Heat Capacities Of Solutions Research Articles

The mechanism of the type III antifreeze protein action: a computational study

The random network model of water quantitatively describes the different hydration heat capacities of polar and apolar solutes in terms of differential distortions of the water–water hydrogen bonding angle in the first hydration shell. This method of hydration analysis is applied here to study the hydration of the wild type III thermal hysteresis protein from eel pout and three mutations at residue 16. Wild type and one mutant have full activity, the other two mutants have little or no anti-freeze (thermal hysteresis) activity. The analysis reveals significant differences in the hydration structure of the ice-binding site (centered on residue 16) among four proteins. For the A16T and A16Y mutants with reduced activity, polar groups have a typical polar-like hydration. For the wild type and mutant A16C with 100% of the wild type activity, polar groups have unusual, very apolar-like hydration. In the latter case, hydrating water molecules form a more ice-like pattern of hydrogen bonding on the ice-binding face, while in the former case water–water H-bonds are more distorted and more heterogenous. Overall, the binding surface of active protein strongly enhances the water tetrahedral structure, i.e. promotes ice-like hydration. It is concluded that the specific shape, residue size and clustering of both polar/apolar groups are essential for the binding surface to recognize, and preferentially interact with nascent ice crystals forming in liquid water.

Thermody namic functions of 1,2-alkanediols in dilute aqueous solutions

The heat capacities of binary aqueous solutions of 1,2-ethanediol, 1,2-propanediol and 1,2-butanediol were measured at temperatures ranging from 283.15 to 338.15 K by differential scanning calorimetry. The partial molar heat capacities at the infinite dilution were then calculated for the respective alkanediols. For 1,2-ethanediol or 1,2-propanediol, the partial molar heat capacities at the infinite dilution of increased with increasing temperature. In contrast, the partial molar heat capacities of 1,2-butanediol at the infinite dilution decreased with increasing temperature.

Partial molar volumes and heat capacities of single ions in aqueous solution over the temperature range 288.15 to 328.15 KElectronic supplementary information (ESI) available: Densities, apparent molar volumes and apparent molar heat capacities of aqueous solutions of NaBPh4, Ph4PBr, Ph4PCl and NaBr. See http://www.rsc.org/suppdata/cp/b4/b407875f/

The partial molar volumes, V02, and partial molar heat capacities,C0p,2, at infinite dilution have been determined for the electrolytes tetraphenylphosphonium chloride, tetraphenylphosphonium bromide, sodium tetraphenylborate and sodium bromide in aqueous solution over the temperature range 288.15 to 328.15 K. These results, along with literature data for sodium chloride, were used to derive V02 and C0p,2 values for the electrolyte tetraphenylphosphonium tetraphenylborate (Ph4PBPh4). An extrathermodynamic assumption based on Ph4PBPh4 as the reference electrolyte was used to evaluate the contributions of the cation and anion and thence to establish scales of single-ion volumes and heat capacities. Using the proton as an example, comparisons were made with other single ion scales reported in the literature.

Heat capacity of solid solutions of deuteromethane in krypton. Nuclear spin conversion in CD4 molecules

A study is made of the heat capacity of the binary solid solutions (CD4)0.01Kr0.99 (in the temperature interval ΔT=0.9–7 K) and (CD4)0.05Kr0.95 (ΔT=0.7–20 K) and of the ternary solid solutions (CD4)0.01Kr0.988(O2)0.002 (ΔT=0.7–11 K) and (CD4)0.05Kr0.948(O2)0.002 (ΔT=0.8–4 K). The contribution of the rotational subsystem to the heat capacity of the solutions is separated out. The influence of temperature, oxygen impurities, and the interaction between the deuteromethane molecules on the effective conversion rate is studied. The energy differences between the lowest-lying levels of the nuclear spin species A and T of deuteromethane is determined, and the effective characteristic conversion times are found. Rapid conversion of isolated CD4 molecules is observed in CD4–Kr solutions. It is shown that in the CD4–Kr solutions, as in CH4–Kr solutions, a hybrid mechanism of conversion is dominant at low temperatures (T<1.4 K), and the “bottleneck” governing the conversion rate is the intermolecular effective octupole–octupole interaction and the related probability of transfer of the conversion energy to phonons.

PVTxand Isochoric Heat Capacity Measurements for Aqueous Methanol Solutions

Isochoric heat capacity and PVTx properties of an aqueous methanol solution (0.50 mass fraction or 0.36 mole fraction of methanol) were measured in the liquid phase with a twin-cell adiabatic calorimeter. Temperatures ranged from 333 to 422 K, and pressures ranged to 20 MPa. The calorimetric cell (70 cm3 capacity) was surrounded by adiabatic thermal shielding (high vacuum) and a steel-sheathed electric heater wound tightly on its surface. By combining the various sources of experimental uncertainty using a root-sum-of-squares formula, the relative uncertainty of C V is estimated to be 2%. The uncertainties of the density, temperature (absolute), and pressure measurements are, respectively, about 0.1%, 40 mK, and ±7 kPa. The measured densities were used to calculate excess molar volumes that were compared with values calculated with a reliable model by Simonson et al. Good agreement within ±0.008 cm3ċmol−1 (or ±0.03% of the density) was found between measured values of excess molar volume and those calculated from the model. Values of saturated liquid densities were determined by extrapolating experimental P-T data to the saturation curve.

Temperature dependence of aqueous solubility of anthracenes: accurate determination by a new generator column apparatus

A new apparatus suitable for accurate measurement of extremely low aqueous solubilities of polynuclear aromatic hydrocarbons (PAH) employing the generator column technique of May et al. has been constructed. In our implementation, the generator column, the extractor column, and the analytical HPLC column are linked by means of a system of three rotary HPLC valves that provide for a complete and convenient control of the measurement sequence including the introduction of calibration samples. The three steps of the measurement, viz. the generation of the saturated solution, the extraction of the solute, and the quantification of the sample are carried out as an in-line procedure, just by switching the valves. The performance of the individual parts and the apparatus as a whole has been optimized and duly validated. The precision of replicate measurements and the day-to-day reproducibility established with anthracene as a test solute are better than ±2%. The agreement of the measured anthracene aqueous solubilities and derived thermodynamic properties (such as the solution enthalpy) with reliable values previously reported in literature confirms the correct performance of the new apparatus. Using the new apparatus aqueous solubilities of anthracene, 2-methylanthracene, 9-methylanthracene, and 9,10-dimethylanthracene have been measured in 5 K increments in the range of temperatures from 273 to 323 K. The data were fitted to a three parameter smoothing equation with approximately 2% relative standard deviation in the mole fraction solubility. Thanks to the precision and the extent of the measured solubility data, reliable thermodynamic properties (Gibbs energy, enthalpy, and heat capacity) of solution for the four anthracenes in water at infinite dilution could be calculated from these fits.

Enthalpy of formation of pargasite by high-temperature solution calorimetry and heat capacity of pargasite and fluoropargasite by differential scanning calorimetry

Synthetic pargasite with near end-member cation stoichiometry (X F = F/((OH) + F) = 0) and natural, gem-quality fluoropargasite from Sri Lanka (X F = 0.605) have been studied by high-temperature lead-borate drop-solution calorimetry under dynamic measuring conditions in flowing Ar. IR-spectroscopy shows that both samples are highly Mg-Al disordered over M(2) and M(3). For synthetic pargasite, five mutually consistent reaction cycles were constructed in which the critical volatile component H 2 O was treated in fundamentally different ways, namely as the simple “oxide” ( i.e. , as the enthalpy difference [H TCal - H 298.15 ] only), or as the (OH)-component in both low and high water-content phases. The molar enthalpies of drop solution of the participating solid phases pargasite, brucite, corundum, jadeite, diopside, enstatite, calcite, magnesite, quartz, tremolite and talc were measured under identical conditions in the same calorimeter. Our refined value for the enthalpy of formation from the elements at 298.15 K for Mg-Al disordered, end-member pargasite with X F = 0 lies between - 12651.4 ± 12.7 and - 12659.4 ± 14.0 kJ/mol, depending on the internally consistent thermodynamic data set used for the reaction components. This value is somewhat less negative than the thermodynamically derived - 12678 ± 19 kJ/mol suggested by Westrich & Holloway (1981), but differs considerably from the value of - 12719.6 ± 5.3 kJ/mol currently used in the data set of Holland & Powell (1998), or the value of - 12623.396 kJ/mol quoted by Helgeson et al. (1978). The enthalpy of solution for natural fluoropargasite is almost identical to that of synthetic pargasite when cationic deviations from end-member composition are accounted for. This result corroborates the enthalpy of formation obtained for synthetic pargasite and shows that not only H 2 O but also fluorine is expelled from the system without interacting with the lead borate melt. Heat capacity data from both samples have been obtained by differential scanning calorimetry in step-scanning mode for the temperature range 50 - 50 °C (pargasite) and 50 - 420 °C (fluoropargasite). The following best-fit equations are valid for compositions corrected to ideal end-member cation stoichiometry (T in Kelvin, C p in J mol −1 K -1 ): synthetic pargasite , NaCa 2 Mg 4 Al 3 Si 6 O 22 (OH) 2 : C p = 1511.14731 - 0.045825 T - 2.00898 10 6 T −2 - 13389.1321 T −0.5 , natural fluoropargasite , NaCa 2 Mg 4 Al 3 Si 6 O 22 (OH) 0.79 F 1.21 : C p = 1585.2892 - 0.10582 T - 1.30629 10 6 T −2 - 14647.5800 T 0.5 . The polynomial for corrected natural fluoropargasite was then used to model the heat capacities of the OH and F end-members of the pargasite-fluoropargasite series employing literature data for brucite, Mg(OH) 2 , and sellaite, MgF 2 , and assuming ideal (OH, F) mixing. The pargasite estimate is within 0.8% of the above polynomial for measured synthetic pargasite. The modelled polynomials differ by about 0.3% (pargasite) and 1% (fluoropargasite) from the respective end-member entries in the internally consistent data set of Holland & Powell (1998). The enthalpy difference [H 985 K - H 298.15 K ] of 625.59 kJ/mol obtained from the modelled polynomial for end-member fluoropargasite compares well (0.6%) with a value of 621.55 ± 0.85 kJ/mol obtained experimentally by Westrich & Navrotsky (1981) for synthetic end-member fluoropargasite by transposed temperature drop calorimetry.

Apparent molar volumes and apparent molar heat capacities of dilute aqueous solutions of ethanol, 1-propanol, and 2-propanol at temperatures from 278.15 K to 393.15 K and at the pressure 0.35 MPa

Apparent molar volumes V φ and apparent molar heat capacities C p, φ have been determined for dilute aqueous solutions of ethanol, 1-propanol, and 2-propanol at temperatures from 278.15 K to 393.15 K and at the pressure 0.35 MPa. The molalities investigated ranged from 0.05 mol· kg −1 to 1.0 mol· kg −1 . We used a vibrating tube densimeter (DMA 512P, Anton PAAR, Austria) to determine the densities and volumetric properties. Heat capacities were obtained using a twin fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter (NanoDSC 6100, Calorimetry Sciences Corporation, American Fork, UT, USA). The results were fit by regression to equations that describe the surfaces ( V φ , T, m) and ( C p, φ , T, m). Infinite dilution partial molar volumes V 2 0 and heat capacities C 0 p,2 were obtained over the range of temperatures by extrapolation of these surfaces to m=0 mol· kg −1 .

The volumetric and thermochemical properties of [formula omitted], [formula omitted], [formula omitted], and [formula omitted] at [formula omitted] K and [formula omitted] MPa

Relative densities and relative massic heat capacities have been measured for aqueous solutions of Y( ClO 4) 3 , Yb( ClO 4) 3 , Dy( ClO 4) 3 , and Sm( ClO 4) 3 at T=(288.15,298.15,313.15, and 328.15) K and p=0.1 MPa. These measurements were made in the concentration range 0.01624⩽m/( mol· kg −1)⩽0.41822 using a Sodev 02D Vibrating Tube Densimeter and a Picker Microflow Calorimeter, respectively. To counter the potential effects of hydrolysis, aqueous solutions of the investigated salts were acidified with perchloric acid. After correcting for the presence of the acid, the measured properties were used to calculate apparent molar volumes and apparent molar heat capacities for solutions of the perchlorate salts in water. The calculated apparent molar properties were modeled at each investigated temperature using Pitzer ion interaction equations to produce estimates of apparent molar volumes and heat capacities at infinite dilution. In addition, the temperature dependences of the apparent molar properties of each salt were found to be well modeled by temperature dependent Pitzer ion interaction equations. The Helgeson, Kirham, and Flowers equations of state were incorporated into the Pitzer ion interaction equations in order to model the infinite dilution properties of the salts. Single ion volumes and heat capacities of the investigated trivalent metal cations have been calculated and compared to previously reported values.

Detection of nuclear spin conversion of CD4 molecules in a solid solution of deuteromethane in krypton

The heat capacity of the solid solutions (CD4)0.05Kr0.95 and (CD4)0.05Kr0.948(O2)0.002 are investigated in the temperature interval 0.7–1.6 K. The contribution to the heat capacity of the solutions from the rotational subsystem is separated out. Conversion between the A and T nuclear spin species of CD4 is detected. It is shown that in unconcentrated solutions of methane isotopes in krypton the rate of conversion is larger for the deuteromethane molecules than for the methane molecules.

Thermodynamics of proton dissociation from aqueous 1-propanoic and 1-butanoic acids at temperatures 278.15⩽( T/K)⩽393.15 and pressure p=0.35 MPa: apparent molar volumes and apparent molar heat capacities of aqueous solutions of the acids and their sodium salts

We have determined apparent molar volumes V φ and apparent molar heat capacities C p, φ for aqueous solutions of 1-propanoic acid and 1-butanoic acid and of their sodium salts. Molalities m ranged from 1.0 to 0.01 mol· kg −1 and temperatures T ranged from 278.15 K to 393.15 K at a constant pressure of p=0.35 MPa. Density measurements from a vibrating-tube densimeter (DMA 512P, Anton Paar, Austria) were used to obtain V φ values. A twin fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter (NanoDSC 6100, Calorimetry Sciences Corporation, American Fork, UT, USA) was used to obtain C p, φ values. We used regressions to empirical equations to describe ( V φ , m, T) and ( C p, φ , m, T) for each solute. Extrapolation of the fitted surfaces to the m=0 mol· kg −1 plane gave standard partial molar volumes V 2 0 and heat capacities C 0 p,2 . We have used literature values of V φ and C p, φ for aqueous NaCl, HCl, and NaOH solutions to calculate estimated Δ r V m and Δ r C p, m values for the volume and heat capacity changes for proton dissociation from each carboxylic acid as functions of m and T. We then determined the ionization enthalpies Δ r H m and equilibrium molality quotients Q a for these dissociations as functions of m and T by integrating (Δ r C p, m , m, T).

Temperature and Concentration Dependence of Heat Capacity of Model Aqueous Solutions

Heat capacities of binary aqueous solutions of different concentrations of sucrose, glucose, fructose, citric acid, malic acid, and inorganic salts were measured with a differential scanning calorimeter in the temperature range from 5°C to 65°C. Heat capacity increased with increasing water content and increasing temperature. At low concentrations, heat capacity approached that of pure water, with a less pronounced effect of temperature, and similar abnormal behavior of pure water with a minimum around 30°C–40°C. Literature data, when available agreed relatively well with experimental values. A correction factor, based on the assumption of chemical equilibrium between liquid and gas phase in the Differential Scanning Calorimeter, was proposed to correct for the water evaporation due to temperature rise. Experimental data were fitted to predictive models. Excess molar heat capacity was calculated using the Redlich–Kister equation to represent the deviation from the additive ideal model.

Modeling and simulation of saline extractive distillation columns for the production of absolute ethanol

A steady-state equilibrium-stage model based on normalized MESH equations is proposed to simulate saline extractive distillation columns and applied to the production of absolute ethanol, using CaCl 2 to break the ethanol–water azeotrope. Among the activity coefficient models available for mixed solvent/salt systems, the NRTL-E model (with refitted parameters), is shown to be the most accurate to correlate the experimental vapor–liquid equilibrium (VLE) data available for the ethanol/water/CaCl 2 system. A model is proposed and implemented to calculate the salt contribution to the liquid phase enthalpy by combining calorimetric quantities of the solute such as its relative apparent enthalpy and the infinite dilution values of its integral heat of solution and partial isobaric heat capacity. The column model is solved by use of the Newton–Raphson method with a damping factor found by minimizing the Euclidean norm of the vector of discrepancy functions associated with the MESH equations.

Temperature dependence of the internal dynamics of a calmodulin-peptide complex.

The temperature dependence of the fast internal dynamics of calcium-saturated calmodulin in complex with a peptide corresponding to the calmodulin-binding domain of the smooth muscle myosin light chain kinase is examined using 15N and 2H NMR relaxation methods. NMR relaxation studies of the complex were carried out at 13 temperatures that span 288-346 K. The dynamics of the backbone and over four dozen methyl-bearing side chains, distributed throughout the calmodulin molecule, were probed. The side chains show a much more variable and often considerably larger response to temperature than the backbone. A significant variation in the temperature dependence of the amplitude of motion of individual side chains is seen. The amplitude of motion of some side chains is essentially temperature-independent while many show a simple roughly linear temperature dependence. In a few cases, angular order increases with temperature, which is interpreted as arising from interactions with neighboring residues. In addition, a number of side chains display a nonlinear temperature dependence. The significance of these and other results is illuminated by several simple interpretative models. Importantly, analysis of these models indicates that changes in generalized order parameters can be robustly related to corresponding changes in residual entropy. A simple cluster model that incorporates features of cooperative or conditional motion reproduces many of the unusual features of the experimentally observed temperature dependence and illustrates that side chain interactions result in a dynamically changing environment that significantly influences the motion of internal side chains. This model also suggests that the intrinsic entropy of interacting clusters of side chains is only modestly reduced from that of independent side chain motion. Finally, estimates of protein heat capacity support the view that the major contribution to the heat capacity of protein solutions largely arises from local bond vibrations and solvent interactions and not from torsional oscillations of side chains.

A new version of differential flow heat capacity calorimeter; tests of heat loss corrections and heat capacities of aqueous NaCl from T = 300 K to T = 623 K

A new differential flow heat capacity calorimeter was constructed. It is designed to operate at temperatures up to 700 K and pressures up to 35 MPa and its primary use is for determining the massic heat capacities at constant pressure of dilute aqueous solutions. The instrument works in the so-called isoperibol regime, where the fluid sample flowing through the cell is heated by an electrical heater and the power necessary to provide a constant temperature rise is measured relative to that for a reference fluid (water). From the two values of power for sample and water the ratio of massic heat capacities of the sample to that of water can be calculated. A thorough investigation of calibration techniques showed that the calorimetric performance is very sensitive to the thermal conductivities of the sample and reference fluids. Measurements under turbulent flow conditions are questionable since there is no guarantee that by changing the flow rate the experiments and the calibrations would be performed at the same flow conditions. The procedure is very accurate and sensitive when measuring the difference in heat capacity between a solvent and a dilute solution of solute in the same solvent. The calorimeter was used to measure heat capacities of aqueous solutions of NaCl at eight temperatures up to 623 K and pressures to 30 MPa. The newly obtained values show consistency with previously published results and enlarge the database of experimental values aboveT= 573 K, where experimental data are rare.

Thermodynamics of aqueous amines: excess molar heat capacities, volumes, and expansibilities of {water + methyldiethanolamine (MDEA)} and {water + 2-amino-2-methyl-1-propanol (AMP)}

The heat capacities of aqueous solutions of methyldiethanolamine (MDEA) and 2-amino-2-methyl-1-propanol (AMP) were measured in a heat-flux differential scanning calorimeter at temperatures 278.15K⩽T⩽ 368.15 K, to yield excess molar heat capacities Cp,mEover the whole mole fraction range. Densities of aqueous AMP were determined with a vibrating tube densimeter over the temperature range 293.15 K⩽T⩽ 353.15 K, and used to derive excess molar volumesVmE and excess molar thermal expansibilitiesEmE= (∂VmE/∂T)p. A modified Redlich–Kister treatment was employed to describe the excess functions. The standard partial molar propertiesCp,2o and V2oatT= 298.15 K for MDEA, as determined by extrapolating the Redlich–Kister expressions to infinite dilution, are consistent with results from a Picker calorimeter and densimeter at mole fractionsx2 (MDEA) ⩽ 0.03. Values ofCp,2o and V2ofor AMP from measurements at low mole fractions { x2(AMP) ⩽ 0.03} differ from the Redlich–Kister extrapolations, suggesting that specific interactions take place in dilute solutions. The results are consistent with Lumry’s model (Faraday Symp. Chem. Soc. 1982, 17, 93–108), with MDEA being less hydrophobic than AMP. The mole-fraction dependence of the reduced excess heat capacities is most pronounced at T< 298.15 K, with clear differences in the behaviour of Cp,mE(water+MDEA) and Cp,mE(water+AMP), while the behaviour ofVmE for the two aqueous alkanolamines is similar.

Heat Capacities of Concentrated Aqueous Solutions of Sodium Sulfate, Sodium Carbonate, and Sodium Hydroxide at 25 °C

Heat capacities of aqueous solutions of Na2SO4 and Na2CO3 up to near saturation (1.9 and 2.5 mol·kg-1, respectively) and of NaOH (to 7 mol·kg-1) have been measured at 25 °C with a Picker flow calorimeter. The calorimeter performance was checked using concentrated NaCl(aq) solutions. On the basis of these measurements, an experimental protocol suitable for the reliable determination of the heat capacities of concentrated electrolyte solutions by Picker calorimetry was established. The heat capacities for Na2SO4(aq), Na2CO3(aq), and NaOH(aq) and literature data for the apparent molar volumes of NaOH(aq) at 25 °C were correlated using the Pitzer formalism. A number of inadequacies in previous models at high concentrations and for extrapolation to infinite dilution are discussed. In particular, it has been confirmed that the heat capacity data for Na2CO3(aq) at low concentrations must be corrected for the hydrolysis of the carbonate ion. Standard partial molar heat capacities for the three salts and the stand...

Thermodynamics of ionization of water at temperatures 278.15⩽⩽⩽T / K⩽⩽⩽ 393.15 and at the pressurep = 0.35 MPa: apparent molar volumes and apparent molar heat capacities of aqueous solutions of potassium and sodium nitrates and nitric acid

The apparent molar volumes Vφand apparent molar heat capacitiesCp, φ of aqueous solutions of potassium and sodium nitrate and of nitric acid have been determined over the temperature range 278.15 ⩽T/K⩽ 393.15 and molality range 0.015 ⩽m/ (mol · kg−1) ⩽ 0.5 at the pressure p= 0.35 MPa. Densities of these solutions were measured using a vibrating-tube densimeter (DMA 512, Anton Paar, Austria). The density results were used to calculateVφ for each solution. The massic heat capacities of these same solutions were measured using a twin fixed-cell, differential-output, temperature-scanning calorimeter (NanoDSC model 6100, Calorimetry Sciences Corporation, Spanish Fork, UT, U.S.A.) and these measurements were used to calculate Cp, φfor each solute. The Vφand Cp, φresults were fit to semi-empirical equations by regression. These equations were used to calculate the change in volume ( m, T,ΔrVm ) and the change in heat capacity ( m,T , ΔrCp, m) for ionization of water in KNO3 (aq) and NaNO 3(aq). The ( m, T,ΔrCp, m ) results were integrated to obtain the change in enthalpy ( m, T, ΔrHm), the molality quotient ( m, T, pQw), and change in entropy ( m,T , ΔrSm) for the ionization reaction of water.

Temperature Dependence of the Excess Molar Heat Capacities for Alcohol−Alkane Mixtures. Experimental Testing of the Predictions from a Two-State Model

A simple association model for alcohol−alkane mixtures, based on the idea that only two energy states are accessible to alcohol molecules in the pure and in the solution states, predicts complex temperature and alcohol concentration dependences of the excess molar heat capacity, . These predictions are tested through the accurate measurement of pure component and solution heat capacities in the 278.15−338.15 K temperature interval. These measurements were performed at low, equimolar, and high alcohol concentrations for a linear alcohol (1-butanol) and a branched alcohol (3-methyl-3-pentanol) mixed with n-decane and with toluene. The qualitative predictions from the two-state model are corroborated by the data. According to this model, the very different behaviors found for the different systems arise simply through the change in hydrogen bonding Gibbs energy occurring on moving from the linear to the branched alcohol and in going from the inert n-decane to the aromatic toluene.

Thermodynamic analysis of the mutual solubilities of hydrocarbons and water

The analysis of mutual solubilities and calorimetric heats of solution carried out for normal alkanes and water close to 298 K [Fluid Phase Equilib. 156 (1999) 21] is extended to normal alkylcyclohexanes, linear 1-alkenes, and normal alkylbenzenes. Extensive solubility data are available for alkylbenzenes, but relatively little is known about alkylcyclohexanes and alkenes in water. All of these hydrocarbons are more soluble than the corresponding alkanes, but are less volatile than the alkanes (their Henry’s constants descending in the order alkanes>alkenes>alkylcyclohexanes>alkylbenzenes). The solubility minimum for hydrocarbons in water is confirmed by calorimetric data for C 6–C 9 alkylbenzenes (∼291 K) and cyclohexane (298.5 K). It appears that the heat capacity of solution is independent of temperature and increases linearly with carbon number, in the descending order alkanes>alkylcyclohexanes>alkylbenzenes. In the absence of calorimetric data, the relatively limited solubility data for alkenes in water do not allow a reliable determination of the solubility minimum or of the heat capacity of solution. As is the case for water in alkanes, the solubility of water in hydrocarbons at 298 K is relatively insensitive to carbon number (CN), but, unlike that in alkanes and alkylcyclohexanes, the solubility of water in alkenes and alkylbenzenes decreases slightly with increasing CN. Less is known about the heat of solution, but for each family it appears to be independent of temperature and CN, covering the range 24 kJ mol −1 (alkylbenzenes) to 35 kJ mol −1 (alkanes and alkylcyclohexanes). The latter is a better measure of the hydrogen bond energy in hydrocarbon/water systems, since alkanes and alkylcyclohexanes have the least affinity for water. This affinity is measured by the heat of hydrophobic interaction, which makes a negative contribution to the heat of solution, and therefore, as observed, the heat of solution of water in alkenes (33 kJ mol −1) and alkylbenzenes is smaller than that in alkanes and alkylcyclohexanes.