J. CHEM. SOC. FARADAY TRANS., 1993, 89(18), 3465-3469 3465

Thermodynamics of Micellization of some Decyl Poly(oxyethy1ene glycol) Ethers in Aqueous Urea Solutions

Rameshwar Jha and Jagdish C. Ahluwalia* Department of Chemistry, Indian Institute of Technology, Hauz Khas , New Delhi I 100 16, India

The enthalpies of solution (H2 - H:) of some homogeneous decyl poly(oxyethy1ene glycol) ether (C,,E,) sur- factants in monomeric and micellar states in 3 and 6 mol dm-3 aqueous urea solutions have been determined calorimetrically in the temperature range 288.1M18.15 K. The enthalpies of micellization (AmiCH) and the heat capacities of monomeric (AC,, ,,,,) and micellar (AC,,, mic) forms as well as the heat capacities of micellization (AC,,, ,,,) have been evaluated from the enthalpy data. AmicH is less positive in aqueous urea solutions than in water and decreases with increasing urea concentration and temperature. AC,, is relatively higher in aqueous urea solutions. The results indicate a decrease in hydration or structure around monomers in the pre-c.m.c. (critical micelle concentration) region and a relative increase in hydration of micelles due to enhanced hydrogen bonding between ethylene oxide groups and urea molecules in the p0stc.m.c. region. Interactions between polyoxyethylene (hydrophilic) groups and urea seem to dominate over decyl (hydrophobic) group-urea inter- actions.

The aggregational and surface properties of surfactants in solutions are very sensitive to the presence of cosolutes.'-'' The nature of the cosolute decides the direction of the changes in the c.m.c. of the surfactants. There are some organic cosolvents which when present in greater amounts even cause disappearance of the micelles.' ' 9 l2 Electrolytes generally decrease the c.m.c. and the cloud points." On the other hand, non-electrolytes may increase or decrease the c . ~ . c . ~ - ' ~ and the cloud points.''~'' The changes in the c.m.c. brought about by cosolutes have been explained on the basis of solvent structural changes, scaled particle theory, pair- pair, pair-triplet interactions" and also on the basis of changes in the chemical potentials of the monomers.

Real systems of industrial interest such as those involving detergent action, solubilization and enhanced oil recovery, as well as biochemical systems bearing similarities to surfactant aggregates, are all multicomponent systems with complex behaviour and are difficult to investigate. Studies of binary and ternary surfactant systems may shed light on the behav- iour of multicomponent systems.

In this paper we report thermodynamic investigations on the surfactant-water-urea ternary system. It is well known that urea increases the c.m.c. appreciably for many sur- factants when present in large quantities.'*' '*14 Va rious experimental studies' 5-23 show that urea is a potential water structure breaker, particularly at concentrations higher than 2 mol dm-3. The work of Wetlaufer et aLZ4 on transfer of solutes containing hydrophobic groups from water to aqueous urea solutions indicates that the strength of the hydrophobic interactions decreases with the addition of urea, while the opposite view has been expressed by Ben Naim and Yaaco bi.

and its role is still not very clear. The denaturation of proteins may be con- sidered to be equivalent to the demicellization of micelles in aqueous urea solutions. Furthermore, it has been suggested that the increase in the c.m.c. on addition of urea also depends on factors like micelle size, the nature of the polar head groups of the surfactants and temperature.22 Often, changes in the thermodynamic parameters of micellization have been attributed to the stabilization of the monomers in aqueous urea solutions, but the interactions of the micelles with urea in the post-micellar solutions are also significant. Besides these, other possibilities such as specific urea- surfactant interactions and binding have also been sug-

Urea is a strong protein

g e ~ t e d . ~ ~ . ~ ' However, it is still not clear which type of interaction is responsible for the behaviour of surfactants in aqueous urea solutions.

Therefore, in order to obtain a better understanding of the nature of the interactions of the surfactants in aqueous urea solutions, studies of various thermodynamic functions in the pre- and post-c.m.c. regions may be significant. Particularly useful in this direction are transfer enthalpies and heat capac- ities, which are sensitive to the structure of the solvent. With this objective in mind, we have studied the thermodynamics of micelle formation of some decyl poly(oxyethy1ene glycol) ether surfactants in aqueous urea solutions at different con- centrations using a procedure which gives the contributions of the changes in the monomeric and micellar states to the net change .in the micellization parameters. Details of this procedure are given e l~ewhere .~ ' -~~

Experimental The enthalpies of solution of tetraoxyethylene glycol mono-n- decyl ether (C, oE4), pentaoxyethylene glycol mono-n-decyl ether (CioE5), hexaoxyethylene glycol mono-n-decyl ether (C loE,) and octaoxyethylene glycol mono-n-decyl ether (CloE,) have been determined in 3 and 6 mol dm-3 aqueous urea solutions in the temperature range 288.15-318.15 K.

The surfactants (> 99% pure) were obtained from Nikko Chemical Company, Japan and used without further treat- ment. Further details regarding their use, the calorimetric setup and the experimental procedure for the determination of enthalpies and heat capacities of micellization are reported el~ewhere.~'-~' Urea (analytical reagent grade, BDH) was used as received after drying in an oven at ca. 333 K for 12 h. Doubly distilled water was deionized by passing through a mixed-bed resin (Cole-Parmer, USA) and was used for making solutions.

Results The mean values of six to eight determinations of the enth- alpies of solution (R, - H:) in monomeric and micellar states, along with the derived values of enthalpies of micelli- zation (Amic H) of the above surfactants in 3 and 6 mol dm-3 aqueous urea solutions, at temperatures ranging from 288.15 to 318.15 K, are given in Table 1. The derived values of the heat capacities (cp2 - CF2) of monomers and micelles, and heat capacities of micellization (AC,,,) are given in Table 2.

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3466 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

Table 1 Enthalpies of solution and micellization of some decyl poly(oxyethy1ene glycol) ethers in aqueous urea solutions

A H B J mol-’

[urea]/mol dm- T K pre-c.m.c.O post-c.m.c.b Amic H‘

‘loE, -35.70 f 0.36 -35.40 f 0.38 -33.55 f 0.33 -33.28 f 0.28 -29.92 f 0.19 -29.20 & 0.24

‘loE, -38.67 & 0.17 -39.69 f 0.23 -31.83 f 0.15 -31.73 f 0.19

Cl OE6 -46.82 f 0.22 -47.09 f 0.16 -40.44 _+ 0.22 -41.62 f 0.20

ClOEfI -60.55 f 0.53 -63.44 f 0.40 -53.28 f 0.15 -56.72 f 0.46 -44.61 f 0.20 -45.36 f 0.31

288.15 288.15 293.15 293.15 298.15 298.15

-22.75 f 0.27 -24.80 f 0.15 -20.82 f 0.28 -23.14 +_ 0.19 -20.41 f 0.28 -21.56 f 0.21

12.95 f 0.45 10.60 f 0.41 12.73 f 0.43 10.14 f 0.40 9.51 f 0.21 7.64 f 0.32

298.15 298.15 308.15 308.15

-28.08 f 0.15 -31.53 f 0.17 -24.20 f 0.20 -26.12 0.16

10.59 f 0.23 8.16 f 0.24 7.63 f 0.25 5.61 f 0.25

298.15 298.15 308.15 308.15

-34.86 f 0.20 -37.57 f 0.18 -31.35 f 0.13 -33.94 & 0.22

11.96 f 0.25 9.52 f 0.24 9.09 f 0.25 7.68 f 0.30

298.15 298.15 308.15 308.15 318.15 318.15

-49.41 f 0.25 -54.77 f 0.32 -44.28 f 0.25 -49.55 f 0.24 -39.67 f 0.25 -40.63 f 0.25

11.14 f 0.59 8.67 f 0.51 9.00 f 0.23 7.18 f 0.52 4.94 f 0.32 4.73 f 0.40

a R; - H ; . R , - H ; . H , - H ; .

The uncertainties reported for the enthalpies of solution are within the 95% confidence limits. The uncertainties in the values of AmicH, AC,,, and AC, of monomers and micelles have been calculated using the following equations. Rep- resenting the uncertainty by e:

are more exothermic. It may be seen from Table 1 that with increase in urea concentration AHs increases in the pre-c.m.c. region and decreases in the post-c.m.c. region. The net effect is that the enthalpy of micellization of these non-ionic sur- factants in aqueous urea solutions is less endothermic than in water, and this decrease in endothermicity increases with increase in urea concentration. The enthalpy of micellization decreases with increase in temperature, but the decrease in the aqueous urea solution is less pronounced than that in water.

e(Amic H) = [x e2(A, - H:)] ‘ I 2

and

e(AC,) = [x e2(R2 - H!3]1/2/7” - T

where the sum extends over all (R, - H:). Heat Capacities The monomeric heat capacities (c,”, - C,”,) of all of the sur- factants are less positive in aqueous urea solutions, whereas

Enthalpies of Solution and Micellization The enthalpies of solution of surfactants (AH,) in aqueous urea solutions in pre-c.m.c. region are less exothermic than those in pure water, whereas those in the post-c.m.c. region

the micellar heat capacities (C,, - C:,) are more positive. Generally, with an increase in urea concentration, the heat

Table 2 Heat capacities of some decyl poly(oxyethy1ene glycol) ethers in aqueous urea solutions

ACp, mic b/J mol - K - ACpsmc/J mol-’ K - ’ [urea]/mol dm- TIK ACp ”/J mol - K -

C10E4 234 f 29 324 f 26

388 f 25 541 f 23

351 & 24 363 f 28

513 & 30 522 f 40 461 f 30 892 f 37

‘10E5

ClOE6

ClOEI3

3 6

293.15 293.15

578 f 41 620 f 45

-344 & 50 -296 5 52

3 6

303.15 303.15

684 & 24 796 f 30

-296 35 -255 f 38

3 6

303.15 303.15

638 f 31 547 f 26

-287 f 39 - 156 f 38

303.15 303.15 313.15 313.15

727 & 55 672 +_ 61 867 & 25

1136 f 55

-214 f 63 - 150 73 -406 f 39 -244 66

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capacities increase in the pre- and post-c.m.c. regions. The net effect is an increase in the heat capacity of micellization (ACp, 3 with increase in urea concentration.

Transfer Parameters

The transfer enthalpies and heat capacities for transferring the surfactants from water to aqueous urea solutions have been obtained using the corresponding data for these sur- factants in pure water.32

= (R2 - HS)mono/mic(aq. urea)

- (A2 - H!)mono/rnic(water) Amic H,, = AmiC H(aq. urea) - Amic H(water)

AH,, refers to the transfer enthalpy of monomers and micelles from water to aqueous urea solution, and Amic H,, to the transfer enthalpy of micellization from water to aqueous urea solution.

Similarly, for the transfer heat capacities :

ACp, tr. mono/mic = (cp. 2 - C:, 2)mono/mic(aq* urea)

AC,, ,, t, = AC,, ,(aq. urea) - AC,, ,(water)

The transfer enthalpies of solution in pre- and post-c.m.c. regions and of micellization from water to 3 and 6 mol dm- urea solutions of the surfactants are given in Table 3 as a function of urea concentration at temperatures ranging from 288.15 to 308.15 K. The transfer enthalpies of micellization have been plotted as a function of the number of oxyethylene groups of the surfactants in Fig. 1.

The transfer heat capacities of monomers and micelles and the transfer heat capacity of micellization from water to aqueous urea solutions for CioE4, C,,E,, C1&6 and C,,E, are listed in Table 4. The A C , m, tr values have been plotted as function of urea concentration in Fig. 2.

Table 3 Transfer enthalpies for the transfer of some decyl poly(oxyethy1ene glycol) ethers from water to aqueous urea solutions

[urea] AHtr, mono A*tr, mic Antic Htr /mol dm-3 T/K /kJ mol-' /kJ mol-' /kJ mol-'

288.15 288.15 293.15 293.15 298.15 298.15

298.15 298.15 308.15 308.15

298.15 298.15 308.15 308.15

298.15 298.15 308.15 308.15 318.15 318.15

ClOE, 4.64 f 0.45 1.35 f 0.33 4.94 f 0.47 2.52 f 0.43 2.79 & 0.39 3.60 f 0.29 4.32 f 0.35

- 3.70 f 0.24 -0.44 f 0.49 -3.18 & 0.37 - 0.79 f 0.3 1 - 1.94 f 0.36

ClOE, 1.92 f 0.20 0.90 f 0.25 0.48 f 0.20 0.58 f 0.23

-1.68 f 0.18 - 5.13 f 0.20 - 1.23 f 0.26 -3.15 f 0.23

'loE, 3.05 f 0.30 2.78 f 0.26

-0.70 f 0.33 - 1.88 f 0.32

- 1.10 f 0.28 -3.81 f 0.27 -2.11 & 0.23 -4.70 f 0.29

ClOE, 1.86 f 0.58

- 1.03 f 0.46 -0.66 f 0.58 -4.10 f 0.52 - 1.13 f 0.27 - 1.88 f 0.36

-3.00 f 0.32 -8.36 f 0.38 -2.56 & 0.29 -7.83 f 0.33 -2.07 f 0.37 - 3.03 f 0.37

5.99 f 0.56 -8.64 f 0.53 -2.96 f 0.65 -5.97 f 0.53 -4.39 f 0.42 -6.26 f 0.50

-3.60 f 0.27 -6.03 f 0.32 -1.71 f 0.33 -3.73 f 0.3

-4.15 f 0.41 -6.59 f 0.37 -1.41 f 0.40 -2.82 & 0.4

-4.86 f 0.66 -7.33 f 0.60 - 1.90 & 0.40 -3.73 f 0.61 -0.94 f 0.46 - 1.15 f 0.5

- 3 t

z- I

- 8 I I 1 I I J 3 4 5 6 7 8 9

number of oxyethylene groups

Fig. 1 Transfer enthalpies of micellization as a function of number of oxyethylene groups of some C,,E, surfactants from water to aqueous 3 (A) and 6 (A) mol dm-3 urea solutions at 298.15 K

Discussion We have not found any calorimetric data in the literature for alkyl poly(oxyethy1ene glycol) ether surfactants in aqueous urea solutions for direct comparison. Even studies on the variation of c.m.c. with temperature and urea concentration are limited for such types of s ~ r f a c t a n t . ~ * ' ~ - ' ~ * ~ ~ The decrease in the AmiCH values in aqueous urea solutions (other than in pure water) is in agreement with the calorimetric data of Benjamin3' for decylamine oxide and with those for other compounds cited in the reviews by Kresheck' and Magid.' Schick,14 from the c.m.c. changes for C10E7, CloE,, and C16E3, in 3 and 6 mol dm-3 aqueous urea solutions at 283.15, 298.15 and 313.15 K, has also shown that the enth- alpy of micellization decreases in aqueous urea solutions. Muller and P l a t k ~ ~ ~ have determined the effect of 2 and 4 mol dm-3 aqueous urea solutions on the c.m.c. of 8,8,8-tri- fluorooctyl hexa(oxyethy1ene glycol) ethers through NMR studies at different temperatures and they found a decrease in the enthalpy of micellization relative to that in water. While Schick" has attributed this decrease in the enthalpy of micel- lization to the enhanced hydration of oxyethylene groups in aqueous urea solution, Muller and P 1 a t k 0 ~ ~ have attributed it to the structural changes brought about by urea in the solution.

Generally, the decrease in AmicH has been attributed to the endothermic transfer of monomers from water to aqueous urea s o l ~ t i o n s , ~ ~ * ~ ~ * ~ ~ but it is evident from our data that the contribution of the transfer enthalpies of micelles in the post- c.m.c. region is more significant than the contribution from

- 400 1 I Y - -

1 0 1 2 3 4 5 6 7 8

Concentration of ureo (mol dm-3 1

Fig. 2 AC, m, of some alkyl poly(oxyethy1ene glycol) ethers from water to aqueous urea solutions: (A) C10E4, 293.15 K; (A) CIoE,, 303.15 K; (0) C10E6y303.15 K; (m) C10E8, 303.15 K

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Table 4 Transfer heat capacities for the transfer of some decyl poly(oxyethy1ene glycol) ethers from water to aqueous urea solutions

[urea]/mol dm - A C , m, JJ mol- K-

3 6

3 6

3 6

3 6 3 6

293.15 293.15

303.15 303.15

303.15 303.15

303.15 303.15 313.15 313.15

- 1 0 4 f 5 5 -62 f 58

-144 f 29 -32 +_ 34

-375 * 44 -466 41

-252 _+ 64 -307 & 69

-47 f 40 222 f 63

ClOE4 56 f 45

146 f 44

ClO% 45 31

198 f 30

Cl OE6 -101 f 36 -89 f 39

44 & 42 53 _+ 50 49 f 46

480 f 51

160 f 71 208 & 7

189 k 42 230 f 45

274 f 57 324 k 70

296 k 76 360 f 85 96 k 61

258 Ifr 81

the transfer of monomers. We therefore propose to discuss our data in the light of the following interactions taking place in the surfactant-water-urea system: (i) the structure- breaking propensity of urea in aqueous solutions; (ii) the interaction of the hydrophobic part of the surfactant with urea; (iii) the interaction of the hydrophilic part of the sur- factant with urea; (iv) the specific urea-surfactant inter- actions.

Transfer enthalpy of solution studies on some highly hydrophobic compounds like Bu4NBr and Am4NBr by Sarma and Ahluwalia” show that AH,, values for transfer of these compounds from water to aqueous urea solutions are positive. Similar results have been obtained by Bright and Jezorek3’ for Bu4NPh4B. On the basis of solubility studies of hydrocarbons in aqueous urea solutions, Wetlaufer et u Z . ~ ~ have inferred that these transfers of hydrocarbons are associ- ated with an increase in enthalpy. Similar conclusions have been drawn for the transfer enthalpies of apolar molecules by Kuharsky and RosskyJ6 using molecular dynamics methods. On the other hand, the enthalpy of transfer of electrolytes like NaCl from water to aqueous urea is seen to be nega-

Negative enthalpies of transfer have also been observed for amino acids by Prasad and Ahluwalia” and Kresheck and Benjamin. Therefore, these results indicate that while the enthalpies of transfer of hydrophobic groups from water to aqueous urea solutions are positive, the enth- alpies of transfer of ionic or hydrophilic groups are negative. A similar conclusion can be drawn from the work of Ahluwa- lia and co-workers20*22 and that of Franks and Clarke.39

The structure-changing propensities of both hydrophobic (structure-making) and hydrophilic (structure-breaking) groups decrease in a less structured medium,37 which results in their opposite contribution to AHtr in urea solutions. Therefore, for decyl poly(oxyethy1ene glycol) ether sur- factants, the contribution of the decyl group to the transfer enthalpy in the pre-c.m.c. region is positive, whereas the con- tribution of the oxyethylene groups (the hydrophilic part) is negative. As the decyl group is the same in all of the sur- factants considered here and the number of oxyethylene groups increases from four to eight on going from CloE, to CloEa, the enthalpy of transfer of monomers from water to aqueous urea must decrease with an increase in the number of oxyethylene groups, and this has indeed been observed in the present study. Thus the values of AHtl,mOnO show the net effect of the contributions made by the hydrophobic and hydrophilic groups. With an increase in urea concentration and number of oxyethylene groups, AH,,, also decreases due to the greater exothermic contributions from the inter- actions between the ethylene oxide groups and urea.

With an increase in temperature, the structure-changing propensities of the hydrophobic and hydrophilic groups decrease and so AHt,,mOnO decreases and even becomes exo- thermic, which shows that the contribution of the poly(oxyethy1ene)-urea interactions dominates over the con- tribution of the hydrophobic (decyl) groupurea interactions and this is also observed with an increase in urea concentra- tion.

The enthalpies of transfer of micelles in the post-c.m.c. region, i.e. AH,,, mic, are negative and become more negative with increase in urea concentration and decrease with increase in temperature. In the micellar state all of the hydro- phobic groups are buried inside the micellar core and hence the endothermic contribution of the interaction between the hydrophobic decyl group and urea essentially remains the same as in the pre-micellar state. The hydrophilic oxyethyl- ene groups, however, are in the palisade layer and their density would be quite high owing to the high aggregation number of the surfactant monomers in the micelles. The hydrophilic groupurea interaction thus becomes the domin- ating interaction leading to negative transfer enthalpies for the transfer of micelles from water to aqueous urea solutions. Here hydrogen-bond formation between oxyethylene groups and urea should also contribute significantly towards the negative values of the transfer parameters. With the increas- ing temperature, as these interactions are weakened, the transfer parameters in the post-c.m.c. region becomes less negative, with the exception of ClOE, in which case the trans- fer enthalpies in both the pre- and post-c.m.c. regions become more negative with increase in temperature and urea concen- tration. This anomaly may be attributed to the drastic increase in the aggregation number4’ of the CIOE6 micelles with increase in temperature: as the aggregation number increases, the number of the ethylene oxide groups in the palisade layer of the micelles also increases and hence the exothermic contribution from the interactions between the ethylene oxide groups and urea will become more pro- nounced.

The overall transfer enthalpies of micellization, AmicHtr are negative owing the decrease in the structure around the monomers in urea solutions.

The heat capacities of micellization of the surfactants are higher in aqueous urea solutions. This may be attributed to the decreased hydration or structure around monomers in urea solutions and the increased hydration of micelles due to enhanced hydrogen bonding between ethylene oxide groups and urea molecules in aqueous urea solutions. This is in accordance with the data obtained by Desnoyers and co- workers’ for nonyltrimethylammonium bromide in aqueous

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urea solutions. The transfer heat capacities of monomers for transfer from water to aqueous urea solutions are negative whereas the reverse is true for the transfer heat capacities of micelles leading to positive values of transfer heat capacities of micellization. AC,,,,,, increases with the number of oxy- ethylene groups and urea concentration. AC,,,,. is less nega- tive in aqueous urea solutions than in water, which suggests a greater extent of penetration of water or solvated urea mol- ecules into the micellar core. This view has also been sup- ported by Baglioni et aL41 from their EPR studies on sodium dodecyl sulfate and dodecyl trimethylammonium bromide micellar solutions in aqueous urea solutions.

Conclusions The thermodynamics of transfer of some decyl poly(oxyethy1ene glycol) ether (C,,E,J surfactants in mono- meric and micellar states from water to 3 and 6 mol dm-j aqueous urea solutions show that the enthalpies of micelliza- tion are less positive in aqueous urea solution than in water and decrease with an increase in urea concentration and tem- perature. The transfer heat capacities of micellization for transfer from water to aqueous urea solutions are positive and increase with increases in urea concentration and number of oxyethylene groups (at a particular temperature and urea concentration), but decrease with an increase in temperature. This may be attributed to the decreased hydra- tion or structure around monomers and the increased hydra- tion of micelles in urea solutions due to the greater extent of hydrogen bonding between ethylene oxide groups and urea molecules in aqueous urea solutions. The dominance of hydrophilic oxyethylene groupurea interactions over the hydrophobic decyl groupurea interactions and the hydrogen-bond formation between oxyethylene groups and urea appear to contribute significantly towards the negative values of the transfer thermodynamic parameters.

References L. Magid, in Solution Chemistry ofSurjiactants, ed. K. L. Mittal, Plenum, New York, 1979, vol. 1, p. 427. G. C. Kresheck, in Water, A Comprehensive Treatise, ed. F. Franks, Plenum, New York, 1974, vol. 4, p. 95. L. G. Ionescu and U. T. DeFavere, in Solution Behaviour of Sur- factants, ed. K. L. Mittal and E. J. Fendler, Plenum, New York, 1982, p. 407. Non-ionic Sujirctants, Physical Chemistry, ed. M. J. Schick, Marcel Dekker, New York, 1987. B. Lindman and H. Wennerstrom, in Micelles, Topics in Current Chemistry, ed. M. J. S. Dewar, K. Hafner, E. Heilbronner, S. Ito, J-M. Lehn, K. Niedenzu, C. W. Rees, K. Schiifer and G. Wittig, Springer, Berlin, 1980, vol. 87, p. 26.

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