2.1. Volumetric Properties
The results obtained from the density tests of the DMF + BuOH mixture (Table S1 Supplementary Materials) show that the values increase with increasing DMF content in the mixture and decrease systematically with increasing temperature over the whole range of mixture compositions. To study the properties of real solutions, excess functions are used, which determine the difference between the magnitude of a given molar thermodynamic function in a real solution and its magnitude in an ideal solution. The excess properties were calculated using the following expression:
where
is the excess quantity of the property
Z and
is the corresponding ideal value [
25].
In this paper will be presented and analyze six excess functions
calculated according to Equation (1). In order to calculate excess molar volume
, the molar volume of the mixture was obtained according to Equation (2):
where:
is the density of DMF + BuOH mixture,
,
and
,
are the mole fractions and molar masses of the mixture components, respectively,
i.e. BuOH (1), DMF (2).
In order to determine the changes taking place in a real solution in relation to an ideal solution in a binary mixture, the values of excess molar volume (
have been executed.
values of the mixture in the whole composition range and at the temperature range (293.15 K–318.15 K) was calculated according to Equation (3) and presented in
Figure 1:
where:
is the molar volume of (DMF+BuOH) mixture,
is the volume of ideal mixture,
,
are the molar volume of pure compounds,
i.e. BuOH (1), DMF (2).
The values of excess functions and among others the
of DMF + BuOH mixture were fitted to the polynomial of Redlich−Kister type:
where
is the polynomial coefficient calculated by the least-squares method using Equation (5).
As can be seen in
Figure 1, the excess molar volume exhibits positive values in a solution with a predominantly BuOH content in the mixture.
increases when small amounts of DMF are added to pure BuOH and passes through a maximum which appears depending on the temperature among
≈ 0.3 and
≈ 0.35 values. Above this value of mole fraction,
values are decreasing and reach the minimum value at
≈ 0.9. It is noteworthy that negative values for
are obtained for different mixture compositions depending on the temperature. The mole fraction of DMF, in which we observe the change of sign of the
function, increases with increasing temperature from the value
≈ 0.5 for 293.15 K to the value
≈ 0.9 for 318.15 K. One can notice that the volume contraction of the DMF + BuOH mixture increases when the molar fraction of DMF increases above 0.5 and decreases with increasing temperature. In the literature, you can find several reports in which an attempt was made to determine the value of
for the DMF + BuOH mixture [
14,
15,
16,
19,
20,
21]
. However, the research results presented in these articles are divergent. Similar results to ours were obtained by Rao and Reddy [
14] at 303 K and Garcia et al. [
21] but only at 298K
. In the other works, the results diverged from each other and from our results. Moreover, some authors obtained negative values of
in the whole range of compositions [
15,
19] which was completely inconsistent with the results obtained by other researchers, including ours. Such ambiguous data and conclusions prompted us to study the properties of this mixture with more accuracy using the three different test methods mentioned earlier.
The sign and magnitude of the excess functions may be attributed to the result of an appropriate combination of the following three major effects. The mutual dissociation of the components due to addition of the second component, the formation of hydrogen bonds between unlike molecules and steric hindrance, as well as geometry of molecular structure, which could be the reason which resist the closeness of the molecules. For DMF + PrOH according to the results obtained by us in our previous work [
26], the values were negative over the entire range of the mixture composition. Excess molar volumes are more negative in systems with lower alcohols, which may be attributed to strong interactions between unlike molecules and different molecular sizes [
27]. Such properties cause volume contraction in these mixtures. For the DMF + BuOH system, the small increase in the size of the alcohol molecule (extra –CH
2 group in 1-butanol compared to DMF + PrOH [
26]) gives completely different results and appears in volume expansion when DMF is added to BuOH. These values are positive in the BuOH rich region. Mostly for the DMF + BuOH system, the results obtained in their absolute value are 1.5 to 2 times smaller than for the DMF + PrOH mixture, however, in some compositions, they are several or ten times smaller than for the DMF + PrOH mixture.
Alcohols are strongly hydrogen-bonded in the pure state. Their molecules are self- and cross-associated [
28,
29]. The degree of association decreases also with increasing alcohol chain length. Thus, the addition of DMF to pure BuOH breaks the hydrogen bonds between molecules in the structure of the alcohol network, which produces a positive contribution to
. The results obtained on the basis of the dielectric study prove that in a solution with a predominant content of BuOH, the largest changes in the structure of hydrogen bonds occur in the DMF + BuOH solution [
30]. At the same time, an increase in the length of the alcohol chain causes a steric hindrance that also contributes to the increase in the real volume of the DMF + BuOH mixture. There is also not much difference in the molecular size of these two compounds, which also makes mutual accommodation difficult. The presence of hydrogen bonds between the components of the mixture tested was also confirmed by other researchers [
4,
20,
30,
31]. Prajapati [
30] based on the analysis of parameters obtained from the dielectric study provides confirmation of the formation of hydrogen bonds between DMF and BuOH molecules and weak dipole-dipole interactions between the components of the mixture. In contrast to the DMF + PrOH mixture, it is known that the interactions between unlike molecules are very weak in DMF + BuOH solution [
32]. Therefore, we observe an increase in
with increasing DMF content in the mixture to
≈ 0.3. The maximum observed at this mole fraction of DMF is related to the fact that in this range of the mixture composition there are probably the weakest interactions between the components of the mixture, which also contributes positively to the
[
32]. Despite the fact that there is probably still the possibility to form some amount of hydrogen bonds between DMF and BuOH molecules, the excess of BuOH amount is a competitive factor in the formation of hydrogen bonds between unlike molecules. According to the researchers [
32], the binding energy –O–H···O=C decreases when another alcohol molecule approaches with its oxygen atom favorablely orientated for “change”. As a consequence, instead of hydrogen bonding between DMF and BuOH molecules, the bonding appears between alcohol molecules rather more. When the amount of BuOH prevails in the DMF + BuOH system, there exists a greater tendency to create hydrogen bonds between BuOH molecules than between unlike species. As a result of the specific arrangement of neighbouring molecules around the DMF and BuOH molecules that form the bond, this interaction is weakened, which is confirmed by the negative
values in the entire concentration range of the solution obtained by Prajapati [
30]. This parameter assumes the largest negative deviation for a mixture with the composition
≈ 0.3, hence we observe a maximum on the relationship
= f(. Dissociation of bonding of pure components presumably became the main reason for volume expansion of the tested system. When > 0.3 values decrease and become negative when > 0.5 (293.15 K). In a solution with a predominant DMF content in the mixture, they can observe small volume contraction. The volume contraction value is similar to that observed for the DMF + PrOH system in the same concentration area of the mixture. When DMF starts to prevail in the solution, dipole-dipole interactions are most likely to begin to prevail in DMF and the creation of hydrogen bonding between DMF and BuOH molecules.
The analysis of the excess partial molar volume of both components of the mixture may help to explain the observed changes in this range of the mixture composition. The volume of the solution, in the case of a binary mixture, is the sum of the partial molar volumes of both components:
where:
,
– mole fraction of BuOH i DMF respectively,
,
– partial molar volume of BuOH i DMF respectively.
and
we can calculate using the following Equations (7) i (8):
where:
– molar volume of the real solution (DMF + BuOH),
,
– mole fraction of DMF and BuOH.
Analysis of changes in the partial molar volume of the components of a mixture can be represented by the excess partial molar volume (
,
). These values can be calculated using Equations (9) and (10):
where:
,
– excess partial molar volume of DMF and BuOH respectively,
,
– molar volume of pure DMF and BuOH.
The results for partial and excess partial molar volume of DMF and BuOH
and
are presented in Tables S4-S7 in Supplementary Materials and presented in
Figure 2 at
T = 293.15 K.
The values of
and
express the difference between the value of the partial molar volume of DMF or BuOH in the solution and the molar volume of each of the components in their pure form. Based on the analysis of
Figure 2, one can notice that for DMF the values of
are negative when
> 0.4 at 293.15 K. This means that DMF contributes negatively to the real volume of the mixture in this composition range of the solution. Although
assumes positive values in the same concentration range, however the amount of DMF prevails over the amount of BuOH when
> 0.4. Having been aware that DMF is a weakly associated liquid in contrast to BuOH [
30], the results obtained allow us to conclude that the negative contribution to the real volume of the mixture is made by the DMF molecules (
This is presumably one of the main factors of the slight contraction of the volume of the mixture causing that we observe
< 0 when
> 0.5. Analysis of data on the partial molar volume of each component in the DMF + BuOH mixture (Tables S4 and S5 Supplementary Materials) allows one to observe that the relationships
and
shows only little change with increasing DMF content in solution. This proves that chemical entities, such as various types of associations or complexes built of both molecules of the solution components, are most likely not formed in the system. Otherwise, we would observe characteristic changes in the course of both functions, along with an increase in the content of one of the components of the mixture, as in the case of an aqueous solution of
N,N-dimethylformamide [
33], which also confirms the conclusions drawn earlier. The partial molar volume of DMF decreases slightly as the DMF content of the mixture increases, with a higher temperature. Garcia et al. [
19] presented a similar course of this dependence at a temperature of 298.15 K. In addition,
takes the highest value at the highest temperature. Determined by Zegers and Somsen [
22] value of
in pure BuOH is equal to 7.779·10
−5 m
3·mol
−1 and is close to the one obtained by us (7.749·10
−5 m
3·mol
−1). The course of the relationship
is opposite to that observed for DMF.
values increase slightly with increasing DMF content. The influence of temperature on the values of
is analogous to the case
. Zegers and Somsen [
22] determined also
in pure DMF, which is equal to 9.197·10
−5 m
3·mol
−1. The value obtained by them is close to the one determined by us (9.208·10
−5 m
3·mol
−1).
Using the density values of the mixture at six temperatures, the coefficient of thermal expansion was calculated (
αp) using Equation (11):
was calculated with Equation (12) [
34]:
The values of molar isobaric expansion (
) were calculated using Equation (13) [
25]:
The obtained results of
as a function of the DMF molar fraction are presented in
Figure 3.
The isobaric molar expansion reaches the highest value for pure BuOH and decreases with increasing DMF in the mixture. The values increase with increasing temperature over the whole composition range of the mixture which seems to be logical due to the increase in thermal movements at higher temperatures. Such a behaviour of the system in the BuOH rich region is most likely related to the breaking of hydrogen bonds in BuOH after adding DMF to the solution. This causes greater changes in volume expansion with an increase in temperature. With a high BuOH content in the mixture, a slight maximum is visible at the two lowest temperatures (293.15 K, 298.15 K). For higher temperatures on the relationship, we observe only a change in the slope of the function. A greater effect of temperature on the value of this function is observed when BuOH prevails in the mixture. As the DMF content increases, the temperature differentiates the values of the mixture increasingly less. Small changes in the value of the isobaric molar expansion in the DMF rich region show that the structure of this solvent remains only slightly associated by dipol-dipol interactions.
Using the calculated values of
and Equations (14) and (15) [
25] excess molar isobaric expansion was determined (
) and presented in
Figure 4:
where:
are volume fraction of BuOH and DMF and
are the coefficient of thermal expansion of pure BuOH and DMF respectively.
Excess isobaric molar expansion in the entire concentration range takes positive values. A real solution has a greater ability to thermally expand relative to the ideal solution. In the range of 0.4 ≤ ≥ 0.5 (depending on the measurement temperature) a maximum appears, which proves the occurrence of characteristic changes in the interactions between molecules in this composition range. The value of is the lowest at T = 318.15 K, which means that at the highest temperature the volumetric expansion of a real solution is the smallest and increases with decreasing temperature. It should be expected that at the lowest temperatures, the interactions between molecules will be stronger in relation to the intermolecular interactions weakening with increasing temperature.
2.2. Sound Velocity and Heat Capacity
Based on density and sound velocity measurements (Tables S1 and S2 in Supplementary Material) the isentropic compressibility coefficient
and molar isentropic compression
were calculated according to Equations (16) and (17) in the whole temperature range:
where
is the isentropic compressibility coefficient,
is the molar isentropic compression
, is the sound velocity of DMF + BuOH mixture,
is the experimental value of solution’s density.
Based on the obtained data like isentropic compressibility coefficient
, the coefficient of isobaric thermal expansibility (
and experimentally gained data of density (
) and specific heat capacity (
of the tested DMF+BuOH solution which (presented in Tables S1 and S3 in Supplementary Materials), the values of isothermal compressibility efficient
and isothermal molar compression
were calculated using Equations (18) and (19):
Obtained results of
and
for the whole composition and temaprature range are presented in Table S8 (Supplementary Materials). The course of changes in isentropic and isothermal molar compression as a function of concentration and temperature is very analogous.
and
reach similar values. Both the isentropic and isothermal molar compression decreases with increasing DMF content in the mixture. This is in agreement with the results of other researchers [
35]. The highest value of
and
is observed for pure BuOH, due to the hydrogen bonds existing in the alcohol structure.
and
increases with concentration increase of alcohol. It is principally associated with an increase in compressibility due to structural changes in the mixture that lead to a decrease in ultrasonic velocity [
36]. Adding DMF to BuOH breaks these bonds and creates new ones, weaker, between the DMF and BuOH molecules. This leads to a closer arrangement of the molecules. In the DMF rich region only the diop-dipol interaction prevail. Most likely, this phenomenon contributes to a decrease in the compressibility of the system with increasing mole fraction of DMF. The compressibility of the system increases with increasing temperature. A greater effect of temperature on the value of isentropic and isothermal molar compression is visible for solutions in which the content of BuOH prevails. When there is more DMF in the system,
and
depends on the temperature increasingly less.
In order to better understand the nature of the interactions between the components of the mixture and the nature of molecular agitation in dissimilar molecules, the excess molar isentropic compression was determined (
) and excess molar isothermal compressibility (
). These are found to be sensitive towards differences in the size and shape of molecules [
37]. For this purpose
and
values were calculated according to Equations (20) – (24)
[25]:
where:
represent excess molar isentropic and isothermal compression,
and
represent the molar isentropic and isothermal compression, and
and
their molar values for an ideal mixture;
the isentropic and isothermal compressibility coefficients of pure components 1 (BuOH) and 2 (DMF),
the volume fraction of the mixture components;
represents the isobaric molar heat capacity of pure BuOH (1) and DMF (2) calculated on the basis of the obtained
values (Table S3 Supplementary Materials).
The courses of both functions
and
in the whole temperature range are shown on
Figure 5.
The excess molar isentropic and isothermal compression take negative values. The same trend can be seen in studies published by Thirumaran et al. [
4], Rao and Reddy [
14], and Acree [
35]. This parameter takes negative values, and the minimum is observed at
≈ 0.45. It can be concluded that the real solution is less compressible than the ideal solution. At
≈ 0.45 where there is a minimum on the function curves
and
visible, apparently there are characteristic changes in the interactions between the particles of the real mixture in relation to the ideal solution, causing more and more difficulties in the compression of the system. With the addition of DMF to the pure BuOH, the excess compressibility rapidly decreases up to
≈ 0.45 caused by the rupture of hydrogen bonds in pure BuOH occurring during mixing. In this range of composition, we also observe probably the weakest interactions between the components of the mixture [
30,
32]. The lower compression in the solution is observed for systems with lower alcohols, which can be attributed to strong interactions between unlike molecules and different molecular sizes causing stronger mutual accommodation of components [
26,
27]. Such situation was observed for DMF + PrOH mixture [
26]. Since the size of both BuOH and DMF molecules is similar, the weakening and breaking of bonds and the lack of interstitial accommodation due to similar molecular sizes may be the reason for decreasing the compressibility of the DMF + BuOH system in the BuOH rich region. In the DMF rich region, with the increasing importance of interactions of unlike molecules and dipole - dipole interaction in DMF, we can observe the decresing negative values for
and
. The value of
decreases with increasing temperature. Thus, the lower the temperature, the greater the compressibility of a real solution relative to that of an ideal solution. At
T = 293.15 K, the real system will show the least negative
and
of the mixture compared to the other temperatures.
The values of isobaric molar heat capacity (
) were calculated using specific heat capacity
obtained from experiment (Table S3 Supplementary Materials). The results in the whole composition range of the mixture and for six temperatures (293.15K–318.15K) were calculated and presented on
Figure 6 and in Table S9 (Supplementary Materials).
Heat capacity and thermal analysis data for the DMF + BuOH system have not been published earlier. As can be seen from the
Figure 6 the
values decrease with increasing DMF content. With increasing temperature
values are increasing. A greater temperature differentiation is also visible in the BuOH rich region compared to the area of DMF that prevails. An analogous course of dependence with an increase in DMF content for this system DMF + BuOH and changes under the influence of temperature was visible in the course of the previously discussed functions
and
, which make us able to conclude that our data are coherent. The analysis of changes in the function
with increasing concentration and temperature allows us to confirm the changes in the nature and strength of interactions between unlike species in the structure of the system occurring and which was discussed earlier. Based on these data, one can observe the transition from strong hydrogen bonds in the structure of BuOH to weak intermolecular interactions in the DMF rich region. With the values of
and
it was also possible to calculate the values of
according to Equation (25) [
25]:
The obtained calculation results for have been collected in Table S9 in Supplementary Materials.
Excess molar isobaric
and isochoric
heat capacities were calculated using Equations (26) and (27).
where:
,
are isobaric and isochoric molar heat capacities of the ideal mixture,
,
are the isothermal and isentropic compressibility coefficients for the ideal mixture and
are the isobaric and isochoric molar heat capacities of pure compounds, BuOH (1) and DMF (2).
is presented as a function of mixture composition in
Figure 7. The obtained relationship of
is analogous to
with the change of temperature. A comparison of both functions at 298.15 K is shown in
Figure 8.
Excess molar isobaric
) and isochoric
) heat capacity of DMF + BuOH mixture shows negative values in the entire composition range of the mixture. The reason of reduction of
is forming of interactions occurring between DMF and BuOH molecules which are presumably weaker compared to the hydrogen bonds between the molecules of these compounds existing in their pure form. As the temperature increases, we observe an increase in the negative values of both functions. With increasing DMF content,
and
decrease and reach a minimum value when
≈ 0.5. The obtained results confirm the changes in
,
and
already analyzed in this paper. All analyzed excess functions, including
and
reach their extreme value when
≈ 0.5. The obtained results of thermal properties for the DMF + BuOH mixture confirm the characteristic change in the strength of interactions between the components of the mixture with the change in the composition of the system. Interactions in the system weaken in the BuOH rich region with an increasing amount of DMF. This allows us to confirm the conclusion that the specific arrangement of neighbouring molecules around the DMF and BuOH molecules forming the bond, this interaction is weakened [
30]. Hence, we observe
and
negative values. Then one can notice an increase in the strength of the interaction when DMF is prevailing. Otherwise, as a result of the formation of intermolecular hydrogen bonds between unlike molecules, stronger than in their pure state, the maximum would be observed [
33]. From
Figure 7 the position of the minimum shifts very slightly towards lower values of the DMF with increasing temperature. An analogous course is observed for
dependence.