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Colloids and Surfaces B: Biointerfaces 109 (2013) 52– 58

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces B: Biointerfaces

jou rn al hom epage: www.elsev ier .com/ locate /co lsur fb

quilibrium and kinetic aspects of sodium cromoglycate adsorptionn chitosan: Mass uptake and surface charging considerations

.R.M. de Lima, M.R. Pereira, J.L.C. Fonseca ∗

nstituto de Química, Universidade Federal do Rio Grande do Norte, Campus Universitário, Lagoa Nova, Natal, RN 59078-970, Brazil

a r t i c l e i n f o

rticle history:eceived 8 February 2013ccepted 19 March 2013vailable online 28 March 2013

a b s t r a c t

Chitosan has more and more been suggested as a material for use as adsorbent in the treatment of efflu-ents as well as in the synthesis of drug-loaded nanoparticles for controlled release. In both cases, a goodunderstanding of the process of adsorption, both kinetically and in terms of equilibrium, has an impor-tance of its own. In this manuscript we study the interaction between sodium cromoglycate, a drug usedin asthma treatment, and chitosan. Equilibrium experiments showed that Sips (or Freundlich–Langmuir)

eywords:hitosanromoglycatedsorptioneta potentialinetics of adsorption

isotherm described well the resultant data and adsorption possibly occurred as in multilayers. A modelbased on ordinary reaction-rate theory, compounded of two processes, each one with a correlated veloc-ity constant, described the kinetics of sorption. Kinetic and equilibrium data suggested the possibility ofsurface rearrangement, favored by the increase of temperature.

© 2013 Elsevier B.V. All rights reserved.

rug delivery

. Introduction

Adsorption has been employed as a technique for purificationince ancient Egyptian eras [1] and its success up to present timeselies on its simplicity, efficacy, and low cost [2]. In effluent treat-ent, it has been used to treat a vast variety of dyes [3] and has

lso been utilized to load drugs on particles for posterior release4].

Sorption associated to effluent treatment has been analyzed,oth in terms of equilibrium and kinetics of sorption for a wideariety of particles [5,6], including biological ones (i.e. microorgan-sms) [7]. As an interesting example, one could cite the work ofong et al., who used macroporous carbon nanoparticles to adsorb

adioactive substances from the human body for subsequent elimi-ation [8]. In this work the idea of remediation by adsorption, used

n the environment when oil spills and chemical leaks [9,10] occur ispplied in the human body, clearly drawing a connection betweenffluent treatment, adsorption and biological systems.

Regarding systems to be directly applied to living beings, it islear that adsorption also is important in processes such as par-icle drug loading: a good understanding of the kinetics of drugoading may be used to understand drug–particle interactions with

bvious consequences for the formulation of plausible hypothe-is’ for the kinetics of drug releasing. Xiao et al. have found thatpirubicin hydrochloride followed a pseudo-second order kinetics

∗ Corresponding author. Tel.: +55 0 84 3211 9224; fax: +55 0 84 3211 9224.E-mail address: jlcfonseca.br@gmail.com.br (J.L.C. Fonseca).

927-7765/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfb.2013.03.025

when adsorbed on magnetic carbon nanotubes [11]. Jansch et al.carried out an interesting study on the kinetics of adsorption ofproteins on ultra-small magnetic particles, however without thedevelopment of mathematical models [12]. Khan et al. have foundthat the adsorption of some bacteria on colloidal silver obeyeda pseudo-first order kinetics [13]. Huang et al. found that theadsorption of a Cathepsin B-sensitive peptide on nanodiamondsfollowed a pseudo-second order kinetics [14] and Cabrita et al.found that acethominophen also followed a pseudo-second orderkinetics when adsorbed on activated carbon [15].

Chitosan, a biopolymer obtained from the deacetylation of chitinhas been the suggested as a cheap and abundant material foreffluent treatment by adsorption [16], as well as raw material forpreparation of nanoparticles to be loaded with drugs [17]. How-ever, to our knowledge, there are few works that try to build akinetic model based on a clear mechanism of adsorption for theresultant systems. Recently we have analyzed the case of adsorp-tion of tetracycline on chitosan, and shown the problems related tothe pseudo-order approach [18]. More specifically, we have shownthat a good curve fitting could be the result of a statistical arti-fact, besides the fact that the mathematical formulation of the rateequation would imply a constant continuous phase concentration:since depletion method is based on the variation of the concentra-tion in this phase, the logical flaw is self-evident. Later, we also havedescribed the adsorption of tetracycline hydrochloride on chitosan

using a kinetic model based on a proposed molecular mechanism[19]. In the present work we intend to extend this analysis to ananionic drug: sodium cromoglycate, a drug used in asthma treat-ment and other allergic conditions [20].

C.R.M. de Lima et al. / Colloids and Surfaces B: Biointerfaces 109 (2013) 52– 58 53

tylatio

2

Pmd1eadU

ssCu

2

2

pottrtdfea

2sr3wt

Fig. 1. Chemical structures of chitosan (I) [the degree of deace

. Experimental

Chitosan (Fig. 1I) used in this work was purchased fromolymar LTD (Brazil) and had a deacetylation degree of 88% deter-ined by CHN elemental analysis and conductometric titration as

escribed elsewhere [21]. Average viscometric molar mass, MV =.6 · 105 g mol−1, was determined using Mark–Houwink-Sakuradaquation [22]. Purified chitosan in the form of powder was obtaineds described in a previous work [18] and its specific surface area wasetermined via BET (BET Nova 1200e, Quantachrome Instruments,SA) as AS = 0.151 m2g−1.

Acetic acid (P.A., Cromato Produtos Químicos LTD, Brazil),odium hydroxide (P.A., Cromoline Química Fina LTD, Brazil), andodium cromoglycate (Fig. 1II, Purity ≈ 100%, Kopalle Pharmahemicals, India), were used as received. Bi-distilled water wassed in all experiments.

.1. Adsorption experiments

.1.1. Equilibrium experimentsA volume of 10 mL of cromoglycate solution (at a given tem-

erature T) with a given concentration [CG]0 was added to a massf chitosan mC = 0.05 g. Before taken to further analyses, the resul-ant dispersion rested for 24 h, at the same temperature T. Afterhis time, the analyses described in the next subsections were car-ied out (at the same temperature T). For experiments designedo surface charge analysis, all the masses and volumes used wereoubled, in order to obtain higher amounts of dispersion, neededor zeta potential measurements. The variables analyzed in thesexperiments were: temperature (T = 5 ◦C, 10 ◦C, 15 ◦C, and 20 ◦C)nd cromoglycate solution concentration, [CG]0.

.1.1.1. Determination of adsorption isotherm. After adsorption, theupernatant was carefully withdrawn using a pipette and the

esultant clear solution underwent centrifugation at 6000 rpm, for0 min. Supernatant sodium cromoglycate concentration, [CG]E,as spectrophotometrically determined using an UV–vis spec-

rophotometer (Genesys 10 UV–vis, Thermo Electron Corporation,

n is given by 100 �D/(�A + �D)], and sodium cromoglycate (II).

USA) at a wavelength of 320 nm, as reported in a previous work[23]. Equilibrium surface excess of adsorbed cromoglycate, �E, wasthen calculated as

�E = ([CG]0 − [CG]E) VA

mCAS, (1)

having in mind that VA is the dispersion volume, mC is the massof chitosan in the dispersion, and AS is the specific surface area ofchitosan.

2.1.1.2. Determination of zeta potential, �. 20 mL of the disper-sion were used to determine electrophoretic mobility, �E, using aZeta-Meter System 3.0+ (Zeta-Meter Inc., USA). Zeta potential wascalculated using Smoluchowski equation [24]:

� = �E�0

ε0εr, (2)

where �E is the electrophoretic mobility, �0 is the continuous phaseviscosity, ε0 is the permittivity of a vacuum, and εr is the relativepermeability of the continuous phase.

2.1.2. Kinetic experiments0.7 L of bi-distilled water was added to a mass of chitosan,

mC = 4 g. The resultant system rested for 20 h, in order to eliminatethe influence of chitosan swelling on the kinetics of cromogly-cate adsorption. Afterwards, at an arbitrary time t = 0, a volumeof 0.1 L of a solution of cromoglycate with a given concentration[CG]T = 0.00145 mol dm−3 (so that, just after addition, overall con-centration at t = 0 was [CG]0 = 0.1 · [CG]T/0.8 = 1.82 · 10−4 mol L−1)was added to the dispersed system, which was maintained understirring for 200 min. During this time a given temperature T washeld at with an associated uncertainty of 0.5 ◦C using a thermostaticbath (Tecnal, model TE-184, Brazil) and aliquots were withdrawnfrom the system (efficient stirring assured that the disperse phasecontents in the aliquots and in the remaining system were the same

during all the experiment). The following analyses were carriedout in these aliquots, using the same methodology described inthe equilibrium experiments: surface excess of cromoglycate, �(t);zeta potential, �(t).

54 C.R.M. de Lima et al. / Colloids and Surfaces B: Biointerfaces 109 (2013) 52– 58

Fig. 2. Equilibrium surface excess, �E , as a function of equilibrium sodium cromo-glycate continuous phase concentration, [CG]E . (a) T = 5 ◦C; (b) T = 10 ◦C; (c) T = 15 ◦C;( ◦

3

3

etaar

i[

�

hnda

�

wiqf

Fig. 3. Parameters from Eqs. (3) and (4) as a function of temperature. (a) �M,S

as a function of T; (b) n as a function of T; (c) van’t Hoff plot of KL; thefirst continuous line represents Eq. (6) for �Hadsorption = (−26 ± 1) kJ mol−1 and

−1 −1

d) T = 20 C. Dashed lines, Eq. (3); continuous lines, Eq. (4).

. Results and discussion

.1. Equilibrium experiments

Fig. 2 depicts the relationship between equilibrium surfacexcess, �E, and equilibrium cromoglycate concentration, [CG]E, atemperatures T = 5 ◦C, 10 ◦C, 15 ◦C, and 20 ◦C. It can be seen that inll the cases, �E asymptotically tended to defined values, so thatnd no rupture of the chitosan particle structure occurred, such aseported in the case of tetracycline hydrochloride sorption [19].

The presence of a saturation value for �E made Langmuirsotherm an obvious choice for describing cromoglycate sorption25]:

E = �M,LKL[CG]E

1 + KL[CG]E(3)

ere �M,L is the saturation value of �E for this adsorption mecha-ism and KL is proportional to the ratio between the adsorption andesorption rates. Sips isotherm, or Freundlich–Langmuir isotherm,lso provides a saturation value of �E [26,27]:

E = �M,SKS[CG]1/n

E

1 + [CG]1/nE

, (4)

here �M,S is the saturation value of � E, KS is a constant and ns a dimensionless constant (more specifically, a parameter thatuantifies the heterogeneity of the adsorption process, derivedrom Freundlich isotherm [28]). One can observe in Fig. 2 that

�Sadsorption = (6 ± 4) J K mol .

Eq. (4) was slightly better fitted to experimental data (for highervalues of [CG]E) than Eq. (3), suggesting that adsorption did notoccur via monolayer adsorption. As a consequence, adsorption wasdescribed using the following parameters: �M,S (to quantify cromo-glycate saturation surface excess), n (to indicate the homogeneityof adsorption process) and KL (to indicate sorption rate desorptionrate ratios at low cromoglycate concentrations). These parametersare plotted as a function of temperature in Fig. 3.

An analysis of Fig. 3a shows that there is an increase in �M,S, butit seems to reach a constant value at T = 15 ◦C. If one assumes thatadsorption occurred in the form of a monolayer, the area occupiedby each cromoglycate molecule, , is given by

= 1�M,SN0

, (5)

where N0 is Avogadro’s number. Eq. (5) yields values of that arebetween 0.7 and 0.9 A2. Using the software Marvin Sketch 5.9, wecalculated the minimal and maximum projected area of the cro-moglycate ion and the values were, respectively, 52 and 121 A2. Interms of adsorption these results could imply:

i. Adsorption occurs in the form of multiple layers.ii. Cromoglycate interacts with the surface, modifying its struc-

ture, so that particle surface is increased (we must have in mindthat surface area of chitosan was measured when it was dry).

iii. Both (i) and (ii).

urfaces B: Biointerfaces 109 (2013) 52– 58 55

ae(mtiFicitso

l

wiwI�oaFtpletosttgtwerfmabmtpcaNnri

pftsta

�

wpi

theory, according to

CG + A − chitkD�kI

CG· · ·A − chit, (8)

Table 1Parameters obtained from curve fitting of Eqs. (3) and (7) to data respectively shownin 4.

C.R.M. de Lima et al. / Colloids and S

Regarding the dependence of n on temperature (Fig. 3b),dsorption has a strong Langmuir character at T = 5 ◦C (n → 1); het-rogeneity of adsorption process increases, reaching a maximumn ∼= 1.5), and, at 15 ◦C it decreases and tends to have, again, a Lang-

uir character. A temperature located between 10 and 15 ◦C seemso be linked to some change on chitosan surface. As heterogene-ty factor increases adsorption tends to have a more pronouncedreundlich character, i.e., it occurs in multiple layers. In fact, whats perceived as adsorption in multiple layers may be a result of spe-ific interactions of cromoglygate with chitosan, resulting in thencrease of chitosan’s original surface area. As this area is addedo the surface, it tends, again, to have a Langmuir character. Thesepecific interactions can be understood by analyzing van’t Hoff plotf KL (Fig. 3c). The van’t Hoff plot is mathematically described as:

n KL = �Sadsorption

R− �Hadsorption

RT, (6)

here �Sadsorption is the entropy of adsorption and �Hadsorptions the enthalpy of adsorption. It can be seen that van’t Hoff plot

orks for part of the graph: for T ≥ 10 ◦C (or 103/T ≤ 3.5 K−1).n this range one has �Hadsorption = (−26 ± 1) kJ mol−1 and

Sadsorption = (6 ± 4)J K−1mol−1. Cestari et al. have found valuesf adsorption enthalpy in the same range when studying thedsorption of indigo carmine dye on cross-linked chitosan [29].or temperatures below 10 ◦C, it seems that there is a drop inhe apparent enthalpy of adsorption to values near zero and aronounced increase in the entropy of adsorption (straight line

inear coefficient). In other words, at lower temperatures, thentropy component of adsorption rules it and at higher tempera-ures the enthalpy of adsorption is the main influence. Interactionsf enthalpic nature usually are based on attractive forces betweenpecific chemical groups of the species involved in adsorption: inhis case, one may cite the electrostatic interactions between pro-onated amino groups of chitosan surface and negatively chargedroups from cromoglycate. An example of entropy-driven interac-ion is the hydrophobic effect, which may drive adsorption evenhen repulsive interactions between adsorbate and adsorbant

xist [30]. In our case, it is the result of association of a hydrophobicegion of chitosan (a region highly acetylated of poorly protonated,or instance) with the hydrophobic region of the cromoglycate

olecule: the increase in entropy is the result of the release ofdsorbed water on these sites. An explanation to the change inehavior is that for temperatures below 10 ◦C adsorption occursainly via hydrophobic effect: there is no kinetic energy sufficient

o promote mobility of chains located on the surface of chitosanarticles in order to expose NH+

3 groups originally located belowhitosan/water interface for electrostatic interactions. As temper-ture is increased, there is an increase in this mobility, so thatH+

3 groups are more available for electrostatic interactions withegatively charged chromoglycate: as a result, there is the occur-ence of a negative enthalpy of adsorption and the decrease in themportance of hydrophobic interactions.

Fig. 4 shows the relationship between zeta potential of chitosanarticles, �, and equilibrium concentration of cromoglyclate, [CG]E,or adsorption experiments at 20 ◦C. As � is approximately equalo the surface potential and this potential is proportional to theurface charge density [24], the variation in �, ��, can be related tohe incorporation of negative charges on the particle surface. If wedapt it to the Langmuir isotherm [19]:

�E,L = ��M,LKL,�[CG]E

1 + KL,�[CG]E⇒ �E,L = �0,L + ��M,L

KL,�[CG]E

1 + KL,�[CG]E,

(7)

here ��E,L is the variation in zeta potential, �0,L is the zetaotential prior to adsorption,

∣∣��M,L

∣∣ is the maximum variationn zeta potential, and KL,� is proportional to the ratio between

Fig. 4. Zeta potential, �, as a function of equilibrium cromoglycate concentration,[CG]E (adsorption experiments carried out at 20 ◦C). Continuous line: Eq. (7).

adsorption/desorption rates. One can see in Fig. 4 that Eq. (7) rea-sonably fits to the data. The values of the parameters related to Eq.(7) are displayed in Table 1 (the values of the parameters obtainedusing sorption data and Eq. (3) are included for the sake of com-parison). It can be seen that the parameter KL,� from Eq. (7) is muchsmaller than the parameter KL related to Eq. (3): it shows that elec-trostatic interactions are just part of the process in the adsorptionof cromoglycate, confirming the importance of hydrophobic inter-actions.

3.2. Kinetic experiments

Fig. 5 shows the dependence between surface excess, �, andtime during the sorption of cromoglycate on chitosan with initialcromoglycate concentration [CG]0 = 1.82 × 10−4mol dm−3, for thetemperatures of 5 and 20 ◦C.

A first analysis of Fig. 5 indicates that at 5 ◦C adsorption is lessintense at shorter times, compared to the experiment at 20 ◦C. Fromthis observation we will divide the sorption process in two (oneoccurring at shorter times and another one at longer times). Thekinetics of a given process will be ruled by ordinary reaction rate

Solute depletion [Eq. (3)] Zeta potential [Eq. (7)]

� M,L(10−4mol m−2) 2.11 ± 0.06 ��M,L (mV) −23.4 ± 0.8KL(104mol−1dm3) 9.4 ± 0.6 KL,�(104mol−1dm3) 6.4 ± 0.9

56 C.R.M. de Lima et al. / Colloids and Surfaces B: Biointerfaces 109 (2013) 52– 58

F[fi

wcihpa

−

titb[

−

[

[s

Table 2Parameters of Eq. (14) for the temperatures of 5 and 20 ◦C.

Parameter T = 5 ◦C T = 20 ◦C

fs 0.751 ± 0.004 0.51 ± 0.01kS(min−1) 0.0092 ± 0.0004 0.018 ± 0.001

ig. 5. Cromoglycate surface excess, �, as a function of time forCG]0 = 1.82 × 10−4mol dm−3. (a) T = 5 ◦C; (b) T = 20 ◦C. Continuous lines, bestt of Eq. (14).

here CG stands for cromoglycate and A is an specific site on thehitosan surface [it could be a fully-acetylated region (if interactions hydrophobic) or protonated/hydroxylated sites (if interactionas an negative enthalpic nature); the dotted line represents thehysical interaction promoting adsorption. In terms of kinetics,dsorption would be described as

d[CG]dt

= d[CG· · ·A − chit]dt

= kD[A − chit][CG] − kI[CG· · ·A − chit].

(9)

Assuming that the amount of sites for cromoglycate adsorp-ion is much higher than the amount of cromoglycate molecules,.e. [A − chit] = [A − chit]0 is constant, and having in mind thathe concentration of adsorbed cromoglycate equals the differenceetween initial and present concentrations of cromoglycate, i.e.CG · · · A − chit] = [CG]0 − [CG], it follows that

d[CG]dt

= −kD[A − chit]0[CG] + kI ([CG]0 − [CG]) . (10)

Rearranging of Eq. (10) yields

1kD[A − chit]0 + kI

d[CG]dt

= [CG] − kI[CG]0

kD[A − chit]0 + kI. (11)

Integration of Eq. (11) results in:

−kt

CG] = [CG]E + ([CG]0 − [CG]E) e . (12)

Here, the equilibrium concentration of cromoglycate isCG]E = kI[CG]0/kD[A − chit]0 + kI, and the apparent velocity con-tant is k = kD[A − chit]0 + kI.

kF(min−1) 0.7 ± 0.3 0.40 ± 0.06kM(min−1) 0.0122 ± 0.0006 0.033 ± 0.003

Using the definition of � from Eq. (1) and applying it to Eq. (12),it follows that:

� (t) = VA ([CG]0 − [CG])mCAS

= �E(1 − e−kt), (13)

where �E = VA ([CG]0 − [CG]E) /mCAS .From this theoretically-derived expression for one process, we

propose the following semi-empirical expression that will take intoaccount slow and fast adsorption processes:

� (t) = �E [fS(1 − e−kSt) + (1 − fS)(1 − e−kF t)] , (14)

here fs is a dimensionless constant between 0 and 1. As it tendsto 0, adsorption tends to be described as driven by fast processes,with a velocity constant kF. As it tends to 1, adsorption tends to bedescribed as governed by slower processes, with velocity constantks.

A second glance at Fig. 5 reveals that Eq. (14) was well fitted tothe data obtained in our experiments. These values are displayedin Table 2. It can be seen that the velocity constant accounting forprocesses related to short times (kS = 0.0092 min−1) increases to thedouble of its value as temperature is increased, while the values ofthe velocity constants accounting to fast processes did not signifi-cantly change (taking into account their associated errors). On theother hand, the value of fs decreases as temperature is increased: inother words, fast processes become more important as temperatureis increased.

We could use an approach already used in dynamic light scat-tering to define an average velocity constant [31]. If an adsorptionprocess is described by single velocity constant, it follows that

�

�E− 1 = e−kt ⇒

∞∫

0

(�

�E− 1

)dt =

∞∫

0

e−ktdt = 1k

. (15)

Therefore, an average velocity constant, kM, may be defined as

kM =

⎡⎣

∞∫

0

(� (t)�E

− 1)

dt

⎤⎦

−1

. (16)

Substitution of Eq. (14) in Eq. (16) yields

kM =(

fSkS

+ 1 − fSkF

)−1

. (17)

The values of kM were calculated and also are displayed in Table 2. Itcan be seen that the kM has the same behavior of ks: as temperatureincreases its value increases. That makes quite clear the importanceof slower processes in determining the kinetics of adsorption. Infact, for values of fs within the same order of magnitude and kF � kS:

kM =(

fSkS

+ 1 − fSkF

)−1∼=

(fSkS

)−1

= kS

fS. (18)

In other words, the kinetics of sorption is determined by the velocityconstant for slow processes and the fraction represented by theseprocesses.

C.R.M. de Lima et al. / Colloids and Surfaces B: Biointerfaces 109 (2013) 52– 58 57

FT

t

E

ytasctbirvlbN

appa

•

•

Fig. 7. Schemme depicting the interaction of cromoglycate with chitosan surface.(I) Cromoglycate is solubilized in the continuous phase and some NH2 groups ofchitosan are in the surface, in its protonated form, NH+

3 , and some of them are in thesubsurface. (II) Cromoglycate is adsorbed on chitosan. (III) Chitosan-chitosan inter-

ig. 6. Zeta potential, �, as a function of time during cromoglycate sorption, at = 20 ◦C. Cromoglycate initial concentration [CG]0 = 1.82 × 10−4mol dm−3.

An apparent energy of activation may be estimated from thesewo values of kM, according to

A =R ln( k

∣∣T=5◦C

/ k∣∣T=20◦C

)(1/293 − 1/278

) , (19)

ielding a value of EA = (44 ± 6)kJ mol−1. As already reported inhe literature, EA values between 5 and 40 kJ mol−1 indicate thatdsorption mainly occurs by physisorption (controlled mainly byurface diffusion) and between 40 and 800 kJ mol−1 indicates thathemisorption (controlled by purely chemical interactions) driveshe process [32]. The value of EA found in this work is in the limitetween the two mechanisms and may be an indication of the

ncidence of the two processes previously cited: slow processes,elated to surface rearrangement, that would have higher acti-ation energies and the process of purely chemical nature, withower activation energies, such as the electrostatic interactionsetween negatively charged cromoglycate and positively chargedH+

3 groups from chitosan.Fig. 6 shows how zeta potential changes with time during

dsorption and it is clear that the behavior is much more com-lex. Although in equilibrium zeta potential is lower than its valuerior to adsorption, it reaches a minimum, before tending to itssymptotic value. That could be due to the following reasons:

Cromoglycate is adsorbed, followed by the adsorption of Na+, itscounter-ion.Cromoglycate is adsorbed via electrostatic interactions orvia hydrophobic effect, decreasing zeta potential (counter-ionadsorption simultaneously occurs). The adsorption of cromo-

glycate at hydrophobic regions of chitosan decreases interfacetension between solid and liquid phase and interactions are bro-ken in the surface, making possible conformational changes inthe surface. As a result, amino groups that were in the subsurface

actions on the surface are weakened, so that bond rotation and/or surface ruptureresults in increase of surface area and the exposition of previously “buried” NH2 forprotonation.

become available at the surface: as part of them are protonated,zeta potential increases. Such a mechanism is depicted in Fig. 7.

The second hypothesis is more consistent with the mechanismsuggested in this paper.

4. Conclusions

Equilibrium and kinetic data on sodium cromoglycate adsorp-tion on chitosan particles indicated that the process of adsorptionmay occur by hydrophobic interactions with nonpolar sites (e.g.,acetylated groups) as well as polar, electrostatic interactions. Atlow temperatures it seems that the hydrophobic effect prevailsbut, as temperature is increased, interactions of enthalpic naturebegin to rule. These data are confirmed by experiments related tothe kinetics of sorption: the process of sorption was divided intotwo simultaneously occurring subprocesses – a fast process, whichis related to adsorption occurring on readily available sites and aslow process, which is related to adsorption occurring in sites thatare being formed as a result of surface rearrangement.

Acknowledgements

The authors thank Brazil’s Conselho Nacional de Desen-volvimento Científico e Tecnológico (CNPq), Coordenac ão deAperfeic oamento de Pessoal de Nível Superior (CAPES), and Pró-Reitoria de Pesquisa da Universidade Federal do Rio Grande doNorte (PROPESQ-UFRN) for financial support during the course ofthis work.

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