Molecular dynamics study of spherical aggregates of chain molecules at different degrees of hydrophilicity in water solution

We present molecular dynamics simulations of three spherical aggregates con stituted of the same number of monomeric chains of increasing hydrophilic c haracter in water solution. The three different chains are dodecane, CH3(CH 2)(10)CH3; dodecan-1-ol, CH3(CH2)(10)CH2OH; and oligoethylenoxide, CH3(CH2) (11)(OCH2CH2)(3)OH, and the systems are denoted C-12, C12E0, and C12E3, res pectively. The three solutions are simulated at the same temperature and pr essure and at about the same concentration in weight of the solute. We inve stigate the structural changes of the aggregate and the conformational chan ges of its chains after increasing the hydrophilicity of the monomers. In t he C12 system, the density of the aggregate is higher than the density of t he pure hydrocarbon liquid in the same thermodynamic conditions. Dodecane c hains are quite rigid and mostly in the all-trans conformation. This is an effect of the strong hydrophobic repulsion exerted on the aggregate by the water. A density depletion develops at the interface between the oil core a nd the solvent because of the mismatch between the two components. This ext ra pressure is gradually released after increasing the hydrophilicity of th e monomers. In the C12E3 system, the density depletion at the interface is completely canceled, and the system is homogeneous through the interface. I n this system, the interfacial regions appear to be divided into an inner p art, where methylene groups, oxyethylene groups, and water molecules are pr esent, and into an outer region formed by a mixture of oxyethylene groups a nd water only. The local density of oxyethylene groups in the interfacial r egion is strongly fluctuating, and its histogram shows an effective attract ion among those groups. Thus, a considerable portion of the interfacial vol ume is filled by the solvent only. We observe a strong tendency for the OCC O dihedrals in the hydrophilic tails to be in the gauche state, and at the same time, we observe the presence of H-bond bridges that stabilize this st ructure.