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Size-dependent freezing of n-alcohols in silicon nanochannels
Publikationstyp
Journal Article
Date Issued
2010-10
Sprache
English
Volume
189
Issue
1
Start Page
239
End Page
249
Citation
European Physical Journal: Special Topics 189 (1): 239-249 (2010-10-01)
Publisher DOI
Scopus ID
Publisher
Springer
We present a study on the phase behavior of several linear n-alcohols (heptanol, nonanol and undecanol) in their bulk state as well as confined in mesoporous silicon. We were able to vary the mean pore radii of the nanochannels from r = 3. 5 nm to 7 nm and to determine the respective temperatures of the freezing and melting transitions using infrared and dielectric spectroscopy. The smaller the chain length the lower the freezing point, both in the bulk and in the confined state. Under confinement the freezing temperature decreases by up to 28 K compared to the bulk value. In accordance with the Gibbs-Thompson model the lowering is proportional to the inverse pore radius, ΔTfr ∝ 1/r. Moreover, the ratio of freezing temperature depression to melting temperature depression is close to the theoretical value of ΔTfr/ΔTmelt = 3/2. The spectra also indicate a structural change: while the solid bulk alcohols are a polycrystalline mixture of the orthorhombic β- and monoclinic γ-form, geometrical confinement forces the alcohol-chains into the more simple orthorhombic structure. In addition, a part of the material does not crystallize. Such an additional amorphous phase seems to be a logical consequence of the size mismatch between molecular crystals and irregular shaped pores.
DDC Class
530: Physik