J. Phys. Chem. 1981, 85, 3385-3387
of the intriguing prospects we are now exploring is to synthesize larger molecules with infrared chromopfiores (such as CF3 groups) at one end which will permit rapid photon absorption at that point and permit study of internal energy migration to another end where a weak bond exists, in competition with collisional deactivation. Studies of such processes become possible with intense laser beams thus opening up entirely new fields of research. Hopefully, we may begin to explore, quantitatively, questions such as the rates of randomization of energy in large chain molecules and the general question of collisional energy
3385
transfer between large molecules excited to high energies. In all cases the use of isotopic labeling permits the assignment of an energy and time scale to the observable branching ratios.
Acknowledgment. The author acknowledges the longterm and current support through a series of grants by the National Science Foundation, CHE-78-26623; the U S . Army Research Office, DAAG29-79-G-0022;and the Air Force Office of Scientific Research, AFOSR-77-3279,which have made possible most of the work described here.
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ARTICLES Phase Transitions in Crystalline Models of Lipid Bilayers Gregory F. Needham and Roger D. Wlllett" Department of Chemistry, Washington State University, Pullman, Washington 99 164 (Received: May 14, 198 1)
A study is reported on the structural phase transitions in two salts, (C12HwNH3)&dC14 and (C12H25NH3)2MnC14, which contain lipidlike bilayers imbedded in a crystalline matrix. Each salt undergoes two first-order transitions involving the onset of dynamic motion of the organic cations, a major transition (AH= 12 kcal/mol) occurring first in Mn salt, but second in the Cd salt. DSC, X-ray, NMR, and Raman studies have shown that this major transition corresponds to chain melting, and the minor transition (-2 kcal/mol) to a 2-fold torsional disorder in the Cd salt, where it precedes the major transition. In the Mn salt, where it follows the major transition, it corresponds to a 2-fold orientational disorder.
Introduction Salts of the type (RNH3)2MX4crystallize in a laminar-type structure in which each metal halide layer is sandwiched between a pair of layers of the organic ammonium ions.' These salts are excellent systems for the study of magnetic interactions in two dimensions.2 More recently, emphasis has turned to phase transitions involving dynamic disorder of the alkylammonium i o n ~ . ~ The structures can alternatively be viewed as a double layer of alkylammonium ions between Mx, sheets and can be considered as crystalline models of lipid bilayers. It is hoped that the study of these salts will help the understanding of the physical nature of actual lipid bilayer^.^ We have undertaken a detailed study of the (CnH2n+lNH3)2MC14 (henceforth, C,M) systems for n = 12 and 14, and for M = Cd2+,Mn2+, and Cu2+using DSC, powder X-ray diffraction, broad-line 'H NMR, and laser Raman investigations. All systems exhibit two characteristic first-order phase transitions: a minor transition with AH& = 1.5-2.5 kcal/mol and a major transition with AHtr = 10-14 kcal/mol. The separation in temperature (1) E.R. Peterson and R. D. Willett, J.Chem. Phys., 56, 1879 (1972). (2) L. J. de Jongh and A. R. Miedema, Adu. Phys., 23, 1 (1974).
(3) R. Blinc, M. Burgar, B. Lozar, J. Seliger, J. Slak, V. Rutar, H. Arend, and R. Kind, J. Chem. Phys., 66, 278 (1977), and references therein. (4) R. E. Jacobs, B. S. Hudson, and H. C. Andersen, Biochemistry, 16, 4349 (1977), and references therein.
(and, indeed, the order of the occurrence of the transitions) varies as n or the metal ion is changed, as seen in Figure 1. We will present the pertinent results for two salts: C12Cd,where the minor transition occurs first; and CI2Mn, where it occurs second. Substantial disagreement concerning the number and temperature of transitions is apparent in previous studies.5-8 We found that this could be due to impurity of the alkylamine starting material or to too rapid DSC scan rates. All of our amines had a purity in excess of 99.7%. DTA and DSC data were collected at heating rates of 1-2 "C/min. The most definitive study to date has been that of Kind et al., on the CloCd ~ y s t e m .This ~ included single-crystal X-ray structure determinations of the room- and hightemperature phases (but not of the intermediate phase), 14Nand 35ClNQR studies, IH NMR linewidth and relaxation-time studies, as well as an analysis of the phase transitions in terms of various structural order parameters. ( 5 ) E. H. Bocanegra, M. J. Tello, M. A. Arriandiaga, and H. Arend, Solid State Commun., 17, 1221 (1975). (6) E. Landi and M. Vacatello, Thermochim. Acta, 13,441 (1975). (7) M. A. Arriandiaga, M. J. Tello, J. Fernandez, H. Arend, and J. Roos, Phys. Status Solidi A, 48, 53 (1978). (8) V. Salerno, A. Grieco, and M. Vacatello, J.Phys. Chem., 80,2444 (1976). (9) R. Kind, S.Plesko, H. Arend, R. Blinc, B. Zeks, J. Seliger, B. Lozar,
J. Slak, A. Levstik, C. Filipic, V. Zagar, G. Lahajnar, F. Milia, and G. Chapuis, J. Chem. Phys., 71,2118 (1979).
0022-3654/81/2085-3385$01.25/00 1981 American Chemical Society
3386 The Journal of Physical Chemistty, Vol. 85,No. 23, 1981
Needham and Willett
LII
t
B
I2i
b 0
0 E
1
8
Y
v
04
20
I
30
40
50
GO
70
TEMPERATURE
4
80
90
100
90
160
[ "C1
Downloaded by UNIV OF SUSSEX on September 6, 2015 | http://pubs.acs.org Publication Date: November 1, 1981 | doi: 10.1021/j150623a007
Figure 2. Second-moment data for C&d,
50
80
70
60
90
TEMPERATURE ("C) Flgure 1. Graphic representation of DSC results.
TABLE I : DSC and X-ray Results A H ,,
T,,kcal/
compd "C mol
AH2, A S l , ( A d / d ) , , T,,ked/ A s z , ( A d / d ) ,
eu
2.6 7.8 C,,Cd 59 C,,Mn 59 11.9 36.0 C,,Cd 35 -1.2 3.8
%
0 7.4 0
"C mol
eu
61 10.4 31.1 63 1.5 4.5 39 -9.2 26.2
%
8.4 0.9 9.7
TABLE 11: Interlayer Spacing (A ) compd
phase I
phase I1
phase I11
CI ZCd C12Mn C,rJCd
28.6 29.7 25.8
28.6 31.9 25.8
32.2 27.3
31.0
The minor transition occurs at 35 "C and the major transition at 39 "C in this salt. The ammonium ion head-group hydrogen bonds tightly into the metal halide lattice in all phases. In the room-temperature phase, the alkylamine is in an all-trans configuration except for a gauche bond (either a t the Cl-Cz or Cz-C3 bond, where C1 is the carbon atom bonded to the NH3 group), causing the alkylamine to be inclined at an angle of 40" from the normal to the layer. In the high-temperature phase the alkylamine is positionally disordered between two sites, with the chain axis parallel to the normal. The various measurements lead to the conclusion that the minor transition was due to the onset of a 2-fold torsional disorder of the whole hydrocarbon chain involving a change from a gauche(+) to gauche(-) configuration at the C1-C2 (or Cz-C,) bond. The major transition involved the onset of chain melting, Le., sapid movement of gauche bonds up and down the chain. The DSC and X-ray results on the ClzCd and C12Mn salts are summarized in Tables I and 11, along with the corresponding data for CloCd. The change in crystal d spacings occurs discontinuously, showing that the transitions are first order. In C&d, no increase in interlayer spacing occurs for the minor transition; thus, there is no
O h
30
Lib
50
60
7b
TEMPERATURE
80
["
Cl
Figure 3. Second-moment data for C1,Mn.
change in the inclination of the trans portion of the hydrocarbon chain. The chain becomes normal to the layer at the major transition. In contrast, the major transition occurs first in ClZMn,where the chain becomes normal to the layer. Only a small increase in interlayer spacing (
