Metal Salt of Aromatic and Aliphatic Dicarboxylic Acids Represented Rectangular-Rod or Board Shaped Morphology in High Crystallinity

The morphology of a substance is a visual appearance reflecting the 3-dimensional orientation and packing of composing molecules. Either by itself or in a mixture, the morphology of a substance affects the physicochemical properties such as electromagneticity of the metal particle,¹ charge transfer in organic semiconductor² and photoelectron transport.³ The morphology of metal or metal oxide could be controlled for a proper catalytic activity⁴ or a designed photochemical property.⁵ The organic materials also represented that morphology-related crystallinity influenced the melting point,⁶ adhesion in a tabuleting,⁷ and thermal behavior of polyester.⁸ Prediction of how solid shapes are formed and how relate to physicochemical properties is beyond current knowledge and requires further study. In a morphology formation from a solution, the solidification process is dependent on solvent, growth rate, and temperature.^7,9 First of all, the primary shape and intermolecular interactions including ionic, dipole, and hydrogen bonding would be the critical issue in the solidification process to give a morphology such as a needle, plate or rectangular type.

Metallic salts of carboxylic acids, composed with hydrophilic dipole as well as lipophilic moiety, are useful component for cosmetic, food, lubrication and coatings.^10-14 The morphology of metal carboxylate is known to affect the physicochemical properties including lubrication,^15,16 and transition temperature.¹⁷ For a study of the morphology-related property of metal carboxylate, we previously reported that the sodium salt of saturated linear aliphatic acid gave branched fibrous SEM (scanning electron microscopy) images.¹⁸ In a continuation effort, we focused on the metal dicarboxylates and their morphology. Metallic salt of dicarboxylic acid contains two ionic-dipoles in a molecule thereby it could result in high crystallinity or strong aggregation via multi-intermolecular interactions with additional hydrophobic intermolecular force. As we expected, we observed and wish to report that the high crystallinity of the metal salt of aromatic (PA, IA, TA) and aliphatic (Su, Se, Dd) dicarboxylic acids was observed via their SEM images.

The metal salts of aliphatic and aromatic dicarboxylic acids were prepared by reaction with alkali metal hydroxides (Li, Na, K) in methylene chloride and water as described in literature¹⁸ to give the corresponding salts.¹⁹ The SEMs were obtained on Ultra plus (Carl Zeiss) at Chungbuk National University and represented in Fig. 2 (for aromatic carboxylic acid) and Fig. 3 (for aliphatic carboxylic acid). The SEM of benzoic acid salt (BA) was also added in Fig. 2, as a comparison SEM for mono-carboxylic acid.

Aromatic Dicarboxylic Salt

Most salts of isophthalic and terephthalic acids (IA, TA) showed clear crystallinity with a rectangular-rod or board-like shapes in their SEM images, particularly in the case of IA-Li, IA-K, and TA-K as in Fig. 2. The SEM images of salts of aromatic mono-acid BA also appeared lamellar-board shape. However, they showed less crystallinity than those of IA and TA. A representative rectangular-rod image appeared in the SEM of IA-Li in µm scale (~2 µm × 10 µm). The potassium salt of isophthalic acid (IA-K) also showed a long rod image (~0.2 µm × 10 µm). In the case of TA-K, characteristic high crystallinity appeared with rectangular rod image (~4 µm × ~20 µm). In the SEM of TA-Na, a thick board (~2 µm thick × ~20 µm length) image was observed. The clear crystallinity of TA series was assumed that the symmetrically separated ionic dipoles facilitated a linear alignment of molecules. In the case of PA, a relatively poor crystallinity appeared with an amorphous shape. Only Na salt of PA gave a partial rectangular rod image. It was suggested that two carboxyl groups at ortho-position might distort planarity hindering the aligned stacking of PA-salt.^17,20

Figure 1. Structures and abbreviations of aliphatic and aromatic dicarboxylic acids.

Figure 2. The SEM images of metal salts (Li, Na and K) of aromatic carboxylic acids (BA, PA, IA and TA) in 50,000 expansion. *Measured thickness of aligned rod or board images.

Aliphatic Dicarboxylic Salt

The SEM images of metal salts of saturated aliphatic C8-C12 dicarboxylic acids (Su, Se, and Dd) are shown in Fig. 3. A branched narrow rod (Su-Na) and crab-meat-like a rod (Dd-Na) or board (Se-K) shaped images appeared in the SEM images of metal salts of aliphatic acids. The size of a branched narrow rod image (Su-Na, Dd-Na) was shown in ~100 nm × ~µm scale. Metal salt Se-K represented a thick board-shaped image (1 µm thick × 10 µm width). Notably, salt Se-Na displayed a unique corn-pillar (2 µm × 5 µm) image on which each corn-image (100 nm × 600 nm) was embedded with grain-like dots in ~10 nm diameter (see also Fig. SI 3). In general, the SEMs of aliphatic dicarboxylic acids showed more crystallinity than those of aliphatic mono-acid salts. Bis-intermolecular interactions at opposite positions, presumably induced the symmetrical linear stacking of molecules.²¹ The crystallinity of metal salts was dependent on the metal. In the case of Na salt of dicarboxylic acids (Su, Se and Dd), a better crystallinity was observed than those of the other metal salts (Li, K) as in SEM images.

Figure 3. The SEM images (×50,000) of metal salts (Li, Na, K) from the aliphatic dicarboxylic acids (Su, Se and Dd).

As a summary, the representative SEM images of the metal salt of aromatic dicarboxylic acids (IA, TA) appeared in shapes of rectangular-rod or lamellar-board in ~µm scale. The crystallinity in metal salts of aromatic dicarboxylic acids decreased in the order of TA> IA> PA in SEM images. For aliphatic C8-C12 dicarboxylic acids, the branched-rod or board-shaped SEM images were observed as in cases of Su-Na, Se-K and Dd-Na. In general, the metal salts of saturated aliphatic dicarboxylic acids (C8-C12) showed better crystallinity than those of mono-carboxylic acids.