Adhesion and Detachment of Solid Colloidal Particles in Aqueous Ionogenic Surfactant Media Eric J. Clayfield and Alec L. Smith’ Shell Research Ltd., Thornton Research Centre, Chester, CH1 3SH, England
The adhesion of carbon black particles to a glass substrate in aqueous media and their subsequent removal by ionic surfactant solutions have been studied by a powder-bed technique and by direct microscopic observation. The electrostatic interaction between the colloidal particles and the substrate was varied by changing the concentrations of surfactant and indifferent electrolyte present in the system. It was shown that considerable removal of adhered particles could be obtained, with no appreciable hydrodynamic displacement action being required; this “spontaneous” detachment of solid colloidal particles thus provides an aqueous analog of the corresponding phenomenon found to occur in nonaqueous media. The experimental results have been considered in conjunction with the electrokinetic potentials of both surfaces, and an explanation offered in terms of electrical double layer theory and the “secondary minimum” adhesion concept.
T
he few previous investigations of the adhesion of solid colloidal particles to a solid substrate, and their subsequent removal by surfactant solutions, have been largely concerned with the particular technical problems of redeposition on textiles in aqueous detergent systems (Durham, 1956, 1961 ; von Lange, 1957, 1961). Such studies have considered that the Dejaguin-Landau-Verwey-Overbeek (DLVO) theory (Derjaguin and Landau, 1941 ; Verwey and Overbeek, 1948) of colloid stability may provide a feasible mechanism for deposition from colloidal suspensions, but that purely electrical effects can have no beneficial influence per se on the removal of particulate soil from a fiber surface (Durham, 1956). Thus it has been concluded (Durham, 1961) that adhered colloidal particles below about 0.2 pm. in size are virtually impossible to remove except by drastic mechanical action, and that removal is difficult even with particles as large as 5 pm. The basis for this view was that the process of soil removal was considered to involve getting the adhered particles out of the deep “primary energy minimum” of the potential energy curve. However, recent investigations have suggested that, in both aqueous (Marshall and Kitchener, 1966) and nonaqueous (Clayfield and Lumb, 1966) systems, deposition of colloidal particles into the “secondary energy minimum” may occur. For such an adhered state, ready removal of the deposited particles by surfactant solutions might well be expected. In
’ Present address :
College of Technology, Liverpool, England.
nonaqueous systems, with polymeric surfactants, it has been demonstrated (Clayfield and Lumb, 1966) that such removal can indeed take place, considerable detachment of adhered carbon black particles being obtained with no appreciable hydrodynamic displacement action being required. I n this case, a quantitative explanation of the experimental results was provided by an entropic repulsion mechanism (Clayfield and Lumb, 1966) of polymeric detergent action. The aim of this present work was to find out whether such appreciable removal of adhered particles could also be obtained in aqueous media and, if so, whether the experimental behavior could be explained in terms of the DLVO electrical double-layer theory. To this end, the adhesion of carbon black particles to a glass substrate in aqueous media and their subsequent removal by ionic surfactant solutions have been studied by a powder-bed technique and by direct microscopic observation. The electrostatic interaction between the colloidal particles and the substrate was varied by changing the concentrations of surfactant and indifferent electrolyte present in the system. Zeta potentials of particles and substrates were determined by microelectrophoresis, with the rectangular electrophoresis cell apparatus also being used for the observation and photographic recording of particle adhesion and detachment behavior. Experimental
Materials. The carbon black, used untreated, was Sterling F T obtained from Cabot Carbon, Ltd.; its average particle diameter, determined from electron micrographs, was 0.22 pm. The glass powder was a narrow size-range (37 to 44 pm. particle diameter) fraction of Grade 20 Ballotini powder of spherical particle shape, obtained from Jencons Scientific, Ltd. Before use this was dry-sieved and then wet-sieved to give the fraction passing 325-mesh, retained by 400-mesh ; washed with chromic acid, followed by washing with distilled water until the washings were neutral; dried for 24 hours at 115” C., and finally resieved dry to give the 37/44 pm. particle size fraction. The Aerosol O T (sodium diethylhexylsulfosuccinate) was obtained from Hardman and Holden and used without any further purification. The sodium undecane-3-sulfate was prepared from octyl bromide and propionaldehyde cia the Grignard reaction to give undecan-3-01; the alcohol was caused to react with chlorsulfonic acid, followed by neutralization with sodium hydrogen carbonate, extraction with pure n-butanol and azeotropic distillation to give the pure alkyl sulfate in aqueous solution form. The sodium dodecyl sulfate was a high-purity sample, homogeneous in chainVolume 4, Number 5, May 1970 413
length as shown by hydrolysis to the corresponding alcohol and subsequent G L C examination, and gave no minimum in its surface tension concentration curve. Powder-bed Procedure. Essentially, this procedure comprised the deposition of carbon black from aqueous dispersion onto a glass powder bed, followed by the attempted removal of the adhered particles by passage of aqueous surfactant solution. The particle size of the glass powder used was chosen to be very much greater than the particle size of the carbon black, so that negligible mechanical filtration took place. Both stages of this procedure were carried out at ambient room temperature (20-22” C.). The deposition stage involved forming 2 grams of glass powder into a cylindrical porous bed of diameter 1 cm., supported o n a 400-mesh stainless steel disk in a n enclosed column apparatus, passing 5 grams of an aqueous dispersion of 60 p.p.m. carbon black through this bed at a controlled constant rate to give a throughput time of 8 min., rinsing through with 5 grams of solution alone to remove dispersion entrained in the bed interstices, and then determining photometrically the amount of carbon black which remained in the dispersion to give the amount remaining adhered to the glass particles. The necessary reproducible dispersion of carbon black was obtained with a Dawe ultrasonic generator type 1150, of 500 watt peak power output at 40 kc./sec. The light transmission measurements were made at 4300 A wavelength, with a Unicam SP 600 spectrophotometer; the Beer-Lambert law applied for these dilute dispersions. The aqueous “deposition solution” used to give a sufficient and reproducible deposition was 60 mM. KC1 0.0133 mM. Aerosol OT. Dispersions in water alone showed negligible deposition onto the glass powder; dispersions in 60 mM. KCI solution alone resulted in essentially irreversible deposition, as discussed later. The “rinse solution’’ used in the preliminary experiments was the same as the “deposition solution”; for the succeeding constant ionic strength experiments, the “rinse solution” was 10 mM. KCl. I n the subsequent removal stage, 5 grams of the ionic surfactant solution was passed through the powder bed a t a constant rate (8 min. throughput time), and the amount of carbon black removed determined photometrically. The reproducibility of the “fraction of adhered carbon black removed” results was about + 8 %. Electrokinetic Measurements. Zeta-potentials of particles and substrates were determined by microelectrophoresis, using both a cylindrical glass cell of the van Gils type (van Gils and Kruyt, 1936; Kruyt, 1952) and a flat cell apparatus mounted vertically, made from a fused silica spectrophotometer cell of length 45 mm., height 10 mm., width (optical path) 1 mm., similar to that described elsewhere by McGown, e t al. (1965). Direct Observation of Adhered Particles. The silica cell apparatus was also used for the direct observation and photomicrographic recording of particle adhesion and detachment behavior. Deposition o n the vertical silica wall was effected using a 15 p.p.m. dispersion of carbon black in 60 mM. KCI 0.0133 m M Aerosol OT, with a deposition time of 30 min.; the rinsing stage was carried out with 20 ml. of 10 mM. KCl, with a throughput time of 45 min.; the removal stage involved passing 50 ml. of the surfactant solution through the cell at a constant flow rate of 0.1 ml./sec. The number of adhered particles before and after the removal stage was determined by direct counting, from photomicrographs taken at 250X magnification with dark-ground illumination.
+
+
414 Environmental Science & Technology
Calculation of’Interaction Energy Curaes Attraction Energy. The attraction energy between a spherical particle (radius 1100 A) and a thick plate was computed from the retarded atom-atom attraction energy expression of Casimir and Polder (Casimir and Polder, 1948) cia the approximation derived by Schenkel and Kitchener (Schenkel and Kitchener, 1960) by use of a method similar to that developed by Hamaker (Hamaker, 1937). The expression thus obtained (Clayfield and Lumb, 1966) for the partially retarded attraction energy, in kTunits, between a particle of r a d’ium ci and N thick plate is
0.000117
X2/.lr2
+ + +
[
38a(3h2 6/ra 4u2) /13(h 243
+
where h is the separation distance between the surfaces, A is the Hamaker constant for the particles in the medium, and X the wavelength corresponding to the intrinsic electronic oscillations of the atoms (taken to be 10-j cm.). Since the glass spheres in the powder bed experiments were of so much greater radius than the carbon black particles (-200 times), it was considered appropriate to use the particle/plate expression in this case, as well as for the “silica wall” adhesion experiments. Repulsion Energy. Repulsion energy curves for the interaction between planar and spherical double layers may be constructed by the approximate method of Derjaguin (Derjaguin, 1934) from expressions for the repulsive energy of plane double layers, If the Debye-Huckel low-potential approximation is used for the latter, the resulting expression for equal effective potential, P,of the two surfaces is V,
=
ea*? In
[I
+ exp( -Kh)]
(1)
where VR is the total repulsive potential energy resulting from the interaction when the sphere of radius a approaches the plane surface to a separation distance, /i, K is the DebyeHuckel reciprocal thickness of double layer, and E the dielectric constant of the dispersion medium. This expression may be used at Ka > 10 and \k up to 50-60 mV without serious error. For higher potentials, though only for small interaction, Equation 2 may be used, where z is the valency of the counter-ion, e the electronic charge, and y = tanh (ze\ll,’ 4kT) VR =
a Z
166 k 2 T 2 y 2 exp(-Kh) e2
In this work, where the effective surface potentials and P? of the plate and sphere differ, \kz in Eq. 1 and y 2 in Eq. 2 have been replaced by \klQ2 and y1y2,respectively. The error involved in this procedure (Hogg, Healey, ef al., 1966), decreases as \kl and Pzapproach, and also as the interaction distance increases; it is therefore more justified in calculations of secondary minimum interactions than would be the case for shorter range interactions. The potential \k required in the above expressions is best thought of as the innermost potential of the Gouy (diffuse) double layer, and has been set equal to the electrokinetically determined zeta potential, .$ in this work. Thus the “origin”
of the repulsive energy is moved out from the surface (the effective origin of the attractive energy curve) by a “displacement distance” which will depend o n the thickness of the adsorbed surfactant layer and the distance of the “slipping plane” from the boundary of the adsorbed layer. The two effects cannot be separated from the data we present. Res~ilrsund Discussion
Preliminary powder-bed results with aqueous Aerosol OT solutions at concentrations up to 20 mM. showed that up to 60% removal of adhered carbon black was readily obtained with minimum hydrodynamic displacement action. Such “spontaneous” detachment of solid colloidal particles thus provided an aqueous analog of the corresponding phenomenon recently found to occur in nonaqueous media (Clayfield and Lumb, 1966). To facilitate the possible explanation of such detergency behavior in terms of the DLVO theory, succeeding removal experiments were carried out with better defined surfactant systems. in which the total ionic strength was maintained constant by the addition of indifferent electrolyte. Figures 1 and 2 show the extent of removal of carbon particles from the glass powder bed and the silica cell wall, respectively, by solutions of sodium dodecyl sulfate and sodium undecane-3-sulfate with the total ionic strength of surfactant solutions made up to 10 mM. with KCI in all cases. The particles were adhered to substrate from 60 mM. KC1 0.0113 mM. Aerosol OT solution, and in the case of the silica cell experiments could be seen to be executing lateral Brownian motion while remaining adhered t o the wall. In contrast to these results, when the particles were adhered from a 60 mM. KC1 solution containing no Aerosol OT, the
+
d 2
3
I JlC€CClE-3-
ILlf31E
-
a
2
2
4
6
I
l 8
0
SURFACTANT CONCENTRATION ( m M )
L
Figure 1. Removal results, glass powder-bed experiments 3
II
>
0
2
4
6
8
5
SURFACTANT C O N C E N T R A T I O N ( r n M )
Figure 3. Zeta potential results, ionic strength kept constant at 10 mhl., with added KCI
fraction of the adhered particles removed by surfactant solutions, even with 10 mM. sodium dodecyl sulfate, was negligibly small (
