Surface charges of wollastonite and xonotlite in aqueous solutions

Surface charges of wollastonite and xonotlite in aqueous

solutions

Citation for published version (APA):

Diemen, van, A. J. G., & Stein, H. N. (1977). Surface charges of wollastonite and xonotlite in aqueous solutions. Science of Ceramics, 9, 264-271.

Document status and date: Published: 01/01/1977

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IN AQUEOUS SOLUTIONS A.J.G. van Diemen and H.N.Stein

Adsorption data for wollastonite and xonotlite in aqueous solutions show, that there exist marked differences between these interfaces: ihe xonotlite sUl'face may be ordered, but the surface of the wollastonite investigated must exhibit some disorder. A surface gel layer is not formed.

Adsorptionsexperimente an Wollastonit und Xonotlit in wässerigen Lösungen zeigen, dass es deutliche Unterschiede gibt zwischen diesen Grenzflächen: die Xonotlitoberfläche kann geordnet sein, doch die Oberfläche des untersuchten Wollasto-nits muss eine gewisse Unordnung vorweisen.

Ein oberflächlicher Gelfilm wird nicht gebildet.

Des expériments d'adsorption SUl' wollastonite et xonotlite

dans des solutions aqueuses montrent, qu'il y a des diffé-rences prononcées entre ces surfaces: celle du xonotlite peut être réglée, mais celle du wollastonite étudié doit avoir un certain désordre. Un film gélatineux superficiel n'est pas formé.

INTRODUCTION

Interfaces between calcium silicate hydrates and aqueous electrolyte solution determine the behaviour of hydrated Portland cement and of sand lime bricks. ~t these interfaces, surface charges arise which may be either positive or negative depending on the electrolyte concentrations (1,2). The present investigation is concerned with the question, what processes are involved in originating these surface charges, and whether there are significant differences between an anhydr;us calcium silicate (wollastonite,

B

-CaSi0

3) and a calcium hydrate of equal Ca/Si atomie ratio (xonotIite, Ca

6Si 0 (OH». The

I_{atter pOlnt lS}" ₀f '_{lmportance Wlt}. h _{a Vlew fo the questlon,}. n ] 7 2 .

whether the adsorption characteristics can be correlated with the bulk crystal structure of the solids, or whether a surface gel layer is formed which has only a remote relationship with the bulk crystal structure of the solids.

Xonotlite and wollastonite were prepared as previously described (3), from CaCO ex Merck "zur Analyse" or CaC0

3 ex Merck "supra pur". If Ca20 "Zür Analyse" is used

(SCaSiO lIL, xonotlite IIIY the samples contain about 500 p.p.m:

~a.

Using CaC0

3 "supra pur" (wollastonite land Il, xonotlite land 11) the content is about 25 p.p.m. Af ter sun-thesis the compounds were ground in a porcelain mortar until the solid partieles were smaller then 250~. Before and af ter grinding the xonotlite was stored in a desiccator, above a saturated CaCI solution in water. Wollastonite was prepared by heating

xon~tlite

for 20 hrs at 9500C (3). X-ray diffrac-tion diagrams were in agreement wi th li terature data (4) •.. The surface areas as determined by means of an areameter (Stroh-lein) were: wollastonite I: 9.6m2g-l, wollastonite 11:1 1.7m2g- 1

2 -] . 2 -1

wollastonite 111: 10.3m g ,xonotllte I: 19.6m g

. 2 -I 2 -I .

xonotlite 11: 20.2m g ,xonotlite 111: 22.5m g • The sodlum content in the solid was determined by means of neutron acti-vation analysis. Neither in xonotlite nor in wollastonite samples separate (e.g. Na+ rich) solidphases could be cletected by SEM.

Methods

Adsorption measurements were carried out in 100 mI polyethylene vessels at constant NaOH concentrations in a nitrogen atmos-phere. 2 gxonotlite or 4 g wollastonite were suspended in 50 mI NaOH solution (0.01 M or 0.002 M) •. CaCI

2 solution (lM) and NaOH solution were added, the lat ter ln the amount necessary

to keep the pH constant during the adsorption as determined by a preceding "dummy" experiment with an automatic titrator (TTT2 Radiometer). The vessel was shaken during 150 min. Af ter shaking the greater part of the suspension was centrifuged (16000 r/min) during 1~ hour. The clear liquid was decanted. In this liquid the concentration of all ions, possibly invol-ved in originating the surface charge, were determined. The concentrations of these ions were also determined in an equal

expe~iment but without the solid. The adsorption or desorp~ion

was calculated from the difference between both concentratlons. Ca2+ was determined spectrophotometrically (5). Na+ and OH-were determined with a specific ion electrode (ORLON); the equivalence point was calculated by the method of Gran (6); OH titration was performed until a pH of 3. Silicate was determi-ned spectrophotometrically (7). With the rest of the suspension

~-potentialmeasurements were performed using an electroosmosis appara tus (2).

(I)

ionic strength) (11)

d~

(I

d<p/dx

A.J.G. van Diemen and H.N.Stein

N~₁

elementary charge

concentration of spftcies i in the bulk liquid

valance of ion i including sign

/ï

- logy Ca

=

4 A [ - - - - 0,2 IJ +/Ï Z. 1 0. 1 where

N':"

1

The correct ion for CaOH+ formatian was carried out by an interative procedure; first m

CaOH+ was calculated from : Ca mCa YOH mOH _{with - log K = J.221 + 2.802 I} 10) (₁₁₁)

Y_{CaOH mCaOH}

K

On the horizontal axis ln fig. 1, log YCamCa is plotted. Here m

Ca is the concentration of free calcium ions (corrected for CaOH+ farmation) and the activity coefficient Y

Ca is calcula-ted from (9):

using uncorrected values for m

Ca and mOH; this was employed ta calculate a carrected I etc.

+

In fig. 1 the adsorption isotherm for Na is ami tted since 0Na+ was very small. With increasing CaC1

2 concentration, bath

2+ d " . 1 l ' 1

Ca and OH-a sorptlon lncrease Slmu taneous y ln near y stoechiometric ratio as observed previously by Siskens c. s. (11)

for a-CaSiO). At large [CaC1

2], DCa and GOH both level off at

lol~0.7

C.m-Z. Essentially the same is found for xonotlite I. With wollastonite land 11, a similar picture is found.

However, both DCa and GOH increase less with increasing

[CaCl

J;

the maximum adsorption is not reached within the

con-ce~trition

range investigated. For samples containing conside-rable amounts of Na (wollastonite 111, xonotlite 111) 0c and 0ÖH increase stronger with increasing [CaCl

Z] ~han with a samples poor in Na. A Na+ desorption is faund increasing with

Z + . +

[CaC1

2]. This indicates exchange of Ca agalnst 2 Na Desorption of silicate is more pronounced than for samples poor in Na+. Desorption Na+ amounts to about ,50% of all Na+

I

r \

I

r ,:

T

L---'---'--'-LLl--'..l.L_-'---'-L.L.L.L..u..i 10-3 10- 2 -+ log YCam_Ca Figure 2

and GOH for different NaOH concentrations.

DCa' O.OI~M NaOH GOH' 0.01 M NaOH DCa' 0.002 M NaOH

°

OH' 0.002 M:MaOH

o

-0.4

a c b d

l

as "surface charge": 0. (in

coulomb-_I 1

with F

=

Faraday (C.mol ), V= volume of 10-3

-+ log YCam_Ca Figure I

Typical adsorption and electrokinetic data for

xonotlite 11. )(: DCa' 8.:osilicate' 0: GOH' .:ç-potential ç(mV) (I:/lll ) t ()

.

~~, -;.

zo

0 -20 G.,l RESULTS

A.J.G. van Diemen and H.N.Stein

:ig. 1 ~hows adsorption isotherms for xonotlite 11, for those lons ~hlCh play ~ role in the origin of the surf ace charge. Negatlve adsorptlon = desorption. "OH- adsorption" consists of the OH- adsorption proper, dissociation of surf ace silan~l groups and silicate adsorption, since the titrationof OH- was continued to a pH

=

3. osilicate was calculated from the

sili-c~te.concentrationin the solution af ter solid/

llqUld contact, awarding a charge -Ze

O to every silicate ion.

Adsorption was calculated -2

m )=z.F.V.~c./(n.s)

1 1

the solution

(I),

~c = difference in concentrations of species

i before and af ter solid/liquid contact (mol.

I-I),

TI

=

amount

of solid (g), s

=

surf ace area of the solid (m2.g-I ). The sur-face charges for calcium and hydroxyl ions were corrected for

th~ electrostatîc charge in the diffuse double I

This formula is based on : (VII) (VIII) Z Z exp(-(u-f) /(Zw » N s w/2ii 6G~/RT, th~ mean value of u its standard deviation, dN. 1 du w where u f

(~*

(solution) -

~* (a~s)

- ZF8.)/RT is assumed. Thus,for a

Ca Ca 1

certain concentration (YCamCa= 0.001 M), INiei/N_s was calculated from DCa' For a Gauss distribution of the sites:

IN.e./N as calculated from DCa must be equal t~: 1 1 S

J

-OO Y m

C exp. u Z Z

Ca a exp(-(u-f) /(2w » du

wl2ri +00 (I +yCamCa exp. u) , since, 8.

=

(y m exp.u)/(I+y

_c

m

C exp.u).

1 Ca Ca a a

For any particular value of w, f was adjusted such as to make Ie.N./N calculated from (VIII) agree with that calculated

1 1 S

from (VI). The value, of f was then used, to calculate

N 8 (1-8 )

J-oo

Y m exp.u Z Z

'i' i i i _I_' ' Ca Ca exp(-(u-f) /(2w »du

i. N (I+y

c mC exp.u)2

s

w/2iï

+00 a a (IX)

Then 0$/0

ln

YCamCa can be calculated from (IV). This was performed both for calcium and for hydroxyl sites.

Fig. 3 shows the results for wollastonite lIj wollastonite I gave similar results. It is seen, that at w

=

0 (a so-called single type site adsorption model) la~/alnYcamcal for both Ca2+ and OH- sites is smaller than laç/alnYcamcal j the same obtains for any kind of average value. Similar result~ ar~

calculated with the square distribution. The electroklnetlc slipping plane, however, cannot be situated ~lose: to the phase boundary than the plane of the adsorptlon sltes. There-for

I

aç/alny m

I

must be <Id~/dlny m

C

I

averaged over the

. Ca Ca Ca a

plane of the adsorption sites. (IV)

(V)

(VI) and xonotlite the surface

-2

0.7 and + 0.7 C.m (see averaged in a certain way over the calcium sites.

O 7 N.

e.

(J

-e .)

..,....,;:-:-_'t'.I...-_] •

I

1 Nl 1

o

ln

YCamCa s

o

~. 1 2 F

=

[I - - . R T

derived the relation:

total amount of Ca2+ ions adsorbed,

total number of Ca2+ adsorption sites, per m2 of N

s

Fig. 2 shows that, for increasing NaOH concentrations at constant [CaC1 2], both DCa and GOH increase. This agtees with the "stimulated adsorption" described by Siskens c.s. (11) for a-CaSi0

3·

enriched in Na+.

surface area,

Ni number of Ca2+ adsorption sites of type i, ei degree of occupation of sites of type i,

~.₁ potential at a site of type i ; Siskens c.s.

o(X/N )

s

DISCUSSION

The levelling of DCa and GOH at large [CaCl

Z] seen in fig. 1, contradicts a "surface precipitation" as postulated by James and Healy (12). Thus, the model of stimulated adsorption of

2+

-Ca and OH (11) appears to be more appropriate in the pre-sent case.

Here, X

If we assume, that for both S-CaSi0 3 charge can vary between the limits _ crystal structure data (13», then:

IN.

e./N

=

(0 d + 0.7)/1.4

1. 1 S a s

can be calculated for all adsorption points. From this,

A.J.G. van Diemen and H.N.Stein A.J.G. van Diemen and H.N.Stein 60 REFERENCES t t (: 40

I) STEIN, H.N.,J.Col1. Sci. j2,578 (1960). 2) STEIN, H.N. ,J.Coll. Int. Sei. 28,203 (1968).

3) SMIT, W. and STEIN H.N.,J.Coll. Int. Sei. ~,208 (1976).

4)

ASTM X-ray Powder Data File, Nr. 23-125 and 19-249. 5) SMIT, W. and STEIN, H.N.,Anal. Chim. Acta 83,297 (1976). 6) GRAN, G., The Analyst 22,661 (1952).

7) LANGE J. Silikatteeh. 20, 167 (1969).

8) AVEYAAD ~. and HAYDON, ~A., "An introduction to the principies of surfaee chemistry",p. 4Z, University Press, 1973.

9) DAVIES, C.A., J. Chem. Soc. 2093 (1938).

la) HOPKINS, H.P. and WULFF, C.A., J. Phys. Chem ~ (I),

6-8(1965).

11) SISKENS, C..A.M., STEIN, H.N. and STEVELS, J.M., J.Coll. Int. Sci. 52, 251 (1975).

12) JAMES, R.O-.-and HEALY, T.W., J. Coll. Int. Sci. 40, 53, 55 (1972).

13) DENT, L.S. and TAYLOR, H.F.W., Aeta Cryst. ~, 1002 (1956). 5 Figure 4 A5 figure 3, for xonotlite 11 c

o

o

20 40

-zo

++ Ca sites, OH sites, a + 2/3 b.

zo

o

-20

r

I 0~----'---'---'---L----'-~5 Fig\,lre 3

o~/otnyc_am

_c

_a for wollastonite 11.

~

Gauss distribution for adsorption sites assumed. w as independent variable.

a for b for

c 1/3

Again, a distinct difference is found between the situations at interfaces 8-CaSi0

3

1

electrolyte solution and

Ca6Si6017(OH)21 electrolyte solution, respectively. This indi-cates that a surface gel layer is not formed under the condi-tions of our experiments.

CONCLUSIONS

A marked difference exists between interfaces wollastonitel electrolyte solution and xonotlite/electrolyte solution. At wollastonite surfaces as prepared in the present investigation some disorder must exist.

If the solid contains Na, then the surf ace layer is enriched in Na towards the bulk.