title: Conformational change in the C form of palmitic acid investigated by Raman spectroscopy and X-ray diffraction.
abstract: Fatty acids are substances found in most living beings in nature. Here we report the effect of the low temperature in the vibrational and structural properties of the C form of palmitic acid, a fatty acid with 16 carbon atoms. The Raman spectra were obtained in the temperature interval from 300 to 18K in the spectral range between 30 and 3100 cm(-1). The assignment of the duly observed bands was done based on the density functional theory. On cooling, the main changes observed in the lattice mode region of the Raman spectra were interpreted as a conformational modification undergone by the palmitic acid molecules in the unit cell. The X-ray diffraction measurements were obtained from 290 to 80K showing a slight modification in the lattice parameters at about 210K. Differential scanning calorimetry (DSC) measurements were recorded between 150 and 300K and no enthalpic anomaly in the DSC thermogram was observed. These techniques provided strong evidence of the conformational change in the molecules of palmitic acid at low temperatures.
authors.affiliation: Instituto de Ciências Exatas, Universidade Federal do Sul e Sudeste do Pará, CEP 68505-080 Marabá, PA, Brazil.
authors.forename: F F
authors.initials: FF
authors.lastname: de Sousa
authors.affiliation: Departamento de Física, Universidade Regional do Cariri, Juazeiro do Norte, CEP 63040-000, Brazil.
authors.forename: C E S
authors.initials: CE
authors.lastname: Nogueira
authors.affiliation: Departamento de Física, Universidade Federal do Ceará, CEP 60455-760 Fortaleza, CE, Brazil.
authors.forename: P T C
authors.initials: PT
authors.lastname: Freire
authors.affiliation: Instituto de Ciências Exatas e Naturais, Universidade Federal do Pará, CEP 66075-110 Belém, PA, Brazil.
authors.forename: S G C
authors.initials: SG
authors.lastname: Moreira
authors.affiliation: Departamento de Física, Universidade Regional do Cariri, Juazeiro do Norte, CEP 63040-000, Brazil.
authors.forename: A M R
authors.initials: AM
authors.lastname: Teixeira
authors.affiliation: Departamento de Física, Universidade Federal do Maranhão, CEP 65080-805 São Luis, MA, Brazil.
authors.forename: A S
authors.initials: AS
authors.lastname: de Menezes
authors.affiliation: Departamento de Física, Universidade Federal do Ceará, CEP 60455-760 Fortaleza, CE, Brazil.
authors.forename: J
authors.initials: J
authors.lastname: Mendes Filho
authors.affiliation: Faculdade de Educação Ciências e Letras do Sertão Central, Universidade Estadual do Ceará, CEP 63900-000 Quixadá, CE, Brazil. Electronic address: gilberto.saraiva@uece.br.
authors.forename: G D
authors.initials: GD
authors.lastname: Saraiva
mesh_terms: D002152:Calorimetry, Differential Scanning; D008958:Models, Molecular; D008968:Molecular Conformation; D019308:Palmitic Acid / Q000737:chemistry*; D033362:Powder Diffraction; D013059:Spectrum Analysis, Raman / Q000379:methods; D013696:Temperature; D014961:X-Ray Diffraction
chemical_list: D019308:Palmitic Acid
keywords: Conformational changes; Palmitic acid crystals; Raman scattering; X-ray powder diffraction