Langmuir 1994,10, 2553-2558
2553
Giant Rodlike Reversed Micelles Formed by Sodium Bis(2-ethylhexyl)Phosphate in n-Heptane Zhi-Jian Yu and Ronald D. Neuman* Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849 Received November 29, 1993. I n Final Form: April 6, 1994@ The solution behaviors of sodiumbis(2-ethylhexyl)phosphate (NaDEHP)in n-heptane were investigated by light-scattering and viscosity measurements. NaDEHP forms giant rodlike reversed micelles, with a radius of gyration as large as 53 nm, which violently contrasts with the literature view that the average micellar aggregation numbers in nonaqueous or apolar media are much smaller (seldom exceeding 20) than those in aqueous media. Significantly, a small amount of water plays the role of an antimicellar growth agent; i.e., the reversed micellar size decreases remarkably when "dry" solutions are exposed to humid air from which water vapor is absorbed or when bulk water is directly added-a behavior which is distinctly opposite to that for sodium bis(2-ethylhexyl)sulfosuccinatelapolar medium systems. Thus, the literature views that large micelles can only be found in aqueous media and that the surfactant headgroups in reversed micelles are linked together by hydrogen bonds are misleading. It is suggested that the primary contribution to the driving force for the growth of rodlike NaDEHP reversed micelles is long-rangeelectrostaticinteractionsamongthe headgroupsofthe surfactant molecules and their counterions, and a possible mechanism for the effect of water is also discussed.
Introduction Reversed micelle is a term given to a surfactant aggregate formed via nonchemical bonds in nonaqueous or apolar media with its structure inverted as compared with a surfactant micelle formed in aqueous media. Such aggregates play a n important role in many practical applications such as hydrometallurgy,' lubricants, cosmetics, and foodstuffs.2 Reversed micelles have been investigated intensively over the last half century. However, many fundamental properties of reversed micelles still remain to be understood. One open debate in the literature is whether or not there exist large reversed micelles in apolar media. Kertes and c o - w o r k e r ~regarded ~?~ reversed micelles as being very small particles with a n aggregation number of usually 4-10. Bourrel and Schechtel.5stated that the aggregation number of reversed micelles seldom ranges above 20 and is often much less. Ruckenstein and Nagaraja1-1~8~ have theoretically studied the aggregation of surfactants in nonaqueous media, and they have concluded that in contrast to aqueous systems there is no strong tendency favoring the formation of large aggregates to the formation of small aggregates. In a recent review, it was further stated that in the absence of additives such as water, the aggregation numbers of reversed micelles are generally so small that analogies with micelles in aqueous media are misleading.8 However, our previous reportg on reversed micellization suggests that very large reversed micelles can form in apolar media.
* To whom correspondence should be addressed.
Abstract published in Advance A C S Abstracts, June 15,1994. (1) Neuman, R. D.; Park, S. J. J.Colloid Interface Sci. 1992,152,41. (2) Luisi, P. L.; Straub, B. E. Reverse Micelles; Plenum Press: New York, 1984. (3)Kertes, A. S.; Gutman, H. In Surface and Colloid Science; Matijevic, E., Ed.; Wiley: New York, 1976; Vol. 8, p 193. (4) Kertes, A. S. In Micellizatwn,Solubilization and Microemulswns; Mittal, K L., Ed.; Plenum Press: New York, 1977; p 445. ( 5 ) Bourrel,M.; Schechter,R. S.Microemulsions andRelated Systems; Marcel Dekker: New York, 1988; p 112. (6) Ruckenstein, E.; Nagarajan, R. J.Phys. Chem. 1980,84, 1349. (7) Ruckenstein, E. In Progress in Microemulsions;Martellucci, S., Chester, A. N., Eds.; Plenum Press: New York, 1989; p 31. ( 8 )Rosen, M. J.Surfactants and Interfacial Phenomena; Wiley: New York, 1989; p 150. (9)Yu, Z.-J.;Zhou, N.-F.; Neuman, R. D. Langmuir 1992, 8, 1885.
Another fundamental problem lies in the effect of water on the size of reversed micelles. Eicke and co-workersloJ1 suggested that water serves as a "gluing" agent or is a prerequisite for micellization in apolar media. This view, which implies that the size of reversed micelles is proportional to the amount ofwater present in the system, has also been accepted by other a u t h o r ~ . ~ JHowever, ~J~ reversed micelles formed by NaDEHP in benzene are small even when in equilbrium with excess water.14 Previously, we have shown that the "gluing" effect of water on the formation of reversed micelles does not exist in the NaDEHPln-heptane ~ y s t e m .Thus, ~ it is compelling to examine whether or not water promotes the growth of NaDEHP reversed micelles. Although there have been a number of investigations concerning the aggregation behaviors of NaDEHP in apolar media,14-19no attention has ever been given to the effects of the environmental humidity or of added trace amount of water on the size ofNaDEHP reversed micelles in a controlled manner. In this paper, we report our findings of the formation of giant reversed micelles and the role of water on the size of the reversed micelles in the NaDEHPln-heptane system.
Experimental Section Bis(2-ethylhexy1)phosphoric acid (HDEHP), sodium bis(2ethylhexyl)phosphate(NaDEHP),and n-heptanewere the same as described previous1y.g High-purity water from a Millipore reverse osmosislSuper-Q system was subsequently doubly distilled with the first distillationbeing from alkalinepermanganate. The NaDEHPln-heptane solutionswere prepared by weighing proper amounts of NaDEHP and n-heptane, and the volume
@
(10) Eicke, H. F.; Christen, H. Helu. Chim. Acta 1978, 61, 2258. (11) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979,83, 480. (12) Schelly, 2. A. In Aggregation Processes in Solution; Wyn-Jones, E., Gormally, J., Eds.; Elsevier Scientific: New York, 1983; p 140. (13) Ueda, M.; Schelly, Z. A. Langmuir 1988, 4, 653. (14) Myers, A. L.; McDowell, W. J.; Coleman, C. F. J.Inorg. Nucl. Chem. 1964,26,2005. (15) Eicke, H. F.; Arnold, V. J.Colloid Interface Sci. 1974,46, 101. (16) Eicke, H. F.; Christen, H. J.Colloid Interface Sci. 1974,48,281. (17) Faure, A,; Tistchenko, A. M.; Zemb, T.; Chachaty, C. J. Phys. Chem. 1986,89, 3373. (18)Faure, A.;Tistchenko, A. M.; Chachaty, C. J.Phys. Chem. 1987, 91, 1827. (19) Shioi, A.; Narada, M.; Matsumoto, K. J.Phys. Chem. 1991,95, 7495.
0743-746319412410-2553$04.5010 0 1994 American Chemical Society
Yu and Neuman
2554 Langmuir, Vol. 10,No. 8,1994
40
I120
i
1.2
F
150
-120
304
1
i
+’,
0
0.8
/7 /
1.6
tso
-2 F-
-
ic- 30
2.4
[NaDEHP] (mM)
Figure 1. Relative viscosity (0)and reduced viscosity (+) of “dry” solutions of NaDEHP in n-heptane as a function of NaDEHP concentration at 293.15 K.
concentration of the solutions was obtained at 20 “C by density correction. All operations for preparing “water-free”or “dry” NaDEHPln-heptane solutions and “optically clean” light-scattering samples were carried out within a glovebox in which an analytical balance and the optical cleaning apparatus described elsewhere20 were installed. The chamber atmosphere of the glovebox was replaced by recirculating ultra-high purity grade nitrogen which was dried by passage through columns of silica gel and sodium hydroxide. The chamber was further dried by PZOSwhich occupied about 50%of the total area of the glovebox. The weight increase of one dish of P20s was traced by the analytical balance, and the removal of the moisture from the chamber was indicated when the weight of the PZOSbecame constant. The prepared NaDEHPln-heptane solutions were transferred to Pyrex light-scattering cells, and any dust was removed by continuous filtration using the optical cleaning apparatus in the “dry”environment of the glovebox. The lightscattering cells were then sealed by tightly fitting Teflon caps which were further sealed with Parafilm. The samples so prepared were immediately subjected to light-scattering measurements. Dynamic and static light-scattering measurements were performed with a Brookhaven BI-2OOSMmultianglegoniometer in conjunction with a Lexel Model 95-4 argon ion laser. The incident laser beam (A = 488 nm) was vertically polarized. A laser power of 200 mW was used in the light-scattering measurements reported herein. A Brookhaven 128-channelBI2030AT digital correlator/computer was used to process the output signal of a Malvern RR51 photomultiplier tube. The viscosity of the NaDEHPln-heptanesolutions was measured by an Ubbelohde viscometer with the temperature controlled to within 0.05 K. In the case of the “dry”NaDEHP/ n-heptane solutions,viscometric measurements were performed in the above-describedglovebox.
Results Figure 1 shows the relative viscosity (qreJ and reduced viscosity ((qrel- l)/(c - cmc)) as a function of the NaDEHP concentration in “dry” NaDEHPln-heptane solutions at 293.15 Kunder controlled humidity conditions. The term “dry” solution as used herein applies to those solutions prepared with the previously dried NaDEHP and nheptane and under the controlled humidity environment. The water content of the “dry” solutions was found to be not measurable by Karl-Fischer titration, which indicates the water content to be P >> l/L3,in which the rodlike particles are overlapped seriously with each other. P can be calculated from
where u is the volume which a surfactant monomer occupies in the reversed micelle and NA is the Avogadro number. We apply the viscosity data measured at the shear rate of about 3 s-l as a first approximation to estimate the reversed micellar size by employing eqs 3 and 4. The results are shown in Table 1,where the values of 0.55nm3 and 1nm were used for u and r, r e ~ p e c t i v e l y .We ~ ~ also observed in our measurements that the solution viscosity is lower at a higher shear rate, which can be interpreted as due to alignment of the rodlike reversed micelles in (22)Porte, G.; Appell, J.; Poggl, Y. J . Phys. Chem. 1980,84,3105. (23)Doi, M.; Edwards, S. F. J . Chem. Soc., Faraday Trans. 2 1978, 74, 918. (24) Lovera, J.; Lovera, P.; Gregoire, P. J . Solid State Chem.1988, 77, 40.
Figure 6. CPK space-filling molecular model of the cross section of rodlike NaDEHP reversed micelles.
flow. Thus, from eqs 3 and 4 one may see that the L values in Table 1reflect a lower limit for the length of the rodlike NaDEHP reversed micelles. The ratio of the length (L) to the mean distance between the rodlike reversed micelles (a = PI”),which describes the degree of overlapping of the reversed micelles, is also listed in Table 1.Lla values of 1.7 to 1.9 shown in Table 1 suggest a serious overlapping of the rodlike NaDEHP reversed micelles. Another significant finding is that the micellar size in the “dry“ solutions can be about 1 order of magnitude larger a t surfactant concentrations which are more than 1order of magnitude smaller than those in “wet”solutions (Table 1). A further tremendous decrease in qrel upon solubilization of a small amount of water in the “wet” solution (Figure 3)implies a remarkable decrease in the size of the NaDEHP reversed micelles, which is confirmed by the light-scattering results shown in Figure 4. Our results show that the size of the reversed micelles formed by the surfactant NaDEHP in apolar media is much larger than the size reported for most aqueous micellar systems. As indicated earlier, it is believed by many that a cmc does not exist in apolar media and the absence of a clear-cut cmc has been interpreted in terms of very small reversed micelles. Previously, however, we observed the existence of what appears to be a cmc in the NaDEHPln-heptane ~ y s t e m .The ~ occurrence of a cmc in the NaDEHPln-heptane system can, therefore, be attributed to the formation of the giant reversed micelles. From spatial considerations, three NaDEHP molecules can be accommodated in the cross section of rodlike NaDEHP reversed micelles (see Figure 6). We propose that a periodic structure of sodium cations and negatively charged oxygen atoms in the core of the NaDEHP reversed micelles can be formed along the axial length of the rodlike reversed micelles. A schematic illustration of such a structure for NaDEHP reversed micelles is shown in Figure 7. According to the Born-Lande the electrostatic lattice energy is
where e is the electronic charge, E is the permittivity, x is the equilibrium distance between ions,p is the Born index, and M is a constant which is known as the Madelung constant in the case of a three-dimensional lattice. Since (25) Barrett,J. Understanding Inorganic Chemistry;Ellis Horwood: New York, 1991; p 127.
Langmuir, Vol. 10, NO.8, 1994 2557
Formation of Giant Rodlike Reversed Micelles
nouo - ponO= AE = - AMNAe2/(4mx)( 1 - (Up)) - yno(A,- A,)
-
%AEpolar
J J A’
A-A’
0‘
0-0‘
Figure 7. Schematic model for NaDEHP reversed micellar core. The negatively charged oxygen atoms of the surfactant headgroups and the sodium cations are arranged in a quasione-dimensional lattice along the axial length of the rodlike reversed micelles. A-A’ corresponds to a typical cross section of the micellar core which contains three NaDEHP molecules. a, a’, b, b’, and c, c’ (not shown) represent the oxygen atoms of the three NaDEHP surfactant headgroups, and g, h, and i represent the sodium cations. Each cross section (e.g., B-B) repeats the adjacentcross section (e.g.,A-A’) after being rotated 60” within the cross-sectionalplane. the length of the rodlike reversed micelles (L) is finite, M for the one-dimensional reversed micelles may vary with L. Thus, the electrostatic energy change in rodlike reversed micellar growth is
Missel et a1.26proposed a “ladder” model for the growth of large rodlike (prolate spherocylindrical) micelles in dilute aqueous solutions. In this model, the number average mean aggregation number ( i i ~can ) be expressed as A,
= no + [K(X - XB)IU2
(7)
where no is the aggregation number of the two hemispherical ends of a rodlike micelle, X is the total mole fraction of surfactant monomers, XB is a characteristic mole fraction which is related to the energy advantage associated with transferring a surfactant monomer from the solvent into the cylindrical region of a micelle, and K is a thermodynamic parameter defined as
K = exp
(-(n&O
- pon,)/kT)
(8)
where pone and ,uoare the standard chemical potentials of the hemispherical region containing no molecules and of a single surfactant molecule in the cylindrical region of the micelle, respectively. Assuming that the “ladder” model can be applied to rodlike reversed micelles and that entropy effects can be neglected we propose that the driving force for reversed micellar growth includes a t least three contributions arising from the long-range electrostatic interactions and the changes in interfacial area and polar interactions. Therefore, nap" - p0,o can be written as (26) Missel, P. J.;Mazer, N. A.; Benedek, G. B.; Young, C. Y.; Carey, M. C. J. Phys. Chem. 1980,84, 1044.
(9)
Combining eqs 7-9 one may see that the larger the IhE( value, the larger the AN. This proposition ofthe driving force for the micellization process differs from that of other author^^-^ who considered the driving force for the micellization process in apolar media to be the dipole-dipole interactions between the polar headgroups of the surfactant molecules. The simple consideration of only dipole-dipole interactions seems appropriate in the case of di(2-ethylhexyl) sulfosuccinate (AOT) which has a large headgroup since AOT is known to form small spherical reversed micelles in apolar media. The solution behaviors of AOT as compared to those of NaDEHP suggest that one or more additional contributions to the driving force must be considered in any analysis of the formation of large rodlike reversed micelles. From geometrical considerations, surfactant molecules pack more tightly in cylinders than in spheres, i.e., A, A, > 0, where A, and A, are the interfacial areas between the solvent and the micellar core per surfactant molecule at the hemispherical end cap and cylindrical regions of a rodlike reversed micelle, respectively. The energy advantage due to the change in interfacial area in the sphereto-rod transition is -y(A, - A,) where y is the interfacial tension between the solvent and the micellar core. Indeed, we have observed by viscosity measurements that NaDEHP forms rodlike reversed micelles of much shorter length in benzene (smaller interfacial tension) than in n-heptane under the same experimental conditions ([NaDEHP], temperature, environmental humidity). Although the changes in the polar interactions (dipole-dipole interactions, hydrogen bonding, etc.) per surfactant molecule (hEp0la) and the interfacial energy contributions are important, they are not sufficient-by themselves-to provide the energy advantage required for the formation and growth of large rodlike NaDEHP reversed micelles, thereby providing an explanation for why large reversed micelles have not been theoretically predicted in the past. An additional contribution, namely, the electrostatic lattice energy, is required, and it is the primary contribution to the driving force for the growth of rodlike NaDEHP reversed micelles. The effect of water on the size of NaDEHP reversed micelles may be understood on the basis of the electrostatic lattice energy term in eq 9. When water is solubilized in the micellar core,the water molecules separate the charges and increase the permittivity in the core of the reversed micelles. From eqs 7-9 one can see that adding a small amount of water (WO> 2) to the reversed micelles (both x and E increase) reduces the size of the rodlike micelles (AN decreases) rather than increases the size as would be expected from the conventional view of the effect of water on reversed micelles (cf., AOT reversed micelles). The experimental verification (see Figures 1-4) of the predicted effect of water provides strong support that the primary force for reversed micellar growth is the electrostatic lattice energy advantage for incorporation of surfactant monomers into the rodlike region of NaDEHP reversed micelles. An alternative interpretation for the tremendous decrease in the reversed micellar size by a trace amount of water either absorbed from the atmosphere or directly added may be hydrolysis of NaDEHP which neutralizes the surfactant charge and consequently reduces the electrostatic interaction forces. However, HDEHP is a
2558 Langmuir, Vol. 10,No. 8, 1994 moderate acid with a dissociation constant of 0.02.27The HDEHP concentration which would result in NaDEHP/ n-heptane solutions at, for example, [NaDEHPl = 30 mM and [HzOI = 45 mM is only about 3 x mM. It can be seen from Figure 4 that the size of the reversed micelles in NaDEHPMDEHP mixtures, even when [HDEHP] = 4-7 mM, does not decrease as much as that in 45 mM HzO (i.e., W,= 1.5). Therefore, HDEHPgenerated through hydrolysis is not responsible for the observed remarkable effect of water on the size of NaDEHP reversed micelles. In conclusion, the formation of giant rodlike reversed micelles in the NaDEHPln-heptane system can be inter~
(27) Ritcey, G. M.; Ashbrook, A. W. Solvent Extraction: Principles and Applications to Process Metallurgy, Part I; Elsevier: New York, 1984; p 101.
Yu and Neuman preted as driven primarily by the electrostatic lattice energy associated with the reversed micellar core. The tremendous effect of a trace amount of water on reducing the reversed micellar size is not due to the hydrolysis of the surfactant, but rather it is due to a decrease in the electrostatic lattice ehergy. At this point it is clear that water can act not only as an antimicellization agent as we reported previouslyg but also as an antimicellar growth agent in NaDEHPln-heptane systems.
Acknowledgment. The authors gratefully acknowledge the financial support provided by the Office of Basic Energy Sciences, Division of Chemical Sciences, Department of Energy, under Grant No. DE-FG05-85ER13357.
