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.

Figure 1

(a) Unit cell of the anhydrous SDS crystal. The white, gray, yellow, red and purple spheres denote the H, C, S, O and Na atoms, respectively. (b) Lamellar structure defined for the anhydrous SDS bulk. The distance tail-tail between the layers measured as the H$•••$H distance is 2.016 Å. Three lamellas are shown in the picture.

Figure 2

(a) Unit cell of the SDS.1H2O crystal. The white, gray, yellow, red and purple spheres denote the H, C, S, O and Na atoms, respectively. (b) Lamellar structure defined for the hydrated SDS bulk. Three lamellae are shown in the picture. A higher lamellae separation than the case of the anhydrous lamellar structure can be observed (compare with Figure 1b).

Figure 3

Molecular graph of the SDS isolated molecule. The gray, white, red, yellow and purple spheres denote the C, H, O, S and Na atoms, respectively. Small gray spheres indicate the bond critical points.

Figure 4

LED isosurfaces for the SDS molecule. (a) LED value of 1.5 au (yellow), containing the core electrons of the C, S and Na atoms while the blue isosurfaces correspond to LED = 0.5 au. These blue areas mainly involve the electron density around the BCPs. The red and white balls represent the O and H atoms, respectively. (b) LED = 1.5 au isosurfaces which encloses the inner core of the heavy atoms. (c) LED = 1.2 au isosurfaces showing that for each oxygen atoms a bell-shaped form around the lone pair electrons appeared. (d) LED = 1.0 au and (e) LED = 0.6 au shows the depicted isosurfaces for the C–H bonds. (f). LED = 0.5 isosurfaces let anyone to visualize and explore the nature of all bonds.

Figure 5

(a) Crystal graph of the heads of the anhydrous SDS system for a unit cell calculated using the AIMUC software [36]. Red, purple and yellow spheres denote the O, Na and S atoms, respectively. Black spheres denote the bond critical points while the dark blue ones highlight the O atoms where the C₁₂ tails are linked. (b) Side-view of the anhydrous SDS graph. Green and yellow lines denote the S–O and Na–O bonds, respectively. Gray lines denote the C–C and C–H bonds. White arrows point to O atoms which are bonded to SO4^- groups located behind the picture.

Figure 6

LED = 0.5 au isosurfaces showing the nature of the bonds involve in the SDS anhydrous crystal. In addition, the Na basins (purple zones), i.e., the space occupied by these atoms is also shown. The white lines highlight the Na–O ionic bonds for just one Na atom. The other Na–O ionic bond is behind the plane of the picture.

Figure 7

Crystal graph for the unit cell of a layer of the H₂₅C₁₂–S$O_4^{-}$ Na.1H₂O system. (a) Side-view on the unit cell. Gray, white, yellow and pink balls denote the C, H, S, and Na atoms, respectively. Red and orange spheres denote the oxygen atoms of the SO₄^- groups and the water molecules, respectively. Green lines denote the S–O bonds. White arrows highlight the H₂O molecules. Light blue lines denote the H–O water bonds. Pink lines denote the hydrogen bonds between the water molecules and the O atoms of the SO₄^- groups. A white ellipse highlights the fact the water molecules form a kind of casing around the surfactant head. (b) Top-view showing the basin of the Na (pink area) and the O atoms (orange area) of the water molecules surrounding the Na atoms.

Figure 8

Simulation box showing the initial conformation of the water phase and the SDS monolayer. Red, white, gray, yellow and pink spheres denote the O, H, C, S, and Na atoms, respectively.

Figure 9

(a) Snapshot of the side-view of the simulation box showing the last configuration of the SDS molecules at the water/vacuum interface. A white circle highpoint just one formed aggregate. (b) Snapshot showing the top-view of the simulation box. A white oval highlights the fact that the SDS heads show the tendency to organize themselves along a line. (c) Top-view of the monolayer formed by 14 SDS molecules heads at the water/vacuum interface at 2 ns and (d) at 8 ns of NVT simulation. For clarity in (c) and (d) the aliphatic chains have been removed. A white square highlights the fact that the Na–SO₄ groups tend to form linear structures.

Figure 10

(a) Side-view of a 2 × 1unit cells showing the Crystal graph of the hydrated crystal. The arrows point to the oxygen atoms where the aliphatic chains are bonded. Yellow arrows point to the SO₄^- groups linked to four Na⁺ ions. For clarity the surfactant hydrophobic tails have been removed. Yellow and white straight lines highlight the Na–OSO₃ and Na–OH₂ bonds, respectively. This graph extends to infinity above and below the picture plane. (b) Bond graph of one of the monolayer aggregates within the white oval of Figure 9b. The gray, white, yellow, red, and purple spheres denote de C, H, S, O and Na toms, respectively. The light blue spheres denote the oxygen atoms of the water molecules and the dark blue spheres denote the O atoms of the SO4⁻groups where the hydrophobic tails are linked. In (b) white circles highlight the SO4^- groups bridging the Na atoms.