de Pauli M, Magalhães-Paniago R, Malachias A
Departamento de Física, Universidade Federal de Minas Gerais, Avenida Antonio Carlos 6627, Belo Horizonte-MG, CEP: 30123-970, Brazil.
Phys Rev E Stat Nonlin Soft Matter Phys. 2013 May;87(5):052402. doi: 10.1103/PhysRevE.87.052402. Epub 2013 May 13.
Melting and premelting phenomena in self-organized organic systems have been extensively explored in the literature, exploring distinct behaviors of different molecule lengths and morphologies. Nevertheless, the influence of the supramolecular assembly configuration on the occurrence of premelting remains poorly explored. Here we use phosphonic acids as model systems for self-organized molecular assemblies. These molecules exhibit long-range order on different types of substrates. The balance between chain-to-chain and head-to-head interactions leads to distinct types of stackings. Although their structural configurations are well understood, very little is known about their behavior near the melting transition. We show here that premelting occurs in lamellar structures and that its behavior depends directly on the ordered configuration assumed in the studied multilayers. Two molecules with different chain lengths were investigated: octadecyl phosphonic and octyl phosphonic acids. Although almost no dependence on the molecule length was observed, the occurrence of premelting is strongly influenced by their lamellar packing configuration. For tilted packings premelting is unfavored while in straight configurations, where alkyl chain interactions are weakened with respect to head-to-head interactions, strong premelting is observed. We find that the onset of premelting occurs at the domain boundaries with straight lamellar configurations and the domain sizes exhibit power law temperature dependences.
自组织有机系统中的熔化和预熔化现象在文献中已得到广泛研究，探讨了不同分子长度和形态的独特行为。然而，超分子组装构型对预熔化发生的影响仍未得到充分探索。在这里，我们使用膦酸作为自组织分子组装的模型系统。这些分子在不同类型的基底上表现出长程有序。链间相互作用和头对头相互作用之间的平衡导致了不同类型的堆积。尽管它们的结构构型已得到很好的理解，但对于它们在熔化转变附近的行为却知之甚少。我们在此表明，预熔化发生在层状结构中，并且其行为直接取决于所研究多层膜中假定的有序构型。研究了两种具有不同链长的分子：十八烷基膦酸和辛基膦酸。虽然几乎未观察到对分子长度的依赖性，但预熔化的发生受到它们层状堆积构型的强烈影响。对于倾斜堆积，预熔化不利，而在直构型中，相对于头对头相互作用，烷基链相互作用减弱，观察到强烈的预熔化。我们发现预熔化的起始发生在具有直层状构型的畴边界处，并且畴尺寸表现出幂律温度依赖性。
