Application of a new theoretical procedure for calculating Kirkwood correlation factors in alkanol + hexane and alkanol + pentane mixtures

Abstract

A recently developed statistical mechanical model for calculating Kirkwood correlation factors gK in self associating liquids and liquid mixtures has been applied for the simultaneous description of gK, the molar enthalpy of mixing HEM and the infrared absorption of monomer alcoholic species as function of the composition in alkanol + hexane and alkanol + pentane mixtures (alkanol: butan-1-ol, pentan-1-ol, hexan-1-ol, heptan-1-ol, sec-butanol, tert-butanol). The majority of parameters involved into the theory are obtained by independent quantum mechanical ab initio calculations of molecular clusters consisting of up to four alcohol molecules. As a consequence only two parameters have to be adjusted freely to each binary system, a third parameter responsible for the non specific intermolecular dispersion interaction has been adjusted within a limited range of possible values given by physical arguments. Excellent agreement between theory and experimental data of gK, HEM and IR absorbance is obtained covering the whole range of concentration including the temperature dependence of these properties without adjusting further parameters. The Kirkwood correlation factor gK turns out to be a sensitive response to peculiarities of the molecular structure of hydrogen bonded systems in the condensed liquid state. The successful application of the theoretical model opens a new way of a deeper and more reliable understanding of such liquid structures.

Speed of sound, density, and heat capacity for (2-methyl-2-butanol+heptane) at pressures up to 100MPa and temperatures from (293 to 318)K. Experimental results and theoretical investigations

“…The results of ab initio calculations point to energetically favourable hydrogen bonding formation for 2-methyl-2-propanol than for ethanol i.e. the Hbonding enthalpies are shifted to more negative values [10,11,31]. This probably explains the differences noted above between (2-methyl-2-butanol + heptane) and (ethanol + heptane) at higher temperatures.…”

“…it seems too high at T = 318.15 K. As mentioned above, the existence of linear associates is assumed in the ERAS model applied in this work. Thus, the ability of the model ERAS to provide a good description of small negative values of C E p at low alcohol concentration (x 1 < 0.02) and a worse description at higher concentration of 2-methyl-2-butanol (especially in the concentration range from x 1 % 0.02 to 0.35) can be explained by comparison with the concentration dependence of the Kirkwood correlation factor, g k , of the (2-methyl-2-propanol + hexane) system [10]. This system seems to be the most alike for the mixtures under investigation.…”

“…This probably explains the differences noted above between (2-methyl-2-butanol + heptane) and (ethanol + heptane) at higher temperatures. Moreover, K c4 , estimated by ab initio studies, is higher for 2-methyl-2-propanol than for ethanol [10,11]. In the ERAS model, applied in this work, the existence of only linear association is assumed.…”

“…The minimum with g k < 1 can be interpreted as a result of the existence of cyclic associates which are assumed to have no dipole moment. Recently, the ERAS model was modified taking into account the cooperative effect of the linear association of alcohol molecules and existence of cyclic tetramers, and applied for the simultaneous description of Kirkwood correlation factor, the excess molar enthalpy and the infrared absorption of monomer alcoholic species as function of the composition in (alkanol + alkane) under atmospheric pressure [10,11]. The ERAS model was also modified, taking into consideration the existence of linear and cyclic structures, in order to describe particularly temperature dependence of excess molar heat capacity under atmospheric pressure [35].…”

“…However, it should be pointed out that the hydroxyl group of the 2-methyl-2-butanol becomes increasingly hindered which effectively reduces the alcohol self-association capability. Moreover, for (alcohol + alkane) systems, including also tertiary alcohols, the results of ab initio calculations show that cyclic trimers are not particularly stable due to the unfavourable steric situation [10,11]. Moreover cyclic associates with more than six molecules are unlikely exist for entropic reasons.…”