Indian Journal of Chemistry Vol. 38A, April 1999, pp. 307 - 310

Non-equilibrium growth patterns of carboxylic acids crystallized on microslides

Ishwar Das*, Anuj Kumar & Namita Rani Agrawal

Department of Chemistry , University of Gorakhpur, Gorakhpur 273009, India

and

R S Lall

Department of Chemistry, St. Andrew's College, Gorakhpur 27300\, India

Received 23 September 1997; revised 10 December 1998

A variety of patterns are observed when carboxylic acids crystallize from aqueous solution admixed with agar agar or from alcoholic solutions. Oxalic acid crystallizes in fractal-like morphology with fractal dimension D =1.65. Succinic acid and adipic acids show tree-like and dendritic morphology respectively. The 0- and p-amino benzoic acids show sheaf-like morphology,· characteristic of spherulitic growth but o-toluic acid shows a radial branched pattern with fractal dimension D=1.53. Mandelic acid shows fractal-like mophology when crystallized from its alcoholic solution and concentric rings on crystallization from a dense aqueous medium. In the latter computer analysis shows periodicity in the number density of crystal nuclei (N) versus distance (r) plot.

A variety of. patterns are observed during non­equilibrium growth processes in physics I , chemist:rl-8

and biology9-15. These patterns are similar to ordered .and disordered natural patternsl6. For spontaneous appearance of patterns, systems must be far from equilibriuml7; patterns result · from competition between the dynamics at different levels I7.18. Many dynamical phenomena in nature are characterized by spatial and temporal fluctuation processes. Stability and transition between well-defined states of patterns can be studied by changing some experimentally­controlled parameters. Morphologies are affected by the addition of dense matrix, which forms a rigid network by creating shearing stress. This shearing stress increases with increase in dense matrix concentration. Rate of evaporation governs the nature of pattern6 and fast evaporation results in a ramified structure while slow evaporation results in linear geometries. During evaporation solvent is removed from the interconnected pore network and large capillary stress can develop during drying when pores are small. This networking may be one of the causes of formation of physical polymer (association of molecules by weak physical forces) . Spherulitic growthl9 may appear when crystallization developes with repeating branching, radiating out from a central

. nucleus. At sufficient distance from primary nuclei , the stack of lamellae are radially oriented with the chain perpendicular to the radius. Spherulitic structures are common in crystalline polymers and aggregates of crystals. Crystallization temperature l9

influence the manner by which the macromolecular segments fold and pack together. Change in packing leads to significant variation of radial growth rate of spherulite as well as change in morphology.

In this paper, we report new results on growth morphologies of different carboxylic acids.

Materials and Methods Oxalic acid (E Merck), phthalic acid (Sigma),

o-toluic acid (Spectro Chern), adipic acid (Qualigens), succinic acid (Qualigens), o-amino benzoic acid (Sigma), p-amino benzoic acid (Sigma), agar agar (Difco, USA) and mandelic acid ( Aldrich) were used as such.

Development of patterns For the development of crystallization patterns,

known amount of compounds were dissolved in a suitable solvent (doubly distilled water or methanol) with or without a dense matrix (agar agar of known concentration). A fixed volume of the solution was poured and spread uniformly over microslides. The slides were placed in an incubator maintained accurate to ± 0.1 dc. After the solvent was completely evaporated, the slides were observed with the help of an Olympus microscope and photographs were taken. Microphotographs are shown in Figs 1 & 2 .

Computation of fractal dimension, D Photographs taken in our experiments were

scanned with the help of a HP Scan Jet 4C Scanner

308 INDIAN J CHEM, SEC A, APRIL 1999

Ollalic a6d Succinic cdc Adip ic acid

Fig. I---Crystalli zation pattcrns of carboxylic acids. Condi tions: a, [oxalic acid]~" " , M; b, [succin ic acid]=O.06 M; c, [adipic acid]=0.06 M. 0. 1 mL of the solution [sprcad uniformly on mien s (7.5 x2.5 cm2

) at 30°C)

Fig. 2---Crystal lizati on patterns of a,o-ami no benzoic acid (0.88 mg/cm2); b,p-ammo benzoic acid (0.044 mg/cm2) ; c, o-toluic acid (0.044 mg/cm2) at 35°C

attached with a computer (HP, PC/AT 486, Model Vectra VL2). The fractal dimension of the ramified object was determined by the box counting method on the computer screen. The relationship between the total number of boxes N(r) inside a circle of radius r should be a power law with non-integer exponent D as N(r) - ,JJ where the thickness Qf the branches is considered as zero dimensionaI20

. The log-log plot of N(r ) versus r was drawn. The fractal dimension D is given by the slope of the best fitted straight line using least squares method.

Computation of number density of crystal nuclei (N) In the case of growth pattern of mandelic acid

(Fig.3b) the number dens ity of crystal nuclei (N) was calculated as a fu nction of di stance (r) . Number of crystal nuclei was determined in a window of width 1 mm by the box counting method on the computer screen. Figure 4 shows the relationship between the average number of boxes N with distance r from the centre of the concentric ring.

Results and Discussion A variety of patterns are observed when

dicarboxylic acids COOH(CH2)nCOOH (where n = 0,

2,4) crystallize from their aqueous solutions, aqueous solution admixed with agar agar or from methanol. Fractal and dendritic patterns were observed for oxalic acid (Fig. I a) and adipic acid (Fig. 1 c) respectively. Succinic acid shows tree like morphology under the same experimental conditions

(Fig. 1 b). As the number of -CI\- group increases,

solubility of acids decreases and value of ionization constant increases. Hydrogen bonding is responsible for physical association of molecules in the form of rings of two, three or more molecules or infinite chains21 (I).

",O "'H-O, ~o " -H- O, " ' (CH ) - c". C-(C Hz'riC ,&c-,"

z 'O-H"'O~ ..... O-H··<>",

These acids exist in different polymorphic modifications with a small difference in internal energy. The hydrogen bond parameters appear to vary insignificantly from one molecule to another. In dicarboxylic acids, the strength of hydrogen bonding decreases with increase in number of -CI\- group as

evident by lowering of melting point, and therefore, power of association in molecules decreases. For example, oxalic acid exists in a- and p-forms. The

intermolecular distance 2.71 (A) undoubtedly indicates a hydrogen bond. Distance between the oxygen atoms of different molecules involved in the formation of hydrogen bonds -O-H-··O vary significantly near 2.70 A (2.66-2.75 A) (ref. 21). Similar to the structure of the p-form of succinic acid and the higher dicarboxylic acids but unlike the structure of the a-form of oxalic acid, hydrogen bonds bind the molecules into infinite chains. The

DAS et al.: GROwrn PA TIERNS OF CARBOXYLIC ACIDS ON MICROSLIDES 309

Fig.3 Crystallizatipn patterns of mandelic acid (1.22 mglcm\ [a, from methanolic solution and b, .from aqueous solution admixed with 0.1% agar agar at 35°C] .

chains are separated by normal intermolecular distances.

Oxalic acid crystallises in the form of fractal-like morphology (Fig. la) with fractal dimension D =

1.65, which is in good agreement with two dimensional DLA cluster. A sudden change was observed in the growth pattern of phthalic acid. Such a phenomenon is very similar to the bacterial growth at low agar concentration22

-24

, hepatocellular carcinoma25 and black sea fans and is being reported for the first time in any chemical system.

Ortho and para-amino benzoic acids in which one -COOH group of phthalic acid is replaced by an amino group, showed a sheaf-like morphology characteristic of spherulitic growth . Both orlho and para forms show two holes in their morpholo~ (Fig. 2a&b) as schematically represented by Hearle 6.

Most spherulites appear to be a radiating array of fi brous crystal between branche~. There is a slight difference in the morphology of ortho- and para­amino benzoic acids. The para fo rm shows a sheaf­like morphology (Fig. 2b), whereas the ortho form (Fig.2a) shows a circular envelop with two holes in

z

40r. ------------------------~

35

30

15

15

5

o~~~~~~~~~~~~~ 0·5 ,., 2 2-5 3 3-5 4·5

Dis lanc(, r

Fig.4 Computer plot of average number density of crystal nuclei (N) versus distance r, [crystallization pattern shown in Fig.3b (window width I mm )].

the centre and appears to be radiating an array of fibrous crystals. Molecular crystallization patterns show similarities with chemical27 and physical structures of molecules. The ion dipole interaction in amino benzoic acid may be responsible for these spherutilic morphology.

When -NH2 group of amino benzoic acid was

replaced by -CH3 group (o-toluic acid), radial

branched pattern (Fig.2c) with fractal dimension D =

1.53 was observed at 35°C when crystallized from its 0.03 M solution. However, when it was crystallised at 10°C, fractal-like pattern was observed with fractal dimension D = 1.61. A crystal can be changed into the corresponding polymorphous body by varying the temperature. Above or below a certain temperature level, one or the other form is stable.

The nature of medium and the rate of evaporation influence the growth morphology. This is evident by the difference in crystallisation patterns of mandelic acid obtained from its methanolic solution and water admixed with agar agar. Mandelic acid shows fractal­like (Fig.3a) morphology with fractal dimension D =

1.84, when rate of evaporation is fast. It crystallises in the form of concentric rings (Fig.3b).in a dense aqueous medium where the rate of evaporation is slow. At low rate of evaporation the diffusion plays an important role.The instability theory proposed by Mullins and Sekerka28 results in concentric morphology autocatalytically. Concentric pattern of mandelic acid crystallized from aqueous solution is shown in Fig. 3b. Gray level analysis of the scanned picture was done on the computer screen. A window of width I mm was chosen for counting the number of crystal nuclei (N) as the function of distance r from the centre of concentric ring. The number density of

310 INDIAN J CHEM, SEC A, APRIL 1999

crystal nuclei (N) varies periodically with distance r from the centre 'of concentric ring as shown in Fig.4. The frictional force exerted by a solvent medium was expected to affect the diffusion of a solute molecule. A quantitative relationship was proposed29 which related diffusion coefficient (Dc) with temperature (1) and frictional ~oefficient of the solute molecules as Dc=kT/f, where k is Boltzmann constant. For spherical particles of radius r,j = 6 1tT1r where TJ is viscosity of the medium (solvent).

The quantity kT is a measure of the thermal or kinetic energy of the molecule and 1] is the measure of the viscous resistance to diffusion . The rate of two opposing quantities kT and 1] determines how early a solute molecule will diffuse in solution. By changing the viscosity of the medium at the same temperature, change in diffusion coefficient leads to a change in morphology. It has been shown30 that the shape of growth surfaces in microscopic crystals may be

correlated with their entropy of fusion M f =Lf IkTm where Lf is the latent heat of fusion, k is Boltzmann

constant and Tm is the melting point. Jackson31

obtained an empirical correlation of M f with the

shape of the crystals. The higher values of the entropy of fusion as well as entropy of formation suggests spherulitic growth while lower values reflect dendritic growth31

• High entropy corresponds to high randomness or roughness in growth surfaces resulting in tendency towards dendritic and spherulitic growth. This depends upon the nature of bonding between the units of molecules in the crystal . The nature of bonding in disaccharides have been calculated with the morphology of crystal growth patterns32.

Acknowledgement Authors wish to thank Prof. R R Y adav, Head

Ctumistry Department, Gorakhpur University Gorakhpur for providing laboratory facilities.

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