Molecular-Scale Description of SPAN80 Desorption from a Squalane-Water Interface

Area of Science:

• Physical Chemistry
• Interface Science
• Molecular Dynamics Simulations

Background:

• Understanding interfacial behavior is crucial for applications like emulsions and drug delivery.
• Sorbitan monooleate (SPAN80) is a non-ionic surfactant commonly used in various formulations.
• Molecular dynamics simulations provide atomistic insights into interfacial phenomena.

Purpose of the Study:

• To investigate the effect of sorbitan monooleate (SPAN80) loading on the water-squalane interface.
• To analyze surface tension, desorption free-energy profiles, and escape times of SPAN80.
• To evaluate the diffusional behavior of SPAN80 at the interface.

Main Methods:

• All-atom molecular dynamics (MD) simulations at T = 300 K.
• Analysis of surface tension equation of state.
• Calculation of desorption free-energy profiles and escape times.
• Modeling of SPAN80 headgroup motion using a diffusional model.

Main Results:

• Loading of SPAN80 has a weak effect on its accommodation at the water-squalane interface.
• Surface tension and desorption free-energy profiles exhibit minimal dependence on SPAN80 loading.
• Perpendicular SPAN80 headgroup motion is well-described by diffusion near the free-energy minimum.
• Escape times: 7 × 10⁻² s into squalane, 3 × 10² h into water.

Conclusions:

• SPAN80's interfacial behavior is robust across a range of loadings.
• The calculated water escape time aligns with experimental observations.
• MD simulations offer valuable predictions for surfactant interfacial dynamics.