The two non-polar hydrocarbons, PFD and
n-hexane, have an atmospheric boiling point of 415 K and 342 K, respectively. The density and dynamic viscosity of neat PFD (1.917 g·cm
−3 and 5.412 mPa·s, respectively) and neat
n-hexane (0.6593 g·cm
−3 and 0.300 mPa·s, respectively) at 298 K are reported in the literature [
17,
18]. These reported values are in good agreement with our measured values (
vide infra). The slight disparities in the values may be attributed to the differences in the instrumentation used, the purity of the chemicals, and the source of the compounds.
3.1. Density of PFD and (PFD + n-Hexane) Mixtures
Experimentally measured densities of PFD and (PFD +
n-hexane) mixtures as a function of temperature in the range (293 to 333) K at select compositions are reported in
Table 1. As expected, with increase in temperature, the densities of PFD,
n-hexane, and their mixtures were found to decrease primarily due to thermal expansion and follow a linear dependence according to the equation:
where
ρ/g·cm
−3 is the density of (PFD +
n-hexane) mixtures. The values of the parameters
(representing density at
T = 0 K) and the slope
a along with the standard deviation of the fits are listed in
Table 2 (measured densities of (PFD +
n-hexane) mixtures along with the fits to a linear expression are presented in
Figure 2).
A careful examination of the density data presented in
Table 1 and
Table 2, along with
Figure 2, indicates the density of PFD to be not only higher than that of water but also that it is significantly higher than that of
n-hexane (almost 3-fold) at all temperatures. It is inferred that biphasic aqueous extractions using PFD would have PFD as the lower phase and water as the higher phase, as opposed to several organic non-polar solvents that have densities lower than that of water. It is also interesting to note that the density of the PFD is much more sensitive to temperature variation compared to the density of
n-hexane (the slope of
vs.
T is −2.4 (± 0.0
5) × 10
−3 g·cm
3·K
−1 for PFD as opposed to only −0.9 (± 0.0
1) × 10
−3 g·cm
3·K
−1 for
n-hexane). Such a high sensitivity of density on temperature for PFD may find uses in several industrial applications and processes and also as temperature sensors based on physical property changes [
19].
As expected, the density of
n-hexane increases monotonically as PFD is gradually added to it (
Figure 3). The increase in density with increasing PFD mole fraction in the mixture is not linear, and it rather shows a downward curvature and best fits a quadratic expression:
where
xPFD is the mole fraction of PFD in the (PFD +
n-hexane) mixtures, and values of parameters
0,xPFD,
b, and
c are listed in
Table 3, while the fits are represented with dark curves in
Figure 3. Quadratic dependence of the density on PFD mole fraction of the (PFD +
n-hexane) mixtures is clearly established. Excess molar volume (
VE) was estimated using equation 3 to assess the extent of molecular-level interactions within (PFD +
n-hexane) mixtures.
Here,
xPFD,
xn-hexane, and
ρPFD,
ρn-hexane refer to the mole fractions and densities, respectively, of PFD and
n-hexane at a given temperature, and
ρm is the density of the mixture.
MPFD and
Mn-hexane are the molecular weights of PFD and
n-hexane, respectively. The
VE at each investigated temperature for (PFD +
n-hexane) mixtures are presented as a function of
xPFD in the inset of
Figure 3. It is clear that, irrespective of the
T,
VE are mostly positive and have maxima at ca.
xPFD = 0.2. The positive
VE points to volume expansion on mixing PFD and
n-hexane and thus hints more at the presence of repulsive interaction (s) between PFD and
n-hexane or weaker interactions between them than the interactions present within neat PFD and
n-hexane, respectively. It may be inferred that the incompatibility of fluorous solvents with most non-fluorous substances brings in the repulsive interaction when the two substances are mixed.
3.2. Dynamic Viscosity of PFD and (PFD + n-Hexane) Mixtures
Experimentally measured dynamic viscosities (
η/mPa·s) of PFD and (PFD +
n-hexane) mixtures in the temperature range (293 to 333) K are reported in
Table 4. It is to be noted that
η of PFD is much higher than that of
n-hexane and is comparable to 2-ethyl-1-hexanol and other mid-chain alkyl alcohols. While in such alcohols, H-bonding usually gives rise to higher
η; in PFD the interaction between fluorine atoms may cause similar
η values [
20].
As expected, with an increase in temperature from (293 to 333) K, a monotonic decrease in
η is observed for a given composition of (PFD +
n-hexane) mixture (
Table 4). The temperature dependence of
η follows the most simplistic Arrhenius-like behavior:
where
Aη is a parameter, and
Ea,η is the activation energy of the viscous flow.
Figure 4 demonstrates the plots of ln
η versus 1/
T for (PFD +
n-hexane) mixtures. The best fit lines are according to Arrhenius expression and the recovered parameters ln
Aη and
Ea,η along with goodness-of-fit are presented in
Table 5. As expected,
Ea,η increases monotonically as the concentration of the component with higher
η PFD is increased in the mixture; neat PFD has the highest
Ea,η. It is established that fluorous liquid, PFD, possesses relatively high activation energy of viscous flow compared to the organic solvent
n-hexane.
The increase in
η with the increasing mole fraction of PFD in the (PFD +
n-hexane) mixture is found to be exponential, as per the equation:
Fits are presented in
Figure 5, whereas the recovered parameters
η0,
d, and
f, along with the goodness-of-the-fit in terms of
R2, are given in
Table 6. In order to assess the interactions within (PFD +
n-hexane) mixtures, deviation in logarithmic viscosities, ∆(ln
η), are estimated from the equation [
21],
where
ηm is the dynamic viscosity of the (PFD +
n-hexane) mixture, and
xPFD,
xn-hexane, and
ηPFD,
ηn-hexane refer to the mole fractions and dynamic viscosities, respectively, of PFD and
n-hexane at a given temperature. Plots of ∆(ln
η) versus for (PFD +
n-hexane) mixtures in a temperature range (293 to 333 K) are presented in the inset of
Figure 5. A careful examination of
Figure 5 reveals that, irrespective of the
T, ∆(ln
η) are negative and that no clear trend exists with variation in
T. The negative ∆(ln
η) further emphasizes the lack of attractive interaction within the (PFD +
n-hexane) mixture; it rather indicates that repulsive interactions are present between PFD and
n-hexane within the mixture, leading to lower viscosities than expected ideally. In this context, the negative ∆(ln
η) corroborates and compliments the positive
VE.
It is clear from the density and dynamic viscosity of the (PFD + n-hexane) mixtures that unfavorable interactions exist between PFD and n-hexane within the mixture, as documented by the nature of the fluorous solvents in general. The fact that the fluorous solvents exhibit contrast in properties as compared to the common organic solvents is established nonetheless.