Molar heat capacities for (2-methyl-2-butanol + heptane) mixtures and cyclopentanol at temperatures from (284 to 353) K
Introduction
The capability of formation of hydrogen-bonds and the type of the associates formed depend on the molecular configuration. Therefore pentanols and their mixtures with non-polar solvents have been chosen for the study of the influence of the chain structure and the position of the hydroxyl group in the alcohol molecule on their thermodynamic properties. Mixtures of this type are of particular interest from the point of view of the self-association of alkanols, of non-specific physical interactions between real species present in the mixture and of interstitial accommodation of alkane molecules in the alkanol multimer structure. Those features lead to excesses in the following molar quantities: heat capacity, enthalpy, Gibbs free energy, volume, entropy, isentropic compressibility, and internal pressure. All these quantities are strongly dependent on temperature.
The heat capacity is a very sensitive indicator of the structure of pure liquids and solutions. Cerdeiriña et al. [7] pointed out that the association effects are exemplified by the different experimental temperature dependence of Cp for pure associated liquids. Cerdeiriña et al. [8] concluded also that Cp (T) is governed by the association energy of the molecules, their self-association capability and molecular size.
The heat capacity of (alcohol + alkane) mixtures is useful for the understanding of the self-association of alcohol molecules in solutions [7], [9], [10], [11], [12], [13], [14], [15], [16]. Mixtures of alkanols with alkanes are liquid systems most frequently studied under atmospheric pressure. However, excess molar properties are reported mainly for alkan-1-ols with alkanes. Relatively few results of the temperature dependence of excess properties of (branched alkanol + alkane) mixtures, especially of this dependence of excess molar heat capacities, are available in the literature. To the best of our knowledge, the temperature dependence of the excess molar heat capacity has been reported only for (3-methyl-3-pentanol + decane) mixtures. The Cp of the above system over the whole composition range at T = (283.15, 298.15, 313.15, and 323.15) K has been reported by Costas and Patterson [11]. The excess molar heat capacity of three mixtures of (3-methyl-3-pentanol + decane) for the temperature range from (278.15 to 338.15) K in 2.5 K steps has been also reported by Cerdeiriña et al. [15]. Moreover Cerdeiriña et al. [7], [8], [15] proposed a two-state association model which is able to describe temperature dependence of heat capacities for pure alcohols and (alcohol + alkane) mixtures. The excess molar heat capacities of the mixtures under test have been measured at T = 298.15 K by Benson and D’arcy [9] and Tanaka and Toyama [14].
Rappon et al. studied properties of (pentanol isomers + heptane) mixtures such as the Kerr effect [1], viscosities [2], 1HNMR chemical shift of the OH group [3], photochromic reaction probe [4], the electro-dilatometric effect [5], and the betaine dye probe [6]. The results obtained show properties of mixtures of (2-methyl-2-butanol + heptane) different from those of other pentanols. That feature was interpreted by different types of associates existing in those mixtures. Reviewing the interpretation presented by Rappon et al. in all their papers [1], [2], [3], [4], [5], [6], it can be seen that at low concentrations of 2-methyl-2-butanol dominate linear dimers and with increasing concentration cyclic trimers are formed from dimers. The authors assumed that each cyclic trimer in the chair conformation may associate with two neighbouring trimers which leads to a ring stacking mode of association. At high concentrations and for pure 2-methyl-2-butanol, the stacking of several rings results in cylinder-like associates.
If the different types of associates were formed with the changing concentration of alcohol, the heat capacity and/or temperature dependence of Cp, as an indicator of the structure, should be different.
In this context, the work is devoted to the study of (2-methyl-2-butanol + heptane) mixtures. The molar heat capacities and excess molar heat capacities of those mixtures over the whole concentration range at temperatures from (284 to 353) K are reported. This work was aimed mainly at the comparison of the effect of temperature on the heat capacities of the mixtures studied. Furthermore, attempts were made to find out whether the differences in the temperature dependences of the heat capacities of the mixtures studied are consistent with the results obtained by Rappon et al. [1], [2], [3], [4], [5], [6]. The excess molar heat capacities are discussed in terms of the influence of the molecular size and configuration of the alkanols on their self-association capability and of the change in molecular structure of the (2-methyl-2-butanol + heptane) mixtures.
Additionally, the isobaric specific heat capacities of 2-methyl-2-butanol and cyclopentanol were measured over the temperature range from (284 to 368) K and (284 to 353) K, respectively. To the best of our knowledge, the temperature dependence of the molar heat capacities of these liquids has been reported for the temperature range from (267.6 to 347.6) K [17], [36], [39] and from (256.3 to 302.9) K [17], respectively.
Section snippets
Experimental
The 2-methyl-2-butanol from Aldrich, minimum 0.99 mass fraction purity C2H5C(CH3)2OH and cyclopentanol from Fluka minimum 0.99 mass fraction purity C5H9OH, were dried over molecular sieves 0.3 nm. Heptane from POCh (Polish Chemicals), minimum 0.99 mass fraction purity C7H16, was used without further purification. The mass fraction of water, determined by the Karl Fischer method, was less than 2 · 10-4 for 2-methyl-2-butanol and 1 · 10−4 for cyclopentanol.
The purities of the chemicals were tested
Results and discussion
The specific isobaric heat capacities of pure cyclopentanol and the (2-methyl-2-butanol + heptane) mixtures were measured over the whole composition range at atmospheric pressure and at temperatures from (284 to 353) K in about 0.02 K steps, while the specific isobaric heat capacities of pure 2-methyl-2-butanol were measured over the (284 to 368) K temperature interval. Thus ca. 3180 experimental points are given for each liquid, and ca. 4070 for 2-methyl-2-butanol. Therefore the values of the molar
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