Aray Yosslen, Parra José G, Paredes Ricardo, Álvarez Luis Javier, Diaz-Barrios Antonio
Universidad de Ciencias Aplicadas y Ambientales, Facultad de Ciencias, Campus Universitario Norte, Calle 222 No 55-37, Bogotá, Colombia.
Universidad de Carabobo, Facultad Experimental de Ciencias y Tecnología, FACYT, Dpto. de Química, Lab. de Química Computacional, Bárbula, Venezuela.
Heliyon. 2020 Jun 26;6(6):e04199. doi: 10.1016/j.heliyon.2020.e04199. eCollection 2020 Jun.
The nature of the interaction between the molecules of the sodium dodecyl sulfate surfactant forming two crystal phases, one anhydrous, NaCHOS and the other, NaCHOS.HO, hydrated with one water molecule for unit cell, has been studied in detail using the quantum theory of atoms in molecules and a localized electron detector function. It was found that for the anhydrous crystal, the head groups of the surfactant molecules are linked into a head-to-head pattern, by a bond path network of Na-O ionic bonds, where each Na atom is attached to four groups. For the hydrated crystal, in addition to these four bonds for Na+, two additional ones appear with the oxygen atoms of the water molecules, forming a bond paths network of ionic Na-O bonds, that link the Na atoms with the S groups and the HO molecules. Each HO molecule is bonded to two groups via hydrogen bonds, while the groups are linked to a maximum of four Na+ atoms. The phenomenon of aggregation of the sodium dodecyl sulfate molecules at the liquid water/vacuum interface was studied using NVT molecular dynamics simulations. We have found that for surfactant aggregates, the Na+ ions are linked to a maximum of three SO groups and three water molecules that form Na-O bonds. Unlike hydrated crystal, each of the O atoms that make these Na-O bonds is linked to only one Na+ ion. Despite these differences, like the crystal phases, the surfactant molecules tend to form a head-to-head network pattern of ionic Na-O bonds that link their heads. The present results indicate that the clustering of anionic surfactant at the water/vacuum interface is a consequence of the electrostatic alignment of the cationic and anionic groups as occurs in the crystalline phases of sodium dodecyl sulfate.
使用分子中的原子量子理论和局域电子探测器函数，详细研究了形成两种晶相的十二烷基硫酸钠表面活性剂分子之间的相互作用性质。一种是无水的NaCHOS，另一种是NaCHOS·HO，每个晶胞中有一个水分子水合。结果发现，对于无水晶体，表面活性剂分子的头基通过Na-O离子键的键径网络连接成头对头模式，其中每个Na原子与四个基团相连。对于水合晶体，除了Na+的这四个键外，还出现了另外两个与水分子的氧原子相连的键，形成了离子Na-O键的键径网络，将Na原子与S基团和HO分子连接起来。每个HO分子通过氢键与两个基团相连，而基团最多与四个Na+原子相连。使用NVT分子动力学模拟研究了十二烷基硫酸钠分子在液态水/真空界面处的聚集现象。我们发现，对于表面活性剂聚集体，Na+离子最多与三个SO基团和三个形成Na-O键的水分子相连。与水合晶体不同，形成这些Na-O键的每个O原子仅与一个Na+离子相连。尽管存在这些差异，但与晶相一样，表面活性剂分子倾向于形成连接其头部的离子Na-O键的头对头网络模式。目前的结果表明，阴离子表面活性剂在水/真空界面处的聚集是阳离子和阴离子基团静电排列的结果，就像十二烷基硫酸钠的晶相那样。
