Conformational disorder in crystalline n-alkanes prior to melting

Journal Article · Thu Oct 19 04:00:00 EDT 1989 · Journal of Physical Chemistry; (USA)

DOI:https://doi.org/10.1021/j100358a050· OSTI ID:6825246

Kim, Y; Strauss, H L; Snyder, R G ^[1]

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1. Univ. of California, Berkeley (USA)

By means of infrared spectroscopy, the concentration of gauche bonds has been monitored as a function of increasing temperature for the crystalline n-alkanes, C{sub n}, n = 17, 25, 36, 50, and 60. For each n-alkane, the number of gauche bonds, and hence the degree of conformational disorder, is found to increase as the melting point is approached. The temperature at which the increase in disorder becomes measurable is 10-40 K below the melting point. For C{sub 17}, C{sub 25}, and C{sub 36}, the increase begins abruptly and occurs in association with a solid-solid phase transition. However, for the C{sub 50} and C{sub 60}, the onset of disorder is not associated with a solid-solid transition and occurs gradually. Therefore, C{sub 50} and C{sub 60} can be said to undergo true premelting. Although the onset of disordering for the shorter chains (C{sub 17}, C{sub 25}, C{sub 36}) occurs abruptly and that for the longer chains (C{sub 50}, C{sub 60}) gradually, there are, otherwise, close similarities. The similarities are in the width of the temperature range over which disordering occurs, in the total concentration of the disorder, and in its distribution. These similarities suggest a common disordering mechanism. Our earlier studies on conformational disorder for crystalline n-alkanes in the hexagonal phase suggest that the disordering mechanism involves longitudinal diffusion. In addition to premelting disorder, we have observed an inherent or background disorder for melt crystallized samples. The concentration of this disorder is temperature independent up to the temperature of premelting and, on a gauche bond per chain basis, has essentially the same value for all the n-alkanes studied here. The origin of this disorder is not clear, but it is probably associated with the lamellar surface.

• OSTI ID: 6825246
• Journal Information: Journal of Physical Chemistry; (USA), Journal Name: Journal of Physical Chemistry; (USA) Vol. 93:21; ISSN JPCHA; ISSN 0022-3654
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ALKANES
CONFORMATIONAL CHANGES
CRYSTAL-PHASE TRANSFORMATIONS
DATA
DATA ANALYSIS
EXPERIMENTAL DATA
HYDROCARBONS
INFORMATION
INFRARED SPECTRA
MEASURING INSTRUMENTS
MEASURING METHODS
MELTING
NUMERICAL DATA
ORGANIC COMPOUNDS
PHASE TRANSFORMATIONS
SPECTRA