Understanding Solute-Hydrotrope Aggregation in Aqueous Solutions: A Molecular Dynamics Approach

Hydrotropy is a phenomenon where an amphiphilic molecule (i.e., the hydrotrope) is able to enhance the aqueous solubility of a hydrophobic solute. Understanding the molecular mechanisms behind this phenomenon is crucial to designing new hydrotropes aimed at enhancing the aqueous solubility of specific target solutes. This study investigates the hydrotropic behavior of 1,2-alkanediols in enhancing the aqueous solubility of syringic acid using molecular dynamics (MD) simulations. The analysis carried out here employs several computational methods, including Kirkwood-Buff integrals, solvation free energies, radial distribution functions, and hydrogen bonding number. The solvation free energy results reported in this work help explain the thermodynamic favorability of syringic acid solubilization in the presence of 1,2-alkanediols, aligning with experimental trends. In addition, MD simulations reveal a pronounced affinity between syringic acid and 1,2-alkanediols, particularly at low hydrotrope concentrations. This high affinity is driven by the alkyl chain of each hydrotrope when water is the main solvent, resulting in an increase in the solubility of the solute as the length of the hydrotrope alkyl chain increases. However, a shift in the solubilization mechanism is seen when water is no longer the main solvent, with the hydrogen bonding capabilities of the hydrotrope playing a larger role than its alkyl chains. Under low water concentration conditions, longer alkyl chains in the hydrotrope have difficulty forming hydrogen bonds, leading to an opposite trend compared to lower hydrotrope concentrations. This different behavior with composition results in a maximum solubility for systems with long alkyl chains at intermediate hydrotrope concentrations.

Temperature Dependence of Hydrotropy

The solubility of hydrophobic solutes increases dramatically with the temperature when hydrotropes are added to water. In this paper, the mechanism of this well-known observation will be explained via statistical thermodynamics through (i) enhanced enthalpy-hydrotrope number correlation locally (around the solute) that promotes the temperature dependence and (ii) hydrotrope self-association in the bulk solution that suppresses the temperature dependence. The contribution from (i), demonstrated to be dominant for urea as a hydrotrope, signifies the weakening of interaction energies around the solute (local) than in the bulk that accompanies incoming hydrotrope molecules. Thus, studying hydrotropic solubilization along the temperature and hydrotrope concentration provides complementary information on the local-bulk difference: the local accumulation of hydrotropes around the solute, driven by the enhanced local hydrotrope self-association, is also accompanied by the overall local weakening of energetic interactions, reflecting the fluctuational nature of hydrotrope association and the mediating role of water molecules.

Progress in the field of hydrotropy: mechanism, applications and green concepts

Sustainability and greenness are the concepts of growing interest in the area of research as well as industries. One of the frequently encountered challenges faced in research and industrial fields is the solubility of the hydrophobic compound. Conventionally organic solvents are used in various applications; however, their contribution to environmental pollution, the huge energy requirement for separation and higher consumption lead to unsustainable practice. We require solvents that curtail the usage of hazardous material, increase the competency of mass and energy and embrace the concept of recyclability or renewability. Hydrotropy is one of the approaches for fulfilling these requirements. The phenomenon of solubilizing hydrophobic compound using hydrotrope is termed hydrotropy. Researchers of various fields are attracted to hydrotropy due to its unique physicochemical properties. In this review article, fundamentals about hydrotropes and various mechanisms involved in hydrotropy have been discussed. Hydrotropes are widely used in separation, heterogeneous chemical reactions, natural product extraction and pharmaceuticals. Applications of hydrotropes in these fields are discussed at length. We have examined the significant outcomes and correlated them with green engineering and green chemistry principles, which could give an overall picture of hydrotropy as a green and sustainable approach for the above applications.