Langmuir 1991, 7,1361-1364
1361
The Krafft Point of Dodecylammonium Chloride: pH Effect Q.Dai and J. S . Laskowaki' Department of Mining and Mineral Process Engineering, The University of British Columbia, Vancouver, British Columbia, Canada Received January 5, 1990. In Final Form: January 7,1991
The Krafft point of dodecylammonium chloride was studied as a function of pH. Because of dynamic effects, the maximum bubble pressure method was found to be the only experimental technique that could be utilized to study the surface tension of dodecylammonium chloride solutions at various pH values. Both the solubility and the critical micelle concentration decrease with pH, but moet importantly, the concentration range over which micelles can form narrows sharply when pH increases and disappears entirely at pH 10. The Krafft point of dodecylammonium chloride at pH 5 is 17.5 O C and decreases to 7 O C when pH is increased up to 9.
Introduction An effect of pH on flotation has been studied extensively. In the flotation of oxide minerals, this effect is especially pronounced and is considered by many to result from the influencethe H+and OH-ions have on the electrical charge of oxides. Since many flotation collectors are weak electrolytes, the effect of pH on the flotation when such collectors are used also results from the influence of the pH on the ionization of the collector. Values of dissociation constants, Ka,for this type of surfactants are known and concentration-pH diagramshave been frequently used to determine which species predominate in the particular pH ranges. Such species are then taken into account in modeling adsorption phenomena. The pH is hence believed to change the ionized/molecular collector ratio; in addition some authors consider the presence of ionomolecular complexes in certain pH ranges.112 In the Gaudin-Fuerstenau adsorption model on charged individual ions are assumed to initially adsorb a t the surface/solution interface with their association into two-dimensional aggregates, hemimicelles,when surfactant concentration is further increased. In the case of a weak electrolyte such as dodecylamine, coadsorption of amine molecules is considered at higher pH values. Neutral molecules are also believed to enhance micelle formation by reducing repulsion between charged heads of organic ions forming micelle. According to Moroi et al.: the Krafft point is the temperature at which solubility of surfactant as monomers becomes high enough for the monomers to start aggregation. Therefore,micelles can form in solutionsonlywhen the temperature exceeds the Krafft point of the surfactant. The use of the concentration-pH diagrams in predicting the species that may appear in the solution is then obviously not sufficient and knowledge of the Krafft point is also needed. Effect of the pH may further complicate the situation. It is of interest to note that Klev(1) Kung, H. C.; Goddard, E. D. Colloid Polym. J. 1969,232, 812. (2) Somaeundaran, P. T. Znt. J. Miner. Process. 1976,3, 35. (3) Gaudin, A. M.; Fuerstenau, D. W. Trona. Am. Znst. Mining Met. Eng. 1956,202,985. (4) Fuerstenau, D. W. In The Chemistv of Biosurfaces; Hair, M. L., Ed.;Marcel Dekker: New York, 1971; Vol. 1, pp 143-176. (5) Chander, S.; Fuerstenau, D. W.; Stigter, D. In Adsorption from Solution; Ottewill, R. H., Rochester, C. H., Smith, A. L., Eds.;Academic Press: London, 1983, pp 197-210. (6) Moroi, Y.; S e i , R.; Matuura, R. J. Colloid interface Sci. 1984,98, 184.
ens and Raison7 demonstrated in 1954 that for some surfactants the pH affects criticalmicelle concentration (cmc), but this subject has never been adequately researched. Cases et al.*tgshowed that the Krafft point should have a significant effect on adsorption behavior of surfactants. Smithloreached similar conclusions. Information on the Krafft point of amines is, however, very limited. The data of Broome et al." may indicate that the Krafft point for dodecylammonium chloride is located at 25 OC. This agrees rather well with Eggenberger and Harwood,12whose results show that the Krafft point for dodecylammonium chloride is somewhere between 20 and 25 "C. For the same compound, Dervichian's solubility seem to locate the Krafft point around 26 OC. He also found clear dependenceof the Krafft point on gegenions. The Krafft points for dodecylammonium bromide and iodide where found in the 30-40 OC range. In all these publications, the pH was not recorded. It must then be assumed that all the reported measurements were carried out at natural pH values, which are slightly acidic for alkylammoniumchlorides. Amines are, however, frequently used in flotation at quite different pH ranges, and dodecylammoniumchloride in particular is a common model cationic collector. The effect of pH on the Krafft point of such Surfactants is entirely unknown. Despite the lack of fundamental information, the literature has recently brought a predictive model for the alkylaminequartz flotation system.14 It is worth pointing out that in deriving the model in the quoted work, all results were plotted versus reduced concentration, that is, the equilibrium concentration to cmc ratio. In order to obtain cmc for various amines, Novich and Ring1' used (7) Klevens,H.B.;Raieon,M.M.In Thelst W o r l d C o ~ o n S u r f a c e Active Agents; Chambre Syndicale Tramegras: Paris, 1964; VoL 1, pp 66-71. (8) Cases, J. M.; Poirier, J. E.; Van Damme, H. In Advances in Mineral Processing; Somaeundaran, P., Ed.;Society of Mining Engineen: L i t tleton, CO, 1986; pp 171-188. (9) Cases, J. M.; Poirier, J. E.; Canet, D. In Znteractiom Solide-Liquide dona le8 Milieux Poreux (Collcque-Bilan, Nancy,February 1984); Collection Collcquea et Seminairea42; Editions Technip: P h , 1986; pp 335-369. (10) Smith, R.W. In Reagents in Mineral Technology;SomaauucLuaa, P., Moudgil, B. M., E&.; Marcel Dekker: New York, 1988; p 219-256. (11) Broome, F. K.; Hoerr, C. W.; Harwood, H. J. J. Am. &%em. Soc. 1961, 73,3350. (12) Eggenberger, D. N.;Harwood, H. J. J. Am. Chem. SOC.1961,73, 3353. (13) Dervichian,D.G.InProceedingsofthe3rdZnternotioMlCo~~ of Surface Actiuity; Gordon and Breach London, 1960; Vol. 1, Sect. A, pp 182-188. (14) Novich, B. E.; Ring, T. A. Langmuir 1986,1,701.
0743-7463/91/2407-1361$02.50/0 0 1991 American Chemical Society
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Watson and Manser'sls as well as their own data obtained from the conductometric measurements. As it will be shown in the subsequent section, this technique can be easily utilized to determine cmc for ionized surfactants, that is, in the case of DDAHCl to determine cmc in acidic solutions, but the technique is highly unreliable for such a surfactant if used to measure cmc in the alkaline environment.
Experimental Section Materials. Dodecylammonium chloride (99 % DDAHC1) for chemical purposes was purchased from Eastman Kodak Co. and was used without further purification. No minimum was observed on the surface tension vs concentration curve near the cmc region. Potassium hydroxide and hydrochloric acid were used to adjust the pH and single-distilled water was utilized throughout. Cmc Determinations. At pH 5, which is close to the natural pH of aqueous DDAHCl solutions, the cmc was determined by the conductivity method as described by Moroi et al.16 As the pH is raised, no sharp change in the slope of the specific conductance vs concentration curves in the region of cmc was observed. A clear break WBB not found on the equivalent conductance vs square root of concentration curve either. Therefore, at higher pH values the cmc was elucidated from the surface tension vs concentration curve. The surface tension of aqueous DDAHCl solutions was measured by the drop-volume method and by using a du Nouy tensiometer. However, the surface tensions of alkaline DDAHCl solutions were found to vary with time as measured by Finch and Smith." These dynamic effects made it impossible to find reproducible values when these two methods were utilized. The Wilhelmy plate method could not provide satisfactory results, probably due to the hydrophobicity of the platinum plate caused by highly surface active species existing in alkaline DDAHCl solutions.1'-19 Finally, the maximum bubble pressure methodmZ1was adopted to measure the surface tension. The apparatus and the method were almost identical with that of Finch and Smith.20 The inner diameter of the capillary tip was 0.23 f 0.01mm, as measured by means of a microscope. A Pyrex glass cell (30 X 30 X 60 mm) containing 25 mL of test solution was placed in a thermostated bath. The difference in height between two menisci in a water manometer was read with a cathetometer to an accuracy of 0.01 mm and the depth of the tip below the surface of the solution was also measured for hydrostatic head correction. During the measurement, the nitrogen gas flow rate was adjusted so that bubbles grew up at the tip at a time interval of 3 min (f10 8 ) . In fact, the surface tension was found not to change significantly when the bubble lifetime exceeds 2.5 min. Solubility Measurements. Different methods were employed in the measurement of DDAHCl solubilities in acidic and alkaline solutions. In the solution of pH 5, the precipitation of DDAHCl forms relatively large hydrated crystals, which settle easily. Excess DDAHCl was first dissolved at higher temperatures (above 30 "C). The solution was kept at about 0 OC for more than 4 h and then put into a bath at a given temperature for more than 8 h, during which the solution was gently shaken every 15 min. The supernatant was sucked through a Millipore filter (0.2 pm) by means of a syringe. The filtrate concentration was determined with a dodecylammonium ion surfactant selective electr0de.n.~3 (16)Watson, D.;Manaer, R. M. !l'ram.-Zmt. Min. Metall., Sect. C 1986,77,C67. (16)Moroi, Y.: Matuura, R.; Kuwamura, T.; Inokuma, S.J. Colloid Interface Sci; 1986,113,226, (17)Finch, J. A.; Smith, G. W. J. Colloid Interface Sci. 1973,4681. (18)Somaaundaran, P.;Ananthapadmanabhan, K. P. In Solutron Chemistry of Surfactants: Mittal. K. L., Ed.:Plenum Prees: New York, 1979; VOK 2;pp f77-800.. 119)Castro.. S.H.: . Vurdela. R. M.: Laskowski. J. S. Colloid Surf. 1986, 21,'87: (20)Finch, J. A.; Smith, G. W. Tram.-Zmt. Min. Metal., Sect. C 1972,81,(2123. (21)Mysels, K.J. Langmuir 1986,2,428.
101
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Figure 1. Surface tension versus logarithm of DDAHCl concentration curves a t various pH values and at 45 OC. Arrows indicate solubility limits. The filtrate was diluted several times in order to measure the concentration of amine in the range in which the electrode acts the best. As pH is increased, free amine appears whose solubility is very 10w.24 Once precipitation occurs, the DDAHCl solution turns to look like milk. In this case clear filtrate cannot be obtained by simple filtration, but the apparent change in turbidity after the solubility limit is reached can be very helpful in determining the onset of precipitation. Therefore, the transmittance measurements were carried out to obtain the solubilities of DDAHCl in alkaline solutions by using a UV-vis spectrophotometer (PerkinElmer, Lambda 3). The wavelength selected for the study was 450 nm. Details of the method are described else~here.'~ True solutions at lower pH were used as blank in each measurement. The pH was controlled within f0.1 and the temperature was kept at a desired level within fO.l OC with a thermostat. Glassware was acid cleaned every time before the set of measurements.
Results For the purpose of discussion, the surface tension as a function of the logarithm of DDAHCl concentration at different pH values and at 45 "C is shown in Figure 1. Arrows indicate the solubility limit for each case. As it can be seen from the figure, at pH 10 the surface tension declines without leveling off, even in the precipitation region. The same behavior was also observed at 23 OC. It is, therefore, considered that no micellization takes place at pH 10. The critical pH of precipitation (pH,) for a given DDAHCl concentration was measured at various temperaturesand, for the purpose of discussion,the concentration vs pH, relations for three temperatures only are presented in Figure 2. These results can be used to evaluate the variation of DDAHCl solubility with temperature at any pH level within the range shown in the figure. The precipitation takes place in the zones above the concentration-pH, curves. In order to identify the form of the precipitate, three precipitate samples were collected and analyzed. The solution conditions under which the three samples were obtained were as follows: (1)5 X mol/L DDAHCl, pH 9.5; (2) 1x 10-3 mol/L DDAHCl, pH 9; (3)1X 10-3 mol/L DDAHCl, pH 11. Each sample was washed 3 times with distilled water of corresponding pH and was either freeze-dried overnight or vacuum dried in a desiccator for 5 days. The analytical results revealed that all three samples contained less than 0.2% chlorine and had similar composition with the C:H:N ratio very (22)Rippin, R.; hkowaki, J. S. Colloid Surf. 1985,15, 277. (23)Castro, S. H.;h k o w s k i , J. S. In Froth Flotation; Castro,S. H., Alvarez, J., Eds.; Elsevier: Amsterdam, 1988;pp 141-154. (24)de Bruyn, P.L. Trans. Am. Inst.Mining Met. Eng. 1966,202,291.
Langmuir, Vol. 7, No. 7, 1991 1363
The Krafft Point of Dedecylammonium Chloride I
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Figure 5. Solubility (0) and cmc ( 0 )of DDAHCl as a function for pH 10. Arrow of temperature at pH 9 and solubilitycurve (0) indicates the Krafft point at pH 9. ,20
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Figure 2. Effectof pH and temperatureon solubilityof DDAHC1. I
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Figure 3. Solubility (0) and cmc ( 0 )of DDAHCl as a function of temperature at pH 5. Arrows indicate the Krafft points.
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Figure 6. Effect of pH on the Krafft point ( 0 )and cmc (0) of DDAHCl in aqueous solutions at 25 "C.
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Figure 4. Solubility (0) and cmc ( 0 )of DDAHCl as a function of temperatureat pH 7 and 8. Arrows indicate the Krafft points.
close to that of molecular dodecylamine. Accordingly,the precipitate can be considered to be composed of free amine. This also confirms that precipitation takes place when the solubility of free amine, formed as a result of DDAHCl hydrolysis, is exceeded. The solubility and thecmc curves for pH 5 are presented together in Figure 3. The Krafft point is indicated by the arrow. As seen, the cmc remains nearly constant while the solubility rises rapidly above the Krafft point. The curves are similar to those for typical ionic surfactants.26126 The same kinds of diagrams were also constructed with the solubility values obtained from Figure 2 for pH 7,8, 9, and 10 and are shown in Figures 4 and 5. Again, the arrows indicate positions of the Krafft points in respective cases. As stated before, cmc was not found at pH 10. (25) Haro, M.; Shinoda, K. Bull. Chem. SOC.Jpn. 1973, 46,3889. (26) Shinoda, K.J. Phys. Chem. 1981,85, 3311.
Consequently, no Krafft point exists a t this pH in the range of temperatures up to 55 "C. Moreover, in contrast to the case of pH 5, the solubility in alkaline solutions does not increase further when temperature is raised up to 55 "C. Such a solubility behavior differs clearly from that of dissociated DDAHCl in the acidic solution. The Krafft points obtained from Figures 3,4, and 5 are plotted against the pH in Figure 6; the effect of pH on the cmc is also shown in this figure. As seen, both the Krafft point and the cmc fall off rapidly as pH increases. Discussion First we wish to point out that, as seen from Figure 3 and perhaps even better from Figure 7, our cmc vs temperature curve measured at pH 5 reveals a distinct minimum around 35 "C. This is in excellent agreement with the cmc vs temperature relationship for ionized doecylammonium chloride reported by Motomura et al.n Shinoda et al.= showed that mixed micelles are formed when aliphatic alcohols are added to a potassium laurate solution and that this leads to a fall off in the cmc of potassium laurate. Our results, shown in Figures 3 , 4 , 5 , and 6, confirm such conclusions. It was found that the higher the pH, the lower the cmc value of DDAHCl measured. Since an increase in pH increases the free amine-to-dodecylammonium ion ratio, a decrease in the cmc can be explained in the same way Shinoda et al.28 (27) Motomuta, K.;Iwanaga,S.;Yamanaka,M.; Aratono,M.; Matuura, R.J. Colloid Interface Sci. 1982,86, 151.
(28) Shinoda, K.; Nakagawa, T.; Tamamushi, B.;Isemura,T. Colloidal Surfactants; Academic Press: New York, 1963; pp 69-71.
1364 Langmuir, Vol. 7, No. 7, 1991
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Figure 7. Phase diagrams for aqueous DDAHCl solutions at various pH.
interpreted the effect of aliphatic alcohols on the cmc of potassium laurate. By the way, such a result was implied in many flotation related publications and usually was used to conclude that the micelleswould form more easily in aqueous solutions of aliphatic amines under alkaline conditions. As seen from Figure 7, this is not necessarily so. A decrease in the cmc caused by increased pH is accompanied by shrinking of the concentration zone in which micelles can exist. At pH 9 (see Figure 51, micelles can still exist in a narrow concentration range from 3 X 104 to 5 X lo4 mol/L, but at pH 10, the concentration of the dissolved surfactant is so low that the Krafft point, defined as the temperature at which solubility of surfactant as monomers is high enough to commence micellization? is not reached and the micelles do not appear in the solution at all. At pH 10, whenever the concentration exceeds 1.2 X 104mol/L, avisible solid precipitate appears in the solution. The Krafft point of DDAHCl was found to depend on pH and decreases from about 17.5 "C for pH 5 to about 7 OC for pH 9. The effect of the pH on the Krafft point is mirrored by the effect of the pH on the cmc, which decreasesfrom 1.2 X 10-2mol/L for pH 5 to 3 X lo4 mol/L for pH 9. According to Smith's estimation,29the cmc of a DDAHCl solution should vary from about 1.2 X mol/L for neutral pH to about 1.3 X mol/L for pH 11-12. As is now obvious,these conclusions resulted from a lack of data that we are presenting in this paper. Watson and Manser,lewith the use of the technique (du Nouy tensiometer) that is not capable of overcoming all experimental problems associated with dynamic effects, measured the cmc of DDAHCl to be about l X 10-8 mol/L at pH 9 and about 1 X lo-' mol/L at pH 11. The analysesof the precipitates obtained from DDAHCl aqueous solutions ranging in pH from 9to 11all indicated that it was a free dodecylamine. Eggenberger and Harwood12 in their conductometric studies of DDAHCl aqueous solutions at various temperatures were able to clearly distinguish between conducting property of the mecelles and nonconducting property of the precipitate, which was postulated to be dodecylammonium chloride (29) Smith, R. W.;In Challenges inMineml Processing; Swtry, K . V. S., Fuerabnau, M. C., Ede.; Society of Mining Ehgineela: Littleton, CO, 19089; p 78.
hemihydrate. Phase diagrams for the alkylammonium chloride-watersystem are very complicated.ll*soThe term "precipitate" as used in our publication seems to coincide with the term "large rod-shaped micelles" as used by some other researchers.g1These precipitated particles (hydrated crystals) have also different electrokineticproperties from micelles. As shown previously, the value of their zeta potential depends clearly on pH and, the i.e.p. for dodecylamine precipitate was found to be situated around pH ll.19*32 Because of this difference we prefer the term "precipitated colloidal particles" to distinguish these species from micelles. Since the Krafft point of DDAHCl was found to vary from 17.5 to 7 OC, and so is always below room temperature (20-25 "C), our results indicate that in the flotation experiments carried out within the pH range up to pH 9, and at room temperature, the effect of the Krafft point will not affect much interpretation of the flotation results. However,it is of interest to note that the flotation of quartz was found to be depressed when the pH exceeded 9 and the concentration of DDAHCl exceeded the solubility limit . 3 2 3 It is to be pointed out that the conclusions related to the formation of micelles, or hemimicelles at the solid/ solution interface, may be affected by our findings. At pH 10 and higher, micelles do not form at all and at any DDAHCl concentration higher than 1.2 X lo4 mol/L a colloidal precipitate will appear in the solution. The bearing of these findings on the hemicellization theory is not, however, clear at this point.
Conclusions The criticalmicelle concentration of dodecylammonium chloride aqueous solutions was found to decline sharply when pH increases;it decreases from 1.2 X mol/L for pH 5 to 3 X lo4 mol/L for pH 9. At pH 10 the concentration of dissolved dodecylammonium chloride is so low that the critical micelle concentration cannot be reached. The Krafft point for dodecylammonium chloride (pH 5 ) was measured to be 17.5 "C and to decrease down to 7 OC when pH is raised to pH 9. With an increase in pH, the concentration range over which micelles can form narrows sharply and the micelles do not form at all at pH 10.
Acknowledgment. The authors gratefully acknowledge financial support provided by the Natural Sciences and Engineering Research Council of Canada. Registry No. Dodecylammonium chloride, 929-73-7. (30)Laughlin, R.G.In Adwnces in Liquid Crystab; Brown,G. H., Ed.; Academic Press: New York, 1978; Vol. 3, pp 41-100. (31) Nery, H.;Marchal, J. P.;Cs.net,D.;Casecl, J. M. J. Colloidlntetface Sci. 1980, 77, 174. (32) h k o m k i , J. S. In Challenges in MineMl Processing; k t r y , K . V. S., Fuelatemu, M. C., Edr.; Society of Mining Engineera: Littleton, CO, 1989; pp 16-34. (33) Laelrowski, J. 5.;Vurdela, R. M.; Liu, Q. In Proceedings of the 16th Internotiom1 ib4ineral Rocessing Congress; Forasberg, K . S. E., Ed.; Elsevier: Amsterdam, 1988;Part A, pp 703-715.
