2.1. DMPH Molecule

In the present study, dimethyl phosphoric acid (DMPH) is used as a model molecule. DMPH contains chemical moieties (atom types) present in the hydrophilic part of phospholipids (Figure ​Figure11A). Moreover, ionic liquids containing dimethyl phosphoric acid is considered as highly efficient in petrochemistry because of their high dissolution capacity.¹⁷ DMPH is a small, water-soluble organic molecule that belongs to the class of dialkyl phosphates. There are two protonation states available for dimethyl phosphate molecule, in which phosphate oxygen is protonated (DMPH) or not (DMP^–). For DMPH, the experimentally measured pK_a of 1.29¹⁸ indicates that it mainly occurs in its anionic form. However, it is worth to mention that the experimental studies¹⁸ revealed dependence between pK_a and length of alkyl substituents in dialkyl phosphates, which increases the probability of appearance of the protonated form of the phosphate moiety in phospholipids or ionic liquids. We tested our procedure on the protonated form of dimethyl phosphoric acid, because for such a state, according to FF parametrization philosophy,¹⁹ we are able to check the influence of the refined parameters on the condensed phase properties of DMPH—known from experimental studies, such as density and enthalpy of vaporization. Reproduction of these properties strongly depends on the proper description of intermolecular interactions.

The DMPH molecule consists of 14 atoms, which can be grouped into following seven atom types: HC, CT, OS, P, O2, OH, and HO (Figure ​Figure11B). Within the DMPH molecule, there are six types of dihedral angles, which are described by the atom types that compose them (Table.1).