Exploring the nature of the interactions between the molecules of the sodium dodecyl sulfate and water in crystal phases and in the water/vacuum interface.

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.