1729
J. Phys. Chem. 1982, 86, 1729-1734
//;,/-----------...\ /,/'
0.021
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'\
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Figure 6. Mdar excess enthalpies HE,as a function of composition, at several pressures and 134.32 K, calculated for argon krypton llquid mixtures, uslng conformal solution theory (-, argon equation of state: ----, Lennarddones equatlon of state): k , 2 = 0.0109.
+
40 J mol-', with k12 = 0.109. It should be noted, however, that the calculations predict a fairly symmetric curve for HE,whereas the experimental results of Lewis et ala6show a markedly asymmetric curve of HE as a function of composition, with a maximum at xAr = 0.35 and HE= 47.3 J mol-'. This seems to be a common feature of several theoretical2and ComputeP studies of liquid Ar Kr, which led McDonalds to point out that 'the variation of GE and HE with composition is rather uninteresting, both functions are nearly symmetrical about x1 = x2 = 0.5". This behavior is not borne out by experiment. The asymmetry is, how-
+
ever, restored by using the argon equation of state for the reference fluid (maximum at x h = 0.37), although qualitatively the agreement is less good than that obtained with the Lennard-Jones equation of state. In Figure 6, we plot results of our calculation of HEas a function of composition and pressure, at a temperature of 134.32 K using both the Lennard-Jones and Gosman et al. equations of state with k12 = 0.0109. Both equations seem to lead to the same qualitative pattern, and although quantitatively the results are different, several conclusions can be drawn from this figure. (a) HE (x = 0.5) is negative at low pressures. Although no experimental results exist for comparison at this or similar temperatures, HEcan be estimated from a plot of GE/Tas a function of 1/T. Lewis et al.6give a plot of this type,which suggests that HE should become negative above 130 K. (b) The dependence of HEon composition seems to be much more interesting than has been suggested previously. The curves at lower pressures, in Figure 6, exhibit marked asymmetry with higher (less negative) values on the krypton-rich side, as in the experimental results of Lewis et al. However, small changes in pressure (and in the density of the mixtures) can result in dramatic changes in the composition dependence of HE. We may conclude by saying that, although the present work supplies part of the information on argon krypton liquid mixtures, which, following Lewis et a1.,6 was necessary for a complete thermodynamic knowledge of these mixtures, there still remains the need for calorimetric measurements of HEat high temperatures and over wide ranges of composition and pressure. Acknowledgment. This work was partly financed by research grants from NATO and Junta Nacional de Investigacgo Cientifica e TecnolBgica, and in part by a grant from the donor of the Petroleum Research Fund, administered by the American Chemical Society.
+
Temperature Dependence of Molecular Motlon in Smectic Liquid Crystals of Hydrated Sodium 4-( 1'-Heptylnonyl) benzenesulfonate Frank D. Blumt and Wllmer 0. Mlller" Department of Chemkby, Unhwsky of Minnesota, Minneapolis, Minnesota 55455 (Received: February 3, 1981; I n Flnal Form: October 2, 1981)
The surfactant sodium 4-(1'-heptylnony1)benzenesulfonate (SHBS), upon hydration to form a smectic liquid crystalline phase, exhibits a thermal transition of 1.62 f 0.22 kcal per mole of SHBS centered at -70 "C. From measurements of the temperature dependence of the carbon-13 NMR spectra and from the calorimetric studies, transition was identified as the freezing-in of motion of the aliphatic chains and can thus be labeled as the so-called gel-liquid crystal transition observed in phospholipids. The enthalpy and the entropy associated with the transition correspond to a change of approximately one trans-gauche rotation per hydrocarbon tail. A more intense, broad thermal transition with a heat of 15 f 3 cal per gram of smectic phase is observed from ca. -10 to -50 "C and shown to come primarily from the freezing-in of bilayer water. At ca. -23 "C water associated with the ionic groups, as well as the motion of the SHBS head, becomes frozen-in. Several comparisons between SHBS and similar studies on synthetic phosphatidylcholine are made. Introduction Carbon-13 nuclear magnetic resonance spectroscopy (NMR) and differential scanning calorimetry (DSC) have
been important tools in the study of biological membranes. Proton-decoupled, natural-abundance 13C NMR spectra have been reported for naturally occurring and model membrane Spectral line widths can be cor-
?Departmentof Chemistry, Drexel University, Philadelphia, PA
(1) E. Oldfield and D. Chapman, Biochem. Biophys. Res. Commun., 43,949 (1971).
19104. 0022-3654/82/2086-1729$01.25/0
0 1982 American Chemical Society
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The Journal of i%ysical Chemistry, Vol. 86, No. 9, 1982
related with thermal transitiondodetected by calorimetry. These and other studies, particularly lH? 2H,11and 31P NMR,12 have allowed insight into structure and motion in multilamellar bilayers and in unilamellar vesicles in phospholipid-water systems. The results of 13C NMR studies clearly identify a motional gradient along the surfactant molecule in a bilayer. In phospholipids fatty acid carbons show increased mobility with increasing distance from the glycerol moiety. The mobility of the large, complex head group is intermediate to the mobilities of the glycerol and fatty acid g r o ~ p s . ~ ~ ~ ~ ' J ~ In this study we report I3C NMR and DSC studies on the surfactant sodium 441'-heptylnony1)benzenesulfonate (SHBS)-water mixtures. SHBS mimics phospholipids in many ways, including the formation of multilamellar bilayers and, upon sonication, vesicles.14-16 This molecule is of interest because of the simplicity of its structure and because of ita stability and resistance to chemical and biochemical degradation. Unlike phosphatidylchlorineand related compounds, which have large head groups and ionic groups separated by several atoms, SHBS has a small hydrophilic head group and two aliphatic chains (tails) attached to an aromatic ring. One may anticipate that the chain dynamics in this system should vary more systematically than in phospholipids. Experimental Section Sodium 441'-heptylnonyl)benzenesulfonate, obtained from the University of Texas, was used as received and also purified by extraction with chloroform, isolated, and subsequently extracted with a 955 (v/v) solution of isobutyl alcohol and water. The results to be reported did not depend on the sample used. Deuterium oxide was 99.7 at. % D (Aldrich). Samples were prepared for NMR by pipetting (designated DA, for direct addition) or vapor sorbing (designated VS) D20 onto surfactant previously weighed into 12-mm NMR tubes. Vapor sorption was performed in a chamber partially evacuated to speed the upake of D20. Some samples (designated SH) were shaken for 30 min in a vortex mixer after preparation. The samples are referred to according to their method of preparation and their composition. Thus, a sample containing 76 wt % surfac(2)J. C. Metacalfe, N. J. M. Birdsall, J. Feeney, A. G. Lee, Y. K. Levine, and P. Partington, Nature (London), 233,199 (1971). (3)J. A. Hamilton, C. Talkowski, E. Williams, E. B. Avila, A. Allerhand. E.H. Cordes. and G. Cameio. Science. 180. 193 (19701. (4)K. M. Keough, E. Oldfield,b'. Chapman, ahd P. Beynon, Chem. Phys. Lipids, 10, 37 (1973). (6) M. P. M. Gent and J. H. Prestgard, Biochem. Biophys. - . Res. Commun., 68, 549 (1974). (6)R E.London. V. H. Kollman. and N. A. Matwivoff. Biochemistrv. '(7)B. Sears', J. Membr. Biol., 20, 59 (1975). (8) P. E.Godici and F. R. Landaberger, Biochemistry, 14,3927(1975). (9)I. W. LanceeHermkens and B. Dekuijff, Biochim. Biophys. .~ Acta, 470,141 (1977). (10)B. D. Ladbrooke and D. Chapman, Chem. Phys. 3, 304 - Lipids, . (1969). (11)For a recent review see H. Mantach, H. Saito, and I. Smith, B o g . N u l . Magn. Reson. Spectrosc., 11, 211 (1977). (12)D. M. Rice, M. D. Meadows, A. D. Scheinman, F. M. Goni, J. C. Gomez-Fernandee, M. A. Moecarello, D. Chapman, and E. Oldfields, Biochemistry, 18,5893(1979). (13)Y.K.Levine, N. J. M. Birdsall, A. G. Lee, and J. C. Metacalfe, Biochemistry, 11, 1416 (1972). (14)E. I. Frames, H. T. Davis, W. G. Miller, and L. E. Scriven, 'Chemistxy of Oil Recovery, R. T. Johaneen and R. L. Berg, Eds., American Chemical Society, Washington, DC, 1979. (15)E.I. Frames, J. E. Puig, Y. Talmon, W. G. Miller, L. E. Scriven, and H. T. Davis, J. Phys. Chem., 84, 1547 (1980). (16)E.I. Franees, Y. Talmon, L. E. Scriven, H. T. Davis, and W. G . Miller, J . Colloid Interface Sci., submitted.
Blum and Miller
9 -90
v-60
-30
0
30
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Flguro 1. Differential scanning thermograms at 10 "C min-' for SHBS-H,O at SHBS concentrations of 100 (A), VS75 (B), DA50 (C), or DA29 (D) at a sensitivity of 1 (A) or 2 (B-D) m a l s-'. See text for sample designations.
tant prepared by vapor sorption, followed by shaking, would be referred to as VS(SH)76. Samples for differential scanning calorimetry were prepared by weighing the surfactant into the sample pan, adding water either by pipet or by vapor sorption, and immediately crimping the cover onto the sample pan. The samples were weighed before and after calorimeter measurement to check for weight losses. Proton-decoupled, natural-abundance 13CNMR spectra were obtained on a Varian XL-100-15 using a frequency of 25.2 MHz and a proton-decoupling power of 9 W. Operating parameters, except where noted, were as follows: spectralwidth, 5120 Hz;acquisition time, 0.4s; data points, 4K (8Ktransform); pulse width (50 ps = goo), 15 ps; sensitivity enhancement, 0.4 s. In the CDC13spectrum a full 8K data points and a 0.8-5acquisition time were used. Chemical shifts were measured relative to internal Me,Si in CDC1, and in D20 to an external sodium 3-trimethylsilopropionate-d4(TSP)-D20 sample in a coaxial capillary. In most samples the chemical shifts were not measured because of the possible sample disruption by the capillary. Samples were not spun in the magnetic field except those whose spectra are shown in Figure 2. Where noted, the temperature was controlled by a Varian temperature controller and measured before and after a spectrum was taken. Where no temperature is noted, spectra were taken at ambient temperature (ca. 35-40 "C). Thermograms were obtained with a Perkin-Elmer DSC-2 differential scanning calorimeter using liquid nitrogen as a coolant. All thermograms presented were measured at a heating rate of 10 "C min-l. Results Thermograms for SHBS-H20 as a function of surfactant concentration are shown in Figure 1. The dry surfactant (Figure 1A) showed no thermal activity except for a small endothermic peak at ca.65 "C (not shown). Vapor sorption to equilibrium uptake of water results in the formation of a hydrated lamellar phase containing ca. 75 wt % surf a ~ t a n t ' ~ J ~and J ' in the appearance of a weak thermal
The Jownal of phvslcal Ch”tty,
Molecular Motion in Smectic Liquki Crystals of SHBS
transition with a maximum at ca. -70 OC, and a stronger, broad transition(s) with apparent maxima at ca. -39,-23, and -13 OC. The -39 OC maximum was not always present; instead, a plateau was observed which fell off rapidly in this temperature region. The sharp transition at 0 OC comes from a trace of extraneous water and did not appear in all samples. Direct addition of water resulted in thermograms similar to the equilibrium vapor-sorbed sample plus an additional component resulting from the melting of bulklike water. With samples containing 50 wt% or more water, the highest temperature maximum coming from the lamellar phase was obscured by the bulk water transition. The relative intensities of the various maxima varied from sample to sample even within the vapor-sorbed samples. It should be noted that, whereas the 75 wt % SHBS sample is a single phase at 25 OC, the 50% and 29% samples are biphasic, the smectic liquid crystalline phase (75f 5% SHBS)in equilibrium with surfactanbsaturated (ca. 0.06 wt % SHBS) water.14J6 The -70 OC transition scaled with the amount of surfactant, having an area of 4.0 f 0.55 cal per gram of surfactant or 1.62 f 0.22 kcal per mole of surfactant. The area under the broad lamellar-phase transition could only be assessed for ca.75% SHBS samples where no bulk water phase was present. Quantitation was made difficult by base-line uncertainties. In addition to instrumental base-line uncertainty at high sensitivity, the heat capacity of the sample above the transition was considerably greater than below the transition, suggesting that some, or perhaps all, of this transition was coming from melting of the water in the bilayer. The breadth of the transition coupled with the difference in heat capacities introduced much base-line uncertainty. The heat involved in the broad transition was estimated to be 15 f 3 Cai per gram of lamellar phase (SHBS H,O). Before presenting the temperature dependence of the NMR spectra, we will first look at the information on motion which can be deduced from ambient temperature data, though it has been discussed elsewhere in slightly different form.l4J6 In Figure 2A the proton-decoupled 18C NMR spectrum of a 20 wt % SHBS solution in CDC13is shown along with the resonance assignments. In chloroform the resonance lines were sharp, and moat are resolved, except for pairs of corresponding carbon atoms on each chain which had similar chemical shifts. The chemical shifts (in ppm) of SHBS resonances (assignments in parentheses) in CDC18relative to internal Me4Si are as follows: 14.16 (7”,9’),22.79 (6”,8’),27.78 (2”,3’),29-41(4”),29.61(69, 29.70 (59, 29.99 (3”,49,32.06 (7’),32.12 (5”),36.75(1”, 29, 46.09 (l’),126.78 (2,6),127.81 (3,5),140.88 (l), 149.8, (4). The assignments were made by comparison with the 13C spectrum of 4-(1’-propylnony1)benzenesulfonatein chloroform. The peak heights of aliphatic resonances due to atoms close to the aromatic ring are reduced and have a larger line width. This is due to efficient relaxation of these carbon atoms as a result of their relative position and motion in the surfactant molecule. Shown in Figure 2B is the spectrum of a 34 wt % SHBS-D20 dispersion @A34 at 25 “C. As stated earlier, samples in this composition range are biphasic. The amount of SHBS in the surfactant-saturated aqueous phase was too small to detect by NMR at this field strength. The spectrum is typical of a liquid crystalline dispersion of SHBS, where many resonances are very broad compared to a SHBS solution (Figure 2A). The resonances from atoms near the unattached end of the aliphatic chains were still intense and resolved, but broader
+
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(17)E.I. Framea, F.D. Blum,, K. R a e , P. S.Rueeo, €2. B. Bryant, and W. G. Miller, J. Colloid Znterjace Sci., to be submitted.
Vol. 86, No. 9, 1982 1731
SQ- Ne+
a
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100
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Flgurr 2. 13C-{‘H) NMR spectra of a solution of 20 wt % SHBS In CDCis (A) and of a liquid crystalline dispersion of 34 wt % SHBS in D,O (E). Assignment of resonances is shown also.
than when in solution. Resonances 1’-3’, 1”. and 2” were usually not resolved individually but often seen as shoulder on the more intense peaks or as a broad hump in the base line. In the aromatic region there was a broad hump corresponding primarily to resonances 2,6and 3,5. In the liquid crystal bilayers the motion of atoms close to the ionic head group was greatly reduced compared to that in the hydrocarbon tails. The motional gradient was apparent either from TIvalues18 or from line widths.’ The appearance of resonances from 2,3,5,6and 1’ as broad but observable suggested anisotropic motion of the benzene ring as well as that of the hydrocarbon chains about the 1’-4 bond. The inability to observe resonances from carbons 1 and 4 is related to the relaxation mechanism of non-proton-appended carbom in the ordered environment. Shown in Figure 3A is a 18C spectrum of a 87 wt ?% sample, VS87,which contained less water than the fully hydrated lamellar phase. The general features of this spectrum were the same as in Figure 2B,except that the line widths were much larger, and only three aliphatic resonances were resolved. Even though the phase diagram has not been determined completely, it is evident from Figure 3A that the sample was not in a two-phase region with fully hydrated liquid crystal in equilibrium with dry surfactant. If this were the case, we would expect to see a superposition of resolved resonances for 4‘-9’and 3“-7” and broad resonances from the dry surfactant. This was also verified by deuterium NMR studie~.’~ Sample VS72,prepared by equilibrium uptake of water (25 f 5% water, ref 13), yielded a spectrum, Figure 3B, where again resonances from 3”-5/’and 4’-7’ are resolved; in addition, a small peak from 1”,2’and a shoulder due to 2“,3’were observed. The additional water gave extra mobility to the aliphatic regions of the surfactant molecules. The effect of the method of sample preparation was (18)E. I. Frames and W. G. Miller, J. Colloid Interface Sci., to be submitted. (19) F. D. Blum and W. G. Miller, to be submitted.
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The Journal of Physical Chemistry, Vol. 86, No. 9, 1982 1
Blum and Miller A
A
34
-05 too PPM
150
b
50
Figure 4. '3C-{'H] NMR spectra of 28 W % SHBS In D,O (VS72) as a function of temperature. Temperatures and vertical expansions are as noted.
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Flgwe 3. %H {-) NMR spectra of #quid crystalline samples VS87 (A), VS72 (B), DA77 (C), and VS(SH)76 (D).
checked by pipetting D20 onto SHBS (sample DA76). The resulting spectrum,Figure 3C, showed the same qualitative features as VS72, Figure 3B. We believe that these samplea are identical and independent of the method of sample preparation provided that annealing procedures are followed." Spectra similar to those found for VS72 and of DA77 are also found for samples in the biphasic region, suggesting that the structure of the liquid crystal is the same across the biphasic region. The effect of shaking on a 76 w t % SHBS sample, VS(SH)76, is shown in Figure 3D. Before shaking, the spectrum was as in Figure 3C, but, after shaking, the resonances have broadened. Resonances 3",4",4'-6' and 5/',7/ are no longer resolved. From 2H NMR and other studies, it is known that mechanical shaking changes the domain size and structure of the smectic phase, while the bilayer spacing remains comt~nt.~'The broadening in the I3C spectra may be caused by a small distribution in chemical shifts or by decreased motion from disrupted lamellae. The same effect is also found for samples in the bisphasic region although the differencesare less dramatic. Inasmuch as the thermal transitions yield only a temperature, '3c NMR was employed to identify the particular groups in the surfactant which were responsible for the thermal activity. To this end two samples were studied, VS72 and DA34. The results are shown in Figure 4 for the VS72 sample. At 34 "C the spectrum of VS72 (Figure 4A) is as discussed previously. Upon cooling to -17 "C (Figure 4B), the aliphatic resonances broaden. Resonances from 3/',4/',4/-6' are no longer resolved from 5",7'. At -27 "C (Figure 4C) the aromatic resonanma are so broad that they are difficult to distinguish from noise; the aliphatic resonances broaden further but are easily observed. A t -45 "C (Figure 4D) the aliphatic region shows a single broad peak; at -85 "C it is indistinguishable from baseline noise (Figure 4B). The 34 w t % sample, not shown, responded similarly. It is apparent in comparing the temperature dependence of the I3C NMR spectra and the thermograms that much
-80
-30
TCC)
Figure 5. DSC thermogram for vapor-sorbed samples (VS75) using H20 (lower) or D20 (upper).
of the thermal activity is not directly associated with the surfactant motion. Therefore, it is probable that some of the transitions are associated with the water. To answer this question, we prepared samples for calorimetry using D20 instead of H20. A typical result is shown in Figure 5. The -70 "C transition is unchanged, but the entire multipeaked higher-temperature transitions are shifted ca. 4 "C, which is the difference in the melting points of bulk H2O and D2O.
Discussion The temperature dependence of the 13CNMR spectra can be compared to the thermal transitions. The -70 "C transition is correlated with the freezing-in of the aliphatic-chain motion in the lamellar phase. It can thus be classified as the gel-liquid crystalline transition temperature and is the lowest that has been reported for an aqueous lamellar system. Motion of the head-group benzene ring, which is highly anisotropic, becomes frozen-in on the NMR time scale between -17 and -27 "C. This does affect the motion of the aliphatic chains resulting in broader resonances. Although there is further slowing of the aliphatic chains from -27 to -45 "C, the cooperative
The Journal of Physical Chemistry, Vol. 86, No. 9, 1982
Molecular Motion in Smectic LiquM Crystals of SHBS I
I
1
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Flgue 8. Entropy per mole of even (0)or odd (A)linear hydrocarbonm (A),2' (O),** and SHBS (X) tails. and of synthetic phospholM (0),20 Inset shows melting points of Itnear hydrocarbons (0,A)and SHBS (X).
freezing-in of their motion, resulting in resonances too broad to be observable, occurs between -45 and -85 "C. Although it is tempting to suggest that the freezing-in of the head group is related to the DSC maximum at -23 "C, further consideration is necessary as much of the thermal activity between -10 and -50 "C was shown to correspond to the freezing of water. If one assumes that the entire broad transition is due to the water, a value of 60 f 12 cal per gram of water is obtained. Inasmuch as bulk water has a heat of fusion of 80 cal g-', and the heat associated with the melting of lamellar-phase water need not be the same, it is entirely plausible that the surfactant contributes only a small amount. There is little to compare with in the literature as calorimetric studies on phospholipidshave not yielded much information about lamellar water melting. It has been reported in hydrated dipalmityphosphotidychlorine that a cooperative melting transition of the water is not observed.20-22 Other studies on synthetic phospholipids are, however, very relevant. Shown in Figure 6 are data on the melting and entropy behavior of linear hydrocarbons, synthetic phospholipids, and SHBS. Unlike saturated-chain phospholipids, where the gel-liquid crystalline transition temperature is considerably higher than the melting point of the corresponding linear hydrocarbon,20the gel-liquid crystalline transition of SHBS is quite comparable. The entropy of melting the fatty acid chains in phospholipids is less than half that of the corresponding linear hydrocarbon. This can be easily rationalized on the basis of restricted motion resulting from being in an ordered phase as well as from covalent attachment to the glycerol moiety. Of greater significance is the ability to explain the entropy as well as the related heat of the gel-liquid crystalline transition in terms of trans-gauche rotations, with a small contribution from changes in van der Waals interaction energy due to volume expansion upon melting.23.24 This (20)D. Chapman, R.M. Williams, and B. D. Ladbrooke, Chem. Phys. Lipids, 1, 445 (1967). (21)H. Hinz and J. M. Sturtevant, J. Biol. Chem., 247,6071 (1972). (22)S.Mabrev and J. M. Sturtevant. R o c . Natl. Acad. Sci. U.S.A.. 73,3862 (1976). (23)J. F. Nagle and D. A. Wilkinson, Biophys. J., 23, 159 (1978). (24)J. F. Nagle, Annu. Rev. Phys. Chem., 31, 157 (1980).
1733
analysis assumes that the fatty acid chains are in the all-trans conformation below the gel-liquid crystalline transition and assumes an energy difference of 0.5 kcal between the trans and gauche conformations. However, we note that in SHBS an all-trans state below the gelliquid Crystalline transition temperature is unlikely as it results in an extended, bulky V shape. If one assumes that the entire enthalpy of the gel-liquid crystalline transition in SHBS is due to changes in trans-gauche population, 1.6 kcal corresponds to three trans-to-gauche rotations per SHBS molecule. This should be an upper limit.24 The corresponding calculated entropy change24is in excess of that observed experimentally. Subtraction of a small amount for the change in van der Waals interaction would bring the calculated enthalpy and entropy changes in line with the experimental values. Thus, an average change of only one gauche rotatmer per hydrocarbon chain at the gel-liquid crystalline transition would be completely compatible with and explain the experimental data. Temperature-dependent NMR on dipalmitoyllecithin shows a progressive broadening of the fatty acid resonances upon cooling until the gel-liquid crystalline temperature is reached, at which point there is a sudden broadening in both 13C and 2H spectra.26*26Our data show similar effects. It is thus to be expected that some contribution to the -10 to -50 "C thermal transitions may come from the aliphatic changes, for example, changes in rotamer population. If the change in trans-gauche population is noncooperative in this region, Le., controlled by a Boltzmann factor with a 0.5-kcal difference in energy, a contribution of 0.5 cal per gram of lamellar phase would be expected assuming rotations about 13 bonds. This should be an overestimate. Contribution from van der Waals interactions should be even smaller. Compared to the observed heat of 15 f 3 cal, there is a very small contribution at best from the aliphatic chains. However, it is very clear from the data in Figure 4 that the benzene ring motion becomes frozen-in between -17 and -27 "C. This may contribute to the thermal transition. Additional data, particularly 23Naas well as 2HNMR data,lg indicate that the sodium ion undergoes a discontinuous change in motion in this same temperature region. The picture that emerges is one in which the water associated with the ionic groups becomes frozen-in in the temperature range where the benzene ring motion becomes frozen-in. We thus feel that the DSC maximum at -23 "C is associated with the ionic groups. The maximum shifts when DzOis substituted for H20as it is associated with water mobility. The correlation in immobilization makes is difficult to separte thermal contributions from water and benzene ring motion. The entire -10 to -50 "C thermal activity is connected with water immobilization, with the water most bulklike freezing in at the highest temperature. Further comparisions with phospholipids are of interest. Phospholipids have not been studied with hydrocarbon tails of such length that the gel-liquid crystalline transition occurs below the melting temperature of the water. It is thus of interest that the gel-liquid crystalline transition in SHBS fits rather well into the pattern of chain-length dependence of hydrocarbon melting even though the water, as well as the surfactant head group, is totally frozen-in. When phospholipids are cooled through the gel-liquid crystalline transition, resulting in gauche-to-trans rotamer changes, the interlamellar spacing increases, the fatty acid spacing decreases, and the density increase^.^^^^^ Although (25)Y.K.Levine, J. M. Birdsall, A. G . Lee, and J. C. Metcalfe, Biochemistry, 11, 1416 (1972). (26)J. H. Davis,Biophys. J., 27,339 (1979).
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7"Journal of Physical Chemistry, Vol, 86, No. 9, 1982
Blum and Miller
the changes in SHBS are expected to be small because of the short hydrocarbon length and small change in transgauche rotamer population, it is not clear how and what changes would occur between frozen aqueous bilayers. In phospholipids I3C line widths are large in the bulk lamellar phase above the gel-liquid crystalline temperature. Sharp resonances from the fatty acid methylene carbons appear only after sonication of the sample, which converts the lamellar phase into vesicles.26 The interpretation of these differences is not entirely clear.n-2B By contrast the methylene resonances of SHBS at room tem-
perature in the bulk lamellar phase are sharp. Sonication to produce vesicles primarily affects the line width of the aromatic carbons.16 However, room temperature is 100 "C above the gel-liquid crystalline transition temperature of SHBS. If one compares line widths at similar ATs (5"Tgel-liq the line widths are comparable. In SHBS the line w ths in the bulk lamellar phase are affected by mechanical shaking of the sample. This was shown in Figure 3. The effect of mechanical shaking is due to domain size and structure, but not to vesicle formation."^^^ Comparable measurements on phospholipids do not appear to have been made.
(27)Y.K.Levine, B o g . Biophys. Mol. Biol., 24, 1 (1972). (28)A. G.Lee,Bog. Biophye. Mol. Biol., 29, 3 (1975). (29)J. M.Pope and B. A. Cornell, B o g . Surf. Membr. Sci., 12, 183 (1978). (30)F.D. Roeeini, K. S. Pitzer, R. L. Amett, R. M. Braun, and G. C. Pimental, 'Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds", Carnegie Press, Carnegie Institute of Technology, Pittaburg, 1953.
Acknowledgment. This work was supported by grants from the National Institutes of Health and the Department of Energy.
T+),
(31) F. D. Blum, Ph.D. Thesis, University of Minnesota, Minneapolis, MN, 1981.
