Composition dependence of the structure of water-swollen nonionic micelles

Composition Dependence of the Structure of Water-Swollen Nonionic Micelles JEAN-CLAUDE RAVEY AND MARTINE BUZIER Laboratoire de Physico-Chimie des Colloides, LESOC, U.A. CNRS No, 406, Universit£ Nancy L Facultd des Sciences, B.P. 239, 54506 Vandoeuvre los Nancy Cedex, France Received September 20, 1985; accepted March 26, 1986 A geometrical model for water-swollen nonionic micelles in decane is proposed in relation to small angle neutron scattering data. It states that the aggregation number of these lamellar particles is monitored by the hydration degree of the oxyethylene (EO) groups along the polyoxyethylene chains: when hydrated, a EO group must be surrounded by a constant given number of water molecules. © 1987 Academic Press,Inc.

The existence of water-swollen micelles of nonionic CmEOn surfactants in hydrocarbon has been reported for many oil-rich systems (1-11). However, their structure which depends on the experimental conditions is not yet satisfactorily understood. But whatever the type of investigation (NMR, conductivity, light/neutron scattering), all the results agree with the fact that the first water molecules added ,to hydrocarbon-surfactant mixtures are bound to the oxyethylene groups. There must be a minimum water/surfactant ratio before water pools with "free" water molecules are eventually obtained (in microemulsions or liquid crystals). The actual inverse micelles with very low water content (or even without water at all) cannot be described in terms of spherical partides, given the length of the hydrophilic head and their small aggregation number N (N -~ 5-15): they must be regarded as bundles of parallel "head to foot" surfactant chains (i.e., "interdigited" EO chains). On the other hand, at least for the C 1 2 E O 4 -J- decane systems below 20°C and with decane content in the range 75-95% (w/w), the m a x i m u m water incorporation is about 2.5 water molecules per EO group (or 2 water molecules per oxygen atom); we have shown that the corresponding micelles

are much larger lamellar particles (N ~ 1500), constituted by two separate layers of hydrated surfactant chains in a rather extended conformation, b u t still without any central water layer for T < 20°C (5). Further addition of water induces a demixing, one of the two phases being a lamellar liquid crystal (6). And for 15 < T < 20°C, further addition of water can lead to the formation of water-in-oil microemulsions (12). Most interestingly, all the structures along these demixing lines are characterized by the same packing of the water/ surfactant molecules: the same area per polar head, tr = 42 A2, due to the same hydration rate and the same extended conformation of the EO chains (6). In other words, according to the water/surfactant ratio, the invariance of the surfactant palisade morphology requires the formation of three different types of structures: large globules, smaller lamellar aggregates, and lamdlar crystal. A GEOMETRICAL MODEL FOR WATER SWOLLEN MICELLES

We consider now the question about the structure of the lamellar swollen micelles for intermediate a water/surfactant molar ratios (0 < a < 10, T < 20°C). Obviously the driving force in the aggregation phenomenon is the 30

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Journalof ColloidandInterfaceScience,Vol. 116,No. 1, March 1987

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WATER-SWOLLEN NONIONIC MICELLES

dipole-dipole interaction between hydrated EO groups (13). Hence the free energy of the micelle should be roughly proportional to the aggregation number since we may assume that the interaction between decane and hydrophobic tails will not change very much by increasing the water amount by a few percents; this should effectively favor the formation of large aggregates. Clearly, their size will markedly depend on the number of water molecules which are necessary to ensure the cohesion between neighboring surfactant molecules, the larger the size the smaller that number. Inversely, from the knowledge of the size variation with the water content, we can infer the number of water molecules "bound" along one polyoxyethylenechain. Of course, in what follows, this bonding must be understood as a dynamic process (17): there may be a rapid exchange between water molecules truly bound (H bondings) and other more labile molecules which are simply located near the EO sites. We propose here, for the present system (C12EO4 + decane, T < 20°C), a simple water binding/location process which can explain the experimental size limitation of the lamellar micelles when a varies from 0 to 10. As outlined above, it is imperative to take into account the two following points: First, practically all the water molecules must be located along the EO chains. Incidentally, let us recall (5) that the addition of a small amount of water to the binary oil/surfactant mixture drastically reduces the concentration of free surfactant molecules. Second, the model must allow an easy change from "interdigited" structures (they exist at least for tiny water contents) to classical (separate) bilayers (they exist at least near the demixing line at low temperatures). The derivation of this geometrical model will mainly rest on the experimental curve of the aggregation number N as a function of the molar water/surfactant ratio a of the sample (Fig. 1) (5). The small angle neutron scattering results allow the determination of the mean

.J.

I00C

100

/° " • ~

"

10

OL

2

~

6

8

10

FIG. 1. Variation of the aggregationnumber of the waterswollen micelles of C~2EO4 in decane, a is the water/surfactant molar ratio. The volume fraction of the disperse phase ~bis in the range 0.10-0.25 (T = 20°C).

"hydration number" of the surfactant chains inside the palisade layers of the aggregates (5, 6). However, theoretical calculations of scattering spectra for model particles compared to the experimental ones (not shown here) indicate that precise information about limited inhomogeneities in this hydration degree cannot be obtained from that technique. For that purpose we used "optimum" deuteration rates 6 of oil (6 = 0.5) and water (fi = l) which, unfortunately, lead to very low levels of coherent scattering at the (large) angles of interest. Besides, these small differences in the spectra are noticeably smeared out by polydispersity and instrumental effects. The best thing we can do is to check the consistency of the model with the neutron scattering spectra previously obtained (5, 6, 11). The basis of our model results from two important observations. First, as noted above, for all the systems along the demixing lines [for weight surfactant/water ratios larger than 1 (6)], the hydrophilic part of the surfactant palisade is characterized by a rather extended chain conformation and by the presence of 10 water molecules evenly distributed along the EO chain, which contains 5 oxygen atoms. Second, from recent studies (19) on ethylene-water interactions, it can be inferred that t w o w a t e r molecules and one ethylene group Journal of Colloid and Interface Science, Vol. 116, No. 1, March 1987

32

RAVEY AND BUZIER

form a ternary complex where the mutual interactions are markedly stronger than they would be in the binary complex and free water dimer. Besides, recent measurements of the initial variation of partial enthalpy of water in nonionic surfactants (20) have been interpreted in terms of water dimers which interact strongly with the EO groups and thus restrict the conformations of that chain. Therefore, the simplest model is the one in which every oxygen of the EO chain, when hydrated, is necessarily "surrounded" by two water molecules (Fig. 2). This maximum and constant local hydration degree induces a constant area per polar head, i.e., a ~ 42 A 2, for every micelle, the conformation of the hydrated part of the EO chain being in the extended form as found for systems with the highest water content. This constant hydration degree appears to be in agreement with the N M R findings of Friberg (21) when the oil is an aliphatic hydrocarbon. This model merely assumes for micelles in samples with intermediate values of a (0 < a < 10) the existence of a hydrated core whose structure is identical to that of the hydrophilic part of the swollen micellesjust before demixing (c~ -~ a o ~ 10). This core has to be surrounded by "dry" EO groups: by all those of the surfactants in the peripheral layer (thickness E), but also eventually by those which are adjacent to the first CH2 groups of the other surfactant chains. Of

E

•

.... •

. . . . .

[

A

FIG. 2. Schematicrepresentationof the models for bilayeredmicelleswith separate(A) and interdigited(B) EO chains. • = water molecule. Journal of Colloid and Interface Science, Vol, 116, No. 1, Match 1987

course, the hypothesis of the existence of dry EO groups is an oversimplification: we just want to interpret the fact that the EO groups directly exposed to the hydrocarbon molecules must be much less hydrated than the other groups into the core of the micelle. We can introduce the parameter K which is the proportion in length of the EO chain which is effectively hydrated for a given overall a value. The validity of this simple model will depend on the relevancy of those values of E and K which allow us to account for the experimental N( a) curve. These parameters are related to the radius R of the micellar core according to . / 1 + CK/a

E

+

c--E-Y D'

[1]

where C is the ratio of the molar volumes of the E O 4 chain and of water. When a --~ OtD, R --~ ~ and we get the demixing phenomenon. The continuity requirements we use throughout the range 0 < a < 10 are a = constant ~. 42 A 2

[2a]

a " ~ a o "" 10

[2b]

K (hank) ~ K (separate layers)

[2c]

E (hank) ~ E (separate layers).

[2d]

K--~ 1

when

From the knowledge of E, K, and a, the morphology of the aggregate may be fully described, and in particular the aggregation number N for both types of lamella of interest here (bilayers with interdigited and separate layers). Let us recall that the number of free surfactants in decane is quite low as soon as some water is present. The research of the best fit between calculated and experimental N(a) curves (Fig. 1), taking into account the above requirements, leads to the thickness value E 1.5-2 A, and to the variation K(a) as shown in Fig. 3. We can note that this value of E is less than the EO thickness in the crystalline phase of the dry surfactant (14). That simply means that there is no water only on the exterior "wall" of the lamellar micelle. Such a

WATER-SWOLLEN NONIONIC MICELLES K

V

J 0

I 10

at •

FIG. 3. Variation of the proportion of the EO chain length which is hydrated as a function of a (E = 2.5/~). --, separate layers;---, interdigitedchains. variation of K strongly suggests that the first water molecules should markedly interact with one of the EO groups, inducing a local restriction in the conformation of the chains. As the water concentration increases, a larger and larger part of these molecules is progressively lodged into this frame from the core to the exterior, which do not interact so strongly with the remaining EO sites. This scenario is quite in agreement with the recent enthalpy measurements by Stenius et al. (20). In the case of the hanklike structure, the first EO groups to be hydrated should be located near the center of the oxyethylene chain; such an occurrence has already been recognized by Friberg (21) from N M R measurements on a longer nonionic surfactant. The K values for the hank structure are just a little less than for the other bilayered structure; at constant N, the free energy will be about the same for each type of aggregation since their number of EO/water bindings is practically equivalent. Therefore it can be conceived that some change in the interparticle interactions can easily promote a transition between these two types of aggregates. This transition can be induced by a particle crowding effect at the constant water/surfactant ratio, as already shown in a previous paper (5); it can also indirectly result from the water incorporation, as will be discussed below.

33

As a matter of fact, we could also imagine that the water swelling is performed at constant K = 1. In that case we should find that E ~ 6 7 A for the hank structure, while for the classical bilayer E becomes a function of a, decreasing from 5 to 2 ~,. In addition to the fact that the two types of aggregates would contain different number of water/EO bindings for the same N, such a large and variable thickness seems unlikely from a thermodynamical point of view, and this model should be discarded (22). At any rate, when a > 2, the overall dimensions of the micelles are not markedly dependent on the model. EXPERIMENTAL EVIDENCE FOR THE EXISTENCE OF TWO TYPES OF AGGREGATES

1. Geometrical considerations. In order to understand the probable transition between the two types of aggregates, we have to consider the orientational excluded volume of these anisometric micelles regarded as "hard bodies." As already noted, the hydrated EO groups are in an extended conformation. Hence the "dry" EO groups will adopt some meanderlike conformation, since they have to occupy the same transverse area as the zigzag groups surrounded by 2-2.5 water molecules (a --~ 42 ~2 at 20°C). A parameter which quantifies the reduction of rotational/translational freedom of an anisometric particle in relation to the interparticle interactions is proportional to the product of its surface (S) by its mean radius of curvature (R) (15). For the case of cylinder (or disk) shapes, we use the expression A=(~.S--~)

=~ l+~r+~-~p+2p,

[31

sphere

where p is the ratio of the two principal dimensions of the hard cylinder. For each type of aggregate the parameter has been calculated as a function of a: typically, A (a ~ 0) values are 1.2 and 1.7, and A (a = 9) are 4 and 2, respectively, for interdigited and separate bilayers. So we get two curves which cross over at some value of a (ac.o. ~ 2-3): at constant Journal of Colloid and Interface Science, Vol. 116, No. 1, March 1987

34

RAVEY AND BUZIER

volume of the particle (V), and due to (geometrical) interparticle interaction, hanldike aggregates should have the most restricted motion when water swelling is the largest; the opposite will be true for particles in lower water content samples. This difference in behavior mainly results from the respective values of the anisometry of the particles, and not from the tiny change with a of the volume fraction ~b of the disperse phase; it can also explain a transition from one type of particle to the other since it is an element in the calculation of the difference in their free energies (13). Another manifestation of these (entropic) interparticle effects can be seen from the outlook of the ternary phase diagram itself (512). It shows that at high off-dilution (90-95%) the lamellar structures seem destabilized. This is probably due to the increasing influence of the attractive interaction between the oil and the hydrophobic surfactant chain, in relation to the penetration rate of the oil molecule into the surfactant film, which depend on the type of aggregates (Fig. 2). This effect markedly depends on the length of the hydrocarbon: this becomes evident when we compare the diagrams with heptane and hexadecane. The present system with decane appears as an intermediate case. On the other hand, in much more concentrated systems (but still with 0 < a < 10), we got evidence ( l l , 18) of the formation of dense packing of disordered multilayered particles (with separate layers). Therefore it appears that the water-swollen micelles are typical aggregates for a < 10 and a volume fraction of the disperse phase in the range 0.1 to 0.25 (at low temperatures). 2. Small angle neutron scattering. A direct evidence of the (transient?) coexistence of two types of aggregates for 5 ~< a ~< 7 has been obtained from particular small angle neutron scattering measurements (the temperature was 15°C, and the oil concentration was 85% w/w). After each successive addition of water in the same cell, we immediately performed the corresponding scattering measurement. For each overall decane/D20/C1EEO4 c o m p o s i t i o n , we used a series of decanes with deuJournal of Colloid and InterfaceScience, Vol. 116, No. 1, March 1987

teration rates in the range 0.1-0.4. So we got, for (nearly) the same aggregates, a series of contrasts in the scattering length of the particle core relative to the continuous oil phase, taking into account the oil penetration into the hydrophobic part of the bilayers. With these particular deuteration rates, the systems are in the vicinity of the so-called matching point for which the sensitivity of the apparent radius of gyration of a particle toward any change in its morphology is quite enhanced, although influenced to a great extent by interparticle effects (5). A few examples of the results are shown in Fig. 4. In the I-l(q2) representation, the slopes of the curves for q --~ 0 are proportional to the square of the apparent radius of gyration, Papp. a Clearly, the curves tend to support the coexistence of two types of particles of different radius of gyration, and it is tempting to ascribe the larger one to hank structures. Given the approximations made for the calculation of the interparticle effects (use of equivalent hard sphere), the evolution of the whole series of curves corresponding to one particular sample appear quite consistent with model theoretical calculations of the spectra I(q). For example, we have found that the experimental results of Fig. 4 (c~ = 6.5) are coherent with a mixture of hank particles (N 550) and bilayers (N "-, 400) in equivalent proportions (curves D, Fig. 4). A full presentation of the equations we used for these calculations is out of the scope of the present paper (5, 11). But let us note that the explanation of such an evolution of the curves rests on the following equation: p2pp ~ i)2 __ k . V 2/3.

[4]

Let og be the geometrical radius of gyration of the hard (supposed homogeneous) particle of volume V. p2 is the square of the apparent radius of the isolated particle, which depends on both its morphology (mainly on pg, for a given model) and the deuteration rate (6) of the solvents (decane). In the present case, the calculations (5) show that p2 is an increasing function of pg, and decreases with ~ (it can even become negative), k is essentially a func-

W A T E R - S W O L L E N NONIONIC MICELLES

f

A

35

C

o . ~ °~° ' ~ ' ° ~ I,C

0,15"

,o.° 0,10.

q2 o,5

+3

0

14

o

2.1b-3

D

B 1,2.

1,0 o o o ~

?

~

1,0

o

O,I

q2 j

i

,

0

i

2 ~'~d3

FIG. 4. The reciprocal of the small angle neutron scattering intensity for a = 6.5 and ~b = 0.15 at 20°C for several deuteration rates, 6, of the decane (A, 6 = 0.2; B, 6 = 0.3; C, 6 = 0.4). Curves D are theoretical

spectra for 6 = 0.3, for mixtures of hank-particles (N = 550) and bilayers(N = 400), in the proportion 3/7 (---) and 5/5 (--). don of the volume fraction of the disperse phase q~. Therefore, at constant ~b and pg, 2 Papp decreases with 6, and may become negative. The smaller pg, the lower the 6 value for which P~pv = 0. Even at constant V (i.e., constant N ~ 400), p~ is the largest for the hank structure; for lower values of 6 (0.1, 0.2), po2 overcomes the term due to interparticle interactions for both aggregates. For intermediate 6 values ('~0.3), p~pp can be positive only for the largest aggregate; it will be zero for about a = 0.4 (curve C, Fig. 4) for this particle. CONCLUSION

The simple model we propose here is then quite consistent with the whole of our neutron scattering results for these micelles. It states that (at lower temperatures) the aggregation number is monitored by the degree of hydration of the EO groups in the parallel packing of the oxyethylene chains: when hydrated, a EO group must be surrounded by a water dimer. Most interestingly, this hydration process appears quite in accordance with the structural results obtained for all the systems with high surfactant/water ratio, whatever the oil content

and including the lamellar phase, as it will be discussed elsewhere (18): all these systems can be characterized by a constant area per polar head. This result is also quite in agreement with conclusions drawn by other authors (9, 23), who have shown that the number of water molecules per EO group is about 2, exceeding this value only at very high water contents. This supports then our K parameter and its interpretation. On the other hand, the model appears to be coherent with the recent N M R and thermodynamic measurements on the isotropic phases of other nonionic surfactant systems with high surfactant/water ratios (20, 21). This structural model is also not at variance with recent N M R quadrupole splitting results on liquid crystalline phases: they show that the water-binding equilibrium involves several water molecules, rather than being a one-step equilibrium involving one water molecule. REFERENCES 1. Ribeiro, A., in "Reverse Micelles" (P. L. Luisi and B. E. Staub, Eds.), p. 113. Plenum, New York, 1984. Journal of Colloid and Interface Science, Vol. 116,No. 1, Match 1987

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RAVEY AND BUZIER

2. Klason, T., and Henriksson, U., in "Surfactants in Solution" (K. L. Mittal and B. Lindman, Eds.), p. 93. Plenum, New York, 1984. 3. Kumar, C., and Balasubramanian, D., J. Colloid Interface Sci. 74, 64 (1980). 4. Christenson, H., Friberg, S. E., and Larsen, D. W., J. Phys. Chem. 84, 3633 (1980). 5. Ravey, J. C., Buzier, M., and Picot, C., J. Colloid Interface Sci. 97, 9 (1984). 6. Ravey, J. C., and Buzier, M., in "Macro- and Microemulsion" (O. Shah Ed.), pp. 253-263. ACS Symp. Ser. No. 272. Amer. Chem. Sot., Washington, D.C., 1985. 7. Boyle, M. H., McDonald, M. P., Rosi, P., and Wood, R. M., in "Microemulsions" (D. I. Robbs, Ed.), p. 103. Plenum, New York, 1982. 8. Hermansky, C., and Merckay, R. A., J. Colloid Interface Sci. 73, 324 (1980). 9. Epstein, B. R., Foster, K. R., and Mackay, R. A., J. Colloid Interface Sci. 95, 218 (1983). 10. Bostock, T. A., McDonald, M. P., and Tiddy, G. J. T., in "Surfactants in Solution" (K. L. Mittal, Ed.), Vol. 3, p. 1805. Plenum, New York, 1984.

JournalofColloidandInterfaceScience,Vol.116.No. 1, March1987

11. Buzier, M.~ Th~se Etat, Universit6 de Nancy I, France (1984). 12. Friberg, S., Buraczewska, I., and Ravey, J. C., in "Micellization, Solubilization and Microemulsions" (K. L. Mittal, Ed.), Vol. 2, p. 901. Plenum, New York, 1977. 13. Ruckenstein, E., and Nagarajan, R., J. Phys. Chem. 84, 1349 (1980). 14. RSseh, M., in "Nonionic Surfactants" (M. J. Schick, Ed.), p. 753. Dekker, New York, 1966. 16. Percus, J. K., and Yevick, G. J., Phys. Rev. 110, 1 (1958). 17. RendaU, K., and Tiddy, G. J. T., J. Chem. Soc., Faraday Trans. 1 80, 3339 (1984). 18. Ravey, J. C., and Friberg S., to be published. 19. Engdahl, A., and Nelander, B., Chem. Phys. Lett. 113, 49 (1985). 20. Olofsson, G., Kizling, J., and Stenius, P., J. Colloid Interface Sci., in press, personal communication. 21. Christenson, H., and Friberg, S., J. Colloid Interface Sci. 75, 276 (1980). 22. The author is indebted to Dr. G. Tiddy for enlighting discussion on this subject. 23. Cheever E., J. Colloid Interface Sci. 104, 121 (1985).