Amphiphilic Organic Ion Pairs in Solution: A Theoretical Study

Abstract

The macroscopic manifestation of hydrophobic interactions for amphiphilic organic ion pairs (tetraalkylammonium-anion) has been shown experimentally by measuring their association constants and their affinity with the organic phase. Beyond a certain size, there is a direct relation between association constants and chain lengths in tetraalkylammonium ions. We propose to cast a bridge between these results and geometrical properties considered at the level of a single ion pair by means of quantum chemistry calculations performed on model systems: trimethylalkylammonium-pentyl sulfate instead of tetraalkylammonium-dodecyl sulfate. Two limiting cases are considered: head-to-head configurations, which yield an optimal electrostatic interaction between polar heads, and parallel configurations with a balance between electrostatic and hydrophobic interactions. All properties (geometries, complexation energies, and atomic charges) were obtained at the MP2 level of calculation, with water described by a continuum model (CPCM). Dispersion forces link hydrocarbon chains of tetraalkylammonium ions and pentyl sulfate, thus yielding (for the largest ion pairs) parallel configurations favored with respect to head-to-head geometries by solute-solvent electrostatic interactions. Given the small experimental association energies, we probe the accuracy limit of the MP2 and CPCM methods. However, clear trends are obtained as a function of chain length, which agree with the experimental observations. The calculated monotonic stabilization of ion pairs when the hydrocarbon chain increases in length is discussed in terms of electrostatic interactions (between ions and between ion pairs and water), dispersion forces, and cavitation energies.

Dimeric Gold Bis(carbene) Complexes by Transmetalation in Water

“…As can be seen in Figure , each Na + cation makes a bridge between two single units in the solid state, with an optimal Na + –( − OOC) interaction and a secondary Na + – – O(OC) one, not accounted for in BNa 2 . Such an electrostatic effect will be drastically lowered in strongly polar solvents . This is confirmed by PCM calculations, which show the relative stability of ANa 2 in water by only Δ E = 19 kcal mol –1 with respect to BNa 2 , without significant changes in the geometries.…”

“…Such an electrostatic effect will be drastically lowered in strongly polar solvents. 20 This is confirmed by PCM calculations, which show the relative stability of ANa 2 in water by only ΔE = 19 kcal mol −1 with respect to BNa 2 , without significant changes in the geometries. The polarizable continuum surroundings involves a small contraction of the structure, which manifests itself in the shortening of the (Au 2 ) 2+ equilibrium distances by 0.15 and 0.05 Å in A and B, respectively.…”

Cationic Porphyrin–Anionic Surfactant Mixtures for the Promotion of Self‐Organized 1:4 Ion Pairs in Water with Strong Aggregation Properties

“…Theoretical calculations were performed to obtain a molecular view of the monomeric ion pair in water by using a polarized continuum model of solvent. According to previous reports on the association of ion pairs in water and the results of NMR experiments, the four surfactants were placed below the porphyrin ring (Figure ). The sulfate polar heads were positioned between the meso and the pyrrole carbons of the porphyrin (Figure ).…”

“…Therefore, our strategy was based on the use of a tetracationic porphyrin as a building block for the aggregates by forming an amphiphilic porphyrin with four long hydrophobic chain anionic counterions in the first step and promoting the formation of higher order aggregates in the second step. According to the literature on “catanionic” surfactants and smaller amphiphilic ions pairs in solutions, the interactions in water between the porphyrin entity and amphiphilic counterions should be strong enough, due to four electrostatic interactions, to guide the hydrophobic interactions between the associated long alkyl chains, which act as the cement of the molecular organization.…”

Influence of a Neighboring Charged Group on Hydrophobic Hydration Shell Structure

“…Interactions between hydrophilic and hydrophobic groups in water, at both macroscopic interfaces − and molecular hydration shells, − are of importance in many biochemical, environmental, and industrial processes. Such interactions may result from both direct charge delocalization and indirect water-mediated mechanisms that shift the delicate balance of electrostatic and dispersion (van der Waals) forces, ,,, and thus influence chemical processes ranging from ligand binding and ion pairing, to the structure and dynamics of aqueous interfaces. , Here we describe experiments, as well as quantum and classical calculations, designed to quantify the length scale over which both neutral and charged hydrophilic groups perturb the structure of water in the hydration shells of neighboring aliphatic chains, in aqueous carboxylic acid and tetraalkylammonium solutions.…”