Adsorption of sodium dodecyl sulfate on layered double hydroxides

Adsorption of sodium dodecyl sulfate on layered double hydroxides

MC 0 ROUSAND ME%1PO0USMATERIAl.S ELSEVIER Microporous and Mesoporous Materials 21 ( 1998) 659-665 Adsorption of sodium dodecyl sulfate on layered do...

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MC 0 ROUSAND ME%1PO0USMATERIAl.S ELSEVIER

Microporous and Mesoporous Materials 21 ( 1998) 659-665

Adsorption of sodium dodecyl sulfate on layered double hydroxides Paulo C. Pavan, Gilmar de A. Gomes, Jo20 B. Valim * Depto. de Quimica, FFCLRP, Universidade de SSo Paulo, Ribeirdo Preto, Brazil Received 2.5August 1997;received in revised form 25 December 1997;accepted 12 January 1998

Abstract

Layered double hydroxides (LDH). or the so-called anionic clays, are hydrotalcite-like compounds which can be represented by the general formula [M:?,M~+(OH),]“+~,~ .nH,O. They can be used as adsorbents, catalysts and catalyst supports and they present many other industrial applications such as polymer stabilisers and anion exchangers. On the other hand, the surfactants have been widely applied in industrial and household uses. so they constitute an important class of materials of commercial interest. The study of the adsorption of surfactants on these clays is thus very important due to the potential application of these adsorbents in many industrial processes. The adsorption of dodecyl sulfate on a magnesium aluminium layered double hydroxide was studied. The results obtained agree well with those found in the literature for the adsorption of this surfactant on other solids. We could observe that the adsorption follows the two-step model, confirmed by zeta potential measurements,We also observed that the amount of adsorbed surfactant increaseswith temperature. 0 1998 Elsevier Science B.V. All rights reserveil. Keywords: Adsorption; Layered double hydroxides; Anionic surfactant; Zeta potential: Anionic clay

1. Introduction Anionic clays, or layered double hydroxides (LDH), are part of (or precursors of) a more general family of compounds designed as pillared layered structures (PLS). These materials exhibit a remarkably broad spectrum of structural, chemical, electronic, ionic, optical and magnetic properties. They also provide supermesh host structures in which chemical reactions or physical processes can proceed under gas-phase conditions, but at liquid/solid state densities [ 11. Layered double hydroxides ( LDH). or the * Corresponding author. 1387-1811/98/%19.00 Q 1998Elsevier ScienceB.V. All rights reserved PII: S1387-1811(98)00054-7

so-called anionic clays, consist of brucite-like layers, which are positively charged due to the isomorphous substitution of bivalent cations by trivalent ones. They contain both intercalated anions, in order to balance the residual charge, and water molecules, resulting in a structure like the hydrotalcite one. The natural occurrence of these materials is rare, but their syntheses can be conducted in a laboratory at relatively low cost. A great variety of anionic clays can be obtained by the combination of M*+, M”” and X”- in the general formula [M:f,M1’(OH),]“+X~,, .nH,O where M* ’ represents the bivalent cation, M 3+ represents the trivalent cation, and X”- represents the intercalated anion with 16 charge. Hydrotalcite is one of the anionic clays that

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occurs in nature. It consists of sheetsof octahedral magnesium/aluminium double hydroxides sharing their edges;its positive residual charge is balanced by intercalated carbonate anions. On the other hand, the adsorption of surfactants in solid/liquid interfaces is an extensively studied system due to its direct relation with colloid stability [ 21. The adsorption of surfactants on mineral oxide surfaces is also an important process in the study of detergency, minerals flotation, dispersion/ flocculation, growth of particles in suspension, enhanced oil recovery, among other processes[36]. Although the adsorption of surfactants from aqueous solutions on charged solid surfaces has been extensively studied, we did not find any systematic work on the adsorption of these surfactants on anionic clays. We can list some factors that have a strong influence on the adsorption of surfactants on the solid/liquid interface: ( 1) the nature of the structural groups at the surface; whether the surface has highly charged sites or essentially non-polar groups, and which atoms these sites or groups are constituted of; (2) the molecular structure of the surfactant, the adsorbate: whether it is ionic or not, whether its hydrophobic chain is long or short, linear or branched, aliphatic or aromatic; (3) the environment of the aqueous phase, its pH, the existence of electrolytes or any other additive such as short chain polar solute (i.e. alcohol and urea) and its temperature. All these factors control both the mechanism in which adsorption occurs and its efficiency [7]. The simple adsorption of binary mixtures of anionic surfactants has been studied for several systems of adsorbates (generally al kylsulfates and/or alkylbenzenesulfonates) and adsorbents. Among the adsorbents we can mention: active carbon [ 81, polymeric resins [9], polymers [2], natural or synthetic fibers [ IO,1 1] and mineral oxides (essentially alumina) [3,12-- 141.The studies of adsorption of surfactants on the related systems are generally completed with zeta potential measurements of its constitutive particles [2,3,10,1 I, 13,151. This procedure is monitored with the aim of achieving a better comprehension of the adsorption process and as a base for a model proposition [7,12,16-191. So, the obtained

results concerned with the adsorption and with the zeta potential measurements could be applied in order to better explain these models (or to give hints for new model propositions), and to explain the adsorption behavior. The aim of this work is to study the behavior of the adsorption of SDS from aqueous solution on a LDH of Mg-Al-CO,. Its importance can be derived from the singular properties of this clay and the wide list of applications of this surfactant.

2. Materials and experimental procedure

2.1. Anionic clay As adsorbent we used the magnesium/ aluminium layered double hydroxide intercalated with carbonate anion, which was prepared according to the Reichle procedure [20] in our laboratory. All reactants used were of high purity degree and were purchased from Merck. The material obtained was characterised by X-ray powder diffraction. elemental and thermal analyses. From the results of these analyses, we found a lamellar material showing a basal distance of 0.754 nm with the approximate minimum molecular formula [Mgl,,,All(OH )s.sBlV333),,.s1 .45(&O). This material, after being submitted to high vacuum at 328 K, presented a specific surface area of 86.7 m2 g-‘, determined by the BET adsorption isotherm. This area corresponds to both the surface and interlamellar area of the LDH particles. 2.2. Surfactant

The anionic surfactant used was sodium dodecyl sulfate (SDS), purchased from Merck in its purest available form (purity > 99%) and presenting a CMC value of 8.23 mM, determined by conductance measurements. The determination of the SDS concentration in aqueous solution was done by a double phase titration [ 2 1] using cetyltrimethylammonium bromide purchased from Merck (purity > 99.0%) as the titrant, chloroform (Merck min. 99.9%) as the other phase and methylene blue (Merck) as the indicator.

P. C’. Pavun ut ul. i Microporous und Mesoporous Materials 21 (1998) 659k56.5

2.3. Adsorption

The adsorption study was carried out using the batch method. The LDH was previously dried under vacuum at room temperature. A constant mass was weighed (200 mg) and suspended with half of the solvent (deionized water) required to produce a total fixed volume (50 cm3) of the solution. This suspensionwas submitted to a ultrasonilication in order to homogenise the particles, and then the other half of the fixed volume was completed by an adsorbate solution with the concentration twice that of the one defined (in the rangeof8x10~4t02x10-2moldm-3);thevalue of pH was in the range of 7.05-0.5. In order to reach the equilibrium, this system was shaken, in a Dubnoff type bath, during 72 h at constant temperature (10.5 K). After this time. half of each suspension was taken away and centrifuged in order to determine the surfactant concentration in the supernatant, while the other part was used to obtain the zeta potentials. 2.4. Electrokinetic meusurements

Zeta potential measurements were done in each suspension after the adsorption equilibrium was reached. A Zetasizer 4 with microcomputer SX/16 from Malvern was used. 2.5. Stunning electronic microscopy

The SEM images were obtained with a scanning electronic microscope LEO model S-440, from three selected solids corresponding to the nonadsorbed material and the adsorbed solid with two different amounts of sodium dodecyl sulfate. 3. Results and discussion

The adsorption study was carried out at three temperatures (288, 298 and 308 K). The results obtained for each temperature are plotted according to Freundlich isotherms represented in Fig. 1. The corresponding curves of zeta potential versus the adsorbate equilibrium concentration are shown in Fig. 2.

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From the isotherms it is possible to observe that the adsorption in this system follows the two-sl;ep model [7,12-141. The first step is due to electrostatic interaction of the positively charged LDH particles with the negatively charged sulfate groups of SDS, also called region 1 in the isotherm. The other step is due to the attraction between the surfactant’s hydrophobic tails, consisting mainly of van der Waals forces. This step is represented by three more regions in the isotherm. The one where this effect starts to be significant and an eventual monolayer or hemimicelle starts to be formed is called region II. After this region, we have the formation of the second layer or the so-called admicelle. this is called region III It finishes at the saturation of the adsorbent that is designated region IV. We can seethat the isotherms obtained show a characteristic behavior which agrees with the adsorption model from the litcrature [ 121.It is possible to notice that the beginning of the saturation point (-2.S g dmw3) between regions III and IV is coincident with the CMC determined for this surfactant, as predicted by the model adopted. The increase in the adsorbed amount of SDS with temperature is explained by considering the enthalpy rather than the entropy as the driving force. since enthalpy has high ncgative values. The entropy. as expected, increases with temperature. This would contribute to decreasing the adsorption, but this is not observed. Therefore, we consider that enthalpy leads to higher amounts of adsorbed SDS, in this case.,for higher temperatures. From the zeta potential values obtained, we could observe the inversion of the potential signal as the adsorption proceeds. as expected. The critical hemimicelle concentration (CHC), which corresponds to the transition from region I to region II, corresponds to the concentration where the zeta potential tends to be constant. It is also possible to see that the zeta potential values are very similar for the three temperatures, which shows the small influence of temperature in the zeta potential. The SEM micrographs were taken to give an idea of the topographic structure. The first image can be seen in Fig. 3(a) and corresponds to the non-adsorbed LDH. Fig. 3(b) shows the image of

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P. C. Pawn et al. i Microporous and Mesoporous Materials 21 (I 998) 659-665 0,001

0.0001

f 0.01

I 0.1

1 1

Equilibrium Concentration of SDS (Q.dm”) Fig. I. Adsorption

isotherms for the system LDH-SDS.

60 40

@ n

20

0

i

9 o-E. 3 aE s -4Orr” LP -6o-

+

= 0 + ip

288 K 298 K 308 K

e).

-6o-

+8 0

-100.

,.,.,.,.,.,', 0

1

2

3

4

5

6

Equilibrium concentration of SDS (g.dm”) Fig. 2. c Potential of the particles relative to the adsorption

the adsorbed material at incomplete monolayer formation (region I of the isotherm), where particles aggregate and then sediment in aqueous media. An image of the adsorbed material at the concentration in which the bilayer is supposed to be formed (region IV) was also taken and is shown in Fig. 3(c). These micrographs show the drastic changes

that occur at the surface of the LDH during the adsorption process. From Fig. 3(a) one can see the porosity and irregular characteristics of the adsorbent. Fig. 3(b) shows a drastic variation of the LDH’s surface, which becomes less irregular and where pores cannot be seen. It also shows diffuse regions, suggesting that non-condensed surfactants are present, with their tails not well

P. C. Pavan et nl. / Microporous and Mesoporous Materials 21 (1998) 659.--665

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(b)

(4 Fig. 3. SEM image of (a) the non-adsorbed LDH; (b. c) the LDH adsorbed with an amount of SDS corresponding to (b) region I and (c) region IV in the isotherm.

bonded to the surface. Fig. 3(c) shows a more condensed state at the surface, which is in agreement with the idea of the formation of a surfactant bilayer. Another piece of information that could be used to emphasise this model concerns covering of the LDH surface by the surfactant anions. We have done an estimate of the ratio of the covered area as follows: we took the area occupied by the adsorbed surfactant anions and considered the theoretical circular cross-sectional area of the dodecyl sulfate as 0.204 nm’; we used the number of anions adsorbed at a temperature of 308 K for each concentration of surfactant and divided this

relative area by the surface specific area of the LDH. which had been determined by the BET method (86.7 m2 g-l). The results obtained are organised in Table 1. As we can see, the total covering of the surface, or the point where the SDS anion area becomes higher than the LDH surface area, occurs at the saturation of the adsorption isotherm, when adsorption reaches 7.1 x 10e4mol of SDS per gram of LDH; in other words, the beginning of region IV. This result would be expected if: ( 1) the SDS anions stayed perpendicular to the LDH surface; (2) only a monolayer was formed until the saturation point is reached; (3) the sites of the LDH surface had

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664 Table 1 Approximate surface

recovery ratio of SDS molecules on the LDH

Adsorption ( 10-4mol g-i)

Relative area (m’)

Recovery (%)

1.8 3.7 3.5 4.3 5.3 5.8 7.1 7.2 ?.3

22.0 33.2 43.0 52.8 65.1 71.3 87.2 88.5 89.7

25 38 50 61 75 82 101 102 103

a Ratio of the area recovered (relative area;LDH face area).

specific sur-

similar energy; (4) the cross-sectional area of the N2 molecules used in the BET method was similar to that used for the SDS anions. Although theseconsiderations will not be exactly true, the first and second ones can be explained if we think that the SDS anions that are forming an eventual bilayer at regions II and III can compensate those that are adsorbed parallel to the surface. This behavior can be accepted by considering that different energy sites are present, and that the formation of the bilayer should be energetically more favorable than the occupation of the lower energy sites by SDS anions. Finally, the crosssectional area of N, molecules is smaller than the one calculated for SDS anion. so this difference could be responsible for bringing the total covering to higher values than predicted by the adopted model, as indicated by the estimate. It should be noticed that the LDH diffraction pattern did not change after the adsorption took place, keeping the original basal spacing. This leads us to conclude that, as expected considering the intercalated anion stability, the interlamellar carbonate anions were not substituted by the SDS anions. 4. Conclusions The results obtained in this work show that the behavior of the adsorption of dodecyl sulfate from

aqueous solution on layered double hydroxides follows the adsorption model proposed in the literature for the adsorption of this surfactant on charged minerals such as alumina. The effectivenessof this adsorbent also encourages the continuation of the study due to its large range of application, its characteristic residual charge and also its relatively high surface area.

Acknowledgement The authors thank the Brazilian Agencies Fundacao de Amparo a Pesquisa do E:stado de Sao Paulo (FAPESP), Conselho National de Desenvolvimento Cientifico e TecnologicoPrograma de Apoio ao Desenvolvimento Cientifico e Tecnolbgico (CNPq/PADCT) and Fundacao Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES ) for financial support. The authors also thank C.V. Santilli for the BET isotherm measurements.

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