Langmuir 1994,10, 2945-2949
2945
O/W Microemulsions and Hydrotropes: The Coupling Action of a Hydrotrope Stig E. Friberg* and Chris Brancewicz Center for Advanced Materials Processing, Clarkson University, Potsdam, New York 13699-5814
David S. Morrison . I
Penreco Division of Pennzoil Products Company, Technology Center, The Woodlands, Texas 77387 Received July 6, 1994@
The influence of a hydrotrope, sodium xylenesulfonate (SXS), was determined on the areas of oil-inwater ( O N ) microemulsions of water, sodium dodecyl sulfate (SDS), pentanol (CsOH), and aliphatic hydrocarbons. The addition of a hydrotrope extended the microemulsion toward higher hydrocarbon content for the aliphatic hydrocarbons hexane and decane, while the improvement for a typical white oil was less pronounced. The regions are reported in the form of three-dimensional diagrams with five components and provide a rational explanation of the coupling action of a hydrotrope molecule.
Introduction The microemulsion systems by now have been investigated from all aspects as revealed by a large number of review articles and monographs.l-’ The related phenomenon of hydrotropic solutions has not received the corresponding attention, and the number of review articles is limited.8-10 The hydrotropes serve in a large number of applications within health care and household products with two main functions. First, they are used as solubilizing agents, and in this capacity they are typically added in high concentrations, resulting in extremelyhigh solubilization. The structural reasons for high concentration and the high solubilization were explained by our group in earlier work1’ as emanating from the fact that the presence of the hydrotrope destabilizes the lyotropic liquid crystalline phase. Their second function is as “couplingagents”to prevent the phase separation in aqueous solubilized systems. As “coupling agents”, their action is presumed to be that of a compound to join oil-in-water ( O N )areas to one region by connecting oil-soluble and water-soluble compounds. Such a function of a short-chain amphiphile that is highly water soluble appeared less likely, and a more rational explanation of the coupling phenomenon was sought. With Abstract published inAduanceACSAbstracts, August 15,1994. (1)Langevin, D. Bull. Soc. Fr. Phys. 1988,68, 11. (2)Langevin, D. Acc. Chem. Res. 1988,21,255. ( 3 )Pfiiller, U.,Mizellen-Vesikel-Mikroemulsionen - Tensidassoziate und ihreAnwendung i n h l y t i c und Biochimie;Springer-verlag: Berlin Heidelberg, New York, 1986. (4)fisica degli antitili: Micelle,vescicole e microemulsioni;DeGiorgio, V., Ed.; Elsevier Sci. Publ. Co.: New York, 1985. (5)Progress in Microemulsion;Martelucci, S., Chester, A. N., Eds.; Plenum Publ. Corp.: New York, 1989. (6) Microemulsions and Emulsions in Food; ElNokaly, M., Ed.; American Chemical Society: Washington, DC, 1991. (7) Friberg, S. E.; Bothorel, P. Microemulsions: Structure and Dynamics; CRC Press: Boca Raton, FL, 1987. (8)Balusubramanian, D.; Friberg, S. E. In Surface and Colloid Science; Matijevid, E., Ed.; Plenum Press: New York, 1993; Vol. 15. (9) Florescu,S.;Rob, M.; Isac, A.; Stefanescu,A. Tenside,Surfactants, Deterg. 1992,29(6), 409. (10) Darwish, I. A.;Ismail, F. A.; El-Khordagui, L. K. J.Pharm. Sci. 1992,6(1),101. (11)Friberg, S. E.; Rydhag, L. Substanzen Tenside 1970,7 , 80. @
0743-7463/94/2410-2945$04.50/0
C5OH
Figure 1. The system water (W), sodium dodecyl sulfate (SDS), and pentanol (CsOH) showing an isotropic liquid solubility region with three kinds of amphiphilic association structures: (I) inverse micellar region; (11) bicontinuous micellar region; (111) aqueous micellar solution. The region of a lamellar liquid crystal is indicated by LLC.
this contribution, we demonstrate that the coupling action of a typical hydrotrope in fact is identical to the solubilizing action. Our results show that the O N and W/O isotropic liquid areas are united to one because the presence of the hydrotrope destabilizes a liquid crystal. As an example, we chose a known O M microemulsion system12that has a W/O microemulsion region close to an O N region in the phase diagram. In addition, the results reveal that the presence of a hydrotrope may enlarge the O N microemulsion regions, which are sometimesnarrow and of dubious stability. Experimental Section Materials. The sodium xylenesulfonate was obtained from Aldrich (Milwaukee, WI),and the n-amyl alcohol and decane, both of certified grade, were from Fisher (Fairlawn, NJ). The hexane, Photrex Reagent grade, was from J. T. Baker (Phillips(12) Podzimek, M.;Friberg, S. E. J. Disp. Sci. Technol. 1980,1(3), 341.
0 1994 American Chemical Society
2946 Langmuir, Vol. 10, No. 9,1994
Friberg et al.
‘lc6
n-C10
n-C6
12.2 wt ?4SXS in
85%
W
Figure 2. Microemulsion regions in the system water (W),
sodium dodecyl sulfate (SDS),pentanol (n-CsOH), hexane (nCS),and sodium xylenesulfonate (SXS). The composition of the aqueous solution of the surfactant and the hydrotrope is given in the left hand corner for parts A-C.
burg, NJ). The sodium dodecyl sulfate was Mallinckrodt Genar Grade (packagedin Paris,KY) twice recrystallized from absolute ethanol and checked for purity by noting the absence of a minimum in a surface tension vs concentration plot. The water was deionized of better than 18 MS2 quality. The white oil (Drakeol7)was a gift from Penreco Division of Pennzoil Products co. Phase Regions. The microemulsion regions of the phase diagrams were determined by titration of water (and the 15 w t % concentration sodium dodecyl sulfate solutions with proper amounts of the hydrotrope sodium xylenesulfonate) to various weight ratio solutions of the hydrocarbon (hexane, decane, and white oil D r a w l 7)inn-pentanol. mephase diagram illustrated in Figure 1 was prepared by titration ofwater into variousweight percent sodium dodecyl sulfate/pentanol mixtures, with vigorous mixing of the sample during the titration. The presence or
n-C&H
15wt%SDS+ 85 Yo W
Figure 3. Microemulsion regions in the system water (W), sodium dodecyl sulfate (SDS), pentanol (n-CsOH), decane (n-
C~O), and sodium xylenesulfonate (SXS). The composition of the aqueous solution of the surfactant and the hydrotrope is given in the left hand corner for parts A-C.
absence of the crystalline sodium dodecyl sulfate component and the formatioddisappearance of the liquid crystal phases were checked by viewing the sample between crossed polarizers and observation of sample birefringence. The phase boundaries of the liquid crystal phase were verified by the use of small-angleX-ray diffraction (SAXS)measurements.
Results The investigations are based on the well-known system water, sodium dodecyl sulfate, and pentanol (Figure 1). It contains the inverse micellar solution 111, the basis for w/o microemulsions, a bicontinuous part [II], and the aqueous micellar solution [I111which forms the basis for the O N microemulsions. The lamellar liquid crystal phase that is present is indicated by LLC. Addition of the
0 I W Microemulsions and Hydrotropes
Langmuir, Vol. 10,No. 9,1994 2947 C5OH
D-7
/ D-7 C5OH
85%W
D-7
Figure 5. Solubility of the system in Figure 1 with sodium xylenesulfonate added to the water.
Discussion
%w Figure 4. Microemulsion regions in the system water (W), sodium dodecyl sulfate (SDS),pentanol (n-CsOH),white oil (D7), and sodium xylenesulfonate(SXS).The composition of the aqueous solution of the surfactant and the hydrotrope is given in the left hand corner for parts A-C. 85
hydrotrope to the aqueous solution caused systematic changes to the solubility regions. The minimum water content to form the inverse micellar solution (I) of the solubilityregion was increased slightly, and the minimum amount of surfactant to form the region was increased (left solubility limit), and the narrow channel [I13 became wider. The “couplingaction”of the hydrotropeis demonstrated in Figures 2A-C and 3A-C. In Figure 2A, the separation of two microemulsion regions I and I1 is a distinct feature of the system water, sodium dodecyl sulfate (SDS), pentanol (CsOH), and hexane. Addition of sodium xylenesulfonate to the water changed the system to give but one microemulsion region (Figure 2B,C). A similar change took place with the microemulsions containing decane (Figure 3A-C) and white oil (Figure 4A-C).
The results support the assumption of the function of the hydrotrope as a coupling agent: addition of the hydrotrope to the water resulted in the divided microemulsion regions uniting to a single solubility region. Figures 2A-C, 3A-C, and4A-C reveal the areas between the microemulsion regions I and I1 to be turbid emulsions and that addition of the hydrotrope changes them to become transparent microemulsions. Hence, the nomen coupling agent is justified. However, it is conceptually not easy to accept a compound with such an insignificant hydrophobic part physically acting as a coupling agent between an O N and a W/O microemulsion. In fact, the present results show that the hydrotrope molecule in reality does not in any capacity act as a “coupling agent”. Instead, the results provide a logical and systematic explanation for hydrotropic coupling action by relating the increased microemulsion areas directly to their solubilization mechanism.11J2 The explanation is found in the changes caused by the hydrotrope on the nonhydrocarbon part of the system (Figure 1).The modifications of the phase regions are discussed in some detail to provide the necessary background. The addition of hydrotrope caused five changes (Figure 5A,B) in the solubility regions 1-111 in Figure 1: (a) The narrow waist I1(Figure 1)was progressively widened with increased amounts of hydrotrope in water, (b)the pentanol solubility in the aqueous solution was increased, (c) the minimum amount of water to form region I1 was slightly increased, (d)the water solubility in pentanol was reduced, and (e) the initial water solubilization into pentanol required a greater amount of surfactant.
Friberg et al.
2948 Langmuir, Vol. 10,No. 9,1994 n-Cfj
SXS
sxs
W SDS n-cs
sxs
W
SDS n-C(j
W
SDS
Figure 6. Combination of the diagrams in Figures 1 and 2 with those in Figure 5.
These phenomena are expected; the hydrotropemolecule is strongly polar with a weak hydrophobic action. Hence, the effects under d and e are rational outcomds of the addition of a salt to a W/O microemulsion base.13 This fact and simple dilution are sufficientto explain the small increase of the minimal water required to form the solubility region II.14 The increaseof pentanol solubility in water, on the other hand, is a result of pentanol hydrotrope association in water, which is in agreement with earlier resu1ts.l’ The widening of the waist (11, Figure 1)is a direct result of the destabilization of the lamellar liquid crystal (LLC) that is indicated in Figure 1. This feature is decisive for the action of the hydrotrope as “couplingagent” as revealed by the changesin the microemulsionregions (Figures2-4). The change of the turbid emulsion region between the transparent regions I and I1 in Figures 2-4 to a transparent microemulsion is in fact a consequence of the (13)Sjoblom,E.; Friberg, S. E. J.CoZZoidInterfaceSci.1978,67,16. (14)Friberg, S.E.; Buraczewska, I. Progr. Colloid PoZym. Sci.1978, 63,1.
SDS
Figure 7. Combination of the diagrams in Figures 1and 3 with those in Figure 5.
destabilizing action of the hydrotrope on the lamellar liquid crystalline phase (LLC, Figure l), a phenomenon that has been amply demonstrated long ago.ll In fact, the disappearance of the solubility gap in the microemulsion regions in Figures 2-4 does not involve any “coupling action”. A combinationof Figures 2-4 with Figure 5A,B directly demonstrates the real function of the hydrotrope. The separation of the O N and W/O branches of the microemulsion regions is a consequence of the presence of the liquid crystalline phase (LLC) in Figure 1. Addition of the hydrotrope to the basic system (Figure 5A,B) destabilizes the lamellar liquid crystalline phase and extends the liquid solution region to higher surfactant concentrations, and the “solubility gap” disappears. The consequence of this change for the microemulsion region is obvious from the combined phase diagrams shown in Figures 6A-C and 7A-C. These diagrams contain both
0I W Microemulsions and Hydrotropes D-7
sxs
D-7
W
D-7
Langmuir, Vol. 10,No.9,1994 2949 extend from regions I and I11 (Figure 1)in long channels toward the hexane (n-Cs)/pentanol(n-C50H)solution. The figure illustrates the fact that the separation of regions I and I1 in Figures 2-4 is a result of the presence of the lamellar liquid crystal. It should be observed that the turbid multiphase region between the isotropic solution I and I1 does not consist of an emulsion of these two liquids. Instead the lamellar liquid crystal (LLC, Figure 1)is involved. Adding 5.6% SXS to the water means that the base plane in the four-component system of aqueous solution of SXS, n-C50H, and SDS is moved to the correct level in the base system W, SDS, n-CSOH, SXS. The “solubility gap”in the base plane is reduced due to the destabilization of the liquid crystal, and the “solubility gap” between the two microemulsions is reduced. The increased destabilization of the lamellar liquid crystal (LLC, Figure 1)by the increased SXS content (Figure 6C) leads to a sufficient widening of the waist (Figure 1)that the baseline aqueous solution of SDS and SXS/n-C50H(Figure 6C) falls entirely within the solution region and the areas I and I1 (Figure 2A) are by now coalesced into a single microemulsion region. The microemulsions with decane (Figure 3A-C) give a similar response to the addition of hydrotrope (Figure 7AC) as do those with the white oil (Figures 4A-C, 8A-C). However, the significantly smaller microemulsion regions for white oil (Figures 4A-C, 8A-C) may be due to structural and molecular weight differencesbetween white oil and the n-alkanes (hexane and decane). Whereas n-hexane and n-decane are straight chain hydrocarbons (“paraffins”), the white oil is a complex mixture of 66% paraffins isoparaffins (branched alkanes) and 34% saturated cyclic hydrocarbons (“naphthenics”) which also may have alkyl substituents. The molecular weight of n-hexane (MW86) and n-decane (MW142) are similar when compared to the molecular weight of Drakeol 7 (average MW 323, by Hirschler). These differences between the white oil and the nalkanes affect hydrocarbon solubilities in the microemulsion systems, reflected by a much smaller microemulsion region for white oil compared to the microemulsionregions for n-hexane and n-decane.
+
Figure 8. Combination of the diagrams in Figures 1 and 4 with those in Figure 5. the basic diagram water (W), sodium dodecyl sulfate (SDS), pentanol (n-C50H) and sodium xylenesulfonate (SXS).In Figure 6A no SXS is added and the microemulsion regions
Acknowledgment. This research was supported by the New York State Commissionof Science and Technology at the Center for Advanced Materials Processing, Clarkson University, Potsdam, NY,and Penreco Division of Pennzoil Products Co.
