КОЛЛОИДНЫЙ ЖУРНАЛ, 2007, том 69, № 3, с. 378-384
УДК 541.183.022
INFLUENCE OF A,0-CARBOXYMETHYL CHITOSAN ON THE PROPERTIES OF OCTADECANOIC ACID/OTCTADECYLAMINE MONOLAYERS AND THE FRACTAL STRUCTURE OF CALCIUM CARBONATE
© 2007 Yu-hua Shen*'**, An-jian Xie*'**, Xue-rong Yu*, Gang Wu*, Ling-guang Qiu*, Shi-kuo Li*'**, Xiang-yun Kong*, Cheng-xiang Han*
* School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, China **Key Laboratory of Environment-friendly Polymer Materials of Anhui Province, Anhui University,
Hefei 230039, China E-mail: s_yuhua@163.com Поступила в редакцию 26.06.2006 г.
The properties of octadecanoic acid/otctadecylamine monolayers and growth of calcium carbonate (CaCO3) induced by the monolayers on the surface of supersaturated CaCO3 solution with ^,0-carboxymethyl chitosan (CMC) were studied. The results suggest that CMC is either adsorbed on or inserted into the monolayers, which is confirmed by n-A, dn/dA-A, and п-t isotherms. The adsorption of CMC changes the properties of the monolayers that results in the transformation of the shape of CaCO3 particles from crystal-like to fractal pattern beneath the monolayers. The different fractal morphologies such as butterfly, and wicker branches consisting of hollow ellipsoidal, solid ellipsoidal, and spherical particles, correspondingly, were observed; these morphologies depended on the CMC concentration in the subphase. The dimensions of fractal patterns were determined. The mechanisms of the formation of CaCO3 crystals and fractal structures are also discussed.
1. INTRODUCTION
Bioinorganic materials such as mollusk shells, teeth and skeletons have a sophisticated structure and higher performance compared with natural inorganic minerals [1]. Almost all mineralized tissues contain distinctive assemblies of acidic protein or glycoproteins, which are capable of influencing the crystal growth in biological systems [2]. There is a wide range of inorganic materials such as calcium carbonate (CaCO3), magnetic iron oxide (Fe3O4), and amorphous silica (SiO2) of exquisite morphology with valuable structure and functions [3, 4].
Calcium carbonate is an important material not only in biomineralization, but also in industrial applications for plastics, rubber, and paper manufacturing [5]. The application of CaCO3 particles is determined by a defined set of parameters such as morphology, structure, size, specific surface area, and brightness, etc. [6]. Among all these, the morphology and size are two most important parameters. To control the shape and size of the particles is, therefore, a fundamental problem from the viewpoint of technical applications.
Biomimetic synthesis of various materials with organic templates and/or additives has been well-developed for decades, and attracted both scientific and industrial interests in the sense that it allows the controlling of crystal size and morphology [7-12]. Water-soluble proteins and insoluble polysaccharides extracted from the naturally biomineralized materials are used as templates to study their effect on the crystallization of in-
organic substances. Recently polyaspartate, synthetic polymers, as well as Langmuir-Blodgett (LB) films were also applied in the mineralization research [13-15].
N,O-Carboxymethyl chitosan (CMC) is one of the most important biomacromolecules, which influences the mineralization process of CaCO3 particles in an essential way. It has been used as template for the deposition of CaCO3 particles [16]. However, the monolayers on subphases containing biomacromolecules have been rarely reported as systems controlling morphology of CaCO3 particles. The CMC belongs to polysaccharides, and octadecanoic acid/otctadecylamine (OA/OAM)
monolayers with -COO- and -NH+ polar groups are similar to those of phospholipids. Polysaccharides and phospholipids are the important constituents of biological membranes. Herein, we studied the system (biom-acromolecules-monolayer) consisted of OA/OAM and CMC as the microenvironment for CaCO3 particle growth, and the influence of CMC on the properties of OA/OAM monolayers. The formation of crystals with regular shape and fractal patterns was promoted by OA/OAM monolayers on aqueous solution with different concentrations of CMC. The obtained results provide a new insight to the mechanism of biomineraliza-tion at interfaces.
2. EXPERIMENTAL PROCEDURES 2.1. Chemicals and instruments
Octadecanoic acid (AR) was obtained from Fluka Chemika; octadecylamine (AR), and chloroform (AR) were purchased from Shanghai Chemical Reagents Co.; calcium chloride and sodium carbonate were all analytic purity and used without further purification; carboxymethyl chitosan was synthesized by ourselves. Double-distilled water was used in all experiments.
Morphology of the particles was determined by scanning electron microscope (SEM) (model DXS-10); n-A isotherms were measured by WM-1 LB trough (Southeast University, China).
2.2. Experimental procedures 2.2.1. Preparation of carboxymethyl chitosan
Chitosan was dissolved in 40-50% (w/w) NaOH solution, appropriate volume of isopropanol was added, and then chloroacetic acid was added into the mixed solution slowly. The pH value of reaction solution was adjusted to 7.0 with dilute aqueous HCl solution and the reaction was allowed to proceed at 60°C. The obtained mixture was precipitated with acetone, washed with anhydrous alcohol at least three times, and the product was dried in vacuum and then was characterized by FTIR spectra, which were omitted here. The main peaks were located at 3446 (vO-H, N-H), 1635
(vCOO-) and 1585 cm-1 (SNH, vC-N), respectively. This
indicated, that CMC was successfully synthesized (molecular weight: 3.05 x 105, degree of substitution: 1.84).
2.2.2. n-A isotherms
To obtain supersaturated CaCO3 solution, equimo-lar aqueous solution of CaCl2 and Na2CO3 was prepared and filtered at room temperature. Double-distilled water, 3 g/L CMC solution and 3 g/L CMC/CaCO3 supersaturated solution were used as subphases. The pH values of these solutions were adjusted to 8.6 by the addition of necessary amounts of aqueous NaOH solution. 100 ^L of the mixed solution containing 0.16 mmol/L OA/OAM in chloroform was spread on the surface of subphase and the chloroform was allowed to evaporate for 15 min. The OA/OAM-mono-layers were compressed at a rate of 20 mm/min. The surface pressure was simultaneously recorded.
2.2.3. Growth of calcium carbonate beneath OA/OAM monolayers
Appropriate amounts of CMC were added into the supersaturated CaCO3 solution to form different CMC/CaCO3 mixtures with concentrations of CMC 0.18, 1.90 and 10.00 g/L, respectively. The OA/OAM monolayer was compressed, and then transferred onto the hydrophobic glass by passing the substrate vertically
Surface pressure, mN/m
Area per molecule, A2
Fig. 1. n-A isotherms of OA/OAM monolayers on different subphases: double-distilled water (1), CMC solution (2), and CMC/CaCO3 supersaturated solution (3); for all subphases pH value was adjusted to 8.6.
through the monolayer at a rate of 1 mm/min and surface pressure of 50 mN/m. The glass substrates with transferred LB films were dried in vacuum for 8 days at 4°C. Then the samples were coated with gold and studied by DXS-10 SEM.
2.2.4. n-t isotherms
The double-distilled water was used as subphase with pH value adjusted to 8.6 by the adding of diluted aqueous NaOH solution. When the surface pressure reached 26.3 mN/m during monolayer compression, the CMC solution (10 g/L, 100 ^L) was injected beneath the monolayer and the n-t isotherm was recorded.
3. RESULTS AND DISCUSSION
3.1 The influence of subphase composition on the properties of OA/OAM monolayers
3.1.1. n-A isotherms
The surface pressure-area per molecule (n-A) isotherms of OA/OAM monolayers on different subphases are shown in Fig. 1. On the surface of double-distilled water, the limiting area per molecule of OA/OAM mixture was 20.6 A2 (curve 1). The molecules in the monolayers were most likely arranged tightly (see Fig. 2a) owing to the following interaction of OA and OAM at the air/water interface
R1COOH + R2NH2 = R1COO- + R2NH+.
In the mixed monolayers, the ion pairs of -COO-and -NH3 groups are formed due to acid-base interactions. These electrostatic interactions result in condensation of OA/OAM monolayers. When the double-distilled water was replaced by aqueous CMC solution, the limiting area per molecule reached 21.8 A2 (Fig. 1,
Fig. 2. Schematic diagram of mixed monolayers on different subphases: double-distilled water (a), CMC solution (b), and CMC/CaCO3 supersaturated solution (c); for all subphases pH value was adjusted to 8.6.
curve 2), that is greater than similar value determined from the curve 1. This effect was obviously caused by the addition of CMC. Residual negative charge on the carboxyl of CMC occurs at pH 8.4 since the isoelectric point (pi 7.28) of CMC is lower than pH value of subphase. Negatively charged CMC molecules interact
with NH3 groups of OAM, and the hydrogen bonds between CMC and OA/OAM are probably formed simultaneously, that is, CMC adsorbs or inserts into the monolayer (Fig. 2b). The limiting area per molecule shifted to 22.2 A2 on CMC/CaCO3 supersaturated solu-
dn/dA, mN/m A2
20 -
10 0 -10 -20 -30
14 16 18 20 22 24 26 28 30 32 34
Area per molecule, Â2
Fig. 3. Differentiated n-A curves of OA/OAM monolayers on different subphases: double-distilled water (1), CMC solution (2), and CMC/CaCO3 supersaturated solution (J); for all subphases pH value was adjusted to 8.6.
tion subphase (Fig. 1, curve 3), that points to the adsorption or insertion of the other components (Ca2+ and
CO3 ) into the monolayer (see Fig. 2c). The strong electrostatic attraction between Ca2+ and -COO-.
or
CO2 and -NH+, and, presumably, hydrogen bonds formed between the monolayer constituents and CMC may result in the increase of the limiting area of OA/OAM monolayer.
The isotherm 1, depicted in Fi
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