Lina Andreoli-Ball

Heat capacity and corresponding states in alkan-1-ol–n-alkane systems

The apparent molar heat capacities, ΦC, and excess heat capacities, CpE, have been obtained at 25 °C for the following alkan-1-ols, Cm H2m+1 OH at low mole fraction, x1 < 0.1, in n-alkanes, Cn H2n+2, (m,n): (1,6), (1,7), (2,10), (4,10), (6,6), (6,9), (6,10), (6,12), (6,16), (7,10), (10,6), (10,10), (10,16), (11,10), (12,10), (14,6), (16,6) and (16,16). The Treszczanowicz–Kehiaian (TK) association theory was applied to the associative part of the apparent heat capacity, ΦC(assoc), obtaining ΔH° for hydrogen-bond formation and the volume-fraction equilibrium constant, K4Φ, for formation of tetramers which are the dominant multimers. K4Φ are related to K4, an equilibrium constant for hydrogen-bond formation in tetramers, which is found to be independent of alkan-1-ol chain length from m= 4 to 16, while K4 for m= 2 and 1 are larger. A common K4 implies a single corresponding states curve (CSC) of ΦC(assoc) either against ψ1, the number of moles of alcohol per mole of segments in the solution, as introduced by Pouchly, or against molarity. The CSC is obeyed by the dilute solution data, with some deviation for methanol and ethanol systems, and also by ΦC(assoc) measured throughout the whole concentration range at 25 °C for m= 6 with n= 7, 8, 12 and 16 and for m= 1 (high and low concentration only), 4, 6 and 10 with n= 7 and m= 2, 4, 6 and 10 with n= 12 (again with m= 2 as exception). The CSC in reduced form allows the determination of the size of the dominant multimer from experimental ΦC data and should be followed by alkan-1-ol–inert solvent data at any temperature. CpE for constant m increases with n, and this is a consequence of the adherence to a CSC as well as being predicted by the TK theory. Together with literature results for other systems it is found that for n= constant, CpE increases as m decreases, reaching a maximum at m= 3, then decreasing again for m= 2 and 1. The behaviour of m= 10–3 is a consequence of the CSC, whereas m= 2 and 1 represent deviations from the CSC.

Heat capacities, self-association and complex formation in alcohol–ester systems

The apparent molar heat capacity, Φc, its associative part, Φc(assoc), and the excess heat capacity, CEp, have been obtained at 25 °C through the concentration range for methanol, hexan-1-ol and decan-1-ol in a series of proton acceptor solvents: methyl acetate, ethyl acetate, octyl acetate, methyl octanoate, decyl acetate and methyl tetradecanoate. The results are explained quantitatively by the Treszczanowicz–Kehiaian (TK) association model and the Flory lattice model in terms of alcohol self-association into tetramers, characterized by an OH—OH equilibrium constant and enthalpy, and by alcohol–ester complex formation characterized by an OH—COO equilibrium constant and enthalpy, each independent of alcohol and ester chain length. As predicted, Φc(assoc) at infinite alcohol dilution is independent of the choice of alcohol, but increases with increasing ester chain length. With increasing alcohol concentration Φc(assoc) passes through a maximum which is much reduced compared with the inert solvent case. For each alcohol, CEp increases with increasing ester chain length, while for the same ester, CEp decreases with increasing alcohol chain length. dCEp/dT is positive for mixtures of methanol and methyl acetate, as predicted by the TK model. It is negative for decanol with methyl acetate, contrary to the TK model, but consistent with a non-random distribution of alcohol tetramers in the solution.

Excess volumes of mixtures of glymes with normal, branched and cyclic alkanes

Abstract The molar excess volume V E has been obtained trough the concentration range at 298.15 K for 41 systems composed of the series of glymes (oxaalkanes), CH 3 –O–(CH 2 –CH 2 –O) m –CH 3 with m =1 to 4 with normal, branched and cyclic alkanes. With literature values, a comprehensive view is given of (a) the change of equimolar V E with the molecular sizes of the components and with alkane structure and (b) concentration dependences which are S-shaped for certain systems. Equimolar H E were obtained at 303.15 K for 43 of the systems and used in applying the Prigogine–Flory (PF) theory to successfully predict the V E features. Evidence for association (Hydrogen bonding) in the pure glymes is reviewed. This accounts for discrepancies between PF theory and experiment which are minor for V E , but major for d V E /d T and C p E as seen through a consideration of literature values of these quantities.

Heat capacity and structure in strongly-interacting systems

The heat capacity has been used to reveal H-bonded multimers, predominantly tetramers, in solutions of 1-alkanols in inert solvents, e.g., alkanes. The Treszczanowicz-Kehiaian association theory has been applied to the association part of the apparent molar heat capacity, rpc, of the alcohol in solution, giving AHo for H-bond formation and the volume-fraction equilibrium constant K4CP for the formation of tetramers. The KqP are related to K4, an equilibrium constant for H-bond formation in tetramers which is found to be independent of alkanol carbon number from 3 to 16 while it is slightly larger for methanol and ethanol. implies a single corresponding states curve (CSC) of rpc(assoc) against g1 the number of moles of hydroxyl segments per mole of segments in the solu- tion (essentially molarity) as introduced by Pouchly. Data for 1-alkanol + n-alkane systems follow a CSC characteristic of tetramers rather than CSC predicted by continuous association models, e.g., Mecke-Kempter or Kretschmer-Wiebe. The CSC approach can be applied to enthalpy and resid- ual entropy data. It predicts characteristic concentration dependences of the excess functions; for instance, SE is positive at low alcohol concen- tration then negative. The replacement of the inert solvent by a proton- acceptor, e.g., benzene, introduces a new association between the alcohol and the proton-acceptor. Characteristic changes occur in C and HE. P Grolier and collaborators have found a wide variety of systems for which CpE has a surprising W-shape concentration dependence with two minima separated by a maximum, or two regions of positive curvature separated by a region of negative curvature. It is suggested that the W-shape is a consequence of local composition non-randomness caused bz large values of HE and GE occurring in strongly-interacting systems. Cp is the superpo- sition of two contributions. One is negative and of parabolic concentra- tion dependence as found in systems where a component is polar or anisotropic in molecular shape. The other contribution due to non-random- ness is positive and large toward the middle of the concentration range, i.e., the critical concentration of the system. A common K4

Excess heat capacities of mixtures of two alcohols

The molar excess heat capacity (CEp) has been obtained through the concentration range at 25 °C for mixtures of alkan-1-ols: methanol with butanol, hexanol and decanol and decanol with butanol and hexanol. CEp is negative and small, increasing in magnitude with difference in chain length of the alkan-1-ols to a maximum of –3 J mol–1 K–1 for methanol–decanol. CEp has also been measured at 25 °C for methanol and other alkanols mixed with iso-, sec- and tert-butyl alcohol (I), 2-methylbutan-2-ol (II), 3-methylpentan-3-ol (III) and 3-ethyl-pentan-3-ol (IV). With increasing steric crowding of the tertiary OH, CEp becomes extremely large, –18 J mol–1 K–1 for methanol–IV at equimolar concentration, the apparent molar Cp of methanol in IV being negative at low concentration. The negative CEp(x) for methanol–I is of normal positive curvature, but with II, CEp has a W-shaped concentration dependence exhibiting two regions of positive CEp(x) curvature separated by a region of negative curvature. An extension of the Treszczanowicz–Kehiaian association model has been made to alcohol–alcohol mixtures with consideration of multimers mainly confined to linear tetramers. Assuming equal enthalpies and equilibrium constants for H-bonding between like alkan-1-ols (AA and BB) and between unlike alkanols (AB) leads to positive CEp predictions. The experimental negative CEp values are associated with a lowering of the equilibrium constant for H-bond formation in the alkan-1-ol with increasing chain length coupled with relatively stronger AB bonding. The equilibrium constant, KBB, for H-bonding between tertiary alcohol hydroxyls is found to be lowered by steric crowding of the tertiary hydroxyl, raising the Cp of the pure liquid. The constant, KAB, for bonding between primary (A) and tertiary (B) hydroxyls in the multimer chain is less affected by the tertiary crowding resulting in lower solution Cp and large negative CEp. The S- and W-shape concentration dependences require modifications of KAB owing to the proximity in the tetramer chain of either A or B alcohols resulting in an effective concentration dependence of KAB.

W-shape excess heat capacities, upper critical solution temperatures and non-randomness in oligomeric oxaalkane–dimethylsiloxane systems

Excess heat capacities, CEp, of oxaalkane (glyme)–dimethylsiloxane systems at 25 °C are reported for 2,5-dioxahexane (monoglyme), 2,5,8-trioxanonane (diglyme), 2,5,8,11-tetraoxadodecane (triglyme) and 2,5,8,11,14-pentaoxapentadecane (tetraglyme) with hexamethyldisiloxane, octamethyltrisiloxane, decamethyltetrasiloxane and octamethylcyclotetrasiloxane, except for tetraglyme with trisiloxane and tetrasiloxane, both incompletely miscible at 25 °C. In addition, CEp was obtained for diglyme mixed with a dimethylsiloxane oligomer corresponding to an average of 15 siloxane segments. W-shaped CEp–composition curves were found for all systems and were attributed to local non-randomness. UCST values were obtained for some systems. Non-randomness was quantified through the concentration–concentration correlation function, Scc=[∂2(G/RT)/∂x2]–1, where G is the solution molar free energy. UCST values were calculated using three models for G, corresponding to the Flory–Huggins combinatorial with interaction between molecular volumes or between molecular surfaces, and a combinatorial due to Prausnitz and collaborators with interaction between surfaces. The last model is best for these systems where there is a large difference in molecular surface/volume ratios. It gives reasonable UCST predictions and a quantitative correlation between the compositions of the maxima in Scc and CEp, as well as a qualitative correlation between the shapes of Scc and CEp curves.

Molecular structure and orientational order effects in enthalpies and heat capacities of solute transfer into n-hexadecane. Part 2.—Cyclic and aromatic solutes

Enthalpies ΔH(b→n) and heat capacities ΔCp(b→n) have been obtained at 25 °C for the transfer of solute molecules into n-hexadecane (n) from the highly branched C16 hydrocarbon 2,2,4,4,6,8,8-heptamethylnonane (b). Solutes (S) include the cycloalkane series, dimethylcyclo-hexane isomers, bicycloalkanes of differet configurations, alkenes, steroids, benzene and its methyl and chlorine derivatives, bicyclic aromatics and two nematogens. Values of ΔH(b→n) and ΔCp(b→n) vary widely but fall on two similar correlation curves. They reflect a balance of two temperature-sensitive ordered contacts: one, between n molecules, is broken on transfer of the solute, while the second is formed between the S and n molecules. The extent to which the S–n contact is ordered varies with solute molecular shape and flexibility, leading to either a net gain or loss of order during transfer. The balance determines the sign of ΔH(b→n) and ΔCp(b→n) and the position of the solute on the correlation curve.

S-Shaped composition dependence of excess thermodynamic quantities for cyclohexane mixtures with globular alkanes

Expansion coefficients α, isothermal compressibilities, thermal pressure coefficients γ and heat capacities have been measured at 25°C for the cyclohexane+trans-decalin system. An S-shaped composition dependence, positivelnegative for highllow cyclohexane compositions is found for Cp′E dVE/dT and the thermal expansion contribution to Cp′E namely ΔαγVT. The thermal motion contribution to Cp′E, namely ΔCv is close to zero. The positive excursion of these mixing quantities at high cyclohexane content is anomalous. Correspondingly, the mixing quantity-ΔγVT deviates strongly in this region from the predicted equality with HE. The literature and this work show that all these excess quantities behave similarly for cyclohexane mixed with cyclooctane, methylcyclohexane and some highly branched alkanes. The unusual composition dependence of the thermodynamic quantities is consistent with ‘order’ occurring when any large alkane molecule of globular shape is added to cyclohexane. This is speculatively associated with an interference by the globular alkane with the relatively free rotation of cyclohexane molecules.