Speaker
Description
Surfactant Adsorption at Interface: An Atomistic Free Energy Approach
Vid Pograjc¹² Matej Kanduč¹
¹ Department of Theoretical Physics, Jožef Stefan Institute, Ljubljana, Slovenia
² Faculty of Mathematics and Physics, University of Ljubljana, Slovenia
Surfactants are amphiphilic molecules that spontaneously adsorb and self-organize at aqueous interfaces, a process central to numerous technological and biological applications ranging from detergency and emulsification to drug delivery and foam stabilization. Their interfacial behavior is governed by the balance between hydrophilic and hydrophobic interactions, which also determines the timescale on which individual molecules exchange between the interface and the bulk solution. The exchange time grows exponentially with alkyl chain length, increasing by a factor of 2–3 per CH₂ group, spanning many orders of magnitude across different surfactant classes.
Short-chain surfactants (≤C8) exchange on nanosecond timescales, allowing adsorption equilibria to be directly monitored in all-atom MD simulations. For intermediate-chain surfactants (C9–C18), which constitute the most industrially prevalent class, exchange times range from microseconds to seconds, far beyond what atomistic MD can directly access, creating a fundamental gap between simulation capabilities and the most practically relevant surfactants.
To overcome this, we employ a molecular thermodynamic theory (MTT) framework enforcing equal chemical potentials across coexisting phases. As input, transfer free energies — defined as the free energy cost of moving a surfactant molecule from the interface into bulk water — are computed via thermodynamic integration in all-atom MD simulations, from which adsorption and pressure isotherms are derived. The approach is validated using a C6 surfactant, for which adsorption is computed both from direct equilibrium MD simulations and from the MTT framework.