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

Abstract

The nature of the interaction between the molecules of the sodium dodecyl sulfate surfactant forming two crystal phases, one anhydrous, NaC₁₂H₂₅O₄S and the other, NaC₁₂H₂₅O₄S.H₂O, 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 $S{O}_4^{-}$ 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 $O_4^{-}$ groups and the H₂O molecules. Each H₂O molecule is bonded to two $S{O}_4^{-}$ groups via hydrogen bonds, while the $S{O}_4^{-}$ 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 SO₄ ^- 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.

Keywords: Aggregation of the surfactant molecules; Chemistry; Clustering of the surfactant molecules at the water/vacuum interface; Crystal phases; Materials chemistry; Physical chemistry; Quantum theory of atoms in molecules; Sodium dodecyl sulfonate; Theoretical chemistry.