Collective adsorption of pheromones at the water-air interface
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.
Summary
This paper investigates the collective adsorption behavior of bombykol, a model insect pheromone, at the water-air interface using all-atom molecular dynamics simulations. The research addresses the poorly understood phase behavior of pheromones, which plays a crucial role in chemical communication, and their interaction with aerosol particles. The simulations reveal the molecular organization of the bombykol monolayer, its adsorption isotherm, and the presence of a two-dimensional liquid-gas phase transition. The authors employed a soft-sticky particle equation of state to accurately describe the monolayer's behavior. The key finding is that collective adsorption stabilizes the pheromone molecules at the interface, resulting in a free energy gain of approximately 2 kBT. This gain is influenced by the condensing surface concentration, potentially enhancing pheromone adsorption onto aerosols. The study highlights the importance of phase transition phenomena in understanding pheromone behavior and its implications for molecular interface science, atmospheric chemistry, and organismal chemical communication. This research matters because it provides insights into how pheromones interact with the environment during transport and detection, potentially influencing the efficiency of chemical communication.
Key Insights
- •All-atom molecular dynamics simulations reveal a two-dimensional liquid-gas phase transition in a bombykol monolayer at the water-air interface.
- •A soft-sticky particle equation of state accurately describes the monolayer's behavior, indicating the importance of soft repulsive interactions at high surface concentrations. This is better than the SIAL EOS which models hard disk repulsion.
- •Collective adsorption stabilizes bombykol molecules at the interface, with a calculated free energy gain of approximately 2 kBT.
- •The activity coefficient of bombykol decreases sharply at low surface concentrations due to attraction between molecules, but becomes more or less constant when the molecules are condensed.
- •The condensing surface concentration (ΓC) strongly influences the activity coefficient and, consequently, the adsorption free energy.
- •The interface widens, reflecting an increase in surface fluctuations with decreasing surface tension. This transition is gradual, without a sudden change in behavior.
- •The authors highlight the limitations of the current simulation in modeling dilute surfaces and the potential for different results if phase separation occurs earlier or if crystallization occurs in the surface layer.
Practical Implications
- •The findings have implications for understanding how pheromones are transported through the air and interact with aerosols, which is relevant for developing more effective pest control strategies.
- •This research can inform the design of synthetic pheromone lures that mimic the natural phase behavior of pheromones, thereby improving their efficacy.
- •The insights into molecular interface science can be applied to the development of new materials with tailored surface properties, such as surfactants and coatings.
- •Future research should focus on modeling more complex and realistic interfaces, including chemically heterogeneous aerosols and antennal cuticles, to better understand pheromone behavior in natural environments.
- •Grand Canonical Monte Carlo simulations could be used to more properly address the initiation of the fluid phase.