384
Langmuir 1989,5, 384-389
performed with DBP in a radioactively contaminated electrodynamic balance yielded similar results except that mass losses as large as 75% and charge losses as large as 63% occurred. The mean value of q-/qRL was 0.73 for these data, which is not statistically different from the previous result. It is likely that the electric fields used to suspend the droplets affect the stability phenomenon, that random thermal fluctuations in local surface charge density
can contribute only in a minor way to the instability, and that surface contamination can alter the fragmentation of the exploding droplet. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the National Science Foundation, Grant No. CBT-8611779, for their support of this research.
State of Water and Surfactant in Lyotropic Liquid Crystals Norbert0 Casillas,? Jorge E. Puig,? Roberto Olayo,J Timothy J. Hart,§ and Elias I. Frames* School of Chemical Engineering, Purdue Uniuersity, West Lafayette, Indiana 47907 Received July 1, 1988. I n Final Form: Nouember 29, 1988 Differential scanning calorimetry (DSC) and 'H NMR spectroscopy were used to study various lyotropic liquid crystalline phases of sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT or AOT) and water. At 25 "C the lamellar liquid crystalline phase contains between surfactant bilayers bulklike water ("water l"),which has a melting point Tlm= 0 "C and an enthalpy of melting AHlm= 80 cal/g. Close to the bilayers, the lamellar liquid crystalline phase contains interfacial water, "water 2", which melts at TZm= -4 f 2 "C and has AH2" = 35 f 8 cal/g of water. After most of water 1 and 2 freezes, the surfactant and the remaining liquid water undergo a phase transition to a nonlamellar liquid crystalline phase. In this phase, "water 3" melts at T3"' = -9 1 "C and has AH3m = 12 f 5 cal/g of water. When water is in ultrathin (510 A) films in lamellar liquid crystal, then water 1 is absent and water 2 can be supercooled to -45 "C or lower. The surfactant has substantial rotational mobility even when the water is frozen at temperatures from -35 to -10 "C.Similar phase and thermal behavior is observed when initially isotropic aqueous micellar solutions of AOT are frozen or melted. Introduction Combining long-range order with considerable local translational and rotational mobility, lyotropic liquid crystals find many applications in foam and emulsion stabilization1v2and enhanced oil r e c o ~ e r y . ~It?is ~ important, therefore, to understand and document their thermodynamic and molecular motional state. The phase behavior and the state of water a t room temperature in these systems are often probed by calorimetry or electron cryomicroscopy, which invariably involve freezing portions of the In this paper, we report DSC and NMR results on the AOT/water binary system, the phase behavior of which has been studied extensively.+l8 AOT is a pure doubletailed anionic surfactant which can form with water a t 25 "C a lamellar phase (GI, an inverse viscous-isotropic phase (V2), or an inverse hexagonal (M,) liquid crystalline phase."18 The single G phase forms from ca. 20 to 75 wt 9'0 AOT and is a t equilibrium with an isotropic micellar solution from 1.4 to 20 wt 70AOT.15 The water spacing in the G phase ranges from ca. 10 to over 100 A.13The V2 and the M2 phases form between 75 and 100 wt 70 AOT.11313-15In this paper it is shown that after freezing of the water the resulting phase microstructures differ from the original lamellar or isotropic microstructures. *Author t o whom all correspondence should be addressed ((317) 494-4078). Facultad d e Ciencias Quimicas, Universidad d e Guadalajara, Guadalajara, Jal., 44430 Mexico. Departamento de Fisica, Universidad Autonoma Metropolitana-Iztalapa, Apdo. Postal 55-534, Mexico City, 09340 Mexico. Now with Texaco USA, Bellaire, Texas 77401.
The thermal behavior a t four concentrations (15,30,40, and 70 wt 70)of AOT/D,O was studied by Czarniecki et a1.16using DSC and adiabatic calorimetry. They detected three endothermic transitions at T I = 276.3 K,T, = 271.33 K,and T3 = 268.40 K. The first transition (starting a t 3 "C), which is absent in the 70 wt 70sample, was attributed to melting of free (unbound) D20. The second transition was attributed to melting of "bound" D20. In their Abstract, Czarniecki et a1.16 report that the third transition (T3= 268.40 K)is due to melting of the surfactant chains. In their Results and Conclusions, they report, however, (1)Friberg, S. J . Colloid Interface Sci. 1971, 37, 291. (2) Fribere. S.: Larsson. K. In Adu. Lia. Crvstals.: Brown. G.. Ed.: Academic Press: New York. 1976: Vol. 2. D 175. (3) Natoli, J.; Benton, W.'J.; Miller, C.'A.; Fort, T., Jr. J . Dispersion Sci. Technol. 1986, 7, 215. (4) Oswald, A. A.; Huang, H.; Huang, J.; Valint, P., Jr. U.S. Patent 4,434,062, 1982. (5) Ter-Minassian-Saraga, L.; Madelmont, G. J . Colloid Interface Sci. 1982, 85, 375. (6) Kodama, M.; Seki, S. Prog. Colloid Polym. Sci. 1983, 68, 158. ( 7 ) Blum, F. D.; Miller, W. G. J . Phys. Chem. 1982, 86, 1729. (8) Zasadzinski, J. A. N.; Schneider, M. B. J. Phys. (Les Ulis, Fr.) 1987, 48, 2001.
(9) Rogers, J.; Winsor, P. A. Nature (London) 1967, 216, 477. (10) Gilchrist, C. A,; Rogers, J.; Steel, G.; Vaal, E. G.; Winsor, P. A. J . Colloid Interface Sci. 1967, 25, 409. (11)Rogers, J.; Winsor, P. A. J . Colloid Interface Sci. 1969,30, 247. (12) Park, D.; Rogers, J.;Toft, R. W.; Winsor, P. A. J . Colloid Interface Sci. 1970, 32, 81. (13) Fontell, K. J . Colloid Interface Sci. 1973, 44, 318. (14) Hart, T. J. M.S. Thesis, Purdue University, 1982. (15) Franses, E. I.; Hart, T. J. J. Colloid Interface Sci. 1983, 94, 1. (16) Czarniecki, K.; Jaich, A,; Janik, J. M.; Rachwalska, M.; Janik, J. A,; Krowczyk, J.; Otnes, K.; Volino, F.; Ramasseul, R. J . Colloid Interface Sci. 1983, 92, 358. (17) Callaghan, P. T.; Soderman, 0. J . Phys. Chem. 1983, 87, 1737. (18) Alexopoulos, A. H.; Puig, J. E.; Franses, E. I. J . Colloid Interface Sei. 1989, in press.
0743-746318912405-0384$01.50/0 0 1989 American Chemical Societv
Langmuir, Vol. 5, No. 2, 1989 385
Water and Surfactant i n Lyotropic Liquid Crystals
that it is due to melting of more strongly bound water and some melting of the aliphatic chains of the surfactant molecules. In this paper we present DSC and NMR evidence which not only demonstrates unambiguously that the third transition is due to water but also explains consistently the previous observations. Moreover, our data yield quantitatively the enthalpies of melting of "water 3" and "water 2", which show that the thermal behavior of water in ultrathin films or small clusters differs substantially from that of bulk water. This information should be of considerable interest in theories of interfacial By providing thin films of controllable and reproducible thickness in large quantities, lamellar lyotropic liquid crystals allow routine thermodynamic and NMR measurements of such films. Finally, our more extensive DSC results include some interesting and previously unreported observations on the supercooling and freezing behavior of water in nonbulk states. Such observations should be important in understanding the behavior of water in microporous materials, gels, certain foods, biological tissues, and other microstructured fluids at subzero temperatures.
0
-0 aJ
k
P
\
__
m P P
D
QJ
1 -30-20-100
t
T, "C
Experimental Section Materials a n d Sample Preparation. Sodium bis-(2-ethylhexy1)sulfosuccinate (Aerosol OT) with a purity greater than 98% was obtained from Fluka AG. It was dried in a vacuum oven for over 24 h a t room temperature and was used without further purification. Distilled water was drawn through a Millipore Milli-Q four-cartridge purification unit. Deuterium oxide from Aldrich was 99.8 atom TOD. Samples were prepared either by direct addition of water to dried AOT, and allowed to equilibrate for a t least 3 days, or by vapor sorption. In the latter method, dried surfactant was weighed in DSC sample pans and allowed to equilibrate with a large volume of NaCl aqueous solution of known water activity in a closed thermostated jar. All samples were used within 2 weeks from their preparation to avoid complications arising from surfactant hydrolysis, which becomes important after months of storage. Methods a n d Procedures. DSC. Thermograms were obtained with a Perkin Elmer DSC-4 calorimeter and an Intracooler I refrigeration unit or a DSC-1B model using liquid nitrogen. All thermograms were determined withheating and cooling rates of 10 "C/min, except where indicated. Sample pans for volatile specimens were used to minimize losses by evaporation. Samples were weighed before and after DSC runs. Results with samples which lost weight were discarded. Transition temperatures were determined (h0.5 "C) upon heating from the point where the base line changed slope significantly. The instruments were calibrated with indium, water, and n-octane standards. Samples were normally cooled to -40 "C or lower to ensure that they were completely frozen (see subsection on 'H NMR Spectroscopy Results). 'H NMR Spectroscopy. 'H NMR spectra were obtained with a 300-MHz Nicolet NMC 1180 spectrometer at the Department of Chemistry of the University of Minnesota. Spectra were obtained after a single 90" pulse under identical spectrometer conditions. The pulse-gradient-spin-echo (PGSE) method was used as described elsewhere?s29 For isotropic diffusion the decay of the echo intensity A is given by A = A. exp(-y2G2DP) (19) (20) (21) (22) (23) (24) (25) (26)
(1)
Hale, B. N.; Han, K. K. J. Phys. (Les U h ,Fr.) 1987, 48,681. Drost-Hansen, W. Znd. Eng. Chem. 1969, 61,10. Etzler, F. M.; Drost-Hansen, W. Croat. Chem. Acta 1983,56,563. Etzler, F. M.; Liles, T. L. Langmuir 1986, 2, 797. Stejskal, E. 0.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288. Stilbs, P. J. Colloid Interface Sci. 1982, 87, 385. Callaghan, P. T. Aust. J . Phys. 1984, 37, 359. Blum, F. D.; Padmanabhan, A. S.; Mohebbi, R. Langmuir 1985,
1. 127.
.ilbs, P. Prig. ?VGk-Spectroscopy 1987, 19, 1.
C
A
B -
A -30-20-10 0
L- I "10 10 20 30
T, "C F i g u r e 1. DSC thermograms for AOT/water a t 10 OC/min: (A-C) 20 mcal/s; (D) vertical scale expanded 10-fold (2 mcal/s). A and D are for micellar solution at 25 "C; at this temperature, B and C are for biphasic dispersions. where p = #(A - 6/3), 6 is the duration of the field gradient pulses, A is the time between gradient pulses (A was constant for each experiment), A , is the signal intensity for /3 = 0, y is the magnetogyric ratio, D is the diffusion coefficient, and G is the field gradient (15-25 G/cm). For benzene, which was used for calibration, a plot of In A vs p gave a straight line with correlation coefficients greater than 0.9996.18 The 5-mm 'H PGSE probe was made by Doty Scientific Inc., Columbia, SC. The sample temperature was controlled to h0.5 "C. Temperature calibration (hl "C) was done by using the frequency difference of the two 'H resonances of methanol containing a trace of HCl. When In A versus @wascurved because of anisotropic diffusion, which is well documented for liquid crystals of AOT,25eq 1 cannot yield the true diffusion coefficient, and a more sophisticated measurement technique and data analysis are needed.%@ In such cases, a rough value of the diffusion coefficient D was estimated from the initial slope of the In A vs plot.'*
Results and Discussion
DSC Results. Thermograms of 1,4, and 15 w t % AOT in water are shown in Figure 1. At a sensitivity of 10-50 mcal/s, the 1 wt % sample, which is a micellar solution exhibits one large peak starting at 0 "C, which at 25 0C,13,15 corresponds to melting of bulk water (Figure 1A). However, with greater sensitivity, a small thermal activity between -10 and 0 "C is detected (Figure 1D). No such peak was observed with pure water at any sensitivity. Both the 4 and 15 wt % samples, which are biphasic dispersions of aqueous solution and lamellar liquid crystal, show two small peaks between -10 and 0 "C and a large peak above
386 Langmuir, Vol. 5, N O .2, 1989
Casillas et al. T,"C IO 0 -10 -20-30'40
0 - 5 -10-15-20
-
-5 -15-25 -35
a
D
QJ
c
D \ 0 D
-
I
-30'20-10
0
0
-30-20-10 0
IO 20 30
T,"C
T,"C
x
QJ
-
30 20 IO 0
T, "C
I O 20
-30-20-100
T,"C
Figure 3. DSC thermograms for 40 wt % AOT in water: (A) cooling (25-40 "C), 10 "C/min, 20 mcal/s; (B) heating (-40 to 25 "C), 10 "C/min, 10 mcal/s; (C) heating (-30 to -1 "C), 10 "C/min, 5 mcal/s; (D) cooling (-1 to -30 "C after C), 5 "C/min, 5 mcal/s; (E) heating (-30 to -40 "C), 10 OC/min, 5 mcal/s; (F) cooling (-4 t o -30 to -4 "C, after E) 5 "C/min, 5 mcal/s. 1
1
1
i
~
30 20 IO 0 IO 20 30 T,"C
Figure 2. DSC thermograms for AOT/water at 10 "C/min and 20 mcal/s. (A) 20 wt % in H20; (B) 20 w t % in D,O; (C) 60 wt % in H20; (D) 60 w t % in DzO. Transition onsets and peaks shift by about 4 "C for all three transitions.
0 "C due to melting of bulk or bulklike water (Figure
lB,C). Between ca. 20 and 75 wt %, AOT forms a one-phase lamellar liquid crystalline phase.15 The 20 wt 70sample has three thermal transitions starting at -9, -5, and 0 "C (Figure 2A). No other transitions were observed between -150 and 60 "C. Since no separate water phase is present in this one-phase sample, one infers that the large peak at 0 "C is due to bulklike water located in thin films (- 100 A)13t30 between the bilayers of the lamellar liquid crystals. All three transitions were detected up to ca. 60 wt 7' AOT (Figure 2C). Moreover, the area of the bulklike water peak decreased relative to the other two as the amount of AOT increased and the aqueous film thickness decreased. To investigate the nature of the three thermal transitions, DSC measurements were done with AOT/D20 samples. Three transitions were detected at -5, -1, and 4 "C (Figure 2). All the peaks and the peak onset temperatures shifted by 4 "C with respect to those of AOT/ H 2 0 samples and agree with previous results16within our experimental uncertainty. Considering that the melting point of pure D20 is about 4 "C higher than that of pure H20,31the results suggest that the three thermal transitions are due to the water in the bilayers, with the 0 "C transition due to bulklike water (water 1) and the other two to interfacial or bound water (water 2 and water 3). For AOT/D20 liquid crystals, Czarniecki et a1.16 reported that transition 3 was due either to melting of surfactant chains or to more strongly bound water or to both. It is, however, unlikely that the short, branched octyl chains of the surfactant melt at such high temperatures (-5 "C). Because the melting points of n-octane and its isomers (30) Kamrath, R. F. Ph.D. Thesis, Purdue University, 1984. (31) Handbook of Chemistry and Physics, 61st ed.; Weast. R. C.. Ed.: CRC Press: Boca Raton, FL, 1980; pp B-100, D-175, F-51.
'
~
Table I. DSC Data for AOT/Water" wt % AOT AHm AlP P,"C F,OC 1 4 10 20 40 50 60 70
80 77 73 69 64 54 62 48
73 67 62 61 61 51 38
0 0 0 0 0 0 0 -6
-15 -16 -18 -18 -16 -22 -43 -60
Enthalpy values represent averages over several runs differing by
