Spectroscopic and Thermo-gravimetric Analysis of Terbium Myristate

Kamal Kishore1*, Manpreet Singh1, Chitra Singh2 and S. K. Upadhyaya3

1 Department of Chemistry, Eternal University, Baru Sahib, Sirmour-173101 (H.P.) INDIA

2 Department of Education Science & Mathematics, Regional Institute of Education,

Shyamla Hills, Bhopal – 462013, INDIA

3 P. G. Department of Chemistry, S. S. L. Jain College, Vidisha-464001 (M.P.) INDIA

* Correspondence: E-mail: k_81kishore@yahoo.co.in

(Received 06 Sept, 2017; Accepted 19 Nov, 2017; Published 29 Dec, 2017 )

ABSTRACT: The terbium myristate has been investigated by IR, X-ray diffraction and thermogravimetric analysis (TGA). The IR result reveals that fatty acids exist as dimer through hydrogen bonding and terbium myristate possess partial ionic character. The X-ray diffraction suggests that molecular axes are somewhat inclined to the basal plane. The metal cation fit into spaces between oxygen atoms of the ionised carboxyl group without giving a large strain of the bond. The thermal decomposition reaction is found kinetically of zero order and value of energy of activation is in the region of 3.84 to 8.55 K cal mol-1.

Keywords: Terbium myristate; diffraction order and thermogravimetric analysis.


INTRODUCTION: Metallic soaps are simple carboxylates of alkaline-earth and other polyvalent metals. The metal is chemically bound to the anionic headgroup of the fatty acid by both ionic and covalent linkages. The anionic headgroup of a fatty acid molecule consists of a terminal carboxylate group. The physico-chemical properties metal soaps have been investigated by several workers. Mehrotra et. al.1-3 studied the ionic nature of rubidium and cesium soaps by infrared techniques. Solanki and Bhandari4 characterised uranyl soaps whereas Varma and Jindal5 analysed cerium soaps by using infrared and electronic absorpiton spectra. Kinetic study of high oleic sunflower oil sponification was carried out by Gaëlle Poulenat et al. 6 by using FTIR spectroscopy. Kambe et al.7 studied the thermal transition of cobalt soaps by differential thermal analysis and thermogravimetric analysis. Mehrotra et al.8 investigated the thermal behaviour of lanthanum and cerium soaps. The thermogravimetric analysis of aluminium soaps was carried out by Rai and Mehrotra9. The thermal stability of potassium soaps was investigated by Ilina et al.10 Gallot and Skoulious 11 used x-ray diffraction to study the polymorphism of the polar groups of sodium, potassium and lithium soaps in ribbon phases. They explained the appearance of the smectic phase in sodium just prior to the melting point as being related to the melting of the polar groups, whereas in lithium stearate no transition from ribbons to lamellae occurred prior to melting. They attributed this difference to relatively weak association of lithium soap headgroups. Marques et al. 12 found liquid crystallinity in cerium (III) soaps with higher alkyl chain and observed the variation on melting point with chain-length. Binnemans et al.13 reported the mesophase behaviour of lanthanum (III) teteradecanoate and higher homologues as a smectic A phase. Only a relatively small amount of literature exists concerning the solution properties of terbium myristate 14 & 15. A thorough knowledge of their physical properties still appears to be lacking. The reported work deals with studies of infrared, X-ray and thermogravimetric analysis (TGA) of terbium myristate in order to investigate the structure and kinetics of thermal decomposition.

MATERIALS AND METHODS : The chemical synthesis of terbium myristate had already communicated. 14 The infrared absorption spectra were obtained with a Thermo Nicolet 370 spectrophotometer in region of 4000-400 cm -1 using KBr disk method. X-ray diffraction patterns of terbium soaps were obtained with a Bruker AXS D 8 Advance X-ray diffractometer using Cu-Ka radiations filtered by a nickel foil over the range of 3-80°. The thermogravimetric analysis was carried out at constant heating rate of 15°C min -1 under nitrogen atmosphere using thermobalance (Perkin Elmer Diamond TGA/DTA).

RESULTS AND DISCUSSION:

Infrared absorption spectra: The infrared absorption bands for terbium myristate has been assigned and compared with the potassium myristate and myristic acid (Table 1). The absorption maxima, which is characteristic of aliphatic portion of the acid molecule, remain unchanged even when acid is converted into potassium or terbium myristate. The absorption maxima of fatty acids near 2660-2640, 1700, 1400, 950-940, 690-680 and 550 cm-1 is associated with the localized carboxyl group of the acid molecule in the dimeric form and confirm the presence of hydrogen bonding between two molecules of fatty acid. The appearance of two new absorption bands due to symmetric and antisymmetric stretching vibrations of carboxylate ion near 1440-1390 cm-1 and 1560-1540 cm-1, respectively, instead of one band of the carboxyl group at 1700cm -1 in the spectra of fatty acid confirms the compound formation and indicate that it is ionic in nature (Figure 1). The assigned frequencies are in good agreement with the results of other workers16 & 17.


Table 1: IR absorption spectral frequencies (cm-1) with their assignments.

S.No.

Assignments

Myristic

Acid

Potassium myristate

Terbium myristate

1

CH3, C-H asymmetric-stretching

2960 vw

2955 w

2958.00 w

2

CH2, C-H asymmetric-stretching

2920 vs

2920 vs

2916.94 vs

3

CH2, C-H symmetric-stretching

2840 vw

2840 vw

2849.22 vs

4

OH, stretching

2640 vw

2640 vw

-

5

C=O, stretching

1700 vs

-

-

6

COO ,C-O asymmetric stretching

-

1550 vs

1541.15 vs

7

CH2, deformation

1465 ms

1460 ms

1468.73 s

8

COO , C-O symmetric stretching

-

1420 m

1410.20 w

9

C-O stretching, O-H in plane deformation

1430 ms

1445 ms

-

10

CH2 (adjacent to COOH group), deformation

1405 vs

-

-

11

CH3, symmetric deformation

1370 w

-

-

12

Progressive bands

(CH2 twisting and wagging)

1350-

1190 w

1340-

1100 w

1355-

1180 vw

13

CH3, rocking

1120 w

1105-

1120 w

1110.72 ms

14

OH, out of plane deformation

940 m

-

940.96 w

15

CH2, rocking

735-725 ms

755-725 ms

721.02 s

16

COOH bending mode

690 ms

700 ms

691.32 m

17

COOH wagging mode

550 ms

580- 545 s

-

Key to abbreviations: vw = very weak ; vs= very strong; s = strong; m = Medium; ms = Medium strong; w = weak.

Table 2: X-ray analysis and determination of long spacings, d.

S. No.

2?

Sin ?

? / 2 Sin ?

d (Å)

Order (n)

1

4.534

0.0395

19.47

38.94

2

2

6.707

0.0585

13.17

39.50

3

3

9.030

0.0787

9.78

39.14

4

4

11.203

0.0976

7.89

39.46

5

5

13.594

0.1183

6.51

39.05

6

6

20.320

0.1764

4.37

39.30

9

7

24.984

0.2163

3.56

39.17

11

8

27.275

0.2358

3.27

39.20

12

9

34.260

0.2945

2.61

39.23

15

10

48.778

0.4129

1.86

39.17

21

Table 3: Weight loss of Terbium myristate with temperature and time.

S. No.

Time

t

(min.)

Temperature

T

(K)

Weight of soap decomposed

w x 105 (gm)

dw/dt x 105

Wr x 105

1

2

303

00.0

0.0

446.0

2

4

333

13.3

3.3

433.0

3

6

363

28.2

4.7

418.0

4

8

393

29.8

3.7

416.0

5

10

423

33.1

3.3

413.0

6

12

453

35.1

2.9

411.0

7

14

483

37.1

2.6

409.0

8

16

513

41.4

2.6

405.0

9

18

543

46.3

2.6

400.0

10

20

573

60.1

3.0

386.0

11

22

603

156.7

7.1

290.0

12

24

633

293.1

12.2

153.0

13

26

663

381.5

14.7

65.0

14

28

693

393.1

14.0

53.0

15

30

723

402.9

13.4

43.0

16

32

753

416.6

13.0

30.0

17

34

783

419.9

12.3

26.0

18

36

813

426.5

11.8

19.0

19

38

843

433.1

11.4

13.0

20

40

873

439.7

11.0

7.0

21

42

903

441.3

10.5

5.0

22

44

933

443.0

10.1

3.0

23

46

963

445.0

9.7

1.0

24

48

993

446.3

9.3

0.0


X-ray Diffraction Patterns: The intensities of the diffracted X-rays as a function of the diffraction angle, 2?, has recorded and the interplanar spacing, d, has been calculated from the position of the intense peak using Bragg’s relationship n ? = 2d sin ?, where ? is wavelength of the radiation. A number of peaks arising from the diffraction of X-ray by planes of metal ion (basal planes) have been observed over the range of 3-80 o diffraction angles. The interplanar spacing for different order diffractions has mentioned in Table 2. The average planar distance i.e. the long spacing for terbium myristate is 39.22 Å. The maximum average d-spacing of the bilayer structure was calculated for an all trans conformation of the myristate chain perpendicular to the metal ion base plan by using equation12.

Where; n is the total number of carbon atoms in the chain, , and . A good agreement is found between the experimental and calculated values.

It is observed that values of average planar distance i.e. long spacing for terbium myristate (39.22 Å) is smaller than the calculated dimensions of myristate ion (42.00 Å) from Pauling’s values 18 of atomic radii and bond angles. This suggest that the molecular axes of terbium myristate are somewhat inclined to the basal plane and the metal ions fit into spaces between oxygen atoms of the ionized carboxyl group without a large strain of the bonds. It is observed that the long spacing peaks are fairly intense while the short spacing peaks are relatively weak. It is, therefore, concluded on the basis of long and short spacings that metal ions in terbium myristate are arranged in a parallel plane, i.e. a basal plane equally spaced in the soap crystal with fully extended zig-zag chain of fatty acid radicals on both directions of each basal plane and these soaps posses double layer structure as proposed by Vold and Hattiangdi19.

Thermogravimetric Analysis (TGA): The thermal decomposition of terbium myristate can be expressed as;

2 (RCOO) 3 Tb ? 3ROR + Tb2O3 + 3CO 2

Where; R is C13H27 for myristate. The TGA curves exhibit three stages of decomposition (Figure 2). The 1st stage is rapid and could not be subjected to kinetic analysis. The 2nd stage represents the major decomposition. Finally, 3 rd stage show very small change with further increase in temperature. The weight loss of Terbium myristate with temperature and time is mentioned in table 3. The TGA shows that the final residue left on heating is the metal oxides as the weight of the residue is in agreement with the theoretically calculated weight of terbium oxide. A white substance is found condensed at the colder part of the sample tube, which is detected as keton. Thermal stability measurement shows that terbium myristate is stable upto 314.74oC.

TGA data have been used to calculate the energy of activation and order of reaction for the decomposition by using the Freeman-Carroll’s 20 rate expression;

where, E = energy of activation, R = gas constant, n = order of decomposition reaction, T = temperature on absolute scale, W r = difference between the total loss and loss in weight at time, t i.e. W0 - Wt and dw/dt = value of rate of weight loss obtained from the in weight vs. time curve at appropriate times. The plots of [log (dw/dt) / log W r] vs 1/T / log Wr have been found to be linear with zero intercept and value of activation energy from slope (-E / 2.303R) of the plot (Figure 3) is calculated to be 8.55 K Cal mol -1.

img2

Figure 1: Infrared Absorption Spectrum of Terbium myristate.

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Figure 2: Thermogram of Terbium myristate.

img2

Figure 3: Freeman-Carroll’s plot.

img2

Figure 4: Horowitz-Metzger’s plot.

img2

Figure 5: Coats-Redfern’s plot.

The energy of activation for the thermal decomposition has also calculated by using the Horowitz-Metzyer’s21 and Coats-Redfern’s22 equations. The values of activation energy calculated from the slopes of plots by using these equations is found in the range of 8.14 - 8.55 K Cal mol-1 (Figure 4 and 5). It is concluded that the decomposition reaction of terbium myristate is kinetically zero order and the activation energy for the decomposition process existed in the range of 3.84 to 8.55 K cal mol-1. The results are found in good agreement with other literature date 23.

CONCLUSION: The IR results confirm that myristic acid exists as dimeric structure due to hydrogen bonding between the carboxyl groups of two acid molecules, whereas terbium myristate is ionic in nature. The X-ray analysis revealed that the molecular axes are slightly inclined to the basal plane. The thermal decomposition found to be kinetically of zero order and the energy of activation for the decomposition is in the range of 3.84 to 8.55 K cal mol-1.

ACKNOWLEDGEMENT: We wish to express our sincere thanks to S.T.I.C. Cochin, India for their valuable assistance to make present study possible by physico-chemical technique. One of authors (K.K.) is grateful to Hon’ble Vice Chancellor, Prof. H.S. Dhaliwal and Dean (PG) Prof. B.S. Sohal, Eternal University, Baru Sahib (Himachal Pradesh) India for their constant encouragement and logistic supports.

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