J. Phys. Chem. 1984, 88, 674-680
674
ARTICLES Phase Transitions in Crystalline Models of Bilayers. 1. Differential Scanning Calorimetric and X-ray Studies of (C,,H,,NH,),MCI, and (C,,H,,NH,),MCI, Salts (M = Mn2+, CdZS,Cu2+) G. F. Needham, R. D. Willett,* Department of Chemistry, Washington State University. Pullman, Washington 991 64
and H. F. Franzen Department of Chemistry, Iowa State University, Ames, Iowa 50010 (Received: February 8, 1983: In Final Form: June 24, 1983)
Structural phase transitions have been studied in a series of (CnH2n+lNH3)2MC14 salts ( n = 12, 14; M = Mn, Cu, Cd) by DSC and powder X-ray diffraction techniques. These laminar materials contain bilayers sandwiched between metal halide layers. Thus, they mimic actual bilayers subject to the constraint of lateral rigidity. DSC and powder X-ray studies on highly purified materials reveal the general existence of two first-order phase transitions. The temperature and order of the phase transitions depend upon the length of the organic chain and on the nature of the metal ion. From AH and A S values, it is concluded that one of the phase transitions corresponds to the onset of "chain melting", e.g., diffusion of gauche bonds along the hydrocarbon chain. A pronounced increase of the interlayer spacing is associated with the onset of the chain melting. The nature of the second transition depends upon whether it occurs at a higher or lower temperature than the chain-melting transition, but in either case corresponds, at a minimum, to the onset of a twofold disorder of the chain orientation. Thermal hysteresis is observed in several of the salts.
1. Introduction The study of structural phase transitions in quasi-two-dimensional systems has been a subject of great interest recently. This interest grew out of the study of low-dimensional materials. Frequently the mathematical models used to describe three-dimensional systems are intractable but are solvable in lower dimensions. The study of materials of low dimensions has led to a greater understanding of the physics of magnetism,' conductivity: and other p h e n ~ m e n a . ~Because of their low-dimensional character and their biological significance, lipid membranes have been the object of much study. They have been shown to exhibit structural phase transitions at temperatures near ambient. These phase transitions have been studied by a variety of experimental techniques, including electron paramagnetic resonance spin lab e l i t ~ g ,fluorescent ~ probe parameter^,^,^ X-ray d i f f r a c t i ~ n , ~ , ~ differential scanning c a l ~ r i m e t r y and , ~ , ~laser ~ Raman spectros~ 0 p y . l ~In addition, several theoretical studies have been undertaken to predict the behavior of these lipid bilayer system^.'^,'^ Lipid bilayers have one important, experimental drawback in that they are inherently a liquid system. Thus, they are not amenable to study by the powerful techniques of solid-state physics. (1) de Jongh, L. J.; Meidima, R. Adv. Phys. 1974, 23, 1. (2) Barisic, S.;Bjelis, A.; Cooper, J. R.;Meontic, B., Eds. In 'Lecture Notes in Physics"; Springer-Verlag: West Berlin, 1979; Vols. 95 and 96. (3) Gamble, F. Ann. N.Y. Acad. Sci. 1978, 313, 86. (4) Hubbell, W. L.; McConnell, H. M. J. Am. Chem. SOC.1971, 93, 314. (5) Sklar, L. A,; Hudson, B. S.; Simoni, R. D. Proc. Narl. Acad. Sci. U.S.A. 1975, 7 2 , 1649. (6) frauble, H.; Eibl, H. Proc. Natl. Acad. Sci. U.S.A. 1974, 71, 214. (7) Ranck, J. L. J. Mol. Biol. 1974, 85, 249. (8) Engleman, D. M. J. Mol. Biol. 1970, 47, 115. (9) Ladbrwke, B. D.; Chapman, D. Chem. Phys. Lipids 1969, 3, 304. (IO) Hinz, H. J.; Sturtevant, J. M . J . Biol. Chem. 1972, 19, 6071. (1 1) Gaber, B. P.; Yager, P.; Petiocolas, W. L. Biophys. J. 1978, 24, 677 and references therein. (12) Jacobs, R.E.; Hudson, B. S.; Andersen, H. C. Proc. Natl. Acad. Sci. U.S.A. 1975, 72, 3993. (13) Nagle, J. F. J. Chem. Phys. 1973, 58, 252. (14) Needham, G. F.; Willett, R. D. J. Phys. Chem. 1981, 85, 3385.
0022-3654/84/2088-0674$01 S O / O
If a simple solid-state model of a lipid bilayer could be developed, the study of this system could help elucidate the phase transitions in the lipid bilayers and further the understanding of low-dimensional materials. When phospholipid bilayers are subjected to thermal stress, they undergo structural phase transitions. In dipalmitoylphosphaticylcholine (DPPC)two phase transitions are observed: a premelting transition at 35 'CIS and a main transition at about 41 OC. The nature of the premelting transition is not well understood. Two groups have recently discussed this transition as a slight reorientation of the ~ h a i n s . l ~ The * ' ~ main transition, on the other hand, has been studied very It has been assigned to the so-called "chain-melting" transition; "chain melting" is defined as the rapid diffusion of one or more gauche bonds up and down the hydrocarbon chain. At the chain-melting transition the chains decrease in length due to the gauche bond formation. This decrease is thought to be about 2O%.I5 In addition there is an accompanying increase in the area occupied by the head groups, estimated to be approximately 25%" The fractional increase in volume accompanying the chain-melting transition is less than 5%.15 With the intent of developing a two-dimensional system which might model the bilayer structure, compounds of the form (RNH&MX4 were synthesized. Here R is a long-chain hydrocarbon, M is a divalen transition metal, and X is a halogen. If the R group is sufficiently long, the RNH3+ cation will have lipid-like properties. These compounds provide the basis for developing systems which combine bilayer characteristics within a crystalline lattice. If the transition metal assumes an octahedral coordination, the metal halide portion will form well-characterized, two-dimensional, (15) Jacobs, R. E.; Hudson, B. S.; Andersen, H. C. Biochemistry 1977, 16, 4349. (16) Janiak, M. J.; Small, D. M.; Shipley, G. G. Biochemisfry 1976, 15,
4575 (1976). (17) Rand, R. P.; Chapman, D.; Larsson, K. Biophys. J. 1975, 15, 1117.
0 1984 American Chemical Society
Phase Transitions in Crystalline Models of Bilayers
The Journal of Physical Chemistry, Vol. 88, No. 4, 1984 615
F
Figure 1. Crystal structure of (CIoH2i)2MnC14 viewed as a bilayer (adapted from ref 18).
perovskite-type sheets of corner-sharing octahedra. When the RNH,+ groups are combined with the MX2- groups, the resulting crystal will have a laminar structure. If the R group is sufficiently long, the layers will be well separated and the material will have quasi-two-dimensional character. The metal ions are bridged by the equatorial chlorines. In addition there are axial chlorines protruding from the layer defining a box with a 5 X 5 A2cross-sectional area. The ammonium groups sit in these boxes, held in place by hydrogen bonding to the chloride ions. The perovskite layer is thus sheathed to two lipid-like layers. Stacking of these “sandwiches” creates a bilayer between each perovskite layer (Figure 1). It is important to note here that the rigidity of the perovskite layer will prohibit the bilayer from accurately mimicking an acutal lipid, since lateral expansion will not be possible. On the other hand, the 5 X 5 A2 metal halide repeat unit defines the crosssectional area available to each hydrocarbon chain. This provides ample room for the chain, even if gauche bonds are present.
2. Literature Review DTA and DSC studies on the (C,H2,+INH3)2MC14series (henceforth, CnM) have been undertaken by a number of different research groups, principally Vacatello and co-workers and Arend and c o - ~ o r k e r s . ’ These ~ ~ ~ groups have concentrated on the Mn2+ and Cu2+salts, with primary emphasis on chain length dependence and of developing a transition mechanism. Vacatello and Corradini have done some DSC work on the Mn2+salts with chain lengths ranging from C9 to C17.18,i9In all cases they observed one to two phase transitions for each salt and reported them to be second order. A 1975 paper by Landi and Vacatello” reported the second-order nature of the transitions in the Co2+salts in the series C9-Cl7. In a separate work they reported22the thermal behavior of the C12M and C16M salts using Mn2+,Cu2+,Co2+, (18) Vacatello, M.; Corradini, P. Gazz. Chim. Iral. 1973, 103, 1027. (19) Vacatello, M.; Corradini, P. Gazz. Chim. Ztal. 1974, 104, 773. (20) Landi, E.; Vacatello, M. Thermochim. Acta 1975, 12, 141. (21) Bocanegra, E. H.; Tello. M. J.; Arriandiaga, M. A,; Arend, H. Solid State Commun. 1975, 17, 1221. (22) Landi, E.; Vacatello, M. Thermochim. Acra 1975, 13, 441. (23) Salerno, V.; G r i m , A,; Vacatello, M. J. Phys. Chem. 1976,80, 2444. (24) Arriandiaga, M. A,; Tello, M. J.; Fernandez, J.; Arend, H.; Roos, J. Phys. Status Solidi A 1978, 48, 53.
TABLE I: Summary of Published DSC Data compd
T, K
c 1 0h.111 C10Mn C I 0Cd C 12Mn C 12Mn C12Mn C14Mn
308 308 308,312 318 3 24 318 339 35 1 345 364 348 34 2 364 308 313 330 337 3 20 344 357 342 354 360
C1621n C16Mn C1641n
c 1 ocu c 12Cu c12cu c14CtI C16Cu
&f. kcal/inol
8.34 8.07 8.94 (total) 9.42 9.80 9.32 12.2 2.13 14.3 2.70 16.5 14.3 2.6
AS,
cal/(mol K ) ref 27.0 26.6 28.8 29.6 30.3 29.6 35.9 5.98 41.6 7.4 1 47.3 41.6 7.4
18 21 25 18 23 22 18
18 23 22 24
10.0 2.39 10.0 11.95 2.63 8.6 1.79 3.3
29.9 4.8 31.3 33.5 5.98 24.9 5 .0 9.3
24 22 24 22
Fe2+,Hg2+,and Zn2+. All of these papers concluded that the main transition in these salts is a chain-melting transition, Le., rapid motion of gauche bonds up and down the chains. Any smaller transitions, if they were observed, were not investigated. Chemical analyses for carbon, hydrogen, and nitrogen were used as a purity check on the compounds. A 1979 paper by Kind and c o - w o r k e r ~gave ~ ~ a very comprehensive explanation of the thermal behavior of the ClOCd salt. They reported two phase transitions, a minor transition at 308 K preceding the main transition at 312 K. The total enthalpy was reported to be 8.9 kcal/mol. They postulated that the minor transition was associated with dynamic twofold rotational disordering of the chains and the major transition was assigned to (25) Kind, R.; Plesko, S.; Arend, H.; Blinc, R.; Zeks, B.; Selinger, J.; Lozar, B.; Slak, J.; Levstik, A,; Filipic, C.; Zagar, V.; Lahajnar, G.;Milia, F.; Chapius, G. J . Chem. Phys. 1979, 71, 2118.
Needham et al.
676 The Journal of Physical Chemistry, Vol. 88, No. 4, 1984
C(6) C(7)
C(8) C(91 C(I0)
La
Id
U
Figure 2. Electron density contours from the crystal structure of the high-temperature form of ClOCd (adapted from ref 25).
the “melting” of the alkyl chains. A summary of the DSC data published on compounds in the series under consideration is given in Table I. As can be seen there is some disagreement as to the correct number of transitions, their transition temperatures, and the associated thermodynamic data. We believe that these discrepancies are due to impurities in the starting amines and/or scanning too rapidly to fully resolve the transitions (see section 3). Therefore, this experiment must be repeated by using pure samples and carefully controlled scan rates. The crystallography of these systems has only recently been investigated since the soaplike quality of these salts makes the growth of the single crystals very difficult. However, several groups have completed single-crystal X-ray structures of some of these salts. Ciajolo et al. have determined the structure of ClOMn in the room-temperature phase?6 The monoclinic structure, space group P2]/a, is illustrated in Figure 1. They claimed that their conclusions could be extended to structures in the CnMn series with n > 10. The MnC142- ions form the “herring-bone” two-dimensional layer, with the MnZf ions in octahedral sites. The RNH,+ moieties were hydrogen bonded to the layer and the straight-chain hydrocarbons separated the layers. Each R group is in the all-trans configuration except for a gauche bond at either the C,-C2 or C2-C, bond where C, is the carbon atom bonded to the N H 3 group), causing the R group to be inclined at an angle of 40’ from the normal to the layer. In 1977 Ciajolo, Corradini, and Pavone determined the crystal structure of C12Zr1.~’ The salt belongs to the monoclinic space group PZ,/c. In this structure the ZnZf ions were in a tetrahedral configuration, in contrast to the ClOMn system, where the Mn2+ ions had octahedral coordination. In C12Zn the chains interdigitated, allowing the longer chain molecule to be grown. In ClZZn, the metal halide lattice defines a box with a cross-sectional area of 7 X 10 A2 in contrast to the 7 X 7 A2 box size in ClOMn. The larger cross-sectional area provided the room necessary for interdigitation. The chains are in the all-trans configuration. Kind et aLz5have completed a single-crystal X-ray structure on the ClOCd system in both the room-temperature and the high-temperature phases. In the low-temperature form the material is monoclinic, space group P2,/n. The R groups have the same configuration in the two halves of the bilayer. In this case the inversion centers are located in the middle of the bilayer, while in ClOMn the Mn2+ ions lie on the inversion centers. The high-temperature form of ClOCd is shown in Figure 2. In this phase the crystal is orthorhombic, space group Amaa, with two molecules in the primitive unit cell. In contrast to lipid ~~~
(26) Ciajolo, M. R.; Corradini, P.; Pavone. V. Gazz. Chim. I f d . 1976, 106, 807. (27) Ciajolo, M . R.; Corradini, P.; Pavone, V. Acfo Crystallogr., Sect. B 1977, 33, 553.
bilayers, the interlayer spacing increases at the chain-melting transition. The high-temperature structure proved that the chains were disordered and occupied two equivalent sites. Kind and co-workers therefore postulated that the minor transition was associated with the twofold flopping and the main transition was the chain-melting transition. In lieu of single-crystal structure work, powder diffraction can be used. Vacate110 and Corradini,’* in addition to their DSC studies, also performed some X-ray diffraction studies on the even-numbered series of the manganese compounds. They determined the interlayer distance in both the room- and hightemperature phases. In all cases, the interlayer spacing increased as the compound went through the phase transitions. The percentage increase, 9% for ClOMn, increased steadily as n increased, reaching a value of 14% for C16Mn. No attempt was made to record the X-ray spectrum at any intermediate temperature to determine the interlayer spacing in the intermediate phase. An important clue to the possible type of motion that must be considered was reported by Salerno, Grieco, and Vacatel10.~~ They studied, by X-ray diffraction, the phase transitions in C 16Mn, C12Mn, and mixtures of the two salts. Results reported for the C12Mn and C16Mn salts indicated that the salts contained impurities (see section 3). They postulated the existence of diffusional motion of the mixed hydrocarbon chains in the high-temperature phase based on experiments on C12Mn/C16Mn mixtures. In the low-temperature phase, the X-ray experiment indicated that two types of structures existed, one characteristic of C12Mn and the other characteristic of C16Mn. When the compound was heated into the high-temperature phase and cooled to rcom temperature, a new diffraction pattern was obtained. They said that two distinct sets of 001 reflections were observed in freshly prepared sample while the diffraction patterns of thermally treated products corresponded to a unique interlayer spacing. They explained this occurrence by postulating that the ammonium ions were diffusing within the layer in the high-temperature phase. These results should be independent of the purity of the starting materials. Included in the paper by Kind et al. describing the ClOCd is a phenomenological model which predicts the behavior of the compound as it undergoes a structural phase transition. They assumed that the hydrocarbon part of ClOCd could be represented as a liquid crystal. Then, from liquid crystal theory,28three order parameters could be defined. 8 is the average tilt of the organic chains with respect to the normal to the layer. Si is the average nematic ordering of the ith segments of the chain. Finally, p is the orientation of the NH3 group between two potential wells. For each order parameter, a Landau-type free-energy expansion can be performed. There is also an expansion due to the interaction of S with p and 8. From the solutions to the minimization of each expansion a phase sequence can be postulated. This phase sequence can accurately predict the behavior of the chains based on the relative order of the two phase transitions. While much of this previous data (except Kind et al.) on the CnMX systems must be questioned because of possible impurities in the starting materials (see next section), some general postulates can be outlined. At this transition, the space group changes, the chains undergo rapid motion, and the interlayer spacing increases. The minor transition is believed to be a flipping motion of the chains between two equivalent positions. At this point a more complete picture of the minor transition is lacking. From the previous work on these salts there are several questions that need to be answered. What is the correct number and transition order of the observed phase transitions? What is the correct motion in each phase? What is the nature of the minor transition? Does lateral diffusion exist and, if so, what is the rate? What is the role of chain length and the role of the metal halide lattice on the transitions? The molecules selected for this study were the C12M and C14M compounds using Mn2+,CdZ+,and Cu2+as the metal ions. These chain lengths were chosen for their proximity to the chain lengths ~
(28) deGennes, P. J. ‘The Physics of Liquid Crystals”;Clarendon Press. Oxford, 1974.
Phase Transitions in Crystalline Models of Bilayers
The Journal of Physical Chemistry, Vol. 88. No. 4, 1984 677
A
t
I
t
0
2 a
0
04
20
I
30
ii0
50
60
70
,
80
90
100
TEMPERRTURE ("CI
5w
I
Figure 3. Second moment data for C14Cd made from 90% (0) and 99% (A) pure amine showing the effect of purifying the starting materials.
occurring in naturally occurring lipids. The results obtained from this study should mimic the behavior of the lipid bilayers more closely than studies on the shorter chain systems. Since no single study had thoroughly examined all of the phases of these systems, a study was undertaken to provide a complete picture of the motions of the molecules in each phase. The intent of the paper is to describe the thermodynamic behavior of each compound. The basic purification and preparation of the molecules will be discussed. The DSC and temperaturedependent X-ray powder diffraction experiments will also be discussed in addition to any hysteresis effects observed in these salts. In the following paper results from the 'H N M R and laser Raman studies will be discussed, a phenomenological approach to phase transitions will be considered, and a comprehensivepicture of the motions will be developed.
D 40
50
70
60
TEMPERATURE
80
90
100
("C)
Figure 4. Effect of scanning rates on resolution: (A) 10.0, (B) 5.0, (C) 2.5, and (D) 1.0 OC/min.
TABLE 11: DSC Results for the Cnhl Salts rehcat
initial
3. Purification and Preparation
coiiipd
T,, K
M, kcal/ niol
The C i 2 H 2 5 N Hamine 2 (abbreviated simply, C12) was purchased from Eastman Organic and had a purity of 99%. Commercially available aliphatic amines of longer chain lengths are generally very impure. Sources quote the purities of the amines to be, at best, 95-97%, but they generally are in the purity range of 70-90%. Principal impurities appear to be of two types: amine N-oxides and other chain length amines. Analysis by GC indicates that the principal amine impurities in a sample of C,H2,+1NH2 are the CnaZamines in approximately equal amounts. Thus, chemical analysis cannot be used as a criteria for purity of the amine or their metal halide salts. The necessity for purification of the C14H29NH2 amine is clearly demonstrated in Figure 3, which shows the second moments of the 'H N M R spectra vs. temperature of C14Cd. At each phase transition, a decrease in second moments is anticipated due to the onset of thermal motion of the aliphatic chains. If the transitions are first order (using the Ehrenfest definition), a discontinuous change is anticipated. In a second-order phase transition, the second moments will decrease continuously to the value in the next phase. The squares show the data for the salt prepared from the 90% amine. The transition clearly exhibits behavior characteristic of a second-order transition. The data represented by the triangles are the data obtained when an amine of 99.7% is used to prepare the salt. The discontinuous nature of the transitions is clearly seen, implying tha the transitions are first order, not second order. It is also observed that the transition temperatures of the impure salt are 4-5 O C lower than those in the higher purity salt. This demonstrates the need for the utmost concern in establishing the purity of the amines. Purification Procedure. The amine N-oxides were removed by filtration of the molten amine. Purity of the resultant amine sample was checked by G C techniques. A Hewlett-Packard gas chromatograph with an Ultrabond R20 column was utilized. If the impurity peaks accounted for more than 3% of the sample, the amine was further purified by liquid-chromatographicmethods. Merck Silica Gel 60 (40-60 wm, 500 g) was used with a solvent of 12% tert-butylamine in hexane. Of the fractions collected, the
C14Mn
345 357 332 336 345 351
13.8 2.2 11.9 1.5 9.1 5.5
40.0 6.3 35.7 4.5 26.2 15.7
332 334 334 356 330 338
2.6 10.4 12.0 2.6 9.95 1.9
7.9 31.1 35.9 7.3 30.3 5.65
Cl2Mn C14Cd C12Cd c14cu C12Cu
AS,
cal/ (mol K ) T c , I
:.:.:.
5
:?::
:.... :.:.
0
IO
C14Cd
.... :y:i
....
.:.:):
c,,cu
5 -
50
'~l , 60
I1
,
I
I
70
80
90
TEMPERATURE
27.8 31.0 35.3 35.3 32.2 36.4 31.1 35.5
Reference 25.
-
0
fiOCda C12Cd C14Cd (reheat) CIOMnb C12Mn C14Mn c12cu c14cu
phase phase phase phase 7c 111, % IV, I, 70 11, A change A change A change A
("C)
Figure 5. Graphic representation of DSC results.
study were obtained at 1.0 OC/min. The summary of the transition enthalpies for the six compounds studied is displayed graphically in Figure 5 and numerically in Table 11. Note that in all cases, except for C12Cd, the main transition precedes the minor transition. In C12Cd the situation is reversed; the minor transition occurs prior to the main transition. The main transition in these systems is believed to be a chain-melting transition. If these systems model lipid bilayers, the enthalpy of the chain-melting transition in the lipid systems Using should be compared to the main transition in these ~a1ts.I~ Flory's value for the enthalpy of a gauche relative to a trans bond (500 cal/mol) we can predict that the minimum required enthalpy for C12M is 9.0 kcal/mol and for C14M is 11.0 cal/mol of compound. There are several values in the literature for the fusion of a normal paraffin chair^.^^.^^ These numbers have been used incorrectly by previous authors for a chain-melting transition. Kind and c o - w o r k e r ~have ~ ~ postulated the entropy for a chain oscillating between two equivalent positions to be R In 2 and R for the free rotation about two axes. From Figure 5 the effect of chain length on the transition temperatures is clearly seen. In the cadmium samples, for example, the large transition occurred first at the longer chain length, but in the shorter chain molecule the smaller transition precedes the major transition. The effect of the metal atom on the transition temperatures is also evident. In the C14M salts, the major transition is only slightly affected by the metal atom, while the minor transition is strongly influenced. Examining the data in Table 11, we see that the data for all compounds, except C14Cd, can be interpreted on this basis. In each case, AH for the main transition exceeds that for predicted for chain melting. Similarly, A S for the minor transition exceeds that for a simple twofold disorder. This implies that additional degrees of freedom, probably torsional in nature and/or involving (30) Sheppard, N.; Szasz, G. J. J . Chem. Phys. 1949, 17, 86. (31) Flory, P. J. "Statistical Mechanics of Chain Molecules";Wiley: New York, 1965. (32) Broadhurst, M. G. J . Res. Natl. Bur. Stand., Secr. A 1962,66,241. (33) Bondi, A. Chem. Rev. 1967,67,565.
5.8 8.4 2.3 3.5 ? 0.9 2.5 2.6 2.9
25.8 28.6 34.5 34.1 26.1 31.9 35.5 30.3
34.5
0 0 6.2 1.5
25.8 28.6 32.5 33.6
7.4 6.1 5.2 4.9
29.7 33.3 28.2 32.9
2.8
32.1
Reference 26
motion of the metal halide framework, are involved. Thus, all of these appear to be well-behaved. In the C14Cd salt, however, the division into two discrete processes is not possible. The value of AH = 9.1 kcal/mol for the major transition is less than that required for chain melting in a C14 salt. Similarly, the value of A S = 15.7 cal/(mol K) is clearly too large for the onset of a simple twofold disorder. Thus, there appears to be a coupling of the motions, with the chain melting apportioned between the two transitions. Both this salt and the C12Cd salt exhibit hysteresis, with reheated samples showing behavior different from that of virgin samples. This will be discussed in detail in a later section.
5. X-ray Powder Diffraction In this study only powdered samples of the six compounds under investigation were available for X-ray use. Since the low-temperature structures are m o n o c l i n i ~ ,we ~ ~were limited to determining the interlayer spacing of the structures in the different phases. A Gunier-Simon X-ray powder diffraction camera was used for collection of the data. The temperature was controlled with heated N 2 gas and an Enraf Nonius temperature controller. Samples were loaded into glass capillaries and a temperature range was selected so the compound would undergo all of the phase transitions observed in the DSC experiment. As the temperature was scanned, the film moved so that temperature vs. diffraction angle was recorded. About 40 h was required for a 30 " C temperature range. For each compound, the intense line at low angle (left-most line in each film) corresponds to the interlayer repeat distance (the 001 or 002 reflection, depending upon the space group). The next four lines are multiples of the interlayer spacings. The less intense lines at higher angle can be assigned to spacings corresponding to intralayer distances. These did not change significantlywith increasing temperature. Results are summarized in Table 111. For C14Mn, there was a large change (6.7%) in the d spacing corresponding to the major transition in the DSC experiment. A small change (2.5%) corresponded to the small transition in the DSC experiment. This is representative of the results for all salts in which the major transition occurs at the lower phase transition. A change in lattice parameter occurs only at the major transition in C12Cd. N o detectable change in lattice spacing occurred at the minor transition. The change in interlayer spacing (8.4%) was significantly larger than that observed for the major transition in the other salts. The increase was about the same as the totat increase for the other C12M salts. Discussion of X-ray Results. The calculated d spacings for each phase of all of the compounds studied are tabulated in Table 111. In contrast to the two transitions observed in DSC experiments, changes in the interlayer spacings were observed only at the major transition in the ClOCd and C12Cd salts. If the minor transition preceded the main transition, no change in d spacing was observed. If the minor transition followed the main transition, the interlayer distances increased at both transitions. This effect implies that a different type of motion takes place in the minor transition depending upon the relative order of the major and minor transitions.
Phase Transitions in Crystalline Models of Bilayers
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TEMPERATURE
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90
The Journal of Physical Chemistry, Vol. 88, No. 4, 1984 679
100
40
60
70
TEMPERATURE
(“C)
Figure 6. (A) Initial DSC heating scan for Cl2Cd and (B) reheating
50
80
90
100
(“C)
Figure 7. (A) Initial DSC heating scan for C14Cd and (B) reheating
scan.
scan.
The main transitions seen in the X-ray results for the C12M and C14M system can be unambiguously assigned to the chainmelting transition on the basis of the analogy to the ClOCd system and the DSC results. When the chains become perpendicular to the layer, the interlayer spacing is required to expand. Since the intralayer distances are essentially unchanged, this means that the volume increases. In the low-temperature phase, the chains tilt to occupy the entire space because their cross-sectional area is less than the 5 X 5 A* box provided by the metal halide lattice. In the high-temperature phases, the chains are perpendicular to the layer and thus effectively occupy the whole cross-sectional area available to them. When the minor transition occurs first, no change in interlayer spacing is observed, indicating that the tilt angle remains unchanged. This would be consistent with a twofold torsional disorder of the main (CH,),, backbone. When the major transition occurs first, a stepwise increase in interlayer spacing is observed, indicating that the chains do not become completely perpendicular to the layer at the chain-melting transition. These results are in contrast to the behavior of the lipid bilayers. In all phases, the chains are normal to the layer. When the lipid bilayer undergoes the chain-melting transition, the interlayer distances decrease about 2O%.l5 This decrease is due to the shorter chain length brought on by the gauche bond formation. Concomitant with this is an increase in the chain-chain distance, so that a net volume increase occurs.
that at least three separate processes were taking place.25 The initial heating showed two transitions, a major transition occurring at 345 K ( A H = 9.1 kcal/mol), followed by a minor transition ( A H = 5.5 kcal/mol) at 351 K. On the basis of these data, it seems that a process different from that in the other salts was occurring. This conclusion was based on the difference in energy distribution between the major and minor transitions. In the reheating scan, a third transition appeared. The major transition was broken into two smaller transitions and the minor transition was unchanged. The first transition ( A H = 5.7 kcal/mol) occurred at the same temperature as the major transition in the initial scan, and the new transition ( A H = 3.8 kcal/mol) occurred at 347 K. The sum of the enthalpies of the first two transitions was the enthalpy of the major transition in the initial heating scan. Thus, the major transition was broken into two smaller processes. This indicated that whatever was taking place in the major transition was composed of at least two processes. Three separate processes clearly occurred in this salt, no one of which corresponded directly to the process which occurred in the other salts. The additional transition is also seen in the X-ray diffraction experiment. Therefore, the motion required some additional increase in interlayer spacing. This motion could possibly be associated with the dynamic puckering of the layers. (Dynamic puckering of the layers means that the “washboard” structure of the metal halide layer becomes dynamic in higher temperature phases.) This would require some latent heat and a slight additional increase in interlayer spacing.
6. Hysteresis Effects As previously mentioned there were significant hysteresis effects in the two cadmium salts. However, the effects in each salt were of a different nature. In the C12Cd salt the minor transition appeared in the DSC experiment only during the initial heating. The DSC scans for the initial and subsequent scans are displayed in Figure 6. All scans after the first one were reproducible. Since A H and AS values for the main transition are the same for virgin and reheated samples, it indicates that disorder has been frozen into the lattice. No difference was observed in the X-ray study between virgin and reheated samples. This is consistent with the freezing in the dynamically torsional disorder postulated for the intermediate phase. The hysteresis in CI4Cd is much different from that in C12Cd. The initial and subsequent scans of the DSC experiment are displayed in Figure 7. From the reheating scans it can be seen
7. Discussion From the X-ray data we can conclude that there seem to be two different pathways to the motion in the high-temperature phase. These pathways are characterized by the relative order of the two phase transitions. If the main transition occurs first, then the minor transition is a twofold positional disorder between two equivalent positions. At the main transition the layers separate, allowing the chains to become nearly normal to the layer. At the minor transition, the chains become disordered and extend perpendicular to the layer. If the minor transition precedes the chain-melting transition, then this transition involves a gauche(+) to gauche(-) conformational change at the kink in the chain (Figure 1 ) leading to a twofold torsional disorder. The main transition again corresponds to chain melting and leads to the same final state as found for the other sequence of transitions. The
J . Phys. Chem. 1984,88, 680-682
680
TABLE IV: Summary of Data for C12M and C14 M Saltsa salt/phase c 1 2 c u I11 I1
I C12Mn I11
I1 I C12Cd I11 I1 I c 1 4 c u 111 I1
I C14Mn Ill I1
I C14Cd I11 I1 I a
z, d, A Cz
5 4-6 2.6 8-16 4.4 2.6 4 8.5 2.5 6 4-5 2.4 8-16 4.7 2.5 25 5.2 2.4
28.8 30.3 31.1 29.7 31.9 32.2 28.6 28.6 31.0 32.9 34.5 35.5 33.3 35.5 36.4 32.5 (32.7, 33.6) 34.5 (34.1) 35.3
AS,
kcal/mol
cal/(mol K )
10.0 1.9
30.3 5.65
11.9 1.5
36.0 4.5
2.6 (0) 10.4
7.8 (0) 31.1
12.0 2.6
35.9 7.3
13.8 2.2
40.0 6.2
9.1 (5.7, 3.8) 5.5 (5.9)
26.4 (16.5, 11.1) 15.7 (16.7)
Values in parentheses are reheated values.
thermodynamic and X-ray data are summarized in T a b l e IV. From t h e d a t a in Table IV it is obvious that some trends exist with regard t o the effect on t h e melting and flopping transition temperatures as the intralayer metal-metal distance is changed.
Klnetlcs of ",-NO
The lack of variation in t h e melting temperature for t h e C14M salts indicates that there is sufficient room in t h e "box" in t h e C14Mn salt. The extra room provided by the copper and cadmium ions is not needed. It is nearly constant for t h e Cuz+ and MnZ+ ions and then drops sharply if t h e metal-metal distance is further increased. This decrease indicates t h a t t h e flopping motion does need the extra room provided by t h e cadmium. In examining t h e effect of change length of t h e R group on transition temperature, we see that the flop transition temperature increases more rapidly with chain length than the melting transition temperature. This is reasonable, considering the nature of two processes. The melting transitions involve the formation of local disorder (gauche trans-gauche pairs) while t h e flop transitions involve a disorder of t h e whole chain. In summary, the (RNH,)2MC12series of salts is shown t o have m a n y physical properties closely related t o those found in liquid-phase bilayers. However, the crystalline nature of these model systems allows for the application of crystal engineering techniques t o systematically vary many of the factors affecting t h e relevent physical processes. In this paper, we have shown how melting and premelting transition temperatures can be systematically varied and interpreted these results in terms of various orderdisorder processes. In a future paper, we will examine how these affect t h e dynamical properties of t h e systems. Registry No. C14Mn, 76317-10-7; C12Mn, 75899-75-1; C14Cd, 53188-92-4; ClZCd, 79001-08-4; C14Cu, 88271-59-4; C12Cu, 7116311-6.
Reactlons on Vanadium Oxide Catalysts
Milton Farber* and Sigmund P. Harris Space Sciences, Inc., Monrovia, California 91 016 (Received: May 26, 1983)
A mass spectrometric study of the reaction of NH3 and NO on vanadium oxide catalysts in the temperature range 300-400 O C has been completed. The re. '+sshow a major reaction product, ",NO, with a minimum lifetime of 100 p. This is a primary step in the reaction mtchanism leading toward N2 and H 2 0 products. Mechanisms should include the adduct formation as an intermediate in the NO reduction reaction. No mass spectrometer evidence for the species N2H was seen although reaction intermediates NH2, N H , and OH were observed.
Although these research efforts have shown prominent reductions Introduction in t h e NO, concentrations, t h e kinetics and reaction mechanisms The universal desire to control combustion effluent pollutants, have been somewhat controversial. For the most part, t h e kinetics especially t h e nitrogen oxides, has accelerated research efforts in have not dealt with t h e possibility that carcinogens and other toxic a number of areas. In recent years t h e use of ammonia injection directly into t h e combustion gases at temperatures of 900-1000 (11) R. D. Matthews, J. A. Horwitz, and L. D. Savage, Fall Meeting, O C 1 or on catalyst beds at lower temperaturesz4 has led to nuStates Section, The Combustion Institute, Berkeley, CA, 1979. merous kinetic studies, both experimental a n d t h e o r e t i ~ a l . ' * ~ - ~ ~ Western (12) M. C. Branch, J. A. Miller, and R. J. Kee, Fall Meeting, Western (1) R. K. Lyon, Exxon Carp., US.Patent No. 3900554, 1975. (2) A. Miyamoto, K. Kobayashi, M. Inomata, and Y.Murakami, J . Phys. Chem., 86, 2945 (1982). (3) K. Otto, M. Shelef, and J. T. Kummer, J . Phys. Chem., 74, 2690 (1970). (4) K. Otto and M. Shelef, J . Phys. Chem., 76, 37 (1972). (5) R. K. Lyon and J. P. Longwell, EPRI NO, Seminar, San Francisco, CA, Feb 1976. (6) C. Castaldini, K. G. Salvesen, and H. B. Mason, "Technical Assessment of Thermal DeNO, Process", Report No. EPA-600/7-79-117, U S . Environmental Protection Agency, Triangle Park, NC, May 1979. (7) R. K. Lyon, fnf. J . Chem. Kinet., 8 , 315 (1976). (8) R. K. Lyon and D. Benn, Symp. (Int.) Combust., [Proc.],17, 1978, 601 (1979). (9) R. K. Lyon and A. R. Tenner, Paper No. 78-8.1.71st Annual Meeting of the Air Pollution Control Association, Houston, TX, June 1978. (10) R. K. Lyon, J. E. Hardy, and D. J. Benn, Fall Meeting, Western States Section, The Combustion Institute, Laguna Beach, CA, 1978.
0022-365418412088-0680$01.50/0
States Section, The Combustion Institute, Berkeley, CA, 1979. (13) M. Farber and A. J. Darnell, J . Chem. Phys., 22, 1261 (1954). (14) L. J. Drummond and S. W. Hiscock, Aust. J . Chem., 20,825 (1967). (15) H. Wise and M. F. Frech, J . Chem. Phys., 22, 1463 (1954). (16) B. B. Fogarty and H. G. Wolfhard, Nature (London), 168, 1112 (1951). (17) C. P. Fenimore and G. W. Jones, J . Phys. Chem., 65, 298 (1961). (18) P. G. R. Andrews and P. Gray, Combust. Flame, 8, 113 (1964). (19) G. Hancock et al., Chem. Phys. Lett., 33, 168 (1975). (20) L. Lesclaux et ai., Chem. Phys. Lett., 35, 493 (1975). (21) D. R. Poole and W. M. Graven, J . Am. Chem. SOC.,83,283 (1961). (22) L. J. Muzio and J. K. Arand, Final Report, Prepared for EPRI FP-253 (Research Project 461-1). Aug 1976, KVB, Inc., Tustin, CA. (23) J. Duxbury and N. H. Pratt, Symp. ( I n t . ) Combust., [Proc.],15, 1974, 843 (1975). (24) T. Takeyama and H. Miyama, Symp. (In?.) Combust., [Proc.],11, 1966, 845 (1976). (25) J . A. Silver and C. E. Kolb, J . Phys. Chem., 86, 3240 (1982). (26) L. J. Stief, W. D. Brobst, D. F. Nava, R. P. Borkowski, and J. V. Michael, J . Chem. Soc., Faraday Trans. 2, 78, 139 (1982).
0 1984 American Chemical Society
