PHYSICAL CHEMISTRY

N. A. AGAEV, I. F. GOLUBEV

VISCOSITY OF n-HEXANE IN THE LIQUID AND GASEOUS STATES AT HIGH PRESSURES AND VARIOUS TEMPERATURES

(Presented by Academician V. A. Kirillin on 20 III 1963)

In the previous article (¹) the layout of the apparatus, the construction of the instrument—the viscosimeter—and the experimental procedure were described. The results of measurements of the viscosity of n-pentane at high pressures and various temperatures were also given there.

In the present article we give the results of measurements of the viscosity of n-hexane over the temperature range from 15 to 275° and pressures from 1 to 500 atm, which were carried out on the same apparatus; however, the glass viscosimeter had somewhat different geometrical dimensions, namely: capillary radius 0.005312 cm; volume of the measuring bulb 0.5775 cm³; volume of the preliminary bulb 0.6711 cm³; capillary length 5.44 cm; difference of mercury levels at the beginning of outflow 7.48 cm, at the end of outflow 5.09 cm; mean height of the mercury column in the viscosimeter during outflow 10.31 cm.

Fig. 1. Viscosity of liquid (a) and gaseous (b) n-hexane at atmospheric pressure and various temperatures according to data of various investigators. The numbers in the figure correspond to the numbers of the cited literature. 1 — data of the authors

There is a very limited number of works in the literature on the viscosity of n-hexane, both for the liquid and for the gaseous state, especially at high pressures. The viscosity of gaseous n-hexane at atmospheric pressure was determined by the capillary method by Titani (²) over the temperature range from 120.7 to 306.6° and by Graven and Lambert (³) by the oscillating-pendulum method over the temperature range 35–77.8°.

The viscosity of liquid n-hexane at atmospheric pressure was measured by the capillary method by Shepard, Henne, and Midgley (⁴) at 25°, by Gartenmeister (⁵) at 20°, by Geist and Cannon (⁶) at 0, 20, and 40°, and by Gillam and Drickamer (⁷) over the range from −98.5 to +20°. It may also be noted

and the earlier work of Thorpe and Rodger ($^8$), carried out by the capillary method in the range from 0.8 to 63.6°.

A comparison of the data on the viscosity of $n$-hexane by the investigators listed with the data obtained in the present work is presented in Fig. 1, from which good agreement is seen for the liquid and gaseous states. At high pressures the viscosity of $n$-hexane was measured by Bridgman ($^9$) by the falling-weight method at 30 and 75° and pressures from 1 to 12,000 kg/cm². Khalilov ($^{10}$), using the capillary method, measured the viscosity of $n$-hexane on the saturation line from 20 to 220°.

For our investigation we used $n$-hexane from the Novocherkassk Synthetic Products Plant, which had the following characteristics: specific gravity $d^{20} = 0.6595$ g/cm³, refractive index $n_D^{20} = 1.3752$, and boiling temperature $t_{760} = 69.00$. This technical $n$-hexane was subjected to additional purification. First it was treated with sulfuric acid to remove impurities of unsaturated hydrocarbons, a small content of which was detected by chromatographic analysis.

Further, the $n$-hexane was distilled on a rectification column, with the middle fraction being collected. After purification the $n$-hexane had the following characteristics: specific gravity $d^{20} = 0.6592$ g/cm³, refractive index $n_D^{20} = 1.3749$, and boiling temperature $t_{760} = 68.88^\circ$. According to the most reliable literature data ($^{11,12}$), for pure $n$-hexane $d^{20} = 0.66937$ g/cm³, $n_D^{20} = 1.37486$, $t_{760} = 68.7^\circ$. As can be seen, there is good agreement. Chromatographic analysis of the product showed a content of $n$-hexane of 99.8% by weight.

Fig. 2. Viscosity of $n$-hexane as a function of temperature at constant pressures

The results of our measurements of the viscosity of $n$-hexane are collected in Table 1. The experimental data were graphically smoothed, as a result of which Table 2 was compiled for the coefficients of dynamic viscosity at constant values of temperature and pressure, and also Table 3 for values on the saturation line. For greater clarity, the general picture of the dependence of viscosity

Table 2

Viscosity \((10^{-7}\ \mathrm{g}\cdot\mathrm{cm}^{-1}\cdot\mathrm{s}^{-1})\) of \(n\)-hexane (smoothed values)

Fig. 3. Dependence of \((\eta_{pT}-\eta_T)\) on density for \(n\)-hexane.

Table 3

Viscosity of n-hexane on the saturation line \((10^{-7}\ \mathrm{g\cdot cm^{-1}\cdot sec^{-1}})\)

The dependence of the viscosity of n-hexane on temperature at constant pressures is shown in Fig. 2.

The error of the experimental data, determined by us on the basis of the general theory of errors, is estimated as \(\pm 1.0\%\) for the liquid state, \(\pm 2.0\%\) for the gaseous state, and \(\pm 3.0\%\) near the critical point.

Figure 3 presents the dependence of the viscosity of n-hexane on density in the coordinates \((\eta_{p,T}-\eta_T)\)—\(\rho\), where \(\eta_{p,T}\) is the viscosity at the given pressure and temperature, and \(\eta_T\) is the viscosity at the same temperature and atmospheric pressure.

The density values were taken from works \(^{(13-16)}\). As is seen from Fig. 3, all the experimental data on the viscosity of n-hexane as a function of density for the temperature range from 25 to 275° and pressures from 1 to 500 atm are well arranged on a single common curve.

Khalilov’s data over the entire temperature interval investigated by him agree satisfactorily with our data; the maximum discrepancy at \(t = 170^\circ\) is 3%. Bridgman’s data agree well with ours at temperatures of 30 and 75° and pressures up to 500 kg/cm². The discrepancy between these data does not exceed 1%.

By the capillary method, the viscosity of n-hexane was measured in the liquid and gaseous states at temperatures from 15 to 275° and pressures from 1 to 500 atm. Values of the experimental and smoothed coefficients of dynamic viscosity are given, as well as graphs of the dependence of viscosity on temperature, pressure, and density.

State Scientific-Research
and Design Institute of the Nitrogen Industry
and Products of Organic Synthesis

Received
19 III 1963

Institute of Power Engineering
Academy of Sciences of the Azerbaijan SSR

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