Standard molar enthalpy of combustion and formation of quaternary ammonium tetrachlorozincate [n-C_nH_2n+1 N(CH₃)₃]₂ ZnCl₄

Abstract

The standard molar enthalpy of combustion (Δ_c H ^o _m) and formation (Δ_f H ^o _m) of quaternary ammonium tetrachlorozincate [n-C_nH_2n+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 Δ_c H ^o _m increased and Δ_f H ^o _m 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 -Δ_c H ^o _m =1440.50n +3730.67 and -Δ_f H ^o _m = −85.32n + 1688.22.

Introduction

The quaternary ammonium tetrachlorometallate with the general formula [n- C_nH_2n+1NR₃]₂MX₄ (M = Cu, Mn, Cd, Zn, Co, …, X = Cl, Br, I, R is alkyl, or aryl) (short notation: C_nC₃M) 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_nC₃M (Francesco et al.2002; Gosniowska et al.2000). However, the thermodynamic properties of the C_nC₃M have been reported rarely in the literature. In the present work, the series of quaternary ammonium tetrachlorozincate [n-C_nH_2n+1 N(CH₃)₃]₂ZnCl₄ (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_nC₃Zn have been determined by an oxygen-bomb combustion calorimeter with increasing chain length at T = 298.15 K.

Experimental procedure

ZnCl₂, concentrated HCl and absolute ethanol were analytical grade. n-Octyltrimethylammonium chloride (A.P.), were purchased from TOKYO CHEMICAL INDUSTRY 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_nC₃Zn, the hot absolute ethanol solutions of ZnCl₂, 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_nC₃Zn were analyzed with an MT-3 CHN elemental analyzers (Japan) are listed in the following: Elemental analyses calc. (%) for C₈C₃Zn: 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 C₁₀C₃Zn: 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 C₁₂C₃Zn: 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. Anal. Calcd for C₁₄C₃Zn: C 56.72, H 10.56, N 3.89, Cl 19.74; Found: C 56.06, H 10.20, N 3.84, Cl 19.03. Anal. Calcd for C₁₆C₃Zn: C 58.84, H 10.84, N 3.61, Cl 18.31; Found: C 58.91, H 10.77, N 3.62,Cl18.72. Anal. Calcd for C₁₈C₃Zn: C 60.65, H 11.07, N 3.37, Cl 17.08; Found: C 60.56, H 11.04, N 3.36, Cl 17.55.

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 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_nC₃Zn was fitted with equation Δ_c U _m = [−ε _(calor) · ΔT+Δm _ign • u _ign +V _NaOH•(−59.7)]/m _CnC3Zn, 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_nC₃Zn 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 HNO₃ (aq) from N₂, O₂, and H₂O(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 cm³ 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):

$$\begin{array}{l}{\left[C_8{H}_{17}N{\left(C{H}_3\right)}_3\right]}_2\mathit{ZnC}{l}_4\left(s\right)+\frac{69}{2}{O}_2\left(g\right)\\ =\mathit{ZnO}\left(s\right)+22C{O}_2\left(g\right)+4\mathit{HCl}\left(l\right)+24H_{2}O\left(l\right)\\ +N_2\left(g\right)\end{array}$$

(1)

$$\begin{array}{l}{\left[C_{10}{H}_{21}N{\left(C{H}_3\right)}_3\right]}_2\mathit{ZnC}{l}_4\left(s\right)+\frac{81}{2}{O}_2\left(g\right)\\ =\mathit{ZnO}\left(s\right)+26C{O}_2\left(g\right)+4\mathit{HCl}\left(l\right)+28H_{2}O\left(l\right)\\ +N_2\left(g\right)\end{array}$$

(2)

$$\begin{array}{l}{\left[C_{12}{H}_{25}N{\left(C{H}_3\right)}_3\right]}_2\mathit{ZnC}{l}_4\left(s\right)+\frac{93}{2}{O}_2\left(g\right)\\ =\mathit{ZnO}\left(s\right)+30C{O}_2\left(g\right)+4\mathit{HCl}\left(l\right)+32H_{2}O\left(l\right)\\ +N_2\left(g\right)\end{array}$$

(3)

$$\begin{array}{l}{\left[C_{14}{H}_{29}N{\left(C{H}_3\right)}_3\right]}_2\mathit{ZnC}{l}_4\left(s\right)+\frac{105}{2}{O}_2\left(g\right)\\ =\mathit{ZnO}\left(s\right)+34C{O}_2\left(g\right)+4\mathit{HCl}\left(l\right)+36H_{2}O\left(l\right)\\ +N_2\left(g\right)\end{array}$$

(4)

$$\begin{array}{l}{\left[C_{16}{H}_{33}N{\left(C{H}_3\right)}_3\right]}_2\mathit{ZnC}{l}_4\left(s\right)+\frac{117}{2}{O}_2\left(g\right)\\ =\mathit{ZnO}\left(s\right)+38C{O}_2\left(g\right)+4\mathit{HCl}\left(l\right)+40H_{2}O\left(l\right)\\ +N_2\left(g\right)\end{array}$$

(5)

$$\begin{array}{l}{\left[C_{18}{H}_{37}N{\left(C{H}_3\right)}_3\right]}_2\mathit{ZnC}{l}_4\left(s\right)+\frac{129}{2}{O}_2\left(g\right)\\ =\mathit{ZnO}\left(s\right)+42C{O}_2\left(g\right)+4\mathit{HCl}\left(l\right)+44H_{2}O\left(l\right)\\ +N_2\left(g\right)\end{array}$$

(6)

$${\Delta }_{\mathrm{c}}{H}^{\mathrm{o}}{\mathrm{m}}_{}={\mathrm{M\Delta }}_{\mathrm{c}}{U}^{\mathrm{o}}{\mathrm{m}}_{}+\Delta n\mathit{RT}$$

(7)

$$\Delta n=n_{\mathrm{g}}\left(\mathrm{product}\right)-n_{\mathrm{g}}\left(\mathrm{reactant}\right)$$

(8)

$${\mathrm{\Delta }}_{\mathrm{f}}{H}^{\mathrm{o}}{\mathrm{m}}_{}\left({\mathrm{C}}_{\text{n}}{\mathrm{C}}_3\mathrm{Zn}\right){=\mathrm{\Sigma V}}_{\mathrm{B}}{\mathrm{\Delta }}_{\mathrm{f}}{H}_{\mathrm{m}}{\mathrm{o}}^{}\left(\mathrm{B}\right){-\mathrm{\Delta }}_{\mathrm{c}}{H}^{\mathrm{o}}{\mathrm{m}}_{}$$

(9)

Table 1 The values of the combustion energies of 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_nC₃Zn. 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), H₂O (l) and CO₂(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_nC₃Zn resulted from the Δ_c H ^o _m by an oxygen-bomb combustion calorimeter at T = 298.15 K. Table 2 lists the values of the standard molar energies Δ_c U ^o _m, the enthalpies of combustion Δ_c H ^o _m and the standard molar enthalpies of formation Δ_f H ^o _m result form Δ_c U ^o _m for the C_nC₃Zn.

Table 2 The value of thermochemical functions of the quaternary ammonium tetrachlorometallate C _n C ₃ Zn

The influence of the hydrocarbon chain length on Δ_c H ^o _m and Δ_f H ^o _m of the C_nC₃Zn has been obtained for chain lengths from 8 to 18 carbon atoms. The values of Δ_c H ^o _m and Δ_f H ^o _m show a linear dependence on the number of carbon atoms from experimental data analysis. Figure 1, Figure 2 show a plot of the calculated -Δ_c H ^o _m and -Δ_f H ^o _m vs. C-atoms (n) that gave a straight line relationship from the values of Table 2. The linear regression equation are -Δ_c H ^o _m =1440.50n +3730.67 with a correlation coefficient r = 0.9998 and - Δ_f H ^o _m = −85.32n + 1688.22 with r =0.9512. A striking feature is that Δ_c H ^o _m increased and Δ_f H ^o _m decreased with increasing the chain length. This reason is that the structures of C_nC₃Zn are characteristic of the piling of sandwiches in which a two-dimensional cavities of ZnCl₄ ^2- tetrahedra is sandwiched between two alkylammonium layers. The layers are bound by van der Waals forces between (CH₂)_nCH₃ groups and by long-range Coulomb forces. The –N(CH)₃ ³⁺ groups of the chains occupy the cavities of the ZnCl₄ ^2- layers 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_nC₃Zn (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.

Figure 1

Plot of Δ _c H ^o _m vs. number( n ) of carbon-atoms in the quaternary ammonium tetrachlorometallate C _n C ₃ Zn.

Figure 2

Plot of Δ _f H ^o _m vs. number( n ) of carbon-atoms in the quaternary ammonium tetrachlorometallate C _n C ₃ Zn.

Conclusions

The standard molar enthalpy of combustion and formation of quaternary ammonium tetrachlorozincate [n-C_nH_2n+1 N(CH₃)₃]₂ZnCl₄ (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.