J . Pkys. Ckem. 1989, 93, 485-490
485
Conformational Disorder in the Binary Mixture n -C50H,02/n-C46HB4: A Vibrational Spectroscopic Study Yesook Kim, H. L. Strauss, and R. G. Snyder* Department of Chemistry, University of California, Berkeley, California 94720 (Received: June 9, 1988)
Conformational disorder in the n-alkane binary mixture n-C50Hlo2/n-C46H94 near 300 K has been characterized by Raman and infrared spectroscopy over the compositional range. Conformational disorder is increased by mixing and is at a maximum near the 1:l composition. The disorder is mainly located near the chain ends. The chain interior is relatively unaffected. The concentrations of gauche bonds at the second and third CC bonds are approximately equal. The gauche concentration at the second and third bonds summed over both ends of the chains i s about 0.20 0.04 gauches/chain. The distribution of conformational disorder for the c5O/c46 binary mixture is similar to that found from our earlier study of the CZ!/Cl9 mixture. However, there are quantitative differences between the two systems in the end-gauche concentration, which is about 2.5 greater for C50/C4. In addition, the concentrations of gauche bonds at the 2- and 3-positions are nearly the same for c5O/c46, but, in the case of CZl/Cl9,the concentration at the 2-position is significantly greater. The differences between the Cso/C4 and CZ1/Cl9systems can be satisfactorily accounted for in terms of the greater chain-length mismatch that occurs for c5O/c46. Therefore, it appears that the important factor determining the distribution of conformational disorder in the mixture is the degree of chain mismatch rather than the average length of the chains. An upper limit to the kink (gtg’) concentration in the 1:l C50/C4 mixture has been estimated at about 0.15 f 0.03 kinks/chain. There is evidence that this value is probably much too high, and therefore we conclude that interior kinks do not play an important role in reducing the chain-length mismatch for either the c5O/c46 or C2,/CI9mixtures. Rather, our results support a model in which the lamellar surface irregularities and the void volume are minimized through a correlation of longitudinal chain translation with gauche-bond formation near the ends of the chains.
*
Introduction n-Alkane chains in the crystalline state pack in an all-trans conformation with their long axes parallel. The methyl end groups lie in parallel planes so that the crystal may be described as lamellar. X-ray studies show that the chains in crystals of binary mixtures of n-alkanes also pack in this way so that the structure of the binary system may also be described as lamellar. However, in this case, since the length of the components is different, packing imperfections result and lead to an increase in the specific volurne.’s2 ,An important question concerns the location of imperfections. Two quite different models have been proposed. In one model, the imperfections are located at the ends of the chains and involve gauche bonds placed in ways that minimize the number and size of interlamellar voids.3 Otherwise, the chains are in their usual all-trans form. The results of X-ray studies of binary mixtures by Asbach and his co-workers4 are consistent with this model. The second model that has been proposed focuses on the interior regions of the chains where the imperfections take the form of localized “kinks”, a term that we use here to mean the specific conformational sequence gtg‘. A kink involves some degree of lateral displacement which serves to shorten the effective length of the longer chains and thus reduce the length m i ~ m a t c h . ~ Recently, we have used vibrational spectroscopy to discriminate between the two models in the case of the n-alkane binary mixture Czl/C19.6 In that study we were able to determine separately the conformational disorder associated with each of the two nalkane components by an infrared method involving selective deuteriation of methylene groups. We found that, for the CZ1/Cl9 system, in both phase I (orthorhombic) and phase I1 (hexagonal), the conformational disorder induced by mixing is confined to the region nearest the ends of the chains, so that the chain interior is left unaffected. It is of course possible that the situation for longer chains may be different and that kinks may play a more significant role in mixtures of such chains. Most recently, this view has been ex(1) Mazee, W. M. Anal. Chim. Acta 1957, 17, 97-106. (2) Mnyukh, Yu.V. J . Struct. Chem. (USSR) 1960, I, 346-365. (3) Lauritzen, J. I.; Passaglia, E.; DiMarzio, E. A. J . Res. Nafl. Bur. Stand. 1967, 71A, 245. (4) Asbach, G. I.; Geiger, K.;Wilke, W. Colloid Polym. Sci. 1979, 257, 1049-1 059. ( 5 ) Blasenbrey, S.;Pechhold, W. Reol. Acta 1964, 6, 174. (6) Maroncelli, M.; Strauss, H. L.; Snyder, R. G. J . Phys. Chem. 1985, 89, 5260-5267.
0022-3654/89/2093-0485$01.50/0
pressed by Dorset’ in a report on an electron diffraction study on the solid solutions C36/C32 and C36/C33. The present paper reports a Raman and infrared study on the n-alkane binary mixture C5o/C46. The chains of this system are more than twice as long as those used in our earlier study and are longer than those used in the electron diffraction study just cited. We note that, unlike the CZl/Cl9system, neither neat C50 or c46, nor their mixture, has a high-temperature form that corresponds to phase I1 of the shorter n-alkanes. One goal of the present work is to learn if the conformational disorder in the c5O/c46 mixture is essentially of the same type observed for the CZl/Cl9mixture in phase I;6 that is, we wish to know if mixing induces disorder near the chain ends only, or if, unlike the CZ1/Cl9case, it also induces a significant concentration of defects along the length of the chain. A longer range goal is to understand the factors that determine disorder in molecular assemblies comprised of long chains. Experimental Procedures The n-CZiH4., sample used in this work is of high purity, 99.6-99.9 mol %.’ The samples of n-CSoHlo2and n-C46H94 (estimated purity 99%) were purchased from Fluka and used without further purification. Solid solutions were prepared by melting together preweighed n-alkanes and then cooling the mixture to room temperature. For the infrared measurements, a film of the n-alkane mixture, about 0.5 mm thick, were prepared by cooling a liquid mixture sandwiched between NaCl windows. For Raman measurements the solid mixture was crushed and sealed in a capillary tube. Calorimetric measurements were made with a Perkin-Elmer DSC-2. The instrument was calibrated with indium and zinc standards. Measurements were made at a scanning rate of 1.25 K/min for each composition. Infrared spectra were measured with an evacuatable Nicolet Model 8000 FTIR spectrometer equipped with a cooled MCT infrared detector. Scanning conditions were chosen to provide a 1-cm-l resolution. Raman spectra were measured with a Spex 1403 double monochromator, which was controlled by a Spex Datamate DMlB computer. Multiple scans were used to improve the signal-to-noise (7) Dorset, D.L. Macromolecules 1988, 20, 2782-2788. (8) Schaerer, A. A,; Busso, C. J.; Smith, A. E.; Skinner, L. B. J . Am. Chem. SOC.1955, 77,2017.
0 1989 American Chemical Society
486
The Journal of Physical Chemistry, Vol. 93, No. 1, 1989
Kim et al.
" i l
TABLE I: Constant-Frequency Methyl Rocking Modes for Long n-Alkanes
"-C21H44
intensity
obsd freq, cm-I
conformers' tt-
gtgg-
tgttg-
m-
n-C21H44c n-C50H,0< 891 875 862 843
891e 871 862 845
calcd freq, cm-I 892 879 862 850 894 -876
Ah
obsd
calcd
1.0 0.45
1.0 0.71 1.2 0.83
0.83
'The conformation at the chain end is indicated. The chain is otherwise all-trans. *Used in eq 1. See text. Cobserved for phase 11. dObservedfor the Cw/C46mixture at 23 OC and for neat Cw at 80 "C (92.4 "C). The frequencies for the gt-, gg-, and tg- modes are the same for the C5,,/C4, mixture at 23 "C and for CS0at 80 "C. For the tt- mode see footnote e below. CTheobserved frequency is 891 cm-I for the CJO/C%mixture at 23 "C and neat C50 at 80 OC and 894 cm-I for neat C50 at 23 OC. ratio. The incident radiation for the Raman spectra was the 514.5-nm line of an argon ion laser. Perpendicularly scattered radiation was collected and passed through a polarization scrambler on to the spectrometer slit. The slits were set for a resolution of 4 cm-l. An estimate of the extent to which the laser excitation radiation heated the sample was made from measurements on a sample of C21near its melting point. We found a negligible temperature effect (
