7782
J . Phys. Chem. 1991, 95,1182-1184
Thermodynamics of Micellitation of Trtton X-I 00 in Aqueous Ethylene Glycol Solutions Rameshwar Jha and J. C . Ahluwalia* Department of Chemistry, Indian Institute of Technology, Hour Khas, New Delhi 1 1 0016 , India (Received: December 13, 1990)
The enthalpies of solution (R2- H Z 0 )of Triton X-100in water and aqueous ethylene glycol solutions have been measured calorimetrically at different concentrations in pre-cmc and post-cmc regions at 298.15 and 308.15 K from which the enthalpies of micellization, AH,,,, and the heat capacities of monomeric, AC?, and micellar, A C ~ forms a as well as the heat capacities of micellization, AC,,,, have been calculated. The results indicate that the ethylene glycol-hydrocarbon interactions are more favorable than water-hydrocarbon interactions.
Introduction The effect of cosolvents on the aggregational and other physicochemical properties of surfactants is of theoretical as well as industrial interest.'*2 There is a group of solvents which is analogous to These include ethylene glycol, hydrazine, hydrogen peroxide, and formamide, etc. The thermodynamic quantities obtained from systematic composition variation in these water analogue mixtures should provide information of central importance for understanding the unique Yaqueousnfluctuation behaviors. Ethylene glycol (EG) is in a very special class of biological solvents5-' as it is highly associated8v9and forms hydrogen-bonded chains leading to predominantly two-dimensional cooperative domains. EG alone or mixed with water stabilizes proteins against although there are some reports which rate EG as a less effective denaturant for globular proteins than higher alcohols, urea, and guanidine hydrochl~ride.I~-'~ Ethylene glycol being a less effective denaturant could be explained by the model studies of Nozaki and Tanford20 which show that the solubility or transfer of hydrophoic side chains from water to ethylene glycol solution is less favorable than the transfer of amino acid side chains to alcohols2' or guanidine hydrochloride22 solution. Surfactants form micelles in the entire concentration range of EG in aqueous EG solutions as well as in pure EG.4,2*28
(1) Magid, L. In Solutions ojSuflacranrs; Mittal, K. L., Ed.; Plenum: New York 1979; Vol. 1, p 427. (2) Kresheck, G. C. In Water, a Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1975; Vol. 4, p 95. (3) Huot, J. Y.; Battistel, E.; Lumry, R.; Villeneuv, G.; Lavellee, J. F.; Anusiem, A., Jolicoeur, C. J . Solution Chem. 1988, 17, 1601. (4) Ray, A. Narure 1971, 231, 313. ( 5 ) Arakawa, T.; Timasheff, S. N . Biochemistry 1982, 21,6536. (6) Bull, H. B.; Breeze, K. Biophysics 1978, 17, 2121. (7) Herskovits, T. T.; Harrington, J. P. Biochemistry 1972, 25, 4800. (8) Gibbson, R. E.; Loeffler, 0. H. J . Am. Chem. Soc. 1941, 63, 898. (9) Dock, M. R. J . Chem. Soc. Reo. 1975, 4, 211. (IO) Gekko, K.; Morikawa, T. J . Eiochem. 1981, 90, 51. (1 1 ) Fujita, Y.; Noda, Y. Bull. Chem. Soc. Jpn. 1984, 57, 2177. (12) Anusiem, A. C. I.; Oshodi, A. A. Arch. Biochem. Biophys. 1978,189, 392. (13) Tanford, C. Ado. Protein Chem. 1968, 23, 121. (14) Gerlsma, S. V. Eur. J . Biochem. 1970, 14, 150. (15) Bock, J. F.; Oakenfull. D.; Smith, M. B. Biochemistry 1979,18, 5191. (16) Gekko, K. J . Biochcm. 1982. 91, 1197. (17) Herskovits, T. T.; Gadegbeku, B.; Jaillet, H. J . Biol. Chem. 1970,245, 2588. (18) Harskovits,T. T.: Jaillet, H.; Gadegbeku, B. J . Biol. Chem. 1970,245, 4544. (19) Herskovits, T. 7.;Jaillet, H. Science 1969, 163, 282. (20) Nozaki, Y.; Tanford, C. J . Biol. Chem. 1965, 240, 3568. (21) Nozaki, Y.; Tanford, C. J . Biol. Chem. 1963, 238, 4074. (22) Nozaki, Y.; Tanford, C. J . Biol. Chem. 1970, 245, 1648. 0022-3654/9 1 /2095-7782$02.50/0
TABLE I: Enthalpies (kJ mor')of Solution nod Micelliution of Triton X-100in Aqueous Ethylene Glycol Solutions
EG, vol % ' temp, K 0 298.15 308.15 0 298.15 IO 308.15 IO 298.15 20 308.15 20 298.15 30 308.15 30 298.15 50 308.15 50 H20
- H20,
AH,,
AHW
pre-cmP -57.18 f 0.22 -48.54 f 0.20 -44.37 f 0.17 -36.58 f 0.16 -32.56 f 0.22 -27.23 f 0.24 -22.05 f 0.14 -17.80 f 0.17 -11.15 f 0.21 -7.46 f 0.11
post-cmc6
b=
H2- H20,
c=
-48.18 -44.14 -42.38 -38.42 -36.18 -32.96 -34.69 -32.20 -20.21 -18.35
f 0.15
f 0.21 f 0.21 f 0.14 f 0.17 f 0.20 f 0.17 f 0.20 f 0.26 f 0.25
N m C
9.00 4.40 1.99 -1.84 -3.62 -5.73 -1 2.64 -14.40 -9.06 -10.89
f 0.27 f 0.29 & 0.27
f 0.21 f 0.28 f 0.31 f 0.22 f 0.31 f 0.33 f 0.27
H2- H 2 0 ,
The study of micellization of surfactants as model compounds in aqueous EG solutions is therefore important in understanding of the effect of ethylene glycol upon noncovalent interactions in proteins. Only a few studies for micelle formation in water EG mixtures are reported.2528 In these studies thermodynamic parameters of micellization have been obtained from the temperature dependence of cmc using a simple two-state model. Besides a number of approximations involved in such analyses, there are some practical difficulties, and precise determination of cmc becomes quite difficult in the presence of organic cosolutes.29*M Sometimes cmc is not a well-defined quantity and is only weakly dependent on temperature. The first and second differentiations of In cmc to obtain AHn,and ACp cause error magnification. Often the data obtained by this procedure are not as reliable as those obtained from calorimetry. Therefore, we thought it worthwhile to determine the thermodynamic parameters of micellization of surfactants in aqueous ethylene glycol solutions of various concentrations by a calorimetric method. Goddard and Benson" have discussed the possible procedures for the determination of AHm calorimetrically. If the integral enthalpy of solution is known as a function of molality ( m ) , we have (R2 - H 2 O ) = (d(mAHs)/dm)r,, The difference of the R2in the micellar and monomeric states, viz., AH,,,, may be determined from such data, when available. (23) Lee, J . C.; Timasheff, S. N . J . Biol. Chem. 1981, 256, 7193. (24) Smith, M. B.; Oakenfull, D. G.; Baek, J. P. Ausr. Biochem. Soc. Proc. 1978. 1 1 , 4. (25) Ray, A. J . Am. Chem. Soc. 1969, 91, 6511. (26) Ray, A.; Nemethy, G. J . Phys. Chem. 1971, 75, 809. (27) Ionescu, L. G.; Fung. D. S.J . Chem. Soc., Faraday Trans. I 1981, 77, 2901. (28) lonescu, L. G.; Romanesco, L. S.;Nome, F. ln Surjacfants in Solulion; Mittal, K. L., Ed.;Plenum: New York, 1979; Vol. I , p 789. (29) Emerson, M. F.; Holtzer, A . J . Phys. Chem. 1967, I / , 3320. (30) Flockhart, B. D.; Ubbelohde, A. R. J . Colloid Sci. 1953, 8, 428. (31) Goddard, E. D.; Benson, G. C. Can. J . Chem. 1957.35, 986.
0 199 1 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7183
Micellization of Triton X-100 TABLE 11: Heat Capacities (J K-' mol-') of Triton X-100 in Aqueous Ethylene Glycol Solutions EG, vol % ACpm0,/ ACpmicb Acme 0
864 779 553 425 369
10 20 30 50 a
=
cp*o
- cp,o.
f 30 f 23 f 32 f 22 f 22
* = c,, - cp*o.
404 396 322 249 186
=
f 26 f 25 f 26 f 32 f 36
-460 -383 -211 -176 -183
TABLE I V Transfer Heat Capacities (J K-' mol-') for the Transfer of Triton X-100 from Water to Aqueous Ethylene Glycol Solutions at 303.15 K
f 40 f 34
c,, - c p , o .
20 20 30 30 50 50
298.15 308.15 298.15 308.15 298.15 308.15
AH,,,,, 12.81 f 0.30 1 I .96 f 0.26 24.62 f 0.31 21.31 f 0.31 35.13 f 0.26 30.74 f 0.26 46.03 f 0.26 41.08 f 0.23
AH,,,, 5.80 f 0.26 7.52 f 0.25 12.00 f 0.23 11.18 f 0.29 13.49 f 0.20 11.94 f 0.34 27.97 f 0.30 29.83 f 0.33
-85 -331 -439 -495
f 38 f 44 f 37
f 38
-8 -155 -155 -218
f 36 f 31 f 39
f 44
77 f 249 f 284 f 277 f
52 57 54 58
"Transfer water to aq. EG.
TABLE 111: Transfer Enthalpies (kJ mol-') for the Transfer of Triton X-100 from Water to Aqueous Ethylene Glycol Solutions
EG," vol % temp, K IO 298.15 IO 308.15
10 20 30 50
f 41 f 39 f 43
AHmsr f 0.40
-7.01 -6.24 -12.62 -10.13 -21.64 -18.80 -18.06 -15.29
f 0.36 f 0.39 f 0.42 f 0.35 f 0.43 f 0.42 f 0.40
"Transfer water to aq. EG. We have adopted a slightly modified procedure (as developed by Singh and A h l ~ w a l i and a ~ ~simultaneously ~~~ by Mazer and Olofsson3' which avoids the differentiation step in numerical calculations. Using very small quantities of samples of surfactants, we have directly determined the differential enthalpies of solution (Rz- H z o ) by using the pure solvent and also a micellar solution of surfactant as a solvent in two sets of experiments. The difference of these two values has been taken as AH,,,. A detailed justification is given elsewhere.3z In addition, our procedure gives the contribution of the changes in monomeric and micellar properties to net change in the micellization parameters AHm and AC,,,,.From the enthalpy of solution data ( H 2- H z o ) in monomeric and micellar states at different temperatures, the enthalpy of micellization AH,,,, the heat capacity changes (CP? - CP2")in premicellar and postmicellar states, and heat capacity of micellization ACpmfor the nonionic surfactant Triton X-100have been obtained.
Experimental Section The calorimetric setup and the procedure for the determination of AH,,, and AC,, are given e1sewhe1-e.~~Triton X-100 was obtained from Sigma and contained 9.5 oxyethylene groups on average as reported by the manufacturer. It was dried over dust-free molecular sieves. EG from E. Merck was distilled for purification before use. Results The enthalpies of solution (Rz- H Z 0 )in pre- and post-cmc regions of the surfactant Triton X-100 in 10,20,30,and 50% (v/v) aqueous EG solutions were measured at 298.15 and 308.15 K. It was not possible to measure the enthalpy of solution values at greater than 50% EG concentration as the solution became quite viscous at higher EG concentration. For the sake of spatial economy, the mean values of six to nine determinations of enthalpies of solution (R2- H20) in pre- and post-cmc regions along with the derived values of enthalpy of micellization AHmat 298.1 5 and 308.15 K are given in Table 1. The derived values of heat capacities - C,,") of monomers and micelles and values of micellization AC,, are listed in Table 11. Combining the above data with the corresponding enthalpies and heat capacities of micellization of Triton X-100 in pure water, the values for the transfer enthalpies AH,r and heat capacities ACpr of Triton X-100
(ep2
(32) Singh. P. K.;Ahluwalia, J . C. J . Sur/. Sci. Technol. 1986, 2, 51. Singh, P. K. Ph.D. Thesis, Indian Institute of Technology, New Delhi, India. (33) Singh, P. K.;Ahluwalia, J. C. In Surfactant in Solution; Mittal, K. L., Ed.; Plenum: New York, 1989, Vol. 7, pp 277-287. (34) Mazer, N . A.; Olofsson, G. J . Phys. Chem. 1982, 86, 4584.
for transfer from water to aqueous EG solutions have been derived and are listed in Tables I11 and IV, respectively. The uncertainties reported for the enthalpies of solution are within the 95% confidence limits. The uncertainties in the various AH,,,, ACm, and ACp of monomers and micelles have been calculated by using the following equations. Representing uncertainty by e e(AH,) = [ C e z ( R z- H z 0 ) ] 1 / 2 and e(ACp) =
1
[E:(& T'- T
- H2°)11'2
where the sum extends over all (Rz- H Z 0 ) . Enthalpies of Solution in Pre- and Post-cmc Regions and Enthalpies of Micellization. The enthalpies of solution at 298.15 and 308.15 K in pre- and post-cmc regions in aqueous EG solution are less negative than in water. The enthalpies of solution (R2 - H Z o )in the post-cmc region are always more exothermic than its pre-cmc counterpart except in 10%EG solution at 298.15 K. The net result is that the enthalpy of micellization AH,,, is exothermic over all EG concentrations except in 10%aqueous EG solution at 298.15 K. The AHm decreases with increase in temperature and EG concentration up to 30% EG. The values AH,,, a t 50% EG concentration is less negative than the values in 30% aqueous EG solution. Thus it seems that the AH,,, values passes through the minimum between 30 and 50% EG solutions. To the best of our knowledge we could not find any reported calorimetric data for the enthalpies of solution (in pre- and post-cmc regions) and the enthalpies of micellization of Triton X-100in aqueous EG solutions. The only data for AH,,, and other thermodynamic parameters for Triton X-100in aqueous EG are due to Ray and Nemethyz6 obtained from the temperature variation of cmc. The trend of AH,,, values of Triton X-100in EG solution at 298.15 K observed by them is similar to that found by us up to 50% EG concentration. However, the AHmvalues reported by Ray and Nemethy26are comparatively more negative than our values at the same.EG concentration. Their values of AH,,, are -4.6,-1 2.5,and -1 2.1 kJ mol-' at 298.15 K and -4.2, -10.5, and -18.8 kJ mol-l at 308.15 K in aqueous EG solutions containing 10,20, and 50% EG, respectively. The discrepancy in the values may be attributed to the higher uncertainty in the method used by Ray and Nemethy.26 Heat Capacitiesof Monomers, Micelles, and Micellization. The heat capacities, (CPz- Cp20),,,ono/mic, of Triton X-100 monomers and micelles in pre- and post-cmc regions in aqueous EG solution are relatively less positive compared to the respective values in pure water, and then decrease with the increase of EG concentration. The decrease in the pre-cmc region is relatively more pronounced than that in the post-cmc region. Since the heat capacity of micellization, AC,,,,, has been taken as the difference of the post- and premicellar heat capacities - C,,zo), the ACpmvalues of Triton X-100in aqueous EG solution is relatively less negative in comparison to the corresponding value in water. Further, the AC,, value increases with the increase in EG concentration UD to 30% and thereafter becomes almost constant till 50% of EG. Thus it seems that probably ACm passes through a maximum between 30 and 50% of the aqueous EG concentration. No heat capacities of Triton X-100either in monomeric or micellar states or the heat capacities of micellization in aqueous
(ep2
7184 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Jha and Ahluwalia
1
~
0
IO
%EG l V / V )
20 '1. E t
Figure 1. Transfer enthalpies of Triton X-100 from water to aqueous ethylene glycol solutions at 298.15 K.
30 LO Concmlrallon ( v / v )
50
60
Figure 3. Transfer heat capacities of Triton X-100as a function of EG concentration at 303.15 K.
TABLE V: Some of the Tbermodymmic Parameters of Micelliution of Triton X-100 in Aaueous Ethvlene Clvcol Solutions at 298.15 K EG, cmc,mol hs,, J AG,,,, vol % L-' X IO' kJ mol-l kJ mol-' K-' mol-' M m T
0
IO 20 30 50
-251
I
I
I
I
I
I
I
6
0
5
IO
15
20
25
30
35
LO
I
45
I
1
50
55
€4
'A EG l V / V )
Figure 2. Transfer enthalpies of Triton X-100from water to aqueous EG solutions at 308.15 K.
EG solutions have been reported in the literature. The Transfer Parameters. The transfer enthalpies and heat capacities for the transfer of Triton X-100 from water to aqueous ethylene glycol solutions have been calculated as
AHtr,mono/mic (H2 - H~~)tr,mono/mic = W 2 - H2°)mom/mie in aq EG) - f(H2 - H~')mono/mic in water) AH,,,,, = (AH,,, in aq EG) - (AH, in water) Similarly for the transfer heat capacities ACp,tr,mono/mic= { ( c p 2 - CpZo)mono/misin eq EG) Kcp2
- CpZ0)mono/mic in water)
ACP,,, = {AC,,, in aq EG) - {ACP in water) The transfer enthalpies, AHw,and heat capacities, AC,,,,?are listed in Tables 111 and IV and have been plotted as a function of EG concentration in Figures 1-3. No literature values have been found for the direct comparison. The pre- and post-cmc transfer enthalpies for the transfer of Triton X-100 monomers and micelles from water to aqueous EG are highly positive at 298.1 5 and 308.15 K respectively. The pre-cmc AH,,,,, is relatively more positive than the corresponding AH,,,,, and hence the AH,,,,,,for the transfer of the enthalpies of micellization in water to aqueous EG are highly negative. The transfer heat capacities AC,,,,,for Triton X-100 monomers and micelles are negative. The pre-cmc values are more negative than the corresponding post-cmc values, so the net result is that AC,,,,, is always positive and increases with the increase in the EG concentration up to 30%. However, AC,,,,,, is found to decrease slightly in 50% aqueous EG solution.
Discussion The enthalpies of solution in both pre- and post-cmc regions are comparatively less negative than in water. This may be attributed to the competition for formation of hydrogen bonds with the oxyethylene groups of Triton X-100 molecules with EG
2.80 2.92 3.89 6.0 17.60
-20.28 -20.17 -19.46 -18.39 -1 5.72
9.00 1.99 -3.62 -12.64 -9.06
98 74 53 19 22
and water in the pre-cmc region. There is a preferential solvation of the monomeric Triton X-100molecules which also causes increase in cmc in aqueous EG. Thus, in the pre- and post-cmc region, a large number of hydrogen bonds are formed between EG and oxyethylene groups of the Triton X-100 and, as these bond formations should be relatively less exothermic than the hydrogen bonds between oxyethylene groups and water molecules, there is an overall decrease in the exothermicity. Thus with the increase in the EG concentration, the enthalpies of solution in the pre-cmc region decrease more sharply than in the post-cmc regions. The net result is that AH,,, becomes negative in aqueous EG becomes more negative solutions and this negative value of AH,,, with the increase in EG concentration. Further, there is a situation where the enthalpy of micellization becomes athermal with EG concentration. It is well-known that EG is a water structure breaker and decreases hydrophobic interactions. This leads to relatively lower heat capacities of Triton X-100in pre- and post-cmc regions in aqueous EG solutions. The result is that ACp,,,.becomes less negative in aqueous EG solution compared to that in water. This phenomenon can be understood on the basis of preferential solvation as proposed by Timasheff and c o - ~ o r k e r s , 3Nozaki ~ * ~ ~ and Tanford,20 and Ray and NemethyaZ6 According to them, the EG-hydrocarbon interactions are more favorable than waterhydrocarbon interactions. That is, the hydrocarbon chains of the Triton X-100 monomers preferentially interact with EG molecules. The local concentration of the EG molecules around the Triton X-100monomers thus becomes larger than the average of bulk. This should be also true in the case of micelles, and so the heat capacities of monomers as well as micelles become less in EG solution. Hence AC,, increases (becomes less negative) with EG concentration. By use of the values of the free energy of micellization:6 AG,, derived from the cmc values of Triton X-100a t different concentration of EG in conjunction with our calorimetric values of enthalpies of micellization, the entropies of micellization in EG solutions are found to be less positive compared to those in pure water and decrease with the increase in EG concentration (Table V). AH,,, values, on the other hand, become more negative within the increase in EG concentration. This indicates that with the increase in EG concentration the formation of aggregates from the monomers becomes largely enthalpy controlled rather than entropy controlled. (35) Timasheff, S. N.; Inoue, H . Biochemistry 1968, 7 , 2501. (36) Gekko, K.; Timasheff, S. N . Biochemistry 1981, 20, 4667.
