(590b) Incorporation of Proton Hopping Mechanism into Dissipative Particle Dynamics

We suggest an approach that incorporates proton dissociation and diffusion in aqueous solutions into dissipative particle dynamics (DPD). Since standard DPD potentials are too simplistic for detailed description of proton transfer, we introduce proton as a separate particle that interacts with bases via the Morse potential, which effectively allows for dissociation of the proton-base pair. Bases are modelled as charged or neutral beads that describe groups which are able to protonate, such as acid anions, water, ammonia etc. The Morse potential is cut and shifted at cut-off distance and thus has a limited range. Therefore, the proton does not entirely “belong” to a particular base, but rather interacts via modified Morse potential with overlapping bases, which makes its stand-alone existence apart from a “base bead” extremely improbable. If a proton interacts with more than one base at the same time, it forms an intermediate complex that can dissociate via  different pathways. As a result, the proton bead is transferred from one base to another in the course of simulation. We demonstrate the capabilities of our approach on several phenomena of different complexity.

1. Proton diffusion in solutions of strong acids. Our model of proton transfer artificially mimics the Grotthius proton hopping mechanism. By varying the Morse parameters, we are able to reproduce the experimental self-diffusion coefficient of proton (with water self-diffusion coefficient serving as a reference) for different bead sizes that include one to four water molecules. If one DPD bead represents one water molecule, the model is able to mimic the local dynamics, such as hopping frequency and characteristic time.
2. Acid dissociation in aqueous solution. By adjusting the Morse parameters, we model dissociation of benzenesulfonic acid, which is a fragment of sulfonated polystyrene proton-exchange ionomers.
3. Phase behavior of ionic surfactant in aqueous solution. As a characteristic example, we consider an aminooxide type of surfactant that can be protonated at low pH. By reducing the concentration of protons we cause a decrease in critical micelle concentration, which results in a partial precipitation of the surfactant,  qualitatively reproducing the experimental behavior.

The approach is applicable in mesoscale modeling of various systems where proton transfer plays an important role, such as fuel cell membranes. We implemented our model in DL_MESO, which is an efficient and widely available software for DPD simulations. This work is funded by NSF grant number 1207239.