16 Influence of Temperature on the Structures of Inverse Nonionic Micelles and Microemulsions J. C. RAVEY and M. BUZIER
Downloaded by PURDUE UNIV on July 7, 2016 | http://pubs.acs.org Publication Date: March 27, 1985 | doi: 10.1021/bk-1985-0272.ch016
Laboratoire de Physico-Chimie des Colloïdes, Université de Nancy I, Faculté des Sciences-1er cycle, B.P. No. 239, 54506 Vandoeuvre les Nancy, France
Just above the P.I.T., the oil-rich phase diagram of the nonionic systems (C12 EO4 + decane + water) is constituted by two separated one phase realms, where the structures have been proved to be very different. If the temperature is slightly raised, the intermediate two phase domain tends to disappear : then we get a single one phase area whose delineation (the maximum surfactant/water ratio) is also temperature dependent. The morphological determinations have been performed by the small angle neutron scattering. It has been found that the structures mainly depend on the overall composition of the sample (the geometrical constraints). They may or may not exist, according to the temperature, but we always get lamellas for lowest water contents which turn into water-in oil globules. One of the most typical characteristics of microemulsions and micelles with nonionic surfactants is their high sensitivity towards the temperature : usually a ternary mixture with a given composition of oil, water and nonionic amphiphile remains monophasic and isotropic only for a narrow temperature range. This well known fact finds expression in a great apparent changeability of the ternary phase diagrams. A pioneering attempt of systematization of the evolution of these phase diagrams was proposed some years ago (1,2). Since then a few more detailed phase behavior investigations have been performed. But they remained purely descriptive (3-5). The phase diagrams reflect the mutual oil-water solubilization properties of the nonionic surfactants, which can be understood, and then also predicted, only i f the structures of the microemulsions (or the micellar aggregates) are known with some degree of certainty. Moreover the thermodynamical explanation of these properties in terms of the hydrophile-hydrophobe forces has to be founded on clear structural evidence and this is far from being the case at the present time (6). Therefore most interesting would be the knowledge of the correlation between the phase diagrams and the structures according to the temperature but also in relation to the chemical nature of the oil 0097-6156/ 85/0272-0253S06.00/0 © 1985 American Chemical Society
Shah; Macro- and Microemulsions ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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and or surfactant molecules : for a given composition does the structure change with any change of the temperature ? And what comes from the difference in the s t a b i l i t y of the molecular aggregates s p e c i f i c to two d i f f e r e n t compositions ? The present report makes our contribution to the solution of that problem, although it concerns only one p a r t i c u l a r nonionic system studied in a small but c r i t i c a l range of temperature. Here we are interested in the correlations (structure-phase behavior) of the tetraethylene glycol dodecyl ether (Cj (EO)^), the oil being the decane and thus for temperatures just above the so-called Phase Inversion Temperature (P.I.T.), i . e . when the surfactant becomes p r e f e r e n t i a l l y soluble in the oil. Indeed, as f a r as i t s phase diagram is concerned, that system exhibits very a t t r a c t i v e features between 18°C and 25°C : there is a progressive coalescence of two d i s t i n c t one phase domains into only one realm. So we get series of "water in o i l systems for which the s t r u c t u r a l determinations from small angle neutron scattering measurements appear to be less d i f f i c u l t than for the "surfactant phase". Now l e t us emphasize that we want a l l the conclusions to be drawn exclusively from the experimental results ( i . e . the neutron scattering spectra), without making use of any theory on microemulsions. In p a r t i c u l a r , at a given temperature and for a c e r t a i n over a l l composition, the structures must be determined independently of any hypothesis about the ( o i l ) d i l u t i o n . Quite the contrary, these structural results should be actually used to support the eventual v a l i d i t y of that theory or other, which can be proposed to explain the evolution of the phase diagrams. 2
Downloaded by PURDUE UNIV on July 7, 2016 | http://pubs.acs.org Publication Date: March 27, 1985 | doi: 10.1021/bk-1985-0272.ch016
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Materials and Methods Chemical and Phase Diagram. The nonionic surfactant was tetraethylene g l y c o l dodecylether C. (EO)^. It was purchased from Nikko Chemicals (Japan). It was used without further p u r i f i c a t i o n (99 % ) , a l though it has been recognized that the s o l u b i l i z a t i o n properties of the pure surfactant may be somewhat influenced by the presence of small quantity of impurities. As a matter of f a c t , the presence of some traces of a polar contaminant only results in a s l i g h t temperature s h i f t of the whole phase diagram, whose exact delineation is a l l the more sensitive that the oil content is very large (more than 95 % ) . At any rate, in the present work, we are not dealing with samples whose decane concentration exceeds 93 %. Besides a l l the measurements have been performed on the same batch of surfactant. The corresponding phase diagram at 20°C may not be quite i d e n t i c a l to the one published by Friberg, it w i l l not at a l l change the conclusions of the present paper, since here we are interested in the evolution of the system, and not in some absolute value of the free energy of the surfactant molecule. Anyway that "equivalence" temperature-contaminant could be explained in terms of an equivalent (but s l i g h t ) modif i c a t i o n of the packing constraints inside the micellar aggregates. Indeed, as it w i l l be shown below, the evolution of the phase diagram corresponds to the progressive introduction of some "disorder" into the amphiphile packing. Both water and oil were mixtures of isotopes. As a matter of fact, the "aqueous component" always contained 80 % D 0 and 20%H 0. 2
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Shah; Macro- and Microemulsions ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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And as f a r as the oil was concerned, we used p a r t i a l l y deuterated de cane molecules of various grades of deuteration. The isotopic compo s i t i o n s of such solvents was chosen so that the v a r i a t i o n contrast method was the most i l l u s t r a t i v e and powerful. On the other hand, we have noted that the exact delineation of the one phase domains was almost i n s e n s i t i v e to any D/H substitution. Let us r e c a l l that the 2 ^ 2 ^ *-° kept constant throughout the present investiga tions : we only made isotopic substitutions on the decane component. And since the phase diagrams is not noticeably modified, we can guess that the small change in Van der Waals forces must influence only s l i g h t l y the structures of these micellar aggregates. This w i l l be confirmed by the results of the present report. The phase diagrams of the system decane-water-C ^(EO)^ are shown in figure 1 f o r s i x temperatures in the range * 17°- 25°C., and for oil concentration above 40 % w/w. Among the samples studied by neutron scattering only a few ones w i l l be considered here. They have o v e r a l l compositions which are represented by points Al to A3. They concern samples with s u r f a c t a n t / o i l r a t i o of O.176 ( i n weight by weight). Most of the results presented here w i l l concern the two points A2 and A3 : they correspond to the water/surfactant ratios of O.50 and 1.17, and occupy very peculiar p o s i t i o n in the phase d i a gram. At low temperatures (17°C) two d i s t i n c t one phase domains e x i s t . At about 20°C., they coalesce in the region very r i c h in oil : the two "wings" of this new realm remain separated by a long " f i n g e r - l i k e " two phase area, which progressively disappears when temperature in creases to 2 5 ° C ; c o r r e l a t i v e l y the l e f t most l i m i t moves toward the oil-surfactant basis. As a r e s u l t , at low temperature ( i . e . 20°C) on l y A2 is in the two phase realm. At intermediate temperature (21°523°C) each sample is a one i s o t r o p i c phase system (although at 21°.5C, A2 and A3 belong to d i f f e r e n t "wings" of the domain). At higher tem peratures samples l i k e A3 undergo a demixing and form emulsions. I t must be emphasized that in that temperature range, a lamellar l i q u i d c r y s t a l always exists for the lower contents of oil (less than about 35 % ) , as shown in figure 2 which represents a part of the phase diagram at 20°C. H
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Neutron Scattering Method. The measurements were carried out at the Institute Laue Langevin in Grenoble (France), using the small angle scattering instruments D l l and D17. The sample-detector distances and the wavelengths (λ) were chosen so that the scattered i n t e n s i t i e s could be measured inside the (q) scattering vector range 0 005 A~* to O.2 Â~l. In fact, the most useful q range was O.02 to O.2 A~* [q = (4 π/λ) s i n (θ/2), θ means the scattering a n g l e ] . A presentation and a discussion of the method we use to analyze the experimental da ta have been presented elsewhere (7,8). That method can be summarized as follows : we have to f i n d the best theoretical spectra correspon ding to "water in oil" micellar aggregates which could f i t the expe rimental spectra in the whole range q .
