In the paper, we discuss the phenomenon of melting and triple point properties deviations from regularity in tangent of slope and “zigzag” progression in n-alkanes, n-alcohols, n-alkanoic amines and n-alkanoic acids. The thermodynamics and chemical aspects of this intriguing phenomenon will be discussed. In graphs of melting and triple point properties of saturated alkanoic (fatty) acids, n-alcohols, n-amines and n-alkanes homologous series have been observed two segments, which are characterized by changes in tangent of slope from the fifth member of homologous series, and harmonic zigzag progression of melting and triple point temperatures and some other thermophysical parameters. The melting and triple point temperatures for even numbers of carbon atoms in homologous series of n-alkanes, n-alkanoic acids, n-alcohols, n-amines and some cyclic and polycyclic substances are larger than average values of the nearest odd numbers. A general character of this phenomenon for a wide number of substances motivates researchers to study the nature and mechanism of it and its implication.
In pharmaceutical and food industries, the phenomenon of crystalline polymorphism of cocoa butter is well-known. It may exist in several different crystal morphologies [
The melting point is a significant parameter in chemistry: it can easily reflect the purity of the substance. For long list of substances as: n-alkanes, n-alcohols, n-alkanoic acids, lipids and some other, different melting points, which often have significant differences, had been presented in literature for the same substance. In some of these observations, the crystalline polymorphism had been pointed out as a cause for the phenomenon, but in several cases the cause was not pointed out. The careful comparison of melting points in several homologous series supports the perception that general differences are not the operator errors and do not result from crystal morphology of samples, but have more meaningful basis.
Despite of these differences some general tendencies in pure substances can be observed, such as the “zigzag” effect: a periodic alteration of some thermophysical properties added to a general smoother change. Among these properties are melting and triple point temperatures, triple point vapor pressure, liquid internal energy and some other properties [
Boese, R. et al. [
In the paper, we present data and calculations for several homologous series, which with no debt demonstrate the zigzag progression of melting and triple point temperatures in homologous series as a general phenomenon. For better perception of this wonderful phenomenon the visual analytics is very helpful. By presentation of graphical images the zigzag phenomenon is clearly visualized and supports generation of possible hypotheses for this intriguing property of molecules in the homologous series.
The possible technical applications [
The visual analysis of diagrams, based on precise experimental data for n-alkanes, permits to analyze the zigzag oscillations of several thermophysical properties. For n-alkanes with the carbon atoms number NC in a molecule from 1 to 12 the zigzag irregularities of some properties near the triple point have been analyzed. The main source of data was the NIST (National Institute of Standards and Technology, USA [
At the triple points of n-alkanes had been analyzed the carbon atoms molar density DC:
Here Dliqtr is the molar density of a liquid n-alkane at its triple point. For the carbon atoms’ molar density DC in liquid n-alkanes at their triple points it is possible to see the zigzag oscillating dependence on the number NC of C atoms in a molecule,
Starting from the NC = 3, the carbon atoms density DC in liquid n-alkanes at their triple points for odd numbers NC is larger than the average value of the nearest even numbers. In liquid methane and ethane, the molar densities of
carbon atoms fall due to a growing proportion of hydrogen atoms for small numbers NC of C atoms in a molecule.
The zigzag dependence of the carbon atoms molar density DC in liquid n-alkanes may be partly explained by the zigzag dependence of the triple point temperature for n-alkanes on the C atoms number NC in a molecule. Linus Pauling in his book General Chemistry [
Here the triple point temperature dependence on the number NC of C atoms in a molecule had been studied utilizing the NIST database [
For the triple point temperature, we see also the zigzag dependence on NC, starting from NC = 3,
The oscillations of the Ttr(NC) are better seen from the second differential ∆2Ttr(NC):
Roberts and Caserio [
Sedunov in 2012 has shown that n-alkanes’ molecules in gaseous dimers are packed in a parallel structure, which bond energy grows almost linearly with the number of carbon atoms, but with zigzag deviations from a straight line [
than ordinary van der Waals bonds. This enforcement of bonds may result from no zero charges of carbon and hydrogen atoms in n-alkanes.
In liquids, the intermolecular bond energy depends on the packing character. It is not surprising that the intermolecular bonding in liquid n-alkanes is like the bonding in the gas phase.
The
The saturation vapor pressure is very sensitive to the bond energy between molecules in a liquid state. For the triple point vapor pressure Ptr the zigzag character of the Ptr(NC) dependence in a series of n-alkanes is clearly seen from the
All analyzed properties of n-alkanes at their triple points are reflected in the
The n-alkanoic acids molecule contains the −COOH head part followed by the
Fluid | NC | Ttr(K) | log(Ptr) | DC (mol/l) |
---|---|---|---|---|
Methane | 1 | 90.69 | −1.93 | 28.14 |
Ethane | 2 | 90.35 | −5.95 | 43.33 |
Propane | 3 | 85.48 | −9.77 | 49.87 |
Butane | 4 | 134.90 | −6.18 | 50.58 |
Pentane | 5 | 143.47 | −7.12 | 52.83 |
Hexane | 6 | 177.83 | −5.89 | 53.04 |
Heptane | 7 | 182.55 | −6.76 | 54.22 |
Octane | 8 | 216.37 | −5.70 | 53.49 |
Nonane | 9 | 219.70 | −6.35 | 54.47 |
Decane | 10 | 243.50 | −5.85 | 54.06 |
Undecane | 11 | 247.58 | ||
Dodecane | 12 | 263.60 | −6.20 | 54.35 |
−Cn−1H2n−1 alkyl tail moiety, where n is the number of carbon atoms in the molecule, designated above as NC for n-alkanes. We can present an n-alkanes molecule also as the −CH3 head part, followed by the −Cn−1H2n−1 alkyl tail moiety. At the same number NC of carbon atoms, the alkyl tail of the n-alkanoic acid molecule is the same as the tail of the n-alkanes molecule. Basing on the group contribution principle [
Among other thermophysical properties the melting point has proven to be very sensitive to the liquid’s molecular structure. If we compare melting points in a series of n-alkanoic acids with the same of n-alkanes, we can see a similarity caused by the same alkyl tails and difference resulting from different head parts. The data are shown at the
Calculating differences between melting temperatures of n-alkanoic acids and triple point temperatures of n-alkanes we ignore the difference between melting temperature and triple point temperature of a fluid under investigation, as a very small value.
The
We see that zigzag oscillations of two curves are similar. It tells about almost the same contribution of the Cn−1H2n−1 tails of n-alkanoic acids’ and n-alkanes’ molecules in the intermolecular bond energy. But there is a strong difference in the regular dependence, growing at small NC due to a growing role of the molecular head parts in the intermolecular bonding. To analyze this regular part, the plot of difference ∆Ttr between two curves versus NC is presented at the
We see that oscillations have disappeared, but at NC = 5 there is a strong change of the tangent of slope, signaling about a change in a molecular packing character in liquid n-alkanoic acids. So, the utilization of n-alkanes as reference liquids permits to reveal the transition in the packing mechanism of molecules in liquid n-alkanoic acids at NC = 5!
Alkanoic acids | Alkanes | Difference | |||
---|---|---|---|---|---|
NC | Fluid | TmeltK | Fluid | TtrK | of TmeltK |
1 | Methanoic | 281.5 | Methane | 90.69 | 190.81 |
2 | Ethanoic | 289.6 | Ethane | 90.35 | 199.25 |
3 | Propanoic | 252 | Propane | 85.48 | 166.52 |
4 | Butanoic | 267.95 | Butane | 134.90 | 133.06 |
5 | Pentanoic | 239.7 | Pentane | 143.47 | 96.23 |
6 | Hexanoic | 269.7 | Hexane | 177.83 | 91.87 |
7 | Heptanoic | 266 | Heptane | 182.55 | 83.45 |
8 | Octanoic | 289.3 | Octane | 216.37 | 72.93 |
9 | Nonanoic | 285 | Nonane | 219.70 | 65.30 |
10 | Decanoic | 304 | Decane | 243.50 | 60.50 |
11 | Undecanoic | 301.65 | Undecane | 247.58 | 54.07 |
12 | Dodecanoic | 317 | Dodecane | 263.60 | 53.40 |
We explain this transition as the competition of attractions between −COOH head parts and n-alkyl tails. The pair of −COOH head parts of two n-alkanoic acids may form a cluster complex like the methanoic acid dimer,
In n-alkanoic acid dimers with NC > 1 the outstanding Hydrogen atoms are substituted by the n-alkyl −Cn−1H2n−1 tails. At NC > 5 the attraction energy between long n-alkyl tails becomes competitive with the hydrogen bonds energy of the head parts. In the liquid n-alkanoic acids at NC > 5 becomes essential the attraction of two parallel molecules, bound by their alkyl tails’ attraction forces.
For this comparison, the melting temperatures of n-alcohols have been measured in Norwegian Drug Control and Drug Discovery Institute (NDCDDI), because the data from different sources [
At NC > 1 the hydrogen atoms, opposite to the −C-O bond, are substituted by the −Cn−1H2n−1 n-alkyl moieties. The experimental data for melting temperatures demonstrate an essential contribution of the −Cn−1H2n−1 n-alkyl moieties. A comparison of n-alcohols’ and n-alkanes’ melting temperatures was performed by the analysis of second differentials
dependence on the number of carbon atoms in a molecule NC,
Unlike n-alkanoic acids, the second differential of the melting point ∆2 Tmeltn
in n-alcohols is smaller than for n-alkanes and falls much quicker at large NC. It is reflected in the difference of melting temperatures for n-alcohols and n-al- kanes,
Comparing the
At NC < 5 may dominate the polar interaction with opposite orientation of interacting molecules, shown at the
The zigzag oscillations of the Tmelt(NC) in n-alcohols demonstrate a strong contribution of the n-alkyl tails in the bond energy of molecules in a liquid state. These oscillations are also clearly seen for numbers NC of carbon atoms in a molecule larger than 12,
The
A deeper analysis of this phenomenon in future may help to discover the physical mechanism of the melting temperature Tmelt(NC) oscillation. It may result from the charge of carbon atoms in n-alkanes’, n-alcohols’ and n-alkanoic acids’ molecules dependence on the number NC. The oscillating bond energy strength between parallel n-alkanes molecules or n-alkyl tails tells that the charge of carbon atoms depends on condition, is the number NC even or odd.
In this section the amines with normal alkyl tails are considered. They contain the ammonia part with one hydrogen atom substituted by normal alkyl tail moiety. The data collected from the NIST database [
The investigation of the melting Tm and boiling Tb temperatures dependences
Systematic Name | NC | Empirical Formula | CAS Number | Molar mass g/mol | Melting point* K | Boiling point* K | Triple point K |
---|---|---|---|---|---|---|---|
Methylamine | 1 | CH5N | 74-89-5 | 31.06 | 180.05 | 266.8 | 179.7 |
Ethylamine | 2 | C2H7N | 75-04-7 | 45.09 | 191 | 291 | |
Propylamine | 3 | C3H9N | 107-10-8 | 59.11 | 190.15 | 322 | 188.38 |
Butylamine | 4 | C4H11N | 109-73-9 | 73.14 | 224 | 351 | |
Pentylamine | 5 | C5H13N | 110-58-7 | 87.17 | 218 | 375 | |
Hexylamine | 6 | C6H15N | 111-26-2 | 101.19 | 251.85 | 403 | |
Heptylamine | 7 | C7H17N | 111-68-2 | 115.136 | 250.15 | 429 | |
Octylamine | 8 | C8H19N | 111-86-4 | 129.247 | 272.75 | 452 | |
Nonylamine | 9 | C9H21N | 112-20-9 | 143.270 | 474.2 | ||
Decylamine | 10 | C10H23N | 2016-57-1 | 157.301 | 288.15 | 490.2 | |
Undecylamine | 11 | C11H25N | 7307-55-3 | 288.15** | 513.2 | ||
Dodecylamine | 12 | C12H27N | 124-22-1 | 185.35 | 301 | 521.2 |
*From [
on the C atoms number NC for n-amines reveals again the zigzag oscillation progression for Tm,
The comparison of melting points progress with NC for n-amines and n-al- kanes and the difference between these temperatures ΔTm(Nc) are shown at the
The
The investigation of ΔTm(Nc) for n-alkanoic acids’, n-alcohols’ and n-alkanoic amines’ melting temperatures differences from the same for n-alkanes reveals the contribution of the head parts of molecules in the molecular interaction in a liquid state. The method to use the n-alkanes’ properties as reference values to study the tangent of slope change in homologous series of n-alkanoic acids, n-alcohols and n-alkanoic amines had not been described before. It may be used to study molecular interactions in other homologous series, including aliphatic substances with cyclic moieties.
The tangent of slope change for n-alkanoic acids’, n-alcohols’ and n-alkanoic amines' melting temperatures differences from the same for n-alkanes at Nc = 5 also has not been described before. The further investigation of this intriguing
phenomenon should be proceeded to understand better the competition between the head parts and the n-alkyl tails’ moieties attractions in a liquid state of these substances.
The phase shift of the zigzag oscillations for n-alkanoic acids’ melting temperatures at large NC numbers of carbon atoms in a molecule has not been described before. It tells about no van der Waals interaction between the n-alkyl tails of molecules in these substances. The further investigation of this intriguing phenomenon should be proceeded for other aliphatic substances.
The hypothesis about no zero charges of carbon and hydrogen atoms in n-alkyl tails has not been formulated before. It may be confirmed by a comparison of the n-alkyl fluids’ and classical van der Waals Noble gases’ critical point temperatures at the same number Z of electrons in a molecule,
Number of electrons | n-alkyl fluids | Noble gases | ||
---|---|---|---|---|
Z | Name | Critical temperature | Name | Critical temperature |
10 | Methane | 190.66 | Neon | 44.4 |
18 | Ethane | 305.33 | Argon | 150.86 |
point temperature reflects the molecular interaction strength and grows with the number of electrons in a molecule of a van der Waals fluid.
The
The difference between critical temperatures of n-alkyl fluids and corresponding Noble gases is impressive! It tells about no van der Waals molecular interactions in n-alkyl fluids, which may be explained by no zero charges of carbon and hydrogen atoms in their molecules.
A symmetric structure of n-alkyl molecules results in zero value for the dipole moment, but higher order moments may have no zero values, providing enlarged molecular attraction as compared to classical van der Waals fluids! The zigzag dependence of the melting temperatures on the number NC of carbon atoms in n-alkyl tail may result from the carbon atoms charge dependence on NC. This intriguing phenomenon, including the phase change at large NC values, should be investigated in future research works more carefully.
1) A general character of thermophysical properties’ zigzag behavior for a wide number of organic substances: saturated alkanoic (fatty) acids, n-alcohols, n-amines and n-alkanes, tells about a universal attraction mechanism between their n-alkyl tails;
2) The change of phase of the Tm(Nc) dependence at large Nc numbers tells about no trivial character of this mechanism;
3) The competition of n-alkyl tails’ attractions with strong polar attractions of the molecular head parts in saturated alkanoic (fatty) acids, n-alcohols and n-amines tells about no zero charges of carbon and hydrogen atoms in the n-alkyl tails;
4) A sharp kink of the ΔTm(Nc) dependence in saturated alkanoic (fatty) acids and n-alcohols tells about structural changes in liquid saturated alkanoic (fatty) acids and n-alcohols at Nc = 5.
Sedunov, B. and Brondz, I. (2017) The Zigzag Progression of Melting and Triple Point Properties of n-Alkanes, n-Alcohols, n-Alkanoic Amines and n-Alkanoic Acids. Voice of the Publisher, 3, 1-14. https://doi.org/10.4236/vp.2017.31001