Enthalpies of Inclusion of Methanol, 1-Propanol, and 1-Pentanol into α-Cyclodextrin in Aqueous Solution at 298.15 K

Abstract

Elucidating the role of asymmetric intermolecular interactions due to the stereospecific structure of a molecule is really important for understanding the mechanisms of reactions in chemisty and biochemistry. One of the present authors measured the enthalpies of mixing aqueous solutions of some optical isomers. Present authors report here the experimental results of microcalorimetry on enthalpy of dilution Q_d11 of aqueous solution of a-cyclodextrin (α-CyD), enthalpy of mixing Q_mix for aqueous ce-CyD solution with aqueous alcoholic solution, and enthalpy of transfer Δ H_tr of alcohol from aqueous to aqueous α-CyD solutions. They are listed in Tables 1 and 3. The equations and parameters used for the least-squares analysis are given in Eqs. (6 ), (7 ), and (10), and Tables 2 and 4 where the standard deviations are also given. Graphical illustrations of them are shown in Figs.1 and 2.
The calorimeter (RMC-II) and detailed procedures used were reported elsewhere 8). The measurements were carried out at 298.15±0.0005 K. The purities of alcohols obtained by GLC were 99.98, 99.95, and 99.84 mass per cent for methanol, 1-propanol, and 1-pentanol, respectively.
The theoretical procedure used for obtaining the enthalpy of transfer is given in Eqs. (1)- (4 ). From these values enthalpies of inclusion were determined as shown in Table 5. The ratio y of alcohols included by α-CyD, defined by Eq. (14), was estimated by the mathematical treatment presented by the authors in section 4.3, where n? n2, n8i and n, mean the amounts of water, α-CyD, alcohol, and 1: 1 complex in the solution, respectively. Since z was fixed to ca.1300 or 5000 in the experiment, Fig.3 was obtained through Eq. (21). Using the slopes of the curves in Fig.4 at f=0.55, we drew the curves in Fig.5. If the assumptions used in the treatment are valid approximately, (100/4Htr, ma. x) (d 4Htr/d f) would be equivalent to (100/Δ H_tr, max) (dΔ H_{tr, MAX}) is the limiting value of Δ H_tr at f→0 as shown in Fig.2. The assumptions involved are (1) all the activity coefficients are unity and (2) the inclusion complexes present in the solution are only of 1: 1 type. We used the values of (100/4Htr, max) (Δ H_tr/d f) to get the values of log₁₀ K and then to obtain Y_MAX through Eq. (20). The main results are summarized in Table 5.1-Pentanol molecules fit into the cavities of α-CyD best as expected. Methanol molecules, however, seem not to distinguish energetically between their circumstances in the cavities of α-CyD and bulk water, because they have the "structure-making" methyl groups only as hydrophobic radicals. Furthurmore we found that the enthalpy of dilution of aqueous α-CyD solution is positive which means that the interactions between α-CyD molecules are energetically more favorable than those between α-CyD and water molecules.