Comparative Study of the Solubility of the Crystalline LayeredSilicates r-Na2Si2O5 and δ-Na2Si2O5 and the Amorphous SilicateNa2Si2O5

Antonio de Lucas, Lourdes Rodrıguez, Paula Sanchez, Manuel Carmona,Pedro Romero, and Justo Lobato*

Facultad de Ciencias Quımicas, Departamento de Ingenierıa Quımica, Universidad de Castilla-La Mancha,13071 Ciudad Real, Spain

The solubility of a crystalline layered silicate, δ-Na2Si2O5, has been evaluated and comparedwith that of a crystalline alpha phase (R-Na2Si2O5) and an amorphous silicate of the samecomposition. Experiments were carried out at 293, 313, and 333 K. It was found that on raisingthe temperature from 293 to 333 K, a large increase in solubility occurred for all silicates studied,whereas increasing the time from 30 to 180 min had a less marked effect. Interactions betweenthe silicates and deionized and tap water are discussed. The solubility in tap water wasdetermined taking into account the alkaline earth metal (Ca2+ and Mg2+) binding capacity ofthe solid phases. A model aimed at understanding the reaction of δ-Na2Si2O5 in deionized waterhas been proposed on the basis of species identified in solution (Na+, Si4+). A good reproductionwas achieved between the amount of NaOH predicted by the model and the pH measured insitu at each temperature studied.

1. Introduction

Detergents of the future will depend on the evolutionof household appliances, substrates, and consumerneeds. In addition, the environmental constraints, whichmay well become more stringent, will also play onimportant role - particularly in terms of formulations.Raw materials will have to be more environmentallyfriendly, with criteria concerning biodegradability andrenewability becoming more prominent.

One of the main ingredients in detergents is builders.In a detergent the builder has to fulfill several functions.The most important function is to remove the calciumand magnesium ions from the tap water. Other func-tions of builders include the ability to supply and bufferthe alkalinity of the wash liquor.

In the field of detergent formulations, the establishedbuilder is sodium tripolyphosphate (STPP). However,the addition of STPP to detergent formulations is nolonger widespread because of the contribution of phos-phates to the eutrophication of waterways.1-4 There area number of alternatives to phosphates and one of theseis zeolites. Unfortunately, however, this material re-mains as a solid in the wash liquor and wastewater untilit is removed with sludge in a wastewater treatmentplant. Zeolites, particularly zeolite A, have proven to besuitable replacements for STPP because they are eco-logically sound water softeners. Despite the fact thatzeolite A possesses a good ion exchange capacity for Ca2+

ions, it has a lower ion exchange capacity for the Mg2+

ion.5,6 For this reason other zeolites, such as zeolite13X,7,8 zeolite P,9 or clinoptilolite10 have been studiedfor use in detergent formulations.

Crystalline layered sodium silicate has recently beenincluded as a new multifunctional builder. This new

builder essentially consists of the δ phase sodiumdisilicate Na2Si2O5, which has a polymeric layeredbidimensional crystal structure in addition to small Rand â and amorphous sodium disilicate contents asimpurities.11 The substance-specific properties of δlayered crystalline silicates are essentially based on theabsence of structural water, the exchangeability ofintermediate layer sodium, and solubility. These prop-erties mean that layered silicates are efficient in remov-ing water hardness, provide suitable washing ability,enhance the performance of surfactants and bleachers,have a corrosion-inhibiting action, and can be used informulations of both liquid and highly compact deter-gents. Moreover, these silicates are inert from anecological point of view and can be mixed with any otherbuilder.

Few papers in the literature have focused on theproperties and washing characteristics of thesesilicates12-14 and extensive studies regarding theirsynthesis have not been performed.15-20

Sodium tripolyphosphate, zeolite, and δ-Na2Si2O5differ in their water-softening mechanism and behaviorin wastewater. δ-Na2Si2O5 is soluble in the wastewaterand forms an extremely dilute waterglass solution. Thehigh solubility of δ-Na2Si2O5 in wastewater means thatthis material makes a very low contribution to sludgeformation in the treatment plant.

With regard to ecological concerns, zeolite and δ-Na2-Si2O5 have clear advantages because they are designedto be phosphate substitutes.13 Zeolite is insoluble inwater and so δ phase sodium disilicate is a goodalternative to sodium triphosphate.

The trend toward more compact powder detergentsputs increasing demands on the detergent builder, andthe layered disilicate δ-Na2Si2O5 meets these demandsbecause it combines high performance per unit weightwith a high degree of multifunctionality.21 An in-depthstudy into the solubility of the new builder δ-Na2Si2O5,used as a substitute for phosphates in detergent for-

* To whom correspondence should be addressed. Tel: +34926 29 53 00. Fax: +34 926 29 53 18. E-mail: Justo.Lobato@uclm.es.

1472 Ind. Eng. Chem. Res. 2004, 43, 1472-1477

10.1021/ie0303909 CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 02/13/2004

mulations, has been performed and is reported here. Theresults are compared with those of a crystalline alphaphase (R-Na2Si2O5) and the amorphous silicate Na2Si2O5used in the industrial synthesis of δ-Na2Si2O5.

The amorphous disilicate removes the water-harden-ing ions by formation of a voluminous alkaline earthmetal silicate precipitate in which the polyvalent cationsare mainly bound by the δ crystalline substance throughan ion exchange process.12

The aim of the work described here was to determinethe influence of temperature on the solubility of differ-ent silicates in deionized or tap water. Furthermore,based on the investigations on the solubility of δ-Na2-Si2O5, this work resulted in a theoretical model thatallows us to understand the different species formedduring the hydrolysis process of δ-Na2Si2O5.

The results described here could also help to identifythe correct detergent dosage per wash, a factor thatwould contribute to decreasing the environmental im-pact of the wash process.

2. Experimental Section

The experimental setup consisted of several Pyrexglass flasks (100 mL each) that were hermetically sealedand vigorously stirred. The flasks contained the deion-ized water solutions and the solid. The flasks weresubmerged in a temperature-controlled thermostaticbath. The temperature was kept constant within (0.5K.

One set of experiments was conducted with deionizedwater, and another set was conducted with tap waterto make the conditions more similar to those of thewashing process. All experiments were carried out at293, 313, or 333 K in the aforementioned bath.

The amount of solid used was 0.3 g in 100 mL andthis represents a typical detergent dosage.

After different times the mixtures were filtered, andthe ionic solutions were analyzed by inductively coupledplasma atomic emission spectroscopy (ICPAES) at 251nm (Si4+) and 589 nm (Na+) or by flame atomic absorp-tion spectroscopy at 589 nm (Na+).

The solubility of δ-Na2Si2O5 in deionized water solu-tions was obtained from sodium ions for different timesand temperature.

The evolution of pH with time was monitored usinga pH meter (Crison 2002) connected to a computer andrun through a data acquisition program developed inour department.

The crystalline disilicates were synthesized in ourlaboratory.22 The amorphous disilicate was supplied byIQE . The mean size of all silicates was 175 µm (range100-250 µm).

The deionized water was conventionally treated in ourlaboratories (conductivity less than 1 µS/cm). The tapwater from our region had the following properties:calcium and magnesium concentrations were 50 and 17ppm, respectively (19.5 °F), conductivity was 384µS/cm, and the pH was 6.8.

3. Results and Discussion

3.1 Kinetic Study of Solubility in DeionizedWater. The δ-Na2Si2O5, alpha, and amorphous-phasesolubilities in deionized water were determined at 293,313, and 333 K. The temperatures studied here encom-pass the usual ranges found in the washing process.Figure 1 shows the influence of temperature and time

on the solubility of the materials in question. It can beseen that the solubilities of the three solid phases usedincrease with increasing temperature. On the otherhand, the effect of time is less marked because after 30and 60 min the solubility was generally constant forδ-Na2Si2O5 and R-Na2Si2O5, whereas the amorphousphase solubility was found to increase with increasingtime over the range studied.

The solubilities reached for the two crystalline phaseswere similar, and the amorphous silicate had lowersolubility values even at the highest temperature andtime. This phenomenon is due to the differences in thestructural build-up of the materials. In particular, thecrystalline silicate units break down under the condi-tions in question.

3.2 Kinetic Study of Solubility in Tap Water. Theδ-Na2Si2O5, alpha, and amorphous phase solubilities intap water were determined at 293, 313, and 333 K.Figure 2 shows the influence of temperature and timeon the solubility. The amounts of Ca2+ and Mg2+ in tapwater before and after solubilization were quantified.It can be seen that the solubility of all three solid phasesincreases with increasing temperature. In each case thesolubility was determined by taking into account thedifference between the total sodium ions in solution andthe amount of sodium exchanged by the alkaline earthmetal (Ca2+ and Mg2+) binding of the solid phases.

Figure 1. Solubility of silicates in deionized water at 293, 313,and 333 K: (a) δ-phase crystalline silicate; (b) R-phase crystallinesilicate; (c) amorphous silicate.

Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1473

The solubility of δ-Na2Si2O5 in deionized water ishigher than its solubility in tap water, which is thoughtto be a consequence of the binding capacity of δ-Na2-Si2O5.

It can be seen that the solubility of the R phase intap water is similar to that in deionized water at thethree temperatures investigated. This phenomenon canbe explained by the fact that this solid has the lowestretention capacity for the Ca2+ and Mg2+ ions.22

Hydrolysis of the amorphous phase occurs to a lesserextent in tap water than in deionized water at 293 and313 K, but is similar at 333 K. This situation is due tothe increase in the retention capacity, caused by pre-cipitation, as the temperature decreases. It can beobserved that the differences in the hydrolysis for bothtypes of water became less marked as the temperatureincreased.

The solubility data (C8) for the different crystalline

silicates at different temperatures were fitted to eq 123

and the constants obtained are shown in Table 1. Thesolubility data (C∞) for the amorphous silicate were notfitted to eq 1 because complete solubility was notachieved during the time interval studied.

where C∞i is the solubility of species i in solution (ppm),i represents each silicate studied, T is the absolutetemperature in Kelvin, and A and B are constants.

3.3 Hydrolysis Reactions in Deionized Water. Atpresent, R-Na2Si2O5 does not have any known industrialapplication, but δ-Na2Si2O5 is used as a multifunctionalbuilder in nonphosphate detergents. For this reasononly the hydrolysis reactions of δ-Na2Si2O5 are coveredin this section. Data published in the literature14

indicate that δ-Na2Si2O5 rapidly releases sodium intosolution. It therefore seems reasonable to propose thatthe following reaction takes place on contact with anaqueous solution with the formation of amorphous silica.

Amorphous silica is known to be partly water soluble,and data published in the literature24,25 indicate thatthe solubility of SiO2 reaches a minimum of 100-130ppm between pH 7 and 8. The solubility then increasessignificantly above pH 9 to reach 400 ppm at pH 10between 292 and 298 K.

In aqueous solution, silicon exhibits a 4-fold coordina-tion of oxygen and forms species generally referred toas silicates (H4SiO4). It therefore seems reasonable topropose that the following reaction occurs on contactwith an aqueous solution:

Orthosilicic acid is understood to be a four protonic,weak acid, and, depending on the causticity of thesolution, one or more protons can be released. In morealkaline media the charges carried by the silicate speciesretard the mutual reactions and depolymerization isfavored under certain conditions.26

The general reaction of orthosilicic acid in alkalinesolution is as follows:

3.3.1 Reaction Schemes. Prediction of the δ phasesodium disilicate reaction in deionized water requiresa knowledge of the kinetics and the equilibrium con-stants for the three temperatures studied.

Sodium and silicon were determined at 293, 313, and333 K in deionized water. Figure 3 shows the sodiumand silicon concentrations generated in the hydrolysisof δ-Na2Si2O5 along with the evolution of these specieswith time and temperature.

The decrease in δ-silicate in deionized water can berepresented by the empirical expression proposed by

Figure 2. Solubility of silicates in tap water at 293, 313, and 333K: (a) δ-phase crystalline silicate; (b) R-phase crystalline silicate;(c) amorphous silicate.

Table 1. Values of the Constants Obtained from theSolubility Equation for Different Types of Water Used

species solution A B r2

δ-Na2Si2O5 deionized water 421.2 4.636 0.978R-Na2Si2O5 deionized water 648.84 5.3532 0.998δ-Na2Si2O5 tap water 539.91 4.8945 0.96R-Na2Si2O5 tap water 559.71 5.0631 0.975

Log C∞i ) - AT

+ B (1)

δ - Na2Si2O5(S) + H2O f SiO2(S) + NaOH(aq) (2)

SiO2(S) + 2H2O T H4SiO4(aq) (3)

H4SiO4(aq) + NaOH(aq) f NaH3SiO4(aq) (4)

1474 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004

Butler27 for crystallization processes:

where n is greater than 1, m is the mass of thecrystalline solid, CA and C∞ are the concentrations ofcrystalline solid at given times and at the saturationconditions (solubility), respectively. The values of C∞ forδ-Na2Si2O5 at each temperature were obtained using eq1. The overall reaction scheme is outlined as follows:

where C* ) CAo - C∞.

The Rosenbrock method28 was used to numericallyintegrate the stiff set of ordinary differential equationsdue to its stability and the low number of integrationsteps required to achieve a satisfactory solution.

The nonlinear system was fitted using Marquardt’salgorithm.29 For this purpose a Fortran 6.0 applicationwas developed to solve this model.

Figure 4 shows the experimental concentrations ofNa+ and Si4+ and their respective fitted data using themodel. These concentrations correspond with the Na+

and Si4+ ions of the species in aqueous solutions (NaOH,H4SiO4, and NaH3SiO4).

Kinetics constants (K0, K1, K2, and K3) were obtainedby fitting the experimental data with the model at therespective temperature. The reaction exponent (n) ob-tained for δ-Na2Si2O5 hydrolysis was constant but notfixed. The kinetics parameters are summarized in Table2.

It can be observed that as the temperature increasesall kinetic constants also increase. However, the in-crease with temperature of the relation K1/K2 for thereversible reaction 3 is remarkable. This behaviorindicates that as the temperature increases, reaction 3btends to be irreversible - as can be observed in Figure3. Thus, at 333 K the Si4+ species is formed rapidly afteronly short times.

In addition, the activation energy and preexponentialfactors for the three reactions could be evaluated fromthe kinetics constants, K, and the temperature. Assum-ing a linear variation of lnK with 1/T, a negative slopeis obtained. These activation energy values and preex-ponential factors are given in Table 3 along with thecorrelation coefficients.

Figure 5 shows the concentrations of the species’ aspredicted by model in deionized water.

The validity of the model was checked by making anadditional comparison between the pH measured ex-perimentally in situ and the concentration of NaOHobtained by the model. The results of this comparisonare shown in Figure 6. The lowest pH value obtainedat 333 K occurs because reaction 3 tends to be irrevers-ible, as indicated above, and therefore the highestconcentration of the acid H4SiO4 is formed and then

Figure 3. Concentrations in deionized water at 293, 313, and 333K: (a) Na+ ions; (b) Si4+ ions.

dmdt

R(CA - C∞)n (5)

rA )dCA

dt) -K0(CA - C*)n (6)

rB )dCB

dt) 2K0(CA - C*)n - K1CB + K2CD (7)

rC )dCC

dt) 2K0(CA - C*)n - K3CDCC (8)

rD )dCD

dt) K1CB - K2CD - K3CDCC (9)

rE )dCE

dt) K3CDCC (10)

Figure 4. Comparison of model (solid lines) and experimentalconcentrations (points) for δ-Na2Si2O5 and silicon species at 293K.

Table 2. Values of Kinetics Constants for DifferentReactions in Deionized Water at 293, 313, and 333 K

T(K)

K0(L2.2‚min-1‚mol-2.2)

K1(min-1)

K2(min-1)

K3(L‚mol-1‚min-1)

293 60000 46.2 1200 114313 100000 915 4120 835333 180000 184000 6150 7120

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neutralized by the NaOH. It can be concluded that thismodel is reliable in providing an insight into thebehavior of δ silicate in desionized water.

4. Conclusions

Crystalline silicates show solubility values higherthan those reached by amorphous silicate in deionizedwater due to the structural differences in the silicatesover the time interval studied.

It has been found that increasing the temperaturefavors the solubility of both crystalline phases. Thesolubility of the δ phase in deionized water is higherthan its solubility in tap water because of the δ-Na2-Si2O5 retention capacity. The solubility of the alphaphase in tap water is similar to its solubility in deionizedwater for the three temperatures studied. This phenom-

enon is due to the alpha phase having the lowestretention capacity for the Ca2+ and Mg2+ ions.

The results obtained in this study show that δ layeredcrystalline disilicate is water-soluble and therefore doesnot remain in wastewater as a solid. This materialtherefore contributes very little to sludge formation inwastewater treatment plants, and also provides andbuffers the alkalinity in the wash liquor.

The model is able to reproduce satisfactorily thehydrolysis of δ-Na2Si2O5 in deionized water at anytemperature. The model also allows evolution of thedifferent species during the hydrolysis process to beobtained and provides information concerning the influ-ence of δ-Na2Si2O5 on the variation of pH in a stirrerreactor.

The new builder, δ-Na2Si2O5, is more soluble thanzeolite 4A, the builder currently used in detergents asa substitute for phosphates. These facts, along withother important characteristics of δ-phase sodium dis-ilicate such as alkalinity supply or good ion-exchangecapacity for Ca2+ and Mg2+ ions, make this materialsuitable for use as a builder or co-builder in nonphos-phate detergents.

Acknowledgment

We thank Industrias Quımicas del Ebro for itsfinancial support (I+D Contact with the University ofCastilla-La Mancha, UCLM).

Nomenclature

n ) reaction exponent of δ-Na2Si2O5 hydrolysis reaction.C∞ ) δ-Na2Si2O5 solubility.C* ) δ-Na2Si2O5 remaining concentration after reaching

saturation.K0 ) kinetic constant for hydrolysis of δ-Na2Si2O5.K1 ) kinetic constant - direct for SiO2 reaction.K2 ) kinetic constant - inverse for SiO2 reaction.K3 ) kinetic constant for reaction between NaOH and H4-

SiO4.CAo ) δ-Na2Si2O5 initial concentration.CA ) δ-Na2Si2O5 concentration with time.CB ) SiO2 concentration with time.CC ) NaOH concentration with time.CD ) H4SiO4 concentration with time.CE ) NaH3SiO4 concentration with time.

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Received for review May 6, 2003Revised manuscript received October 20, 2003

Accepted November 5, 2003

IE0303909

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