J. Phys. Chem. 1986,90, 3314-3311
3374
vesicles to form interstitial sulfur, as reported by Gratze13 in polymer-stabilized CdS colloids. In fact, no anion effect was found upon changing C1- by NO< in the DHP vesicle system,22where NO,- was reported to produce red fluorescence in polymer-stabilized system^.^ However, we cannot exclude an interaction between the phosphate head groups of the vesicles and the CdS colloids. This could create surface states and radiative recombination centers. The present data do not tell whether the crystal structure of DHP-stabilized CdS colloids is cubic (zinc blende), hexagonal (wurtzite), or even amorphous. A controversy exists about which form is more active for photochemical hydrogen generation.l’~~~ In vesicle systems, neither the location of CdS nor the type of vesicle (polymerized or not) was found critical for hydrogen production rate, but rather the nature of the electron donor and the charge of the vesicles (positive or negative) were found determinant.24 A cubic structure was found for CdS colloids in both water and water/acetonitrile mixture^.^^'^*'^ Further work is under current investigation to determine the crystal structure and the fluorescence mechanism of vesicle-stabilized colloidal CdS particles. Summary and Conclusions In situ generation of colloidal CdS in DHP vesicles has been demonstrated to be a very flexible technique allowing wide var(31) Matsumura, M.; Furukawa, S.; Saho, y.; Tsubomura, H.J . Phys. Chem. 1985, 89, 1327.
iations of CdS photophysical properties in aqueous medium. Vesicles possess both an inner and an outer interface, and these two sites have different properties. This asymmetry makes it possible to generate two different populations of CdS colloids, either separately or on the same vesicle. Variation of the surface density of Cd2+is an easily controllable parameter which alters the size and properties of CdS particles. Unlike in other preparation media,9 the fluorescence excitation threshold was blueshifted from the absorption edge due to the presence of different particle sizes, even when the particles were all at the inner surface. Partial growth of colloidal CdS produced only the fraction having a very small size (ca. 25-A diameter). This fraction was responsible for the fluorescence, and its absorption edge (ca. 430 nm) corresponded to the excitation threshold. The growth of colloidal CdS was shown to occur by distinct steps with increasing amounts of H2S and could be stoppped at any of these steps. The capability of such control over CdS properties may be very useful to optimize the coupling of CdS with other semiconductors such as ZnS32or to achieve interparticle electron transfers.33 Acknowledgment. Support of this work by the US. Department of Energy is gratefully acknowledged. Registry No. CdS, 1306-23-6; DHP,2197-63-9. (32) Emeren, A.; Tricot, Y.-M.; Fendler, J. H., unpublished results. (33) Serpone, N.; Borgarello, E.; Grltzel, M. J. Chem. SOC.,Chem. Commum 1984, 342.
A Fourier Transform Infrared Study of Bllayer Membranes of Double-Chain Ammonium Amphiphlles Naotoshi Nakashima, Norihiro Yamada, Toyoki Kunitake,* Department of Organic Synthesis,t Faculty of Engineering, Kyushu University, Fukuoka 812, Japan
Junzo Umemura, and Tohru Takenaka Institute for Chemical Research, Kyoto University, Kyoto 61 1, Japan (Received: September 12, 1985; In Final Form: March 14, 1986)
Fourier transform infrared spectroscopy was applied to an examination of the phase transition behavior of aqueous bilayer membranes of double-chain ammonium amphiphiles. Large spectral changes were observed at the respective gel-to-liquid crystal phase transitions (T,) of the bilayers. The frequency change in the antisymmetric CH2 stretching band indicated formation of the gauche conformation at Tc The ester groups became either hydrated or disordered in the liquid crystalline state, but the amide units remained strongly associated. It is concluded that intermolecular hydrogen bonding is not necessarily weakened at T,, although the molecular packing is loosened.
Introduction It has been established that a large number of synthetic amphiphiles produce stable bilayer membranes.’ These synthetic bilayers possess self-assembling properties which are fundamentally the same as those of biolipid bilayers. The structures of some of the synthetic bilayers have been analyzed by X-ray diffraction of their single crystals and ordered f i l m ~ . ~ 9It~ is desirable, however, that the structure and dynamics of synthetic bilayers be studied directly in the aqueous phase. Fourier-transform infrared spectroscopy has been increasingly used for examining the nature of aqueous molecular aggregates such as micelle formation of aqueous surfactant^?^ and phase transitions of phospholipids.@ Contribution No. 807.
Fine review articles on the latter subject have been published re~entIy.~J~ (1) Kunitake, T.; Okahata, Y. J. Am. Chem. SOC.1977, 97, 3860, and subsequent publications from these and other laboratories. (2) Okuyama, K.; Soboi, Y.; Hirabayashi, K.; Harada, A.; Kumano, A,; Kajiyama, T.; Takayanagi, M.; Kunitake, T. Chem. Lett. 1984, 21 17. (3) Shimomura, M.;Kunitake, T.; Kajiyama, T.; Harada, A.; Okuyama, K.; Takayanagi, M.Thin Solid Films 1984, 121, L89. (4) Umemura, J.; Cameron, D. G.; Mantsch, H. H . J . Phys. Chem. 1980, 84, 2272. ( 5 ) Kawai, T.; Umemura, J.; Takenaka, T. Colloid Polym. Sci. 1984,262, 61. (6) Cameron, D. G.; Casal, H. L.; Mantsch, H. H. Biochemistry 1980, 19, 3665. (7) Mantsch, H. H.; Martin, A.; Cameron, D. G. Biochemistry 1981, 20, 3138.
0022-365418612090-3374%01.5010 0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 3375
FT-IR Study of Ammonium Amphiphiles TABLE I: Frequencies of Major Infrared Absorption Bands of an Aqueous Bilayer Membrane and a Cast Bilayer Film of ZC~~-L-CIU-CIIN' freq, cm-' cast bilayer film, 26.5 OC 2920 2851 1735 1652 1537 1469 1 I92
aqueous bilayer 20 "C 40 OC 2923.4 2853.0 1734.7 1654.4 -1540 1468.3 1194.9
2925.4 2854.7 1738.6 1651' -1540 1466.9 1198.8
assignment V,,(CHz) dCH2) v ( C 4 ) ; ester amide I amide I1 6,(CHz); (trans) ~ ~ ~ ( C - 0 - cester );
Wavenumber / cm-'
Figure 1. Difference IR spectra of aqueous 2Cl2-~-GIu-C1IN' bilayer (50 m M ) between adjacent temperatures.
Insufficient precision due to interference of the water absorption.
In the present paper, we discuss the FT-IR characteristics of a bilayer membrane of double-chain ammonium amphiphiles (2CI2-~-Glu-C,NtX-, n = 2, 6, 11).
.-V
E al
5 0 C
w
0
-1 -2 3
w
33.0'C
n = 2,
f=C i
n=6 , n=11,
f =B f
~C~~-L-GIU-C~N* ~C,~-L-GIU-C~N'
K=BB~
ZC~~-L-GLU-C~,N+
These amphiphiles produce well-developed bilayer vesicles and, in some cases, helical superstructures.12-'6 They are particularly suited for examining the effect of the molecular structure on the molecular conformation and packing by the FT-IR technique, because they contain ester and amide units in addition to the ammonium head group and the double alkyl tails. Experimental Section Preparations of amphiphiles 1, 2, and 3 have been reported e l ~ e w h e r e . ' ~ * 'Differential ~~'~ scanning calorimetry (DSC) was performed by using a Seiko Electronics SSC/560 instrument at a heating rate of 2 0C/min.17 Aqueous bilayer samples (50mM) were obtained by sonication (Branson Sonifier Model 185, sonic power 40 W, 2 min) and aged for 2 days at room temperature and 15 min a t T < T, (-5 "C). FT-IR measurements were conducted according to the published literature.l* Results and Discussion Temperature-Dependent Spectra. IR spectra of aqueous 2CI2-~-Glu-C,N+ (n = 2,6, and 11) were measured a t several temperatures. Selected bands and their assignments for the 2CI2-~-Glu-CIIN+bilayer are summarized in Table I. The assignments were made by referring to those of phospholipids?**'' The CHI group in the alkyl chain gives the antisymmetric and (8) Cameron. D. G.: Mantsch. H. H. Bioohvs. J . 1982. 38. 175. (9j Mantsch,". H.; Cameron, D. G.; Tredblay, P. A.; Kat-, M. Biochim. Biophys. Acta 1982, 689, 63.
(10) Casal, H. L.; Mantsch, H. H. Biochim. Biophys. Acta 1984, 779,381. (1 1) Amey, R. L.; Chapman, D. Biomembrane Structure and Function, Chapman, D., Ed.; Verlag Chemie: London, 1984; Chapter 4, pp 199-256. (12) Nakashima, N.; Fukushima, H.; Kunitake, T. Chem. Len. 1981, 1207. ._. .
(13) Kunitake, T.; Okahata, Y.; Yasunami, Y . J . Am. Chem. SOC.1982, 104, 5547. (14) Nakashima, N.; Asakuma, S.; Kim, J.-M.; Kunitake, T. Chem. Lett. 1984, 1709. (15) Nakashima, N.; Asakuma, S.; Kunitake, T. J . Am. Chem. SOC.1985, 107, 509. (16) Kunitake, T.; Asakuma, A,; Higashi, N.; Nakashima, N. Rep. Asahi Glass Found. Ind. Technol. 1984, 45, 163. (17) Okahata, Y.; Ando, R.; Kunitake, T. Ber. Bunsenges. Phys. Chem. 1981, 85, 789. (18) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S, J . Colloid Interface Sci. 1985, 103, 56.
2926
-
2921
c
I
E292; Y
tl
n~ 2 9 2 ( 3 C
C
I/ c=o(ester)
? 173!
2
173: 173! 173:
0
10
20
30
40
Temp.l'C
Figure 2. DSC thermograms (A) and temperature dependence of the antisymmetric C H 2 stretching frequency (B) and the ester carbonyl stretching frequency (C) of 2CI2-~-Glu-C,,N' bilayer membranes.
symmetric stretching bands at 2923 and 2853 cm-', respectively, at 20 OC. They shift to 2925 and 2855 cm-l, respectively, at 40 OC. The CH3stretching bands show similar shifts. In the carbonyl stretching region, the temperature rise causes shifts of the C=O (ester) stretching band from 1735 to 1739 cm-I, while the amide I band is shifted from 1654 to 1651 cm-'. The antisymmetric C-0-C stretching band becomes broader with temperature and shifts from 1195 to 1199 cm-I. Other bilayers give very similar IR characteristics. The spectral changes due to changes in the molecular conformation of the bilayer component occur simultaneously in a certain temperature region. They are apparently related to the gel-to-liquid crystal phase transition of the bilayer, as is known already for phospholipid bilayers.611 The two temperatures of Table I correspond to the gel and liquid crystalline states. Frequency shifts between the two states are apparent. Difference spectra between those obtained at adjacent temperatures were subsequently produced from spectra prior to subtraction of the water spectrum. Figure 1 shows difference spectra for bilayer 3 (n = 10). Large peaks at ca. 1650 cm-l are due to the O H bending vibration of surrounding water which gives rise to different spectra at different temperatures. The spectral changes found
3376 The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 in this figure are also observed for bilayer 2 (n = 6), but not for bilayer 1 (n = 2). The gel-to-liquid crystal phase transition temperatures (peak top) as determined for aqueous 2C12-~Glu-C,N+ bilayers by DSC are as follows: 3.5 OC for n = 2,14J9 12.0 OC for n = 6,4 and 33.0 O C for n = 1 l.I4 These T, values correspond to the temperature ranges where most remarkable difference spectra are obtained: complex spectral changes are observed at ca. 10 and 30 O C for bilayers of 2CI,-~-Glu-C,N+ and 2C12-~-Glu-ClIN+, respectively, but are not found for 2CI2-~-GIu-C2N+at 5-40 OC. Therefore, it is concluded that these spectral changes are indeed produced by the changes in the bilayer physical state. In the following are discussed details of the changes in the characteristic bands. Alkyl Chain Since the CH2 stretching bands have large extinction coefficients and do not overlap with the water absorption, their temperature dependence is a reliable indication of the change of the bilayer physical state. Figure 2 includes the variation of the peak position of the antisymmetric CH2stretching band with temperature. For the 2C12-~-Gl~
