Standard molar enthalpy of combustion and formation of quaternary ammonium tetrachlorozincate [n-CnH2n+1 N(CH3)3]2 ZnCl4

The standard molar enthalpy of combustion (ΔcHom) and formation (ΔfHom) of quaternary ammonium tetrachlorozincate [n-CnH2n+1N(CH3)3]2ZnCl4 have been determined for the hydrocarbon chain length from even number 8 to 18 of carbon atoms (n) by an oxygen-bomb combustion calorimeter. The results indicated that the values of ΔcHom increased and ΔfHom decreased with increasing chain length and showed a linear dependence on the number of carbon atoms, which were caused by that the order and rigidity of the hydrocarbon chain decreased with increasing the carbon atoms. The linear regression equations are -ΔcHom =1440.50n +3730.67 and -ΔfHom = −85.32n + 1688.22.


Introduction
The quaternary ammonium tetrachlorometallate with the general formula [n-C n H 2n+1 NR 3 ] 2 MX 4 (M = Cu, Mn, Cd, Zn, Co, . . ., X = Cl, Br, I, R is alkyl, or aryl) (short notation: C n C 3 M) have been attracted considerable attention because of their physical properties including ferro-, piezo-or pyroelectricity, ferri-, antiferro-or piezomagnetism and their technical application for electro-or magneto-optical devices (Blachnik et al. 1996;Kezhong et al. 2010). The advances in synthesis along with the ease of controlling various structural parameters (metal, halogen and number of carbon atoms in the alkylammonium ion) have made them ideal objects for studies by spectroscopy, calorimetry, diffraction, and a variety of other techniques (Abid et al. 2011;Donghua et al. 2011;Shymkiv et al. 2011). In addition, several theoretical studies have been undertaken to predict the behavior of the C n C 3 M (Francesco et al. 2002;Gosniowska et al. 2000). However, the thermodynamic properties of the C n C 3 M have been reported rarely in the literature. In the present work, the series of quaternary ammonium tetrachlorozincate [n-C n H 2n+1 N(CH 3 ) 3 ] 2 ZnCl 4 (n = 8, 10,12,14,16,18) are synthesized from ethanol solutions. The standard molar enthalpy of formation (Δ f H o m ) and the standard molar enthalpy of combustion (Δ c H o m ) of the C n C 3 Zn have been determined by an oxygenbomb combustion calorimeter with increasing chain length at T = 298.15 K.

Experimental procedure
ZnCl 2 , concentrated HCl and absolute ethanol were analytical grade. n-Octyltrimethylammonium chloride (A. P.), were purchased from TOKYO CHEMICAL INDUS-TRY CO LTD (Japan). n-Decyltrimethylammonium chloride(A.P.), n-Dodecyltrimethylammonium chloride (A.P.), n-Tetradecyltrimethylammonium chloride(A. P.), n-Hexadecyltrimethylammonium chloride(A.P.), n-Trimethylstearylammonium chloride(A.P.) were purchased from J & K CHEMICAL LTD. For the synthesis of C n C 3 Zn, the hot absolute ethanol solutions of ZnCl 2 , concentrated HCl and the corresponding quaternary ammonium were mixed in a 1:2:2 molar ratios. The solutions were concentrated by boiling for 1 h, and then cooled to room temperature. After filtration, the products were recrystallized twice from absolute ethanol and then were placed in a vacuum desiccator for 10 h at about 353 K.
The C n C 3 Zn were analyzed with an MT-3 CHN elemental analyzers (Japan) are listed in the following: Elemental analyses calc. (%) for C 8 C 3 Zn: C 47. 88,H 9.43,N 5.08,Cl 25.75;Found: C 47.45,H 9.50,N 5.13,Cl 24.99. Anal. Calcd for C 10 C 3 Zn: C 51. 37,H 9.88,N 4.61,Cl 23.38;Found: C 50.98,H 9.95,N 4.58,Cl 22.81. Anal. calcd for C 12 C 3 Zn: C 54. 26,H 10.25,N 4.22,Cl 21.41;Found: C 53.93,H 10.34,N 4.26,Cl 21.25 The combustion experiments were performed with a static bomb calorimeter (XRY-1A Shanghai). Benzoic acid (Thermochemical Standard, BCS-CRM-190r) was used as calibrant of the bomb calorimeter. Its massic Table 1 The values of the combustion energies of the quaternary ammonium tetrachlorometallate C n C 3 Zn energy of combustion is Δ c U = −(26460 ± 3.8) J · g −1 under certificate conditions. The massic energy of combustion Δ c U m for each C n C 3 Zn was fitted with equation where ε cal is the energy equivalent of the calorimeter, ΔT is the calorimeter temperature change corrected, Δm ign is the mass of the Nickel-chromium alloy for ignition and the massic energy is u ign = −3.245 kJ · g −1 (U ign = Δm ign • u ign ). m CnC3Zn is the mass of the C n C 3 Zn which were burned, V NaOH is the volume of sodium hydroxid which consumed by nitric acid, the corrections for nitric acid formation were based on −59.7 kJ · mol −1 for the molar energy of formation of 0.1 mol · dm −3 HNO 3 (aq) from N 2 , O 2 , and H 2 O(l) (Matos et al. 2002). The calibration results were corrected to the average mass of water added to the calorimeter: 2500.0 g and the volume of oxygen bomb was 300 ml. From five independent calibration experiments between T = 295.15 K and T = 299.15 K, the energy equivalent ε cal = (13965.4 ± 4.7) J · K -1 was obtained, where the uncertainty quoted is the standard deviation of the mean. For all experiments, ignition was made at T = (298.150 ± 0.001) K. Combustion experiments were performed in oxygen at a pressure p = 3.00 MPa and in the presence of 10.00 cm 3 of water added to the bomb (Matos et al. 2002).

Results and discussion
The individual results of all combustion experiments, together with the mean values and their standard deviations, are given for each compound in Table 1. In accordance with normal thermochemical practice, the uncertainties assigned to the standard molar enthalpies of combustion are, in each case, equal to twice the overall standard deviation of the mean and include the uncertainties in calibration (Henoc et al. 2009). The results are referred to the following reactions (1~6) and the following equation (7~9):  Plot of Δ f H o m vs. number(n) of carbon-atoms in the quaternary ammonium tetrachlorometallate C n C 3 Zn.
Where R is the molar gas constant and M is the molar mass of the C n C 3 Zn. The V B is the stoichiometric coefficient and the Δ f H o m (B) is the standard molar enthalpy of formation of the combustion products. The standard molar enthalpies of formation of ZnO(s), H 2 O (l) and CO 2 (g) at T = 298.15 K, −(348.28) kJ · mol −1 , −(285.830 ± 0.042) kJ · mol −1 and − (393.51 ± 0.13) kJ · mol −1 (Manuel et al. 2010). The Δ f H o m of the C n C 3 Zn resulted from the Δ c H o m by an oxygen-bomb combustion calorimeter at T = 298.15 K. Table 2   This reason is that the structures of C n C 3 Zn are characteristic of the piling of sandwiches in which a twodimensional cavities of ZnCl 4 2tetrahedra is sandwiched between two alkylammonium layers. The layers are bound by van der Waals forces between (CH 2 ) n CH 3 groups and by long-range Coulomb forces. The -N(CH) 3 3+ groups of the chains occupy the cavities of the ZnCl 4 2layers and are bonded by ion bonds to the chlorine atoms (Weizhen et al. 2011). As the hydrocarbon chain length increases, the formation of the chain conformer plays a more important role in the structural phase transitions. It is known that the order and rigidity of the hydrocarbon chain were decreased with increasing the carbon atoms, that is with increasing mean number of conformationally flexible chain in C n C 3 Zn (Nobuaki et al. 2011), furthermore, the intensities of the ion bonds and van der Walls force decrease with increasing the carbon atoms resulting in that the values of Δ c H o m and Δ f H o m show a linear dependence on the carbon atoms.

Conclusions
The standard molar enthalpy of combustion and formation of quaternary ammonium tetrachlorozincate [n-C n H 2n+1 N(CH 3 ) 3 ] 2 ZnCl 4 (n = 8, 10, 12, 14, 16, 18) have been measured by an oxygen-bomb combustion calorimeter. The results indicated that the values of the standard molar combustion enthalpies Δ c H o m of these compounds increased with increasing chain length and the standard molar formation enthalpies Δ f H o m of these compounds decreased with increasing chain length and showed a linear dependence on the number of carbon atoms.