surfactant systems

Powder Technology, 27 (1980) 251- 259 @ Ekevier Sequoia S-A., Lausanne -Printed

The Stability NADA “Ruder

of Zeolite/Surfactant

FILIPOVI6VINCEKOVI~, BoskouiE”

(Received

251 in the Netherlands

Institute.

June 19,198O;

41000

LAVOSLAV Zagreb.

Systems SEKOVANIC*

Bijenicka

in revised form August

54. POB

The effect of various surfactants on the stability properties of zeolite dispersions has been studied in aqueous solutions. The results. show a close relation between the structural properties of the interfacial solid/liquid region and the association equilibria in surfactant aqueous solutions_

INTRODUCTION

In the last few years, synthetic zeolites of the NaX and NaA types with high capacity for multivalent ions have shown excellent properties as phosphate substitutes [l - 3]_ Commercial laundry detergents are composed of water-soluble components and it is of utmost importance to prepare stable zeolite dispersions during washing in order to minimize incrustation. The experiments to be discussed in this paper are concerned with the stability of zeolite dispersions in model surfactant soIutions_ Two different groups of surfact.a.nt.s were used to determine the stability of zeoiite dispersions and the manner of surfactant incorporation at the solid/liquid interface_ (i) The zeolite chosen for the present study was full crystalline zeolite NaA. The unit cell of the zeolite of this type is composed of (12 SiOz + 12 A10zj-12, compensated by 12 Na* ions. The structure is characterized by a three-dimensional network consisting of large and small cavities. Interstices are occupied by 12 sodium ions and 27 water molecules. The water molecules affect specific positions of exchangeable cations, but they appear to have no primary structural function and can be re-

47000

Karlovac,

Yugoslavia.

ZITNIK

11,198O)

SUMMARY

*KGK,

and DORICA 1 OI 6 (Yugoslavia)

moved without disrupting the framework structure. When hydrated, the zeolite can be regarded as a polyanionic framework surrounding a solution of positive ions in water_ Suspended in water, the zeolite produces hydroxyl ions due to hydrolysis, and the pH of the water slurry is above 7 [5] : Na’ _ _ . 0-(Al,

Si) + H20 ;I? OH-(Al,

Si) +

+ NaOH

(1)

If the pH of the zeolite water suspension is reduced to a pH below 5 by addition of an aqueous acid, aluminium ions are removed (leached) from the framework and the structure is destroyed [6] _ (iij Aqueous surfactant solutions may be differently structured depending on surfactant concentrations. With varying surfactant concentrations, the structure of aqueous solutions undergoes changes, resulting in the formation of submicelles (premicellar association), micelles and micelle agglomerates (postmicellar association) [ 7,8] _ (iii) n-Dodecylamine is a weak base having a pK of 10.6. With a pH below 8.6 the amine is virtually all in the cationic form. Above pH 8.6 hydrolysis of the cationic form is promoted: Cr2HzsNHs+ + OIi-

;=*Ci2H2sNH2

f Hz0

(2)

The amine generated by hydrolysis forms a complex with the cationic form remaining in the solution [ 91: ~(Cr2HzsNH;j

+ ~(C~zHas.NHzj T=*

~(C~zHasNIGjY(GaH&‘JHz)

(3)

The hydrolysis of sodium n-dodecyl sulfate is not important in the pH range from 4 - 10. The formation of dodecanol has been reported in strongly acidic solution [ 101.

252 EXPERIMENTAL

Materials Zeolite was prepared by crystallization of sodium-containing aluminosihcate gel. After preparation, the material was washed several times in order to remove small amounts of free alkali entrained within the structure during synthesis until the pH of the slurry was 9.3 [ 5]_ Chemical and structural analysis (Xray diffraction) of the dried material (at 105 “C) showed that the material was full crystalline zeolite NaA with a high capacity for multivalent ions (2.4 X 10d3 mol Ca2*/g hydrated sample) and a mean particle size of 13 X lo3 nm [ll] _ n-Dodecylamine nitrate, DDAN03, was prepared from distilled n-dodecylamine and p-a-nitric acid. The material was recrystallized several times from an ethanol-water mixture and dried in vacua at 293 K. The equivalent conductivity and surface tension versus surfactant concentration curve indicates transition concentrations in the structure of aqueous DDAN03 solutions as shown in Fig. 1 [ 8]_ Sodium n-dodecyl sulfate, NaDDSO,, of special purity grade was supplied by BDH and purified twice from ethanol. The critical micelle concentration, c.m.c., was determined from surface tension isotherms and the value is indicated on the abscissa (Fig_ 4). Systems The systems were prepared by dispersing 0.2 g zeolite in 100 ml of aqueous surfactanc solutions of predetermined concentrations_ Zeolite was dispersed for 10 min by means of an electromagnetic high-speed stirrer and then aged in a bath at 293 K before anaiysis.

EXPERIMENTAL

phoretic mobility, u (cm2 V-’ s-l), were determined by ultramicroelectrophoresis (after Smith-Lisse).

pH measurements The pH value was measured with a pH Meter 26 (Radiometer, Copenhagen).

Adsorption isotherm The amount of adsorbed surfactant on zeolite particles was determined after equilibration of the solid phase in s-urfactant aqueous solutions of various concentrations. The solid phase was removed by centrifugation. The amount of surfactant remaining in the liquid phase was determined analytically [12, 13]_ The amount of adsorbed surfactant per mass of adsorbent was plotted vers=s equilibrium concentrations_

RESULTS

AND

DISCUSSION

It is known that surfactants interact very strongly with stable inorganic sols of opposite charges causing various changes of the properties of the systems [ 143 _ The interaction mechanism of inorganic SOIS and surfactants is a complex one and depends on the type and concentration of the surfactant present in respect to flocculation, stabilization and mutual coagulation of ~01s. Similar phenomena were observed in systems composed of inorganic sols and surfactant of the same charge [15]. Zeolite NaA was dispersed in water and aqueous solutions of cationic and anionic surfactant and therefore results obtained are presented in the following three parts.

TECHNIQUES

Turbidity The change in turbidity, r, of the systems as a function of time (dr/df) was used in some experiments as a criterion to determine the rate of sedimentation. The measurements of tyndallometric values were performed on a Pulfrich photometer combined with a turbidimetric extension (Carl Zeiss, Jena).

(A)

Dispersion of zeolite in water The surface of zeolite in aqueous solutions is covered with Si-OH and Al-OH due to: (i) hydroxyl groups which terminate the lattice at the crystal faces at points where bonding would occur were the solid phase extended: -Si-(O)zH, -Al-(O),*H; (ii) rapid hydration of broken Si-0 and Al-O bonds:

Microeiectrophoresis The particle charge and change in electro-

aluminol

*(o)”

= non-framework oxygen)_

oxygen

(siloxane

or

253

9.33, as a result of hydrolysis. A small decrease in pH after prolonged aging revealed a small consumption of OH- ions as a consequence of the formation of polynuclear hydrolysed aluminate cations. From literature data, at pH > 4, aluminium exists as hydrolysed and partially polymerized cations

(iii) hydrolysis

ClS] Once dispersed, zeolite is stable for 100 min. After that time, settling-down of particles under gravity occurs.

of zeolite:

H

Al

19Si

Any further change in surface properties depends upon the solution concerned. According to James [16], the probable zeolitic surface reaction with water is leaching of Al’s ions from the surface. The number of surface StOH groups increases. The Ieaching of the zeolite surface is more pronounced in the slightly acidic region and could extend to a few molecular layers in the crystals. In the strongly acidic region zeolite crystals are decomposed. A consequence of the removal of surface aluminium is that the electrophoretic mobility of leached aluminosilicate crystals shows higher negative electrophoretic mobility [ 17]_ Washed and dried zeolite samples were dispersed in bidistihed water. Suspended in water zeolite particles become negatively charged with changes in electrophoretic mobility, pH and system turbidity, r, as indicated in Table 1. TABLE

1

The change of electrophoretic mobility, stability of systems zeolite + Hz0 f*

(min)

30 100 1500

u- (cm2 2.8 3.2 3.2

V-l

s-l )

pH and

PH

i

9.33 9.26 9.23

4000 4100 181

The change of electrophoretic mobility and pH during aging in water can be explained by the slow leaching of Ai+3 from the zeolite surface. The system aged for 30 min has a pH of

(B) Dispersion of zeoiite in cationic surfactant solutions The change in pH of systems is indicated in Table 2 and the turbidity of systems and the electrophoretic mobility of particles are shown in Fig. 1. The results show a strong dependence on cationic surfactant concentrations_ TABLE

2

The change in pH of systems zeolite +- DDANO3 Hz0 DDANO3

0.00001 0.00005 0.0001 0.0005 0.001 0.003 0.007 0.01 0.1

(mol dme3)

fA (min) PH 6.12 6.10 6.09 5.82 5.58 5.53 5.10 4.i7 4.53

= 0

+

fA (min) = 100 PH 8.73 8.23 8.20 8.03 7.91 7.80 7.63 7.55 7.05

At the time t, = 0 (the state o? DDANO, aqueous solutions before addition of dry zeolite), the pH decreases with increasing surfactant concentrations. During aging the resulting change in systems pH is a consequence of zeolite hydrolysis and leaching of Ale3 from the zeolite surface. At lo-* mol dmM3 of DDANO,, positive surfactant associates (or complexes) possess good properties which can be observed by means of an ultramicroscope (Fig. I)_ The number of visible entities and electiophoretic mobility increase with increasing DDAN03 concentrations. The critical association concentrations (c.a,.c. and c.az.c.), the critical micelle concentrations (cm-c_) and the critical micelle agglomeration concentra-

254

L :;om3L

‘;;i 5.

_______

)

I

=_

JF /---n ------------

CCL,

4 -5

log @DA%]

cc+c

c

,

cmc

!

I(

,CPmc

9

,

hn&i3)

, -1

Fig. 1. Turbidity, r, of systems and electrophoretic mobility of surfactant associates and zeolite particles changes plotted against the logarithm of the initial DDANOa concentrations. 0.2 g of zeolite was dispersed in aqueous DDANOa solutions of predetermined concentrations_ The systems were thermostated at 293 K and aged for: (A) 0 min, (e) 30 min. (X ) 1500 min. The c.a~_c. and the c.az.c. are the transition concentrations at which associates are formed; the c_m.c_ is the critical micelle concentration and the c.a.m.c_ is the concentration at which micelle-micelle interactions take place (agglomeration of micelles) [S] _

tion (c.a.m.c.) determined from discontinuity of equivalent conductivity and surface tension isotherms [S] are denoted on the abscissa in Fig. 1. An analysis of turbidity curves shows the following stability regions: (I) range of stable dispersions (dispersions have the same stability as zeolite dispersion in water) ; (II) range of flocculated dispersions (dispersed zeolite particles sedimented immediately after the zeolite had been dispersed); (III) range of stabilized dispersions (dispersions were stable after aging for 1500 mm).

At low surfactant concentrations the electrophoretic mobility of zeolite particles exhibits no dependence on surfactant addition. With increasing DDANOs concentrations the electrophoretic mobility of negative zeolite particles decreases to zero. The zero point of electrophoretic mobility lies between 0.0001 and 0.00015 mol dmm3 of DDANOs. It is a range of rapid sedimentation. On increasing the concentration the originally negative charge is reversed to positive and the electrophoretic mobility increases to a constant value. This is a range of stabilized dispersions. Dispersions are stabilized due to increased electrostatic repulsion between overcharged zeolite particles_ Figure 2 shows results obtained for DDANOa adsorption on zeolite aged for 1500 min. The adsorption isotherm for region I is not drawn; there is no measurable amount of surfactant remaining in the liquid phase after adsorption and the initial part of the isotherm is vertical_ The surfactant is in the form of monomers. The first part of the isotherm drawn in Fig_ 2 corresponds to region II. This concentration region is characterized by associates formation. The isotherm changes its slope at the DDANOs concentrations where second-kind of associates occur (at the c.a,.c.). In the range of the c-m-c. and the c.a.m.c. the isotherm exhibits a minimum. The decrease of surfactant adsorption in the range of postmicellar association was accounted for by the establishment of an equilibrium between micelles, monomers and adsorption film [ 19]_ The probable interactions of alhylammonium ions and zeolite are: (i) incomplete ion exchange with Nd ions located in the intra-crystalline space (C,), C,zH,sNH> F=PNa+; (ii) the electrostatic adsorption of cationic surfactant in the interfacial zeolite/liquid region Alkylammonium ions with lengths which are much greater than the channel diameter can pass through the 0.45 nm window (8membered ring) in a stretched configuration [ZO] . The space available is insufficient for the next ion exchange and thus the ion exchange is incomplete (the length of CmHss is 1.67 nm). The concentration of Na’ ions which may be exchanged by n-dodecyl-

255

ZEOLITE

*

ZEOLITE/9i’ t,/rml T/K=

OOANO,

+ Hz0

= 2 = 1500

293 ./@\.,/ . /’ .I-

,/

_/*

I -5

I -6

I log

Fig. 2. Adsorption ordinate -adsorbed

I

I -1

-2 { [ DDAN;&3l

drri3)

of DDANOa on zeolite aged for 1500 DDANOa(mo!) per gram of zeolite.

ammonium ions is about 1.83 X low6 mol g-’ (see Appendix). At low surfactant concentrations (region I), the mechanism of DDANOa adsorption involved both types of interactions. For stability regions II and III the part of adsorption due to incomplete ion exchange is too small to play any significant role in overall interactions and thus the interaction involved is mainly of type (ii)_ It is evident that bulk association has a clear influence on the solid/liquid phenomena_ Discontinuities in the structure of aqueous DDANOa solutions are reflected in discontinuous changes of systems stability as a result of direct electrostatic adsorption of monomers and surfactant associates in the interfacial zeolite/solution region (Zimmels model [ 21])_ The possible sites for monomer or associate adsorption on zeolite NaA are shown in Fig. 3. (C) Dispersion of zeolite in anionic surfactant solutions Addition of a surfactant to a colloidal solution with particles of like surface charge produces a change in systems stability which is similar to a change in systems with surfactants of the opposite charge. An important constituent of ccmmercial laundry detergents is the anionic surfactant and thus the interactions between the negatively charged zeolite

min.

Abscissa

--

equilibrium

surfactant

concentrations;

Fig. 3. Schematic presentation of possible adsorption DDANOa sites on zeolite surface: (a, b) faces of 4, 6 membered rings (S6R, S4R); (c) faces of cubic units (DSR); (d) adsorption of associates; 0, siloxane or aluminoi non-framework oxygen; e=x., C12H2sNH.$ ions.

particles and anionic surfactant are of particular interest. Dry zeolite samples were dispersed in aqueous surfactant solutions_ Figure 4 shows the change of pH and turbidity of systems and the electrophoretic mobility of surfactant associates and zeolite particles. In pure NaDDS04 aqueous solutions pH decreases with increasing surfactant concentrations (aging time t, = 0). Negatively charged surfactant associates and micelles have a higher electrophoretic mobility than zeolite particles. The surfactant associates were observed at 10m4 mol dmm3 of NaDDS04. The value of the c.m.c. is indicated on the abscissa. After aging of zeolite in NaDDS04 aqueous solutions, the pHs were shifted in the alkaline

256

-5

-‘

-3

tog{ [NoOOSOL]

-t

was the same as in simple zeolite/water systems_ These data are important for application of zeolite in detergents because they revealed no significant interactions which could lead to change of stability of the systems_ Figure 5 shows results for NaDDS04 adsorption on zeolite aged 1500 min. The initial portion of the isotherm is of the S-type. After the plateau the adsorption abruptly increases and at the c.m.c_ the isotherm changes its slope. The second plateau is reached in the range of high surfactant concentrations. Owing to the large negative charge on the zeolite framework, anions are excluded from the crystals and thus the measured adsorption can be explained by ion exchange of surface OH- groups with n-dodecyl sulfate ions (DDSO,)_ The first part of the isotherm is indicative of the vertical orientation of adsorbed surfactant [22]_ A plateau was reached at an adsorption density of 1.7 X 10d5 mol NaDDS04 per gram of zeolite. The approximate concentration of OH- groups at the zeolite surface is 7.33 X 10m6 mol g-l (see Appendix)_ If we assume that all OHgroups are ion exchanged by DDSO, ions, at the monolayer coverage, the rest to the plateau value corresponds to the electrostatic adsorption of aluminium-surfactant positively charged complexes. The Ale3 ions, leached from the zeolite surface,_exist in bulk phase at

-I

I mol drr?}

Fig. 4. Turbidity, T, and pH change of systems and electrophoretic mobility of NaDDS04 associates and micelles and zeolite particles plotted against initial surfactant concentrations_ The c.m_c_ is the critical micelie concentration_ Systems were aged at 293 K_

region, similarly to the systems with DDANOs. The change of systems turbidity did not show marked differences for all surfactant concentrations_ The settling-down rate

/ . .-.

c-’

-

1s

ZEOLITE

T/K= 5:

l

NaDDSO,

* Hz0

./-

ZEOUTE/9i’ In/mm - 1500 = 2

293 ./-

-‘-

g 9

/

z i

Fig. 5. Adsorption of NaDDSOa on zeolite aged for 1500 min. Abscissa -equilibrium ordinate -adsorbed NaDDS04 (mol) per gram of zeoiite.

NaDDS04

concentrations;

257

a pH above 4 as polynuclear hydrolysed cations_ The polynuclear aluminium cation in interaction with anionic surfactant yields positive complexes at low surfactant concentrations, which are electrostatically adsorbed on the zeolite surface. The results of change in electrophoretic mobility and charge of observed particles in model aluminium + n-dodecyl sulfate systems (Fig. 6) confirmed the existence of positive ahuninium-surfactant complexes up to 2 X low4 mol dmm3 of NaDDS04. The rapid increase in adsorption after the plateau is due to subsequent surfactant adsorption. In this concentration region the associative structures in the liquid phase are negatively charged (Fig. 6) and adsorption can be explained by surfactant adsorption due to attractive forces between alkyl chains (interactions of hydrophobic chains of oncoming surfactants ions with previously adsorbed surfactant). In the first step a second layer of surfactant ions with the orientation of the ionic

head

towards

solution

is formed.

With

increasing surfactant concentrations, DDSO, ions adsorbed previously by ion exchange can serve as hydrophobic centres for the formation of surface associates_

At2 ( SO‘ Ia

l

[ntz I SO‘ l,]/ml

Na00S0~

-

The ageing of zeolite in aqueous surfactant solutions includes several physicochemical processes: hydrolysis, leaching of surface Al and adsorption of surfactant. Differently structured cationic surfactant solutions cause various changes in stability of zeolite dispersions. These changes are due to electrostatic direct adsorption of monomers or associative DDAN03 structures onto negatively charged zeolite particles. The adsorption due to incomplete ion exchange is significant only at very low surfactant concentrations. The dispersion of zeolite in anionic surfactant solutions shows no significant change in systems stability. Zeolite particles sedimented spontaneously as in the simple zeolite/water system. At low surfactant concentrations the obtained adsorption revealed a combined mechanism of adsorption: ion exchange and eltictrostatic adsorption. At higher surfactant concentrations hydrophobic interactions between alkyl chains of oncoming surfactant ions with those previously adsorbed by ion exchange cause further adsorption_

NaOH . HZ0

drri3

=

~6’

PH

=

9

-

tn/mm

=

1500

-

T/K

=

293

Cl0005

CONCLUSION

-aocKl5

Fig. 6. The electrophoretic concentration.

mobility

change of aluminium-surfactant

complexes

plotted

against NaDDS04

258 ACKNOWLEDGEMEW

We thank Dr. R. J. Mikovsky (Central Research Division, Princeton, New Jersey 08540) for valuable discussion fcr calculation of the number of surface hydroxyl groups in zeolite NaA.

mol of Na’ ions per gram of zeoiite may be involved in incomplete ion exchange. (ii) Surface OH~(-0)”

= 24(N,&

Approximate calculations of concentration of Na* ions involved in incomplete ion exchange (i) and conce@ration of surface OH(ii) groups of zeolite NaA were based on crystallographic parameters, a density 1.99 and a mean particle size of 1.8 X lo3 nm. (i) Concentration of Nd ions involved in incomplete ion exchange: Owing to the space limit, it is assumed that only one Nd ion located in a surface unit cell (a0 = 12.32 A) may be exchanged by C12H25NH3+ ion. The number of exchanged Na* ions in I,2 and 3 sheets of a crystal with an edge length L, = 3a. is (lj _ _ _ (L,a;;1)2, (2) _ _ _ (Lcao1)2 (3) _ _ _ (Lcao1)2,

-

(L,a;l

= (-OH),

(A3)

For a zeolite NaA type crystal with an edge length L,, the number of (-OH), groups is (-OH),

= (6)(4)(L,/ao)

2

(A4)

and the number of hydroxyl groups per gram of zeolite is (-OH),/g

= (24)(L,/ao)2/(L,j3p

(A5)

Fig. 8. (A) The position of a truncated octahedron on the surface of zeolite NaA, with denoted 4-membered rings as carriers of (-O),. (B) The truncated octahedron as part of the zeolite-crystal surface with (-OH),

2)2,

-

(A2)

Assuming that alI (-0), groups are bonded with H’ ions one can write Z(-0),

APPENDIX

groups of zeolite NaA:

respectively.

grOUPS_

The totai number of Nd ions which may be exchanged by C,,H,,NH,’ ions in surface C, is N (Nd, = (L,a;1)3

-

(L,a;l

-

2)3

(Al)

LIST

For crystals with a mean particle size L, = 1.8 X lo3 nm, the number of possible sites for incomplete exchange is N (Na+)= 12.8 X lo6

or NcNa+)/g = 1.1 X 101’

It follows from eqn. (Al)

that 1.83 X 10m6

LC (-O), (-OH),

Fig. 7. (A) Ion exchange of CIZH~~NH~ ions with Na’ ions in a C, cage. (B) Possibieposition and number of C12H2sNHs ions in C, cages which are part of the zeolitecrystal surface (L, = 3~).

Nc~a*)

OF SYMBOLS

unit cell dimension 26-hedron type I (truncated cubooctahedron) 14-hedron type I (truncated octahedron) free dimension of C, (11.4 K for inscribed sphere) the length of alkyl chain (stretched configuration) (rest of alkyl chain Ia > I-ddc, outside C,) edge length non-framework oxygen total number of OH- ions on the surface of zeoIite NaA number of Na* ions involved in incomplete ion exchange number of truncated.octahedra on edge length

259

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