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Boundary layer and the stability of hydrophilic dispersions
Colloids and Surfaces A: Physicochemical and Engineering Aspects 149 (1999) 171–178
Boundary layer and the stability of hydrophilic dispersions V.N. Moraru *, F.D. Ovcharenko, D.V. Moraru Institute of Biocolloidal Chemistry, National Academy of Sciences of Ukraine, 42 Vernadsky blvd, 252142 Kyiv, Ukraine Received 26 August 1997; accepted 29 July 1998
Abstract This article presents the results of an experimental study of the influence of different factors on the structure and extent of the hydration boundary layers (BL) of oxidised synthetic diamond (OSD) and oxidised graphite (OG). The experimental technique allows changes in the hydrodynamically immobile BL thickness to be recorded by means of an electrokinetic method. It also provides independent information about the surface charge (s), f potential and sol stability over a wide range of pH and concentration of the inert electrolyte ( KCl ). To check the correctness and the reliability of the technique we studied the influence of the four different factors on the BL thickness: (1) hydrophilization of the graphite and diamond surface (by means of an increase in the degree of oxidation); (2) preliminary thermotreatment of the OSD micropowder in an inert gas flow or in a vacuum; (3) introduction into aqueous dispersions of OSD and OG of the non-ionogenic compounds, promoting the destruction of the BL structure; (4) influence of temperature on the electrophoretic mobility and the dispersion stability. On the basis of the influence of each of these factors on the BL, it is established that the sign of the f potential change correlates strictly with the direction of the shift in the slip-plane boundary, supporting the basic concept of the method. On the basis of the dependences obtained, f(C ) for constant s(pH ), and f(s) for constant ionic strength, we el estimated the thickness h of the hydrodynamically immobile hydration layer on the surfaces of OSD and OG, applying the diffuse double electric layer theory. Using the different approaches we found that h varied in the range h=1.0 –3.0 nm. The values obtained for h on average correspond to the BL thickness of 5–7 molecular water layers. These results are in quantitative agreement with experimental data on the stability of the dispersions under consideration and with the results of a calculation of the pair interaction potential curves for OSD and OG particles in the framework of the generalised DLVO theory taking into account the structural forces. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Hydration boundary layer thickness; Oxidised graphite and diamond dispersions; Hydrophilicity; Electrosurface properties; Stability
1. Introduction The development of methods for the quantitative study of the boundary layer (BL) is important in the context of the theory of the structural forces and stability of hydrophilic colloids [1]. One way * Corresponding author. Fax: +380 44 444 8078; e-mail: [email protected]
to obtain information about the BL consists in a complex study of the electrosurface phenomena in model systems satisfying the conditions of quantitative interpretation of the f potential: absence of a gel layer; small surface roughness; the possibility to change the surface hydrophilicity and charge and of their quantitative determination; absence of pores [2]. If one assumes the hydrodynamic immobility of
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the BL (or, at least, of that part of it which is subjected to the maximal influence of the surface forces field ), then under certain conditions (moderate ionic strength j, low specific adsorption of ions) one can expect considerable electrokinetic effects due to destruction or formation of the BL. Therefore, it is interesting to trace how the BL manifests itself in the electrokinetic phenomena and in the hydrophilic dispersion stability under the influence of the different factors. The aim of this work is a quantitative test of the ‘‘slip plane’’ concept, and the consequent estimation of the width of the hydrodynamically immobile BL of water and the contribution of the BL to the dispersion stability. Aggregatively stable aqueous dispersions of oxidised synthetic diamond (OSD) and oxidised graphite (OG) were chosen as model systems. Interest in dispersions of OSD and OG arise because they are good models for the study of electrosurface phenomena on passing from lyophobic to lyophilic colloids and vice versa. The essence of our approach consists in influencing the dispersion by any factor that changes the structure and the extent of the BL, subsequently recording the given effect by the electrokinetic method and obtaining independent information about the surface charge and the sol stability over wide ranges of pH and concentration of inert electrolyte ( KCl ) To check the correctness and reliability of the method we decided to study the influence of two groups of factors on the BL, by means of affecting the surface chemistry and the neighbouring liquid: (1) hydrophilization (increase in the degree of oxidation) of the graphite and diamond surface; (2) preliminary thermotreatment of the OSD micropowder in an inert gas flow or in vacuum; (3) introduction of a saccharose destroying the hydration BL into aqueous dispersions of OSD and OG; (4) influence of the temperature on electrophoresis mobility and dispersion stability. For constant surface charge and ionic strength of the inert electrolyte background solution, the slip-plane coordinate (h) can be determined by means of the well-known equation of the theory
of the diffuse double electric layer (DEL) structure [3]: th[zef/4kT ]=th[zey /4kT ] exp(−xh) d
(1)
where z is the valence of the counterions, e is the elementary charge, x is the value of the inverse Debye screening radius, f is the slip boundary potential, y is the Stern layer potential, k is the d Boltzmann constant and T is the absolute temperature. Logarithmic presentation of Eq. (1) takes form ln[th(zef/4kT )]=ln[th(zey /4kT )]−xh d
(2)
The dependence of ln[th(zef/4kT )] on x is linear, with h being the tangent of its slope angle. Thus, substituting in Eq. (2) the experimental dependences of the f potential of OG and OSD on inert electrolyte concentration expressed through x we obtain a number of straight lines, the tangents of whose slope angles allow the distance to the slip plane (h) to be determined even if the y potential d value is unknown. Moreover, from the intercept of each line on the ordinate axis it is also possible to calculate the corresponding y value. d The second possibility to determine h consists in the following. In the case of initial graphite and diamond whose surface is only slightly oxidised, no continuous BL is formed and the slip boundary occurs near the surface. Therefore, the f potential measured at low electrolyte concentrations is approximately equal to the y potential. Possessing d now a number of the f potential values for the oxidised graphite and diamond samples (at s= const and j=const) we can calculate from Eq. (1) the slip plane shift (h) as a result of the BL formation. The distinctive feature of the applied approach for BL detection is the use of the ‘‘sparing’’ method of microelectrophoresis at low electric field strengths (E#5–6 V cm−1) and the shear rates. It is easy to show that the maximum value of the shear rate gradient c˙ =∂v/∂x will be reached at the distance from the particle centre equal x =앀6r o and its order of magnitude will be #0.1v /r (where o v is the particle velocity, and r is the particle o radius), i.e. about 1–2 s−1. At such low shear rates, one can assume that the water layers nearest to
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the surface are hydrodynamically immobile and that the slip boundary is remote from the surface at some distance depending on the BL thickness.
of the conductometric and potentiometric titration of the H+ form with a 0.1 M solution of KOH in a 0.2 M background solution of KCl. To calculate s we used the formula s=C
2. Experimental The micropowder of synthetic diamond of the ACM-1 trade mark 0.3/0 (GOST 9206-80) used in this work had a narrow maximum in the particle size distribution in the region of the 0.3–0.5 mm and a specific BET surface area of 23.0 m2 g−1. During the dispersing and subsequent chemical purification from admixtures in strong oxidizing media (conc. H SO , HNO , CrO , K Cr O and 2 4 3 3 2 2 7 others) the diamond surface is oxidized and becomes hydrophilic. In order to transform the surface ionogen groups to the H+ form, the OSD micropowder was treated several times with 1.0 M HCl solution, and then washed with bidistillate to a constant pH value of 4.0. The characteristics of OSD are given in Table 1. Natural graphite of high purity (99.8%) of mark C-O, with the particle size 0.2–2.0 mm and specific surface area 20 m2 g−1, was subjected to different degrees of the oxidation by means of the concentrated H SO and HNO treatment in the presence 2 4 3 of a catalyser, after which it was washed off with a 1 M solution HCL and finally with bidistillate to a pH of 3.5–4.0. Table 1 gives the characteristics of the oxidised graphite obtained. The determination the surface charge density (s) of OG and OSD was carried out by the methods
F/S OH–H where C is the adsorption of the potentialOH–H determining ions, F is the Faraday number, and S is the specific surface. The values of S were determined by the adsorption of dye (methylene blue) from the water solution. The electrokinetic potential (f) of the dispersions was determined by the microelectrophoresis method according to the procedure described earlier in Ref. [4]. In order not to introduce the corrections for the polarisation of the DEL in the Helmholtz–Smoluchowsky formula for f f=4pgv /eE ef where v is the electrophoresis velocity, g and e ef are the medium viscosity and dielectric constant, respectively, and E is the field strength, we chose the experimental parameters which satisfied the condition xr≥100 (were x is the value inverse to the DEL thickness, and r is the particle radius) and Rel#[exp(ef/2kT )−1]/xr<1 where Rel is the polarisation criterion [2]. The coagulation thresholds (CT ) and stability were determined from the dependences of the optical density (D) of the OSD and OG suspensions on the electrolyte concentration. The specific heat
Table 1 The potentiometric surface charge density (s) and specific heat of wetting (q ) of OG and OSD, the optical density (D) and f-potential s of their aqueous suspensions, as well as the calculated thickness (h) of the boundary layer for the objects investigated Object
s (C m−2) at pH 7.0
q (J m−2) s
D at pH 6.0
f (mV ) at pH 6.0
h (nm)
Natural graphite C-0 OG-1 OG-2 OG-3 OG-4 OSD OSD thermally treated at 180°C
0.08 0.30 0.68 1.00 1.20 0.29 0.15
0.06 0.14 0.30 0.41 0.46 0.15 0.11
0.05 0.27 0.46 0.80 0.95 0.90 a 0.40 a
72 58 51 43 36 28 a 41 a
0.0 0.9 1.6 2.5 3.2 2.1 0.6
aAt pH 5.0.
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of wetting of the OG and OSD micropowders (q ), quantitatively characterising the surface s hydrophilicity, was determined using a DAK-1M microcalorimeter.
3. Results and discussion 3.1. The influence of the graphite surface oxidation on the stability and electrosurface properties on the graphite aqueous dispersions The data of Table 1 and Fig. 1 show that the oxidation of graphite is accompanied by increases in the surface charge, the specific heat of wetting and the optical suspension density. However, consideration of the dependences f=f (pH ) and j=const f=f (C ) for the series of the OG samples KCl pH=const with different degrees of oxidation shows that, in spite of the growth of s, these dependences are at lower levels, the higher are the degree of oxidation and the graphite hydrophilicity. While interpreting these data it is necessary to take into account the influence of two opposing factors: the increase of the f potential at the expense of the charge growth on oxidation, and the decrease of f due to the thickening of the BL and the moving of the slip boundary away from the surface. Since the degree of the mutual compen-
Fig. 1. The surface charge density determined by conductometric method (curve 1), specific heat of wetting of graphite (curve 2), optical density (curve 3) and f-potential (curve 4) of graphite aqueous dispersions as functions of the content of oxygencontaining ionogenic groups (C ) on the graphite surface. ox
sation of the factors mentioned is not open to quantitative analysis, to exclude the charge influence we have plotted the dependence f= f(s) for the same series of OG samples j=const ( Fig. 2). It can be seen that at low charge densities, when the potential within the DEL changes relatively slowly, the f(s) curves for all OG samples, including the natural sample, are close to each other or coincide. This fact means either that the BL is weakly developed and the slip plane passes near the surface, or that the displacement of the slip boundary weakly affects the f potential. This result agrees with the data of Haydon [5], and Davies and Rideal [6 ], who established that at s<0.1 C m−2 the slip plane, in the emulsions they studied, coincided with the plane of the most proximity of the charged groups on the hydrocarbon drop surface, as f#y , where y is the d d Stern potential. At higher values of the charge densities on the OG particle surface a continuous BL is formed, which strongly manifests itself in the electrokinetic phenomena owing to which the f(s) curves diverge: they are located at lower levels the thicker is the BL (Fig. 2). Using Eq. (1), we calculated the shift of the slip plane (h) as a result of the surface oxidation and hydrophilization for every OG sample, with
Fig. 2. The relationship between the experimentally determined f-potential and the surface charge density (s) for different degrees of graphite oxidation C ×10−6 mol m−2: 0.9 (curve ox 1), 3.1 (curve 2), 5.4 (curve 3), 7.25 (curve 4) and 10.5 (curve 5) (ionic strength j=5×10−3 M KCl ).
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the f potential of the initial natural graphite taken as 75 mV assumed equal to the Stern layer potential. At C =5×10−3 mol dm−3 KCl (z=1, x=2.32×108 m−1) and s=0.3 C m−2 we obtained the h value, which for different OG varied within 1–3 nm (Table 1). From Fig. 3 one can see that dependences of the f potential for the initial and oxidised graphites on the KCl concentration represented in the coordinates of Eq. (2) are straight lines whose slope angles increase with increasing degree of oxidation and graphite hydrophilicity. The h values for OG obtained on this basis are within the same 1–3 nm limits. Such limits to the change in water BL thickness on the hydrate object surface seems to us quite real. Moreover, as follows from the calculations given below for the pair interaction energy of the OG and OSD particles according to the generalised DLVO theory [1], the structural forces are important only at the distances ≤5.0 nm (doubled BL thickness). At the same time the fact that at equal s values the BL thickness differs considerably for OG with various degrees of oxidation may cause surprise. In explaining such a divergence one should take into consideration that during the graphite oxidation in addition to iongenic groups the non-
Fig. 3. The dependences of ln[th(zef/4kT )] on x for the graphite with different degrees of oxidation C ×10−6 mol m−2: 0.9 ox (curve 1), 3.1 (curve 2), 5.4 (curve 3), 7.2 (curve 4) and 8.6 (curve 5) ( j=5×10−3).
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iongenic groups whose content increases in proportion to the degree of oxidation are formed. Although these groups make no contribution to the total surface charge they are, nevertheless, capable of strong hydration and structuring of the surface water. 3.2. The influence of the preliminary thermotreatment of the OSD micropowder Thermotreatment of the OSD in flowing helium at T=453 K (3 h) resulted in a decrease in the number of surface groups and in the titrated charge as well as a decrease in the stability of its dispersions [7] (see also Table 1). However, in spite of the decrease of s, the dependence f=f (s) (Fig. 4) obtained by recalculation of the f(pH ) and s(pH ) dependences and the dependence f=f (C ) ( Fig. 5) in the case of KCl calcinated OSD both shift to the higher potentials. For other s, the observed growth of the OSD f potential after calcination over wide range of pH and C can be explained only by partial destrucKCl tion of the BL and the approach of the slip boundary to the surface. Using Eq. (1) we calculated the thickness of the hydrodynamically immobile BL at different s and C values (Figs. 4 and 5). As can be seen, the h KCl values obtained for the OSD vary within the reasonable limits 0.6–2.1 nm. This means that the
Fig. 4. The relationship between the f-potential and the surface charge density (s) for initial (curve 1) and thermally treated OSD (curve 2) and the thickness of boundary layer h (curve 3) calculated from Eq. (1) as a function of s.
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particles (H ), we used the following formulae:
Fig. 5. The dependences of the f-potential for initial (curve 1) and thermally treated OSD (curve 2) and the thickness of boundary layer h (curve 3), calculated on the basis of Eq. (1), on KCl concentration at pH 6.0.
OSD calcination resulted in thinning of the BL to maximum of seven water molecular layers that gave rise to the decrease in the dispersion stability. One more important conclusion follows from Fig. 5: the increase in the electrolyte concentration destroys the BL and, at C #0.1 M, h is equal to KCl the thickness of one or two molecular layers of water on the OSD surface. The non-monotonic character of the h(s) dependence observed on diamond ( Fig. 4) can be associated with the stronger specific adsorption of the K+ counterions. So, excessive growth of the OSD charge (s>0.3 C m−2) probably involves very high fillings of the Stern layer by counterions so Eq. (1) is no longer applicable for the calculation of h. The presence of a BL on the initial OSD and its practical absence on the calcinated sample undoubtedly must be reflected in the character of the potential curves of the pair interaction of particles of the corresponding dispersions, calculated according to the DLVO theory and its generalised version taking into account the structural forces [1]. To calculate energy of the electrostatic (U ), molecular (U ) and structural (U ) forces as i m s functions of the distance between the diamond
U (H )=2pee ry2 ln[1+exp(−xH )] (3) i o d U (H )=−Ar/12H (4) m U (H )=prl2K exp(−H/l ) (5) s were y #f, the Hamaker constant A= d 0.78×10−19 J, r=0.3 mm, e =8.85×10−12 F m−1; 0 empirical parameters K=5×106 N m−2 and l= 1×10−9 m [1]. Fig. 6 shows the total potential interaction curves U(H )/kT obtained for a KCl concentration of 0.02 mol dm−3 (the coagulation threshold of the OSD dispersion). One can see that the generalised DLVO theory correctly predicts the disappearance of the energetic barrier and the threshold of the rapid coagulation in the case of the initial OSD dispersion, whereas for the calcinated OSD sol, where the BL is absent, this theory predicts a high system stability (U #100 kT ) that contramax dicts the experimental data [7]. In this case the stability is better described within the framework of the usual DLVO theory. Finally, the fact that the point of the potential minimum in the U(H ) curves corresponds to the doubled BL thickness, in our opinion, is not accidental and indirectly confirms our estimations for h. Thus, a good agreement of the DLVO theory with the experiment is found.
Fig. 6. The total potential curves of the pair particle interaction for initial (curves 1, 1∞) and thermally treated OSD (curves 2, 2∞) calculated according to the DLVO theory (curves 1∞, 2∞) and taking into account the structural forces (curves 1, 2); ionic strength j=0.02 M KCl.
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3.3. The influence of saccharose on stability and electrosurface properties of OSD–aqueous dispersions As follows from Fig. 7, on increasing the saccharose concentration from zero to 0.25% the f potential of OSD increases from 29.5 to 40 mV, whereas the surface charge density (s) and the dispersion optical density (D) do not change. Starting from C >0.25%, the f potential, the surface charge sac density and the optical density decrease synchronously, resulting in the complete coagulation and clarification of the OSD suspension at C #1–2%. To our knowledge, this is the first sac report of an increase in f due to introduction of saccharose in a dispersion. Joint analysis of the dependences f(C ) and s(C ) leads to the conclusac sac sion that the observed initial increase in f should be accounted for only by the destruction of the BL and shift of the slip boundary closer to the surface. This is also confirmed by the f=f (pH ) dependence at fixed saccharose (0.25%) and background electrolyte concentrations ( Fig. 8), which over all the pH region shifts to higher potentials as compared with the analogous dependence at C =0. The shift of the slip plane at different pH sac values (Fig. 8) calculated from these dependences according to Eq. (1) shows that the thickness of the destroyed BL varies in the reasonable limits
Fig. 7. Variation of the OSD surface charge (curve 1), optical density (curve 2) and f-potential (curve 3) of OSD aqueous dispersions as a function of the saccharose concentration at pH 6.0 ( j=5×10−3).
Fig. 8. The dependences of the f-potential of OSD dispersions on medium pH in the background KCl solution (5×10−3 M ) (curve 1) and in the background solution with addition of 0.25% saccharose (curve 2); curve 3, calculated thickness of the OSD boundary layer as a function of pH.
0.6–2.8 nm and that the maximum BL destruction under the influence of saccharose takes place in acidic media. The non-monotonic h(pH ) dependence is probably connected with the change in the degree of dissociation of the diamond surface groups and, consequently, with the different ability of these groups to bond to the surrounding water
Fig. 9. The temperature dependences of the f-potential (curves 1, 2) and the coagulation threshold (curve 3) for OG (curve 1) and OSD suspensions (curves 2, 3) in the background solution of KCl (5×10−3 M ) at pH#6.5.
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and saccharose molecules by the formation of the hydrogen bonds.
a convincing argument in the favour of the concept of a hydrodynamic immobile near-wall layer.
3.4. Temperature dependence of the f potential and the coagulation threshold of OG and OSD dispersions
4. Conclusions
The temperature dependence of the f potential and of the coagulation threshold of OG and OSD dispersions (Fig. 9) also indicates the increase of f and the decrease of the sol stability in a certain Dt° interval, which is different for OG and OSD. Undoubtedly, the effect of the f increase with the increase of t° also is associated with the BL destruction, which is confirmed by the sharp decrease of the coagulation threshold. The h estimations in this case also give reasonable values of 1.3–1.6 nm. The observed bends in the temperature dependences of OG and OSD f potential at t=35–40°C and t=55–60°C are probably connected with some rearrangements of the ‘‘open-work architecture’’ of the hydrogen bonds in the liquid near-wall water. The difference of the temperature intervals within which the growth of f is observed for OG and OSD indicates that on the surface of OSD, unlike OG, water is bound not only by H bonds but also by other strong forces (probably the formation of coordinative bonds with such surface ions as Fe3+, Cr6+ or Ni2+, Co2+, Mn2+, passed from the catalyst and implanted in the lattice during its synthesis and purification). Finally, the fact that irrespective of the method of the modification of a disperse system, the sign of the change in the f potential is strictly correlated with direction of the slip boundary shift serves as
The features of boundary layers manifested in electrokinetic phenomena of the hydrophilic dispersions of oxidised graphite and diamond were studied by modifying their surface chemistry and the adjoining liquid layer. It is shown that irrespectively of the method of modification of a disperse system, the sign of change in f potential correlates strictly with the direction of slipping boundary displacement, which is related to the formation or collapse of the boundary layer. The possibility of estimating quantitatively the boundary layer thickness (h) and its contribution to the stability of dispersions was demonstrated. A good agreement was obtained for different approaches with h values fluctuating within 1.0–3.0 nm limits.
References [1] N.V. Churaev, Kolloidn. Zh. 46 (1984) 302. [2] S.S. Dukhin, B.V. Derjaguin, Electrophoresis, Nauka, Moscow, 1976, p. 332. [3] H.R. Kruyt, Nauka o kolloidakh, vol. I, Ed. Inostr. Lit., Moscow, 1955. [4] V.N. Moraru, F.D. Ovcharenko, Kolloidn. Zh. 48 (1986) 90. [5] D.A. Haydon, Proc. R. Soc. London, Ser. A 258 (1960) 319. [6 ] J.T. Davies, E.K. Rideal, in: Interfacial Phenomena, Academic Press, New York, 1961, p. 352. [7] V.N. Moraru, F.D. Ovcharenko, L.A. Totskaya, Kolloidn. Zh. 53 (1991) 874.
Boundary layer and the stability of hydrophilic dispersions V.N. Moraru *, F.D. Ovcharenko, D.V. Moraru Institute of Biocolloidal Chemistry, National Academy of Sciences of Ukraine, 42 Vernadsky blvd, 252142 Kyiv, Ukraine Received 26 August 1997; accepted 29 July 1998
Abstract This article presents the results of an experimental study of the influence of different factors on the structure and extent of the hydration boundary layers (BL) of oxidised synthetic diamond (OSD) and oxidised graphite (OG). The experimental technique allows changes in the hydrodynamically immobile BL thickness to be recorded by means of an electrokinetic method. It also provides independent information about the surface charge (s), f potential and sol stability over a wide range of pH and concentration of the inert electrolyte ( KCl ). To check the correctness and the reliability of the technique we studied the influence of the four different factors on the BL thickness: (1) hydrophilization of the graphite and diamond surface (by means of an increase in the degree of oxidation); (2) preliminary thermotreatment of the OSD micropowder in an inert gas flow or in a vacuum; (3) introduction into aqueous dispersions of OSD and OG of the non-ionogenic compounds, promoting the destruction of the BL structure; (4) influence of temperature on the electrophoretic mobility and the dispersion stability. On the basis of the influence of each of these factors on the BL, it is established that the sign of the f potential change correlates strictly with the direction of the shift in the slip-plane boundary, supporting the basic concept of the method. On the basis of the dependences obtained, f(C ) for constant s(pH ), and f(s) for constant ionic strength, we el estimated the thickness h of the hydrodynamically immobile hydration layer on the surfaces of OSD and OG, applying the diffuse double electric layer theory. Using the different approaches we found that h varied in the range h=1.0 –3.0 nm. The values obtained for h on average correspond to the BL thickness of 5–7 molecular water layers. These results are in quantitative agreement with experimental data on the stability of the dispersions under consideration and with the results of a calculation of the pair interaction potential curves for OSD and OG particles in the framework of the generalised DLVO theory taking into account the structural forces. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Hydration boundary layer thickness; Oxidised graphite and diamond dispersions; Hydrophilicity; Electrosurface properties; Stability
1. Introduction The development of methods for the quantitative study of the boundary layer (BL) is important in the context of the theory of the structural forces and stability of hydrophilic colloids [1]. One way * Corresponding author. Fax: +380 44 444 8078; e-mail: [email protected]
to obtain information about the BL consists in a complex study of the electrosurface phenomena in model systems satisfying the conditions of quantitative interpretation of the f potential: absence of a gel layer; small surface roughness; the possibility to change the surface hydrophilicity and charge and of their quantitative determination; absence of pores [2]. If one assumes the hydrodynamic immobility of
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the BL (or, at least, of that part of it which is subjected to the maximal influence of the surface forces field ), then under certain conditions (moderate ionic strength j, low specific adsorption of ions) one can expect considerable electrokinetic effects due to destruction or formation of the BL. Therefore, it is interesting to trace how the BL manifests itself in the electrokinetic phenomena and in the hydrophilic dispersion stability under the influence of the different factors. The aim of this work is a quantitative test of the ‘‘slip plane’’ concept, and the consequent estimation of the width of the hydrodynamically immobile BL of water and the contribution of the BL to the dispersion stability. Aggregatively stable aqueous dispersions of oxidised synthetic diamond (OSD) and oxidised graphite (OG) were chosen as model systems. Interest in dispersions of OSD and OG arise because they are good models for the study of electrosurface phenomena on passing from lyophobic to lyophilic colloids and vice versa. The essence of our approach consists in influencing the dispersion by any factor that changes the structure and the extent of the BL, subsequently recording the given effect by the electrokinetic method and obtaining independent information about the surface charge and the sol stability over wide ranges of pH and concentration of inert electrolyte ( KCl ) To check the correctness and reliability of the method we decided to study the influence of two groups of factors on the BL, by means of affecting the surface chemistry and the neighbouring liquid: (1) hydrophilization (increase in the degree of oxidation) of the graphite and diamond surface; (2) preliminary thermotreatment of the OSD micropowder in an inert gas flow or in vacuum; (3) introduction of a saccharose destroying the hydration BL into aqueous dispersions of OSD and OG; (4) influence of the temperature on electrophoresis mobility and dispersion stability. For constant surface charge and ionic strength of the inert electrolyte background solution, the slip-plane coordinate (h) can be determined by means of the well-known equation of the theory
of the diffuse double electric layer (DEL) structure [3]: th[zef/4kT ]=th[zey /4kT ] exp(−xh) d
(1)
where z is the valence of the counterions, e is the elementary charge, x is the value of the inverse Debye screening radius, f is the slip boundary potential, y is the Stern layer potential, k is the d Boltzmann constant and T is the absolute temperature. Logarithmic presentation of Eq. (1) takes form ln[th(zef/4kT )]=ln[th(zey /4kT )]−xh d
(2)
The dependence of ln[th(zef/4kT )] on x is linear, with h being the tangent of its slope angle. Thus, substituting in Eq. (2) the experimental dependences of the f potential of OG and OSD on inert electrolyte concentration expressed through x we obtain a number of straight lines, the tangents of whose slope angles allow the distance to the slip plane (h) to be determined even if the y potential d value is unknown. Moreover, from the intercept of each line on the ordinate axis it is also possible to calculate the corresponding y value. d The second possibility to determine h consists in the following. In the case of initial graphite and diamond whose surface is only slightly oxidised, no continuous BL is formed and the slip boundary occurs near the surface. Therefore, the f potential measured at low electrolyte concentrations is approximately equal to the y potential. Possessing d now a number of the f potential values for the oxidised graphite and diamond samples (at s= const and j=const) we can calculate from Eq. (1) the slip plane shift (h) as a result of the BL formation. The distinctive feature of the applied approach for BL detection is the use of the ‘‘sparing’’ method of microelectrophoresis at low electric field strengths (E#5–6 V cm−1) and the shear rates. It is easy to show that the maximum value of the shear rate gradient c˙ =∂v/∂x will be reached at the distance from the particle centre equal x =앀6r o and its order of magnitude will be #0.1v /r (where o v is the particle velocity, and r is the particle o radius), i.e. about 1–2 s−1. At such low shear rates, one can assume that the water layers nearest to
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the surface are hydrodynamically immobile and that the slip boundary is remote from the surface at some distance depending on the BL thickness.
of the conductometric and potentiometric titration of the H+ form with a 0.1 M solution of KOH in a 0.2 M background solution of KCl. To calculate s we used the formula s=C
2. Experimental The micropowder of synthetic diamond of the ACM-1 trade mark 0.3/0 (GOST 9206-80) used in this work had a narrow maximum in the particle size distribution in the region of the 0.3–0.5 mm and a specific BET surface area of 23.0 m2 g−1. During the dispersing and subsequent chemical purification from admixtures in strong oxidizing media (conc. H SO , HNO , CrO , K Cr O and 2 4 3 3 2 2 7 others) the diamond surface is oxidized and becomes hydrophilic. In order to transform the surface ionogen groups to the H+ form, the OSD micropowder was treated several times with 1.0 M HCl solution, and then washed with bidistillate to a constant pH value of 4.0. The characteristics of OSD are given in Table 1. Natural graphite of high purity (99.8%) of mark C-O, with the particle size 0.2–2.0 mm and specific surface area 20 m2 g−1, was subjected to different degrees of the oxidation by means of the concentrated H SO and HNO treatment in the presence 2 4 3 of a catalyser, after which it was washed off with a 1 M solution HCL and finally with bidistillate to a pH of 3.5–4.0. Table 1 gives the characteristics of the oxidised graphite obtained. The determination the surface charge density (s) of OG and OSD was carried out by the methods
F/S OH–H where C is the adsorption of the potentialOH–H determining ions, F is the Faraday number, and S is the specific surface. The values of S were determined by the adsorption of dye (methylene blue) from the water solution. The electrokinetic potential (f) of the dispersions was determined by the microelectrophoresis method according to the procedure described earlier in Ref. [4]. In order not to introduce the corrections for the polarisation of the DEL in the Helmholtz–Smoluchowsky formula for f f=4pgv /eE ef where v is the electrophoresis velocity, g and e ef are the medium viscosity and dielectric constant, respectively, and E is the field strength, we chose the experimental parameters which satisfied the condition xr≥100 (were x is the value inverse to the DEL thickness, and r is the particle radius) and Rel#[exp(ef/2kT )−1]/xr<1 where Rel is the polarisation criterion [2]. The coagulation thresholds (CT ) and stability were determined from the dependences of the optical density (D) of the OSD and OG suspensions on the electrolyte concentration. The specific heat
Table 1 The potentiometric surface charge density (s) and specific heat of wetting (q ) of OG and OSD, the optical density (D) and f-potential s of their aqueous suspensions, as well as the calculated thickness (h) of the boundary layer for the objects investigated Object
s (C m−2) at pH 7.0
q (J m−2) s
D at pH 6.0
f (mV ) at pH 6.0
h (nm)
Natural graphite C-0 OG-1 OG-2 OG-3 OG-4 OSD OSD thermally treated at 180°C
0.08 0.30 0.68 1.00 1.20 0.29 0.15
0.06 0.14 0.30 0.41 0.46 0.15 0.11
0.05 0.27 0.46 0.80 0.95 0.90 a 0.40 a
72 58 51 43 36 28 a 41 a
0.0 0.9 1.6 2.5 3.2 2.1 0.6
aAt pH 5.0.
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of wetting of the OG and OSD micropowders (q ), quantitatively characterising the surface s hydrophilicity, was determined using a DAK-1M microcalorimeter.
3. Results and discussion 3.1. The influence of the graphite surface oxidation on the stability and electrosurface properties on the graphite aqueous dispersions The data of Table 1 and Fig. 1 show that the oxidation of graphite is accompanied by increases in the surface charge, the specific heat of wetting and the optical suspension density. However, consideration of the dependences f=f (pH ) and j=const f=f (C ) for the series of the OG samples KCl pH=const with different degrees of oxidation shows that, in spite of the growth of s, these dependences are at lower levels, the higher are the degree of oxidation and the graphite hydrophilicity. While interpreting these data it is necessary to take into account the influence of two opposing factors: the increase of the f potential at the expense of the charge growth on oxidation, and the decrease of f due to the thickening of the BL and the moving of the slip boundary away from the surface. Since the degree of the mutual compen-
Fig. 1. The surface charge density determined by conductometric method (curve 1), specific heat of wetting of graphite (curve 2), optical density (curve 3) and f-potential (curve 4) of graphite aqueous dispersions as functions of the content of oxygencontaining ionogenic groups (C ) on the graphite surface. ox
sation of the factors mentioned is not open to quantitative analysis, to exclude the charge influence we have plotted the dependence f= f(s) for the same series of OG samples j=const ( Fig. 2). It can be seen that at low charge densities, when the potential within the DEL changes relatively slowly, the f(s) curves for all OG samples, including the natural sample, are close to each other or coincide. This fact means either that the BL is weakly developed and the slip plane passes near the surface, or that the displacement of the slip boundary weakly affects the f potential. This result agrees with the data of Haydon [5], and Davies and Rideal [6 ], who established that at s<0.1 C m−2 the slip plane, in the emulsions they studied, coincided with the plane of the most proximity of the charged groups on the hydrocarbon drop surface, as f#y , where y is the d d Stern potential. At higher values of the charge densities on the OG particle surface a continuous BL is formed, which strongly manifests itself in the electrokinetic phenomena owing to which the f(s) curves diverge: they are located at lower levels the thicker is the BL (Fig. 2). Using Eq. (1), we calculated the shift of the slip plane (h) as a result of the surface oxidation and hydrophilization for every OG sample, with
Fig. 2. The relationship between the experimentally determined f-potential and the surface charge density (s) for different degrees of graphite oxidation C ×10−6 mol m−2: 0.9 (curve ox 1), 3.1 (curve 2), 5.4 (curve 3), 7.25 (curve 4) and 10.5 (curve 5) (ionic strength j=5×10−3 M KCl ).
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the f potential of the initial natural graphite taken as 75 mV assumed equal to the Stern layer potential. At C =5×10−3 mol dm−3 KCl (z=1, x=2.32×108 m−1) and s=0.3 C m−2 we obtained the h value, which for different OG varied within 1–3 nm (Table 1). From Fig. 3 one can see that dependences of the f potential for the initial and oxidised graphites on the KCl concentration represented in the coordinates of Eq. (2) are straight lines whose slope angles increase with increasing degree of oxidation and graphite hydrophilicity. The h values for OG obtained on this basis are within the same 1–3 nm limits. Such limits to the change in water BL thickness on the hydrate object surface seems to us quite real. Moreover, as follows from the calculations given below for the pair interaction energy of the OG and OSD particles according to the generalised DLVO theory [1], the structural forces are important only at the distances ≤5.0 nm (doubled BL thickness). At the same time the fact that at equal s values the BL thickness differs considerably for OG with various degrees of oxidation may cause surprise. In explaining such a divergence one should take into consideration that during the graphite oxidation in addition to iongenic groups the non-
Fig. 3. The dependences of ln[th(zef/4kT )] on x for the graphite with different degrees of oxidation C ×10−6 mol m−2: 0.9 ox (curve 1), 3.1 (curve 2), 5.4 (curve 3), 7.2 (curve 4) and 8.6 (curve 5) ( j=5×10−3).
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iongenic groups whose content increases in proportion to the degree of oxidation are formed. Although these groups make no contribution to the total surface charge they are, nevertheless, capable of strong hydration and structuring of the surface water. 3.2. The influence of the preliminary thermotreatment of the OSD micropowder Thermotreatment of the OSD in flowing helium at T=453 K (3 h) resulted in a decrease in the number of surface groups and in the titrated charge as well as a decrease in the stability of its dispersions [7] (see also Table 1). However, in spite of the decrease of s, the dependence f=f (s) (Fig. 4) obtained by recalculation of the f(pH ) and s(pH ) dependences and the dependence f=f (C ) ( Fig. 5) in the case of KCl calcinated OSD both shift to the higher potentials. For other s, the observed growth of the OSD f potential after calcination over wide range of pH and C can be explained only by partial destrucKCl tion of the BL and the approach of the slip boundary to the surface. Using Eq. (1) we calculated the thickness of the hydrodynamically immobile BL at different s and C values (Figs. 4 and 5). As can be seen, the h KCl values obtained for the OSD vary within the reasonable limits 0.6–2.1 nm. This means that the
Fig. 4. The relationship between the f-potential and the surface charge density (s) for initial (curve 1) and thermally treated OSD (curve 2) and the thickness of boundary layer h (curve 3) calculated from Eq. (1) as a function of s.
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particles (H ), we used the following formulae:
Fig. 5. The dependences of the f-potential for initial (curve 1) and thermally treated OSD (curve 2) and the thickness of boundary layer h (curve 3), calculated on the basis of Eq. (1), on KCl concentration at pH 6.0.
OSD calcination resulted in thinning of the BL to maximum of seven water molecular layers that gave rise to the decrease in the dispersion stability. One more important conclusion follows from Fig. 5: the increase in the electrolyte concentration destroys the BL and, at C #0.1 M, h is equal to KCl the thickness of one or two molecular layers of water on the OSD surface. The non-monotonic character of the h(s) dependence observed on diamond ( Fig. 4) can be associated with the stronger specific adsorption of the K+ counterions. So, excessive growth of the OSD charge (s>0.3 C m−2) probably involves very high fillings of the Stern layer by counterions so Eq. (1) is no longer applicable for the calculation of h. The presence of a BL on the initial OSD and its practical absence on the calcinated sample undoubtedly must be reflected in the character of the potential curves of the pair interaction of particles of the corresponding dispersions, calculated according to the DLVO theory and its generalised version taking into account the structural forces [1]. To calculate energy of the electrostatic (U ), molecular (U ) and structural (U ) forces as i m s functions of the distance between the diamond
U (H )=2pee ry2 ln[1+exp(−xH )] (3) i o d U (H )=−Ar/12H (4) m U (H )=prl2K exp(−H/l ) (5) s were y #f, the Hamaker constant A= d 0.78×10−19 J, r=0.3 mm, e =8.85×10−12 F m−1; 0 empirical parameters K=5×106 N m−2 and l= 1×10−9 m [1]. Fig. 6 shows the total potential interaction curves U(H )/kT obtained for a KCl concentration of 0.02 mol dm−3 (the coagulation threshold of the OSD dispersion). One can see that the generalised DLVO theory correctly predicts the disappearance of the energetic barrier and the threshold of the rapid coagulation in the case of the initial OSD dispersion, whereas for the calcinated OSD sol, where the BL is absent, this theory predicts a high system stability (U #100 kT ) that contramax dicts the experimental data [7]. In this case the stability is better described within the framework of the usual DLVO theory. Finally, the fact that the point of the potential minimum in the U(H ) curves corresponds to the doubled BL thickness, in our opinion, is not accidental and indirectly confirms our estimations for h. Thus, a good agreement of the DLVO theory with the experiment is found.
Fig. 6. The total potential curves of the pair particle interaction for initial (curves 1, 1∞) and thermally treated OSD (curves 2, 2∞) calculated according to the DLVO theory (curves 1∞, 2∞) and taking into account the structural forces (curves 1, 2); ionic strength j=0.02 M KCl.
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3.3. The influence of saccharose on stability and electrosurface properties of OSD–aqueous dispersions As follows from Fig. 7, on increasing the saccharose concentration from zero to 0.25% the f potential of OSD increases from 29.5 to 40 mV, whereas the surface charge density (s) and the dispersion optical density (D) do not change. Starting from C >0.25%, the f potential, the surface charge sac density and the optical density decrease synchronously, resulting in the complete coagulation and clarification of the OSD suspension at C #1–2%. To our knowledge, this is the first sac report of an increase in f due to introduction of saccharose in a dispersion. Joint analysis of the dependences f(C ) and s(C ) leads to the conclusac sac sion that the observed initial increase in f should be accounted for only by the destruction of the BL and shift of the slip boundary closer to the surface. This is also confirmed by the f=f (pH ) dependence at fixed saccharose (0.25%) and background electrolyte concentrations ( Fig. 8), which over all the pH region shifts to higher potentials as compared with the analogous dependence at C =0. The shift of the slip plane at different pH sac values (Fig. 8) calculated from these dependences according to Eq. (1) shows that the thickness of the destroyed BL varies in the reasonable limits
Fig. 7. Variation of the OSD surface charge (curve 1), optical density (curve 2) and f-potential (curve 3) of OSD aqueous dispersions as a function of the saccharose concentration at pH 6.0 ( j=5×10−3).
Fig. 8. The dependences of the f-potential of OSD dispersions on medium pH in the background KCl solution (5×10−3 M ) (curve 1) and in the background solution with addition of 0.25% saccharose (curve 2); curve 3, calculated thickness of the OSD boundary layer as a function of pH.
0.6–2.8 nm and that the maximum BL destruction under the influence of saccharose takes place in acidic media. The non-monotonic h(pH ) dependence is probably connected with the change in the degree of dissociation of the diamond surface groups and, consequently, with the different ability of these groups to bond to the surrounding water
Fig. 9. The temperature dependences of the f-potential (curves 1, 2) and the coagulation threshold (curve 3) for OG (curve 1) and OSD suspensions (curves 2, 3) in the background solution of KCl (5×10−3 M ) at pH#6.5.
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and saccharose molecules by the formation of the hydrogen bonds.
a convincing argument in the favour of the concept of a hydrodynamic immobile near-wall layer.
3.4. Temperature dependence of the f potential and the coagulation threshold of OG and OSD dispersions
4. Conclusions
The temperature dependence of the f potential and of the coagulation threshold of OG and OSD dispersions (Fig. 9) also indicates the increase of f and the decrease of the sol stability in a certain Dt° interval, which is different for OG and OSD. Undoubtedly, the effect of the f increase with the increase of t° also is associated with the BL destruction, which is confirmed by the sharp decrease of the coagulation threshold. The h estimations in this case also give reasonable values of 1.3–1.6 nm. The observed bends in the temperature dependences of OG and OSD f potential at t=35–40°C and t=55–60°C are probably connected with some rearrangements of the ‘‘open-work architecture’’ of the hydrogen bonds in the liquid near-wall water. The difference of the temperature intervals within which the growth of f is observed for OG and OSD indicates that on the surface of OSD, unlike OG, water is bound not only by H bonds but also by other strong forces (probably the formation of coordinative bonds with such surface ions as Fe3+, Cr6+ or Ni2+, Co2+, Mn2+, passed from the catalyst and implanted in the lattice during its synthesis and purification). Finally, the fact that irrespective of the method of the modification of a disperse system, the sign of the change in the f potential is strictly correlated with direction of the slip boundary shift serves as
The features of boundary layers manifested in electrokinetic phenomena of the hydrophilic dispersions of oxidised graphite and diamond were studied by modifying their surface chemistry and the adjoining liquid layer. It is shown that irrespectively of the method of modification of a disperse system, the sign of change in f potential correlates strictly with the direction of slipping boundary displacement, which is related to the formation or collapse of the boundary layer. The possibility of estimating quantitatively the boundary layer thickness (h) and its contribution to the stability of dispersions was demonstrated. A good agreement was obtained for different approaches with h values fluctuating within 1.0–3.0 nm limits.
References [1] N.V. Churaev, Kolloidn. Zh. 46 (1984) 302. [2] S.S. Dukhin, B.V. Derjaguin, Electrophoresis, Nauka, Moscow, 1976, p. 332. [3] H.R. Kruyt, Nauka o kolloidakh, vol. I, Ed. Inostr. Lit., Moscow, 1955. [4] V.N. Moraru, F.D. Ovcharenko, Kolloidn. Zh. 48 (1986) 90. [5] D.A. Haydon, Proc. R. Soc. London, Ser. A 258 (1960) 319. [6 ] J.T. Davies, E.K. Rideal, in: Interfacial Phenomena, Academic Press, New York, 1961, p. 352. [7] V.N. Moraru, F.D. Ovcharenko, L.A. Totskaya, Kolloidn. Zh. 53 (1991) 874.