Ion mobility and clustering of sodium hydroxybenzoates in aqueous solutions: a molecular dynamics simulation study

Abstract

The relative position of the hydroxylic and the carboxylic group in the isomeric hydroxybenzoate (HB) anions is known to have a large impact on transport properties of this species. It also influences crucially the self-organisation of cationic surfactants. In this article a systematic investigation of aqueous solutions of the ortho, meta, and para isomers of the HB anion is presented. Molecular dynamics simulations of all three HB isomers were conducted for two different concentrations at 298.15 K and using two separate water models. From the resulting trajectories we calculated the self-diffusion coefficient of each isomer. According to the calculated self-diffusion coefficients, isomers were ranked in the order o-HB > m-HB > p-HB at both concentrations for both the used SPC and SPC/E water models, which agrees very well with the experiment. The structural analysis revealed that at lower concentration, where the tendency for dimerisation or cluster formation is low, hydrogen bonding with water determines the mobility of the HB anion. o-HB forms the least hydrogen bonds and is therefore the most mobile, and p-HB, which forms the most hydrogen bonds with water, is the least mobile isomer. At higher concentration the formation of clusters also needs to be considered. The ortho isomer predominantly forms dimers with 2 hydrogen bonds per dimer between one OH and one carboxylate group of each anion. m-HB mostly forms clusters of sizes around 5 and p-HB forms clusters of sizes even larger than 10, which can be either rings or chains.

Structure and Stability of Long Rod-like Dodecyltrimethylammonium Chloride Micelles in Solutions of Hydroxybenzoates: A Molecular Dynamics Simulation Study

“…However, it might also be due to the fact that o-HB can form very stable dimers. 25 In any case, this feature provides further evidence that o-HB interacts with the micelle in two distinct ways.…”

“…Most likely, they form o-HB dimers in which the two HB anions are connected by two hydrogen bonds, similar to the ones discussed in detail in our previous work. 25 Once more, it should be mentioned that practically all HB anions were attached to or incorporated into the DTAC micelle during the sampling time interval.…”

“…The Ryckaert–Bellemans torsion potential was used for the −C–C–C–C– torsions. Structures and partial atomic charges (Figure ) of all HB isomers were taken from our previous work . The structure of the DTAC molecule was prepared with VMD (1.9.1), and partial atomic charges were taken from the literature .…”

“…Structures and partial atomic charges (Figure 1) of all HB isomers were taken from our previous work. 25 The structure of the DTAC molecule was prepared with VMD (1.9.1), 26 and partial atomic charges were taken from the literature. 27 In DTA + the headgroup methyl groups and the methylene group adjacent to the headgroup nitrogen atom bear a charge of +0.25 (Figure 1).…”

“…The rationale for using the SPC and SPC/E water models is that in our previous study 25 of hydroxybenzoates in pure water we observed that the SPC/E model solvates HB anions better than does SPC water, which results in a lower self-diffusion coefficient of HB (this is partially due to the lower self-diffusion coefficient of the solvent itself) and fewer HB−HB hydrogen bonds and thus less clustering of HB anions in SPC/E water. It is also commonly assumed that the different solvation of the isomers plays a role in their different effects on the selforganization of DTAC micelles.…”