Langmuir 1989, 5, 924-926
924
Phase Studies of Zwitterionic Surfactants: Alkyl Sulfobetaines Patrick G. Faulkner and Anthony J. I. Ward Chemistry Department, University College, Belfield, Dublin 4, Ireland
David W. Osborne* Drug Delivery R&D, The Upjohn Co., Kalamazoo, Michigan 49001 Received August 10, 1988. I n Final Form: December 15, 1988 Binary phase behavior of synthesized sulfobetaine zwitterionic surfactants of the form CH3-
(CH2)11N+(CH3)z(CH2),S03with m = 2,3, or 4 has been determined by using laboratory robotics, optical
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microscopy, and NMR studies. A Krafft boundary of 70 "C was found for the case m = 2. For the cases of m = 3 and 4, the phase changes, isotropic micellar stiff isotropic stiff anisotropic hydrated solid plus crystalline surfactant, as the surfactant concentration increases. Liquid-liquid miscibility gaps were observed for both these surfactants with an upper critical temperature of -2 "C for m = 3 and 90 "C for m = 4. Decreasing the hydrophilicity of the headgroup moves the composition range of the liquid crystalline phases to higher surfactant concentrations (ca. 5% at 25 "C). Hydroxylation of the m = 3 surfactant (CH2CHOHCH2)dramatically changes the phase behavior with the loss of both the liquid-liquid miscibility gap and the stiff isotropic phase. The stiff isotropic and anisotropic phases have been tentatively assigned as cubic and hexagonal phases, respectively. The changes in phase behavior resulting from changes in the headgroup structure are discussed in terms of the hydrophilicity of the surfactant.
from 95% aqueous ethanol. Elemental analyses agreed within experimental error with those expected, and a surface tension concentration scan showed no minimum. 1-(N-n-Dodecyl-N,N-dimethylammonio)-4-butanesulfonate (DDBS). A 1%excess of 1,4butane sultone was added dropwise to a solution of NJJ-dimethyldodecylaminein ethylene dichloride with gentle continuous stirring. The reaction mixture was heated to a gentle reflux for 20 min and was then evaporated to dryness under vacuum. The crude product was recrystallized 3 times from absolute ethanol when the elemental analyses agreed with those expected, and the surface tension scan showed no minimum. 1-(N-n-Dodecyl-N,N-dimethylammonio)-2-ethanesulfonate (DDES). A 10-foldexcess of ethylene dibromide was stirred with N,N-dimethyldodecylaminefor 72 h at room temperature. The excess dibromide was then removed under vacuum and the resultant product sulfonated with sodium sulfite in water at equimolar ratio. The surfactant was recrystallized from water. Methods. Samples were prepared by using a computerized robotic system as describeds previously. The fluid micellar phases were readily mixed with a vibro-mixer, while the viscous liquid crystalline phases required mixing by use of a spatula or repeated centrifugation of the matrix through a constriction placed in the center of a sealed 7-mm glass tube. Phase boundaries were checked by means of optical microscopy using a Zeiss Universal R polarizing light microscope. Deuterium NMR spectra were obtained with a multinuclear Fourier transform spectrometer (JEOL FX9OQ) operating at a resonance frequently of 13.71 MHz.
Introduction The utility of surfactant phase behavior in the characterization of a wide range of chemical systems has been demonstrated. It has led to understanding of phenomena such as hydrophobic foam stability,' sustained drug release,2and emulsion ~ t a b i l i t y . ~Most studies have concentrated in systems based on either ionic or nonionic surfactants, and to our knowledge only a limited phase behavior study4 has been published in efforts to characterize zwitterionic sulfobetaine surfactant systems. These surfactants are of interest since they show many useful and unique properties including tolerance to hard water, extremes of pH, strong electrolytes, and oxidizing and reducing agents. They have utility, therefore, in extreme conditions where ionic surfactants would be unusable. Zwitterionic surfactants of the type alkyl carboxybetaine have achieved some commercial importance in shampoos, but the use of alkyl sulfobetaines has been more limited. This paper reports a study of such zwitterionic surfactant/water systems to determine the effect of changing charge separation of the headgroup upon phase behavior. Experimental Section Materials. N,N-Dimethyldodecylamine, 1,4-butane sultone, epichlorohydrin, sodium sulfite, sodium bisulfite, and sodium carbonate (all from Aldrich) were of highest available purity and used as received.
Results and Discussion
l-(N-n-Dodecyl-NJJ-dimethylammonio)-3-propanesulfonate
(DDPS) from Serva Ltd. was twice recrystallized from methanol-acetone (20930 v/v) mixture and dried under vacuum. Synthetic Procedures. 1-(N-n-Dodecyl-N,N-dimethylammonio)-2-hydroxy-3-propanesulfonate (DDHPS). Synthesis was by the reaction of N,N-dimethyldodecylamineand sodium l-chloro-2-hydroxy-3-propanesulfonate according to the method of Parris et aL5 The product was recrystallized 3 times (1) Friberg, S. E.; Blute, I.; Kunieda, H.; Stenius, P. Langmuir 1986, 2, 659.
(2) World Patent 84/02076, 1984. (3) Friberg, S. E.; Solans, C. Langmuir 1986, 2, 121. (4) Nilsson, P.-G.;Lindman, B.; Laughlin, R. G. J. Phys. Chem. 1984, 88, 6357. (5) Parris, N.; Weil, J. K.; Linfield, W. M. J.Am. Oil Chem. SOC.1976, 53.60.
0743-746318912405-0924$01.50/0
The conditions of temperature and concentration over which each type of phase exists may be indicated by a phase diagram. Figures 1-3 show the binary phase diagrams for DDPS/water, DDBS/water, and DDHPS/water, respectively. For DDPS and DDBS, as the surfactant concentration increases, the order of phase change is nmicellar stiff isotropic stiff anisotropic hydrated solid plus crystalline surfactant. The n-micellar region of DDPS is interrupted by a liquid-liquid miscibility region having an upper critical temperature of -2 "C. For DDBS, this gap is much larger with an upper critical temperature
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(6) Pesheck, C. V.; O'Neill, K. J.; Osborne, D. W. J. Colloid Interface Sci. 1987, 119, 289.
0 1989 American Chemical Society
Langmuir, Vol. 5, No. 4, 1989 925
Phase Studies of Zwitterionic Surfactants 100-
soHydrated
LL SOlld
Surfactant
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Ice
1 WATER
20
40
~
.:.
\\
60
CONCENTRATION
80
I OOPS
20
40
60
- - a' \ ice
Figure 1. Temperature-composition diagram for the binary system DDPS/water. The phases are denoted L,, fluid isotropic micellar; I stiff isotropic cubic liquid crystalline;E, stiff anisotropic hexagonal liquid crystalline. Ice formed below approximately -4 " C for the fluid phases upon cooling, while formation of ice was more difficult to detect in the viscous phases. Depression of the ice formation from 0 "C is likely an artifact due to hysteresis. The three-phase line at the upper limit of the cubic phase was not experimentally established.
WATER
0.
(%I
so
DDBS
CONCENTRATION (%I
Figure 2. Temperature-composition diagram for the binary system DDBS/water. The phases are denoted L,, fluid isotropic micellar; I, stiff isotropic cubic liquid crystalline;E, stiff anisotropic hexagonal liquid crystalline. Slight yellowing was noted for samples above 70 O C , and thus the diagram is considered tentative above this temperature (- - -). Depression of the ice formation from 0 to -4 O C was observed upon cooling and is likely due to hysteresis. The three-phase line at the upper limit of the cubic phase was not experimentally established. of greater than 80 "C. Above this temperature, distinct yellowing of the DDBS samples occurred, indicating possible decomposition of the surfactant. Thus, while the miscibility gap appeared to close between 80 and 100 "C, the chemical integrity of the samples was considered suspect, and the phase boundaries were considered only approximate in this range. The phase regions differ for DDHPS only in the absence of the stiff isotropic phase. The n-micellar phase extends up to 55% (w/w) surfactant, while a single stiff anisotropic region exists in the range 60-85% (w/w) of surfactant. Examination by polarized light microscopy of the stiff anisotropic liquid-crystal regions showed a texture indicative of a hexagonal phase. However, from our experience this texture is not unique to the normal hexagonal phase but can also be seen for lamellar phases, particularly prior to shearing between a glass slide and cover slip. A preliminary SAXS study was carried out in this region for 75-85% (w/w) DDPS in water. The samples gave only first-order reflections corresponding to an interlayer spacing of approximately 32 A for each of the samples. Absence of the higher order
WATER
\
I
\
25 50 CONCENTRATION
I
75
I DDHPS
(%)
Figure 3. Temperature-composition diagram for the binary system DDHPS/water. The phases are denoted L1,fluid isotropic micellar; E, stiff anisotropic hexagonal liquid crystalline. Depression of the ice formation from 0 to -4 "C was observed upon cooling and is likely due to hysteresis. Bragg reflections is more common for hexagonal phases than for lamellar liquid crystal^.^ A definite assignment of the phase from this data, however, was not possible. A 2H NMR study of the DDPS and DDHPS systems was carried out in this region (60-85% w/w) in an attempt to further clarify the structure. Quadrupolar splittings were obtained from the observed powder spectra of samples made with 2H20 in this composition range. Values in the range 150-550 Hz were found to be more consistent with those for hexagonal rather than lamellar phases. Further assignment as to whether the hexagonal phase is normal or reversed in structure is less clear. In the absence of a third component, surfactants of a single dodecyl chain length so far studied have not displayed inverse hexagonal phases. On the basis of these observations, we tentatively assign the stiff anisotropic regions to be a normal hexagonal phase. The stiff isotropic phase has been considered by Nilsson et al.4 to be a cubic liquid crystal for the DDPS/water system. On the basis of the physical appearance and properties of the stiff isotropic phases formed by DDBS and DDPS, the assignment of a cubic liquid crystal seems justified. As the temperature of the DDPS/water system (Figure 1) is raised, the fluid isotropic n-micellar region becomes of special interest. Previous studies by Nilsson et aL4v5 indicated that selected zwitterionic surfactants display a liquid-liquid miscibility gap, thus showing an upper critical temperature rather than a lower critical temperature as characteristic for nonionic surfactants. For DDPS, the miscibility gap (Figure 1) is near the ice crystallization temperature and is thus difficult to determine. Raising the temperature above the upper consolute temperature resulted in the formation of a clear isotropic solution up to the highest temperature (ca. 100 "C) studied. The most dramatic difference between DDPS and DDBS is found in this region of composition and temperature. There is a large increase in the liquid-liquid miscibility gap (Figure 2) which extends now to temperatures above 70 "C. Lengthening of the lipophilic chain in the headgroup by a single methylene unit significantly enlarges, therefore, the composition and temperature range of the miscibility gap for DDBS/water. (7) Fontell, K. In Liguid Crystals and Plastic Crystals; Gray, G. W., Winsor, P. A., Eds.; Ellis Horwood Ltd.: Chichester, 1974; Vol. 2, p 80.
926 Langmuir, Vol. 5 , No. 4, 1989
The DDHPS/water system (Figure 3) shows how placing the hydroxyl group on the propyl chain of the headgroup increases its hydrophilic nature. This results in the formation of a n-micellar solution at all the temperatures studied; i.e., no liquid-liquid miscibility gap was observed for this surfactant system. The Krafft boundary was not encountered for the systems studied except for DDES, which had a Krafft boundary at approximately 70 OC, in agreement with previous resultss for dilute compositions. The relatively high value for DDES presumably is a result of high crystal lattice energy arising from the ability of the headgroups to form a regular ionic array. Further characterization of the phase behavior of DDES was not pursued. The differences in the miscibility gap between DDPS and DDBS can be explained in terms of the degree of interaction between the charged groups allowed by the lipophilic chain of the headgroup. The propyl chain is not sufficiently long to allow the sulfonate to adopt a conformation which allows an increased degree of intramolecular interaction between the positively and negatively charged portions of the headgroup. Thus, for DDPS an essentially polar the relatively hydrophilic overall headgroup results, whereas for DDBS the additional length of the hydrophobic chain is sufficient to allow more complete interaction between the oppositely charged portions of the surfactant, giving a headgroup which is less hydrophilic. A similar description of change in the miscibility gap as a function of headgroup hydrophilicity is supportedg by observations in nonionic surfactants based on polyoxyethylene headgroups. As the intrinsic hydrophilicity of the polar functional group is decreased, the miscibility gap is enlarged.1w12 Note that the actual distance between the charged groups of a zwitterionic surfactant may be significantly different in solution than in the solid state. Increasing the number of bridge methylene groups, therefore, does not necessarily increase the distance between the charged groups. Laughlin'O concluded from thin-layer chromatographic and pK, studies of a series of compounds of the type H3N+(CH2),C02H(n = 1-5,lO) that the average degree of separation of the charged substituents in aqueous solution increases monotonically as the length of the bridging groups increases. Direct interactions between the charged centers "level off" at n = 3 or 4 and become essentially structure independent. These data combined with the virtually identical appearance of the ammonio(8) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. I 1983, 79, 975. (9) Laughlin, R. G. In Advances in Liquid Crystals;Brown, G. H., Ed.; Academic Press: New York, 1978; Vol. 3, p 42. (10) Laughlin, R. G. In Aduances in Liquid Crystals; Brown, G . H., Ed.; Acacemic Press: New York, 1978; Vol. 3, p 99. (11) Raphael, L. Proceedings of the 1st International Congress on Surface Activity, Paris, France, 1964; Vol. 1, p 36. (12) Elworthy, P. H.; McDonald, C. Kolloid-Z. 1964, 195, 16.
Faulkner et al. ethyl sulfate/water and ammoniopropyl sulfate/water phase diagrams'O led Laughlin to conclude that (1)the least hydrophilic zwitterionic headgroup is the system with n = 1 (2) insertion of the second methylene results in a large increase in hydrophilicity, while addition of a third methylene group results in a smaller increase, and (3) further lengthening of the bridge has little effect upon hydrophilicity. These conclusions are not consistent with the phase behavior (Figures 1and 2) for the ammonioalkanesulfonates. For DDES, the rigidity of the headgroups allows the formation of a regular ionic array in the solid state which results in a high Krafft boundary. Addition of a third methylene produces the most hydrophilic surfactant of the series, as indicated by the small miscibility gap in Figure 1. The hydrophilicity of the ammoniobutanesulfonate headgroup is decreased through interactions between the charged centers. A further increase of the bridge length, however, should show less distinct changes in headgroup hydrophilicity. The DDHPS/water phase diagram (Figure 3) does not follow the same pattern as shown by the other surfactants. The cubic phase is no longer present, the stiff anisotropic phase begins at a lower surfactant concentration, and no miscibility gap was observed. Such behavior can again be related to the increased hydrophilicity of the headgroup due to the presence of the hydroxyl group on the propyl bridge. Since cubic phases typically exist over narrow concentration ranges, it is not surprising that significantly increasing the headgroup hydrophilicity eliminates the cubic phase. The increased hydrophilicity resulting from the addition of a hydroxyl group to the propyl linkage may be similar, although greater in magnitude, to the increase caused by the ester linkage of the ammonioalkyl sulfates.l0 It may further be anticipated that changes of hydroxyl group position on either propyl or butyl bridges will have little effect on the phase behavior. It is also of interest to note that neither of the previously studiedlo ammonioalkyl sulfate/water diagrams exhibited a viscous isotropic phase region. These results indicate that the association behavior of this type of zwitterionic surfactant is closer to that of nonionic rather than ionic surfactants even though they carry formal charges. Considering that these solutions are nonconductive and possess no net charge, this is not surprising. This was also the conclusion from a light-scattering study13of aggregation in zwitterionic and nonionic surfactants in simple micellar solutions.
Acknowledgment. The review by Dr. Robert Laughlin of this manuscript was greatly appreciated. R8gistW NO.DDHPS, 13197-76-7; DDBS, 64463-49-6; DDES, 24020-67-5;N,N-dimethyldodecylamine,112-18-5; sodium 1chloro-2-hydroxy-3-propanesulfonate, 126-83-0;1,4butane sultone, 1633-83-6. (13) Herrman, K. W.
J. Colloid Interface Sci. 1966, 22, 352.
