Multilayer adsorption of amino acids on oxidized cellulose

Multilayer adsorption of amino acids on oxidized cellulose

Journal of Colloid and Interface Science 285 (2005) 502–508 www.elsevier.com/locate/jcis Multilayer adsorption of amino acids on oxidized cellulose D...

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Journal of Colloid and Interface Science 285 (2005) 502–508 www.elsevier.com/locate/jcis

Multilayer adsorption of amino acids on oxidized cellulose Dmitry S. Zimnitsky ∗ , Tatiana L. Yurkshtovich, Pavel M. Bychkovsky Belarusian State University, Research Institute for Physical Chemical Problems, 14 Leningradskaya Street, Minsk 220050, Belarus Received 1 October 2004; accepted 1 December 2004 Available online 29 January 2005

Abstract The adsorption of amino acids (AA) (glycine, L-alanine, L-proline) on oxidized cellulose (OC) with various carboxyl contents and degrees of crystallinity from aqueous and water/ethanol solutions was studied. It was found that multilayer adsorption occurs in concentrated solutions of AA. It proceeds according to successive mechanisms via adsorption of AA zwitterions onto carboxyls of already adsorbed AA. This leads to formation of chain AA associates in the OC phase. A sharp increase in swelling accompanies multilayer adsorption. It was established that structural characteristics and degree of polymerization of OC are the main factors that affect multilayer adsorption. The distribution of carboxyls in the OC phase also plays an important role. Multilayer adsorption does not proceed in water/ethanol solutions and in the case of the cationic form of AA.  2004 Elsevier Inc. All rights reserved. Keywords: Adsorption; Amino acids; Oxidized cellulose; Multilayer; FT-IR; Crystallinity; Degree of polymerization

1. Introduction Many recent studies have been devoted to investigation of adsorption of amino acids (AA) on ion exchangers. They have dealt with the separation and purification of AA [1,2], the mechanism of their binding with adsorbent [3–5], the effect of temperature [6], adsorbent [7], and AA nature [8] on adsorption, and so on. The interest to AA is caused by their extensive use in the pharmaceutical and health industries. Glycine and alanine are used in the treatment of benign prostatic hyperplasia [9] and schizophrenia [10]. Other AA (proline, tryptophane, and 4-aminobutanoic and 6-aminocaproic acid) are also widespread drugs. Because AA are constituents of human body, side effects during their use are rare. However, rapid elimination of AA requires frequent and massive introduction of drugs for effective therapy. For this reason, these medicines are preferable in prolonged forms. The prolongation of drug action can be achieved by adsorption immobilization of drugs on polymer carriers. * Corresponding author. Fax: +375-17-2066229.

E-mail address: [email protected] (D.S. Zimnitsky). 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.12.021

Oxidized cellulose (OC) containing carboxyl groups is a polymer carrier that has several useful medical characteristics [11]. It is one of the most widespread hemostatics, used almost in all types of surgery [12]. OC has been shown to possess immunostimulant [13], wound-healing [14], and adhesion-prevention properties [15]. The presence of carboxyl groups in the OC permits the immobilization of drugs by adsorption [16] and preparation of polymer drugs with a variety of therapeutic effects. These properties, as well as complete bioabsorption [17], characterize OC as a medical material with very high potential. One of the key questions of drug immobilization on OC is the achievement of a sufficient therapeutic concentration of the drug in the OC phase. This requires profound investigation of regularities of drug adsorption on OC and factors that affect the adsorption. The aim of this work was to investigate adsorption of AA from their aqueous and water/ethanol solutions on samples of OC with different exchange capacities and structure characteristics. The investigation of adsorption regularities is necessary for determination of optimal conditions for preparation of polymer drugs with high AA content in the OC phase. The adsorption from concentrated AA solutions was a problem of special interest.

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2. Experimental

ethylenediamine. Thus, the calculated DP must be considered only for a demonstration of changes of this parameter in the series of OC samples. Fourier transform infrared (FT-IR) spectra of samples as KBr pellets were obtained with a Thermo Nicolet FT-IR Nexus spectrometer. The X-ray diffraction measurements were performed using a Carl Zeiss diffractometer (CuKα, Ni filter, HZGb-4A).

2.1. Preparations

2.3. Adsorption procedure

The phenomenon of multilayer adsorption of AA on OC in concentrated solutions of AA has been found for the first time. The effect of solution composition, structural characteristics, and exchange capacity of OC on multilayer adsorption of AA was investigated.

The OC samples were obtained by oxidation of native and mercerized celluloses at 292 ± 1 K over various periods of time. Cellulose in the form of coarse calico with cellulose I structural modification was taken as a starting material. Solutions with different concentrations (10–40%) of N2 O4 in CCl4 were used as oxidants. The ratio of cellulose mass to solution volume was 1:10 (g/ml). The mercerized cellulose was obtained by treatment of cellulose with 20% solution of sodium hydroxide over a period of 3 h at 273 K [18]. The physical form and appearance of cellulose were unchanged after the oxidation procedure. γ -ray in radiation of OC samples was performed with 137 Cs as a source of radiation over a period of 90 h. The absorbed dose was 100 kGr. 2.2. Characterization of adsorbents The OC samples were analyzed for carboxyl content with calcium acetate [19], and for carbonyl content with an ethanol solution of hydroxylamine hydrochloride (HCl that formed in the reaction was titrated potentiometrically with an NaOH solution up to pH 3.2) [20]. A detailed description of the preparation and characterization of the OC samples is given in the paper [21]. The surface areas S (m2 /g) of the OC samples were determined by nitrogen adsorption at 77 K on a NOVA 2200 device (Quantachrome Corp., USA). The samples were outgassed at 333 K and 1 mPa for 2 h. Because immobilization of AA occurs in water solution, an attempt was made to evaluate surface areas of OC samples swelled in water. The water replacement procedure was used [22]; that is, samples of OC were swelled in water and treated three times with acetone and hexane. After this, samples were dried in vacuo at 323 ± 2 K until they reached a constant weight. The swelling of OC was analyzed gravimetrically with centrifugation [23]. The degree of polymerization (DP) of OC was determined by the viscosity method with cadmium ethylenediamine solution as a solvent. The DP values of OC were calculated with the Staudinger equation [24] using viscosity constants for solutions of cellulose in this solvent. The analogous approach to calculation of DP of OC samples has been reported elsewhere [25]. Since OC has low stability in basic media, it should be noted that DP values obtained demonstrate destructive processes, not only in the process of oxidation but also during the dissolution of OC in cadmium

The adsorption of AA from aqueous and water/ethanol solutions was studied by a batch method at 298 ± 0.5 K in the range of initial concentration 0.01–2.7 mol/L. The pH values of initial solutions corresponded to the isoelectric point of AA and were unchanged after the adsorption. The various pH values of initial solutions and water–ethanol mixtures were prepared by adding HCl, NaOH, and ethanol, respectively. The determination of adsorbed AA (q, mmol/g) was performed by their desorption from the OC phase to a 0.1 M HCl solution. Preliminary experiments showed that complete desorption of AA occurs in these conditions. AA in the desorption solution were dyed using the Ninhydrin procedure [26]. Their concentrations were determined with an SF-26 spectrophotometer. This procedure was repeated three times for each concentration and the amount of AA adsorbed was taken as the average of the three replications. The confidence interval of experimental concentration was not higher than 5%. The ability of AA molecules to associate in the concentrated solution was evaluated by cryoscopy with Beckman thermometer. AA used in work were purchased from Diaem (Moscow, Russia). All other chemicals were analytical grade and were purchased from Five Oceans (Minsk, Belarus).

3. Results and discussion Four samples of OC obtained in various conditions (see Table 1) were used as sorbents. Three of them (OC-1, OC2, and OC-4) had the same carboxyl content, 1.8 mmol/g. Their diffraction patterns (Fig. 1) and indexes of crystallinity (IC) (Table 1), calculated according to Segal [27], show that the concentration of nitrogen(IV) oxide solutions in CCl4 strongly affects the structural characteristics of OC. The samples of OC-2 and OC-4 were obtained by oxidation of native and mercerized celluloses using 10 and 15% solutions of N2 O4 , respectively. The diffraction patterns of OC-2 and OC-4 and IC values are almost similar to those of starting cellulose materials. This shows that the oxidation with a 10–15% solution of N2 O4 in CCl4 proceeds mainly in amorphous regions and on the surface of cellulose crystallites. Compared to the dilute solutions (10–15%) of N2 O4 in CCl4 , a 40% solution significantly affects the degree of crystallinity; that is, oxidation proceeds not only on the surface of crystallites, but

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Table 1 Preparation conditions and characteristics of OC Sample

Oxidation time (h)

Concentration of N2 O4 in CCl4 (%)

Carboxyl content (mmol/g)

IC

DP

Swelling (g/g)

S (m2 /g)

OC-1 OC-2 OC-3 OC-4

5 24 24 24

40 15 40 10

1.8 (2.0)a 1.8 (2.0)a 3.8 (4.0)a 1.8 (2.0)a

0.40 0.82 0.40 0.75

67.7 (43.5)a 139.0 (65.9)a 26.0 (18.6)a 80.1 (56.7)a

0.52 0.37 0.58 0.42

12.5 6.2 14.8 8.4

a Characteristics of OC samples irradiated with γ -rays.

Fig. 1. Diffraction patterns of OC-1 (1), OC-2 (2), OC-3 (3), OC-4 (4), and native (5) and mercerized (6) celluloses.

also inside them, destroying their structure. The calculated IC values for OC-1 and OC-3 are 0.4. However, it should be noted that existing methods of distinction of amorphous and crystalline fractions of cellulose do not take into account portion of irregularity (defects in the lattice), which contributes to widening diffraction bands. Thus, OC-1 and OC-3 have very small and defective crystallites and are hardly comparable on crystallinity with native cellulose and OC-2. The analysis of diffraction patterns of native cellulose, OC-2 and OC-1 shows that the latter has one order of magnitude smaller crystallites. The decrease of crystallinity leads to the growth of surface area and degree of swelling in water (Table 1). The dependencies of adsorbed AA amount on the equilibrium AA concentration (Ceq ) for the adsorption on the OC samples and native cellulose are shown in Fig. 2 (the adsorption time was 20 h and the ratio of solution volume to the sorbent mass was 200 ml/g). The AA content in the OC phase increases with growth of exchange capacity and slightly depends on structural characteristics of OC. The curves for AA adsorption on the OC-2 and OC-4 coincide almost entirely. Some increase in the adsorption of glycine and alanine on OC-1 compared to OC-2 and OC-4 can be connected with the larger values of surface area and swelling. The sample of OC-3 has exchange capacity 3.8 mmol/g and structural characteristics analogous to those of OC-1. The adsorption curve on OC-3 resembles the BET isotherm of multilayer gas adsorption. The AA content in the OC phase is higher than the exchange capacity (carboxyl con-

tent) of a sorbent; i.e., multilayer adsorption of AA on OC-3 occurs. Because the multilayer adsorption of all AA causes violation of cellulose fiber structure, it can be assumed that the adsorption equilibrium was not achieved during 20 h. This implies the dependence of the amount of adsorbed AA on the time of adsorption and on the ratio of solution volume to the sorbent mass. Fig. 3 demonstrates the dependence of the amount of adsorbed glycine on the ratio of solution volume to sorbent mass (initial glycine concentration in the solution was 0.75 mol/L and the adsorption time was 20 h). Table 2 shows that amount of adsorbed AA increases with time and exceeds exchange capacity of all OC samples; i.e., multilayer adsorption also occurs in the cases of OC-1, OC-2, and OC-4. However, the rate of this process strongly depends on the method of OC preparation. It is higher in the case of amorphous OC-1 and lower for OC-4 obtained by oxidation of mercerized cellulose. The multilayer adsorption causes almost complete destruction of fibers and formation of gel-like products for all OC samples, which was established by visual observation. The swelling of these products is 300–400 g/g and AA content is 70–80% of dry weight. The DP of OC in these products is approximately two times less than for untreated OC. The formation of such products proceeds over a period of 6 min for OC-3 and 265, 480, and 840 h for OC-1, OC-2, and OC-4, respectively. The difference in times and reactivity of OC-1 and OC-2 allows us to assume that the degree of crystallinity is one of the main factors that affect the multilayer adsorption. The diffraction patterns of OC-2 with adsorbed glycine are represented in Fig. 4. It is clearly seen that the formation of crystalline structure of glycine proceeds with growth of AA content in the OC phase. Both crystalline structures of OC and glycine are seen simultaneously. However, IC values of OC-2 calculated according to Segal for products of multilayer adsorption remain unchanged even for samples with high AA content. This shows that multilayer adsorption does not affect crystallites of OC and proceeds only in amorphous regions and on the surfaces of crystallites. The OC-3 sample exhibits a much higher rate of adsorption compared to OC-1 (Table 2) and visually lower stability in the concentrated AA solutions. This cannot be explained by different degrees of crystallinity, because they have similar IC values. Both samples of OC-1 and OC-3 were oxidized by a 40% solution of N2 O4 in CCl4 but over various

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(a) Fig. 3. Dependence of glycine amount in the OC-3 phase on the ratio of solution volume to the sorbent mass. Table 2 Effect of sorption time on glycine content in the OC phasea Time (h) 0.1 50 116 265 480 840

Glycine content in the OC phase (mmol/g) OC-1

OC-2

OC-3

OC-4

– 2.03 4.92 8.76 – –

– 1.53 3.78 6.73 9.02 –

8.01 – – – –

– – 1.90 3.43 5.83 9.10

a Initial glycine concentration in the solution 2.5 mol/L. Ratio of cellu-

(b)

lose mass to solution volume 1:50 g/ml.

(c) Fig. 2. Dependencies of glycine (a), alanine (b), and proline (c) amounts in the sorbent phase on their equilibrium concentrations.

Fig. 4. Diffraction patterns of OC-2 (1) and products of glycine adsorption on OC-2. Glycine content in the OC phase 1.5 mmol/g (2), 3.8 mmol/g (3), 6.7 mmol/g (4), 9.0 mmol/g (5).

periods of time, so they differ in exchange capacity and DP. It is quite probable that the extent of OC destruction and decrease in DP would be dependent on time of the action of oxidant on cellulose. For an additional estimate of effect of DP on multilayer adsorption, the adsorption of glycine on γ -ray radiated OC was studied. Fig. 5 shows that a sharp increase on the curve of adsorption on γ -ray radiated OC begins at lower AA con-

centration in the case of OC-3 and occurs in the case of OC-1 and OC-2 during 20 h. The content of carboxyls and IC values change insignificantly during radiation and cannot be a reason for such different behavior in concentrated AA solutions. However, radiation strongly affects the DP values of the OC samples (Table 1). Because the decrease of DP occurs during multilayer adsorption, it can be concluded that DP plays a crucial role in this process.

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are bands of initial AA 3100–3000 cm−1 (antisymmetric valence stretchings of NH+ 3 and intensive bands at 1550– 1480 cm−1 (bending vibrations of NH+ 3 ). The absorption bands of native glycine at 1000–500 and 1340–1300 cm−1 are also present. The growth of glycine concentration in the OC phase increases the intensities of these bands. We assume two possible mechanisms of multilayer AA adsorption on OC: 1. The successive mechanism via adsorption of AA molecules on carboxyls of already adsorbed AA and formation of AA associates in the OC phase. 2. The adsorption of AA associates of different sizes formed already in the AA solution. Fig. 5. Dependences of glycine amount in the sorbent phase on its equilibrium concentration for adsorption on γ -irradiated OC-samples.

The cryoscopis analysis of AA solutions with various concentrations up to 1.5 mol/L showed that no association of AA molecules occurs in their concentrated solutions. So it was concluded that multilayer adsorption proceeds via successive adsorption with formation of AA associates already in the OC phase according to the scheme − Cell–COOH + NH+ 3 –R–COO + − → Cell–COO NH3 –R–COOH, + − Cell–COO− NH+ 3 R–COOH + NH3 –R–COO + + − − → Cell–COO NH3 –R–COO NH3 –R–COOH, etc.

Fig. 6. FT-IR spectra of OC-3 (1) and products of glycine adsorption on OC-3. Glycine content in the OC phase 0.7 mmol/g (2), 3.6 mmol/g (3), 6.0 mmol/g (4).

The mechanism of monolayer AA adsorption on ion exchangers was described elsewhere [28] and can be represented by the following equation: − Cell–COO− H+ + NH+ 3 –R–COO + − → Cell–COO NH3 –R–COOH.

The total electroneutrality remains constant at the expense of proton transfer from the carboxyl group of the ion exchanger to the carboxylate ion of the zwitterion of adsorbed AA. The evidences for this mechanism are unchanged pH values of the AA solution after adsorption and data of FT-IR analysis (Fig. 6). The spectrum of OC with adsorbed glycine (concentration in the OC phase 0.7 mmol/g) shows slightly increased intensity of the band at 1420 cm−1 and considerable growth of the band at 1610 cm−1 . Such changes are characteristic of salt formation in the OC phase [29]. The intensity of the band at 1750 cm−1 (undissociated carboxylic group) remains unchanged. There are no bands of glycine in the spectrum. The spectra of OC-3 with multilayer adsorbed glycine (Fig. 6, curves 3 and 4) show some additional changes. There

This chain associates form the crystalline structure of AA in the OC phase during the drying of adsorption product. The appearance of the crystalline structure of AA is seen from Fig. 4. The formation of chain AA associates in the OC phase causes macromolecules of OC to slide apart. This leads to sharp growth of swelling and reduction of cohesion between fibers. It is the reason of violation of fiber structure and gel formation. It can be assumed that change of conformation of OC molecules takes place during multilayer adsorption under the action of chain associates. The mutual distribution of carboxyls in polymer chain and in the volume of the sample is an important factor in the formation of associates. Each third glucose residue in OC-1 and two-thirds of such residues in OC-3 possess carboxylic groups. The macromolecule of OC-3 with lower DP and higher number of sorption centers more easily adopts a conformation at which multilayer adsorption proceeds. The different reactivity of OC-2 and OC-4 in the process of multilayer sorption is connected with a different distribution of carboxyls in the volume of the sample. The decrease of mercerized cellulose surface area compared to native material after washing and drying was established by the authors [22]. During mercerization, alkali penetrates deep into the cellulose fiber, which swells significantly. The fibers become more elastic and their net structure more uniform. The distance between polymer chains averages. Washing and drying shrink the system, so it has a lower volume of accessible pores; i.e., fiber collapse occurs. However, the total

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Fig. 7. Dependencies of proline amount in the OC-3 phase on the concentration of its aqueous and water/ethanol solutions.

structural order of macromolecules and IC value are less in the case of mercerized cellulose that in native cellulose. The mercerized cellulose has a larger value of surface area, which was determined after swelling in water and water replacement, than native cellulose has, according to researchers [22] and data represented in Table 1. Water penetrates into cellulose fibers, expanding their pores, and facilitates transport in the volume of the sample. The water replacement procedure retains the structure of a fiber swelled in water. However, the oxidation of mercerized cellulose proceeded after drying, in the CCl4 solution, where swelling of cellulose was negligible. So the pores of mercerized cellulose were closed during oxidation and the mechanism of capillary transport was hindered. It can be assumed that the oxidation of sites on the surface of mercerized cellulose fibers most probably proceeded. The distribution of carboxyls in the volume of OC-4 is less uniform than in OC-2, which was obtained by oxidation of native cellulose with unhindered capillary transport. The sites of unoxidized cellulose resistant to the concentrated AA solutions occupy a greater volume in the case of OC-4. It should be noted that a sharp increase on the adsorption curve begins long before achievement of the exchange capacity of OC-3 (Fig. 2). This evidences the different accessibility of OC carboxyls for AA molecules and more beneficial adsorption on carboxyls of AA than on OC carboxyls. The addition of ethanol to the concentrated AA solutions vanishes the effect of multilayer adsorption (there is no sharp increase on adsorption curve) and the fiber structure of cellulose remains unchanged. However, the numerical value of monolayer uptake increases (Fig. 7). The degree of swelling in water/ethanol mixtures is reduced considerably compared to that in water solutions [30]. There is also an alteration in the system of hydrogen bonds in the AA water/ethanol solution. These are the reasons for impossibility of formation of the AA chain associates in the OC phase. Thus, the multilayer sorption does not occur even at the small ethanol content in the solution.

507

Fig. 8. Dependencies of glycine adsorption by OC-3 on pH of equilibrium solution.

Fig. 8 illustrates the dependence of glycine monolayer and multilayer adsorption by OC-3 on the pH of the equilibrium AA solutions. Both patterns have maxima, but in different ranges of pH. There is a maximum of monolayer adsorption at pH 3–3.5, i.e., in the region of co-existence of cations and zwitterions of AA. The uptake of cations is more favorable than that of zwitterions. The low adsorption at pH < 3 is connected with the low degree of dissociation of OC carboxyls in this pH region. The decreased adsorption at pH > pHmax is caused both by decreasing cation concentration and by the rival adsorption of Na+ . The multilayer adsorption is maximal at pH 5. It should be noted that the violation of fiber structure, which is characteristic of the multilayer adsorption, begins at pH > 3. This evidences that the formation of chain associates and multilayer adsorption is possible only for zwitterions of AA. The different pH value of maximal uptake is caused by multilayer adsorption of zwitterions, which is the reason for increased affinity of concentrated AA solution to the OC phase. The addition of NaOH leads to rival adsorption of Na+ and some decrease in the adsorption of AA.

4. Summary Thus, it is established that multilayer AA adsorption on OC occurs in concentrated solutions of glycine, alanine, and proline. Presumably, it is provided by successive mechanisms via adsorption of AA molecules on carboxyls of already adsorbed AA. This leads to formation of AA chain associates in the OC phase. Such adsorption is accompanied by sharp increase of swelling and decreasing DP. The swelling increase is the reason of violation of fiber structure and gel formation. It is also established, that the structure characteristics and DP of OC are the main factors that affect multilayer adsorption. The distribution of carboxyls in the OC phase also plays important role. The multilayer adsorption does not proceed in water/ethanol solutions and in the case of cationic form of AA. The studied regularities of

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multilayer adsorption can be used for preparation of polymer drugs with high AA content in the OC phase.

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