Langmuir 1985,1, 701-708 In aqueous solutions (KF electrolyte) at a platinum electrode, the primary oxidation product (probably (SCN),) is rapidly hydrolyzed to CO,, SO4,-, and NOz-. There is no evidence of CN-as has been reported previously. The oxidation reaction of SCN- at silver electrodes is complicated by the concurrent oxidation and reaction of the silver with SCN-. The first voltammetric wave represents the oxidation of silver to form Ag(SCN),- which is present in the solution phase. At +0.4 V, there is further oxidation and the formation of AgSCN which remains as a film on the silver electrode surface. The process is chemically reversible. The reaction is slow enough to be monitored by simple time-resolved FTIR spectroelectrochemistry.
701
In aqueous solutions at a silver electrode, adsorption of water inhibits nucleation of AgSCN, which does not occur until high positive overpotentials ( + O X V vs. SCE). On subsequent excursions, the required overpotential is much less. The intermediacy of the Ag(SCN)2-ion is not indicated in aqueous solutions. The study of adsorption of SCN- at gold and other electrodes in the double-layer region has been undertaken and will be the subject of a forthcoming report.
Acknowledgment. We thank the Office of Naval Research for support of this work. Registry No. SCN-, 302-04-5;(SCN)z,505-14-6;Ag(SCN)y, 18444-31-0;AgSCN, 1701-93-5;COZ, 124-38-9;Sod2-,14808-79-8; NOz-, 14797-65-0;Pt, 7440-06-4; Ag, 7440-22-4.
A Predictive Model for the Alkylamine-Quartz Flotation System B. E. Novich and T.A. Ring* Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39 Received April 18, 1985. In Final Form: September 3, 1985
A model for predicting the flotation behavior for the alkylamine-quartz flotation system from the alkylamine solution chemistry was developed. The flotation prediction model is based on the finding that the formation of micelles in the quartz double layer corresponds to the inception of optimum flotation behavior and that the formation of micelles in the bulk solution corresponds to the termination of optimum flotation behavior. Critical micelle concentrations, adsorption isotherms, bubble contact angles, and flotation recoveries were determined for the alkylamine-quartz system to validate the flotation prediction model. Introduction Prediction of the flotation behavior of a mineral-surfactant system as a function of surfactant concentration and solution pH would be a powerful tool in the design and regulation of industrial flotation operations. Incomplete understanding of both surfactant adsorption and mineral flotation processes has resulted in the lack of an accurate, quantitative prediction model; this has led to inefficient flotation practice resulting in increased reagent costs. Attempts to formulate predictive models from correlation studies of flotation recovery have been qualitative, showing parallel relationships between flotation recovery and interfacial properties such as contact angle, { potential, and adsorption density.lS These models have formed the basis for understanding the interfacial interplay between phases but have limited use in design and regulation. Predictive models based on theory and/or experiment have been only marginally successful due to the lack of a coherent and comprehensive flotation model that accounts for (1)mineral-surfactant interaction (adsorption), (2) bubble-surfactant interaction (adsorption), (3) mineral-bubble interaction (contact angle and collection efficiency), (4) surfactant-surfactant interaction (micellization and solubility), and (5) mineral-mineral interaction (coagulation and collection efficiency). (1) Fuerstenau, D. W. Trans. Am. Inst. Min., Metall. Pet. Eng.1957, 208,1365-1367. (2) Fuerstenau, D. S.Plenary Lecture, 22nd Intemational Congress of Pure and Applied Chemistry, Sydney, Australia, 1970.
0743-7463/85/2401-0701$01.50/0
In addition an adsorption model for ionic surfactant adsorption onto charged surfaces that is consistent with the experimental adsorption and flotation data is not available. For example, adsorption isotherms for dodecylammonium ions onto quartz at 25 “ C and pH 6-7 determined by deBruyn3 indicate multilayer adsorption, in which the first layer adsorbs via a different adsorption mechanism than the subsequent layer or layers. This is inconsistent with the currently accepted Gaudin-Fuerstenau4 adsorption model5which predicts that the monolayer adsorption density for the first layer equals the saturation adsorption density for the second layer. deBruyn’s3 data indicate that the monolayer adsorption density for the first layer is far less than the saturation adsorption density for the second layer. There is no verifiable explanation for the increase of flotation recovery in the quartz dodecylamine system with increasing pH from 6 to 10 at constant surfactant concentration. Smith6 has proposed that the increased flotation is due to the coadsorption of the dodecylamine molecule with the dodecylammonium ion. Smith‘s theory is inconsistent with hydrolysis calculations, which show less than 0.001% hydrolysis at pH 6.0, less than 0.2% (3) deBruyn, P. L. Sc.D. Thesis, MIT, Cambridge, MA, 1952. (4) Gaudin, A. M.; Fuerstenau, D. W. Trans. Am. Inst. Min., Metall. Pet. Eng. 1955,202, 958. ( 5 ) Smith, R. W.; Akhtar, S. “Flotation, A. M. Gaudin Memorial Volume”; AIME: New York, 1976; Vol. 1, pp 87-116. (6) Smith, R.W.Trans. Am. Inst. Min., Metall. Pet. Eng. 1963,226, 427-433.
0 1985 American Chemical Society
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hydrolysis at pH 8.0, and less than 6% hydrolysis at pH 9.5, indicating that there is not a sufficient amount of amine base to affect the ion adsorption mechanism significantly. This work investigates the fundamental phase interactions comprising the flotation system, whose interrelationships cause the observed flotation effect. A qualitative adsorption model consistent with the experimental data and a quantitative adsorption analysis based on a modified Brunauer-Emmett-Teller (BET)7analysis are presented. The adsorption model and analysis form the basis for the flotation prediction model which integrates the fundamental phase interaction parameters. Discussion Alkylamine Solution Chemistry. The flotation behavior of the quartz-alkylamine system is strongly dependent on the adsorption behavior of alkylamine onto the quartz and bubble surfaces. The adsorption behavior is strongly dependent on the alkylammonium ion RxNH4-,+ activity in solution. There are several modes of a R,NHk+ activity reduction: hydrolysis, precipitation, and micellization. These solution processes, which can both depress flotation behavior by lowering cation solution activity and activate flotation behavior by increasing the amine surface activity, will define the system operating boundaries as a function of surfactant concentration and solution pH. Alkylammonium salts are used as flotation surfactants principally due to their amphiphilic character and because of their relatively high solubility. Dissolution of the alkylamine salt yields the cationic surfactant which adsorbs at the quartz and bubble surfaces. The dissolution reaction for the amines used in this study is R,NH,-,Cl(s)
H20
R,NH4-,+(aq)
+ Cl-(aq)
(1)
where R, is the symbol for the hydrocarbon chain, CnH2n+l, n is the carbon number, and 3c is the number of identical carbon chains ranging from 1 to 3. In alkaline solutions, the cationic amine reacts with hydroxyl ions to form free amine, R,NH3-,, and water according to the following reaction: R,NH,-,+
+ OH-
-
R,NH3-,
+ H20
(2)
The equilibrium constant for this reaction is the base hydrolysis constant, Kb, given by (3)
where a is the activity of the subscript species. Below the solubility of the free amine, the fraction of cationic amine in solution is given by
, (4)
Above the solubility of the free amine, the activity of free amine in solution is constant and further reaction of the cationic species with OH- gives a precipitate, R,NH3,(s). Hoerr et al.’ and Smithg give Kb values for n-propyln-hexylamine (4.4 X lo-,), di-namine (2.5 X hexylamine (10.2 X lo“), and n-dodecylamine (4.3 X lo4), (7) Brunauer, S.; Emmett, P. H.; Teller, E . J . Am. Chem. SOC. 1943, 65. 328. (8) Hoerr, C. W.; McCorkle, M. R.; Ralston, A. W . J . Am. Chem. SOC. 1943, 65, 328. (9) Smith, J. W. “The Chemistry of the Amino Groups”;Plenum Press: New York, 1968. I
~~~
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1
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e
f
9
7 I o-6
lo-‘
I 7 8 9 IO 11 12 Solution pH
Figure 1. Dodecylamine solution properties; 4.0
X
M total
amine concentration.
indicating that the alkylamines are slightly stronger bases than ammonia. Base hydrolysis constanb for trialkylamine compounds have been reported between 5.6 X and 1.0 x 10-4. Solubility data for the alkylamine compounds in water are very limited. Aplan and Fuerstenau’O give the following solubilitiesof n-alkylamines in water at 25 OC: decylamine (10 C), 5 X loW4 M; dodecylamine (12 C), 2 X M; tetradecylamine (14 C), 1 X lo4 M. The third solution process that can reduce the cationic amine activity is micellization, an aggregation phenomena occurring at relatively high surfactant concentrations, close to the solubility limit. Micelles are aggregates of 50-100 surfactant ions with the charged heads directed outward into the water solution, giving a spherical or ellipsoid geometry” excluding water from their interior. Micellization is favored energetically as the free energy decreases by 650-750 cal when 1 mol of CH2 groups is removed from water.12 The surfactant concentration at which micelles form is the critical micelle concentration, cmc. The cmc is strongly a function of salt concentration and pH,I6J7length of the hydrocarbon tail,18-20and t e m p e r a t ~ r e . ~ l - ~ ~ The solution properties for dodecylamine are summa(10) Aplan, F. F.; Fuerstenau, D. W. “Froth Flotation-50th Anniversary Volume”;AIME New York, 1962; pp 170-214. (11) Tanford, C. “The Hydrophobic Effect: Formation of Micelled and Biological Membranes”; Wiley Interscience: New York, 1973. (12) Fuerstenau, D. W.; Modi, H. J. J . Electrochem. SOC.1959, 106, 336-34 1. (13) Corrin, M. L.; Harkins, W. D. J. Am. Chem. SOC.1947,69, 684. (14) Lange, H. Kolloid-2. 1951, 121, 66-71. (15) Kushner, L.; Hubbard, W. D. J. Colloid Sci. 1955, 10, 428-435. (16) Klevens, H. B.; Raison, M. lreCongres mondial de la detergence et des produits tension-actifs (World Congress on Surface Active Agents), Paris, 1952, Vol. 1, pp 66-71. (17) Watson, D.; Manger, R. M. Trans.-Inst. Min. Metal., Sect. C 1968, 77, C57-C60. (18) Ralston, A. W. J. Am. Chem. SOC.1948, 70, 977-979. (19) Ralston, A. W.; Eggenberger, D. N.; DuBrow, P. L. J. Am. Chem. SOC.1949, 71, 671-672. (20) Ralston, A. W.; Groome, F. K.; Harwood, J. H. J . Am. Chem. SOC. 1949, 71, 671-672. (21) Klevens, H. B. J. Colloid Sci. 1947, 2, 301-303. (22) Klevens, H. B. J. Phys. Colloid Chem. 1947, 51, 1143-1154. (23) Stainsby, G.; Alesander, A. E. Trans. Faraday SOC.1950, 46, 587-597.
Langmuir, Vol. 1, No. 6, 1985 703
Model for the Alkylamine-Quartz Flotation System rized in Figure 1 as a function of pH at a total amine M. The distribution of cationic concentration of 4.0 X amine and free amine is plotted. The concentration of cationic amine starts to decrease drastically above pH 10. Micelle formation appears to be the important solution process directly affecting flotation and adsorption behavior. For example, Watson and Manser17 have shown that the cmc coincides with flotation cessation for a variety of silicate minerals and dodecylamine. Connor and 0ttewi1la have shown that the cmc correponds to the limiting adsorption density for a homologous series of n-alkyltrimethylammonium cations on polystyrene latex. Tamamushi and TamakiZ5observed that the cmc is the saturation adsorption condition for several cationic surfactants on alumina. When placed in aqueous suspension, quartz surfaces will adsorb potential determining ions, H+ and OH-. The relative amount of proton and hydroxyl adsorbed will depend primarily on the suspension pH. If the suspension pH is above the point of zero charge (pH -2.0), the quartz surface will be negatively charged. When cationic amine is placed in solution with the negatively charged quartz surface, the surfactant will adsorb. The adsorption process is experimentally monitored by determining adsorption isotherms. This involves determining the amount of surfactant adsorbed on the quartz surface relative to the equilibrium surfactant concentration in solution at constant temperature and solution properties. Critical Micelle Concentration, In addition, hydrolysis may play an important role in the micellization process. Neutral amine molecules, formed from the reaction of cationic amine and OH-, may act m templates for micelle formation, analogous to a heterogeneous nucleation process. Fuerstenau and Palmerz6 suggest that neutral molecules formed by hydrolysis reduce the repulsion between charged heads, enhancing micelle formation and lowering the cmc. The neutral amine theory is supported by the parallel relationship between cmc and hydrolysis with pH. The effect of pH on micellization has not been fundamentally addressed in the literature, even though Klevens and Raison16first discussed the pH effect over 30 years ago. For example, Mukerjee and MyselsZ7 offer the most comprehensive compilation of cmc’s in aqueous solution, citing measurement technique, temperature, and salt type and concentration with no mention of the effect of solution pH. Critical micelle concentrations were determined for the alkylammonium chlorides as a function of solution pH by using a conductometric technique.% The results are shown in Figure 2, where cmc is plotted against solution pH. The primary amine compounds 1-C6 and 1-C12 gave decreasing cmc with increasing pH. The cmc’s for the secondary and tertiary compounds either varied much less than the primary compounds or were essentially constant over the pH range 6-11. Adsorption. The adsorption of dodecylammonium ions onto the surface of quartz from an aqueous solution has been studied by Bloecher and G a ~ d i n deBr~yn,~BO ,~~ (24)Conn&.,-P.; Ottewill, R. H. J. Colloid Interface Sci. 1971,37, 642-651. (25)Tamamushi, B.; Tamaki, K. “Proceedings, 2nd International Conference of Surface Activity”; Butterworths: London, 1957;Vol. 3,pp 449-456. (26)Fuerstenau, M. C.; Palmer, B. R. ‘Flotation, A. M. Gaudin Memorial Volume”; AIME: New York, 1976;Vol. 1, pp 148-196. (27)Mukerjee, P.; Mysels, K. J. Natl. Stand. Ref. Data Ser. (V.S. Natl. Bur. Stand.) 1971,“Critical Micelle Concentrations of Aqueous Surfactant Svstems”. ._ (28)Ralstk, A. W.; Eggenberger, D. N.; Broome, F. K. J. Am. Chem. SOC.1949,71,2145-2149.
I
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I
I
I
I
I
I
I
I
I
1
,
6
7
8
9
10
I1
12
Solutlon pn
Figure 2. Critical micelle concentration as a function of pH: (0) n-propylamine, ( 0 )n-hexylamine, (w) di-n-hexylamine, (+) tridi-n-dodecylamine. n-hexylamine, (A)n-dodecylamine, (0)
Schultz and Cooke,3l Klyucharev et al.,32 Waksmundzki and M a r u s ~ a kLi , ~and deBr~yn,3~ and Ball and F ~ e r s t e n a u . ~Mitrofanov ~ and K u ~ h n i k o v astudied ~~ tridecylammonium chloride adsorption on quartz; TerMinassian-Saraga3* studied the dodecyltrimethylammonium bromide-quartz adsorption system. Adsorption data for other alkylamine-quartz systems are not available in the literature. With the exception of deBruyn’s data, the effect of solution pH on the adsorption isotherm has not been investigated. The specific values of adsorption density as a function of equilibrium bulk concentration obtained in the studies mentioned above show wide variance. Decker33and Li39 attribute the variation of adsorption density to surface preparation, since similar quartz was used in all of the studies. For example, Bloecher and GaudinZ9and deBruyn3 used the same chemical treatment on quartz, consecutive HC1 and deionized water washed, and obtained similar adsorption results. Li39and Decker33used quartz treated with HF and obtained similar results, which were significantly lower than those measured by Bloecher, Gaudin, and deBruyn. Adsorption data obtained by Ball and Fuerstenau40 show that surface treatment is not the (29)Bloecher, F. W.; Gaudin, A. M. Trans. Am. Inst. Min.,Metall. Pet. Eng. 1950,187,499-505. (30)deBruyn, P. L.flans. Am. Inst. Min., Metall. Pet. Eng. 1955,202, 291-296. (31)Schultz, N. F.;Cooke, S. R. B. Ind. Eng. Chem. 1953,45,2767. (32)Klyucharev, A. P.; Plaskin, I. N.; Myasnikova, G. A. Russ. Metall. Fuels 1960,2,86. (33)Decker, T. Ph.D. Thesis, MIT, Cambridge, MA, 1964. (34)Waksmundzki, A.; Maruszak, E. Rocz. Chem. 1964,38,835-842. (35)Li, H.C.; deBruyn, P. L. Surf. Sci. 1966,5,203-220. (36)Ball, B.; Fuerstenau, D. W. Discuss. Faraday SOC.1971, 52, 361-371. (37)Mitrofanov, S. I.; Kushnikova, V. G. Izu. Metally 1960,33,(lo), 1. (38)Ter-Minassian-Saraga, L. Adu. Chem. Ser. 1964,43,232. (39)Li, H.C. Sc.D. Thesis, MIT, Cambridge, MA, 1958. (40)Ball, B.; Fuerstenau, D. W. Discuss. Faraday SOC.1971, 52, 361-371.
704 Langmuir, Vol. 1, No. 6, 1985
Nouich and Ring
to the surface was approximately equal to the cmc. Observing that the sharp break in these interfacial properties is analogous to the sharp transitions in the solution properties at the cmc, Gaudin and Fuerstenau postulated the presence of surface micelles. The hemimicelle concentration, hmc, is defined as the solution concentration a t which an abrupt change in interfacial properties (e.g., adsorption density, { potential) occurs. This is consistent with the definition of cmc, relative to the solution properties (e.g., conductivity, surface tension). The relationship between micelle and hemimicelle formation is supported by parallel relationships between cmc and hmc as a function of solution P H , chain ~ ~ length,41s46147 and t e m p e r a t ~ r e . On ~ ~ the basis of a theoretical monolayer adsorption density for amine on quartz of 7.1 X 10-lo mol/cm2, hemimicelle formation was considered to occur on the surface prior to monolayer coverage. After mono1 1 I I I I I layer coverage, Gaudin and Fuerstenau suggested that a -4 -3 -2 -I -6 -5 second layer forms with the hydrocarbon tails oriented lop Dodecylamine Concentration toward the surface and the charged amine heads toward Figure 3. The Gaudin-Fuerstenau adsorption model: adsorption the solution. This accounted for the charge reversal that ~ c h e m a t i cadsorption ,~ i~otherm,~ { potentiaL4 the { potential experiences at the beginning of the highconcentration region. In the high-concentration region, only cause for the variation. Ball and Fuerstenau used a Gaudin and Fuerstenau postulate that adsorption is by van quartz and surface treatment similar to deBruyn’s3 but der Waals attraction of the hydrocarbon chains only. obtained adsorption densities 2 orders of magnitude lower Adsorption ceases when the now positively charged infor a given equilibrium solution concentration. terface is full, which would correspond to an adsorption Although the specific values for adsorption density bedensity of only 14.2 X 10-lo mol/cm2. tween studies do not agree, all of the adsorption isotherms Somasundaran et a1.45-49investigated the process of exhibit the same general “S”shape. Curves of this type hemimicelle formation and found a straight line relationare typical for other ionic surfactant-mineral s y s t e m ~ . ~ l - ~ ship between the number of carbons in the alkyl chain and On the basis of this general shape of the adsorption isothe surfactant concentration at zero { potential for a hotherm and electrokinetic data for quartz and dodecylmologous series of straight-chain alkylamine and quartz. ammonium acetate, Gaudin and F ~ e r s t e n a udeveloped ~~ The slope of the line gave a van der Waals cohesive energy the currently accepted model for alkylamine adsorption per CH2 group of 0.97 kT or 580 cal/mol at 25 “C. on quartz. The principal deficiency of the Gaudin-Fuerstenau adThe Gaudin-Fuerstenau” adsorption theory consists of sorption model is that it does not adequately address three principal stages: (1)single ion adsorption at low monolayer and subsequently layer formation. The original surfactant concentration, (2) surface micelle formation at radiometric adsorption studies performed in the early intermediate concentration, and (3) multilayer adsorption 1950’s by deBruyn? and Gaudin and at high surfactant concentrations. The experimental basis on quartz, hematite, and sphalerite introduced a theoretical for the model is the parallel relationship between admonolayer adsorption density of 7.1 X mol/cm2 for sorption, electrophoresis, and micellization data. The quartz based on crystal chemical considerations.” Subadsorption isotherm, {-potential curve, and schematic sequent studies and reviews have made use of this value diagram of the Gaudin-Fuerstenau model are given in to explain adsorption and flotation behavior.1~5J0~39~55*51 Figure 3. Decker33 considering the cross-sectional area of the amine In the low-concentration region, alkylammonium ions functional group, 25 A2, gives a theoretical monolayer specifically adsorb at negative sites on the quartz surface, adsorption density of 6.64 X mol/cm2. As shown in oriented with the charged head toward the surface and the Figure 3, this adsorption density corresponds to a { pohydrocarbon tail into the solution. This is shown by an +40 mV, indicating that charge reversal occurs tential of increase in adsorption density with increasing equilibrium before a theoretical monolayer of amine has been adsorbed. solution concentration. The {-potential measurements in Therefore Gaudin and Fuerstenau’s hypothesis about this region are suspect, since the potential decreases to more negative values as positive charge is being added to (45) Somasundaran, P.; Fuerstenau, D. W . J.Phys. Chem. 1966, 70, the interface. 90-96. The beginning of the intermediate concentration range (46) Fuerstenau, D. W.; Healy, T. W.; Somasundaran,P. Trans. Am. is marked by a rapid increase in {potential and an increase Inst. Min., Metall. Pet. Eng. 1964,229, 321-325. in adsorption density. Using the Gouy-Chapman theory, (47) Somasundaran, P.; Kulkasni, R. D. Trans.-Inst. Min. Metal., Sect. C 1973,82, C164-C167. Gaudin and Fuerstenau” calculated the ion concentration (48) Somasundaran, P.; Fuerstenau, D. W . Trans. Am. Inst. Min., in the diffuse double layer by using a solution concentraMetall. Pet. Eng. 1972, 252, 275. tion corresponding to the inflection points for the { po(49) Somasundaran,P. Ph.D. Thesis, University of California,Berkley, CA, 1964. tential and adsorption density. The ion concentration close I
(41) Wakamatsu, T.; Fuerstenau, D. W . Adu. Chem. Ser. 1968, 79,
lfil-172. - - - .-.
(42) Tamamushi, B.; Tamake, K. Proc. Int. Miner. Process. Congr., IOth, 1973, 1974, 473-492. (43) Predali, J. J.; Cases, J . M.; Proc. Int. Miner. Process. Congr.; loth, 1973, 1974, 473-492. (44) Gaudin, A. M.; Fuerstenau, D. W . Trans. AIME, 1955,202,958.
(50) Novich, B. E. Ph.D. Thesis, MIT, Cambridge, MA, 1984. (51) Leja, J. “Surface Chemistry of Froth Flotation”: Plenum Press: New York, 1982. (52)’Morrow,J. G. Sc.D. Thesis, MIT, Cambridge, MA, 1952. (53) Gaudin. A. M.: Morrow. J . G. Trans. Am. Inst. Min.. Metall. Pet. Eng. 1954, 299, 1196. (54) Gaudin, A. M.; Rizo-Patron, A. Trans. A m . Inst. Min., Metall. Pet. Eng. 1943,153, 462. (55) Somasundaran, P. Interfacial Chem. Part. Flotation 1975, 71, 1-15.
Langmuir, Vol. 1, No. 6, 1985 705
Model for the Alkylamine-Quartz Flotation System
& 1.6 F
1.4
I *,.&W
0.0
' -4
-3 X
Reduc8d Equilibrium Concentration, X
= Ce,/CMC
Figure 4. Adsorption isotherms at 23-25 "C plotted as a function of reduced concentration: (0) n-propylamine (1-C3)pH 11.0;(+) n-hexylamine (1-C6) pH 6.7,8.0,10.0;(m) di-n-hexylamine(2-C6) pH 7.0; (A)n-dodecylamine (l-Cl2) pH 6.5-7.0; (A)n-dodecylamine pH 6-11.
-
-2 CeJCMC
-I
Figure 5. Adsorption isotherms for n-alkylammonium ions on Biotite at pH 5.5, 25 0C:57 ( 0 )1410, (0) 1-C12, (U) l-Cl4, (0) 1416, (+) 1418.
-9
%
monolayer coverage is made without strict adherence to experimental evidence.
Results Amine Adsorption. Adsorption isotherms of alkylamine on quartz were determined5, at 23 f 2 OC as a function of amine concentration and solution pH. Analysis was performed spectrophotometrically by using an amine-picrate complex in ~ h l o r o f o r m .The ~ ~ pH ranges surveyed were from 5.80 to 10.50. The amount of amine adsorbed at the quartz surface, I?, was determined by a difference method as a function of equilibrium solution concentration, Ceq,and pH. As shown in Figure 4, the adsorption density was plotted as a function of reduced concentration, X,given by X = C,,/cmc (5) The isotherms for (1-C6), (2-C6), and (1-Cl2) exhibit three adsorption regions, consistent with the GaudinFuerstenauU model. In the low-concentration range, log,, r increased linearly with log,, X. At a reduced concentration of approximately 0.01-0.04, adsorption curves flatten and then abruptly steepen and continue to rise linearly. This was observed by Aplan and Fuerstenau,lo who state that the condition required for the association of adsorbed ammonium ions at the quartz-solution interface a t pH 7 is constant at X = 0.01, based on electrokinetic data for C8, Cl0, C12,C14,and CI6n-alkyl amines. The second break in the isotherm occurred at a reduced concentration of approximately X = 0.5-1, close to the cmc. Normalizing the adsorption data to the cmc has been shown to cancel the effects of surfactant type, chain length, branching, and solution pH on adsorption density for a variety of surfaces. CasesM measured five distinct isotherms for a homologous series of five alkylamine compounds on biotite at constant pH. As shown in Figure 5, Predali and Cases5' found that the five curves converge (56) Cases, J. M. Int. Miner. Process. Congr., 8th 1968. (57) Predali, J. J.; Cases, J. M. Proc. Znt.Miner. Process. Congr., loth, 1973 1974,473-492.
I
-5
i
I -4
-3
I
I
-2
-/
I ]
0
l o g l ~ Reduced Concentralion, X-C/CMC
Figure 6. Alkylamine adsorption on Latex-G at pH 8 in
M
KBr:24 (0)octyltrimethylamine,(A)decyltrimethylamine,(0) dodecyltrimethylamine. 10-
I
I
I
I
Reducbd I4uilibrium C ~ n c ~ n t n t i e n X-C,@C .
Figure 7. Adsorption isotherm for didecylamine on quartz at 23-25 "C over pH range from 6 to 10.5 ( 0 )this work, pH 6.5-7.0; (A)ref 3, 4 X M, pH 6.6-10.5; ( 6 ) ref 3, 4 X lo4 M, pH 6.8-10.5; (u) ref 3, pH 6-7. (- - -) Modified BET isotherm.
to one (after normalizing Cases' adsorption data to the cmc), giving two breaks in the isotherm at X = 0.01-0.03 and 0.5-1. As shown in Figure 6, this same type of behavior was observed for Connor and Ottewill's24adsorption data for a homologous series of branched alkylamines on polystyrene latex. The pH dependence on adsorption behavior has been observed by several investigators. deBruyn3 found a proportional relationship between r and pH for dodecylamine on quartz over the range of amine concentra-
706 Langmuir, Vol. 1, No. 6, 1985
Novich and Ring
Table I. Results of Modified BET Analysis second step
______
first step
rM,mol/cm2 n-propylamine (1-03) n-hexylamine (1-C6) 2.7 X di-n-hexylamine (2-C6) 1.56 X n-dodecylamine (1-Cl2) 1.24 X
K,
mol/cm2
=
/i
MONOLAYER C O V E M G E
1.24 X 280.5 1.04 X 28.1 7.57 X 148.6 3.96 X
Ks 0.464 0.340 0.202 2.64
tions (4 X lO*)-(l X 1 0 3 M and for a range of pH values between 6 and 11. When plotted with amine concentration scaled with cmc for each solution pH, all data converge to a single curve, as shown in Figure 7. Breaks in the adsorption isotherm occur at approximately X N 0.01 and 0.5. The convergence of the adsorption isotherms, measured as a function of amine concentration and solution pH, to one curve when normalized with the cmc suggests that the solution processes govern the adsorption behavior. A quantitative analysis of the adsorption data is given next, with particular emphasis on the transition regions occurring at X N 0.01 and 0.5. Isotherm Analysis. Quantitative analysis of the adsorption data was carried out using a composite isotherm based on a modified Brunauer-Emmett-Teller (BET) analysis." This modified analysis is the solution analogue to gas adsorption, accounting for solution and surface activity. Adsorption data analysis involved treating each step of the isotherm separately, using the following equations: for the first step
r
120t
rS,
~
rM1 +K1X KIX
(6)
and for the second step
(7) where K1 is the distribution constant for the first layer, Ks is the distribution constant for the subsequent layers, r M is the monolayer adsorption density, and l?s is the saturation adsorption density. The results of this analysis aie shown in Table I. In the first step adsorption only a monolayer is possible. The monolayer adsorption density is 1.24 X 10-lomol/cm2 (134 A2/molecule) for dodecylamine which is about 19% of the theoretical monolayer (6.6 X 1O1O mol/cm2, or 25 A2/molecule), calculated from quartz surface chemical chemistry and the diameter of the cationic amine head. There is no consideration given to hydrocarbon chain length or branching. A more realistic picture of monolayer adsorption begins with the fundamental driving force for adsorption, to minimize the free energy of the adsorption system. The charged surface minimizes its free energy by satisfying the surface charge with adsorption which affects the diffuse double layer. The free energy for the adsorbing amine is equally distributed between neutralizing the suriace charge, steric hinderance between interacting hydrocarbon tails, and repulsion of the amine heads. The equilibrium condition seeks to minimize the steric hinderance by increasing the adsorption area per molecule with the increasing length or number of branches on the hydrocarbon chain. According to the modified BET analysis, the adsorbed amine ions are at great distances apart relative to the close-packed theoretical monolayer, so that lateral interaction between hydrocarbon chains is also minimized. This is a necessary condition for BET adsorption to be applied. One of the principal arguments against using BET or Langmuir analysis to obtain param-
3
6
I2
Total Numbtr of Carbons
Figure 8. Variation of adsorption area at monolayer coverage with.t.ata1carbon number.
eters with physical significance has been the presence of lateral interaction among adsorbed species. It is interesting to note that some investigators who argue against using the BET or Langmuir equations routinely use the SternGrahame which is derived from Langmuir adsorption theory. The relationship between monolayer adsorption area and hydrocarbon chain length observed in Figure 8 shows that as the carbon number is reduced from 12 to 3, npropylamine, the adsorption area decreases from 128 to 28 A2/moleculeand approaches the theoretical limit of 25 A2f molecule. The increase in adsorption area to decrease steric hinderance between adsorbing and adsorbed amine ions is strongly supported by the linear correlation between adsorption area and chain length. The effect of branching on the area per molecule is demonstrated by di-n-hexylamine (2-C6), which gives a monolayer adsorption area between that of hexylamine (1-C6), with the same chain length, and dodecylamine (l-C12), with the same carbon number. The saturation adsorption densities for the second step, rs,were much greater than those for the first step, r M , Consequently, the second step adsorption areas were significantly less than those for the first step. The saturation adsorption areas ranged from 4.19 A2/molecule for dodecylamine (1-Cl2) to 0.13 A2/moleculefor n-propylamine. Consistent with the first-step results, the area per molecule was proportional to chain length, suggesting that steric hinderance due to molecular size and head/tail interaction also plays an important role in the second step. Multilayer adsorption is indicated for the second step, because the adsorption areas are smaller than the molecular area for the alkylamine surfactants, 23 A2/molecule.51 The adsorption results suggest that a step in the adsorption isotherm is governed by adsorption energetics. The first step can be modeled as a monolayer of ionically attracted alkylammonium ions oriented with the cationic head toward the quartz surface and the hydrocarbon tail into the solution. The second step can be modeled as a multilayer of van der Waals attracted hydrocarbons oriented in all directions. With this view of adsorption we can examine its effect on flotation recovery. Flotation Recovery. Flotation experiments were performed at 23 f 2 "C in a Hallimond microflotation ce1l.l Nitrogen gas was used at a flow rate of 60 cm3/min. for aeration. Air could not be used for flotation experiments since the C02 present in air would adsorb into the basic amine solutions and decrease their pH during the course
Langmuir, Vol. 1, No. 6,1985 707
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100
90 -
‘2
80
-
70-
60-
e
. I
d
-
50-
I
40-
20 -
30
IO 01 A -3
I
I
-2
-I
0
I
Figure 9. Reduced concentration flotation recovery for hexylamine and dodecylamine: (A)pH 6.0, (0) pH 8.0, (0) pH 10; open symbols 1426, closed symbols 1-C12. of the flotation experiments. To assure that solution pH did not change during flotation, it was monitored before and after flotation. When a sample showed a deviation of more than 0.2 units, the data were discarded. The details of this technique are described by N o v i ~ h . ~ ~ Flotation recoveries were determined as a function of collector concentration and solution pH. The results obtained for the quartz-dodecylamine (1412) system agree very well with those reported at lower pH values by Bleier et al.% and Fuerstenau et At high pH values COz can be expected to decrease the pH values reported in the literature, so it is not surprising that our results are slightly different (shifted to higher pH’s) from those reported. Noting the importance of the cmc as a function of pH in the adsorption isotherms, we investigated its effect on flotation recovery. By making a plot of flotation recovery vs. reduced amine concentration X , we found that flotation curves for various solution pH’s converged to one curve, as shown in Figure 9. This trend was observed with all the surfactants investigated, l-C3,1-C6, 2-C6, and 2-C12. The onset of flotation occurred at a reduced concentration, X 0.01, which is the same as that of monolayer coverage observed in the adsorption studies. Further, flotation recovery decreased to zero at a reduced concentration of X = 1.0. Thus flotation begins when a monolayer is present on the quartz surface and ends when the surface is fully saturated. It is believed that the reason flotation ceases at the cmc is that the bubble surface is also saturated with amine cations and it is not possible to push these two thickly populated, positively charged surfaces together. Indeed, the bubble contact angle exhibits a maximum value ( 7 8 O for dodecylamine on quartz) at the cmq50 supporting this argument. A cross-correlation of flotation recovery, adsorption density, { potential, and bubble contact angle for dodecylamine is shown in Figure 10. It can be seen that the monolayer coverage at a reduced concentration of X = 0.01 is responsible for the onset of flotation and for the beginning of a decrease in { potential; it has no significant effect on contact angle. Charge reversal corresponds to some location in the multilayer section where the charge
X=MME
Figure 10. Variation of interfacial properties of the quartzdodecylamine solution-nitrogen bubble system at pH 8.0: (A) flotation recovery, (B)adsorption isotherm, (C) {potential: (D) bubble contact angle. HYDROLYSIS RXN: RNH;
S
r r
I
II.llICYC
!
O b =RNH2
+
H20
\
-
(58)Bleier, A.; Goddard, E. D.; Kulkami, R. D. “Flotation, A. M. Gaudin Memorial Volume”; AIME: New York,1976;Vol. 1,pp 117-147. (59)Grahme, D.C. Chem. Reu. 1947,41,441.
“i Solution pH
Figure 11. Hydrolysis-micellization curve for dodecylamine as a function of concentration and pH. on the quartz is balanced by the adsorption of cationic amine molecules. The attainment of the maximum positive { potential corresponds to the attainment of a fully saturated quartz surface, as well as the maximum bubble contact angle. Predictive Model. The results of studies of solution chemistry, cmc, and adsorption as a function of pH can be used to predict flotation behavior. An example of this M dodecylamine is shown in Figure prediction for 4 X 11. Since a reduced concentration of 0.01 is responsible for the onset of flotation, a curve parallel to the cmc at 1/100 of its value is constructed. The intersection of this
Langmuir 1985, 1, 708-713
708
covery is predicted. All experiments performed for dodecylamine are plotted in Figure 12. These results show excellent agreement with the predicted flotation behavior. Lower recoveries are shown outside the boundary region, following a pattern that conforms to the boundary edges.
ICG
‘+’ 7
8
9
10
12
Solution pH
Figure 12. Flotation prediction curve for the dodecylaminequartz system: (-) model (M) 100% recovery, (A)
