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Doctoral Thesis
DOI
https://doi.org/10.11606/T.54.1984.tde-16032015-112502
Document
Author
Full name
Ireno Denicolo
Institute/School/College
Knowledge Area
Date of Defense
Published
São Carlos, 1984
Supervisor
Committee
Craievich, Aldo Felix (President)
Amaral, Lia Queiroz do
Mascarenhas, Yvonne Primerano
Rodrigues, Antonio Ricardo Droher
Tsallis, Constantino
Title in Portuguese
Estudo estrutural das parafinas normais com 17 ≤ n ≤ 34
Keywords in Portuguese
Não disponível
Abstract in Portuguese
O presente trabalho experimental é devotado ao estudo das estruturas médias e suas desordens, nas parafinas normais (CnH2n+2), abrangendo os compostos com 17 ≤ n ≤ 34 exceto as com n = 30, 31 e 33. A técnica aqui utilizada é a da difração de raio x, cuja radiação é proveniente de geradores convencionais e de sincrotron. As parafinas acima, com o aumento da temperatura exibem as fases cristalina, líquida e uma intermediária denominada rotatória cuja estrutura média era tida como hexagonal e que ora é objeto de um estudo mais detalhado. A simetria molecular das parafinas com n impar difere das com n par, assim também ocorrendo com suas estruturas lamelares médias. As parafinas ímpares, na fase cristalina exibem uma estrutura com rede ortorrômbica de camada dupla, grupo espacial Pcam, 4 moléculas por cela, parâmetros de ‌a‌ = 7,5 R, ‌b‌ = 5,0 R e ‌c‌ = 2 LM (LM, comprimento molecular) e a razão ‌a‌/‌b‌ é de 1,50. Nos compostos com n = 17, 19 e 21, elevando-se a temperatura, ocorrem as transições de fase cristalina → rotatória I (RI) → líquida. No domínio térmico da fase RI ‌a‌/‌b‌ evolue de ˜ 1,56 ao limite superior ˓ 1,69, devido à expansão de ‌a‌ e contração de ‌b‌, o que é conseqüente de uma desordem orientacional do pIano molecular (oscilação do plano molecular). Como na fase precedente, a estrutura pseudo-hexagonal da fase RI é ortorrômbica, de dupla camada, com 4 moléculas por cela, porém de grupo espacial Fm2m. Nas parafinas com n = 23 e 25, além da fase RI, observou-se uma fraca transição de primeira ordem RI → rotatória II (RII) e posteriormente, à fase líquida. Na fase RII, ‌a‌/‌b‌ atinge o valor de √, caracterizando uma estrutura hexagonal (romboédrica) de camada tripla, do tipo ABC ABC ... , grupo espacial R ۙM, cada cela contendo 3 moléculas e sendo a distância entre as moléculas de 4,77 Å. As moléculas que oscilavam na fase RI , passam a girar ao redor de seu eixo longo na fase RII, formando cilindros. As parafinas com n= 27 e 29 exibem as transições cristalina → rotatótia III (RIII) → Rotatória IV (RIV) → liquida. As estruturas das fases RIII e RIV são triclínicas e monoclínica respectivamente, onde as moléculas também giram ao redor de seu eixo longo. Nas parafinas pares com 18 ≤ n ≤ 24., observou-se na fase cristalina, uma estrutura triclínica ‌a‌ ≅ 4,2 Å, ‌b‌ ≅ 4, 8, Å, ‌c‌ ≅ LM, α ≅ 98° , β ≅ 61°, o que se observa também nos compostos com n=28 e 34. Já a parafina C32H66 difere na forma que é só monoclínica, com os parâmetros ‌a‌ ≅ 5,6 Å, ‌b‌ ≅ 7,5 Å, ‌c‌ ≅ 2LM e β ≅ 81°. A parafina C28H58 passa pelas transformações cristalina → RIV → liquida, e as com n= 32, 34, cristalina → RIII → liquida. O estudo de desordem estrutural efetuado envolveu as parafinas com 17 ≤ n ≤ 28, que exibissem a fase rotatória estável - exceto n= 18 e 20. Constatou-se na fase rotatória - considerando- se que a fase cristalina está ausente de desordens estruturais - deslocamentos moleculares longitudinais crescentes com n, devido à desordem orientacional dos planos moleculares. Também verificou-se que a concentração de defeitos "Kink" com n, independentemente se n é par ou impar
Title in English
Not available
Keywords in English
Not available
Abstract in English
This thesis is devoted to the study of the average structure and molecular disorders of normal paraffins (CnH2n+2), with 17 ≤ n ≤ 34, excepted n = 30, 31 and 33. The main technique which was used is X-ray diffraction. Conventional generators and synchrotron radiation sources have provided the X-ray beams which were used in the several types of studies. Paraffins exhibit several solid phases. At low temperature the equilibrium phase is named crystalline phase. Between this phase and the liquid, a mesophase which was called rotator phase was discovered. This phase has an average lattice with a symmetry wich was taken as hexagonal. The aim of this work is a detailed study of the structure of this mesophase. The symmetry of the molecules of paraffins with odd n differs' from that of even n. The average structure of the crystalline phases of odd and even numbered paraffins are also different. The structure of the crystalline phase of odd paraffins exhibits an orthorombic lattice with double layers, spatial group Pcam ,4 molecules per unit cell, lattice parameters ‌a‌ = 7,5 R, ‌b‌ = 5,0 R e ‌c‌ = 2 LM (LM being the molecular length) and the ratio ‌a‌/‌b‌ = 1,5. Odd compounds with n = 17,19 and 21 exhibit two phase transitions at increasing temperatures: crystal (c) → rotator I (RI) → liquid. Within the temperature domain of stability of the RI phase, a/b evolves from 1.56 to 1.69, as a consequence of an expansion of the parameter a and a contraction of b, wich are associated with the orientational disorder of the molecules (libration of the molecular plane). The lattice of the pseudo-hexagonal RI phase is orthorombic, double layered, with 4 molecules per unit cell and spatial group Fm2m. Odd - numbered C23H48 and C25H52 also exhibit the RI phase. An additional weak transition to a rotator II (RII) phase, some degrees below the liquid, has been detected. For RII, a/b reaches the value 𕔇 which corresponds to a rhombohedral lattice associated with a triply layered structure ABC ABC, spatial group R3m 3 molecules per unit cell and a distance between neighbouring molecules of 4.77 Å. The molecules, which librate in phase RI are rotating around their long axis in RII phase. Paraffins with n = 27 and 29 exhibit the transitions: crystal → rotator III (RIII) → rotator IV (RIV) → liquid. The structures of phases RIII and RIV have triclinic and monoclinic lattices, respectively, and the molecules also rotate around their long axis. Even paraffins with 18 ≤ n ≤ 24 present a crystalline phase with triclinic lattice. Their lattice parameters are ‌a‌ ≅ 4,2 Å, ‌b‌ ≅ 4, 8, Å, ‌c‌ ≅ LM, α ≅ 98° , β ≅ 61°. By increasing the temperature, the transitions crystal → RII → liquid occur. The paraffin C26H54 exhibit, simultaneously, structures with triclinic and monoclinic modifications are ‌a‌ ≅ 5,6 Å, ‌b‌ ≅ 7,5 Å, ‌c‌ ≅ 2LM and β ≅ 61°. The same lattice has been found in the C28H58 and C29H60 compounds. The paraffin C32H,sub>66 presents a monoclinic modification. Their lattice parameters are a = 7.5 Å, b = 5.0 Å , c = 2 LM and β = 81°. The phase transitions in C28H58 are crystal rotator IV liquid and, in C32H,sub>66 and C34H,sub>70, They are: crystal → rotator III → liquid. This study focused the attention to paraffins with 17 ≤ n ≤ 28, excepted n = 18 and 20 which do not exhibit rotator phases. It has been detected the presence of longitudinal displacements of the molecules with increasing amplitude for increasing molecular length. It has also been established the existence of intra molecular defects (kinks) which increase in number for increasing molecular length
 
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