Morphology of Metal Salt of Carboxylic Acid: Metal and Acid Dependency on Branched Round Cluster Images

The morphology of a substance is a visual appearance that reflects the three-dimensional orientation and the packing state of its constituent molecules. It is known that the physicochemical properties of a substance are affected by its morphology whether a single component or a mixture.¹ Electromagnetic properties,² charge transfer, photoelectron transport,³ and catalytic reactivity⁴ are known to be dependent on the morphology of the composing metallic components. In the organic compounds, their shape and crystallinity also affect the melting point,⁵ adhesive strength of tablets,⁶ and lubricity.^2,3 The morphology of crystals precipitate out of solution depends on the solvent, crystal growth rate, concentration and temperature.^6,7 In a crystallization process of a molecule, the original structure, and the intermolecular force are considered as one of the fundamental factors for its morphology determination to form a morphology of plate, rod, amorphous, and fiber.^6,8

Metallic salt of carboxylic acid, composed of the hydrophilic dipole as well as lipophilic moiety, is used as an additive for soap, food, lubrication and coating. The morphology of metal carboxylate also affects the physicochemical properties in their applications. For instance, Ca salts of C18 carboxylic acids (stearic acid, oleic acid) with round plate shapes showed better lubricity than less crystalline linoleic acid (18:2) salts.⁹ Similarly, lamellar calcium N-lauroyl taurate exhibited superior lubricity compared to other amorphous metal salts.¹⁰ Interested in the relationship between the morphology and structure of metal salts of carboxylic acids we reported that sodium salts of aliphatic carboxylic acids and aromatic carboxylic acids showed a fiber and rectangular-rod or plate morphology.^11,12 In a continuing effort to the relationship of the structure and morphology of the metallic carboxylates, the carboxylic acids as in Fig. 1 were converted to metal salts and their SEM (scanning electron microscope) images were analyzed.

Figure1.

Structure and abbreviation of carboxylic acids for metal salts.

The metal salts of aliphatic and aromatic carboxylic acids were prepared by reaction with metallic hydroxides (Li, Na, K, Mg, Ca, Ba, Al) in methylene chloride and water as in the literature procedure with minor modifications.^11,12 For a typical procedure; 16-hydroxypalmitic acid (150 mg, 0.55 mmol) dissolved in methylene chloride (20 ml) was dropped slowly into an aqueous NaOH (26 mg, 0.66 mmol) solution with stirring at room temperature. For 30 min more stirring at 50 ℃, the reaction mixture was evaporated under a vacuum to give a white solid, which was then washed with methylene chloride and water alternately. The solids were dried in a vacuum oven and then crashed. The powders were washed in a glass filter with methylene chloride (or ethanol) and water alternatively, then dried in a vacuum oven to give 97 mg of white sodium palmitate, HPA-Na in 60% yield (mp; 210 ℃). Likewise, other metal salts of C10–18 aliphatic acids and dicarboxylic acids (abbreviated in Fig. 1) were prepared by the reaction of acids with the corresponding metal hydroxides (Li, Na, K, Mg, Ca, Ba, Al).13 The SEM images of those were obtained on model Ultra Plus (Carl Zeiss) at Chungbuk National University and represented in Figs. 2, 3, 4 and 5.

For the preparation of metallic salts, aliphatic carboxylic aid (LA, MA, PA and SA), hydroxylaliphatic carboxylic acid (HLA, HPA and HAS), dicarboxylic acid (SuA) and isophthalic acid (IA) were reacted with the metallic hydroxide (MOH). Monovalent (Li, Na, K) and multivalent ions (Mg, Ca, Ba, Al) were included as the metal ions. The SEM images of the metal carboxylates, categorized as fiber, rod, plate, and amorphous, are analyzed focussed on their dependency on the metal and the carboxylic acids.

In Figs. 2A and 2B, the representative SEM images of the salts from the aliphatic acids (MA, PA, SA, SuA) and aromatic acid (IA) with monovalent (Li, Na, K) and divalent (Mg, Ba) ions are displayed.

Metal Effect

As shown in Fig. 2A, metallic salt of aromatic dicarboxylic acids (IA) represented a clear crystallinity¹² with either mono or divalent metals resulting in the image of rectangular-rod (IA-Li), branched-rod (IA-K, Ba) and plate-shape (IA-Mg) in ~µm scale. In the case of diacid SuA, fiber (SuA-Na) or rod-like branched image (SuA-Ba) appeared. Most of the aliphatic carboxylic acids (MA, PA and SA) showed amorphous glassy morphology with monovalent (Li, K) metals. But, their Na salt represented a clear branched fiber image as in Fig. 2B.¹¹ The amorphous glassy-plate images also appeared in divalent Mg salt of aliphatic carboxylic acid (MA-Mg, PA-Mg, SA-Mg).

Figure2.

The SEM images of metal salts (Li, Na, K, Mg, Ba); (A) for SuA and IA and (B) for C14-18 aliphatic carboxylic acids (50K expand, except 10K for PA-K, PA-Ba). *The value in parenthesis is the radius of the metal cation in pm. The SEMs of Li, Na and K of carboxylic acids are from ref (11) for MA, PA, SA and ref (12) for SuA and IA.

It is noticeable that divalent Ba salt of aliphatic carboxylic acid (SA-Ba) showed a branched round cluster (BRC effect) image with 87 nm width as shown in Fig. 3. This effect, however, did not appear in the case of carboxylic acids (MA, LA, DA: decanoic acid) with a shorter chain than C-16. The Ba salts of the carboxylic acids DA, LA and MA showed a glassy image as shown in Fig. 3.

Figure3.

The SEM images of divalent Ba salt of aliphatic carboxylic acids depend on carbon length (50 K expand). DA: Decanoic acid.

To find out how the cation size¹⁴ affects the morphology of carboxylate, the SEM images of paired Li/Mg (78 pm) and K/Ba (133, 143 pm) salts were compared in Figs. 2A and 2B. The Li and Mg salts of aliphatic carboxylic acids (MA, PA, SA) showed a similarly glassy plate shape (Fig. 2B). Even in the case of high crystalline aromatic acid (IA) salt,¹² both Li and Mg salts exhibited a similar plate-like morphology (Fig. 2A).

The K and Ba ionic salt of crystalline aromatic acid (IA) both showed a branched rod-like shape as in Fig. 2A. In the case of diacid SuA, its K and Ba metal salts looked also similar in rectangular crystalline. Likewise, the K/Ba ionic salts of aliphatic acid (MA) showed a similar amorphous glassy plate image. With the carbon chain increased over C-16, however, the glassy image of Ba ionic salt appeared to be a branched round cluster as shown in PA-Ba and SA-Ba (Fig. 2B).

The Ca (106 pm) and Na (98 pm) ions, even similar in size, both salts of C-18, the metal salt of SA represented a completely different morphology: a fiber for SA-Na and a branched round cluster for SA-Ca, respectively. It is noticeable that Ca ion-induced branched round cluster was observed only in the carboxylic acid with C-18 chain not in short C-10 or C-12 carboxylic aids (DA-Ca, LA-Ca) as shown in Fig. S1 (see Supporting Information).

In the case of the trivalent Al ion with a very small ionic size (57 pm), its carboxylate showed a distinct amorphous glassy surface in the SEM images. Even in the case of TA (terephthalic acid), known excellent crystallinity of its metal salt,¹² the TA-Al carboxylate clearly showed a glassy image (Fig. S2, in Supporting Information).

Carboxylic Acid Effect

The metal (Li, Na) salts of hydroxyaliphatic carboxylic acids (HLA, HPA and HSA) are prepared to study how the hydroxyl group induces the morphology, and the SEM images are represented in Fig. 4. In their metal salts, the branching and crystallinity are observed generally rather than noticeable morphological deformation from the SEM images of the metallic non-hydroxy carboxylates. Especially, HSA-Li with an internal OH showed a cotton-like fiber image with a finely branched crystallinity. In the case of HSA-Na, its SEM image showed a 100 nm thicker (176, 167, 206 nm) fiber image compared to that (78, 81, 107 nm) of the saturated carboxylic acid SA-Na. In general, the salt of terminal-OH (HLA, HPA) gave a thickly branched fiber image. However, the salt of internal-OH (12-HSA) represented a cotton-fiber image (HSA-Li) or a little kinked fiber (HSA-Na). This result suggested that the hydrogen bonding is less critical than the ionic interaction to the morphology formation in the metallic salt.

Figure4.

The SEM images of metal salts (Li, Na) of LA, PA and SA and the hydroxycarboxylic acids HLA, HPA and HSA (50 K expand except [a] 10 K). Width of the rod: LA-Na (336 nm), SA-Na (78, 81, 107 nm), HPA-Na (32, 36, 56 nm), HSA-Na (176, 167, 206 nm).

The SEM images of metal salts of unsaturated C-18 carboxylic acids, oleic acid (OlA), linoleic acid (LeA) and linolenic acid (LnA) containing cis-alkene geometry are represented in Fig. 5. In the SEM of LeA-Na with two cisdouble bonds, a glassy image is observed. It was suggested that a cis-alkene geometry inhibits the linear elongation of an aliphatic chain inducing a kink arrangement, thereby resulting in an amorphous image with less crystallinity. The Na salt of oleic acid (OlA-Na), with one cisalkene functionality, showed a fiber image like that of SA-Na. However, it appeared in a more amorphous and less branched shape than that of SA-Na. In the case of LeA-Li, the SEM image appeared a brick-like crystallinity but, also displayed a glassy image on the crystalline surface. In the case of linolenic acid (LnA), with three cis-alkene functionalities, its metallic salts (LnA-Li, Na) also displayed the amorphous image with decreased crystallinity as shown in Fig. S3 (see Supporting Information).⁹

Figure5.

The SEM of Metal (Li, Na) Salts of C-18 carboxylic acids OlA, LeA and SA (expand 10K, ^[a] 5 K).

In conclusion, the morphology of metallic carboxylates appeared in the SEM image was dependent on metal ions and carboxylic acid. In general, the metal salt of dicarboxylic acid displayed high crystallinity with a rectangular rod and thick-plate images. Meanwhile, metal salts of aliphatic carboxylic acid represented noticeably metal and chain-length dependent SEM images; a fiber-shape with Na, and glassy-amorphous with Li, K, Mg and trivalent Al ion. With Ba and Ca ions, the aliphatic acids with C-16 and C-18 carbon chains showed a branched round cluster image. Depending on the size of the metal ion, the Li/Mg (78 pm) ions showed a similar glassy plate image. Likewise, Na/Ca (98, 106 pm) salts induced a branching effect resulting a fiber or round granular images. With the OH group, the metal salt of aliphatic carboxylic acid showed a finely divided crystalline image (HSA-Li). The SEM analysis showed a glassy-amorphous image of the cis-aliphatic C-18 carboxylic acid. These results suggested that metal salt of carboxylic acids can be designed for the morphology to be required.