Chemical Transfer Energetics of the −CH₂− Group:  A Possible Probe for the Solvent Effect on Hydrophobic Hydration and the 3D-Structuredness of Solvents

Abstract

Transfer Gibbs energies ΔG°t, and entropies, ΔS°t, of −CH2− have been evolved in aqueous mixtures of methanol (MeOH), ethanol (EtOH), 2-propanol (2-PrOH), tert-butyl alcohol (t-BuOH), and acetonitrile (ACN) by determining the solubilities of Ag salts of acetate (OAc-), propionate (OPr-), n-butyrate (OBu-), as well as picrate (Pi-) ions from 15 to 35 °C by spectrophotometric measurements. The chemical contributions of these energetics of the ions (i), ΔG°t,ch(i) and TΔS°t,ch(i), at T = 298.15 K have been evolved by subtracting the cavity effect and Born-type and ion−dipole-type electrostatic interaction effects. ΔG°t,ch(i) values of carboxylates (RCOO-) are guided by solvent acidity induced hydrophilic hydration (HlH) of the COO- ion and cosolvent induced hydrophobic hydration (HbH) of the R group and the back-bonding interaction of d10 electrons in the case of Ag+ ion, while TΔS°t,ch(i) values are partly guided by structural effects as well. ΔG°t,ch and TΔS°t,ch values of (−CH2−) are found to be more or less same, indicating their additivity. The increase in ΔG°t,ch (−CH2−)−composition profiles and the “characteristic maximum” of TΔS°t,ch (−CH2−)− composition profiles indicate the effect of increasingly reduced HbH caused by increasing 3D-structure promotion of these alkanols. The decrease in ΔG°t,ch (−CH2−) and the broad minimum in ΔS°t,ch (−CH2−) in aqueous ACN indicate the effect of increased HbH caused by 3D structure breaking of ACN. Thus the chemical transfer energetics and especially entropies of −CH2− reflect not only the solvent effect on HbH but also the 3D-structuredness of aquo-organic cosolvents.

Hydrophobic hydration from solubilities of aliphatic hydrocarbons in water

“…In case of -CH 3 , 3 atoms of H are exposed to solvent water for the formation of buffer bonds [11][12][13][14]. Thus in this case not only 3D structuredness of water is somewhat enhanced but also due to the formation of buffer bonds between H 2 O molecule and H atom, each of the 3H atoms and attached C atom of -CH 3 group become fixed and unmovable, losing a large amount of entropy.…”

“…Moreover, the direct experimental determination of these are also not directly viable [1,2]. On the other hand it is well known [3][4][5][6][7][8][9] that hydrophobic hydration or 'the hydration of the second kind' arises due to the strong attraction or affinity of water molecules for one another. The hydrophobic, amphiphalic or amphiphilic solutes with a polar sites, not being strongly attracted by water, pushes them closer and thus easily induce more water molecules around them with the formation of a cage in a way similar to clathrate hydrates.…”

“…With this aim in mind, the phenomenon here chosen is one that provides plausibly the simplest manifestation of the hydrophobic effect -the meager solubility of hydrocarbons in water. In fact, in most of the previous studies [11][12][13][14][15][16][17][18][19], as found from the literature, H b H has been estimated and understood in association with the dispersion interaction, which is somewhat akin to hydrophobic interaction (HI) and acted as a reversal of H b H. From statistical mechanical consideration, Arieh Ben-Naim [20,21] expressed his doubt regarding the relative importance of the hydrophobic interaction because such interaction arises not due to unique peculiarities of liquid water but to strong solvent-solvent interaction. In some old studies [15,16,[22][23][24] efforts made to understand HI and H b H from a set of data where effect of all three different types of involved interactions namely, solute-solute, solute-solvent and solvent-solvent interactions are intermixed.…”

“…where suffix t indicates the transfer process from standard state of HC to that of water, V i the molar volume of the ith liquid and all other quantities are as usual [11][12][13][14]. Here all the terms of RHS of Eq.…”

Enthalpies of Transfer of the −CH2− Moiety into Aqueous Amide Solvent Systems; Values from n-Alcohol Solutes

“…This suggests that the maximum may be related to the hydrophobicity of the cosolvent. [2][3][4] In the present paper, we report enthalpies of solution of n-alcohols in aqueous mixtures of the amides formamide, N,Ndimethylacetamide (DMA), and N-methylpyrrolidinone (NMPy). These, along with literature data for the aqueous N,N-dimethylformamide (DMF) system, 5 provide ∆ t H(CH 2 ) for a second series of aqueous mixtures with increasing hydrophobicity of the organic cosolvent.…”