4756
J. Phys. Chem. 1983, 87,4756-4761
Water Self-Diffusion in Nonionic Surfactant Solutions. Hydration and Obstruction Effects Per Gunnar Niisson and BJornLindman’ Physical Chemistry 1, Chemical Center, Lund University, S-220 07 Lund, Sweden (Received November 29, 1982; In Final Form: March 29, 1983)
The self-diffusion coefficient of water (DzO)in aqueous solutions of nonionic surfactantsof different concentrations and temperatures was determined by means of the NMR spin-echo pulsed field gradient method. The nonionic surfactants chosen were pentaethyleneglycol dodecyl ether (CIzE5)and octaethylene glycol dodecyl ether (C12E8). The observed self-diffusioncoefficient of water in a nonionic surfactant solution is lower than that of neat water because the surfactant aggregates obstruct the paths of the water molecules and because a fraction of the water is bound to the surfactant (hydration) and hence has a lower translational mobility. The hydration is found to be approximately four to six water molecules per ethylene oxide group for a 10% surfactant solution and to decrease with increasing surfactant concentration. A small decrease in hydration with increasing temperature was also found, but the surfactants are still strongly hydrated close to the cloud point. The nonionic surfactants were compared with a poly(ethy1ene glycol) (PEG). The comparison shows that the hydration of the ethylene oxide groups seems to be independent of the particular system and mainly determined by the temperature and the composition of the system. The consequences of this for the understanding of the clouding phenomenon are discussed. It is shown that there is no direct relation between surfactant dehydration and the clouding phenomenon.
Introduction The properties of an aqueous solution of a nonionic surfactant of the ethylene oxide variety are determined to a large extent by the interaction between the water molecules and the ethylene oxide groups. The interaction is temperature dependent and it has been concluded from different observations that the hydration of the ethylene oxide chains decreases with increasing temperature.l+ There is, however, very little direct experimental evidence to be found in the literature. Hydration numbers have been reported,”14 but often aggregate shape and hydration are inseparable; consequently, prior knowledge of the aggregate shape is required to estimate the hydration. As regards the concentration and temperature dependence of the hydration, which is very important for the understanding of the phase behavior, few results have been p~blished.~-l~J~ In the present work, the self-diffusion coefficient of DzO in aqueous solutions of nonionic surfactants at different temperatures and concentrations has been determined by means of the NMR spin-echo pulsed field gradient method. The surfactants chosen are pentaethylene glycol dodecyl ether (C12E5) and ðylene glycol dodecyl ether (ClzE&; approximate phase diagrams of these surfactant-water systems are shown in Figure 1. The water self-diffusion is influenced by the monomer and micelle hydration and (1) J. C. Lang and R. D. Morgan, J. Chem. Phys., 73, 5849 (1980). (2) G. J. T. Tiddy, Phys. Rep., 57, 1 (1980). (3) R. R. Balmbra, J. S. Clunie, J. M. Corkill, and J. F. Goodman, Trans. Faraday Soc., 60, 979 (1964). (4) R. R. Balmbra, J. S. Clunie, J. M. Corkill, and J. F. Goodman, Trans. Faraday Soc., 58, 1661 (1962). (5) K. Shinoda and H. Kunieda, J. Colloid Interface Sci., 42, 381 (1973). (6) K. Shinoda, H. Kunieda, N. Obi, and S. E. Friberg, J . Colloid Interface Sci., 80, 304 (1980). (7) P. Becher and H. Arai, J. Colloid Interface Sci., 27, 634 (1968). (8) D. Attwood, J . Phys. Chem., 72, 339 (1968). (9) P. H. Elworthy and C. B. Macfarlane, J . Chem. Soc., 311 (1964). (10) P. H. Elworthy and C. B. Macfarlane, J . Chem. SOC.,907 (1963). (11) J. M. Corkill and T. Walker, J . Colloid Interface Sci., 39, 621 (1972). (12) C. Tanford, Y. Nozaki, and M. F. Rohde, J. Phys. Chem., 81,1555 (1977). (13) T. Arnarson and P. H. Elworthy, J. Pharm. Pharmacol., 33, 141 (1981). (14) T. Arnarson and P. H. Elworthy, J . Pharm. Pharmacol., 34, 87 (1982). (15) C. J. Clemett, J. Chem. SOC.A , 2251 (1970).
by an obstruction effect due to the micelles. It has been demonstrated that the latter can be quantified and it is then possible to deduce information on micelle hydration and how it varies with temperature and concentration. For comparison, a poly(ethy1eneglycol) has also been studied.
Experimental Section C12E5 and C12E8of high quality were obtained form Nikko Chemicals, Tokyo, Japan. Poly(ethy1ene glycol) (mol wt 20 O00, for gas chromatography)was obtained from Merck. D20was obtained from Ciba-Geigy and was of >99.7 at. 5% D isotopic purity. All components were used without further purification. Solutions were prepared by weighing the components. All concentrations are in percent by weight unless otherwise indicated. The diffusion studies were performed on a Bruker 3223 pulsed NMR spectrometer using 2H NMR at 13.8 MHz for the water diffusion (DzO). The pulsed field gradient technique innovated by Stejskal and T a n n e P was used, the spectrometer being equipped with a specially made pulsed magnetic field gradient unit. With this technique, a spin echo is produced by a 90°-r-1800 pulse sequence and the magnetic field gradient, g, is applied during a time, 6, as two pulses, separated by a time, A, one before and one after the 180° pulse. For a given chemical species, the diffusion attenuates the contribution, E, from the particular compound to the echo amplitude at time 27 according to
E = E, exp(-(yg6)2DD(A- 6/3)]
(1)
where Eo is the contribution to the echo in the absence of field gradients and y is the magneto gyric ratio of the nucleus studied. The self-diffusion coefficient, DD,was determined by measuring E for a series of 6 values, keeping g and A fixed (typically, A = 52 ms) at values suitable for a reliable determination of DD. By minimizing E - Eo exp[-B62(A - 6/31] using a least-squares procedure, one determines the diffusion coefficient from the optimal value of B. To establish the absolute values of the self-diffusion coefficients, measurements were performed on a reference DzO sample with known17 self-diffusion coefficient. (16) E. 0. Stejskal and J. E. Tanner, J. Chem. Phys., 42, 288 (1965). (17) J. R. Mills, J. Phys. Chem., 77, 685 (1973).
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983 4757
Water Self-Diffusion in Nonionic Surfactant Solutions
20
5 1 0
%C12E5
I
I
I
10
I
I
I
I
I
30
20
I
I
50
40
%C12E5 Flgure 2. Observed selfdiffusion coefficient of D,O vs. the C12E5 content for the samples investigated in the system Cl,E5-D,0.
f
-i
2 05 1 0
,
CUB.fl (C 1
,
20
I
40
:,'\f?T 1 I
I
60
I
I
80
%C12E8 Flgure 1. Schematic composition-temperature diagrams for the systems C,&-water and C12E8-water. HEX., LAM., and CUB. are hexagonal, lamellar, and cubic liquid crystalline phases, respectively. L1and are isotropic phases and S is solld surfactant. Ll is the reglon investigated in the present attick. (After: F. Harusawa, S.Nakamura, and T. Mitsui, Colloid Polym. Sci., 252, 613 (1974); K. Shinoda, J . Colloid Interface Sci., 34, 278 (1970). (The C1 phase was not presented In the reference.))
In the absence of compounds containing exchangeable hydrogens (and other molecules containing deuterium, of course), DD gives the DzO self-diffusion directly. In the studies of self-diffusion in the presence of compounds containing exchangeable hydrogens (in this study OH in the amphiphile), one must take into coyideration that the deuterium diffusion measures an average over the different environments of the deuterium. If the diffusion of the compound containing the exchangeable hydrogen is studied separately,'* the D20diffusion can easily be corrected for the exchange. The observed self-diffusion coefficient is
ON
n n
!
(18)P.G.Nilsson, H.WennerstrBm, and B. Lindman, J.Phys. Chem., 87,1377 (1983).
35.6.C
A
t
B
I
0.40
0
10
20
30
40
50
%C12E5 FQwe 3. Relative selfdiffusion coefficient of D,O (relative to neat D20) vs. the C,,E5 content for the samples investigated in the system C12E5-020.
50
I
0
where D D -and ~ D,are the self-diffusioncoefficients of D20 and the amphiphile respectively, and X denotes mole fraction. Equation 2 is valid in the fast exchange limit and when the mass transport due to the exchange is negligible compared to diffusion; it can be assumed that eq 2 applies for all cases studied in this work. The values presented for the surfactant-water systems have been corrected for the effect of the deuterium exchange. For solutions with more than 50% D20 (by weight), the correction was in all cases less than 2%, while the correction increases to nearly 15% for solutions with about 10% D20. In the polymer-water system, no correction is necessary. For the samples with the lowest water contents, the spin echoes were accumulated on a computer in order to obtain a better signal/noise ratio.
45.1' C
. ^.^.^
I
100
1 3 A
1
20
40
60
80
100
%C12E8 Figure 4. Observed selfdiffusion coefficient of D,O vs. the CI2E8 content for the samples investigated in the system C1,E8-D,O.
The temperature was measured with a calibrated copper-constantan thermocouple which was placed in an NMR tube with glycerol. The temperature was measured immediately before and after the spin-echo experiment. The temperatures given are accurate to within f0.5 "C. Results The observed self-diffusion coefficients of D20 and the where DD20 relative self-diffusion coefficients Dow/DD20, means the self-diffusioncoefficient of neat D20 at the same temperature as the experimental one, are presented as graphs of the experimental values vs. weight percentage amphiphile. Figure 2 shows the self-diffusion coefficients of D 2 0 in the system ClzE5-D20and Figure 3 shows the relative self-diffusion coefficients Dobsd/DDz~ for the same
4758
The Journal of Physical Chemistty, Vol. 87,
No. 23, 1983
662 C 493 c 254C
A b
1
cubic
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Nilsson and Lindman
7;: i
-
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40
20
0
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1
I
60
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l
80
100
0.60
I
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%C12E8 Flgm 5. Relative setfdiftusi coefficient of D20(relative to neat D,O) vs. the C12El content for the samples Investigated in the system C12Ea-D20. 1
1
ON
n n
2 0
n
I
I
0
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20
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80 100 % PEG
Flgure 8. Relative selfdiffusbn coefflcienlof D,O (relative to neat 40) vs. the PEG content for the samples investigated in the system P E G D20.
system. Figure 4 shows the self-diffusion coefficients of D,O in the system CI2E8-D,O and Figure 5 shows the relative self-diffusion coefficients DoM/DD50for the same system. Figure 6 shows the relative self-diffusion coefficients for the system poly(ethy1ene glycol) (PEG)-D20.
Discussion Water Self-Diffusionin Surfactant Solutions. General Considerations. The observed self-diffusion coefficient of water in a nonionic surfactant solution is lower than that of neat water. Our model assumes that this decrease in self-diffusion coefficient is caused by two phenomena: obstruction and hydration. Self-diffusion experiments with the NMR spin-echo method measure the average change in position in the direction of the gradient that occurs during the time between the two gradient pulses. The aggregates present in the solution (mainly the hydrocarbon part) obstruct the paths of the water molecules. If a water molecule is close to obstructing volumes, the change in position that occurs during the diffusion time will be lower than in the absence of obstructing volumes, thus making the observed self-diffusion coefficient lower. In ref 19, an expression for the obstruction effect in a system with spherical obstructing particles is derived. The theory is extended to spheroidal particles in ref 20. In this derivation, it is assumed that the cell model can be used and that the cell is spheroidal of the same type as the spheroidal particle. Results for some different particle shapes (19) G . M. Bell, Trans. Faraday SOC.,60, 1752 (1965). (20) B. Jonsson, P. Linse, and P . 4 . Nilsson, to be submitted for publication.
Here, D is the observed self-diffusion coefficient, 4 the excluded volume (volume fraction), and Do the self-diffusion coefficient in the absence of obstruction. Equation 3 has been tested experimentally (water self-diffusion in the presence of spherical latex particles) and with Monte Carlo simulations.20 The equation was found to satisfactorily describe the obstruction effect over large concentration ranges. The second effect in our model that decreases the observed self-diffusion coefficient is that a fraction of the water molecules is bound to or affected by the head groups of the surfactants and hence has a lower translational mobility. This effect can be described by eq 4 D = PfDf + P@b (4) where D is the observed self-diffusion coefficient, P f and Pb are the fractions of free and bound water, respectively, and Df and Db are the self-diffusion coefficients of free and bound water, respectively. We assume that the obstruction only affects the free water molecules and combine the two effects. For spherical volumes (5) where D, is the free-water self-diffusion coefficient in the absence of obstructions, i.e., in the absence of surfactant micelles but in the presence of surfactant monomers.
Dreland Dn,can be measured (Ofand Dm are more or less equal). If we assume that the bound water molecules have
Water Self-Diffusion in Nonionic Surfactant Solutlons
the same mobility as the surfactant, D b is also obtainable from experiments. 4 can be calculated under certain assumptions of the obstructing volumes and the densities of the components. From pb, the hydration (expressed as the number of bound water molecules per ethylene oxide group) can easily be calculated: = PbnD,O/(nC,E,m) (7)
The Journal of Physical Chemistry, Vol. 87,
No. 23, 1983 4759
I
p
I
356’C e 248’C 14.3’C A
5
41
Here, nD,Ois the number of D20 molecules in the system, nCnEm the number of amphiphile molecules, and m the number of ethylene oxide groups in a nonionic surfactant molecule. Because of the small difference in the obstruction correction between spherical and prolate micelles (see Figure 7), only a small error will arise if the expression for spherical obstructing volumes is used for prolate aggregates. This is especially true for strongly hydrated aggregates (as in the present case) where the obstruction term only plays a minor role. It may be noted that the obstruction effect for spheres and rods can only account for an approximately 20% reduction in the observed selfdiffusion coefficient (4 = 0.5) while the experimentally seen reduction is considerably larger. Even for oblate structures the effect is only 33 % . If oblate particles with a large axial ratio are present in the system, this should be deduced from the concentration dependence of the observed self-diffusion coefficients. The only real problem is for oblate aggregates with moderate axial ratios (for example, 1:5) where incorrect hydration values can be obtained if the aggregate shape is unknown and an inapplicable expression for the obstructing volumes is used. We assume that oblate aggregates do not occur in the present systems (cf. below). We will hence use eq 6 and 7 to calculate the hydration numbers. We assume that all surfactant is micellized, that only the hydrocarbon cores of the aggreates obstruct the paths for the water molecules, and that, furthermore, the hydrocarbon chains of the surfactants have the same density as dodecane and that the density of the ethylene oxide chains is 1.1g/cm3. The self-diffusion coefficients of the surfactants are taken from ref 18. At low surfactant contents, the factor 1/(1+4/2) - Dml is small and very sensitive to experimental errors; at the same time, nc is small and nDBis large, thus amplifying the error. Tf% means that it is not possible to obtain reliable hydration numbers at the lowest surfactant concentrations. Also at the highest surfactant concentrations, the deduced hydration values must be treated with care, since distinct aggregates in a water-continuous medium are assumed in the model, a situation not applicable for the most concentrated surfactant solutions. The high D20 selfdiffusion coefficients obtained also at rather high surfactant concentrations show, however, that the isotropic solution is water continuous over large concentration regions and that the model is applicable. Hydration. The solubility of the nonionic surfactants in water is due to the interaction between water and the ether oxygens in the ethylene oxide chain. Although there is little direct experimental evidence, there is strong indirect evidence that the interaction between the ethylene oxide groups and water is temperature dependent, i.e., becomes less favorable with increasing temperature. It has been shown21~22 that a closed solubility gap can be generated if a temperature-dependent directionality of
hydrogen bonding is assumed. When the temperature is increased, the directionality of the hydrogen bonding is successively broken down due to the thermal motion leading to a decreased interaction resulting in phase separation. At still higher temperatures, entropic contributions will dominate leading to miscibility. The general features of this model can be expected to be valid also for nonionic surfactant system^,^.^^^^^ since closed-loop coexistence has also been found in some of these systems. A further indication for a decreased interaction between water and the ethylene oxide chain with increasing temperature is found in phase equilibria studies. As shown by Shinoda and Kunieda: the effect of a temperature rise seems similar to a decrease in ethylene oxide chain length. In Figures 8 and 9, the hydration numbers calculated from eq 6 and 7 are presented for the two surfactants studied. It should be noted that the concept of a hydration number is a questionable simplification and that “hydration” is defined somewhat differently depending on the experimental method used. In the DzO self-diffusion studies, the hydration is defined as the water molecules effectively moving with the micelles as kinetic entity. The hydration numbers obtained are for the lowest surfactant concentrations presented, approximately four to six D 2 0 molecules/ethylene oxide group. The hydrogen-bonding
(21) G . R. Andersen and J. C. Wheeler, J. Chem. Phys., 69, 3403 (1978). (22) J. A. Barker and W. Fock, Discuss. Faraday SOC.,15,188 (1953).
(23) R. Kjellander and E. Florin, J. Chem. SOC.,Faraday Tram. I, 77, 2053 (1981). (24) R. Kjellander, J. Chem. SOC.,Faraday Tram. 2, 78, 2025 (1982).
-
3
e
2
1
0
w
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20
1
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% C12E5 Flgure 8. Hydration H (no. of D,O molecules/ethylene oxide group) calculated from eq 6 ,and 7 vs. the C,,E, content. The values for samples with less than 10% C,& have been excluded due to the very large uncertainty.
a
4-
321-
-
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YJC12E8 Flgure 9. Hydration H (no. of bound D,O molecules/ethylene oxide group) calculated from eq 6 and 7 vs. the C12E, content. The values for samples with less than 10% CI2E8 have been excluded due to the large uncertainty.
4760
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983
Nilsson and Lindman
capacity of the ethylene oxide chain is two water molecules per ether oxygen. The fact that the observed hydration numbers are higher than the hydrogen-bonding capacity implies that the observed hydration numbers contain contributions from structured water around the ethylene oxide chains as well as hydrogen bonding. The obtained hydration numbers show the expected temperature dependencies, i.e., a decrease with increasing temperature. It should be noted that the decrease in hydration with increasing temperature is relatively small and that the surfactant aggregates remain strongly hydrated also close to the cloud point. The hydration number shows a slow monotonic decrease with increasing surfactant concentration over the concentration range studied. One would expect micellar hydration to be rather constant as long as the surfactant concentration is low and the micelles are well separated from each other. At higher surfactant concentrations, a decreasing hydration with increasing surfactant concentration is expected. Structural Considerations. From Figures 2 and 4, it is seen that there is a monotonic slow decrease in the water self-diffusion coefficient with increasing surfactant concentration over the entire extension of the isotropic phases. It is interesting to note that the water self-diffusion seems to be independent of the distance from the critical point. It is according to expectation and to studies of simple systems that self-diffusion coefficients are not influenced by critical phenomena.z5 Any effects on water self-diffusion (these effects are, however, not found to be marked) should, therefore, be associated with hydration (or obstruction) phenomena. For the ClzE8-Water system, the DzOself-diffusion was also measured in the cubic C1 phase at 4.9 OC and in the cubic C2phase at 25.4 OC. No irregularity was found on passing from one phase to another. A t least in the C12E8 system, as the C12E8 concentration increases, one passes from solutions with bulk water to regions where no bulk water would be expected. No irregularity is, however, found. If the self-diffusion coefficients of D20 are compared to the self-diffusioncoefficients of the surfactant,18 it is seen that, even at the highest surfactant concentrations, the DzO self-diffusion coefficients are more than 1 order of magnitude higher than the surfactant self-diffusion coefficients. If all water were bound to be surfactant head groups, the diffusion coefficients of the two species would be similar. The fact that D D p >> DC,&shows that there must be large domains that are water continuous at high concentrations. It seems probable that the solution consists of hydrocarbon domains and water/ethylene oxide domains. The surfactant must be strongly associated even at the lowest water concentrations. As shown earlier (Figure 71, the obstruction is very large for oblate particles with large axial ratios already at low surfactant concentrations, while for long prolates the obstruction is small. This result can be used to distinguish between prolate and oblate micelles if large micelles are known to exist in the solution. If the relative self-diffusion coefficients presented in Figures 3 and 5 are compared with Figure 7, it can be concluded that no oblate aggregates with large axial ratios exist in the systems at least at low or moderate surfactant concentrations. In the system C12E5-water,large aggregates have been found already at low surfactant concentrations; l8 it was suggested that the aggregates were prolate shaped. The present results provide further evidence for prolate-shaped aggregates.
Comparison with Poly(ethy1ene glycol). It seems profitable for a good understanding of the nonionic surfactant systems to compare their behavior with that of poly(ethy1ene glycol) (PEG)-water systems. A polymer with an average molecular weight of 20 OOO was chosen for study. It can be inferred from the literaturez6that the cloud point of this polymer is higher than 100 "C. In Figure 6, the observed self-diffusion coefficient of DzO is presented for two different temperatures. It is seen that the relative self-diffusion coefficient has the same type of concentration and temperature behavior as was found in the nonionic surfactant systems. To better compare the diffusion data for the different systems, it is favorable to replot the data. In Figure 10, the relative self-diffusion coefficients of D20 vs. the molar ratio D,O/ethylene oxide groups are plotted for C12E5,C & & ,and PEG at approximately 5 "C. The corresponding plot for CI2E, and PEG at approximately 66 "C is presented in Figure 11. If there were no interactions between the hydrocarbon chains and water and a negligible obstruction effect and if, furthermore, the interaction (as measured with the present method) between the ethylene oxide groups and water were equal in the different systems for a certain temperature and ratio DaO/ethylene oxide groups, then all systems would be expected to fall on the same line in these plots. This is, indeed, found to be the case to a very good approximation. Since the hydrocarbon-water contact in
(25) H. Hamann, C. Hoheisel, and H. Richtering, Ber. Bunsenges. Phys. Chem., 76, 249 (1972).
(26) S. Saeki, N. Kuwahara, M. Nakata, and M . Kaneko, Polymer, 17, 685 (1976).
n
0.60
i
P 0
0.40
5
2
50 100 200
10 20
500
/
molD20 mol EO Figure 10. Relative selfdiffusion coefficient of D20(relative to neat D,O) vs. the molar ratio D,O/ethylene oxide groups for PEG, CE ,, and CI2EB at approximately 5 O C .
0.80
I 0 a
0.601 a
P
C
0.40
a
i
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a
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".
3a I
l
0.5 1
l
2
I
l
I
l
5 10 20
l
l
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molD20 mol EO Figure 11. Relative selfdiffusion coefficient of D20 (relative to neat D20)vs. the molar ratio D,O/ethylene oxide groups for PEG and C12Es at approximately 66 OC.
The Journal of Physical Chemistry, Vol. 87, No. 23, 7983 4761
Water Self-Diff usion in Nonionic Surfactant Solutions
1.0 I
n
0.6 O-a;
ii
n
.
B
0.4
0.
1
0
5
O 0
10 m
% =
50 2 100 200
20
500 m
/
molD20 mol EO Figure 12. Fraction of bound water, P, vs. the molar ratio D,O/ ethylene oxide groups for PEG, C12E5,and C1,E8 at approximately 5 0u.
0.8 0.6 0
n
.U
0.4
'"1 0
b
,
,
0.5 1
, 2
5
:bbi
b
o,o
b 1
~
1
10 20 50 100200 500
I
molD20 mol EO Flgure 13. Fraction of bound water, P,, vs. the molar ratio D,O/ ethylene oxide groups for PEG and C12E8at approximately 66 OC.
nonionic micelles seems in~ignificant,'~.~',~~ the ethylene oxidewater interaction can be compared if the obstruction term is compensated for. The obstruction term can be eliminated if pb (or equivalently the hydration H)instead of D,, is plotted vs. the molar ratio D20/ethylene oxide groups. Pb is calculated from eq 6 assuming spherical obstructing particles for the nonionic surfactant systems. For the polymer system, eq 4 is used for the calculation of Pb Because of the very large difference in self-diffusion coefficient between D20 and PEG, the self-diffusion coefficient of PEG can be set equal to zero. In Figures 12 and 13,pb is plotted. All values fall on the same line within the experimental uncertainty. This result indicates that the obstruction term is small and that eq 5 gives a sufficiently good description. It is very unlikely, in view of (27) B. Lmdman and H. Wennerstr6m, Top. Curr. Chem., 87,1(1980). (28) F. Podo, A. Ray, and G. Nemethy, J.Am. Chem. SOC.,95,6164 (1973).
these findings, that there is any appreciable amount of oblate aggregates with large axial ratios in these systems. The present results indicate that the hydration of the ethylene oxide groups is very much independent of the particular system. It was found to be dependent only upon the temperature and the concentration of ethylene oxide groups in the system. The results on the cubic phases suggest that the hydration is also independent of the structure of the phase as has been previously suggested.29B30 It has been suggested in the literature that the clouding phenomenon is caused by a dehydration of the ethylene oxide chains with increasing temperature. As pb is found to be independent of the sample position with respect to the cloud point, the clouding phenomenon can, at most in an indirect way, be caused by dehydration. The clouding phenomenon and the position of the cloud point are influenced by a number of different factors and an analysis involving all these will necessarily be complicated. The dominating contribution undoubtedly comes from the interaction between different ethylene oxide chains and from the interaction between the ethylene oxide chains and water. Other important contributions may come from van der Waals forces between the hydrocarbon cores in the micelles. Micellar size and shape are expected to have a great influence on the position of the cloud point. The head group-head group interactions and the head group-water interactions as well as the micellar shape and size are expected to be highly temperature dependent and are all connected with the here observed dehydration of the ethylene oxide chains with increasing temperature. It has been suggested1 that the different interactions are interdependent; a modification of the hydrocarbon part may change the forces of hydration. In a recent article,24 Kjellander has theoretically analyzed the phase separation of nonionic surfactant solutions. The analysis takes into consideration several of the factors mentioned above. It has been suggested that the micellar hydration (per ethylene oxide group) increases with increasing ethylene oxide chain length because water becomes physically trapped in the micelles by the ethylene oxide chains. This is not supported by the present findings. It is possible that the ethylene oxide chains of the nonionic surfactants in the present investigation are too short for this effect to be noticeable.
Acknowledgment. Grants have been obtained from Stiftelsen Bengt Lundqvista Minne, The Swedish Natural Sciences Research Council, and The Swedish Board of Technical Development. Helpful discussions with Bengt Jonsson and HIkan Wennerstrom are gratefully acknowledged. Registry No. CI2Ea,3055-95-6; CI2EB,3055-98-9; water, 7732-18-5. (29) J. M.Corkill, J. F. Goodman, and J. Wyer, Trans. Faraday SOC., 65, 9 (1969). (30) J. S.Clunie. J. F. Goodman, and P. C. Svmons. Trans. Faradav
