J. Phys. Chem. 1083, 87, 4981-4991 ( A ) Difference Spectrum [ F i g . I ( C ) ]
( 6 ) Residual Spectrum
1330 I
1500
1000
x
(cm-') Flgure 2. Analysis of spectral data in Figure 1: (A) scale-expanded display of difference spectrum, Figure 1C in the frequency range of 900-1500 cm-'; (B) resldual spectrum (see text). Absorbance scale expanded by (X5) as compared with Figure 1. I/
where (ref), refers to the concentration of a reference compound at t min of W irradiation. Both C2H4and C3H, served as convenient reference compounds, since the relative decay rates of CH3HgCH3and these olefinic compounds were found to be comparable, i.e., within a factor of two. The photooxidation of mixtures containing CpH,ONO and NO at approximately 10 ppm each provided the HO radical source for these runs. The relative decay rates of the CH3HgCH3 and reference compounds were
4981
measured as a function of their mixing ratios. The extent of conversion of these reactants was kept low to minimize possible secondary reactions involving species such as 0 atom and 03.','These results are summarized in Table 11. Values of kC2H4/kDMM listed in the last column of this table give an average of 0.428 f 0.033 (a),and similarly, kC3Hs/kDMM = 1.413 f 0.114. An absolute value of k D M M = 1.9 (*13%) X lo-'' cm3 molecule-' s-l can be derived from these relative rate constants combined with the litand kCBHs = erature values of kC2HI = 8.48 ( i 5 % ) X 2.52 ( i 5 % ) X lo-" cm3 molecule-' s-l.12 On the thermochemical basis, this value appears to be more consistent with the displacement channel, reaction la, rather than the abstraction channel, reaction lb. Namely, the C-H bond energy of CH3HgCH3should be comparable to that of the CH, groups in C3H8.13 The H-atom abstraction from all the primary C-H bonds of C3H8by HO has an estimated rate constant of approximately 2 X lo-', cm3 molecule-' s-' at 300 K.14 If such a correlation between C-H bond energy and HO reactivity also holds for CH3HgCH3, the rate constant for reaction l b would be two orders of magnitude smaller than that observed experimentally. On the other hand, similarly to the reaction C1 + CH3HgCH3 CH3HgC1+ CH3 reported earlier,2reaction l a can be expected to be reasonably fast, since the major driving force for this reaction would be the longrange interaction between HO and Hg. Thus, both the kinetic and mechanistic results appear to be mutually consistent. Registry NO.HO, 3352-57-6;CH3HgCH3,593-74-8;CZHSONO, 109-95-5;NO, 10102-43-9; CHSONO, 624-91-9.
-
(12) R. Atkinson, S. M. Aschmann, A. M. Winer, and J. N. Pitts, Jr., Int. J. Chem. Kinet., 14, 507 (1982). (13) S. W. Benson, "Thermochemical Kinetics", Wiley, New York, 1968. (14) R. Atkinson, K. R. Darnall, A. C. Llyod, A. M. Winer, and J. N. Pitts, Jr., Adu. Photochem., 11, 375 (1979).
Lyotropic Liquid Crystalline Phases and Dispersions In Dilute Anionic Surfactant-Alcohol-Brine Systems. 1. Patterns of Phase Behavior W. J. Benton and C. A. Mlller' Depadment of Chemical Engineering, Rice Universitv. Houston, Texas 7725 1 (Receive& June 22, 1982; In Final Form: March 23, 1983)
The phase behavior of dilute aqueous saline solutions of several anionic surfactants was studied with short-chain alcohols present. Most of the work dealt with systems containing less than 10 wt % surfactant. Although complete phase diagrams were not developed, both temperature and composition were varied and some general patterns of phase behavior were found. The sequence of phases observed with increasing salinity in all systems was an isotropic aqueous solution, a single lamellar liquid crystalline phase, and an optically isotropic phase which scatters light and exhibits streaming birefringence. This last phase also persisted to higher temperatures than the lamellar phase. Various two-phase and three-phase regions were also found so that the overall phase behavior was rather complex. However, the relative oil solubility and water solubility of the surfactant-alcohol mixture present and the disruption of large, organized structures at high temperatures by thermal motion were clearly two major influences on phase behavior.
Introduction Aggregation or self-assembly of surfactant molecules in solvents takes various forms. Micellar solutions, lyotropic liquid crystals, and microemulsions are all phases whose internal structure is influenced by the aggregation prop0022-385418312087-4981$01.50/0
erties of surfactants. Such phases have received considerable theoretical and experimental attention in recent (1) G. J. T. Tiddy, Phys. Rep., 57, 1-46 (1980).
0 1983 American Chemical Society
4982
Benton and Miller
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
TABLE I: Pure Systems surfactant
mol wt
purity, %
source
cosurfactant
electrolyte
sodium 4-( 1-heptylnony1)benzenesulfonate(SHBS) sodium dodecvl sulfate f SDS) sodium octanoate (SO)
404.59 288.38 166.20
99+ 99.8 98.0
U. of Texas BDH K & K and BDH
n-propyl alcohol (NPA) n-hexyl alcohol (“A) n-decyl alcohol (NDA)
NaCl NaCl
~I
TABLE 11: Commercial Systems surfactant, chemical type monoethanolamine salt of predom (62%) dodecyl-o-xylenesulfonic acid monoethanolamine salt of predom (95%) nonyl-o-xylenesulfonic acid conventional sodium pet monosulfonate conventional sodium pet mono- and disulfonate conventional sodium pet mono- and disulfonste + alcohol ethoxy sulfate
av mol wt
active
-400
85
PDM-337
Exxon
tert-amyl alcohol (TA-4)
NaCl
360
84
PDM-484
Exxon
tert-amyl alcohol
NaCl
430
60
AMOCO
62
Witco
424
62
TRS 10-410
Witco
isopropyl alcohol ( IPA ) isobutyl alcohol ( IBA )
NaCl
424
Mahogany AA T R S 10-410
.,
%
trade design
437
59
Neodol 25-35
(2) R. G. Laughlin, Adu. Liq. Cryst., 3, 42-147 (1978). (3) K. Shinoda and S. Friberg, Adu. Colloid Interface Sci., 4, 281-300 (1975). (4) J. N. Israelachvili, D. T. Mitchell, and B. W. Ninham, Biochim. Rioohvs. 185-201 r ,-Acta. - - - - 470. ~ - --- - (1977). (5) C. L. Murphy, CanadianPatent 921 690, 1973. (6) R. N. Healy and R. L. Reed, SOC.Pet. Eng. J. Trans. AIME, 257, 491-501 (1974). (7) R. Hwan, W. J. Benton, J. Natoli, C. A. Miller, and T . Fort, Jr., presented at the ERDA Symposium on Enhanced Oil and Gas Recovery, Tulsa, OK, 1976, B-4/1-B-10. - 7
cosurfactant
electrolyte
NaCl NaCl
isobutyl alcohol
Most of the work on lyotropic liquid crystals has been on phases relatively rich in surfactant. Here we deal with the little-studied situation of liquid crystal formation at much lower surfactant concentrations. We report observations of a single lamellar liquid crystalline phase containing only a few percent of an anionic surfactant and a comparable amount of a short-chain alcohol, the remainder being sodium chloride brine. We also describe some general patterns of phase behavior in these systems produced by changing composition and temperature. The same patterns occur for both commercial and pure anionic surfactants. Changes such as increasing salinity, for instance, which reduce solubility of the surfactant in water, cause successive transitions from an isotropic aqueous solution to a dispersion of lamellar liquid crystal in the aqueous solution to a single lamellar phase to an optically isotropic phase which scatters light and exhibits streaming birefringence. With increasing temperature thermal motion disrupts the liquid crystalline structure with the result that the lamellar phase ultimately disappears. Out attention was drawn to these systems during our study of processes now under development in which surfactants are used to increase the recovery of petroleum from underground formations. In many of these processes the fluid injected into the formation is an aqueous saline “solution” of a petroleum sulfonate surfactant, often with short-chain alcohol as a cosurfactant. Proposed surfactant concentrations are typically 5 1 0 % by weight. Some of the early literature on these processes contained suggestions that liquid crystalline phases might be i n v o l ~ e d . ~ . ~ Our initial studies with the polarizing microscope of several ‘solutions” of this type supplied by industry showed that most of them did indeed contain liquid crystals.’ Among the objectives of our work, therefore was the determination both of the structure of the liquid crystal in the injected
I
source
Shell
NaCl
+ CaC1,
fluids and of the effect of the liquid crystal on oil displacement and recovery. An important aspect of this work has been the use of polarized light in both macroscopic and microscopic experiments. The latter, involving the polarizing microscope and, in recent years, Hoffman modulation contrast optics, are discussed in part 2 along with rheological measurements. Here we emphasize the macroscopic results obtained by using a polarized light screening (PLS) technique which is described below and which we have developed for rapid determination of phase behavior. We discuss the general trends of phase behavior seen in dilute anionic aqueous surfactant systems and give some explanations for the observations in terms of changes in the state of aggregation of the surfactant.
Materials Both pure and commercial surfactants have been investigated in our laboratory. The pure surfactants were sodium 4-(l-heptylnonyl)benzenesulfonate(SHBS or Texas l),sodium dodecyl sulfate, and sodium n-octanoate. The molecular weights, purities, and formulas are listed in Table I. However, results obtained with only the first of these materials are presented here as the others are to be discussed elsewhere. Sodium 4-(l-heptylnonyl)benzenesulfonate,an isomerically pure dialkylaryl sulfonate, was obtained from the University of Texas.8 This surfactant was further purified in our laboratory by liquid chromatography. First two solvent filtrations were conducted successively as 10 wt % solutions in chloroform and methanol. After each filtration the solvent was vacuum distilled in a rotovapor (Buchi, Switzerland) and dried in a vacuum oven for 48 h at 60 O C . Finally, a 10 wt % methanol solution was filtered through a 0 . 2 ~ mMillipore filter under pressure and then passed through a Sephadex LH-20 (Pharmacia Fine Chemicals, Sweden) chromatographic column. The collected fractions were dried first by rotovapor and then in a vacuum oven at 60 “C for 48 h. The final product, which was stored in a desiccator, was a white, slightly waxy solid. Purity was checked by elemental analysis and conductivity measurements. Details of the elemental analysis (Galbraith Analytical Laboratories) indicated a purity ex(8) P. H. Doe, M. El-Emary, and W. H. Wade, J.Am. Oil Chem. SOC., 54, 570-7 (1977).
Anionic Surfactant-Alcohol-Brine
Systems
ceeding 99% and have been reported el~ewhere.~ The commercial surfactants and formulas used in this study are listed in Table 11. They include both synthetic and refinery stream petroleum sulfonates. The latter generally have fairly broad molecular weight distributions and contain both monosulfonates and disulfonates. In column 3 of Table I1 the surfactant "activity" in percent is listed. This refers to the percentage of surfactants in the material as supplied. The impurities vary from system to system but consist mainly of unreacted oil, water, and salts. All the surfactants of Table I1 were used as received from the respective sources. The cosurfactants used in this study were all of reagent grade as were the electrolytes. They were used as received. The water was deionized and triple distilled, once in metal, then in glass, and finally from a potassium permanganate solution in glass. Solutions were mixed in 13-mm i.d. Teflon-capped glass test tubes. Stock solutions were prepared in glassware cleaned in chromic acid solutions and thoroughly rinsed with distilled water. Solution Formulation and Mixing Procedures As SHBS forms a dispersion of liquid crystalline particles when mixed directly with water, we first dissolved the solid surfactant in the alcohol cosurfactant to obtain a true solution. Then water or brine was added. Solutions of the commercial sulfonates were all prepared by the same procedure, as they are generally highly viscous materials. NaCl was always added to a solution as brine, never as a solid. Generally the procedure was to formulate 10-g or 10-mL samples of aqueous solutions. After addition of each component, the systems were heated to 60 "C and mixed thoroughly by vortex mixing and ultrasonic dispersion until homogeneous solutions were achieved. In some cases more than one phase was present at various stages of formulation. The final solutions were all heated at 60 "C for 2 h and mixed by vortex mixer, centrifuging, and ultrasonic dispersion. After 24 h the solutions were again mixed, without heating, and then allowed to equilibrate. Methods a n d Criteria for Determining Phase Equilibria The application of polarized light for observing the macroscopic phase behavior of surfactant solutions is not novel,'*13 but what we report here is development of a polarized light screening (PLS) system with improved optics for observing several solutions simultaneously under isothermal conditions. A transparent rectangular tank (50 x 18 x 30 cm) was constructed of Pyrex glass plates and placed inside a chamber. The light source consisted of a bank of fluorescent bulbs (12 X 125 mm, 20 W). Between the bulbs and the tank the light was first diffused (opal flashed glass) Polaroid and then passed through a polarizer ("-22. Corp.). An analyzer ("-22) with its optic axis rotated 90" with respect to the polarizer was placed in front of the tank. Flat- and round-bottom calibrated 15-mL test tubes of 13-mm i.d. and with Teflon insert screw-on caps were (9) C. A. Miller and T. Fort,Jr., Quaterly Technical Progress Report to US. D.O.E. (Contract-No. DE-AS19-79-BC10007) covering period March 1-May 31,1979. (10) P. A. Winsor, "Solvent-Propertiesof Amphiphilic Compounds", Butterworths, London, 1954. (11) R. G. Laughlin, J. Colloid Interface Sci., 55, 239-41 (1976). (12) J. C. Lang and R. D. Morgan, J. Chem. Phys., 73,5849-61 (1980). (13) A. S. C. Lawrence and J. T. Pearson in "Proceedingsof the 4th International Congress on Surface Active Substances",Vol. 11, J. Th. G. Overbreek, Ed., Gordon and Breach, London, 1967, pp 709-19.
The Journal of Physical Chemistry, Vol. 87, C R O S S - S E C T I O N OF PLS
No. 24, 1983 4983
11 O P T I C S
Bmk
d View
tight F i x f ? r e Support
Flgure 1. Schematic of polarized light screening (PLS) technique optics.
suspended and submerged within the tank and observed through the analyzer. The overall arrangement is shown schematically in Figure 1. With this simple optical system isotropy, anisotropy, and scattering of solutions under isothermal conditions can be easily differentiated. Interfaces are more easily observed than in ordinary light, as are the diffuse regions between phases associated with critical phenomena. Liquid crystalline phases appear as various birefringent textures. These textures have been correlated with textures from polarized microscopy which will be discussed in detail in part 2. The water (doubly distilled) in the tank was thermostatically controlled (3tO.1 "C) over a range from 5 to 85 "C. The water was pumped continuously (7.5 gal/min) and the level monitored continuously and held constant from a secondary reservoir. After solutions came to the desired temperature, they were either rotated back and forth by hand or vortex mixed for 15 s and then replaced to equilibrate. The desired temperature was approached in most cases by heating solutions from below and subsequently confirming the equilibrium state by cooling them from above. Phase equilibrium was said to have been achieved when no further change with time in solution appearance or phase volumes was apparent under constant experimental conditions. The time for equilibrium to be attained varied and was dependent on composition, temperature, viscosity, and structure. Samples below 40 "C were allowed a minimum of 5 weeks for equilibration. At higher temperatures equilibrium was reached more rapidly and shorter times were used. Even a t the highest temperature of 85 "C, however, at least 3 days was allowed for equilibration. In most cases the solutions were maintained at constant temperature for much longer than these minimum times. In some solutions where dispersions were present the polarizing microscope was used to determine when no further change occurred on a microscopic scale (see part 2). Reproducibility of solution formulation and phase behavior was achieved within experimental error for numerous compositions of both the pure and the commercial systems of Tables I and 11. Additional data were collected from samples held under isothermal conditions in a constant-temperature room over periods of up to 3 years. It is worth noting that at no time were solutions submitted to high g forces, such as those generated by an
4084
The Journal of Physical Chemistty, Vol. 87, No. 24, 1983
Benton and Mlller
1.5% SHBSI~O.O%NPAIX'/,N~CII T'C=35 I.+ I
olo
1lo
I
210
310
4.0
5 10
% NaCl
I1 = Isotropic micellar solution S = Dispersion of tpherulitas
L = lamellar phesa C = Cubic phase
Flgure 2. Sequence of structures observed wlth Increasing sallnky for 1.5% SHBS and 10% n-propyl alcohol at T = 35 O C .
ultracentrifugal field, in contrast to some other studies of phase b e h a ~ i 0 r . l ~A high gravitational field can produce nonuniform compositions within a phase and modify the conditions for phase transiti~n.'~J"''
Phase Behavior Results We have used the PLS technique described above to study various aspects of phase behavior of the pure and commercial surfactants listed in Tables I and 11. Virtually all work has been restricted to the water-rich portions of the phase diagrams with surfactant and alcohol concentrations below about 15 wt % and NaCl concentrations below about 5 wt %. Even with the pure surfactants we already have quaternary systems so that a very large effort would be required to obtain complete phase behavior information, and a three-dimensional form would be required to present it, viz., the portion of a tetrahedron near the vertex representing pure water. With the petroleum sulfonate surfactants which are complex mixtures and whose exact compositions are unknown, a complete and rigorous thermodynamic description of the phase behavior is out of the question. Accordingly, we do not present complete phase diagrams but concentrate on particular items of interest. One is the effect of salt on phase behavior, which is of considerable importance for enhanced oil recovery. As we shall see, a general pattern of phase behavior is found in this case. Next we consider the effects of surfactant and alcohol concentrations at constant salinity. With the assumption that sodium chloride partitions with the water so that the NaC1-to-H20ratio is the same in all phases, such systems are pseudoternary with components surfactant, alcohol, and brine. They can thus be represented in the usual types of two-dimensional diagrams. Finally, we present information on temperature effects. Although changing temperature introduces still another variable into these already complicated systems, knowledge of temperature effects is of some fundamental interest since increased thermal motion disrupts liquid crystal structure. Because many petroleum reservoirs are a t elevated temperatures study of temperature effects is also of practical interest. Salinity Effects. We begin with salt effects. Figure 2 shows the sequence of phases observed with increasing salinity in a system containing 1.5% by weight of the pure sulfonate surfactant SHBS and 10% n-propyl alcohol with temperature maintained constant at 35 OC. An isotropic solution (1') occurs a t low salinities. The state of aggregation of surfactant in this phase has not been (14)L.Mandell and P. Ekwall, Acta Poly. Scand., 74,1-116 (1968). (15) E. A. Guggenheim, "Thermodynamics", Wiley, New York, 1967. (16)S. Friberg, private communication. (17)R. Friman, I. Danielsson, and P. Stenius, J. Colloid Interface Sci., 86, 501-14 (1982).
Flgve 3. Photograph from PLS of 1.5% SHBS-10% n-propyl alcohol at T = 35 OC for 2.0, 2.2, 2.4, 2.6. 2.8, and 3.0 wt % NaCl showlng change In texture of solutions (negatlve print).
established. In the absence of salt and alcohol some workers suggest that micelles exist'* while others argue that they do n ~ t . ~In~any p ~case, ~ the solubility limit is very low under these conditions-about 5 X mol/L, as would be expected for a double-chained surfactant. With substantial amounts of alcohol present as in the system of Figure 2, surfactant solubility in the aqueous phase increases considerably. The shaded region of Figure 2 is a two-phase region where mixtures of I1 and the lamellar phase L exist. Within this region various textures are observed. Although we have seen the same textures in all the systems studied, so that they appear to be general, it is conceivable that mixing procedures quite different from ours would produce different particle size distributions leading to different textures. In the narrow unlabeled region between I1 and S an unstable dispersion of liquid crystalline particles in I1 is found in which the particles settle out. At salinities above about 1 wt % NaCl stable dispersions S are found. They exhibit a slight cloudiness when viewed in polarized light and exhibit regions of birefringence when disturbed which relax slowly to the original cloudiness. That they are indeed dispersions has been confirmed by optical microscopy,21p22but they exhibit no tendency to separate even after 1 year or more. Beyond about 1.3 wt % NaCl the S texture coexists with domains of a bright birefringent texture characteristic of the lamellar liquid crystalline phase L. In the S + L and L + S regions gross phase separation does not ordinarily occur when the samples are prepared in test tubes though an exception is discussed below. But separation on a smaller scale can be detected by microscopy (see ref 21 and 22 and part 2). Figure 3 is a negative photograph of several compositions near 2.5 wt % NaCl and illustrates the abrupt change in texture which characterizes the transition from S + L, where S predominates, to L + S, where L predominates. The photograph was taken after the solutions had been subjected to vortex mixing a t 35 "C and then allowed to (18)L.Magid, R. J. Shaver, E. Gulari, B. Bedwell, and S. Alkhaafaji, presented at the Symposium of Chemistry of Enhanced Oil Recovery, Division of Petroleum Chemistry, American Chemical Society, Atlanta, GA, March 1982,pp 93-109. (19)J. E. Puig, H. T. Davis, W. G. Miller, and L. E. Scriven, unpublished results, 1980. (20)W. J. Benton and D. F. Evans, unpublished results, 1980. (21)W. J. Benton, C. A. Miller, and T. Fort, Jr., presented a t the 53rd Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME, Houston, TX, Oct 1-3, 1978,paper S P E 1579. (22)C. A. Miller, S. Mukherjee, W. J. Benton, J. Natoli, S. Qutubuddin, and T. Fort, Jr. in "Interfacial Phenomena in Enhanced Oil Recovery," D. T. Wassan and A. Payatakes, Eds., American Institute of Chemical Engineers, New York, 1982,AIChE Symp. Ser. No. 212, pp 28-41.
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
Anionic Surfactant-Alcohol-Brine Systems
3-
4985
~%surf.(5-Y%TRS10-410:Y%Neodol25-3S)3%IBA X%NaCI. Aa.O T-22%
2-
v)
? VI
-n 0
0 0
z &? i
Figure 4. Photograph from PLS of 5.0 % TRS 10-410-3.0% isobutyl alcohol with lncreaslng salinky showing sequence of structure changes from 0.0 to 2.4 wt % NaCI. T = 30 O C .
equilibrate for several days at that temperature. Further observations and comments regarding this transition are made below. The pure lamellar phase L exists over only a relatively narrow salinity range. At higher salinities an optically isotropic phase C is seen which scatters light and exhibits streaming birefringence. Between L and C and beyond the salinity range of the pure C phase are two-phase regions in which gross phase separation is observed. The I1 phase of the I, + C region is the same I, phase as found at lower salinities. If the salt scan of Figure 2 is repeated for solutions with the same surfactant but higher n-propyl alcohol content, the same sequence of phases is found but the phase boundaries are shifted to higher salinites. Also the width (in percent NaC1) of the L and C phases increases. Figure 4 is a photograph of test tubes in the PLS apparatus for a system containing 5 wt 70 of the petroleum sulfonate TRS 10-410 with 3 wt % isobutyl alcohol as cosurfactant. The temperature is 30 OC and the salinity range is 0-2.4 wt % NaC1. The sequence of phases is basically the same as in Figure 2 except that at very low salinities a turbid, opalescent dispersion (T) is seen. The C phase, which appears over only a narrow range of temperatures in this system, is not shown in Figure 4, but we have in fact observed it in the expected salinity range. The S L to L S transition is not as striking as in Figure 3, where the test tubes were equilibrated in a water bath with better temperature control and where vortex mixing was employed. The same basic pattern of salinity effects on phase behavior has been observed for the other commercial surfactant systems listed in Table I1 and for the SDS-n-hexyl alcohol system of Table Figure 5 shows, for instance, phase behavior as a function of salinity in a system containing various ratios of the petroleum sulfonate TRS 10-410 and the ethoxy sulfate Neodol25-38 with the same isobutyl alcohol content as in Figure 4. Note that, when
+
+
~~
(23) W. J. Benton, J. Natoli, S. Qutubuddin, S. Mukherjee, C. A. Miller, and T. Fort, Jr., Soc. Pet. Eng. J., 53-60 (Feb 1982). (24) J. Natoli, Ph.D. Dissertation, Carnegie-Mellon University, Pittsburgh, PA, 1980. (25) G. B. Frankforter and F. C. Frary, J. Phys. Chern., 17, 402-73 (1913). (26) R. DeSantis, L. Marrelli, and P. N. Musceka, Chem. Eng. J. (Lausanne),11, 207-14 (1976). (27) E. I. Frances, J. E. Puig, Y. Talmon, W. G. Miller, L. E. Scriven, and H. T. Davis, J . Phys. Chern., 84, 1547-56 (1980). (28) W. J. Benton and C. A. Miller, 'Proceedings of the International Symposium on Surfactants in Solution, Lund, Sweden, June 1982", in press.
1-
2
4
6
i
io
wt. X NaCl
Figure 5. Effect on structure on the addition of Neodol 25-3s to the TRS 10-410-isobutyl alcohol system.
Flgure 6. Three faces of the tetrahedron at the H,O vertex for the system SHBS-n-propyl alcohol-H,0, and NaCI.
Neodol is added, the sequence of phases remains unchanged, but all the phase boundaries are shifted to higher salinities. Note also that the region I, + ppt of unstable liquid crystal dispersions becomes larger than the region S of stable dispersions with increasing Neodol content. As insufficient data points were available to draw exact boundaries of the narrow S + L, L + S, C + L, and C regions, they are indicated by dotted lines. All these textures were actually observed, however. Still another example of the general pattern is the ternary system Aerosol OT-H,O-NaCl. With increasing salinity one finds transformation from a lamellar phase to an optically isotropic phase, as first reported by F ~ n t e l l . ~ ~ We have repeated the experiments in our laboratory and confirmed the results. The optically isotropic phase is evidently the same as the C phase of Figures 2 and 5. No alcohol is required, however, to produce it in this system. We note that in this system and in the PDM-337-TAA system of Table I1 the C phase is less dense than the lamellar phase L, while the opposite was observed in all other systems. We conclude from the above results that the pattern that we have found is a general one for anionic surfactantalcohol-brine systems. Ternary and Pseudoternary Diagrams. In the absence of NaCl the SHBS-H,O-n-propyl alcohol system is a true (29) F. D. Blum and W. G. Miller, J. Phys. Chem., 86, 1729-34 (1982).
4986
The Journal of Physical Chemistry, Vol. 87,
Benton and Miller
No. 24, 1983
SHBS
H20
Figure 7. Pseudoternary diagram at 1.0 wt % NaCl In the system SHBS-n-propyl alcohol-H,O-NaCI. T = 22 OC.
IO
10
30
40
0
% NaCl
ternary. The central triangle of Figure 6 summarizes our observations. The other two triangles are included to provide further perspective on overall phase behavior since the three triangles are the faces of the tetrahedron representing the phase behavior of the SHBS-H,O-NaC1-npropyl alcohol system in the vicinity of the vertex corresponding to pure water. As indicated previously, surfactant solubility is very low in the absence of alcohol. As shown in Figure 6, adding alcohol produces a significant increase in surfactant solubility. The isotropic phase shown is the I, phase of the preceding section, and indeed the zero salt end of the salt scan of Figure 2 is indicated by the star in Figure 6. The two-phase region at the left side of the diagram indicates that partial miscibility of aqueous and alcohol phases occurs a t sufficiently high salinities and alcohol content^.^^^^^ At low surfactant concentrations Figure 6 indicates that the liquid crystal is present as a dispersion where the particles settle in the water. Such dispersions were first reported in this system by Frances et al.27 At higher surfactant concentrations no settling occurs and viscous, birefringent, gel-like solutions are seen. Their structure is currently being studied. When salt is present, the system may still be considered a pseudoternary at constant salinity. Figure 7 shows phase behavior of a system with pseudocomponents SHBS, npropyl alcohol, and 1wt % NaCl brine. Note that the I, region has shrunk from that of Figure 6. Also, within the shaded region of Figure 7 the dispersion of liquid crystalline particles is unstable only a t low alcohol concentrations while the stable dispersion S of Figure 2 now appears and is present over a significant portion of the diagram. The S L texture is also seen, but salinity is too low for the L and C phases of Figure 2 to exist. Addition of sufficient alcohol shifts the system into the I, region, the same as seen in Figure 6 with no salt. SHBS is a rather hydrophobic, double-chain surfactant, and n-propyl alcohol is quite water soluble. We have also studied the opposite situation with a water-soluble surfactant (SDS) and an oil-soluble alcohol (n-hexyl alcohol) in constant-salinity brine (see ref 23). In this case all the phases (except T) of Figures 2-5 are found. Adding alcohol L C, while shifts phase behavior in the direction I, adding SDS shifts it in the opposite direction. In contrast to the SHBS system of Figure 7 , the I,, L, and C phases extend more or less radially from the brine corner of the diagram and occur over relatively narrow ranges of alcohol-to-surfactant ratio. Similar behavior occurs in the extensively studied system sodium octanoate-n-decyl alcohol-water.14 Our recent
+
--
Figure 8. Phase diagram of temperature and NaCl for 1.5 % SHBS and 10% n-propyl alcohol.
studies have revealed a previously undetected C phase in this system. It contains over 90 wt % water and has an alcohol-to-surfadant ratio greater than that of the lamellar phase. Temperature Effects. An initial investigation of the influence of temperature on phase behavior was carried out in the SHBS system. Temperature was varied from 15 to 65 "C in 5 "C increments (2.5 "C increments in the higher temperature range) for 24 solutions containing 1.5% SHBS, 10% n-propyl alcohol, and various salinities between 0 and 5 wt % NaC1. Observations were made with the temperature raised at each step and again with the temperature lowered at each step to confirm that the phase behavior seen was indeed for equilibrium conditions. The solutions were also remixed a t each temperature. Figure 8 shows the results. Between temperatures of about 30 and 45 "C the phase behavior is basically that shown in Figure 2 for 35 OC although the salinity range over which the various liquid crystalline textures exist decreases significantly at higher temperatures. In particular, the I, phase extends to higher salinities a t higher temperatures owing to the increase in surfactant solubility with temperature. Above about 50 "C, however, the stable dispersions characteristic of the S and S + L regions separate into two phases, I, and L, with I1 having the greater density. The single-phase L region becomes very narrow, and single-phase and two-phase regions involving the C phase predominate above about 3% NaC1. It is known that alcohol solubility in brine decreases with increasing temperature in this s y ~ t e m As . ~a~result, ~ ~ ~an alcohol-rich isotropic phase 1, appears at high salinities for temperatures above about 60 "C. Because the total alcohol content of the system is relatively low, the I2 phase is always seen in equilibrium with other phases. Of particular interest is a three-phase region where the C phase coexists with I, and I2 simultaneously. We note that we have seen such a three-phase region and indeed some two-phase regions involving I2 at room temperature in both the SDSn-hexyl alcohol-brine systemz3 and the sodium octanoate-n-decyl alcohol-water system.28 In these systems the longer alcohol chain length makes mutual solubility of alcohol and brine relatively low at all salinities and temperature of interest. Near 60 "C the phase mapping is incomplete. More data points at more closely spaced temperatures and salinities would be required to map accurately the various regions with two or more phases. Above about 65 "C, however,
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
Anionic Surfactant-Alcohol-Brine Systems
I
,
.
.
.
.
0.2
0.4
0.0
0.8
10
,
,
.
.
.
.
J
1.2
1.4
18
1.8
2.0
22
2L
wl.% N i C l
Flgure 9. Phase diagram of temperature and NaCl for 5.0% TRS 10-410 and 3.0% isobutyl alcohol.
4987
T'C = 80
b
I
lo
I I
,
I
I
the liquid crystal disappears altogether and one sees simply I, at low salinities and I1 + Iz a t high salinities. Below the dashed line of Figure 8 a crystalline solid phase separates from the aqueous phase. Presumably, this behavior results from some transition in either the head group or aliphatic chain region of the surfactant bilayers. Blum and Miller2Bhave, by means of NMR and DSC studies, detected several such transitions at low temperatures in aqueous SHBS systems in the absence of salt and alcohol. The symbols at and a i in Figure 8 indicate that at salinities below 2.5 wt % NaCl the solid settles to the bottom of the test tube, whereas at salinities above 2.5 wt % it rises to the top. This salinity is the same as that where the abrupt change in texture was seen at higher temperatures (Figure 3 and vertical dotted line in Figure
I I
,
II 1I
Vf
05-
T
'1
; I
I I
,
I
I I I
I
,,
I
I
,
06
12
16
1 2 Yb N i C l
16
8).
Figure 9 shows the effects of temperature and salinity in a petroleum sulfonate system containing 5 wt % TRS 10-410 and 3 wt 9% isobutyl alcohol. The salinity was varied in 0.1 w t % increments up to 2.4 wt % NaC1. Here some 500 data points were obtained-enough to locate the various two-phase and three-phase regions at higher temperatures. Despite the surfactant being only 60% active and having a distribution of various surface-active components, the observed behavior is basically the same as that described above for the pure quaternary SHBS system. Below about 50 "C we see the same sequence of phases with increasing salinity as in the SHBS system. The shaded regions of S and S + L are again the two-phase region of I1 + L where no gross phase separation is seen with the exception discussed below. At high salinities and low temperatures (below the dotted line) a solid phase again separates. It is a yellowish, semicrystalline, oily mixture which rises to the top of the solutions, in contrast to the crystalline phase found in the SHBS system a t low temperatures. This difference in appearance is due to the impurities such as oil which are known to be present in the petroleum sulfonate. Below 1.5 wt % NaCl the solutions remain homogeneous even at the lowest temperatures studied (about 5 "C). Above about 50 "C no dispersion S is seen but a region of coexisting I, and L develops as in the SHBS system. The single L phase disappears while the single C phase expands. Above 62.5 "C the L phase melts and is no longer seen even in equilibrium with C or I,. The single C phase persists up to about 67.5 "C, however, and multiphase regions with C are seen up to 80 OC. Thus, the C phase,
0 6 11
Flgure 10. Volume fraction of phases increasing NaCi at T = (a) 85, (b) 80, and (c) 75 "C for 5.0% lRS 10-410 and 3.0% isobutyl alcohol.
which probably has bilayers of smaller lateral extent than the lamellar phase (L), is less easily melted than the L phase. As in the SHBS system, alcohol insolubility in brine is sufficient to produce an I2 phase only at elevated temperatures. The three-phase region I, + C + Iz is more extensive here than in Figure 8 for SHBS. Figures 10-12 are a series of plots showing changes in phase volumes with salinity at various temperatures. Further details of phase behavior are made clear by these plots. For example, in Figure 12b at 50 "C some separation of the dispersions in the S + L region was seen except at 1.5 wt % NaCl. It should be noted that only on rare occasions in the past 5 years in any systems studied have we seen such macroscopic separation in a test tube of dispersions in the S + L region. The viscosity in this system is relatively low as will be discussed in part 2. Perhaps the further reduction in viscosity caused by increased temperature and the reduction in natural con-
The Journal of Physical Chemistty, Vol. 87, No. 24, 1983
Benton and Miller
T C =70
T’C
~
T
T C = 3 5
T C=6
T
0 6
Figure 11. Volume fraction of phases with increasing NaCl at T = (a) 70, (b) 65, and (c) 60 OC for 5.0% TRS 10-410 and 3.0% isobutyl alcohol.
vection produced by improved temperature control allow the separation to take place. Near 65 “C and 1.4 wt % NaCl the I1phase scatters light and becomes similar in appearance to the C phase with which it coexists, as indicated by the shaded region of Figure llb. Perhaps the system is not far from a criticality condition between I1 and C, but further investigation is required to confirm this conjecture. The “turbid” region (T) was mentioned previously in connection with Figure 4. It occurs below about 0.1 w t % NaCl a t low temperatures but, as shown in Figure 9, its range extends up to about 0.4 w t % NaCl at 75 OC, the highest temperature for which data were recorded. Its structure is unknown, but neither birefringence nor any particles or drops were found upon examination by microscopy. Thus, the aggregates causing the turbidity are smaller than the wavelength of light. It is not known
c
L
c
1 2 wt % NICl
1 8
f
Figure 12. Volume fraction of phases with increasing NaCl at T = (a) 55, (b) 50, and (c) 35 O C for 5.0% TRS 10-410 and 3.0% Isobutyl alcohol.
whether the T region represents equilibrium conditions. Further study of these turbid solutions is clearly needed.
Discussion Figures 2 and 4 demonstrate the general pattern of phase behavior produced by changes in salinity. Several aspects of this behavior are of interest. In the first place we note that in the transition region between I1 and the stable dispersion, S, a narrow unstable region exists where the liquid crystalline particles settle. Addition of salt shifts the system into the S region and thus stabilizes the dispersion, just the opposite of what would be expected on the basis of the usual DLVO theory of colloid stability. Possible explanations for the observed behavior are that adding salt, first, reduces liquid crystal viscosity so that a given stirring procedure can produce smaller particles and, second, reduces the density difference between particles and brine. Since the surfactant is
Anionic Surfactant-Alcohol-Brine Systems
denser than water, the latter effect is due to the increase in brine density with salinity and the shift of the relatively low density alcohol from the brine to the surfactant bilayers. Both smaller particles and a lower density difference tend to make the dispersion more stable. If they outweigh the decrease in electrical repulsion between particles brought about by adding salt, the observed stabilization can be understood. As indicated previously, observations both above and below the solid-liquid crystal boundary in Figure 8 show an abrupt change in behavior at 2.5 wt % NaCl in the SHBS system. Together with the microscope observations to be discussed in part 2, these results hint that phase behavior may be more complex than so far indicated. That is, a transition may occur between two different liquid crystalline phases with different surfactant-to-alcohol ratios and hence different densities in passing from the S to the L textures. Further studies with other experimental techniques are needed to determine whether or not such a phase transition does exist. Our emphasis on the S and L regions and the possible transition between them is related to the use of surfactants for enhanced oil recovery. In order to assure uniform distribution of surfactant throughout the underground formation, the injected “solution” should not exhibit macroscopic phase separation. Thus, both single phases and stable dispersions are suitable, but unstable dispersions and two-phase mixtures such as I1 + C are not. Experience shows that, while the I1 phase meets this injection criterion, the surfactant is usually too hydrophilic under these conditions to produce the ultralow interfacial tensions required for oil displacement. The stable dispersions S, S + L, and L + S and the lamellar phase L, where the surfactant is somewhat more hydrophobic and where no gross phase separation occurs, therefore become very attractive for the injected solution. Apparently various workers in the industry have reached this conclusion empirically without having the overall view of phase behavior presented here. Much of the observed phase behavior can be understood in terms of changes in the relative oil and water solubility of the surfactant-alcohol aggregates. Adding salt in all cases makes the aggregates more oil soluble and produces transformations in the direction I1 L C. Adding the oil-soluble alcohols n-hexyl alcohol and n-decyl alcohol in the SDS and sodium octanoate systems causes similar behavior, while adding the water-soluble, anionic surfactants reverses the sequence of transformations. The reverse sequence is also seen when Neodol 25-38 with its hydrophilic ethylene oxide chain is added to the more oil-soluble surfactant TRS 10-410 (Figure 5 ) . As several previous workers have suggested, addition of salt reduces the effective repulsion between adjacent surfactant ions in surfactant aggregates such as micelles and allows the ionic groups to approach one another more closely. Transformation from micelles (which need not be spherical) to a lamellar liquid crystal is the result. Oilsoluble alcohol molecules occupy positions between the surfactant ions in the aggregates, thus allowing closer packing of the polar groups and again promoting formation of the bilayers of lamellar liquid crystals. For the SDS and sodium octanoate systems and probably for the commercial surfactants as well, the stable dispersion S and the unstable dispersion at slightly lower salinities consist of liquid crystalline particles in an aqueous micellar solution. The liquid crystal has a higher alcohol-to-surfactant ratio than the micelles. A simple mass balance shows that addition of alcohol produces an
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The Journal of Physical Chemlstry, Vol. 87, No. 24, 1983
4989
increase in the amount of liquid crystal and a decrease in the amount of micellar solution. Adding surfactant has the opposite effect. Adding salt, on the other hand, increases the amount of liquid crystal because, while the overall alcohol-to-surfactant ratio remains constant, its value in both liquid crystal and micelles decreases, in view of the effect on packing described above. The SHBS system is somewhat different. In the first place, Figure 7 shows that the various regions are not limited to narrow ranges of alcohol-to-surfactant ratios, as is the case in the SDS and sodium octanoate systems. Addition of the oil-soluble surfactant SHBS does produce the expected shift from I1 at very low concentrations to liquid crystalline dispersions at higher concentrations. Addition of the water-soluble alcohol ultimately leads to the I1 phase as expected, but it can also cause transformation from the unstable dispersion to the stable dispersion S to S + L, which is the reverse of the expected behavior. Note that at high alcohol concentrations the II phase can contain a substantial amount of surfactant. It has been reported, however, that no micelles are present. The results for SHBS can be understood in view of the very low surfactant content in the isotropic phase for this system at low alcohol concentrations where liquid crystal is present (see Figure 7). Thus, surfactant cannot transfer between aqueous solution and liquid crystal as previously but must stay in the liquid crystal. But alcohol can partition between brine and bilayers. Because surfactant is present in significant amounts only in the liquid crystal, adding surfactant must increase the proportion of liquid crystal present. This argument is consistent with Figure 7, which shows a shift from S to S + L with increasing surfactant content. Note that in the SDS-n-hexyl alcohol and the sodium octanoate-n-decyl alcohol systems mentioned above adding surfactant caused transformation to occur in the opposite direction. Adding n-propyl alcohol increases the alcohol concentration both in the brine and in the bilayers of the SHBS system. That is the surfactant-to-alcohol ratio decreases in the bilayers. As the propyl alcohol molecule is much shorter than the surfactant molecule, the result is a decrease in bilayer thickness. Now the thickness of the brine layers in the liquid crystal is, according to a few calculations that we have made, not very sensitive to small changes in bilayer thickness and surface charge density and, in any case, the effects on brine layer thickness of their respective decreases here would be in opposite directions. Hence, the major effect on the liquid crystal of adding alcohol is an increase in its brine content. The result is an increase in the volume fraction of liquid crystal in the system. Our observations of a shift from S to S + L, i.e., toward more concentrated dispersions, are consistent with such behavior. If propyl alcohol is replaced by a longer chain alcohol, less alcohol is required to bring about a given phase t r a n s i t i ~ n . ~The ~ ? ~reason ~ is apparently that a larger proportion of the longer chain alcohol partitions into the bilayers. In a similar manner, adding salt decreases solubility of the alcohol in the aqueous phase so that some alcohol transfers from brine to bilayers. By the same argument as just given, the result is an increase in the brine content of the liquid crystal and an increase in liquid crystal content, in agreement with our observations. (30) P. K. Kilpatrick, F. D. Blum, H. T. Davis, A. H. Falls, E. W. Kaler, W. G. Miller, J. E. Puig, L. E. Scriven, Y. Talmon, and N. A. Woodbury in “Microemulsions”, I. D. Robb, Ed., Plenum Press, New York, 1982.
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In all of the systems studied our observations-primarily the microscopy of part 2 3 h o w that a single lamellar phase L can exist for situations where the surfactant and alcohol content of the bilayers is 10% or less of the total phase. The brine layers must therefore be quite thick. Lamellar phases containing only a few percent surfactant and having a wide spacing have also been reported recently in phosphonic acid-water systems.31 Adding salt to the L phase evidently decreases the lateral extent of the bilayers to some degree with the result that the optically isotropic C phase is formed. It is not clear whether the disklike aggregates in the C phase are separate or are fused into a regular three-dimensional structure as has been proposed for cubic liquid crystalline phases in systems with lower water c ~ n t e n t . ~Further ~ ! ~ studies with other techniques are needed to establish the structure of this phase. The known cubic phases which most closely resemble the C phase-at least as concerns their position in the phase diagram-are those which have been reported in the potassium oleate-n-decyl alcohol-water system34and in the octylammonium chloride-n-decyl alcohol-water sysLike the C phase that we have seen in the sodium octanoate-decyl alcohol-water system,28they have higher alcohol-to-water ratios than the corresponding lamellar phases. On the other hand, they contain considerably less water than our C phase (about 65 wt % vs. 90 wt 70).They are also highly viscous, in contrast to the C phase which is quite fluid and thus likely to have less long-range structure. The low viscosity of the C phase allowed us to observe its streaming birefringence by viewing it in polarized light during agitation by a magnetic stirrer. This technique allows observations to be made a t different rates of agitation and as the system returns to rest after stirring ceases. When applicable, it seems preferable to inserting a glass rod, the technique used by Gilchrist et al.36 for viewing streaming birefringence in highly viscous cubic liquid crystalline phases. Increasing temperature seems to have an effect similar to that of increasing salinity since Figures 8 and 9 show that the L phase yields to the C phase as the temperature is raised. In this case, however, it is the increased thermal motion rather than the more hydrophobic nature of the surfactant which produces the L to C transition. With even further increases in temperature the C phase also disappears, perhaps due to “melting” or perhaps just because all the surfactant present can dissolve in the I, and I, phases. We note that the 1, phase in the SHBS system sometimes exhibits a bluish color due to scattering, suggesting that it contains some surfactant aggregates. Others have reported similar observations at lower temperature for longer chain alcohols.31 Near 62.5 “C the sequence of phases observed with increasing salinity in Figure 9 is rather complex. As the dashed line indicates, the following interesting sequence beginning and ending with I, + C occurs: I, C C C + I, C I, + C. Figure 13 helps clarify how this behavior is possible. For a quaternary system at constant
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+
-
-+
(31) G. Klose, A. G. Petrov, F. Volke, H. W. Meyer, G. ForRter, and D. Rettig, Mol. Cryst. Liq. Cryst., 88, 109-29 (1982). (32) G. Lindblom, K. Larsson, L. Johanssen, K. Fontell, and S. Forsen, J. Am. Chem. Soc., 101, 5465 (1979). (33) W. Helfrich in ‘Physics of Defects”, R. Balian, M. Kleman, and J.-P. Poirier, Eds., North-Holland Publishing Co., Amsterdam, 1981. (34) P. Ekwall, L. Mandell, and K. Fontell, J. Colloid Interface Sci., 31, 508-29 (1969). (35) P. Ekwall, Adu. Lip. Cryst., 1, 1 (1975). (36) C. A. Gilchrist, J. Rogers, G. Steel, E. G. Vaal, and P. A. Winsor, J. Colloid Interface Sci., 25, 409-20 (1967).
Benton and Miller
Figure 13. Schematic showing a three-phase region within its tetrahedral form (upper part of diagram) and the effect of curvature along one edge where the phases I , C I,, I, C, C and C I, meet (lower part of diagram) (see text).
+ +
+
+
temperature phase behavior can be represented by a tetrahedron. The three-phase region I1 C I, is a curved three-dimensional region which may be viewed as an infinite stack of triangles, as shown schematically in the upper diagram of Figure 13. It bounds three two-phase regions along its surfaces app’, and apy and three single-phase regions along its edges. The lower diagram of Figure 13 is an expanded view of a portion pllplll of the edge bounding the C phase. The wedge-shaped region between ap”c and d p ‘ e contains the C phase. The surfaces a/3”/3”’eand c@”fl‘”d separate C from I, + C and C + I,, respectively. The shaded area is a portion of one of the triangles of the I, + C 1, region. The salt scan represented by the dashed line in Figure 9 is the line 13-18 of Figure 13. It passes successively through the I, + C(13-141, C(14-151, C + 1,(15-16), C(16-17), and I, + C(17-18), in agreement with the observed results. Note that some twisting of the three-phase region is required to have this sequence of phases. We note that a similar argument can be used to justify the following sequence found in Figure 9 at slightly higher temperatures: I, C I, C 1, C I2 C I, + C. It also explains why the C phase meets the I, + C + I, region at two different points. These and other unusual features of Figure 9 are consequences of the fact that it shows the path of a single line through a tetrahedron (assuming that this surfactant can be considered a single pseudocomponent) at various temperatures. The positions of the various single and multiphase regions shift within the tetrahedron as temperature varies with the result that the line passes through different regions.
+ +
+
+
-
+ +
-+--
Summary and Conclusions Study of several systems containing an anionic surfactant, a short-chain alcohol, and water or brine has revealed some general patterns of phase behavior in the dilute region where surfactant concentration is less than about 15 wt %. Adding salt or making the surfactant-alcohol mixture more hydrophobic in some other way produces transformation from an isotropic, aqueous solution I, to a single lamellar phase L to an isotropic phase C which scatters light and exhibits streaming birefringence. Adding a water-soluble surfactant or otherwise making the surfactant-alcohol mixture more hydrophilic reverses the
J. Phys. Chem. 1983, 87,4991-4995
sequence. Increasing the temperature disrupts the lamellar structure so that it eventually “melts”. The C phase persists to somewhat higher temperatures, but at the highest temperatures we found only the I1 phase and, a t higher salinities where alcohol and brine are no longer completely miscible, an alcohol-rich Iz phase as well. Acknowledgment. Discussions with T. Fort, Jr., R. Griffiths, J. Natoli, E. Toor, and S. Qutubuddin were of
499 1
value. This work was supported by the U S . Department of Energy with supplementary grants from Gulf Research and Development Co., Amoco Production Co., and the Mobil Foundation. Registry No. SHBS, 67267-95-2;SDS, 151-21-3;SO, 1984-06-1; PDM-337, 69494-75-3; PDM-484, 55121-84-1; TRS 10-410, 63194-35-4; NPA, 71-23-8; NHA, 111-27-3;NDA, 112-30-1;TAA, 75-85-4; IPA, 67-63-0;IBA, 78-83-1;Mahogany AA, 80209-51-4; Neodol 25-35, 12688-28-7.
Thermotropic Ionic Liquid Crystals. 3. Sodium-23 Nuclear Magnetic Resonance Study of the Ionic Mesophase of Sodium n-Alkyl Carboxylates J. E. Bonekamp, I. Artakl, M. L. Phillips, S. Plesko, and J. Jonas’ Department of Chemistry. School of Chemical Sciences, Universlv of Illlnols, Urbana, Illinois 6 180 1 (Received: JuV 6, 1982; In Final Form: April 11, 1983)
The 23Naspin-lattice relaxation times, T I ,are reported for molten sodium n-alkyl carboxylates including n-valerate, n-hexanote, and n-heptanoate. The temperature range studied covers both the liquid crystalline smectic mesophase and the isotropic melt. The Fourier transform 23NaNMR spectra of the mesophase show the presence of the second-order quadrupole shift in the 23Nacentral transition in spite of the rapid Na+ and ~ 1). This second-order quadrupole shift was observed earlier for sodium isovalerate carboxylate motion ( w , , ~
