Journal of Colloid and Interface Science 299 (2006) 155–162www.elsevier.com/locate/jcis

Equilibrium and kinetics for the sorption of promethazine hydrochlorideonto K10 montmorillonite

Gürhan Gereli a, Yoldas Seki a, I. Murat Kusoglu b, Kadir Yurdakoç a,∗

a Department of Chemistry, Faculty of Arts & Sciences, Dokuz Eylül University, 35160 Buca, Izmir, Turkeyb Department of Metallurgical and Materials Engineering, Faculty of Engineering, Dokuz Eylül University, 35100 Bornova, Izmir, Turkey

Received 14 December 2005; accepted 8 February 2006

Available online 9 March 2006

Abstract

This study presents the adsorption of cationic drug, promethazine hydrochloride from aqueous solution onto K10 montmorillonite. The effects ofpH and temperature on adsorption process were investigated. Maximum adsorption pH was obtained to be about 7.5. Thermodynamic parametersfound in this study depict the exothermic nature of adsorption. The process was favorable and spontaneous. From kinetic studies, it was found thatadsorption process obeyed the pseudo-second-order kinetic model. The Langmuir, Freundlich, Dubinin–Radushkevich (DR) models were appliedto describe the equilibrium isotherms and the isotherm constants were determined. The fit of the Langmuir and DR models appeared to be good.Physisorption mainly controls the whole adsorption process but chemisorption also shows a particular contribution.© 2006 Elsevier Inc. All rights reserved.

Keywords: K10; Adsorption; Kinetic; Thermodynamic parameters; SEM

1. Introduction

The interaction between clay minerals and drugs is one ofthe concerns in pharmaceutical industry for a long time due torelease properties of clay minerals. The presence of the claysin these pharmaceutical formulations affects the drug activitybecause of its interactions with the clay [1]. Interactions be-tween clay minerals and drugs may increase or decrease thebioavailability of drug after administration [1–3]. Promethazineis a cationic drug which possesses high solubility in water. Thisdrug is used due to sedative effects including sedation of youngchildren. In some countries, promethazine is available as a 2%cream for the treatment of allergic skin conditions; however,topical use is not recommended due to skin sensitization reac-tions [4].

Clays such as montmorillonite, sepiolite, and halloysite canbe considered as low cost adsorbent for the release formula-tions. The wide range of application of clays is owing to theirlarge surface area, high chemical and mechanical stability, andhigh cation exchange capacity [5]. The chemical nature and

* Corresponding author. Fax: +90 2324534188.E-mail address: k.yurdakoc@deu.edu.tr (K. Yurdakoç).

0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2006.02.012

pore structure usually determine the sorption ability of clays[5,6].

Some comparative studies that concerned adsorption ofcationic drug have been reported in the literature [7,8]. Theyinferred that the interactions between cationic drug and mont-morillonite prolong the release of cationic drug. They have beenfocusing on the intercalation of promethazine chloride in theinterlayer space [7]. However, the kinetic and thermodynamicparameters and surface characterization were not examined indetail.

In this study, equilibrium, kinetic and thermodynamic mod-els were used to examine the adsorption behavior. They werealso performed to find out the mechanism of interactions usingFTIR, SEM/EDS by observing changes before and after adsorp-tion process. We hope that the results obtained in experimentalstudy may give information for further understanding of the ef-fective administration of promethazine chloride in formulationsthat contain K10 montmorillonite.

2. Materials and methods

K10 montmorillonite was supplied from Fluka (FlukaNo. 69867). The chemical composition of K10 is given in

156 G. Gereli et al. / Journal of Colloid and Interface Science 299 (2006) 155–162

Table 1Chemical composition of K10 montmorillonite

Component %

SiO2 69.0Al2O3 14.0Fe2O3 4.5MgO 2.0CaO 1.5Na2O <0.5K2O <1.5Ignition loss 7.0

Fig. 1. The structure formula of PHCl.

Table 1. Promethazine hydrochloride (PHCl) was used as re-ceived, without any treatment or purification. The structureformula of PHCl (FW: 320.9 g/mol) is shown in Fig. 1.

PHCl stock solution (1 mM) was prepared by dissolving inthe distilled water. Further solutions were freshly prepared fromstock solution for each experimental run. Adsorption experi-ments were evaluated in batch equilibrium technique.

0.1 g of K10 montmorillonite was mixed with 25 mL aque-ous solutions of various concentrations of PHCl for 21 h toreach an equilibrium of the solid solution mixture and kept un-der a constant speed 150 rpm in an isothermal shaker (25 ±1 ◦C), unless otherwise stated. The pH of the drug clay suspen-sions were adjusted with 0.1 N NaOH and HCl solutions usinga pH meter

Samples were withdrawn after 21 h and centrifuged at 4000rpm for 20 min. The pH values of equilibrium solutions were atabout 4.5. The final concentrations of PHCl were measured us-ing Shimadzu UV–vis 1601 model spectrophotometer at wave-length 250 nm (λmax). Similar procedures were carried out toobtain isotherms and thermodynamic parameters at solutiontemperatures of 17.5 and 30 ◦C. The equilibrium concentrationsof the samples were determined by using a standard curve. Theamounts of drug adsorbed onto K10, qs (mmol/g), were ob-tained by subtracting the final solution concentration of drugsolution as follows:

(1)qs = C0 − Ce

msV,

where C0 and Ce (mmol/L) are the initial and final concentra-tions of PHCl, ms is the weight of K10 in solution and V isthe volume of the solution (L). Experiments were conducted atvarious time intervals to determine the kinetic parameters.

FTIR spectroscopy was used to obtain the interactions be-tween drug and clay sample. The FTIR spectra of drug, K10,drug loaded K10 were obtained using KBr pellets with PerkinElmer Spectrum BX-II Model FTIR spectrophotometer. K10montmorillonite, promethazine hydrochloride and drug loadedK10 montmorillonite were brought to constant weight in a dry-ing oven at 40 ◦C for 24 h. Afterwards 100 mg of fine KBr

Fig. 2. Isotherms for promethazine chloride adsorption on K10 at 17.5, 25, and30 ◦C.

powder was dried at 110 ◦C and then mixed with 1 mg of sam-ples. For pellets obtained, the infrared spectra were recordedin the range of 4000–400 cm−1. The background spectrum ofeach KBr pellet was obtained by subtracting it from the samplespectra.

The elemental analysis of K10 and drug loaded was deter-mined using X-ray Florescence (XRF system, Inc., 500 Dig-ital Processing) attached to a scanning electron microscope(Jeol JSM 60 SEM). The surface morphologies of the K10and drug loaded K10 were studied using a scanning electronmicroscope (SEM) at an accelerating voltage of 20 kV. Allsamples were dried and coated with gold before scanning. Pho-tographs were taken at various magnifications (between 1000×and 5000×).

3. Results and discussion

3.1. Adsorption isotherms

Equilibrium data of promethazine hydrochloride (PHCl)onto K10 at different solution temperatures is shown in Fig. 2.As shown in corresponding figure, the adsorbed amount in-creased as the equilibrium concentration increased. The curvestend to reach a plateau. In terms of the slope of initial portion ofthe curves, the shapes may be classified as L type of the Gilesclassification at 17.5 ◦C [9]. This indicates relatively high affin-ity between K10 and PHCl. This type of isotherm usually showschemisorption. The adsorption isotherms corresponding to 25and 30 ◦C may be classified as H type of the Giles classifica-tion, this fact indicating strong K10 and PHCl interaction. TheH and L types are considered typical shapes for chemisorption.Yet one should always remember that the isotherm shape itselfcan never prove the adsorption mechanism involved; it can onlypoint to a reasonable mechanism [10].

PHCl adsorption data at different temperatures were inves-tigated to fit the models of Langmuir and Freundlich. It is ofimportance to optimize the design of an adsorption system. TheLangmuir adsorption isotherm which assumes monolayer cov-

G. Gereli et al. / Journal of Colloid and Interface Science 299 (2006) 155–162 157

Table 2The sorption parameters of Langmuir equation for the adsorption PHCl on K10clay at different temperatures

Temperature (◦C) R2 Cm (mmol/g) L (g/dm3)

17.5 0.980 0.333 64.1025 0.992 0.252 337.1530 0.989 0.245 469.96

Table 3Freundlich isotherm constants for the adsorption of PHCl onto K10 at differenttemperatures

Temperature (◦C) R2 Kf (mmol/g) nf

17.5 0.893 1.015 0.43425 0.745 0.465 0.17630 0.984 0.365 0.143

erage of the adsorbate at the outer surface of the adsorbent canbe represented as follows:

(2)Ce/Cs = 1/CmL + Ce/Cm,

where Ce is the equilibrium drug concentration in solution(mmol/L), Cs is the equilibrium drug concentration on adsor-bent (mmol/g), Cm is the monolayer capacity of the adsorbent(mmol/g) and L is the constant for the adsorption process. Thevalues of Cm and L were estimated from the slope and interceptof the linear plot of Ce/Cs versus Ce and are given in Table 2.As can be seen from Table 2, the values of correlation coeffi-cient (R2

L) demonstrated a good agreement of the experimentaldata with the Langmuir model. This, however, is indicative ofmonolayer coverage at the outer surface of the adsorbent. Thisfurther assumes that once the drug molecule occupies a siteon clay particles, no further adsorption can take place at thatsite [5].

The adsorption capacity values obtained from Langmuirequation increase from 0.245 to 0.333 mmol/g when the tem-perature decreases from 30 to 17.5 ◦C. This offers that lowtemperatures favor adsorption of PHCl on K10 in the range of17.5–30 ◦C.

The empirical Freundlich equation corresponds to the het-erogeneous adsorbent surface. The Freundlich equation is em-ployed to describe reversible adsorption and is not restricted tothe formation of monolayer [11]. The well-known expressionof the Freundlich model is

(3)Cs = KfCnfe ,

where Kf and nf are the constants which show adsorption ca-pacity and adsorption intensity, respectively. A linear form ofthe Freundlich model can be written by taking logarithms ofthe equation as follows

(4)lnCs = lnKf + nf lnCe.

A plot of lnCs versus lnCe gives a straight line of slope nf,and intercept lnKf. The values of Kf (relative adsorption ca-pacity), nf (adsorption intensity) and R2 (the correlation coef-ficients for Freundlich equation) are presented in Table 3. Ascan be seen from Table 3, the values of nf for K10 are less

Table 4The sorption parameters of DR equation for promethazine hydrochloride onK10 at different temperatures

Temperature (◦C) R2 k × 102 Xm × 104 (mol/g) E (kJ/mol)

17.5 0.988 0.40 27.67 11.2425 0.940 0.14 6.07 18.9730 0.994 0.12 5.78 20.85

than 1 at all temperatures, indicative of high adsorption inten-sity. If the values of Kf and nf are compared, it can be seenthat higher values of nf and Kf were obtained at lower temper-atures.

It appears that the Langmuir model gives a much better fitthan the Freundlich model when the R2 values are compared inTables 2 and 3. The values of relative adsorption capacity (Kf)increase from 0.365 to 1.015 mmol/g when the temperature de-creases from 30 to 17 ◦C. This result is in good agreement withthe result found in Langmuir equation. The Freundlich constantnf is also considered the measurement of linearity. If the valueof nf is smaller than 1, reflecting favorable adsorption, thenthe sorption capacity increases and new adsorption sites occur.When the value of nf is larger than 1, the adsorption bond be-comes weak; unfavorable adsorption takes place, as a result ofthe decrease in adsorption capacity [12–14].

In order to determine the adsorption type, the Dubinin–Radushkevich equation was employed to adsorption data. TheDR equation can be expressed as [15]

(5)lnCs = lnXm − kε2,

where ε (Polanyi Potentiali) can be written as RT ln(1+1/Ce),Cs is the amount of drug adsorbed per unit weight of adsor-bent (mol/g), Xm is the adsorption capacity (mol/g), Ce is theequilibrium concentration of drug in solution (mol/L), k is theconstant related to adsorption energy (mol2/kJ2), R is the gasconstant (kJ/(mol K)), T is temperature (K). A plot of lnCs vsε2 should be linear. The values of Xm and k were calculatedfrom intercept and slope of this plot at different temperatures.The results were summarized in Table 4. The values of k in-creased from 0.12 × 10−2 to 0.40 × 10−2 with the decrease oftemperature from 30 to 17.5 ◦C. The mean free energy changeof adsorption (E) can be considered as the free energy changeduring the transfer of one mol of molecule from infinity in so-lution to the surface of the solid. The value can be calculatedfrom the following equation:

(6)E = −(2k)−0.5.

It is noted that magnitude of E is useful for estimating thetype of adsorption and if this value is between 8 and 16 kJ/mol,adsorption type can be explained by ion exchange [15]. Adsorp-tion energy for the adsorption of As(III) on activated aluminawas found to be 7.45 kJ/mol. It was also assumed that the ad-sorption type is physical adsorption [16]. The values of E foundin this study are 11.24, 18.97, and 20.85 kJ/mol for 17.5, 25,and 30 ◦C, respectively. The adsorption type at low tempera-tures such as 17.5 ◦C can be considered as ion exchange. How-ever, the type of adsorption at higher temperatures such as 25

158 G. Gereli et al. / Journal of Colloid and Interface Science 299 (2006) 155–162

Fig. 3. pH effect for the adsorption of promethazine chloride on K10 montmo-rillonite.

and 30 ◦C can be assumed as chemical adsorption. The adsorp-tion capacity values obtained from DR isotherm are decreasingwith the increase of temperature from 17.5 to 30 ◦C. As wasmentioned earlier, the values of adsorption capacity calculatedfrom the Langmuir and Freundlich equations were decreasingwith the increase in temperature in the range of 17.5–30 ◦C.This result is consistent with the findings obtained from the Fre-undlich equation.

3.2. Effect of pH

In order to determine the optimum pH for adsorption ofPHCl on K10, the amount of drug adsorbed as a function ofpH was investigated. As seen from Fig. 3, maximum adsorptionachieved in the pH range of 7–8 for initial drug concentrationof 1 mM. The adsorption decreased at lower and higher pH val-ues than 7.5. It is apparent that adsorption is dependent on thepH of the solution. However, it can be said that it is not so muchsensitive to the pH change due to low difference between maxand min values of adsorbed drug. It is probable that pH affectsthe surface charge of K10 and degree of ionization of drug andspeciation of drug. As it was anticipated, at lower pH, moreprotons are available, thereby decreasing electrostatic attractionbetween cationic drug and positively charged sites of adsorbent.This case may be the reason of decrease of adsorption belowpH 7.5. When the pH value of the solution was increased, thesurface of the adsorbent becomes negatively charged. In addi-tion to this, the concentration of OH− ion also increased withthe increase of pH. Moreover, the species of neutral promet-hazine will also increase due to increase of degree of ionization.Consequently, at higher pH values (especially above pH 7.5),no significant ionic attraction will be observed between drugmolecules and negatively charged surface. According to the ex-planations [14,17,18], there are no exchangeable anions on theouter surface of the adsorbent at higher pH values. Due to theseresults, the amount of adsorbed PHCl decreased at higher pHvalues.

Fig. 4. Effect of contact time for the adsorption of promethazine hydrochlorideonto K10.

3.3. The effect of contact time

In order to determine the optimum contact time for adsorp-tion of PHCl onto montmorillonite K10, the equilibrium con-centrations were measured at definite times. The curve wasplotted of equilibrium concentration of PHCl (Cs, mmol/L)measured in the definite time intervals versus time at room tem-perature (Fig. 4). As can be seen from Fig. 4, when the contacttime increased, the amount of adsorption also increased. Theoptimum contact time for the adsorption of PHCl onto mont-morillonite K10 was found to be 21 h.

3.4. Adsorption kinetics

In order to understand an adsorption and desorption process-es properly, one should deal with two basic ingredients: equilib-ria and kinetics [19]. In order to investigate the consistency ofadsorption data with the kinetic models, experimental data wereapplied to some kinetic equations such as pseudo-first-ordermodel, pseudo-second-order model and intraparticle diffusionmodel. The kinetic models can be presented as follows:

(7)1/qt = (k1/q1)(1/t) + 1/q1,

(8)t/qt = 1/(k2q

22

) + t/q2,

(9)qt = kpt1/2 + C,

where qt is the amount of dye adsorbed (mg/g) at time t , q1is the maximum adsorption capacity (mg/g) for pseudo-first-order adsorption, k1 is the rate constant of pseudo-first-orderadsorption (min−1), q2 is the maximum adsorption capacity(mg/g) for the second-order adsorption, k2 is the second-orderrate constant for the adsorption process (g/(mg min)), C isthe intercept and kp is the intraparticle diffusion rate constant(mg/(g min1/2)). From plots of t/qt versus t for the second or-der reactions at various temperatures, the k2 and q2 values werecalculated by using the values of intercept and slope. Accord-ing to the equation (7), a plot of 1/qt versus 1/t should be astraight line with a slope of k1/q1 and intercept 1/q1, when theadsorption process follows the first-order equation. The slopeof the plot qt vs t1/2 shown in Fig. 5 yields kp values and inter-cept gives C values. It was obtained that correlation coefficients

G. Gereli et al. / Journal of Colloid and Interface Science 299 (2006) 155–162 159

Fig. 5. Intraparticle diffusion plots for the adsorption of promethazine hy-drochloride on K10.

Table 5Kinetic parameters for the adsorption of promethazine on K10 at various tem-peraturesa

Temper-ature (◦C)

k2 × 102

(g/(mg min))

q2(mg/g)

R22 kp × 10 (mg/

(g min1/2))

C

(mg/g)R2

p

17.5 1.05 63.69 1 0.554 62.35 0.99625 1.21 63.37 1 0.708 61.87 0.99730 1.23 63.53 1 0.542 62.24 0.966

a k2, pseudo-second-order rate constant; q2, the maximum adsorption ca-pacity for the pseudo-second-order adsorption; R2

2 , correlation coefficientsfor pseudo-second-order kinetic; kp, the intraparticle diffusion rate constant;

C, a constant; R2p , correlation coefficients for intraparticle diffusion.

for pseudo-first-order model are significantly small. Thereforethese results were not presented here. The results belongingto the intraparticle diffusion and pseudo-second-order modelsfor PHCl adsorption onto K10 at various temperatures weresummarized in Table 5. Based on the R2 values for promet-hazine sorption, pseudo-second-order kinetic model appears toproduce the best fit. The greatest adsorption capacity valueswere estimated at low temperature (17.5 ◦C). This result sup-ports the idea that low temperature favors PHCl adsorption onK10. This effect offers that adsorption type can be consideredas physisorption process for the sorption of promethazine onK10.

The pseudo-second-order rate constants are decreased from1.23 × 10−2 to 1.05 × 10−2 for the PHCl adsorption onto K10,when the temperature is decreased from 30 to 17.5 ◦C. As wasmentioned by [14,20], increasing temperature usually increasesthe rate of approach to equilibrium in conventional physisorp-tion systems. Since the pseudo-second-order model could notreveal itself the diffusion mechanism, the fit of intraparticlediffusion model was investigated. The results imply that goodapplicability was observed with high correlation coefficients.

As was shown in Table 5, the value of intraparticle diffusionrate constant is increased in the range of 17.5–25 ◦C. When thePHCl diffused in the pore of adsorbent, the diffusion resistancedecreased, that caused the diffusion rate to increase. However,

Fig. 6. Arrhenius plots of PHCl adsorption onto K10.

with the increase of temperature to 30 ◦C, the diffusion rate be-came lower. The values of the intercept give an idea about thethickness of boundary layer. This means that the larger the inter-cept, the greater the boundary layer effect [21]. In comparisonwith the results found from [22], it can be said that C valuescalculated in our study are greater than the values estimated forphosphate adsorption onto dolomite. As a result, it can be saidthat adsorption process can be also controlled by intraparticlediffusion.

Comparing the values of R2 of pseudo-first-order and sec-ond-order equations, the second one is better and can be usedto predict the adsorption kinetics of promethazine chloride onK10.

3.5. Thermodynamic parameters

Activation energy of adsorption process can be estimated bythe Arrhenius equation,

(10)ln k = lnA − Ea

RT,

where Ea is the Arrhenius activation energy for the sorptionprocess indicating the minimum energy that reactants must havefor the reaction to proceed, A is the Arrhenius factor, R is thegas constant (8.314 J/(mol K)), k is the rate constant for thepseudo-second-order kinetics and T is the solution tempera-ture [23]. The activation can be found by plotting lnk versus1/T . As can be observed in Fig. 6, Ea and A values can be es-timated from slope and intercept value of this plot, respectively.In our study, the activation energy value (Ea) was computed tobe −9.40 kJ/mol, which can be considered as a low energy bar-rier.

The other thermodynamic parameters, standard free energychange (�G0), enthalpy change (�H 0), and entropy change(�S0) for the sorption of drug and K10 were estimated fromthe following equations and represented in Table 6.

(11)�G0 = −RT lnKc,

(12)Kc = Cs/Ce,

160 G. Gereli et al. / Journal of Colloid and Interface Science 299 (2006) 155–162

Table 6Thermodynamic parameters for PHCl adsorption on K10a

T (K) Kc �G0 (kJ/mol) �S0 (kJ/(mol K)) �H 0 (kJ/mol)

290.5 10.026 −28.15 0.06 −11.29298 13.021 −28.59303 11.708 −28.88

a Kc, equilibrium constant; �G0, standard free energy; �S0, standard en-tropy change; �H 0, standard enthalpy change.

(13)lnKc = −�H 0ads/RT + �S0/R,

where Cs is the equilibrium concentration of drug on adsor-bent (mg/L), Ce is the equilibrium concentration of drug in thesolution (mg/L). Kc is the equilibrium constant, R is the gasconstant (8.314 J/(mol K)), and T is temperature (K).

As was seen in Fig. 7, lnKc is plotted against 1/T and theslope is equal to −�H 0

ads/R. The intercept value is equal to�S0/R. Generally, the absolute magnitude of the change infree energy for physisorption is between 0 and −20 kJ/mol,chemisorption has a range of −80 and −400 kJ/mol. The re-sults found in this study are −28.15, −28.59, and −28.88 kJ/

Fig. 7. Van’t Hoff plot of PHCl adsorption onto K10.

mol for 17.5, 25, and 30 ◦C, respectively. These values are inthe middle between physisorption and chemisorption. It canbe interpreted that physical adsorption was enhanced by achemical effect. In addition to this, since �G0 values are be-tween 20 and 80 kJ/mol, adsorption type can be explained asion exchange, which is consistent with the results found fromDR isotherm. Presumably ion-exchange has a range from −20to −80 kJ/mol. On the other hand, the standard free energychange for multilayer adsorption is greater than 20 kJ/mol andless then zero. Hence, �G0 values found in this study can beconsidered as multilayer adsorption, namely physical adsorp-tion. The negative values of �G0 imply that the adsorptionof PHCl on K10 is spontaneous by nature. Since the value ofthe standard enthalpy change (�H 0) is −11.29 kJ/mol, in-teraction of the drug molecule with K10 is exothermic by na-ture. Moreover, the standard enthalpy change is lower than thevalue of 40 kJ/mol. It indicates that the adsorption is physi-cal by nature and involves weak forces of attraction [24]. Ifadsorption decreases with increasing temperature, it may beindicative of physical adsorption, and the reverse is generallytrue for chemisorption. However, there is a number of con-tradictory cases in the literature [22]. It can be inferred thatadsorption process was administrated by combined control ofion exchange and physisorption. However, it can be said that thephysical adsorption is the predominant type of adsorption, con-sidering the value for the sorption enthalpy (−11.29 kJ/mol).Apparently, the negative values of standard enthalpy changereveal decreased adsorption with increased solution tempera-ture.

3.6. FTIR results

The FTIR spectra of K10, PHCl, K10-PHCl can be seenin Fig. 8. The absorption at 3620 cm−1, found in the spec-trum of K10 montmorillonite, is typical for smectites with highamount of Al in the octahedral [25,26]. A band at 3428 cm−1

in the spectra of K10 shows H–O–H hydrogen bonded wa-

Fig. 8. FTIR spectrum of PHCl, K10, PHCl treated K10 samples.

G. Gereli et al. / Journal of Colloid and Interface Science 299 (2006) 155–162 161

Fig. 9. SEM micrographs of (a) K10 montmorillonite (×1900), (b) promethazine hydrochloride loaded K10 (×1500), (c) promethazine hydrochloride loaded K10(×1000), (d) promethazine hydrochloride loaded K10 (×3500), (e) EDS analysis of K10, (f) EDS analysis promethazine hydrochloride loaded K10.

ter. After drug adsorption, these peaks shifted to 3624 and3437 cm−1. It seems that hydrogen bonding may occur betweenwater molecules and PHCl. In the spectrum of K10 Montmoril-lonite, intensive band at 1048 cm−1 can be assigned to Si–Ostretching vibrations. The Si–O–Al and Si–O–Si bending vi-brations appear at 525 and 469 cm−1, respectively. The smallband at 1635 cm−1 corresponds to the HOH deformation vi-bration. After PHCl treatment, OH deformation band shiftedto 1633 cm−1 in the spectrum of promethazine hydrochloridetreated K10. There are not so much shifts in that peak afterdrug treatment. A band at 2380 cm−1 in the spectra of promet-hazine hydrochloride indicates the N–H stretching vibrations inthe quaternary ammonium group of the molecule. After the ad-sorption onto K10, this band shifted drastically to 2534 cm−1

and cannot be seen easily. In contrast, the bands characteristicof skeletal vibrations of the aromatic ring, below 1600 cm−1,shifted only slightly. In comparison between two spectra (K10and drug treated K10), a new small band appears at 1461 cm−1.The band located at 1456 cm−1 in the spectrum of PHCl couldbe attributed to substituted aryl compound. In the spectra of

K10, the stretching band at 1048 cm−1 is replaced by a broadband at about 1045 cm−1 in the spectrum of K10-PHCl. It in-dicates that the symmetry of the tetrahedral sheet is probablydistorted after treatment with the drug.

3.7. SEM results

The morphologies of the K10 and drug treated K10 werestudied by SEM. The micrographs obtained for these materi-als are shown in Fig. 9. A general view of K10 can be seenin Fig. 9a. Figs. 9b, 9c and 9d present the PHCl loaded K10Montmorillonite with different magnifications. SEM images il-lustrate that promethazine hydrochloride are well dispersed onthe montmorillonite K10. As can be seen from Figs. 9a, theparticles of K10 Montmorillonite are of irregular shape. In ob-serving the clay specimens using SEM, the drug treated K10montmorillonite appear, some aggregated and some massive.In the drug modified K10, the particles are bound into largeragglomerations. It can be seen from elemental analysis deter-mined by using EDS analysis (Figs. 9e and 9f) that with respect

162 G. Gereli et al. / Journal of Colloid and Interface Science 299 (2006) 155–162

to the percentage of C weight, a clear difference was observedbetween drug-treated K10 and K10. The percentage of C el-ement of drug modified clay is higher as compared with K10montmorillonite.

4. Conclusions

As a result of data obtained from DR isotherm, it can beconcluded that the type of the adsorption tends to chemical ad-sorption with the increase of temperature. The results of kineticexperiment indicate that adsorption process is pseudo-secondorder and adsorption process can be also controlled by intra-particle diffusion. It seems to be that adsorption process wasadministrated by combined control of ion exchange, intraparti-cle diffusion.

The computed apparent activation energy for the adsorptionof PHCl onto K10 is −9.40 kJ/mol, implying that the adsorp-tion has a low potential barrier. The negative values of �H 0

and �G0 indicate exothermic nature and spontaneity, respec-tively. The positive value of �S0 suggests increase in disorderof PHCl adsorption. According to the SEM results, some ag-gregated and some massive was observed after PHCl treatmentof K10.

It is probable that physisorption is observed at low tempera-tures. It can be inferred from pH experiments that adsorption isdependent on the pH of the solution and maximum adsorptioncan be reached at about 7.5 of pH. Presumably, this may notallow the release of promethazine hydrochloride in intestinalmedium due to proximity of pH values of maximum adsorptionand intestinal juice.

The results of present study show that K10 montmorilloniteis an effective adsorbent for promethazine hydrochloride due tohigh adsorption capacity.

Acknowledgments

The authors would like to thank Prof. Dr. Dieter Hönicke,Chemnitz Technical University/Germany for his help. Thescholarship support for Dr. K. Yurdakoç by DAAD/Germanyis gratefully appreciated. The authors are also grateful to Re-

search Foundation of Dokuz Eylül University (Project No:03.KB.FEN.021) for financial support.

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