Microhydration of sorbitan monostearate (Span 60) investigated using hybrid QM/MM calculations in the gas phase

Abstract

The binding interactions and structural and electronic properties involved in microhydration of Span 60•(H2O)n (n = 1–10) complexes were investigated using hybrid QM/MM calculations in the gas phase. The Span 60 head group and its hydrating water molecules were treated using the QM method, while the Span 60 tail group was treated with the MM method. For the monohydration complex, it was revealed that the hydration energy and binding force are strongly dependent on the hydration sites (O1–O6) of the Span 60 head group. The ether oxygen (O4) exhibited the highest hydration energy and the strongest hydrogen bonds. An ester oxygen atom (O2) showed a lower binding force due to its delocalized lone pair electrons. For polyhydration complexes, it was found that the average hydration energy of each polyhydration complex is close to that of the monohydrated form because of steric hindrance and electrostatic repulsions between water molecules. Interaction between the ether oxygen (O4) and water molecules is still prominent for all polyhydration complexes. The charge transfer phenomenon is noticeably altered when a few water molecules bind with a particular oxygen atom of the Span 60 head group, leading to an increase in the polarity of this group. Hydrogen bond lengths tend to decrease slightly and the conformation change of the Span 60 head group is significantly affected as the number of hydrated water molecules increases.