The dependence of zeta potential on pH of an aqueous Si carbide suspension was studied by an electroosmosis method. The effect of pH on the Si carbide electrical double layer is attributed to the participation of H and OH ions in the surface dissociation reactions of weak acid OH groups on an oxidized carbide surface. Maximum stability of the suspensions was observed at maximum negative zeta potentials

Eremenko, B. V.

Kolesnik, V. O.

English

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NASA TECHNICAL MENORANDUM 
(blASA-TPi-77 49 2) 6LBCTilGKli NB TIC POTBIY3XAL N85-12984 
BPJD S T B B U I T X  OP Sic SUSEBBSICHS IN 
ELBCTROLY TE SOLDT XI IS (Na t i c r  n a1 Aerouau tios 
and Space Administration) 8 p ffc AO2/HP A01 Unclas 
CSCL Q7D 63/25 24498 
ELECTROKINETIC POTENTIAL AND STABILITY O F  
S i c  SUSPENSIONS IN ELECTROLYTE SOLUTIONS 
B.V. Eremenko and V.0, Kolesnik 
Trans la t ion  of “ E l e k t r o k i n e t i c h i n i y  P o t e n t s i a l  
t a  S t i y k i s t  I Suspenziy  Karbidu Kremniyu U ROZ- 
ch inakh E l e k t r o l i t i v ” , V i s n i k  Ki ivs’koho Univer- 
s i t e t u ,  S e r i y a  Rhimii ,  No. 13, 1972, pp. 53-56. 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASBE NGTON D C . 28546 J U L Y  1 9  8 1  
https://ntrs.nasa.gov/search.jsp?R=19850004676 2020-03-20T20:06:36+00:00Z
iC SUSPZNSIOk3 IN BLECTRO- , 
B.V. Eremenko and V . 0 .  Kolesnik 
- _ _  1 
111 L I I ~ ~ ~  
Translat ion of " ~ l e k t r o k i n e t i c h i n i y  Po ten t s i a l  ta  S t i y k i s t  Sus- 
penziy Karbidu Kremniyu U Rozchinakh E lek t ro l i t i v '  , Visn ik  Kiivs' 
koho Ucrversitetu,  Ser iya Khimii, No.  13, 1972, pp. 53-56. 
The attpendence of zeta  po ten t i a l  on pH of a n  aq. S i  carbide 
suspension was s tudied by an  electroosmosis method. The e f f e c t  
of pS on the  S i  carbide e l e c  
pa r t i c ipa t ion  of I I  and OH ions  i n  t h e  sur face  dissocn. reac t ions  
of  weak ac id  OH groups on an oxidized carbide surface.  Nax 
s t a b i l i t y  of t h e  suspensions w a s  obsd. a t  m x .  neg. ze ta  
poten t ia l s .  
double layer  is a t t r i b u t e d  t o  the  
ELECTROKINETIC POTENTIAL AND STABILITY OF Sic SUSPENSIONS 
B.V. Eremenko and V.O. Kolesnik 
IN ELECTROLYTE SOLUTIONS /53* 
summary 
Zeta potential of Sic in solutions oE XC1, 
NaOH, Na2Si03 and Na2C03 determinied by the electro- 
osmosis method is compared wikh the stability of 
suspensions of Sic. Results are discussed from the 
point of view of taking part of the H+ and OH- ions 
in dissociation reactions of the surface of weak acid 
OH groups. 
The ina5ility to use slip casting as a method of 
forming products made of refractory compounds of the 
carbide, boride, nitride and silicide types is primarily 
associated with the difficulties arishg in the preparation 
of stable suspensions of these compounds [31.  Up to now 
the causes conditioning the stability of these suspensions 
remain unknown. 
Deryagin's physical theory of stability [21 is con- 
structed on investigations of the balance between the 
farces of attraction and repulsion of separate particles 
in the dispersion phase of the system. 
theory, the forces of repulsion are of an electrical nature, 
and in a first approximation they can be evaluated according 
to their electrokinetic potential. 
According to this 
This report presents the results found in research on 
the effect of the electrolytes of HCl, NaOH,  PJa2Si03 and 
N a p 3  en the zeta potential and the stability of suspensions 
of silicon carbide, which are used more and more commonly in 
technology e5 I . 
* N u m b e r s  in margins refer to foreign pagination 
We studied the silicon carbide both in the industrial 
form and as admixtures treated with hydrochhoric acid and 
washed until the rinsed liquid had a stable electrical 
conductivity. We measured the zeta potential with the 
electroosmosis method [41. We evaluated the stability of 
the suspensions according to the optical density of the centri- 
fuged suspension. The research results can be seen in 
Figures 1-3. h 
Figure 1. Dependence of the zeta potential and 
stability of a suspension of technical silicon 
carbide on the pH. 
Figure 2. Dependence of the zeta potential and 
stability of a suspension of refined silicon 
carbide on the pH with additives of HCl, NaOH (1, 4 1 ,  
Na2co3 12) and Na2C03 ( 3 )  . 
2 
Figure 3. The dependence of the 
and stability of a suspension of 
carbide on the concentrations of 
(a) and sodium carbonates (b). 
zeta potential 
refined silicon 
sodium silicates 
Suspensions of silicon carbide in distilled water are 
characterized by small quantities of negative zeta potential. 
The zeta potential of refined material (-39mw) is higher in 
absolute value than that of the technical product (-18mw). 
This is certainly associated with the presence in the 
industrial product of admixtures of iron, which were 
revealed by chemical analysis methods. Iron oxides in 
water have a positive surface charge [81, and their 
presence masks the negative charge of the surface of the 
silicon carbide [61. With the addition of alkali, the most 
negative zeta potentials are observed at pHa11.2 for tech- 
nical and pH"11.4 for refined specimens. A further increase 
in the alkali concentration in the supply system sharply 
reduces the zeta potential of the carbide. 
The addition of hydrochloric acid to the suspension 
reduces the negative zeta potential. The existence of an 
iron admixture in the technical product greatly affects the 
results of measurements of the zeta potential in the acidic 
environment. This effect is based on the contact time 
bewteen the suspension and the acid solution. 
time is short (up to five minutes), the pH ratio of the zeta 
If the contact 
3 
potential is almost the same as for pure carbide, while 
increasing the time shifts the curve C = f(pH) for the 
technical product to a higher alkali zone. This shift 
embraces almost 4 pH units with contact longer than 24 hours. 
These results are not shown in Figure 1, since those measure- /54 
rnents were intended only for orientation. The zeta potential 
dependence on the pH renders it possible to determine the 
isoelectrical point, which is pW3.2 for refined silicon 
carbide. 
In adding sodium carbonates and silicates to a suspension 
of refined silicon carbide, the negative zeta potential 
(Figure 3) increases and reaches the highest valaes in an 
electrolyte concentration of S-10-3n. The effect of these 
electrolytes on the electrical double layer of carbide may 
be produced by the following causes. First of all, the 
growth in the zeta potential with an increase in the 
concentration of these salts can be connected with an in- 
crease in the electrothermodynamic potential of the silicon 
carb,de surface in the calculation of the possible (by the 
Panet-Fayance rule1 potential occasioned by the aclsorption 
of ions of silicate and carbonate. Secondly, the h-irolysis 
of these salts in an aqueous solution cannot help raising the 
alkalinity of the medium, vhich, as the data from Figure 2 
confirm, leads to an increase in the negative zeta potential. 
As can be seen from Figure 2, the dependence of the zeta 
potential on the pH in rhe presence of sodium silicate is 
almost the same as in the presence of caustic soda. The 
maxiumum zeta potential is found at pH-11.3. For sodium 
carbonate, the maximum zeta potential is observed at pH-7.8, 
at which an NaOH solution still does not provoke noticeable 
changes in the zeta potential. 
In this way it is possible to assume that the action of 
the sodium silicate is conditioned only by hydrolysis; for the 
carbonate a potential conditioned by anion adsorption prevails. 
4 
In general the stability of sodium carbide suspensions 
is not high. 
justifies the observation that, just as for titanium 
carbide [ 4 1 ,  the major role in the mechanism of stabilizing 
the silicon carbide.suspension in aqueous solutions is 
played by the electrical factor. 
In all solutions the maximal stability correlates 
with the greatest negative zeta potential values. This /5 5 
In this way it is possible to draw a conclusion 
regarding the surface effect of the OH- and Hc ions 
on the double electrical layer of the silicon carbide surface 
in aqueous solutions. In analogy with some other carbides 111, 
it can be observed that the silicon carbide surface is coated 
with a film of oxide, which our data can explain from the 
standpoint of the pH effect on the acidic dissociation of 
these molecular groups, raising the negative charge on the 
surface, revealed in the increase in the negative values 
of the zeta potential. Acidification of the solution 
retards the dissociation of the OH groups, reducing the 
negative charge and the zeta potential of the surface. 
Such an explanation is also credible because of the 
fact that the isoelectrical point of the silicon carbide 
(pH = 3.2) correlates quite well with the zero charge 
(pH = 3.6) and the isoelectrical point. (pH = 3.7) 171 of 
roughly an9 highly dispersed quartz. 
5 
References 
1. Avgustinnik, A.I., Drozdetskaya, G.V. and Ordan'yan, 
S.S., Poroshkovaya metallurgiya, N o .  6, 53, 1967. 
2. Deryagin, B.V., Kolloidi Zh., 7, 285, 1941; Works of the 
Third All-Union Conference on Colloid Chemistry. Moscow, 
USSR Academy of Sciences, 1956, p. 225. 
3 .  Dobrovol'skiy, A.C., "Slip Casting," Metallurgizdat, 
Moscow, 19 6 7. 
4. Eremenko, B.V., Lyubchenko, I . N .  and USKOV, I.A., "Physico- 
chemical Mechanics and the Lyophilic Nature of Dispersed 
Systems," Vol. 3, 1971. 
5, Kosolapova, T. Ya., "Carbides," Metallurgiz' Moscow, 1968. 
6. Chernoberezhskiy, Yu. M., Kuleshova, L.N., adi Kolesnikova, 
L.f.,  Vestnik Leningradskogo Unta., No. 10, 114, 1969. 
7. Ahmed, S.M., Can. J. Chem. 44, 1663, 1966. 
8 .  Parks, G.A. and de 3ruyn, P . L . ,  J. Pliiys. Chem., 66, 967, 
1962. 
6 


Electrokinetic potential and stability of SiC suspensions in electrolyte solutions

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