Escuela de Quı́mica , Universidad de Costa Rica , San José 11501-2060 , Costa Rica.
Centro de Investigación en Ciencia e Ingenierı́a de Materiales (CICIMA) , Universidad de Costa Rica , San José 11501-2060 , Costa Rica.
J Phys Chem B. 2020 Feb 6;124(5):890-899. doi: 10.1021/acs.jpcb.9b09567. Epub 2020 Jan 24.
A thermotropic nematic liquid crystal (LC) becomes an isotropic liquid at the nematic-isotropic transition temperature (), which depends on the molecular order of the mesophase. By means of a polarized optical microscope and a differential scanning calorimeter, it was found that doping the nematic liquid crystal 4--pentyl-4'-cyanobiphenyl (5CB) with molecularly rigid carboxylic acids (benzoic, 1-naphthoic, 2-naphthoic, and biphenyl-4-carboxylic acids) increases without modification of the nematic-isotropic transition enthalpy. This increment in is due to the increased order caused by the formation of molecularly rigid and elongated dimers of carboxylic acids in the nematic LC, as confirmed with infrared spectra. Furthermore, increased with the length of the molecularly rigid dimers at the same concentration level. Conversely, doping the LC with molecularly flexible acids caused lowering of . A quantitative correlation was established between the increase of the rigid carboxylic acids and the length of the dimers of these acids; a predictive model for these values as a function of acid molar fraction was developed. It was also demonstrated that the doping of 5CB with rigid carboxylic acids increases the rotational viscosity of the liquid crystal.
热致向列型液晶（LC）在向列-各向同性转变温度（）下变为各向同性液体，这取决于中间相的分子有序性。通过偏光显微镜和差示扫描量热仪发现，向列液晶 4--戊基-4'-氰基联苯（5CB）掺杂分子刚性羧酸（苯甲酸、1-萘酸、2-萘酸和联苯-4-羧酸）时，在不改变向列-各向同性转变焓的情况下，增加了。这种的增加是由于在向列 LC 中形成了分子刚性和伸长的二聚羧酸，导致了有序性的增加，这一点通过红外光谱得到了证实。此外，在相同浓度水平下，随着分子刚性二聚体长度的增加而增加。相反，向 LC 中掺杂分子柔性酸会降低。刚性羧酸的增加与这些酸的二聚体长度之间建立了定量相关性；还开发了一个预测这些值的模型，作为酸摩尔分数的函数。还证明了向列液晶 5CB 掺杂刚性羧酸会增加液晶的旋转粘度。
