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Adsorption of polyacrylamide to Na kaolinite: Correlation between clay structure and surface properties
Adsorption of Polyacrylamide to Na Kaolinite: Correlation between Clay Structure and Surface Properties E. PEFFERKORN, L. NABZAR, AND A. C A R R O Y Centre de Recherches sur les Macromol~cules (CNRS), 6, rue Boussingault, 67083 Strasbourg Cedex, France Received July 9, 1984; accepted October 25, 1984
Interactions between nonionic polyacrylamideand Na kaolinite in aqueous suspensions are discussed in terms of amide-hydroxyl H bonding. Correlations obtained by comparingthe extent of adsorption of the polymer onto the clay mineral and onto synthetic aluminosilicates at different pH reveal the dominant effect of the surface density of "free" hydroxyl groups acting as adsorbing sites on both adsorbent types. Most interesting is the opposingbehavior of the basal and edge faces of the kaolinite with regard to the polymer/surface affinity. Indeed, two crystallographically different surfaces are exposed by the plate-like panicles: in relation to a polymer present in aqueous suspension, the basal faces act as repulsive surfaces and the edge faces act as adsorbing ones. The pH controls the extent of polymer adsorption. Variation of this parameter influences the sign and magnitude of the electrical charge of the edges and thus leads to changes in the surface density of "free" hydroxyls,present either as silanol in the acidic range or as aluminol in the alkaline range. Results concerningthe variation of the surface coveragewith polymerconcentration are discussed in terms of the nature of the interactions between the un-ionized polyacrylamideand Na kaolinite. © 1985AcademicPress.Inc. a priori the respective participation of the crystal edges and faces in the polymer uptake. Any rationalization requires an investigation of the nature of the forces acting among monomers and the constitutive elements of edge and basal faces. In this context, it is important to obtain information on the adsorption of polyacrylamide on particles of simpler morphology but with a chemical composition approaching that of either the clay edges or the basal faces. Following this line of approach, we have synthesized aluminosilicates of spherical shape with a definite AIlSi surface composition. This modeling, although still crude, nevertheless provides better insight into the adsorption mechanism of polar macromolecules onto clay sorbents of more complex structure. In this work, for simplicity, we have restricted the study to the adsorption of nonhydrolyzed polyacrylamide. Indeed, since the surface of kaolinite and other layer silicates may acquire negative and positive charges, depending on the p H of the medium, long-
INTRODUCTION
The ability of polymers to destabilize clay suspensions is well known. This subject owing to its practical interest received widespread attention during the last decade (1-4); nevertheless, one is still in search of a comprehensive and consistent picture of the relevant factors describing the interactions between clays and hydrosoluble polymers. In the present work, we have focused attention on the adsorption of nonhydrolyzed polyacrylamide on Na kaolinite in aqueous media; our aim is to elucidate the mechanism of clay-polyacrylamide interactions in terms of the crystalline structure of the mineral. Kaolinite is a hydrous aluminosilicate of layer structure comprising silica and alumina sheets stacked on top of each other. It is widely recognized that the edge surfaces of kaolinite plates differ fundamentally in their structural and electrochemical properties from the cleavage surfaces. In view of this property, inherent in all clays, it is difficult to predict 94 0021-9797/85 $3.00 Copyright© 1985 by AcademicPress, Inc. All rights of reproduction in any form reserved.
Journalof Colloidand InterfaceScience. Vol. 106, No. 1, July 1985
ADSORPTION OF POLYACRYLAMIDE TO KAOLINITE range electrostatic interactions a m o n g clays and polyelectrolytes are expected to occur and the situation is in reality more complicated. MATERIALS AND METHOD
Polymer. A statistical copolymer (acrylamide-acrolein) with an (acrolein)/(acrylamide) m o n o m e r ratio of 10 -3 was prepared by radical copolymerization in aqueous media according to known techniques (5):
TABLE I Na kaolinite Average dimensions Plate diameter Plate thickness Edge specific surface Base specific surface Particle size distribution Finer than 0.1 ~m Finer than 0.16 um Coarser than 10 um
I
C=O
HC=O
I NH2 n
10_ 3
0.21 ~m 0.04 ~m 8.3 rn2 g-i 18.5 m2 g-~ 12% 50% negligible
Spherical beads modified by A1C13
~ CH2-- C H ] w - - [ C H 2 - - C H J~ n,
I
95
Sample type code
Average diameter (#m)
Specific surface area (ttm)
Surface site (AIOH)/(SiOH) ratio
AS 1 (b)(d) AS 2 (b) (d) AS 3 (c) (d)
34 34 1.2
7.8 × 10-2 7.8 × 10-2 4
5% 12% 33%
[1]
m
[r/] = 8.42 × 10-3M~ 757
(cm3/g).
[2]
After purification and fractionation (6), the polymer samples were characterized by viscosity and light scattering in 0.1 M NaC1 at 25°C and relationship [2] was found. A polymer sample of molecular weight 1.2 × 106 (Mw/Mn -- 1.3) was then selected and reacted in alkaline aqueous solution (pH 9.5, T = 25°C) with tritiated potassium borohydride. By reduction of acrolein to the propenol function CHz-CH(CaHzOH) -, a radioactive polyacrylamide of specific radioactivity 0.1 mCi/g was obtained. During this reaction, hydrolysis of acrylamide functions is less than 1%. Adsorbents. Adsorption measurements were performed for polyacrylamide (PAM) on Na kaolinite clay and on a d s o r b e n t s - denoted AS 1-3 in Table I - - w h i c h were either nonporous spherical glass beads (AS 1 and AS 2) or spherical amorphous silica beads (AS 3) treated with A1CI3. The characteristics of the adsorbents are reported in Table I. (a) Kaolin des Charentes (French origin) was supplied by the Centre de Recherches sur la Valorisation des Minerals, Nancy).
The sample with code-type name G Z A IV (7) corresponds to a kaolin of 86% kaolinite. The granulometric and morphological characteristics are listed in Table I. Compared to kaolinite of different origin (8), our sample is characterized by a large SE/(SE + SB) ratio, SE and SR being the edge and basal specific surface areas, respectively. Previous analysis did not show the presence of organic materials in the sample and purification by oxidizing agents was not judged necessary. In order to prevent any interaction of polyacrylamide with di- and trivalent cations such as A13+ or Fe z+ always present in crude kaolin, the heteroionic clay was converted to the homoionic sodium form according to the method r e c o m m e n d e d by Van Olphen (9). The clay powder was suspended in 1-2 M NaC1 distilled water and left under gentle shaking for 24 h. After multiple decantations and resuspensions in fresh electrolyte solution, the sediment was resuspended in pure water at p H 6-7 until the clay remained dispersed. The water treatment was stopped after the electrolyte concentration reached 1.5 × 10 -4 M NaC1 at p H 6-7. This value was chosen to inhibit hydrolysis of the clays and also to Journal of Colloid and Interface Science, Vol. 106, No. 1, July 1985
96
PEFFERKORN, NABZAR, AND CARROY
prevent Na + adsorption onto the clay surfaces (10). Finally, the clay dispersion was dialyzed against 1.5 × 10-4 M NaC1 at the pH at which adsorption experiments were to be performed. (b) The commercial glass beads AS 1, AS 2 (Verre et Industrie) were treated with hot chlorhydric acid in order to extract or exchange with H + all surface ionic exchangeable species. This treatment was performed until the supernatant liquor remained water white. The beads were then washed free of acid. The granulometry was determined with a particle counter (Coulter Counter, Coultronics). The harmonic mean diameter of the weight distribution and the specific surface area are reported in Table I, columns 2 and 3. (c) The microspheres AS 3 were synthesized by hydrolyzing ethyl silicate in a mixture of ammonia, water, and alcohol (11). Selected conditions yielded microspheres of 1.2-~zm average diameter with a narrow size distribution (a = 0.08) and a density of 1.28. The density differs from the density of pure silica (2.2) because, as already reported (12), the spherical silica particles are in fact a closepacked assembly of smaller elementary spherical silica granules of 40-A, diameter. 1 The interstices of this framework, however, are not sufficiently wide to permit diffusion of polymers of large size and the specific surface accessible for polymer adsorption was computed from the mean diameter determined from electron microscopy. ( d ) For surface modification of silica, glass or precipitated silica beads preheated at 110°C for 24 h under reduced pressure were treated chemically with aluminum chloride in anhydrous chloroform. To prepare a chloLet f be the volume fraction of silica and d the apparent density; then d = 2 . 2 f The Coulter counter was used for the determination of f After calibration this instrument gives the volume as well as the number o f particles in a known amount of dilute dispersion. The value d = 1.28 is close to 1.38, which corresponds to the density of an array of close-packed nonporous silica spheres. Journal of Colloid and Interface Science, Vol. 106, No. 1, July 1985
roform solution saturated with AIC13, excess AICI3 was first added to anhydrous chloroform at -40°C. This mixture was brought to 20°C and left at this temperature for about 24 h. The supernatant saturated in A1C13(0.8 g/liter) was poured under moisture proof conditions over the glass beads or the silica powder. After removing all free A1C13 by washing with chloroform the material was dried and stored in the presence of P205. Surface composition determination. In a detailed study, Peri has concluded that the hydrolysis at 25°C and pH 5-7 of silica surfaces treated with aluminum chloride gives rise to the presence of at least three different chemical units (13): OH
I
/.O~ ../AI
/AI~
°\si/°
°\si/°
o/AI
.~. O
\si /
(a)
(b) OH
OH
I
J
[3]
- - S i - - O - - S i --
(c) As shown by Sears, a titration of the modified silica surface with sodium hydroxide enables rapid determination of the relative proportions of A1-O-AI, A1-OH, and SiOH units (14). In analyzing our surface composition, we followed this procedure, which entails essentially two steps: (i) base is first added to a suspension of modified silica beads in water and a first pH rise observed at pH 7.5 corresponds to the titration of the aluminol \ / A1-OH groups only; (ii) the total number of aluminum atoms grafted to the silica surface is then determined after strong acid treatment at pH 1.5. Strong acid hydrolysis disrupts the Si-O-A1 links and the aluminum recovered in the supernatant solution in the hexacoordinate form is again titrated with sodium hydroxide. According to this analysis, the surface of our samples (AS 1-3) consists ofaluminol (b) and silanol (c) units, structure
ADSORPTION OF POLYACRYLAMIDE TO KAOLINITE (a) being present only in negligible proportions. The molar ratios (A1OH)/(SiOH) of surface units are reported in Table I for each sample.
Adsorption measurements of radioactive PAM. Sorbents AS 1 and AS 2 were dispersed in water in the presence of the polymer. The suspension was gently shaken to reduce the adsorption time. Suspensions of AS 3 and Na kaolinite were first subjected to ultrasonic treatment for 5 min to ensure complete dispersion prior to polymer addition. Experiments were performed using the device represented in Fig. 1. Each cylindrical test tube was fitted with a multiblade stirrer controlled by a single central stirring gear, and a dialysis membrane was clamped at the bottom of the tube. This membrane retains the polymer and the colloid in the test tube but allows free diffusion of the electrolyte and of water between the upper tube and the water below the membrane. Equilibrium conditions of the colloid suspension were maintained by controlling the pH and the ionic strength of the equilibrium liquid, which also assured thermostation. The PAM was dissolved in pure water and the pH was adjusted before addition to the suspension. The concentration of the polymer solution was calculated on the basis of the specific radioactivity, and the amount adsorbed per unit area of sorbent A was calculated, from the specific radioactivities Rb and R a of the supernatant solution
~ G
E
IHIIIIII IIIIIIIII I IIlllltlI I Ill]Ill f IWIII I IIIJlIIE AT~
DM
I
M
I
WR
FIG. 1. Diagrammatic representation of the experimental device for adsorption studies: SG, stirring gear; M, magnetic stirrer; MS, multiblade stirrer; E, electrode; AT, automatic pH titrator; TT, test tube; DM, dialysis membrane; S, suspension; WR, distilled water reservoir at constant pH, ionic strength, and temperature.
97
determined respectively before and after adsorption (the supernatant liquid was cleared by centrifugation), according to A - Rb -- R a
(mg m -z)
[4]
mSoE where m and S o represent the mass and the specific adsorption surface of the adsorbent and E, equal to 1.15 X 105 dpm mg -l, is the specific radioactivity of the polymer. RESULTS
1. Adsorption Isotherms The amounts of polymer adsorbed at 25°C per unit area of Na kaolinite and AS 3 adsorbent are represented in Figs. 2 and 3, respectively, as a function of the polymer equilibrium concentration of the suspension. Each curve represents results obtained at a fixed pH of the suspension. For Na kaolinite, the adsorption is expressed in milligrams per square centimeter of the edge surface (to the extent that adsorption proceeds on the edge face of Na kaolinite, this representation is convenient). The amount of adsorbed polymer as a function of the solution concentration follows the usual trend. In both cases, already at relatively low solution concentrations, adsorption tends to level off, although a horizontal plateau is not reached in either case. It should be noted that the adsorption isotherms on AS 3 were determined for a 1 wt% suspension (ca. 0.8 cm3/100 ml), whereas those on Na kaolinite were obtained for a 0.2 wt% suspension (ca. 0.08 cm3/100 ml). In the latter case, adsorption was found to depend on the colloid concentration (at pH 4, the value decreases from 36 to 26 mg/g in the concentration range 0-2 wt% suspension). Nevertheless, the value obtained at 0.2 wt% is close to the maximum adsorption, corresponding to the value extrapolated to zero concentration of colloid. For AS 3, a slight dependency of adsorption on the colloid concentration was noted at polymer concentrations below 5 × 10 -2 mg m1-1. At low Journal of Colloid and Interface Science, Vol. 106, No. 1, July 1985
98
PEFFERKORN, NABZAR, A N D C A R R O Y
fects, could reduce the available area for polymer adsorption; therefore only qualitative consideration should be given to results reported in Fig. 3 to the left of the dotted line. Adsorption isotherms for AS 1 and 2 have already been reported in a previous publication (15).
5 i A x 104(mg/cm 2)
2. pH Dependency of Adsorption The amounts of polymer adsorbed onto Na-kaolinite and onto adsorbents AS 1, AS 2, and AS 3 are represented as a function of ~ , , 4 - 1 + ~ 0 . . - . - - - - - - ~ 5.5 ; 7.2 the pH of the suspension in Figs. 4 and 5, 1 . /" ...-'. ./ l _ .. . _ . respectively. The ordinate As represents the value corresponding to the equilibrium polymer concentration 0.2 mg ml -l of the adz_ 0~ ] , 1 I 2 5 C x 10(mg/ml) sorption isotherms. For natural and the synthetic aluminosilicates, surface coverage deFIG. 2. Adsorption isothermsof polyacrylamide(Mw = 1.2 X 106) on Na kaolinite,0.2 wWo suspensions,1.5 pends strongly on the pH and a minimum × 10-4 M NaCl, at various pH. The number on the and a maximum are observed with varycurve is the pH of the suspension. ing pH. polymer concentrations steric stabilization does not operate and some particle association, occurring through polymer bridging ef-
A x 104(mg/cm 2)
DISCUSSION
In order to elucidate the origin of the polymer-surface interactions leading to the PAM-kaolinite complexes, which are central in our study, we first discuss the properties
pH4 i
,,
AsX 104(mg/cm 2)
pH 5
4 J
pH 6
5 pH 3
,~
=
pH
7
C x 10(mg/ml) FIG. 3. Adsorption isotherms of polyacrylamide (Mw = 1.2 × 106) on sorbent AS 3, 1 wt% suspensions, at various pH. The dotted line delimits the domain of the low polymer concentrations where flocculation falsifies the adsorbed amount. Journal of Colloid and Interface Science, VoL 106, No. 1, July 1985
2
1 pH 4
6
8
10
FIG. 4. Adsorption of polyacrylamide (Mw = 1.2 X 106) on Na kaolinite as a function of pH.
ADSORPTION OF POLYACRYLAMIDE TO KAOLINITE
A s x 104(mg/cm 2)
4
'~
fl •"
3
(b) N ~ a I 4
[ 5
~
I 6
I 7
pH =
of PAM adsorption onto the structurally simpler synthetic aluminosilicates.
1. Nature of PAM-AS 1-3 Interactions The effect of the surface density of A1OH groups on the adsorption seen in Fig. 5 suggests that PAM interacts with this surface site. In the same way, taking into account the variation of the adsorption with pH, one m a y infer that PAM reacts only with the neutral A1OH forms. The aluminol unit acts as an amphoteric hydroxyl and the dominant equilibrium reactions between positive, negative, and neutral surface sites in the indicated p H range are the following:
A1OH ~ A I O - + H
+
the requisite condition for PAM adsorption, i.e., the presence of "free" silanol groups on the surface, is obtained on fresh, nonhydrated silica or on silica regenerated by heat treatment. The strong adsorption on A1OH sites located on the silica surface is in line with this observation. The grafted aluminol group which protrudes away from the underlying primary silicate surface is separated from silanol groups by a greater distance than the separation of the residual silanols a m o n g themselves. Therefore, hydrogen bonds between aluminols and silanols cannot occur.
t
FIG. 5. Adsorption of polyacrylamide (Mw 1.2 X 106) on synthetic adsorbents as a function of pH. (a) AS 1, c = 5 X 10-3 mg/ml, (A1OH)/(SiOH) = 5%; (b) AS 2, c = 5 X 10-3 mg/ml, (AIOH)/(SiOH) = 12%; (c) AS 3, c = 0.1 mg/ml, (A1OH)/(SiOH) = 33%.
A1OH + H + ~ AlOHa-
99
p H < 4.5
[5]
pH>4.5.
[6]
It is well known from previous results (16) that aged pure silica, like precipitated silica, does not interact with PAM. However, PAM hydrogen bonds with silica in the presence of "free" silanol groups. By "free" silanols, we mean silanols which are not engaged in hydrogen bonding with nearby surface located hydroxyls. According to Griot and Kitchener,
2. Nature of PAM-Na Kaolinite Interactions The crystallographic structure of kaolinite corresponds to a 1:1 layer structure as shown in Fig. 6. The principal building elements are a single silica tetrahedral sheet and a single alumina octahedral sheet oriented so that the tips of the silica tetrahedrons and one of the layers of the octahedral sheet form a c o m m o n layer. This duality is preserved on the kaolinite flakes, which are generated by mechanical processes and cleavage. One basal face of the kaolinite plate (face A in Fig. 6) is poorly hydrated in aqueous media and possesses a small negative charge due to isomorphic exchange between SiO2 and AlOe. Interactions between surface oxygen atoms and hydrogen bonding agents are generally inexistant. Indeed, hydrogen bonds between water and the silicate lattice are weaker than water-water hydrogen bonding (17).
basalFaceA oo
sr ,
I /
tbasalfaceB FIG. 6. Atom arrangement in the unit cell of the kaolinite mineral [schematic, after ref. (9)]. Journal of Colloid and Interface Science, Vol. 106, No. l, July 1985
100
PEFFERKORN, NABZAR, AND CARROY
This holds true for formamide which, as shown previously, does not adsorb onto the basal faces of clay minerals from a water suspension (17). Ledoux and White pointed out that hydrogen bonds with the hydroxyls and oxygens of the kaolinite basal face (intercalation) is weaker than the intermolecular hydrogen bonds in pure formamide (18). It is also necessary to consider the possible effect on the amide groups of the counter ions of the permanently charged silica face (19). In anhydrous media, amide protonation has been observed at montmorillonite and kaolinite surfaces with the exchangeable cations H +, A13+, and Fe 3+ but not with Na + (20, 21). In water, protonation seems to be even weaker as a result of the polarization forces which are distributed among a large number of water molecules. Moreover, from the small cec of the clay and the low A13+/ Na + ratio resulting from the careful pretreatment of the clay, no quantitative adsorption was expected. In addition, previous work has shown that the basal face of the alumina sheet (face B in Fig. 6) is hydrated in aqueous suspension and unaffected by acid or alkaline conditions (22). On account of the regular hexagonal organization of the aluminols and their chemical stability, this aluminol surface resembles in its chemical properties an "aged" silica surface in so far as internal hydrogen bonds preclude interactions with hydrogen bonding agents. Let us further point out that Dodson and Somasundaran were able to desorb up to 50% of adsorbed PAM from Na kaolinite by addition of polyphosphate to a kaolinitePAM suspension (23) and that polyphosphate has in fact been shown to interact specifically with the alumina groups on the lateral face of kaolinite (24, 25). From those foregoing arguments, it may be concluded that PAM probably does not interact with the basal faces of Na kaolinite. We shall now attempt to explain the nature of the interactions occurring between PAM and the constitutive components of the edge faces and the depenJournal of Colloid and Interface Science, Vol. 106, No. 1, July 1985
dency of adsorption on pH (Fig. 4) and concentration. a. Adsorption-pH relationship. Let us consider the charge characteristics of the edge faces (26). According to Schofield and Samson (27), the broken edges of the kaolin platelets are composed of exposed silica and alumina and, owing to the aluminol units, are positively charged in acidic media and negatively charged in alkaline media (cf. Fig. 7). The zero electrokinetic potential is in the range pH 5 to 8 (28). Following a method described by Fripiat (29) to determine the surface of the clay, we have titrated the kaolinite sample with NaOH and HC1, starting at pH 7. The consumed base may be attributed to neutralization of the edge faces. However, protonation of aluminum on the edges and ion exchange between H + and the cations of the basal permanent charge cannot be analyzed separately. Disregarding at first approximation the latter effect which is in any case small, we report in Fig. 8 the net charge of the edge faces as a function of pH. One observes very little variation &the net charge in the region of the isoelectric point of the edge faces (IEPS -- 7). There is on the contrary a large variation below pH 5.5 or above pH 7.5. The acidity of silanol groups on pure silica and on aluminosilicates has been found to be identical (30) and dissociation of silanol becomes important only at pH > 7.5 (31). From the titration data reported in Fig. 8 and taking into account the amide-"free" hydroxyl interactions previously discussed in the context of adsorption onto the aluminosilicate of simpler structure (cf. Sect. 1 under Discussion), it is possible to
AI
|
~A[ I "~l-I .J (o) (b)
I ~OH21
sAI I~)H (c)
F]G. 7. Yafiations in charge characteristics of the edge face of kaolinite under acid (a), neutral (b), and alkaline (c) conditions [after ref. (27)].
ADSORPTION OF POLYACRYLAMIDE TO KAOLINITE
b. Adsorption-concentration
[FH+-FOHJ (/~e'q./g)
/
!
I /
i
101
////
-3 pH 4 5 6 7 8 9 FIG. 8. Relative electrical charge of the edge face of Na kaolinite as a function of pH. -4
establish a straightforward correlation between the adsorption of PAM and the kaolinite edge surface properties. (i) pH 5.5-7.5 corresponds to H ÷ concentrations at which neutral silanol and neutral aluminol groups coexist on the edge as inferred from Fig. 8. This leads to a minimum in adsorption as seen in Fig. 4. The only way to account for this minimum is in terms of s i l a n o l - a l u m i n o l interactions, perhaps through hydrogen bonding of water between the silanol oxygen and the c o m m o n Si-A1 oxygen (edge-ligand water) (32), which would prevent hydroxyl-amide association at the edge. (ii) Protonation of the aluminol or ionization of the silanol groups on the edge (cf. Fig. 7) promotes interactions between amide groups and the remaining "free" hydroxyls. Therefore, the amount of polymer adsorbed should correspond to the density of the "free" hydroxyl groups; i.e., the adsorption should parallel the variation of the absolute net charge of the kaolinite active faces. According to Figs. 8 and 4, it is clear that this correlation is at least qualitatively verified.
relationship.
Theoretical relations for the adsorption isotherms of polymers are based on considerations of the interfacial structure of adsorbed macromolecules and evaluation of the energetic gain for systems in thermodynamic equilibrium. The last point, in view of the dynamic properties of interfacial polymeric systems in contact with a polymeric solution, is subject to controversy (15). Nevertheless, the Langmuir equation has been used to obtain the "equilibrium" parameters characterizing polymer adsorption (33, 34). We have tested in Fig. 9 the applicability of the Langmuir relation [7] over the entire pH range, As 1 ~- c = ~ . + c, [71 where A = amount adsorbed per unit area adsorbent at solution concentration c, As = value of A at saturation coverage, and K = adsorptivity constant. For acidic and al-
A .C s (mg/ml) A OJ~
//s•l• / 0.1( • ///'" /,,~" / •
/ o °
+
i
~o61 0.04
0.02
(" )
%-
0.02
0.04
0.06
C(mg/ml) 0.08
FIG. 9. Langmuir's plot for adsorption isotherms of polyacrylarnide on Na kaolinite at various pH: 4 (v), 4.5 (~), 5.5 (O), 7.2 (A), 8.5 (O), 9 (+), 10 (0). Journal of Colloid and Interface Science. Vol. 106, No. 1, July 1985
102
PEFFERKORN, NABZAR, AND CARROY
kaline conditions, the Langmuir representation yields a single straight line with the constant K equal to 0.08 liter/mg. At p H 5.5 and 7.2, the extrapolated value of K is, according to Eq. [7], equal to 0.03 liter/mg. These results support the hypothesis of edge adsorption; i.e., (i) in acidic and alkaline media, interactions between PAM and Na kaolinite occur through hydrogen bonding between hydroxyls and amide groups, while (ii) in neutral media, the P A M - N a kaolinite affinity decreases. CONCLUSION It has been shown that interactions between nonhydrolyzed PAM and aluminosilicates are governed by hydrogen bonding. Although the p H has a negligible effect on the conformational properties of PAM in aqueous solution (at least in the explored domain), this parameter controls the interfacial behavior of the polymer. The variations in adsorption closely follow the modifications of the electrochemical surface properties. The most interesting observation concerns the opposing behavior of the two faces of kaolinite: whereas the basal faces act as nonadsorbing surfaces, the edge faces modulate the adsorptive capacity. This mechanism has recently been invoked by the authors in the interpretation of the thermodynamic stability o f Na kaolinite suspensions in the presence of PAM (35). The question raised in the introduction regarding the adsorption of partially hydrolyzed polyacrylamide onto aluminosilicates remains open: it is of course not relevant to generalize the mechanism proposed for the nonhydrolyzed polymer. Recent experimental results appear to confirm that in this case, despite long-range polyelectrolyte-charged surface interactions, hydrogen bonding is still operating (23). Nevertheless, the adsorption should vary differently on both sides of the isoelectric point of kaolinite, as a result of positive-negative charge attraction and negative-negative charge repulsion. In addition, the structural characteristics of the polyelecJournal of Colloid and Interface Science, Vol. 106, No. 1, July 1985
trolytes are altered on adsorption and this subject would be of interest in future studies. ACKNOWLEDGMENTS The research program is supported by Project ATP No. 269 PIRSEM "Energie et Mati~res Premieres 1982" of the Centre National de la Recherche Scientifique Franqaise. A. Carroy thanks the Institut Franqais du P~trole and L. Nabzar is grateful to the Centre Technique du Papier of Grenoble for grants enabling his stay at the CRM. We also thank Dr. R. Varoqui for many helpful discussions. REFERENCES 1. Vincent, B., and Whittington, S. G., in "Surface and Colloid Science" (E. Matijevic Ed.), Vol. 12. Plenum, New York, 1982. 2. de Gennes, P. G., Macromolecules 14, 1637 (1981). de Gennes, P.G., Macromolecules 15, 492 (1982). 3. Fleer, G. J., and Lyklema, J., in "Adsorption from Solution at the Solid/Liquid Interface" (C. D. Parfitt and C. A. Rochester, Eds.). Academic Press, New York, 1983. 4. Schmitt, A., "Macromolecules aux interfaces. Adsorption ~ l'interfacesolide/liquide," Course, Ecole d'Et6 d'Aussois: "Colloides et Interfaces" (1983). 5. Schulz, R. C., Kaiser, E., and Kern, W., Makromol. Chem. 58, 160 (1962). 6. Franqois, J., Sarazin, D., Schwartz, T., and Weill, G., Polymer 20, 969 (1979). 7. Thomas, F,, Th~se, Universit~ de Nancy I, Nancy (1982). 8. Grim, R. E., "Clay Mineralogy," Chap. 7. McGrawHill, New York, 1968. 9. Van Olphen, H., "An Introduction to Clay Colloid Chemistry." Wiley, New York, 1977. 10. Ferris, A. P., and Jepson, W. B., J. Co[[oidInterface Sci. 51, 245 (1975), 11. Stober, W., Fink, A., and Bohn, E., J. Colloid Interface Sci. 26, 62 (1968), 12. McMillan, D., U.S. Patent 3, 591, 518 (Du Pont) (1971). 13. Peri, J. B., J. Catal. 41, 227 (1976). 14. Sears, G. W., Anal. Chem. 28, 1981 (1956). 15. Pefferkorn, E., Carroy, A., and Varoqui, R., J. Polym. Sci., in press. 16. Griot, O., and Kitchener, J. A., Trans. Faraday Soc. 61, 1026 (1965); 61, 1032 (1965). 17. Mortland, M. M., Adv. Agron. 57, 22 (1970). 18. Ledoux, R. L., and White, J. L., J. Colloid Interface Sci. 21, 127 (1966). 19. Weiss, A., and Russow, J., Proc. Int. Clay Conf. Stockholm 203 (1963).
ADSORPTION OF POLYACRYLAMIDE TO KAOLINITE 20. Mortland, M. M., Clay Min. 6, 143 (1966). 21. Dekking, H. G. G., Clays Clay Miner. 12, 603 (1964). 22. Theng, B. K. G., "Formation and Properties of Clay-Polymer Complexes." Elsevier/North-Holland, Amsterdam, 1979. 23. Dodson, P. J., and Somasundaran, P., J. Colloid Interface Sci. 97, 481 (1984). 24. Swartzen-Allen, S. L., and Matijevic, E., Chem. Rev. 74, 385 (1974). 25. Lyons, J. W., J. ColloidSci. 19, 399 (1964). 26. Flegman, A. W., Goodwin, J. W., and Ottewill, R. H., Proc. Brit. Ceram. Soc. 13, 31 (1969). 27. Schofield, R. K., and Samson, M. R., Clay Miner. Bull. 2, 45 (1953).
103
28. Williams, D. J. A., and Williams, K. P., J. Colloid Interface Sci. 65, 79 (1978). 29. Fripiat, J. J., Bull. Groupe Fr. Argiles N.S. 9(4), 23 (1957). 30. Hair, M. L., and Hertl, W., J. Phys. Chem. 91, 74 (1970). 31. ller, R. K., "The Chemistry of Silica." Wiley, New York, 1979. 32. Conley, R. F., and Althoff, A. C., J. Colloid Interface Sci. 37, 186 (1971). 33. Lipatov, Y. S., and Sergeeva, L. M. "Adsorption of Polymers." Wiley, New York, 1972. 34. Schamp, N., and Huylebroeck, J., J. Polym. Sci. Symp. 42, 553 (1973). 35. Nabzar, L., Pefferkorn, E., and Varoqui, R., J. Colloid Interface Sci. 102, 380 (1984).
Journal of Colloid and Interface Science, Vol. 106, No. 1, July 1985
Interactions between nonionic polyacrylamideand Na kaolinite in aqueous suspensions are discussed in terms of amide-hydroxyl H bonding. Correlations obtained by comparingthe extent of adsorption of the polymer onto the clay mineral and onto synthetic aluminosilicates at different pH reveal the dominant effect of the surface density of "free" hydroxyl groups acting as adsorbing sites on both adsorbent types. Most interesting is the opposingbehavior of the basal and edge faces of the kaolinite with regard to the polymer/surface affinity. Indeed, two crystallographically different surfaces are exposed by the plate-like panicles: in relation to a polymer present in aqueous suspension, the basal faces act as repulsive surfaces and the edge faces act as adsorbing ones. The pH controls the extent of polymer adsorption. Variation of this parameter influences the sign and magnitude of the electrical charge of the edges and thus leads to changes in the surface density of "free" hydroxyls,present either as silanol in the acidic range or as aluminol in the alkaline range. Results concerningthe variation of the surface coveragewith polymerconcentration are discussed in terms of the nature of the interactions between the un-ionized polyacrylamideand Na kaolinite. © 1985AcademicPress.Inc. a priori the respective participation of the crystal edges and faces in the polymer uptake. Any rationalization requires an investigation of the nature of the forces acting among monomers and the constitutive elements of edge and basal faces. In this context, it is important to obtain information on the adsorption of polyacrylamide on particles of simpler morphology but with a chemical composition approaching that of either the clay edges or the basal faces. Following this line of approach, we have synthesized aluminosilicates of spherical shape with a definite AIlSi surface composition. This modeling, although still crude, nevertheless provides better insight into the adsorption mechanism of polar macromolecules onto clay sorbents of more complex structure. In this work, for simplicity, we have restricted the study to the adsorption of nonhydrolyzed polyacrylamide. Indeed, since the surface of kaolinite and other layer silicates may acquire negative and positive charges, depending on the p H of the medium, long-
INTRODUCTION
The ability of polymers to destabilize clay suspensions is well known. This subject owing to its practical interest received widespread attention during the last decade (1-4); nevertheless, one is still in search of a comprehensive and consistent picture of the relevant factors describing the interactions between clays and hydrosoluble polymers. In the present work, we have focused attention on the adsorption of nonhydrolyzed polyacrylamide on Na kaolinite in aqueous media; our aim is to elucidate the mechanism of clay-polyacrylamide interactions in terms of the crystalline structure of the mineral. Kaolinite is a hydrous aluminosilicate of layer structure comprising silica and alumina sheets stacked on top of each other. It is widely recognized that the edge surfaces of kaolinite plates differ fundamentally in their structural and electrochemical properties from the cleavage surfaces. In view of this property, inherent in all clays, it is difficult to predict 94 0021-9797/85 $3.00 Copyright© 1985 by AcademicPress, Inc. All rights of reproduction in any form reserved.
Journalof Colloidand InterfaceScience. Vol. 106, No. 1, July 1985
ADSORPTION OF POLYACRYLAMIDE TO KAOLINITE range electrostatic interactions a m o n g clays and polyelectrolytes are expected to occur and the situation is in reality more complicated. MATERIALS AND METHOD
Polymer. A statistical copolymer (acrylamide-acrolein) with an (acrolein)/(acrylamide) m o n o m e r ratio of 10 -3 was prepared by radical copolymerization in aqueous media according to known techniques (5):
TABLE I Na kaolinite Average dimensions Plate diameter Plate thickness Edge specific surface Base specific surface Particle size distribution Finer than 0.1 ~m Finer than 0.16 um Coarser than 10 um
I
C=O
HC=O
I NH2 n
10_ 3
0.21 ~m 0.04 ~m 8.3 rn2 g-i 18.5 m2 g-~ 12% 50% negligible
Spherical beads modified by A1C13
~ CH2-- C H ] w - - [ C H 2 - - C H J~ n,
I
95
Sample type code
Average diameter (#m)
Specific surface area (ttm)
Surface site (AIOH)/(SiOH) ratio
AS 1 (b)(d) AS 2 (b) (d) AS 3 (c) (d)
34 34 1.2
7.8 × 10-2 7.8 × 10-2 4
5% 12% 33%
[1]
m
[r/] = 8.42 × 10-3M~ 757
(cm3/g).
[2]
After purification and fractionation (6), the polymer samples were characterized by viscosity and light scattering in 0.1 M NaC1 at 25°C and relationship [2] was found. A polymer sample of molecular weight 1.2 × 106 (Mw/Mn -- 1.3) was then selected and reacted in alkaline aqueous solution (pH 9.5, T = 25°C) with tritiated potassium borohydride. By reduction of acrolein to the propenol function CHz-CH(CaHzOH) -, a radioactive polyacrylamide of specific radioactivity 0.1 mCi/g was obtained. During this reaction, hydrolysis of acrylamide functions is less than 1%. Adsorbents. Adsorption measurements were performed for polyacrylamide (PAM) on Na kaolinite clay and on a d s o r b e n t s - denoted AS 1-3 in Table I - - w h i c h were either nonporous spherical glass beads (AS 1 and AS 2) or spherical amorphous silica beads (AS 3) treated with A1CI3. The characteristics of the adsorbents are reported in Table I. (a) Kaolin des Charentes (French origin) was supplied by the Centre de Recherches sur la Valorisation des Minerals, Nancy).
The sample with code-type name G Z A IV (7) corresponds to a kaolin of 86% kaolinite. The granulometric and morphological characteristics are listed in Table I. Compared to kaolinite of different origin (8), our sample is characterized by a large SE/(SE + SB) ratio, SE and SR being the edge and basal specific surface areas, respectively. Previous analysis did not show the presence of organic materials in the sample and purification by oxidizing agents was not judged necessary. In order to prevent any interaction of polyacrylamide with di- and trivalent cations such as A13+ or Fe z+ always present in crude kaolin, the heteroionic clay was converted to the homoionic sodium form according to the method r e c o m m e n d e d by Van Olphen (9). The clay powder was suspended in 1-2 M NaC1 distilled water and left under gentle shaking for 24 h. After multiple decantations and resuspensions in fresh electrolyte solution, the sediment was resuspended in pure water at p H 6-7 until the clay remained dispersed. The water treatment was stopped after the electrolyte concentration reached 1.5 × 10 -4 M NaC1 at p H 6-7. This value was chosen to inhibit hydrolysis of the clays and also to Journal of Colloid and Interface Science, Vol. 106, No. 1, July 1985
96
PEFFERKORN, NABZAR, AND CARROY
prevent Na + adsorption onto the clay surfaces (10). Finally, the clay dispersion was dialyzed against 1.5 × 10-4 M NaC1 at the pH at which adsorption experiments were to be performed. (b) The commercial glass beads AS 1, AS 2 (Verre et Industrie) were treated with hot chlorhydric acid in order to extract or exchange with H + all surface ionic exchangeable species. This treatment was performed until the supernatant liquor remained water white. The beads were then washed free of acid. The granulometry was determined with a particle counter (Coulter Counter, Coultronics). The harmonic mean diameter of the weight distribution and the specific surface area are reported in Table I, columns 2 and 3. (c) The microspheres AS 3 were synthesized by hydrolyzing ethyl silicate in a mixture of ammonia, water, and alcohol (11). Selected conditions yielded microspheres of 1.2-~zm average diameter with a narrow size distribution (a = 0.08) and a density of 1.28. The density differs from the density of pure silica (2.2) because, as already reported (12), the spherical silica particles are in fact a closepacked assembly of smaller elementary spherical silica granules of 40-A, diameter. 1 The interstices of this framework, however, are not sufficiently wide to permit diffusion of polymers of large size and the specific surface accessible for polymer adsorption was computed from the mean diameter determined from electron microscopy. ( d ) For surface modification of silica, glass or precipitated silica beads preheated at 110°C for 24 h under reduced pressure were treated chemically with aluminum chloride in anhydrous chloroform. To prepare a chloLet f be the volume fraction of silica and d the apparent density; then d = 2 . 2 f The Coulter counter was used for the determination of f After calibration this instrument gives the volume as well as the number o f particles in a known amount of dilute dispersion. The value d = 1.28 is close to 1.38, which corresponds to the density of an array of close-packed nonporous silica spheres. Journal of Colloid and Interface Science, Vol. 106, No. 1, July 1985
roform solution saturated with AIC13, excess AICI3 was first added to anhydrous chloroform at -40°C. This mixture was brought to 20°C and left at this temperature for about 24 h. The supernatant saturated in A1C13(0.8 g/liter) was poured under moisture proof conditions over the glass beads or the silica powder. After removing all free A1C13 by washing with chloroform the material was dried and stored in the presence of P205. Surface composition determination. In a detailed study, Peri has concluded that the hydrolysis at 25°C and pH 5-7 of silica surfaces treated with aluminum chloride gives rise to the presence of at least three different chemical units (13): OH
I
/.O~ ../AI
/AI~
°\si/°
°\si/°
o/AI
.~. O
\si /
(a)
(b) OH
OH
I
J
[3]
- - S i - - O - - S i --
(c) As shown by Sears, a titration of the modified silica surface with sodium hydroxide enables rapid determination of the relative proportions of A1-O-AI, A1-OH, and SiOH units (14). In analyzing our surface composition, we followed this procedure, which entails essentially two steps: (i) base is first added to a suspension of modified silica beads in water and a first pH rise observed at pH 7.5 corresponds to the titration of the aluminol \ / A1-OH groups only; (ii) the total number of aluminum atoms grafted to the silica surface is then determined after strong acid treatment at pH 1.5. Strong acid hydrolysis disrupts the Si-O-A1 links and the aluminum recovered in the supernatant solution in the hexacoordinate form is again titrated with sodium hydroxide. According to this analysis, the surface of our samples (AS 1-3) consists ofaluminol (b) and silanol (c) units, structure
ADSORPTION OF POLYACRYLAMIDE TO KAOLINITE (a) being present only in negligible proportions. The molar ratios (A1OH)/(SiOH) of surface units are reported in Table I for each sample.
Adsorption measurements of radioactive PAM. Sorbents AS 1 and AS 2 were dispersed in water in the presence of the polymer. The suspension was gently shaken to reduce the adsorption time. Suspensions of AS 3 and Na kaolinite were first subjected to ultrasonic treatment for 5 min to ensure complete dispersion prior to polymer addition. Experiments were performed using the device represented in Fig. 1. Each cylindrical test tube was fitted with a multiblade stirrer controlled by a single central stirring gear, and a dialysis membrane was clamped at the bottom of the tube. This membrane retains the polymer and the colloid in the test tube but allows free diffusion of the electrolyte and of water between the upper tube and the water below the membrane. Equilibrium conditions of the colloid suspension were maintained by controlling the pH and the ionic strength of the equilibrium liquid, which also assured thermostation. The PAM was dissolved in pure water and the pH was adjusted before addition to the suspension. The concentration of the polymer solution was calculated on the basis of the specific radioactivity, and the amount adsorbed per unit area of sorbent A was calculated, from the specific radioactivities Rb and R a of the supernatant solution
~ G
E
IHIIIIII IIIIIIIII I IIlllltlI I Ill]Ill f IWIII I IIIJlIIE AT~
DM
I
M
I
WR
FIG. 1. Diagrammatic representation of the experimental device for adsorption studies: SG, stirring gear; M, magnetic stirrer; MS, multiblade stirrer; E, electrode; AT, automatic pH titrator; TT, test tube; DM, dialysis membrane; S, suspension; WR, distilled water reservoir at constant pH, ionic strength, and temperature.
97
determined respectively before and after adsorption (the supernatant liquid was cleared by centrifugation), according to A - Rb -- R a
(mg m -z)
[4]
mSoE where m and S o represent the mass and the specific adsorption surface of the adsorbent and E, equal to 1.15 X 105 dpm mg -l, is the specific radioactivity of the polymer. RESULTS
1. Adsorption Isotherms The amounts of polymer adsorbed at 25°C per unit area of Na kaolinite and AS 3 adsorbent are represented in Figs. 2 and 3, respectively, as a function of the polymer equilibrium concentration of the suspension. Each curve represents results obtained at a fixed pH of the suspension. For Na kaolinite, the adsorption is expressed in milligrams per square centimeter of the edge surface (to the extent that adsorption proceeds on the edge face of Na kaolinite, this representation is convenient). The amount of adsorbed polymer as a function of the solution concentration follows the usual trend. In both cases, already at relatively low solution concentrations, adsorption tends to level off, although a horizontal plateau is not reached in either case. It should be noted that the adsorption isotherms on AS 3 were determined for a 1 wt% suspension (ca. 0.8 cm3/100 ml), whereas those on Na kaolinite were obtained for a 0.2 wt% suspension (ca. 0.08 cm3/100 ml). In the latter case, adsorption was found to depend on the colloid concentration (at pH 4, the value decreases from 36 to 26 mg/g in the concentration range 0-2 wt% suspension). Nevertheless, the value obtained at 0.2 wt% is close to the maximum adsorption, corresponding to the value extrapolated to zero concentration of colloid. For AS 3, a slight dependency of adsorption on the colloid concentration was noted at polymer concentrations below 5 × 10 -2 mg m1-1. At low Journal of Colloid and Interface Science, Vol. 106, No. 1, July 1985
98
PEFFERKORN, NABZAR, A N D C A R R O Y
fects, could reduce the available area for polymer adsorption; therefore only qualitative consideration should be given to results reported in Fig. 3 to the left of the dotted line. Adsorption isotherms for AS 1 and 2 have already been reported in a previous publication (15).
5 i A x 104(mg/cm 2)
2. pH Dependency of Adsorption The amounts of polymer adsorbed onto Na-kaolinite and onto adsorbents AS 1, AS 2, and AS 3 are represented as a function of ~ , , 4 - 1 + ~ 0 . . - . - - - - - - ~ 5.5 ; 7.2 the pH of the suspension in Figs. 4 and 5, 1 . /" ...-'. ./ l _ .. . _ . respectively. The ordinate As represents the value corresponding to the equilibrium polymer concentration 0.2 mg ml -l of the adz_ 0~ ] , 1 I 2 5 C x 10(mg/ml) sorption isotherms. For natural and the synthetic aluminosilicates, surface coverage deFIG. 2. Adsorption isothermsof polyacrylamide(Mw = 1.2 X 106) on Na kaolinite,0.2 wWo suspensions,1.5 pends strongly on the pH and a minimum × 10-4 M NaCl, at various pH. The number on the and a maximum are observed with varycurve is the pH of the suspension. ing pH. polymer concentrations steric stabilization does not operate and some particle association, occurring through polymer bridging ef-
A x 104(mg/cm 2)
DISCUSSION
In order to elucidate the origin of the polymer-surface interactions leading to the PAM-kaolinite complexes, which are central in our study, we first discuss the properties
pH4 i
,,
AsX 104(mg/cm 2)
pH 5
4 J
pH 6
5 pH 3
,~
=
pH
7
C x 10(mg/ml) FIG. 3. Adsorption isotherms of polyacrylamide (Mw = 1.2 × 106) on sorbent AS 3, 1 wt% suspensions, at various pH. The dotted line delimits the domain of the low polymer concentrations where flocculation falsifies the adsorbed amount. Journal of Colloid and Interface Science, VoL 106, No. 1, July 1985
2
1 pH 4
6
8
10
FIG. 4. Adsorption of polyacrylamide (Mw = 1.2 X 106) on Na kaolinite as a function of pH.
ADSORPTION OF POLYACRYLAMIDE TO KAOLINITE
A s x 104(mg/cm 2)
4
'~
fl •"
3
(b) N ~ a I 4
[ 5
~
I 6
I 7
pH =
of PAM adsorption onto the structurally simpler synthetic aluminosilicates.
1. Nature of PAM-AS 1-3 Interactions The effect of the surface density of A1OH groups on the adsorption seen in Fig. 5 suggests that PAM interacts with this surface site. In the same way, taking into account the variation of the adsorption with pH, one m a y infer that PAM reacts only with the neutral A1OH forms. The aluminol unit acts as an amphoteric hydroxyl and the dominant equilibrium reactions between positive, negative, and neutral surface sites in the indicated p H range are the following:
A1OH ~ A I O - + H
+
the requisite condition for PAM adsorption, i.e., the presence of "free" silanol groups on the surface, is obtained on fresh, nonhydrated silica or on silica regenerated by heat treatment. The strong adsorption on A1OH sites located on the silica surface is in line with this observation. The grafted aluminol group which protrudes away from the underlying primary silicate surface is separated from silanol groups by a greater distance than the separation of the residual silanols a m o n g themselves. Therefore, hydrogen bonds between aluminols and silanols cannot occur.
t
FIG. 5. Adsorption of polyacrylamide (Mw 1.2 X 106) on synthetic adsorbents as a function of pH. (a) AS 1, c = 5 X 10-3 mg/ml, (A1OH)/(SiOH) = 5%; (b) AS 2, c = 5 X 10-3 mg/ml, (AIOH)/(SiOH) = 12%; (c) AS 3, c = 0.1 mg/ml, (A1OH)/(SiOH) = 33%.
A1OH + H + ~ AlOHa-
99
p H < 4.5
[5]
pH>4.5.
[6]
It is well known from previous results (16) that aged pure silica, like precipitated silica, does not interact with PAM. However, PAM hydrogen bonds with silica in the presence of "free" silanol groups. By "free" silanols, we mean silanols which are not engaged in hydrogen bonding with nearby surface located hydroxyls. According to Griot and Kitchener,
2. Nature of PAM-Na Kaolinite Interactions The crystallographic structure of kaolinite corresponds to a 1:1 layer structure as shown in Fig. 6. The principal building elements are a single silica tetrahedral sheet and a single alumina octahedral sheet oriented so that the tips of the silica tetrahedrons and one of the layers of the octahedral sheet form a c o m m o n layer. This duality is preserved on the kaolinite flakes, which are generated by mechanical processes and cleavage. One basal face of the kaolinite plate (face A in Fig. 6) is poorly hydrated in aqueous media and possesses a small negative charge due to isomorphic exchange between SiO2 and AlOe. Interactions between surface oxygen atoms and hydrogen bonding agents are generally inexistant. Indeed, hydrogen bonds between water and the silicate lattice are weaker than water-water hydrogen bonding (17).
basalFaceA oo
sr ,
I /
tbasalfaceB FIG. 6. Atom arrangement in the unit cell of the kaolinite mineral [schematic, after ref. (9)]. Journal of Colloid and Interface Science, Vol. 106, No. l, July 1985
100
PEFFERKORN, NABZAR, AND CARROY
This holds true for formamide which, as shown previously, does not adsorb onto the basal faces of clay minerals from a water suspension (17). Ledoux and White pointed out that hydrogen bonds with the hydroxyls and oxygens of the kaolinite basal face (intercalation) is weaker than the intermolecular hydrogen bonds in pure formamide (18). It is also necessary to consider the possible effect on the amide groups of the counter ions of the permanently charged silica face (19). In anhydrous media, amide protonation has been observed at montmorillonite and kaolinite surfaces with the exchangeable cations H +, A13+, and Fe 3+ but not with Na + (20, 21). In water, protonation seems to be even weaker as a result of the polarization forces which are distributed among a large number of water molecules. Moreover, from the small cec of the clay and the low A13+/ Na + ratio resulting from the careful pretreatment of the clay, no quantitative adsorption was expected. In addition, previous work has shown that the basal face of the alumina sheet (face B in Fig. 6) is hydrated in aqueous suspension and unaffected by acid or alkaline conditions (22). On account of the regular hexagonal organization of the aluminols and their chemical stability, this aluminol surface resembles in its chemical properties an "aged" silica surface in so far as internal hydrogen bonds preclude interactions with hydrogen bonding agents. Let us further point out that Dodson and Somasundaran were able to desorb up to 50% of adsorbed PAM from Na kaolinite by addition of polyphosphate to a kaolinitePAM suspension (23) and that polyphosphate has in fact been shown to interact specifically with the alumina groups on the lateral face of kaolinite (24, 25). From those foregoing arguments, it may be concluded that PAM probably does not interact with the basal faces of Na kaolinite. We shall now attempt to explain the nature of the interactions occurring between PAM and the constitutive components of the edge faces and the depenJournal of Colloid and Interface Science, Vol. 106, No. 1, July 1985
dency of adsorption on pH (Fig. 4) and concentration. a. Adsorption-pH relationship. Let us consider the charge characteristics of the edge faces (26). According to Schofield and Samson (27), the broken edges of the kaolin platelets are composed of exposed silica and alumina and, owing to the aluminol units, are positively charged in acidic media and negatively charged in alkaline media (cf. Fig. 7). The zero electrokinetic potential is in the range pH 5 to 8 (28). Following a method described by Fripiat (29) to determine the surface of the clay, we have titrated the kaolinite sample with NaOH and HC1, starting at pH 7. The consumed base may be attributed to neutralization of the edge faces. However, protonation of aluminum on the edges and ion exchange between H + and the cations of the basal permanent charge cannot be analyzed separately. Disregarding at first approximation the latter effect which is in any case small, we report in Fig. 8 the net charge of the edge faces as a function of pH. One observes very little variation &the net charge in the region of the isoelectric point of the edge faces (IEPS -- 7). There is on the contrary a large variation below pH 5.5 or above pH 7.5. The acidity of silanol groups on pure silica and on aluminosilicates has been found to be identical (30) and dissociation of silanol becomes important only at pH > 7.5 (31). From the titration data reported in Fig. 8 and taking into account the amide-"free" hydroxyl interactions previously discussed in the context of adsorption onto the aluminosilicate of simpler structure (cf. Sect. 1 under Discussion), it is possible to
AI
|
~A[ I "~l-I .J (o) (b)
I ~OH21
sAI I~)H (c)
F]G. 7. Yafiations in charge characteristics of the edge face of kaolinite under acid (a), neutral (b), and alkaline (c) conditions [after ref. (27)].
ADSORPTION OF POLYACRYLAMIDE TO KAOLINITE
b. Adsorption-concentration
[FH+-FOHJ (/~e'q./g)
/
!
I /
i
101
////
-3 pH 4 5 6 7 8 9 FIG. 8. Relative electrical charge of the edge face of Na kaolinite as a function of pH. -4
establish a straightforward correlation between the adsorption of PAM and the kaolinite edge surface properties. (i) pH 5.5-7.5 corresponds to H ÷ concentrations at which neutral silanol and neutral aluminol groups coexist on the edge as inferred from Fig. 8. This leads to a minimum in adsorption as seen in Fig. 4. The only way to account for this minimum is in terms of s i l a n o l - a l u m i n o l interactions, perhaps through hydrogen bonding of water between the silanol oxygen and the c o m m o n Si-A1 oxygen (edge-ligand water) (32), which would prevent hydroxyl-amide association at the edge. (ii) Protonation of the aluminol or ionization of the silanol groups on the edge (cf. Fig. 7) promotes interactions between amide groups and the remaining "free" hydroxyls. Therefore, the amount of polymer adsorbed should correspond to the density of the "free" hydroxyl groups; i.e., the adsorption should parallel the variation of the absolute net charge of the kaolinite active faces. According to Figs. 8 and 4, it is clear that this correlation is at least qualitatively verified.
relationship.
Theoretical relations for the adsorption isotherms of polymers are based on considerations of the interfacial structure of adsorbed macromolecules and evaluation of the energetic gain for systems in thermodynamic equilibrium. The last point, in view of the dynamic properties of interfacial polymeric systems in contact with a polymeric solution, is subject to controversy (15). Nevertheless, the Langmuir equation has been used to obtain the "equilibrium" parameters characterizing polymer adsorption (33, 34). We have tested in Fig. 9 the applicability of the Langmuir relation [7] over the entire pH range, As 1 ~- c = ~ . + c, [71 where A = amount adsorbed per unit area adsorbent at solution concentration c, As = value of A at saturation coverage, and K = adsorptivity constant. For acidic and al-
A .C s (mg/ml) A OJ~
//s•l• / 0.1( • ///'" /,,~" / •
/ o °
+
i
~o61 0.04
0.02
(" )
%-
0.02
0.04
0.06
C(mg/ml) 0.08
FIG. 9. Langmuir's plot for adsorption isotherms of polyacrylarnide on Na kaolinite at various pH: 4 (v), 4.5 (~), 5.5 (O), 7.2 (A), 8.5 (O), 9 (+), 10 (0). Journal of Colloid and Interface Science. Vol. 106, No. 1, July 1985
102
PEFFERKORN, NABZAR, AND CARROY
kaline conditions, the Langmuir representation yields a single straight line with the constant K equal to 0.08 liter/mg. At p H 5.5 and 7.2, the extrapolated value of K is, according to Eq. [7], equal to 0.03 liter/mg. These results support the hypothesis of edge adsorption; i.e., (i) in acidic and alkaline media, interactions between PAM and Na kaolinite occur through hydrogen bonding between hydroxyls and amide groups, while (ii) in neutral media, the P A M - N a kaolinite affinity decreases. CONCLUSION It has been shown that interactions between nonhydrolyzed PAM and aluminosilicates are governed by hydrogen bonding. Although the p H has a negligible effect on the conformational properties of PAM in aqueous solution (at least in the explored domain), this parameter controls the interfacial behavior of the polymer. The variations in adsorption closely follow the modifications of the electrochemical surface properties. The most interesting observation concerns the opposing behavior of the two faces of kaolinite: whereas the basal faces act as nonadsorbing surfaces, the edge faces modulate the adsorptive capacity. This mechanism has recently been invoked by the authors in the interpretation of the thermodynamic stability o f Na kaolinite suspensions in the presence of PAM (35). The question raised in the introduction regarding the adsorption of partially hydrolyzed polyacrylamide onto aluminosilicates remains open: it is of course not relevant to generalize the mechanism proposed for the nonhydrolyzed polymer. Recent experimental results appear to confirm that in this case, despite long-range polyelectrolyte-charged surface interactions, hydrogen bonding is still operating (23). Nevertheless, the adsorption should vary differently on both sides of the isoelectric point of kaolinite, as a result of positive-negative charge attraction and negative-negative charge repulsion. In addition, the structural characteristics of the polyelecJournal of Colloid and Interface Science, Vol. 106, No. 1, July 1985
trolytes are altered on adsorption and this subject would be of interest in future studies. ACKNOWLEDGMENTS The research program is supported by Project ATP No. 269 PIRSEM "Energie et Mati~res Premieres 1982" of the Centre National de la Recherche Scientifique Franqaise. A. Carroy thanks the Institut Franqais du P~trole and L. Nabzar is grateful to the Centre Technique du Papier of Grenoble for grants enabling his stay at the CRM. We also thank Dr. R. Varoqui for many helpful discussions. REFERENCES 1. Vincent, B., and Whittington, S. G., in "Surface and Colloid Science" (E. Matijevic Ed.), Vol. 12. Plenum, New York, 1982. 2. de Gennes, P. G., Macromolecules 14, 1637 (1981). de Gennes, P.G., Macromolecules 15, 492 (1982). 3. Fleer, G. J., and Lyklema, J., in "Adsorption from Solution at the Solid/Liquid Interface" (C. D. Parfitt and C. A. Rochester, Eds.). Academic Press, New York, 1983. 4. Schmitt, A., "Macromolecules aux interfaces. Adsorption ~ l'interfacesolide/liquide," Course, Ecole d'Et6 d'Aussois: "Colloides et Interfaces" (1983). 5. Schulz, R. C., Kaiser, E., and Kern, W., Makromol. Chem. 58, 160 (1962). 6. Franqois, J., Sarazin, D., Schwartz, T., and Weill, G., Polymer 20, 969 (1979). 7. Thomas, F,, Th~se, Universit~ de Nancy I, Nancy (1982). 8. Grim, R. E., "Clay Mineralogy," Chap. 7. McGrawHill, New York, 1968. 9. Van Olphen, H., "An Introduction to Clay Colloid Chemistry." Wiley, New York, 1977. 10. Ferris, A. P., and Jepson, W. B., J. Co[[oidInterface Sci. 51, 245 (1975), 11. Stober, W., Fink, A., and Bohn, E., J. Colloid Interface Sci. 26, 62 (1968), 12. McMillan, D., U.S. Patent 3, 591, 518 (Du Pont) (1971). 13. Peri, J. B., J. Catal. 41, 227 (1976). 14. Sears, G. W., Anal. Chem. 28, 1981 (1956). 15. Pefferkorn, E., Carroy, A., and Varoqui, R., J. Polym. Sci., in press. 16. Griot, O., and Kitchener, J. A., Trans. Faraday Soc. 61, 1026 (1965); 61, 1032 (1965). 17. Mortland, M. M., Adv. Agron. 57, 22 (1970). 18. Ledoux, R. L., and White, J. L., J. Colloid Interface Sci. 21, 127 (1966). 19. Weiss, A., and Russow, J., Proc. Int. Clay Conf. Stockholm 203 (1963).
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Journal of Colloid and Interface Science, Vol. 106, No. 1, July 1985