (a) C0 at 308 and 313 K; (b) C1 at 293 and 308 K (inset, C1 at 313 K); (c) C3 at 308 and 313 K (inset, C2 at 313 K).
Figure 2. Tensiometric plots for C0, C1, C2, and C3 at different temperatures: (a) C0 at 293, 298, and 313 K; (b) C1 at 293, 298, 303, 308, and 313 K; (c) C2 at 293, 298, 303, 308, and 313 K; (d) C3 at 293, 298, 303, 308, and 313 K.
is one unit higher than the pH of water. The hydrolysis of the salts in solution was thus neglected. The use of high or low pH and addition of salt (to control ionic strength) would affect the self-assembling properties of the surfactants. The objective of the present study was to explore the solution behavior of the amphiphiles in the absence of the addition of any external electrolyte. We wanted to collect basic data on the selfaggregation of the surfactants. The results herein presented refer to this state of physicochemistry. In the experimental solutions, only Br- ions were present as the counterions by way of dissociation from the surfactants. Micelle Formation of C0, C1, C2, and C3. The surfactants herein studied were found to yield low cmcs. The conductance method was found to be less sensitive than tensiometry. The isothermal titration calorimetry was also found to be much less sensitive in this respect and, therefore, could not be used. Conductometric observations are presented in Figure 1. While mild breaks in the κ versus concentration plots were observed for C0, C2, and C3, C1 did not show any break, which were expected at 0.120, 0.087, and 0.079 mM at 293, 308, and 313
K, respectively, according to tensiometry, as indicated in Figure 2b. The absence of a break was also observed for C0 at 303 K (not illustrated). The observed mild breaks and its absence in the plots meant low and no counterion (Br-) binding by the formed micelles, respectively. For the detection of cmc as well as for the evaluation of thermodynamic parameters, tensiometric results were considered. The observations are depicted in Figure 2. Fluorimetric determination of cmc was also attempted as supportive evidence (Figure 3a,b). Differentiation of the sigmoidal plots between I1/I3 and [surfactant] was considered to estimate cmc from the points of minima. The cmc values of the surfactants at different temperatures obtained from the abovedescribed methods are presented in Table 2. On a comparative basis, cmcC1 > cmcC0 > cmcC3 > cmcC2. Fluorimetry has produced two cmcs for C1 (Figure 3a) but a single cmc for C0, C2, and C3 amphiphiles at 293 and 298 K (Figure 3b). The fluorescence spectra of pyrene with varied [C1] are depicted in the inset of Figure 3a. We have also observed reproducible two-plateau regions in the tensiometric plot for C1 at 303 K. These corresponded to two cmc values at 0.0042 (cmc1) and
11318 J. Phys. Chem. B, Vol. 110, No. 23, 2006
Mitra et al. TABLE 2: cmca and Thermodynamic Parametersb,c for the Micellization of C0, C1, C2, and C3 T (K)
ST
293 298 303 308 313
36.3 34.7 30.2 27.5 24.0
cmc/µM condd
FLe,f
∆H°m ∆S°m -∆G°m (kJ mol-1) (kJ mol-1) (J K-1 mol-1) C0
33.5 28.8 24.8
34.7 35.4 36.3 37.2 38.1
16.1
173
31.8 32.5 33.4 34.2 35.0
16.2
163
35.4 36.1 37.6 39.0 41.9
-9.85 25.1 58.8 91.5 123
87.1 205 318 424 527
32.3 35.5 37.1 38.5 39.4
159 112 66.9 23.1 -19.3
653 495 343 200 64.3
C1 293 120 298 110 303 4.2, 98 308 86.7 313 79.2
12.1, 105
C2 293 298 303 308 313
27.5 26.2 18.2 13.5 5.57
293 298 303 308 313
96.2 33.8 21.9 16.3 14.8
29.6 5.58 C3
Figure 3. I1/I3 vs [surfactant] plots for C0, C1, C2, and C3: (a) C1 at 298 K (inset, the basic fluorescence spectra for C1 using pyrene as a probe); (b) C2 at 298 K and C3 at 293 K (inset, C0 at 298 K); (c) tensiometric plot for C1 at 303 K.
0.098 mM (cmc2), respectively (Figure 3c). The above was a striking feature for the studied C1 amphiphile. We have recently reported double cmc formation for cationic surfactants and their mixtures by different methods.32,33 In footnote e of Table 2, the polarity index I1/I3 values at the cmc points for C0, C1, C2, and C3 are presented. The interior of the micelles was found to be fairly nonpolar, with the polarity index following the trend C1cmc1 ≈ C2 ≈ C3 > C1cmc2 > C0. The differences were ascribed to the disparity in the degree of water penetration in the micelles depending on the variations in the compactness of the surfactant head groups.34 Water penetration was less in C0 and C1cmc2 compared with C1cmc1, C2, and C3; the compactness in the head groups was more in the former than the latter. Compared to the first, the interior of the second micelle of C1 was more nonpolar. The thermodynamic parameters of micellization were obtained using the normal procedure of temperature dependence of cmc obtained from tensiometry. In the calculation of the standard Gibbs energy of micellization, counterion binding was not considered, since it was very small (Table 2, footnote). C1 did not evidence its manifestation, so also C0 at 303 K. Thus,
∆G°m ) RT ln Xcmc ∆H°m )
[
]
(1)
∂(∆G°m/T) ∂(1/T)
(2) p
∆S°m ) (∆H°m - ∆G°m)/T
( )
∂∆H°m ∆C°pm ) ∂T
p
(3) (4)
The standard state was considered as the hypothetical state of ideal solution of unit mole fraction. In the calculation of ∆H°m by eq 2, ∆G°m/T was found to vary linearly with 1/T for C0 and C1 (Figure 4a) so that single values of 16.1 and 16.2 kJ mol-1, respectively, for ∆H°m were obtained. For C2 and C3,
94.0 31.7 17.0 14.9
a The average error in the cmc is (2%. b The average errors in ∆G°m, ∆H°m, and ∆S°m are (4, (6, and (8%, respectively. c ∆C°pm ) 6.55 ( 0.07 and -8.44 ( 0.2 kJ K-1 mol-1 for the C2 and C3 derived processes. d Extent of counterion (Br-) binding: 0.14 (308 K) and 0.2 (313 K) for C0; 0.12 (313 K) for C2; 0.11 (308 K) and 0.10 (313 K) for C3. e (I1/I3) at the cmc and at 298 K: C0 ) 0.75; C1 ) 1.29 (cmc1) and 1.13 (cmc2); C2 ) 1.27; C3 ) 1.28. f Aggregation number at 298 K: n ) 3 (C0 at 5cmc); n ) 2 (C1 at 5cmc for cmc1) and n ) 5 (C1 at 5cmc for cmc2); n ) 6, 36, and 88 (C2 at 5, 100, and 400cmc, respectively); n ) 8 and 27 (C3 at 8 and 100cmc, respectively). The fluorescence data processed in terms of eq 8 produced very low n values for C0 and C1 (at cmc1). Although the measurements were doubly checked, low values such as 2 and 3 were doubtful.
∆G°m/T varied nonlinearly with 1/T, and ∆H°m at the studied temperatures were obtained from second-degree polynomial fitting of the data (Figure 4b). The results in column 6 of Table 2 show significant variation of ∆H°m with temperature. At 293 K for C2 and 313 K for C3, the enthalpy values were negative. The endothermicity of ∆H°m at other temperatures indicated disorder in the system, which expectedly reflected on large positive ∆S°m. The ∆H°m evidenced a linear dependence on temperature (not illustrated) with constant ∆C°pm values (Table 2, footnote) for C2 and C3. The positive ∆C°pm value for C2 suggested prominent contribution of the hydrophobic part of monomers as compared with the ionic hydrophilic head of the former, which was the reverse in the latter. Interfacial Adsorption. The air/solution interfacial adsorption of the surfactants was examined from the dependence of γ on log C according to the Gibbs adsorption equation
Γmax ) -
dγ 1 Lt 2.303xRT Cfcmc d log C
(5)
where x ) 2. Representative γ versus log [surfactant] plots are presented in Figure 2 where the break points correspond to the cmcs. The Amin value was obtained from the relation
Amin ) (1018/NAΓmax) nm2 molecule-1
(6)
CTAB and Its Modified Head Group Analogues
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11319
Figure 4. Gibbs-Helmholtz plot for the evaluation of the standard enthalpy of micellization: (a) for C0 and C1; (b) for C2 and C3.
TABLE 3: Gibbs Surface Excess (Γmax), Area Minimum (Amin), Standard Gibbs Energy of Adsorptiona (∆G°ads), Standard Enthalpy of Adsorptiona (∆H°ads), and Standard Entropy of Adsorptiona (∆S°ads) for the Amphiphiles C0, C1, C2, and C3 at Different Temperatures Amin -∆G°ads ∆H°ads ∆S°ads T 106Γmax (K) (mol m-2) (nm2 molecule-1) (kJ mol-1) (kJ mol-1) (J K-1 mol-1) C0 293 298 303 308 313
2.33 2.62 2.68 2.75 2.84
0.713 0.634 0.620 0.604 0.585
53.9 53.0 52.7 53.2 53.0
293 298 303 308 313
1.91 1.90 1.88 1.58 1.44
0.869 0.874 0.885 1.05 1.16
C1 (cmc2) 46.0 52.4 58.5 65.8 69.8
293 298 303 308 313
2.20 2.21 2.34 2.33 2.50
0.754 0.750 0.710 0.712 0.664
293 298 303 308 313
1.92 2.14 2.21 2.26 2.48
0.865 0.774 0.753 0.733 0.669
-60.3
-23.2
307
1206
Figure 5. Gibbs-Helmholtz plot for the evaluation of the standard enthalpy of adsorption: (a) C2 and C3; (b) C0 and C1.
C2 54.0 55.9 57.2 59.3 61.5
53.6
367
54.6 56.0 57.5 58.8 60.1
26.3
276
C3
a The error limits of ∆G°ads, ∆H°ads, and ∆S°ads are (4, (6, and (8%, respectively.
∆G°ads was calculated from the relation
∆G°ads ) ∆G°m - (πcmc/Γmax)
(7)
Here, πcmc is given by the relation πcmc ) γH2O - γcmc. The results are presented in Table 3. The Γmax quantities have shown moderate variation with the type of surfactant and temperature. Similar were the variations of Amin which followed the order C1 > C3 > C2 > C0. The ∆G°ads values were all greater than ∆G°m (Table 2), and the former process was more spontaneous than the latter. The interfacial adsorption started from the beginning of the addition of an amphiphile in water; micellization was a subsequent process. For the evaluation of ∆H°ads and ∆S°ads, relations equivalent to eqs 2 and 3 were used. The results are indicated in Table 3. The trend of temperature dependence of interfacial adsorption of C0 was opposite to that of C1, C2, and C3. Hence, the ∆H°ads values were also opposite (Figure 5), negative for C0
and positive for C1, C2, and C3. The corresponding entropies were also negative and of high positive values, respectively. The adsorption of C1 ended up with a large positive ∆H°ads value, which appreciably declined for C2 and C3; the corresponding positive entropy values declined proportionately. Positive ∆H°ads and ∆S°ads values suggested production of randomness at the interface by the transfer of C1, C2, and C3 molecules from the bulk which was not overcompensated for by interfacial rearrangement. By the transfer of C0 from the bulk to the interface, it became more ordered, producing negative ∆H°ads and ∆S°ads values. However, why C1 with one hydroxyethyl group in the amphiphile head produced a larger ∆H°ads value than the rest remains to be explained. Aggregation Number of Micelles. The aggregation number of the studied surfactants was determined from the static fluorescence quenching of pyrene using CPC as a quencher on the basis of the equation
ln
()
I0 [Q]n ) IQ [surfactant] - cmc
(8)
The slope of the linear plot between ln(I0/IQ) and [Q] at constant [surfactant] (5-400 times the cmc) was used to get n (Figure 6). The results are given in footnote f of Table 2. It has been found that all of the surfactants produced a low aggregation number at 5-8 times the cmc. Higher [surfactant] produced higher n. The concentration dependent n for micelles has been well documented in the literature.35 Microemulsion Formation with C0, C1, C2, and C3. In w/o microemulsions, nano water droplets are stabilized by amphiphiles in oil continuum. In this study, the surfactants (C0,
11320 J. Phys. Chem. B, Vol. 110, No. 23, 2006
Mitra et al. Related Thermodynamics. For the formation of a stable microemulsion, two sets of constants are considered:25,28-30 (1) the ratio between noa and no in the system and (2) the ratio between Xia and Xoa . Thus,
k ) noa /no
(9)
Kd ) Xia/Xoa
(10)
and
Figure 6. Plot of ln(I0/IQ) vs [CPC] for the determination of the aggregation number of C2 and C3 ([surfactant] ) 100 cmc) at 298 K.
C1, C2, and C3) and the cosurfactant (n-hexanol) were the stabilizers for the water droplets in isooctane. It is normally considered that the total surfactant resides at the interface and the cosurfactant (alkanol) remains distributed in the water, interface, and oil. The alkanols higher than butanol essentially remain partitioned between the interface and oil because of their negligible solubility in water. Thus, at a fixed [surfactant], a critical concentration of the cosurfactant is required for the stabilization of the microemulsion. Addition of extra oil extracts the cosurfactant from the interface to destabilize the system, which can be stabilized by the addition of extra cosurfactant in the system. This is the fundamental basis of the dilution experiment described in the Experimental Section.
Addition of oil in the system should affect both k and Kd; the system gets destabilized. At this stage, addition of alkanol is needed to restore the values of k and Kd, that is, to restabilize it. For a given set of components at a fixed temperature, the droplet structure, composition, and geometry remain invariant at their stable states of existence; only their population density decreases because of the increasing volume of the system with the addition of oil and alkanol. Interdroplet interaction, if any, decreases with increased states of dilution. In a just formed w/o microemulsion system with a critical alkanol concentration, nta is given by the sum27-30
nta ) nwa + nia + noa
(11)
By eq 9, the normalized (expressed per mole of surfactant) form of eq 11 becomes26,28-30
no nta nwa + nia ) +k ns ns ns
(12)
In the experiment, both ns and nw remain unchanged so as to
Figure 7. Plot of nta/ns vs no/ns for different systems (a and b, at ω ) 8; c and d, at ω ) 52): (a) with 125 µmol of C0 at (1) 308 K, (2) 313 K, and (3) 318 K; (b) with C1 at 313 K and at (1) 54.5 µmol, (2) 80.4 µmol, and (3) 109.0 µmol (inset, with 56 µmol of C1 at (4) 313 K, (5) 318 K, and (6) 323 K); (c) with C2 at 308 K and at (1) 12.0 µmol, (2) 24.2 µmol, (3) 49.3 µmol, and (4) 73.7 µmol (inset, with 73 µmol of C2 at (5) 318 K, (6) 313 K, (7) 308 K, and (8) 303 K); (d) with C3 at 313 K and at (1) 22.2 µmol, (2) 44.0 µmol, and (3) 66.2 µmol (inset, with 22 µmol of C3 at (4) 318 K, (5) 313 K, and (6) 308 K).
CTAB and Its Modified Head Group Analogues
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11321
TABLE 4: ns Dependent nia, K, Kd ,a and β for C0, C1, C2, and C3 for w/o Microemulsion Formationb with n-Hexanolc in Isooctanec at Different Temperatures 105ns (mol) (105nia (mol)) C0
C1
β
k (Kd)
C2
C3
C0
C1
C2
C3
C0
C1
C2
C3
303 K 1.20 (13.0) 2.44 (12.7) 4.93 (12.1) 7.37 (11.8)
0.020 (47.8) 0.021 (41.3) 0.021 (35.0) 0.019 (33.6)
566 251 118 86
308 K 1.20 (12.7) 2.42 (12.0) 4.93 (11.0) 7.37 (10.7)
0.020 (46.2) 0.022 (38.3) 0.021 (33.8) 0.018 (33.3)
2.22 (21.5) 4.48 (11.4) 6.70 (9.38)
12.5 (72.1)
0.024 (39.2) 0.031 (24.2) 0.035 (17.4)
0.069 (13.2)
526 224 107 80
408 183 40
395 148 59 35
338 111 37
319 97 41 25
198 71 40
83.6
313 K 5.45 (11.9) 8.04 (15.4) 10.9 (19.0)
1.22 (10.1) 2.44 (8.26) 4.90 (7.64) 7.32 (7.31)
2.22 (21.7) 4.40 (16.7) 6.62 (8.55)
12.3 (16.9)
0.020 (35.0) 0.020 (32.9) 0.020 (32.3)
0.021 (43.5) 0.023 (34.6) 0.027 (23.6) 0.029 (17.8)
0.029 (32.3) 0.034 (23.9) 0.035 (16.7)
109 94 86
0.055 (11.0)
31.1
318 K 5.67 (11.4) 8.31 (13.6) 11.1 (16.6)
1.20 (9.25) 2.46 (7.85) 4.90 (6.75) 7.29 (5.97)
2.18 (14.5) 4.57 (10.1) 6.57 (9.76)
12.8 (7.30)
0.020 (34.1) 0.022 (28.7) 0.022 (28.2)
0.024 (37.4) 0.033 (24.0) 0.034 (17.7) 0.033 (14.1)
0.033 (26.6) 0.031 (22.8) 0.037 (16.6)
100 74 69
0.040 (9.43)
14.2
323 K 0.019 (33.7) 0.023 (25.7) 0.023 (25.3)
5.51 (9.8) 8.26 (11.6) 11.0 (14.3)
92 60 57
The error limits of nia and Kd are (2 and (3%, respectively. b ω ) 8 (for C0 and C1) and 52 (for C2 and C3). c The isooctane/n-hexanol (v/v) ratio for initial microemulsion stabilization is 26.3:1 (C0); 32.3:1 (C1); 50:1 (C2); 44:1 (C3). Density of isooctane ) 0.692 g/mL, n-hexanol ) 0.814 g/mL. a
get a series of nta/ns and no/ns values that can be graphically plotted according to eq 12 to get nia from the I value if nwa is known or negligible. The constant k can be obtained from S. Kd can be written in the form
Kd )
Xia Xoa
)
nia(noa noa (nia
+ no)
R(1 + S) ) S(1 + R) + ns)
(13)
where R ) nia/ns ) I - (nwa /ns) and S ) k. To get Kd, knowledge of nwa is required which can be obtained from the solubility data of alkanols in water. The higher alkanols other than n-butanol have very low or negligible solubility,36 which makes R ) I, reducing eq 13 to
Kd ) I(1 + S)/S(1 + I)
(14)
In the present study, eq 14 has been used, since the cosurfactant (n-hexanol) had negligible water solubility. ∆G°t can be obtained from the relation
∆G°t ) -RT ln Kd
(15)
∆H°t and ∆S°t then follow from the relations
∆H°t )
[
]
∂(∆G°t /T) ∂(1/T)
(16)
p
and
∆S°t ) (∆H°t - ∆G°t)/T
(17)
For C2, the enthalpy was temperature dependent; for its evaluation, a second-degree polynomial of the following form was used.
∆G°t ) a + bT + cT2
(18)
The relation for ∆H°t then follows as
∆H°t ) a - cT2
(19)
The evaluation of ∆C°pt follows from the relation
∆C°pt )
( ) ∂∆H°t ∂T
p
(20)
The validity of eq 12 both at different surfactant concentrations and temperatures exemplified in the plots has resulted in good correlations (Figure 7). The evaluated parameters nia, k, and Kd are presented in Table 4 for all of the four studied surfactants (C0, C1, C2, and C3). The differences that arose due to differences in the surfactant head group type and temperature are evident from the results. In the calculation of Kd, concentrations in mole fraction scale were used: the distribution constant was not activity derived for want of such knowledge for the cosurfactant in oil and at the interface.29 Although the distribution of alkanols between water and micelles in terms of activity has been reported,37 its partition between oil and interface in a microemulsion is a nonidentical issue which is yet to be quantified in terms of activity. We have herein considered equivalent nonideality effects on the cosurfactant in the inter-
11322 J. Phys. Chem. B, Vol. 110, No. 23, 2006
Mitra et al.
Figure 8. Plot of nia vs ns for the w/o microemulsion systems: (a) with C3 at 313 and 318 K (inset, with C1 at (1) 323 K, (2) 318 K, and (3) 313 K); (b) with C2 at 303, 308, 313, and 318 K.
TABLE 5: Thermodynamic Parametersa for the Transfer of n-Hexanol from Oil to the Interface -∆G°t (kJ mol-1) (-∆H°t (kJ mol-1)) ∆S°t (J K-1 mol-1)
105ns (mol) C0
C1
C2
C3
C0
C1
C2b
C3
303 K 1.20 2.44 4.93 7.37
9.74 (-1.9) 38.3 9.37 (-8.7) 59.8 8.95 (6.52) 8.02 8.86 (23.8) -49.4 308 K
1.20 2.42 4.93 7.37
2.22 4.48 6.70
12.5
6.61 (25.4) -61.0 313 K
5.45 8.04 10.9
1.22 2.44 4.90 7.32
2.22 4.40 6.62 6.24 (25.4) -61.0
12.3
9.25 (3.59) 18.1 9.09 (23.0) -44.4 9.04 (22.6) -43.2 318 K
5.67 8.31 11.1 12.8
1.20 2.46 4.90 7.29
2.18 4.57 6.57 5.93 (25.4) -61.0
5.51 8.26 11.0
9.33 (3.59) 18.0 8.88 (23.0) -44.4 8.82 (22.6) -43.2
9.81 (6.86) 9.59 9.33 (13.3) -12.7 9.02 (25.6) -53.9 8.98 (40.3) -102
9.39 (31.5) -71.9 8.16 (5.08) 10.0 7.31 (3.54) 12.2
9.82 (17.3) -24.0 9.22 (39.7) -97.2 8.23 (48.5) -129 7.49 (60.0) -168
9.05 (31.5) -71.8 8.26 (5.08) 10.2 7.33 (3.54) 12.1
9.58 (26.1) -51.8 8.40 (61.7) -167.0 7.60 (67.6) -189.0 7.00 (76.5) -218.0
8.68 (31.5) -71.9 8.26 (5.08) 10.0 7.43 (3.54) 12.2
323 K 9.44 (3.59) 18.0 8.71 (23.0) -44.4 8.67 (22.6) -43.2
a The error limits of ∆G°t, ∆H°t, and ∆S°t are (3, (5, and (8%, respectively. b ∆C°pt ) -1.89, -4.75, -4.12, and -3.55 kJ K-1 mol-1 at 1.20, 2.50, 4.90, and 7.30 mol of C2, respectively.
facial region and oil to consider the concentration ratio derived Kd value as a thermodynamic quantity. The interfacial composition of the cosurfactant is essentially decided by the concentration of the surfactant at a given temperature for a given system. In Figure 8, nia versus ns profiles are presented for the surfactants C1, C2, and C3; C0 is not represented for the reason of its low solubility and difficulty in performing experiments with varied concentration of the amphiphile. The feature of C1 was different from C2 and C3; while nia increased linearly with ns for C1, it decreased exponentially with ns for C2 and linearly with ns for C3. In the past, a nonlinear dependence of nia on ns has been reported for cationic and anionic surfactants (viz., cetyltrimethylammonium bromide, cetylpyridinium chloride, and sodium dodecyl sulfate) and n-butanol in isopropyl myristate.29 The linear direct and
inverse dependences of nia on ns observed for C1 and C3 were different observations. The intercept (I) and the slope (S) of eq 12 have been found to depend oppositely on temperature, as reported earlier.29 However, their ratio (I/S) linearly decreased with temperature. The ratio was defined as β28-30 and is given by the relation considering nwa ) 0 as i w i I (na + na )no nano β) ) ≡ S n no n no s a
(21)
s a
β is considered as a measure of the relative adjustment between the surfactant and the cosurfactant populated at the interface and that of the latter in the oil phase for the sake of droplet stability. The β values are also presented in Table 4.
CTAB and Its Modified Head Group Analogues
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11323
Figure 9. Gibbs-Helmholtz plot for the evaluation of the standard enthalpy of transfer of n-hexanol from oil to the interface: (a) with C0 at 125 µmol, with C1 ((1) 56 µmol, (2) 82 µmol, and (3) 110 µmol), and with C3 ((1) 22 µmol, (2) 45 µmol, and (3) 66 µmol); (b) with C2 at (1) 73 µmol, (2) 25 µmol, and (3) 12 µmol (inset, plot of ∆H°t vs T at varying initial [C2]; (1) 73 µmol, (2) 49 µmol, and (3) 12 µmol).
The Kd values obtained were fairly large and decreased with increasing [surfactant] and temperature, implying that the transfer process of the cosurfactant from oil to the interface was exothermic. The ∆H°t values were obtained according to eq 16 for C0, C1, and C3; for C2, ∆H°t was evaluated in terms of eqs 18 and 19. The derived thermodynamic parameters for the transfer process are presented in Table 5. The plots of ∆G°t/T versus 1/T are presented in Figure 9. Since ∆H°t for C2 became a function of temperature, the ∆C°pt values were also obtained for the systems from the slopes of the plots according to eq 20 (inset of Figure 9b). Structural Parameters. The structural parameters of the microemulsion droplets were estimated following a simplified model.28-30,38-40 The droplets were assumed to be monodisperse and spherical, having a monomolecular film of amphiphile at the interface in their dispersion in oil. Vd and Ad are given by the relations
Vd ) (4/3)πRe3Nd
(22)
Figure 10. Interdependence of Nd, nia, and ns for the w/o microemulsion systems: (a) with C1 at different temperatures of (1) 313 K, (2) 318 K, and (3) 323 K; (b) with C2 at different temperatures of (1) 303 K, (2) 308 K, (3) 313 K, and (4) 318 K.
above helps to get Via according to the relation
Via ) niaMa/Fa
For the evaluation of Vs, a relation equivalent to eq 25 has been used. Equation 23 for Ad can also be written as
Ad ) [ns(As + IAa) - nwa Aa]NA
(23)
Vd is related with Vw, Vs, and Via in the form
Vd ) Vw + Vs + Via
(24)
The knowledge of nia from the dilution protocol discussed
(26)
For negligible solubility of the cosurfactant in water (as in the present study with n-hexanol), nwa ≈ 0; eq 26 then becomes
Ad ) [ns(As + IAa)]NA
(27)
In the present study, the Amin values of the surfactants obtained from the tensiometric measurements (given in Table 3) were used for As, and Aa ) 0.2 nm2 was used for the cosurfactant in the calculation.30 Re and Nd can be evaluated from the relations
and
Ad ) 4πRe2Nd
(25)
Re ) 3Vd/Ad
(28)
Nd ) 3Vd/4πRe3
(29)
and
N ˜ s and N ˜ a can be estimated in terms of the relations
N ˜ s ) nsNA/Nd
(30)
11324 J. Phys. Chem. B, Vol. 110, No. 23, 2006
Mitra et al.
TABLE 6: Structural Parametersa for the Transfer of n-Hexanolb from the Oil to the Interface 105ns (mol) C0
C1
C2
C3
C0
C1
C2
N ˜ s (N ˜ a)
10-16Nd
Rw (Re) (nm) C3
C0
C1
C2
C3
C0
C1
C2
C3
303 K 1.20
3.77 (5.31) 5.50 (7.02) 7.94 (9.51) 9.64 (11.3)
2.44 4.93 7.37
5.13
140 (1526) 445 (2318) 1353 (3322) 2381 (3813)
3.30 2.19 1.86 308 K
1.20 2.42
2.22
4.93
4.48
7.37
6.70
12.5
3.72 (5.27) 5.59 (7.12) 8.29 (9.87) 9.90 (11.5)
5.11 3.97 (5.50) 7.81 (9.38) 9.98 (11.6)
1.93 (3.90)
3.07
8.03
1.93
2.10
1.70
1.50
59.7
141 (1498) 474 (2354) 1539 (3434) 2615 (3798)
166 (1610) 1283 (3272) 2693 (3766)
213 (1761) 864 (2924) 2420 (3770) 3822 (3817)
164 (1601) 716 (2719) 2948 (3812)
232 (1797) 935 (2978) 2780 (3826) 4559 (3731)
299 (1995) 1548 (3413) 3116 (3812)
126 (727)
313 K 1.22 2.44
2.22
5.45
4.90
4.40
8.04
7.32
6.62
2.20 (3.85) 2.23 (3.87) 2.26 (3.89)
10.9 12.3
4.30 (5.83) 6.86 (8.40) 9.65 (11.3) 11.2 (12.9)
3.45 3.95 (5.48) 6.42 (7.95) 10.3 (12.0)
1.70
8.14
17.9
1.22
3.70
24.8
1.15
1.35
183 (401) 196 (376) 206 (357)
32.0
3.05 (4.98)
15.2
489 (671)
318 K 1.20 2.46
2.18
5.67
4.90
4.57
8.31
7.29
6.57
2.20 (3.85) 2.30 (3.92) 2.34 (3.95)
11.1 12.8
4.39 (5.92) 7.02 (8.57) 10.1 (11.7) 11.9 (13.6)
3.10 4.81 (6.33) 8.29 (9.88) 10.5 (12.1)
1.59
4.39
18.0
1.06
1.78
23.4
0.96
1.27
190 (381) 214 (349) 224 (334)
29.8
3.56 (5.55)
9.54
811 (461)
323 K 5.51
2.28 (3.90) 2.37 (3.97) 2.38 (3.97)
8.26 11.0
16.2
205 (365) 231 (324) 236 (308)
21.5 28.0
a Density of the surfactants ∼1.01 g/mL (assumed to be nearly equal to the density of CTAB). b Area of the head group of n-hexanol taken as 0.2 nm2.30
and
and
N ˜ a ) niaNA/Nd
(31)
Rw is related with the effective droplet radius (Re) [Re ) Rw + L (the thickness of the adsorbed monomolecular amphiphile film)] as
Rw ) [(VH2O + Vhs + Vha )/Vd]1/3Re
(32)
where Vhs and Vha are given by the following relations
Vhs )
4 As3/2N ˜s 3π1/2
(33)
Vha )
4 Aa3/2N ˜a 3π1/2
(34)
The structural parameters Rw, Re, Nd, N ˜ s, and N ˜ a are presented in Table 6. The parameter values used in the calculations to derive them are given in the footnotes of Table 6. The representative interdependence of nia, ns, and Nd is exemplified in parts a and b of Figure 10 for C1 and C2 bearing systems, respectively, at different temperatures. The C1 derived system witnessed linear dependence, while those for C2 and C3 (not illustrated) were exponential in nature. With increasing temperature, nia declined with a consequent decline in Nd.
CTAB and Its Modified Head Group Analogues
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11325 comparable conditions, the number was more with C3 than C2. The population of surfactant and cosurfactant on the droplet interface was comparatively lower for C3 than C2. The C3/nhexanol/isooctane bearing microemulsion is thus expected to be more convenient for enzyme accommodation at the interface to augment better catalytic activity. This has agreed with the observed lipase activity found toward the hydrolysis of pnitrophenyl-n-hexanoate.14,15 Acknowledgment. D.M., I.C., S.R., and D.D. thank CSIR, Govt. of India, for the financial support. P.K.D. is also thankful to CSIR for a research grant. S.P.M. thanks INSA, Govt. of India, for a senior scientist position. References and Notes
Figure 11. Profiles of (a) Nd vs nw, (b) N ˜ s vs ns, and (c) N ˜ a vs ns, all with C2 and C3 at 313 K.
It has been reported that lower concentration of n-hexanol and surfactant at the droplet interface and increased size of the head group of the surfactant produced better lipase activity.14,15 In Figure 11a, Nd versus nw profiles at 313 K show more efficient dispersion of water in C3 than in C2; the formation of a bigger interface with a larger number of lower dimension water droplets was envisaged. At equal moles of surfactants (ns), the average numbers of surfactant and cosurfactant molecules ˜ a, respectively) at the present per droplet surface (N ˜ s and N studied temperatures were both lower for C3 than for C2 (Figure 11b,c). The observed lower population of surfactant and cosurfactant makes more room for water to solvate the w/o interface in the case of C3 than C2. As a consequence, an enzyme has more access to the interface to exhibit better catalytic activity which was observed by us14,15 with lipase. It is to be noted that C0 and C1 were kept out of the above comparison for their inability to form stable w/o microemulsions at ω > 8. Conclusions The surfactants C0, C1, C2, and C3 have shown very low cmc values in aqueous medium. C1 exhibited two cmcs at 298 and 303 K. The micelles only bound nominal counterions (Br-). Their aggregation numbers were low which increased appreciably with concentration above 8 times their cmc. The energetic parameters for the micellization and interfacial adsorption processes of the surfactants were large. The transfer of n-hexanol from oil to the droplet interface was exothermic in nature and occurred with a negative entropy change. The droplet number increased with decreasing temperature, and under
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