Collective adsorption of pheromones at the water-air interface

TL;DR

This work tackles how amphiphilic pheromones behave at the air–water interface, using all-atom MD of bombykol to map its monolayer formation and phase behavior. By computing surface tension across varying surface coverages and applying Gibbs-based thermodynamics, the authors extract adsorption isotherms and test two interfacial EOS models (SIAL and soft-sticky) together with a Maxwell construction to describe a 2D gas–liquid transition. The results show a gradual reorientation and clustering of bombykol at increasing coverage, with condensing and vaporisation coverages at roughly Gamma_C ≈ 0.24 nm^-2 and Gamma_V ≈ 1.2 nm^-2, and an adsorption free-energy gain of about $2 k_B T$ per molecule from lateral interactions. While this energy gain is not sufficient to drive strong adsorption on pristine aqueous aerosols under typical conditions, the study highlights how phase transitions and surface heterogeneity could enable collective adsorption relevant to atmospheric transport and chemical communication.

Abstract

Understanding the phase behaviour of pheromones and other messaging molecules remains a significant and largely unexplored challenge, even though it plays a central role in chemical communication. Here, we present all-atom molecular dynamics simulations to investigate the behavior of bombykol, a model insect pheromone, adsorbed at the water-air interface. This system serves as a proxy for studying the amphiphilic nature of pheromones and their interactions with aerosol particles in the atmosphere. Our simulations reveal the molecular organization of the bombykol monolayer and its adsorption isotherm. A soft-sticky particle equation of state accurately describes the monolayer's behavior. The analysis uncovers a two-dimensional liquid-gas phase transition within the monolayer. Collective adsorption stabilises the molecules at the interface and the calculated free energy gain is approximately $2\:k_\mathrm{B}T$. This value increases under lower estimates of the condensing surface concentration, thereby enhancing pheromone adsorption onto aerosols. Overall, our findings hold broad relevance for molecular interface science, atmospheric chemistry, and organismal chemical communication, particularly in highlighting the critical role of phase transition phenomena.

Figures (5)

• Figure 1: Illustration of the simulated system of bombykol molecules at the air water interface. OH is the oxygen atom of the alcohol head group of the bombykol, C8 is the carbon atom at the center of its hydrophobic chain and C16 is its terminal carbon atom. In molecular dynamics snapshots, saturated carbons atoms are represented in black and unsaturated carbons in grey. Oxygens and hydrogens of the alcohol group are drawn in red and in white respectively. Oxygens and hydrogens of the water molecules are respectively represented in dark and light blue.
• Figure 2: Potential of interaction of two bombykol molecules at the water-air interface calculated for different sites of the molecule represented in figure 1: medium carbon (C8), oxygen of the hydroxyl head group (OH), terminal carbon (C16) and center of mass (COM). The dashed line represents the fitted function equation \ref{['eq:fitw2']}. It is calculated for different points of the bombykol molecules represented in Fig. \ref{['fig:box']}. COM is the center of mass.
• Figure 3: Snapshot, surface tension (isotherm), histogram in $z$ and pair correlation functions $g_r$ of several sites in bombykols molecules. For the surface tension plot, the solid curves correspond to the overall fits, while the dashed curves correspond to the two EOS, extended for $\Gamma_a>1.4\:$nm$^{-2}$.
• Figure 4: Average z position of the bombykol atoms as a function of the surface concentration. The bombykol molecules stands more and more vertical as the surface is densily covered.
• Figure 5: Chemical potential (left) and activity coefficient (right) in $k_\mathrm{B}T$ unit of a monolayer of bombykol as a function of the surface concentration $\Gamma_a$. Since the chemical potential is only known to within an additive constant, the standard term has been removed: $\Delta \mu_a = \mu_a - \mu_a(\Gamma_a=\frac{1}{L_x^2})$ and $\Delta \mu_a^{\mathrm{ideal}} = k_\mathrm{B}T \ln \left( \Gamma_a L_x^2 \right)$