Langmuir 1995,11, 3269-3271
3269
Notes Phase Studies of Water/AlcohoY l-Octy1-2-pyrrolidone/Alkane Systems
I,A,
S. T. Adamyt Colgate-Palmolive Co. Technology Center, 909 River Road, Piscataway, New Jersey 08855
Figure 1. Structure of l-octyl-2-pyrrolidone (C8P).
Received November 14,1994. I n Final Form: May 10,1995
I. Introduction A great deal of research has been done on the fundamental properties of alkylpyrrolidone surfactants and how these surfactants interact with many anionic and zwitterionic surfactants. ld4 However, few, if any, investigations have been performed on the phase behavior of alkylpyrrolidone systems. We have therefore conducted a study of phase behavior in pyrrolidone systems. We have specifically focused on systems containing water, ethanol, l-octy1-2-pyrrolidone, and an n-alkane having a chain length of 6 to 12 carbons. The structure of l-octyl-2-pyrrolidoneis shown in Figure 1. With regards to phase behavior, the octylpyrrolidone systems are of particular interest, since many of the systems possess a three-phase (i.e., phase inversion) region between 0 and 100 "C.Surfactant systemswhere the three-phase region is within an easily accessible temperature range may be of great applicability, since minima in the oil-water interfacial tension and maxima in the detergency occur near the three-phase region^.^ We have further investigated how varying the ethanol concentration affects the position of the three-phaseregion. It is well-known that changes in the phase behavior of surfactant systems are imparted by the addition of alcohols, and the phenomena may be interpeted through the Gibbs-Duhem expression.
Water
n-Hexane 1-0ctyl-2-Pyrrolidone
A
11. Experimental Section Materials. All water used was filtered and deionized. The hydrocarbons n-hexane, n-octane, n-decane, and n-dodecane all had a purity greater than 99% and were all obtained from Aldrich Chemical Co., Inc. They were used without further purification. The l-octyl-2-pyrrolidone( l-odyl-2-pyrrolidinone)also possessed a purity of a t least 99% and was used without further purification. It was obtained from ISP Technologies, Inc. Ethanol, approximately 98% pure, was obtained from Quantum Chemical Corp. Finally, sodium chloride, 99.9% pure, was obtained from J. T. Baker, Inc. Phase Behavior. Samples for phase diagram determination were prepared robotically (Zymark Corp.) in screw-topcentrifuge tubes at a resolution of between 1 and 5%, depending on the characteristics of the phase region. The samples were then placed Current address: National Starch and Chemical Co., 10 Findeme Ave, Bridgewater, NJ 08807. (1)Rosen, M. J.; Zhu, Z. H.; Gu, B.; Murphy, D. S. Langmuir 1988, 4, 1273.
(2)Rosen, M. J.; Gu, B.; Murphy, D. S.;Zhu, Z. H. J. Colloid Interface Sci. 1989, 129,468. (3) Zhu, Z . H.; Yang,D.; Rosen, M. J. J.Am. Oil Chem. Soc. 1989, 66, 998. (4) Rosen, M. J. Langmuir 1991, 7, 885. ( 5 ) Kriissman, H.;Bercovici, R. Tenside, Surfactants, Deterg. 1993, 30, 99.
n-Dodecane
Water
Figure 2. Ternary phase diagrams at 25.0 "C of water, C8P, and either hexane (a) or dodecane (b). in a thermostated bath for 24 to 72 h in order to obtain equilibrium. Phase behavior was determined by visual observation.
111. Results and Discussion
T w o representative ternary phase diagrams displaying the phase behavior of water/l-octyl-2-pyrrolidone(C8P)/ alkane systems are shown in Figure 2. In Figure 2a, the phase diagram of the system containing water, CSP, and hexane displays a large two-phase region which extends from the water-hexane side to the water-C8P side. The fact that the surfactant is soluble in the oil component and negligibly soluble in the aqueous component (the solubility in water is 0.12%),6 means that at room temperature the two-phase region is a Winsor I1 system (6) Homby, J. C.; Jon, D. Soap, Cosmet. Chem.Spec. 1992 (Sept),52.
0743-7463/95/2411-3269$09.00/00 1995 American Chemical Society
3270 Langmuir, Vol. 11, No. 8, 1995
Notes
Water: n-Wxane = 1:l
Watrr : n-Octano I1:l
501 3b)
3a)
a w
40
40
F w
30
1
f
L
20
+I
10
n
0
20
40
60
-0
80
Water : n-Decanr= 1:l
Wetar : n-Do&c"
50
= 1:l
50r---l 40
f e
30
B
H
20
+
f
30 2o
10
10
0
60
40
I1-0ctyl-2-Pyrrolidone
G
aE
20
% 1-0ctyl-2-Pyrrolidone
3d)
g!
20
f 10
40
G e
30
E
0
3c)
50
0
40
60
80
X 1Qctyl-2-Pynolidone
0
0
'
.
"
20
' 40
.
'
80
.
'
80
X l-Octyl-2-Pyrrolidone
Figure 3. Temperature versus % C8P phase diagrams of systems with water, C8P,and either hexane (a), octane (b), decane (c), or dodecane (d). In each case the weight ratio water:alkane = 1:l. (denoted by 5 in Figure 2a), where an oily phase containing solubilized water (i.e., a w/o micellar phase) is in equilibrium with an aqueous phase. The miscibility gap along the water-C8P side is the result ofthe upper (closed loop) miscibility gap in the water-C8P binary phase diagram. Figure 2a also approximates the phase behavior in the corresponding system with octane or decane as the oil, with slight variations of the extent of the two-phase region within the interior of the triangle. We therefore do not show these diagrams. The second type of ternary diagram is shown in Figure 2b. In this system, composed of water, C8P, and dodecane,a three-phase region exists (denoted by 3 in Figure 2b), which is representative of a Winsor I11 system. In a Winsor I11 system, there is an equilibrium between the oil-in-water (o/w) micellar phase, the water-in-oil (w/o) micellar phase, and the surfactant-rich microemulsion phase. The appearance of the three-phase region as the length of the oil chain is increased shows that the C8P amphiphile has less tendency to partition into the oily phase as the chain length of the alkane increases. The evolution of the three-phase region is better illustrated by examining the variable temperature phase diagrams. Figure 3 displays temperature-composition phase diagrams for the hexane, octane, decane, and dodecane systems with water and C8P. Each diagram shows increasing amphiphile concentration along the x-axis and increasing temperature along the y-axis. The ratio ofwater/oil in each case is l/1. Figure 3 demonstrates why the hexane, octane, and decane system triangular diagrams at 25 "C do not show a three-phase region while that for the dodecane system does. Figure 3a shows that in the hexane system, the top of the three-phase region lies around 7 "C, with the remainder of the three-phase
region presumably existing below 0 "C. Hence, its 25 "C triangular diagram (as in Figure 2a) does not show a threephase region. Similar circumstances as in the hexane system apply in the octane (Figure 3b) and decane (Figure 3c) systems. When octane is the oil, the top of the three-phase region extends just above 10 "C. When decane is employed, the "fish" structure of the three-phase region becomes apparent, with half of the fish existing above 5 "C, and the maximum in the three-phase region lying at about 20% C8P and T = 18 "C. Again, as in the hexane system, the three-phase region is absent at room temperature. The three-phase region in the dodecane system, as shown in Figure 2d, is quite large and clearly visible in the temperature interval studied. Below the fish presumably lies the Winsor I region (denoted by 2 in Figure 3d), where an equilibrium composed of an aqueous phase with solubilized oil (i.e., an olw micellar phase) plus an oily phase exists. Above the fish lies the Winsor I1 system, and within the body of the fish lies the Winsor I11 system. Since the body of the fish crosses the 25 "C line, the threephase region is correspondingly present in the phase triangle of Figure 2b. The trends in the phase behavior of the C8P systems qualitatively agree with results from previous studies of nonionic surfactants. I t is well established in systems containing water, hydrocarbons, and ethoxylated alcohol nonionic surfactants that the three-phase region occurs at higher temperatures and that the size of the region increases with increasing oil chain length (or degree of ~
(7) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990,94,3881.
Langmuir, Vol. 11, No.8, 1995 3271
Notes
hydrophobi~ity).~!~ Such behavior is aptly justified by the Gibbs-Duhem e x p r e s ~ i o n . ~ - ~ We have also examined the effect of incorporating a fourth component-ethanol-into the water/C8P/alkane systems. The concept of altering the phase properties of a ternary system by adding a fourth component has been implemented bef~re.~JO Figure 4a illustrates the effect on the phase behavior of the water/C8P/hexane system when ethanol is added. The percentages of ethanol described in Figure 4a are the weight percentages in the corresponding water-ethanol psuedocomponents, even though the ethanol-water mixture probably does not distribute itself between the different phases as if it were a pure component. Although in each case much of the fish structure is hidden below the minimum temperature of investigation, it is clear that adding ethanol to the system raises the position of the three-phase region as well as shrinks its size. The rise in temperature of the fish is demonstrated by noting how the one-phase region (the tail of the fish) approaches the three-phase region with the increasing amount of ethanol, showing that the point at which the two regions join is moving up. Because only a very small three-phase region could be found at 60% ethanol and no three-phase region could be found at 63%ethanol, it is hypothesized that a tricritical point lies between these two ethanol concentrations. The addition of ethanol has a similar effect on the phase behavior in the water/CSP/octane system as in the hexane system, as shown in Figure 4b. The position of the fish is moved dramatically between a concentration of 45% ethanol, where a portion lies below 0 "C, to 55% ethanol, where the entire three-phase region lies completely above 25 "C. Addition of higher amounts of ethanol drives the fish to even higher temperatures while shrinking its size. A tricritical point no doubt lies somewhere above 110 "C at higher concentrations of ethanol. Finally, the effect of adding ethanol to the water/ pyrrolidone/decane system is shown in Figure 4c. In addition to the three-phase regions being larger than corresponding fish in the hexane and octane systems, it can also be seen that the bodies lie at higher temperatures. This is of course due to the increased hydrophobicity of the oil. The trends in phase behavior where the three-phase region is elevated to higher temperatures and is ultimately shrunk to a tricritical point by the addition of ethanol have previously been noted in ref 10. The systems investigated in ref 10 were similar to those in this study, being comprised of water, cyclohexane, C12HdOCH2OCH2)60H, and ethanol. The effect of ethanol on the phase behavior in nonionic surfactant systems has also been discussed in ref 9. In particular, a water-miscible alcohol primarily partitions into the aqueous phase but also makes the psuedocomponent of surfactant alcohol more hydrophilic. It is well understood that as the nonionic surfactant becomes more hydrophilic, the three-phase region rise^.^^^ It may also be reasonable to view the pyrrolidone systems in this manner. However, we note that in this study, significant quantities of ethanol, e.g., 45%, have been added to the
+
(8) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1986,24,
654.
(9)Kahlweit, M.;Strey, R.; Busse, G. J.Phys. Chem. 1991,95,5344. (10)Yoshida, M.;Kunieda, H. J . Colloid Interface Sci. 1990, 138, 273.
(Ethanol+Water) : n-Hexane I 1:l
4a) 1 - 108
1
60?4
a
1
I I 40
50% 60%
1 i
69%
,
20
n
20
0
40
BO
60
K 1Qctyl-2-Pyrrolldone (Ethanol+Weter) :n-Oc(ane I 1:l
4b)
10
30
20
50
40
60
K 1-0ctyl-2-Pyrrolldone (Ethanol+Water) : n-Decane = 1:l
4c) 100
a
80
1
1
e
40
20 0 10
20
30
40
50
60
70
I
80
X 1Qctyl-2-Pyrrolldone
Figure 4. Temperature versus % C8P phase diagrams of systems with water + ethanol, C8P, and either hexane (a), octane (b), or decane (c). Symbols denote systems with either 45% (0),50%(O),55% (A),60%(A),63%(01,or 66%(m) ethanol
in the psuedocomponent. In each case the weight ratio (water ethano1):alkane = 1:l.
+
+
water ethanol psuedocomponent. Perhaps the rise in the three-phase region with increasing ethanol content could also viewed in terms of the polarity of the aqueous phase changing so that C8P preferentially partitions into the aqueous phase. The system then goes from being a Winsor I1 type to a Winsor I type, with the position of the Winsor I11 region moving to a higher temperature. LA9409079
