Oct., 1957
ELECTROKINETIC BEHAVIOR OF INORGANIC SUBSTANCES
1281
THE ELECTROKINETIC BEHAVIOR OF INORGANIC SUBSTANCES IN THE PRESENCE OF SURFACE ACTIVE AGENTS’ BY HAROLD 0. STRANGE AND J. FRED HAZEL Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania Received February 66, 1967
The electrokinetic behavior of several positive and negative inorganic colloidal systems has been studied in the presence of anionic and cationic surface active agents. Both electrophoretic and streaming potential methods were employed in the investigation. The inorganic systems and the surface active agents were prepared and purified by methods cited in the literature, and were the subjects of a stability study that has been reported previously. Iron(II1) oxide and aluminum oxide were used as positive colloidal systems and manganese dioxide and arsenic( 111)sulfide as representatives of negative systems. Potassium salts of a series of fatty acids, sodium alkyl sulfates, alkyl amine hydrochlorides and alkylpyridinium chlorides were emplo ed a8 colloidal electrolytes. A slit-ultramicrosco e was used for the electrophoretic measurements and a modification o r t h e apparatus described by Briggs was used for &e streaming potential determinations. The concentration of surface active agent required to decrease the mobility of the particles to zero called the isoelectric concentrations (IEC), was determined by plotting mobility of particles against concentration of surfactant. Plots of the logarithms of the isoelectric concentrations against chain length yielded straight lines in most cases. The curves could be represented by an equation of the type log (IEC) = A B N , where A is a constant for each homologous series, B is a general constant, and N is the number of carbon atoms in the hydrocarbon chain. Streaming potential studies were limited to iron(II1) oxide and aluminum oxide owders with potassium laurate and sodium dodecyl sulfate as electrolytes. There wa8 poor agreement between the resuis from the two different types of measurements, which was attributed to the different methods of preparing the inorganic substances in the two cases.
+
Introduction Studies of the effect of pure surface active compounds on the stability of dialyzed inorganic colloidal suspensions, and of the effect of the inorganic substances on the critical concentration of surface active agents2 indicated that the properties were electrical in origin and suggested the present investigation. The literature contains references to the coagulation and recharging of the inorganic substances, iron(II1) oxide3+ and arsenic(II1) sulfide3t4 with univalent surface active ions of sign opposite to that of the original particles, but no systematic study of the effect of the higher chain length compounds on the electrokinetic behavior of colloidal inorganic substances appears to have been made. Materials.-The colloidal suspensions used in the electrophoresis experiments were dilutions of the sols used in the flocculation studies.2 Information on these systems is given in Table I. TABLE I INORGANIC COLLOIDAL SYSTEMS Syatem
Iron(Il1) oxide Aluminum oxide Manganese( IV) oxide Arsenic(II1) sulfide
Met,hod of preparation
Hydrolysis of iron(II1) chloride Aluminum hydroxide dil. HCl (“peptization”) Dil. potassium permanganate 3% hydrogen peroxide Arsenious acid hydrogen sulfide gas
+
+
+
The arsenic(II1) sulfide sol was purified by bubbling hydrogen gas through it. The other colloidal systems were purified by dialysis. Streaming potential measurements were made on solid powders of iron(II1) oxide and aluminum oxide prepared from the respective chlorides by precipitation with ammonia. (1) This work waa supported by the Office of Naval Research. (2) J. F. Hazel and H. 0. Strange, paper presented before the Sympoaium on Adsorption from Solution, Miami Meeting of the American Chemical Society, April 7. 1957. (3) H. Freundlich and C.V. Slottman, 2. phyaik. Chem., 129, 305 (1927). (4) H. Freundlich and V. Birstein, KoEloidchem. Beihefte, 22, 95 (1923). (5) M. Nonaka, J . SOC.Chem. Ind., Japan, 32, 115 (1929).
The ammonia precipitates were washed with hot water and dried in a furnace at elevated temperatures: 800’ for iron(II1) oxide and 1200” for aluminum oxide. The solids were ground in a mortar to pass a 100 mesh screen. The homologous series of surface active agents employed in the present study were prepared by methods the details of which have been presented previously.2 Information on these compounds is given here for ready reference. The compounds contained from 8 to 14 carbon atoms. TABLE I1 SURFACE ACTIVE AGENTS Compound
Potassium soaps
Type
Method of preparation
+
Anionic
Fatty acid alcoholic KOH Sodium alkyl Anionic Fatty alcohol H$301 sulfates (concd.), or chlorosulfonic acid; followed by neutr. with NasCOa or NaOH Alkyl amine Cationic HC1 (dry gas) amine hydrochlorides (ether soln.) Alkyl pyridinium Cationic Pyridine alkyl chloride chlorides (reflux) The compounds were recrystallized several times from the appropriate solvents before use. Purification was considered complete when no change in the equivalent conductivity was noted after successive crystallizations. The critical concentrations of the compounds, as determined by the break in the conductance-concentration curve, compared favorably with literature values. Plots of the logarithms of the CMC values against chain length yielded straight lines.
+
+
+
Experimental Electrophoretic mobilities were determined with the aid of a slit ultramicroscope. The carbon arc image was focused in the electrophoresis cell6.’ after passing through an adjustable slit. The beam was passed through a solution of copper( 11) chloride to remove excessive heat radiation, thus preventing convection currents in the cell, and drifting of the particles. The ocular was equipped with a ruled grating which covered 42 length on the objective image. The microscope was focused a t a point 0.7 of the radius away from the center of the cylindrical cell for purposes of observing the motion of single particles. The electroosmotic mobility of the liquid is zero under these conditions.8~9 8. Mattson, THISJOURNAL, Sa, 1532 (1928). (7) J. F. Hazel and G. H. Ayres, ibid., 86, 2930 (1931). (8) M. von Smoluchowski, in Graetz, Handbuch Elsktr. Maon., 7 , (6)
383 (1921). (9) A. R. Wiley and J. F. Hazel, TRISJOURNAL, 41, 699 (1937).
HAROLD 0. STRANGE AND J. FRED HAZEL
1282
The streaming potential assembly and procedures employed in the present work have been described previously .l0J1
Results and Discussion The mobility of the colloidal particles of a given sign was lower at a given concentration of ionic surfactant the longer the hydrocarbon chain. The particles were recharged by the higher molecular weight compounds as indicated by the data in Table 111. TABLE I11 MINIMUM C H A I N LENGTH O F SURFACE ACTIVE ION REQUIREDFOR RECHARGING INORGANIC PARTICLES Inorganic particle (Positive)
Chain length
Nature of oompound
Vol. 61
gests that adsorption is relatively greater in dilute sols than in more concentrated At the isoelectric concentration, the number of ions adsorbed is just sufficient to offset the original charge on the colloid particle. The linear relationship between the logarithm of IEC values and chain length of the surface active ions may be construed as an illustration of Traube's rule.16 The adsorp+ 10.0 n
9n
v
2-
F f
p z b
CS C'1 ClO Cl2
K-salts n-fatty acids K-salts n-fatty acids Na-Alkyl sulfates Na-Alkyl sulfates
ClO ClO
n-Alkyl ammonium chlorides n-Akyl ammonium chlorides *Alkyl pyridinium chlorides %-Alkylpyridinium chlorides
ClZ
ClO
The concentration of surface active agent required to decrease the mobility of the particles to zero may be termed the isoelectric concentration (IEC). These concentrations were determined by plotting mobility of particles against concentration of surfactant. The isoelectric concentrations were found t o decrease sharply with increase in chain length. The logarithms of the IEC values when plotted against chain length were found to be linear with one exception: aluminum oxide with sodium alkyl sulfates. Moreover, the curves were found t o be parallel for different detergent types used with the same colloidal system. The curves may be represented by an equation of the type log (IEC)= A + BN where A is a constant for each homologous series, B is an empirical constant and N is the number of carbon atoms in the hydrocarbon chain. The equation is similar to the one proposed for the linear relationship between the logarithms of the CMC values and chain length of the surface active agent 2, a log CMC = A + B N where the terms have the same meaning as above. In a related study with the same systems it was found that plots of the logarithm of the flocculation value (F.V.) versus chain length approached straight lines as the inorganic colloid was diluted.2 There were marked deviations from linearity a t higher concentrations of sols, a fact not surprising in view of the uncertain relationship between adsorption and flocculation. That plots of log F.V. versus chain length tend to be linear with dilute sols sug(10) R.Edelberg and J. F. Hazel, J . Blecfrochem. Soc., 96, 13 (1949). (11) H. 0. Strange, "Interaction of Inorganic Macromolecular Systems with Surface Active Agents," p. 76, O.T.S. No. PB120, O 5 c e of Technical Services, U. S. Department of Commerce, Washington 25. D. C. (12) H. B. Klevens, THIBJOURNAL, 52, 130 (1948). (13) E. K. Goette, J . Colloid Sci., 4, 459 (1949).
U
t; N 0
0.5
1.0
1.5
2.0
CONCENTRATION OF SURFACTANT (rnilhnoles/ liter).
Fig. 1.-Zeta potentials of metal oxides in presence of anionic surface active agents: A, aluminum oxide-sodium dodecyl sulfate; B iron( 111) oxide-potassium laurate; C, iron( 111) oxide-sodium dodecyl sulfate; D, aluminum oxide -potassium laurate.
tion of soap ions by a colloid particle possessing a charge opposite that of the ion may consist of the polar group entering the electric double layer while the hydrocarbon chain extends into the solution. Recharging of the particles at higher soap ion concentrations may be postulated as being due to association of hydrocarbon chains, with the polar groups of the second layer extending into the solution. This results in the particle being surrounded by a sheath of charges which conveys to the particle its sign of charge. Streaming Potential Studies.-The streaming potential studies were conducted with powdered iron(II1) oxide and aluminum oxide using potassium laurate and sodium dodecyl sulfate as surface active agents. Zeta potentials were calculated from streaming potentials by use of the Briggsvon Smoluchowski-Helmholtz equation.11 The results are given in Fig. 1. Higher concentrations of surfactant were required to recharge the iron(II1) oxide and aluminum oxide surfaces in the case of the sols, as indicated by a reversal in the direction of electrophoretic migration of the particles, than with the solid powders, except for potassium laurate with iron(II1) oxide, It might be expected that a lower Concentration of electrolytes would be required to recharge the solid powders because of the large particle size and hence small specific surface. The streaming potential data were quite reproducible but since the powders were prepared by different methods than the sols it was felt that the powders did not fairly represent the colloid (14) H. 8.Weiser and W. 0. Millipan, J . Am. Chem. Soc., 65, 1924 (1940). (15) J. F.Hazel, THISJOURNAL, 45,738(1941). (16) von J. Traube, Liebigs Ann. Chsm., 565, 27 (1891).
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Oct., 1957
FREEENERGY CHANGE FOR SODIUMLAURYL SULFATE MICELLEFORMATION 1283
system surface. Reliance was placed, accordingly, on the electrophoretic method of examining the
effect of surface active agents on the electrokinetic potentials.
THE STANDARD FREE ENERGY CHANGE FOR THE FORMATION OF THE SODIUM LAURYL SULFATE MICELLE BY HORSTW. HOYER Department of Chemistry, Hunter College, New York 81, N . Y . Received February $6,1067
Equilibrium concentrations of sodium ions, lauryl sulfate ions and micelles are calculated for solutions of sodium lauryl sulfate from light scattering, electrophoresis and conductivit data. Values of - AFoo/n, the standard free energy change per molecule associated with micelle formation are calculateffrom these values and are found to be linear functions of the ionic strength of the solution. Extrapolation to zero ionic strength gives a value of - A F o o / n equal to 6,100 oal. per mole of monomer.
Introduction Within the past few years considerable reliable data has been obtained concerning the physical properties of solutions of colloidal electrolytes. Mysels and his eo-workers have concentrated their attention upon carefully prepared and specially purified sodium lauryl sulfate. A series of investigations with this compound have established the nature of the relationship between critical micelle concentration and total salt concentration,’ the influence of salt concentration upon the molecular weight of the micelle,2 and upon its electrophoretic m ~ b i l i t y . ~Stigter and Myselsa have interpreted their mobility data in terms of Booth’s theory4 of the electrophoresis of colloidal particles and have calculated the change in micelle charge with total salt concentration. The conductivity of solutions of this colloidal electrolyte has been determined by K. Mysels and Dulin.6 Myselss has re-examined the question of the scattering of light by charged particles and found that the effect of the micelle charge is to increase the molecular weights slightly above those obtained experimentally by this method. His corrected degree of association of n monomer units into a micelle is given by log n = 2.152 0.091 log (C CO) (1) where C is the molarity of the added salt and Co is the critical micelle concentration in moles per liter in the presence of the added salt. The equation connecting critical micelle concentration with added salt is of similar form
+
log Co = -3.509
from n - z sodium ions and n lauryl sulfate ions, LS-, may be written nLS-
+ ( n - z)Na+ Jr
M-S
(3)
The equilibrium constant for this reaction will then be
For the sake of completeness we list below the various formulas, relationships and definitions to be used, starting with the equation relating the specific conductivity, K , of the colloidal solution to the molar concentrations of the ions and their equivalent conductances, A.
+
+ CMZAM
1 0 0 0 ~= C N ~ + A N ~ *CLS-ALECLS-
cz 1000~- - [AN,+ + A M ] n = (n AN&+- 5 A ALE- 4- 7
(5) (54
n M
where C is the formality of the sodium lauryl sulfate solutions.
+
- 0.679 log (C + Co)
(2)
We wish to show how the above data may be used t o calculate the equilibrium constant and standard free energy change for the formation of the sodium lauryl sulfate micelle. Theory and Method The equilibrium involving the formation of the sodium lauryl sulfate micelle, M , with charge z, (1) R. J. Williams, J. N. Phillips and K. J. Mysels, Trans. Faraday BOG.,61, 728 (1955). (2) J. N. Phillips and K. J. Mysels, THIS JOURNAL, 69, 325 (1955). (3) D. Stigter and K. J. Mysels, ibid., 69, 45 (1955). (4) F. Booth, Proc. Roy. Soc. (London), 11203, 514 (1950). (5) K. J. Mysels and C. I. Dulin, J . Colloid S‘ci., 10, 481 (1955). (6) K. J. Mysels, ibid., 10, 507 (1955).
+
C N ~= + CLE- ZCM M’ = 265% 23.0(n - z )
+
(7) (8)
where M’is the molecular weight of the micelle and the concentrations are in moles or gram-ions per liter. Equations 5a, 6 and 7 were used to calculate the concentrations of the different ionic species present in the colloidal solution. Since n, z and the equivalent ionic conductances are all functions of the ionic strength of the solution, equation 5a was first solved by a series of approximations until successive values of CLS- agreed to within 2%. This last value was then used to calculate CMand C N ~ . The basic assumption of our method rests upon what seems to us a logical interpretation of the experimental data involving micelle charge, molecular weight and sodium chloride concentrations. The charge and molecular weight were determined a t the critical micelle concentrations for micelles existing in equilibrium with known concentrationa
