A general review [1,2] is presented concerning jointly the crystallographic structures and the transition physicochemical data (T fus , ∆ fus H, ∆ fus S, ∆ eb H, ∆ sub H, Cp) in the pure normal alkanes, as well as the structural and thermodynamic behavior of their synthetic binary, ternary and multinary model mixtures and of real petroleum waxes, particularly the solubility and mixture properties [3][4][5][6].A major part of the structural and thermodynamic data of the literature and our experimental results are listed from methane up to the alkane with carbon atom number equal to 390 [1, 2] as well as their variations versus the atom carbon number.The experimental results [3] and those of literature concerning the pure n-alkane dissolution enthalpies allow to determine the enthalpy of the formation of one or several crystallized solid solutions of the mixtures [4] and to highlight the influence of the characteristic parameters of the paraffinic distribution (theoretical average chain length, n-alkanes number, monophasic or polyphasic state) on the excess properties in the solid state.This data bank permits to test the predictive capacities of the UNIQUAC thermodynamic model.After modifying the expression of the interaction energy to take into account the effect of the size difference between the n-alkanes of the mixtures and the internal disorder induced by the nalkane length distribution, this model give very good results for the prediction of the excess enthalpy of complex multialkane samples [4].

The purpose of this paper is to display the influence of isoalkanes on the structure and mechanical properties of waxes. This work was carried out by means of X-ray diffraction, differential thermal analyses, and dynamic mechanical analyses. Isoalkanes coming from an industrial wax were added in different quantities to a synthetic mixture of linear alkanes. The disorder generated by this addition was estimated by X-ray diffraction; it modifies the structural transformations versus temperature and therefore the mechanical properties. The branched chains of isoalkanes hinder the molecules motions, especially the rotation along the long axis. Thus, some phases usually formed at high temperature are not observed when increasing the temperature. With regard to the mechanical properties, the effect of the branched alkanes was studied using two industrial waxes with Gaussian distribution composition. The dynamic mechanical analyses have shown that the storage modulus (E') is also affected by the isoalkane quantity.

The aim of this paper is to determine the liquidus surface of the pseudoternary system {C14H30 tetradecane/ C14H10 phenanthrene/wax} to display the influence of the phenanthrene C14H10 on the solubility of a petroleum wax in the normal tetradecane C14H30. The binary diagram {C14H30 tetradecane/C14H10 phenanthrene}, the two pseudobinary diagrams {C14H30 tetradecane/wax} and {C14H30 phenanthrene/wax} and three vertical sections, through ternary space diagram from pure tetradecane (C14H30) to three points on the opposite binary {C14H10 phenanthrene/wax}, are established by X-ray diffraction and differential thermal analyses. The polythermal projection of the liquidus surface displays three eutectic valleys in the pseudoternary system. The eutectic valley, whose origin is the binary eutectic of the system (C14H10 phenanthrene/wax) borders the pseudobinary system (C14H30 tetradecane/wax) with lower crystallization temperatures, and therefore, for a mixture, whose wax/C14H30 tetradecane ratio is constant, the addition of phenanthrene decreases the amount of the wax deposited as far as C14H10 phenanthrene concentrations close to the eutectic valley.

Thermodynamic and structural analyses were carried out by X-ray diffraction and differential thermal analyses as functions of the temperature on multi-alkane samples whose distribution of mole fractions shows a shape of the ‘exponential decreasing’ type, as observed in petroleum cuts. Nine samples, whose number of normal-alkanes, Cns, varied from 15 up to 23, were studied with continuous distributions of mole fractions going from the C22–C36 series to the C14–C36 series: C22 and C14 corresponding respectively to the first Cns of the two terminal series and C36 is always the last Cn of each series: each mixture differed from the previous sample by the addition of a lighter Cn−1. At the solid state and according to literature, the multi-alkane samples of C22–C36, C21–C36 and C20–C36 series are in a two-phase solid thermodynamic state, C19–C36 to C15–C36, three-phase, the broader distribution C14–C36, four-phase and the mixtures with numbers of Cns≤11, single-phase. Thus, the Cns crystallise in the solid phase, where the thickness of molecule layers is compatible with their own chain length. The results allow for extracting the recurring structural and thermodynamic properties for these types of mixtures during crystallisation, and to clarify the molecular mechanisms involved in the number of observed solid phases, in the order of their appearance and in their thermodynamic and structural behaviour during the course of cooling from the liquid state.

In this paper, the results of thermomechanical analyses (TMA), dynamic mechanical analyses (DMA) on pure linear alkanes and a commercial wax are discussed with the help of structural analyses and differential thermal analyses (DTA) performed at various temperature. Examination of the results show that thermal expansion of pure alkanes is around 10−4K−1 at room temperature. It decreases continuously until 0.3×10−4K−1 in the structural state corresponding to the rotator phase (α-RII). As concerns DMA, the storage modulus is ranged between 750MPa (for the commercial wax) at room temperature and 1MPa near the melting point. It decreases by stages when temperature is increasing. This general evolution is observed with pure linear alkanes as well as with the studied commercial product. Each stage corresponds to the stabilization of a solid phase, whereas a solid–solid transition is highlighted by an increase of the curve's slope. Density measurements performed at various temperature confirm such an evolution. In fact, the density (included between 0.92 and 0.94gcm−3 at room temperature) decreases by stages versus temperature.

Structural analyses were carried out by X-ray diffraction, at ambient temperature, on multi-alkane samples whose mole fraction distribution shows a shape of the ‘exponential decreasing’ type, as observed in petroleum cuts. Nine samples, whose normal-alkane number varies from 15 to 23, have been studied with mole fraction continuous distributions of normal-alkanes going from C22–C36 to C14–C36: each mixture differs from the previous sample by the addition of a lighter n-alkane. At the solid state, the multi-alkane solid samples C22–C36 and C21–C36 are two-phase, C20–C36 to C15–C36, three-phase, and the broader distribution C14–C36, four-phase. In these polyphase solid systems, whose heaviest n-alkane is always C36, the average composition of the heavy and middle phases are constant and their structure are isostructural to the β′ ordered intermediate solid solution, observed in n-alkane binary or ternary molecular alloys; the mean carbon atom number of the light phase decreases as the global average carbon atom number of the synthetic mixtures in relation to the addition of light n-alkanes and its structure simultaneously evolves from the β′ ordered intermediate solid solution towards the β-RI(Fmmm) and the α‐RII(R3¯m) disordered solid solutions, observed in pure n-alkanes: the light n-alkane added between each distribution intercalates itself into the structure whose molecule stacking period (thickness) is compatible with its own carbon chain length, in order to reduce the molecular gaps.

The phase diagram of (naphthalene + n-pentacosane) has been determined by thermometrical and differential thermal analyses and X-ray diffraction carried out on the two pure compounds and twelve binary mixtures. The solids phases of the pure compounds are immiscible, but there is cosolubility in the liquid state. With increasing temperature, the binary mixtures undergo structural changes corresponding to those of pure n-pentacosane, while naphthalene seems to retain its initial crystalline structure until fusion occurs. The binary diagram, determined by experiments, displays an eutectic solidification with immiscibility in the solid state:liquid <----> delta(0)-C10H8 +beta(o)'(Pbnm)-C25H52.

Measurements of the dissolution enthalpies in heptane at ambient temperature were carried out by mixing calorimetry on multialkane synthetic mixtures with an exponentially decreasing distribution of the n-alkane mole compositions, as observed in petroleum cuts. These experimental results and those of literature concerning the pure n-alkane dissolution enthalpies in the same experimental conditions allow us to determine the enthalpy of the formation of one or several crystallized solid solutions of these mixtures and thus, to highlight the influence of the characteristic parameters of the paraffinic distribution (theoretical average chain length, n-alkanes number, monophasic or polyphasic state) on the excess properties in the solid state. Second, this data bank permits us to test the predictive capacities of the UNIQUAC thermodynamic model. After modifying the expression of the interaction energy λij to take into account the effect of the size difference between the n-alkanes of the mixture and the intern...

The thermodynamic behaviour and structural evolution of 13 different mixtures of a multiparaffinic wax with a molar concentration distribution of the normal logarithmic type (Prolabo 52–54 °C) and (C10H8)—naphthalene were investigated at temperatures ranging from 288.15 to 373.15 K. The wax and (C10H8)—naphthalene are not miscible in the solid state but show co-solubility in the liquid state. The diagram representing the isobaric pseudo-binary (T,x) cross section of the system ((C10H8)—naphthalene+multiparaffinic wax) displays a curve of crystallization which resembles that of a binary eutectic solidification with an invariant temperature TE=312.25±0.7K; below this TE temperature the equilibrium state corresponds to a two-phase domain containing the solid phases of the two compounds ((C10H8)-naphtalene and multiparaffinic solid solution of wax) separately.

This paper displays a study of the system associating a commercial multi-paraffinic sample and pure n-eicosane: (C20H42). It has been studied by means of calorimetric and structural analyses. The X-ray analysis of the commercial product performed at room temperature has shown that it forms a single solid-solution of orthorhombic structure isostructural to that previously observed with binary or ternary alkane mixtures. The structural characterizations carried out on molecular alloys whose concentration in C-20 is in range from 0 to 100% have allowed to determine the following phase appearance sequence: beta'/beta' + gamma(0)/gamma(0) where beta' is the solid solution of the wax and gamma(0) is the triclinic structure of C-20.The solid-solid transitions of the molecular alloys have been highlighted by DTA and characterized by X-ray diffraction. Compilation of the results led to the proposal of the isopleth section of the system: multi-alkane product-eicosane. In this system, the crystallization is of eutectic' type and several 'invariant' transformations ('eutectoid' or 'peritectoid' reactions) are reported. (C) 2004 Elsevier Ltd. All rights reserved.

Measurements of enthalpy increments from the temperature of the ordered phases of low temperatures at 293.4 K up to the liquid phase above the melting point were carried out by differential scanning calorimetry using discontinuous mode temperature programming on 18 multiparaffinic mixtures, prepared by melting pure alkanes, and 4 commercial petroleum waxes. Temperatures and enthalpies of the solid/solid transitions and melting were determined, as well as the total enthalpy variations from the order/disorder transition onset temperature up to the melting end temperature and the heat capacity variations in the ordered solid solution and the liquid phase, as a function of temperature. The comparison of the experimental results with the values, calculated for equivalent ideal mixtures from the thermodynamic data of pure n-alkanes, allows us to highlight a deviation in relation to the ideality in the solid state for all of these mixtures: the ordered solid phase reveals a significant gap in relation to ideality, whereas the liquid mixtures show athermal behavior. The composition characteristic parameters of the mixtures (shape of the n-alkane mole fraction distribution, number of n-alkanes, and percentage of the other nonlinear hydrocarbons in the petroleum mixtures) are closely related to the degree of internal disorder in the solid solutions and thus to the deviation in relation to the ideality in the solid state.

The crystallization of n-alkanes is known to be responsible for the solid deposit in flowlines of middle distillate fuels or petroleum cuts. To solve the problems of solid deposits in industrial equipment, it is essential to determine the thermodynamic and structural behaviour of the components of crude oils. The aim of the study was to determine the phase diagram of (n-tetradecane + n-pentacosane) was determined by simple and differential thermal analyses and X-ray diffraction; then it was compared with the pseudo-binary isopleth of the (n-tetradecane + multiparaffinic wax) multicomponent system. In both cases, the solid phases of compounds (gamma(o)(P (1) over bar)-C(14)H(30); beta(o)(Pbcm)-C(25)H(52); beta'-wax multiparaffinic solid solution) were observed separately, but there was co-solubility in the liquid state. In the C(25)-rich side of the diagram. the binary mixtures (C(14) + C(25)) undergo structural changes corresponding to those of pure C(25) when the temperature increases and those of beta' orthorhombic multiparaffinic solid solution of the wax are observed for the multicomponent system (C(14) + wax) in the wax-rich side. The binary diagram (C(14) + C(25)) displays an eutectic solidification (liquid <----> gamma(o)(P (1) over bar )C(14)H(30) + beta(o)(Pbcm)-C(25)H(52)) and the isopleth (C(14) + wax), a solidification that resembles an eutectic binary equilibrium (liquid <----> gamma(o)(P (1) over bar)-C(14)H(30) + beta'-wax). The comparison of the two diagrams, particularly the curves of the crystallisation in the heavy n-alkane-rich side, shows that the solubility in C(14) of the multiparaffinic solid solution, whose medium number of carbon atoms per molecule is equal to 25.6, is greater than that Of C(25). (C) 2003 Elsevier Ltd. All rights reserved.

L'étude de la solubilité d'une cire paraffinique, dite de type « normale », issue de déparaffinage des huiles lubrifiantes et celle d'un n-alcane pur dont le nombre de carbones est égal au nombre de carbones moyen de la cire, dans un solvant est réalisée par analyse thermique simple et différentielle.Les résultats obtenus montrent, d'une part, que les deux systèmes (Cire, C14) et (C25, C14) ont des diagrammes de phases superposables dans tout le domaine de composition et d'autre part, les deux diagrammes présentent une solidification eutectique binaire.Ce qui nous a donc amener à considérer la cire comme un pseudo constituant pur ou « composé définit ».La prédiction de température de cristallisation commençante du système (C25, C14) est réalisée à l'aide du modèle de Flory Huggins.

This paper displays recent works relative to the effect on structural state and thermodynamic properties of n-alkanes added to a commercial multi-n-alkane product. The G.P.C. and X-ray diffraction analyses have shown on one hand that the n-alkanes molar concentration distribution is of "normal logarithmic"- type and on the other hand that this wax forms a single solid solution of orthorhombic structure, isostructural to the intermediate phase previously characterized with the binary and ternary n-alkane mixtures. Pure components (n-eicosane or n-pentacontane) have been added to the wax and the molecular alloys resulting were examined by means of X-ray diffraction (at various temperatures) and by Differential Thermal Analysis. Compilation of the results led to the proposal of the isopleth sections of both the systems: n-eicosane/commercial product and commercial product/n-pentacontane. In the first system, the crystallization is of eutectic type while in the second, it is of peritectic type. Nevertheless, whatever the. diagram, no intermediate solid solution was observed, in fact both the phases characteristic of the component coexist in a large concentration domain. Thus, several invariant transformation are proposed in order to respect. the-law of adjoining phase regions.

Measurements of dissolution enthalpies in heptane at ambient temperature were carried out by mixing calorimetry on the n-alkanes going from C20H42 to C36H74. These dissolution enthalpy values increase linearly as a function of the carbon atom number. Furthermore, they reveal an alternating effect between odd- and even-numbered n-alkanes and highlight that the mixing of an n-alkane in the liquid phase with heptane is an athermal process: the variation of its enthalpy from the dissolution temperature up to its melting temperature corresponds to its dissolution enthalpy in heptane.

Measurements of enthalpy increments from the temperature of the ordered phases of low temperatures at 293.4 K and 298.3 K up to the liquid phase above the melting point were carried out by differential scanning calorimetry using a discontinuous mode of temperature programming on the n-alkanes from C-18 to C-38 and C-41, C-44, C-46, C-50, C-54, and C-60. The temperatures and enthalpies of the solid-solid transitions and of the fusion were also determined and compared with the values of the literature.

Simple predictive relationships for the estimation of pure normal alkanes' thermodynamic properties, like transition temperatures, phase change enthalpies, and heat capacities in the solid or liquid state, are established as a function of carbon atom number n(c), by combining our own calorimetric experimental results and a general review of literature data. This study underlines that total and melting enthalpies vary linearly with n(c) carbon atom number, whatever the parity of n(c), while order-disorder transition enthalpies show a quadratic variation and an alternating effect between odd- and even-numbered n-alkanes. Heat capacity variations versus temperature and the carbon chain length were represented by a model of group contributions for the liquid phase and by an expression derived from Einstein's model for the solid state.

This paper displays a study of binary mixtures of n-alkanes whose ratio of chain length is around two. The systems composed of n-tricosane (n-C23H48)–n-pentacontane (n-C50H102) and n-pentacosane (n-C25H52)–n-pentacontane (n-C50H102) have been studied by means of X-ray analyses. These latter, performed at room temperature, showed in both cases, the existence of a large domain where the phases characteristic of each pure component coexist. These mixtures obey Kravchenko's rule relative to the solubility of the n-alkanes according to the chain length of each component. The mixtures studied do not form an intermediate solid solution. In other words, there is no particular arrangement of the shorter molecules inside the crystallographic unit of the longer.