Adsorption of sodium dodecylsulfate on a hydrotalcite-like compound. Effect of temperature, pH and ionic strength

Colloids and Surfaces A: Physicochemical and Engineering Aspects 154 (1999) 399 – 410

Adsorption of sodium dodecylsulfate on a hydrotalcite-like compound. Effect of temperature, pH and ionic strength Paulo C. Pavan, Eduardo L. Crepaldi, Gilmar de A. Gomes, Joa˜o B. Valim * Departamento de Quı´mica, FFCLRP, Uni6ersidade de Sa˜o Paulo, A6. Dos Bandeirantes 3900, 14040 -901 Ribeira˜o Preto, Brazil Received 6 July 1998; received in revised form 14 October 1998; accepted 16 October 1998

Abstract The study of the adsorption of surfactants on mineral oxides has received considerable attention. Extensive work has been done in order to better understand such processes. In the present study we investigated the influence of the temperature, pH and ionic strength on the adsorption of sodium dodecylsulfate (SDS) on a layered double hydroxide (LDH). The results were compared to those obtained for the adsorption of surfactants on mineral oxides which has already been explained by reported models. The influence of the analyzed variables was similar to those predicted for these models. Measurements of the electrokinetic potential of the suspensions of SDS-adsorbed LDH particles were made in order to monitor the variation of this potential as a function of the adsorption. The results showed a characteristic behavior of SDS-adsorbed LDH that can be related to an overpopulation of unimers in the electric double layer at concentrations near the critical micelle concentration (CMC). Scanning electron microscopy was used to obtain high magnification images of the SDS-adsorbed LDH particles, which showed a characteristic image for particles adsorbed at equilibrium concentrations ranging from the CMC to the highest concentration used. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Surfactant; Sodium dodecylsulfate; Layered double hydroxides; Scanning electron microscopy

1. Introduction The adsorption of surfactants at the solid/liquid interface has been extensively studied due to its direct relationship with colloid stability [1]. The adsorption of surfactants on mineral oxides is also important in the study of detergency, mineral flotation, dispersion/flocculation, particle growth * Corresponding author. Tel.: +55-016-602-3766; fax: + 55-016-633-8151. E-mail address: [email protected] (J.B. Valim)

in suspension, oil enhanced recovery, lubrication and chromatography, among other processes [2– 5]. Even though this is an extensively studied process, few reports are available about the adsorption of surfactants on LDH. We recently reported some results related to the adsorption of SDS on LDH at three different temperatures [6]. Another research group published the results of the sorption of an anionic surfactant (sodium dodecylbenzenesulfonate) by LDH and its calcined product, focusing both on its potential application as a removal sorbent and on the

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possibility of its recycling [7]. It is amazing that the application of such material as an adsorbent at solid/liquid interfaces has not yet received the deserved attention. Structurally, LDHs consist of brucite-like layers, which are positively charged due to the isomorphous substitution of bivalent cations by trivalent ones. The layers are intercalated with anions, in order to balance the residual charge. Together with the anions, water molecules are also present in the interlayer domain. The natural occurrence of these materials is rare, but they can be synthesized in the laboratory at relatively low cost. Hydrotalcite is one of the anionic clays that occur in nature, consisting of sheets of octahedral magnesium/aluminum double hydroxides which share their edges. Its positive residual charge is balanced by intercalated carbonate anions. A wide variety of anionic clays can be obtained by the combination of: M2 + , M3 + and X m − in x+ m− 3+ X x/m the general formula: [M21 + − xMx (OH)2] ·nH2O, where: M2 + represents a bivalent cation; M3 + represents a trivalent cation; X m − represents an intercalated anion with an m − charge. Many research groups have studied LDHs that can be fitted into the above formula by varying the cationic and anionic species [8 – 20]. LDHs can be applied in three broad areas: in catalysis as catalysts [21 – 24], catalyst precursors or catalyst supports [25,26], in adsorption processes as adsorbents [6], and in exchange processes as anion exchangers [13,14,16]. The specific surface area and the porosity of LDHs are very important properties concerning to their application as adsorbents and catalysts. These properties mainly depend on the method of preparation and on the parameters involved. The specific surface area of LDHs typically ranges from 50 to 80 m2 g − 1. The porosity of hydrotalcite-like compounds has also been studied [12], showing the existence of pores measuring 75–300 A, . Although the charge of LDHs is balanced by interlamellar anions, their exposed surface presents a positive residual charge which strongly contributes to processes like anion adsorption. These materials can also be used as sorbents in

‘sorption’ process. In this case, LDHs can be calcined with consequent elimination of the interlamellar anion. The calcined product can then be regenerated by contact with an aqueous solution of an anion of interest, with the concomitant adsorption of this anion on its surface. These results have been reported in a study in which adsorption was investigated concomitant with the intercalation of the surfactant [7]. The adsorption of anionic surfactants or their binary mixtures has been studied for various adsorbate (alkylsulfates and/or alkylbenzenesulfonates) and adsorbent systems (active carbon [27], polymeric resins [28], polymers [1], natural and synthetic fibers [29,30] and especially alumina and other mineral oxides [2,31–33]). The process of surfactant adsorption is complex to understand due to: (i) the multicomponent nature of these adsorptive systems, which results in adsorption isotherms interpreted as consisting of a series of step changes in surface covering. This in turn corresponds to a surface with different energy patches [32]; (ii) the surface heterogeneity of mineral oxides which is related to the existence of different pore sizes and irregular topography; (iii) the importance of the water structure in surfactant aggregates (micelles); and (iv) the complex interactions that may occur between surfactant molecules, surfactant aggregates, counter-ions, potential determining ions and surface groups of the mineral oxides. In this way, one can see that the simultaneous existence of different surfactant aggregates is possible. In view of the above considerations, the treatment of the adsorption data for a surfactant is not easy. Therefore, several models have been proposed for this treatment [34–42]. The model that is most frequently used to explain the adsorption data is the one in which the isotherm is divided into three or four regions and was first reported by Somasundaran and Fuerstenau [34]. These regions are related to the different phenomena present in multicomponent adsorption, as can be seen the Fig. 1 [31]. In the past, many investigators used to consider that: (a) region I was characterized by non-aggregative adsorption, in other words, only non-associated surfactant ions are present at this stage;

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(b) the transition between regions I and II was characterized by the beginning of aggregate formation; and (c) regions II and III were characterized by the existence of a surfactant monolayer and bilayer with a consequent saturation of the surface (occurring near the CMC of the surfactant). On this basis, the surface was treated as being energetically uniform. Today, new techniques have provided different information. There is now agreement about the existence of aggregates even in region I, whose density depends on the energy of the patches and their availability on the surface. Thus, region I is characterized by low coverage of the surface by the surfactant species. In this region, the adsorption obeys Henry’s law. Nowadays it is believed that aggregates can be formed in this region, even on a small scale. Region I/region II transition occurs when the surfactant aggregates are formed more quickly. Therefore, region II is characterized by a rapid increase in adsorption with concentration. In region III, the curve slope is lower than in region II. This decrease is attributed to the electrostatic repulsion between the unimers (i.e. surfactant monomers) and to the lower availability of high energy patches for adsorption. The transition between regions III and IV occurs near or at

Fig. 1. Illustration of the adsorption regions for surfactant adsorption on mineral oxide surfaces.

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Fig. 2. Configuration of the adsorbed surfactant unimers.

the CMC of the surfactant, when the adsorption patches become saturated. Therefore, region IV corresponds to the adsorption plateau and is characterized by a near zero slope [31–35]. The adsorbed surfactant configurations for each region are schematically illustrated in Fig. 2, taking into consideration each set of the above assumptions. There are various factors that strongly influence surfactant adsorption at the solid/liquid interface concerning either its efficiency, defined here as the log of the reciprocal of equilibrium bulk concentration of surfactant in the aqueous phase when the adsorption on the adsorbent has reached half of its saturation, or its effectiveness, defined as the amount of surfactant adsorbed per surface unit area of substrate when the surface has become saturated with one or two layers of surfactant. These factors are: 1. The nature of the structural groups on the surface; whether the surface contains highly charged sites or essentially nonpolar groups, and what atoms these sites or groups consist of. 2. The molecular structure of the surfactant, the adsorbate; whether it is ionic, and whether the hydrophobic chain is long or short, straight or branched, aliphatic or aromatic. The efficiency of adsorption increases with the hydrophobic group length because the free energy decreases.

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This results from the removal of the hydrophobic chain from the bulk phase to the substrate. Thus, the tendency to aggregate or adsorb via dispersive forces will be enhanced by the increase in chain length. 3. The environment of the aqueous phase; its pH value, the electrolyte content, the presence of any other additives such as short chain polar solutes, e.g. alcohol and urea, and its temperature. Changes in the pH of the aqueous phase usually cause remarkable changes in the adsorption of ionic surfactants onto charged solid substrates. As the pH of the aqueous phase is reduced, the solid surface becomes more positive, or less negative, due to the adsorption of protons from the solution onto charged sites. This consequently leads to an increase in the adsorption of anionic surfactants and to a decrease in the adsorption of cationic ones. The reverse is true when the pH of the aqueous phase is raised. An increase in the ionic strength by the addition of a neutral salt also causes an increase in the ionic surfactant adsorption efficiency on charged adsorbents. This effect is probably due to the reduction of the repulsion between polar portions of the surfactants adsorbed on the surface at a higher ionic strength. The increase in the temperature at which the adsorption is conducted often reduces the adsorption of ionic surfactants in the case of a physiosorption. Data concerning this decrease have been used to calculate the enthalpy and entropy of adsorption [43]. Taken together, these factors determine the mechanism and efficiency of the adsorption process [44]. Many techniques have been used to study the adsorption process [45 – 54]. For instance, measurements of the electrokinetic potential of the equilibrated suspensions are usually made in order to study the behavior of the electric double layer following adsorption, since they provide information about the mechanism involved in this process. Thus, many investigators have used this technique for various adsorbent/adsorbate systems [1,6,55 – 59]. Another technique that can be used to study the adsorption process is scanning electron microscopy (SEM). Although it is a pow-

erful technique generally used to characterize the topography of surfaces, it has scarcely been used for the study of adsorption [6]. The aim of the present investigation was to study the influence of pH, ionic strength and temperature on the adsorption of SDS from aqueous solution onto a magnesium-aluminumcarbonate LDH.

2. Materials and experimental procedures

2.1. Anionic clay As adsorbent, we used a magnesium/aluminum layered double hydroxide intercalated with carbonate anions, which was prepared in our laboratory according to the procedure of Reichle [15]. All reactants used were of high purity degree and were purchased from Merck. The obtained material was characterized by X-ray powder diffraction and by elemental and thermal analyses, which permitted us to identify a lamellar material showing a basal distance of 7.6 A, with the approximate minimum molecular formula: [Mg1.92Al1(OH)5.84]. (CO3)0.5 2.31(H2O) After being submitted to high vacuum at 328 K, this material presented a specific surface area of 87.1 m2 g − 1 as determined by the N2-BET adsorption isotherm. This area corresponds to the surface, porous and interlamellar area of the LDH particles. Gas adsorption analysis also showed an average pore diameter of 240 A, .

2.2. Surfactant The anionic surfactant used was sodium dodecylsulfate (SDS) purchased from Merck in its purest available form (\99% purity), which presented a CMC value of 8.2 mM determined by conductance measurements. SDS concentration in aqueous solution was determined by double phase titration [60] using cetyltrimetylammonium bromide purchased from Merck (\ 99.0% purity) as the titrant, chloroform (Merck min. 99.9%) as the other phase, and methylene blue (Merck) as the indicator.

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2.3. Adsorption

3. Results and discussion

The adsorption study was carried out using the batch method. LDH was previously dried under vacuum at room temperature, a constant mass was weighed (200.0 mg) and then suspended in half the solvent volume (deionized water) required to produce the total fixed volume (50 cm3) of the solution. This suspension was submitted to sonication in order to homogenize the particles. Next, the other half of the fixed volume was completed with an adsorbate solution at double the concentration as that of the one defined solution (in the 8 × 10 − 4 to 2×10 − 2 mol dm − 3 range). In order to reach equilibrium, these systems were left in contact in a Dubnoff type bath for 72 h under constant temperature (90.5 K) and shaking. After this time, half of each suspension was removed and centrifuged in order to determine the surfactant concentration in the supernatant. The other part was used for the measurement of the z potentials. The pH and the ionic strength were adjusted by the addition of the appropriate amount of NaOH and NaCl, respectively, to the water used as solvent for the preparation of all solutions and suspensions.

The adsorption was carried out at two temperatures (298 and 313 K), under two conditions of ionic strength (0 and 0.1 M of NaCl) and at two pH values (7 and 9). It is possible to observe from the isotherms illustrated in Fig. 3 that the adsorption presented important features related to the variables involved. The LDH diffraction pattern did not change after the occurrence of adsorption, but kept the original basal spacing. This led us to conclude that SDS anions did not substitute the interlamellar carbonate anions, as expected considering the stability of the intercalated anion. Thus, only the external surface with its sites (charged or not) is available for SDS adsorption, as is also the case for surfactant adsorption on mineral oxides. Therefore, the four-region model will be used to discuss the results of SDS adsorption on LDH obtained here.

2.4. Electrokinetic measurements z Potential measurements were made in each suspension after adsorption equilibrium was reached. A Zetasizer 4 coupled to a Malvern SX/16 microcomputer was used to make the measurements.

2.5. Scanning electron microscopy The SEM images were obtained with a Zeiss DSM 960 Digital Scanning Microscope. The images were taken from each of the residual solids obtained from the centrifugation of equilibrated suspensions. These solids were drained, dried under vacuum at room temperature, supported on the proper sample holder and sputtered with gold with a Sputter Coater Balzers SCD 050.

3.1. Effect of temperature It can be seen that an increase in temperature leads to considerable differences in the maximum quantity of adsorbed SDS. The lower the temperature, the higher the maximum adsorbed. This behavior was expected since an increase in temperature can be understood as an increase in the kinetic energy of the species. Consequently, there is an increase in the entropy of the system which results in a decrease of aggregate organization on the surface of the adsorbent. Similar results were obtained by Fridriksberg et al. when studying the adsorption of SDS on quartz powder [58], Somasundaran and Fuerstenau reported a shift to lower values in all the extension of the adsorption isotherm of dodecyl sulfonate on alumina [61]. As discussed in the introduction, this effect of temperature on adsorption is predicted by the models reported in the literature.

3.2. Effect of ionic strength Fig. 3 clearly shows that the adsorption process is strongly affected by variations in ionic strength. As discussed above, the ionic strength and the pH

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Fig. 3. Isotherms for the SDS/LDH system under various conditions.

value are the factors that should most influence the process and in the mechanism of adsorption due to their effects on some adsorption steps (regions). This effect is more pronounced in terms of ionic strength. In region I of the isotherm, the rate of increase in adsorption was the same for both ionic strength conditions, even though the amount of adsorbed surfactant (expressed as absolute values) varies with other parameters (pH value and temperature). In region II, the rate of increase in adsorption was considerably enhanced in the presence of higher ionic strength, resulting in a decrease in the range of concentrations at which region II is identified. Thus, region III started at a lower concentration for adsorption at the higher ionic strength. This displacement of region III is in agreement with the CMC of SDS at both ionic strengths (which is : 1.3 mol dm − 3 for an ionic strength resulting from the addition of 0.1‘M NaCl, and 8.2 mol dm − 3 for the ionic strength attained without the addition of NaCl [62]).

This behavior has already been observed in various studies of surfactant adsorption [35,63] and agrees with the theories about the mechanisms involved in adsorption. According to the proposed mechanisms and experimental observations, an increase in the ionic strength should lead to a decrease in the repulsive forces between the surfactants polar groups. In addition, an enhancement of the hydrophobic effect due to the more ionic environment felt by the surfactant hydrophobic chain should induce a shift of the equilibrium in the direction of surfactant aggregate formation. Consequently, these two effects should lead to an increase in the adsorption rate in region II of the isotherm since in this region the adsorption of unimers is improved by the formation of surface aggregates. These effects are also responsible for the increase in the total amount of adsorbed SDS on the isotherm plateau, as demonstrated by a shift to higher adsorption values in region IV.

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Fig. 4. Variation of the electrokinetic potential of the suspended particles in SDS equilibrium solutions related to the studied isotherms.

3.3. Effect of pH Before discussing the influence of pH on adsorption, it is important to make it clear that the CMC value for SDS does not change at pH values ranging from 6 to 10 [64]. An increase in pH also resulted in a decrease in the amount of adsorbed surfactant. Any variation observed in the isotherm must be due to the effect of pH on the surface of the adsorbent, since the hydroxyl anion is a potential determining ion for the LDH surface. As reported in the literature [61,63], the adsorption (especially region I) is directly related to the adsorbent surface charge, however the occurrence of drastic effects on the electrokinetic potential of the LDH surface only begins near pH 10 and its point of zero charge lies at pH  12.2 [65]. The isotherm of the adsorption conducted at pH 9 showed a slight shift to smaller amount of adsorbed SDS when compared to the adsorption conducted at pH 7. In this way, we can attribute this effect to a small change in the adsorbent

surface due to OH − concentration, which is related to the surface charge. The decrease in surfactant adsorption on mineral oxides and clays (positively charged) with an increase in pH has been previously reported [34,35,37,63,65].

3.4. Beha6ior of the electrokinetic (z) potential during adsorption Measurements of this potential were carried out in each experiment for suspensions at all the SDS equilibrium concentrations, including the blank. The results obtained are plotted against the equilibrium concentration and are shown in Fig. 4. As mentioned above, it should be taken into account that the LDH electrokinetic potential only changes its signal at pH  12.2 and that it decreases from around +30 to + 20 mV on going from pH 7 to pH 9. This change is actually slight if compared to the strong influence of surfactant adsorption on this potential. Fig. 4 shows the discrete influence of pH on the electrokinetic potential for this system. As can be

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seen, the electrokinetic potential curves obtained at different pH values, but at the same ionic strength, were closely similar in the range of low surfactant concentrations. For the adsorption isotherms obtained at a lower ionic strength (where no NaCl was added), we observed a similar variation in the z potential with concentration ranging from the lowest equilibrium concentration to the end of region II in the isotherm. For higher equilibrium concentrations, the curves obtained for the same condition of ionic strength presented irregularities near the CMC of the surfactant. With the increase in the ionic strength (i.e. 0.1 M NaCl), the z potential curves showed a different behavior. There was a sharp decay of potential values up to the region near the CMC, followed by a slight increase with the equilibrium SDS concentration, producing a valley, since the potential became nearly constant, at a value slightly higher than the minimum observed, for higher equilibrium concentrations. Fig. 4 shows the existence of a valley near CMC for each condition of ionic strength. The existence of these minima in the curves suggests that the electric double layer is more negatively charged when compared to the regions that come after and before these minima. This behavior may be related to an increase in the unimer concentration in the electric double layer near the already existing surfactant aggregates on the particle surface. In other words, at concentrations immediately near the CMC, the unimers in solution are located close to the aggregates already formed on the particles probably due to the hydrophobic effect. As a consequence, the module of the surface potential increases, producing this valley. When the CMC is achieved, these unimers near the electric double layer leave it in order to form the micelles in solution. As a result, the module of the potential decreases until it reaches a constant value. It is important to make clear that this behavior is not usually found or even explained, suggesting that it may be a particular effect produced only by the adsorption of surfactant anions on LDH.

3.5. Scanning electron microscopy applied to solid particles Scanning electron microscopy (SEM) is an important tool to study the surface topography of solids due to the good resolution that can be achieved when dealing with high magnification images. On this basis we took SEM images of the particles obtained from the adsorption experiment conducted at 298 K, pH 7, and without modification in the ionic strength (no NaCl added). Fig. 5 shows the SEM image of the prepared LDH without any surfactant adsorption (used as a blank). This figure illustrates the surface irregularity and the existence of pores on the adsorbent. Fig. 6 represents the characteristic image observed for the adsorbed solids in the equilibrium concentration ranging from the lowest to the one just before the CMC (i.e. from 2.6×10 − 4 to 6.6× 10 − 3 mol dm − 3 of SDS at equilibrium). The images of the adsorbed solids in this concentration range did not show any significant differences, but in fact presented the same features. A different image pattern was obtained for the adsorbed solids obtained from an SDS equilibrium concentration of 8.61 mmol dm − 3 (point nearest the CMC) to the last equilibrium concentration analyzed in the isotherm, without any further considerable modifications. This characteristic image could be seen in some particles, whereas it was not observed for other particles of the same sample. As mentioned above, the formation of this different image structures was observed only for adsorbed solids from equilibrium SDS concentrations immediately higher than the CMC. Actually, this coincides with the first point of region IV in the isotherm, the adsorption plateau. In Fig. 7, one can see these ‘band-like’ image structures with an approximate width of 1 mm. Moreover, Fig. 8 clearly shows that they are anchored to the surface of the LDH. First of all, it is important to emphasize that the solids were obtained by centrifuging each suspension. After removing the supernatant, the tubes containing the remaining solid were drained and vacuum dried. Further experiments in which the sample holders were quickly washed by im-

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Fig. 5. SEM micrograph of the prepared Mg-Al-CO3-LDH (20 000 ×).

mersion in fresh water were also performed. In this case the ‘band-like’ structures disappeared. Two hypotheses may be formulated on the basis of these behaviors. The first is that these ‘band-like’ images are due to crystallized SDS molecules. The amount of solid sample that could be separated by centrifugation was about 100 mg. The amount of surfactant that could be crystallized should come from the solution that just wetted the material. Considering, as an overestimating, that all of this 100 mg amount is composed of equilibrium solution, the amount of surfactant would be in a range of 0.25 to 1.5 mg since the equilibrium concentration of the surfactant from the points above CMC was in the range of 0.25 to 1.5% in mass. This approximate amount is negligible when compared to the adsorbed amount of surfactant (:20 mg, the adsorption at the plateau was  20%). Therefore, it is difficult to observe a characteristic image due to this small amount of crystallized surfactant resulting from the simple crystallization of SDS molecules from the wetting equilibrium solution. However, we may propose that crystallization can be induced at sites where the adsorption has reached admicelle formation. Moreover, this crys-

tallization may be related to the surfactant aggregates in solution, and not only to a conventional unimer crystallization, since the fact that these bands appeared only at concentrations from the CMC upwards does not seem to be a coincidence. Another hypothesis is to consider that these ‘band-like’ image structures may be associated with a composite formed between the smallest LDH particles and surfactant molecules which would link these particles together. This aggregation, however, could only be formed when micelles are present in the media, justifying the observed data. Surfactant aggregates (surface micelles) in this concentration range are believed to be in their closest packing form; in this configuration SDS-adsorbed nanometric LDH particles could be linked together by lateral interactions between the hydrophobic chains of the surfactant. The formation of this composite is almost to idealize since a complete study of the adsorption of different surfactants is necessary to propose a model for surfactant adsorption on LDHs. Actually, a combination of both hypotheses should explain the existence of such structures. Therefore, the crystallization of the surfactant together with its surface aggregates is probably

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Fig. 6. SEM micrograph of the solid obtained at an SDS equilibrium concentration of 6.63 mM (20 000 × ).

occurring, carrying the smallest LDH particles. This crystallization should occur at the more energetic sites, where a maximum of adsorption is believed to occur. This explain why these structures are apparently anchored to the surface, and why a simple water washing of the material resulted in the extinction of such structures. Although the results obtained from image analysis are not conclusive, they clearly show the importance of this technique for the study of the adsorption of surfactants on mineral oxides and LDHs. Therefore, our aim is to apply this technique again in a future study using different types of sampling, such as direct dip-coating in the adsorption suspension.

4. Conclusions The present results show that the behavior of the adsorption of dodecylsulfate from aqueous solution on layered double hydroxides basically follows the adsorption model proposed in the literature for the adsorption of such surfactant on charged minerals such as alumina. The behavior of the electrokinetic potential, which had a char-

acteristic aspect attributed to an eventual accumulation of surfactant anions over the electric double layer at concentrations near the CMC, is also an important aspect since such behavior has not been previously reported in literature. The SEM images could be interpreted as an innovative technique for the study of adsorption of surfactants at the solid/liquid interface. The micrographs showed interesting aspects of the surface of the SDS-adsorbed LDH, which may eventually be related to the adsorption process and to adsorbent morphology. The effectiveness of this adsorbent also encourages a continuation of the study due to the possibility of its recycling, its large range of application, its characteristic residual charge and also its relatively high surface area and porous structure.

Acknowledgements The authors thank the Brazilian agencies Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP-95/5752-0, 95/3735-1, 96/6030-1

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Fig. 7. SEM micrographs of the solid obtained at an SDS equilibrium concentration of 8.61 mM: (10 000 × ).

and 96/12373-9), Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico-Programa de Apoio ao Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq/PADCT) and Fundac¸a˜o Coorde-

nac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior (CAPES) for financial support. The authors also thank C.V. Santilli for the BET isotherm measurements.

Fig. 8. SEM micrograph of the solid obtained at an SDS equilibrium concentration of 25.7 mM (5000× ).

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