Cationic surfactants in organic solvents. II. Viscosity and conductance of trilaurylammonium salts in carbon tetrachloride and benzene

Cationic Surfactants in Organic Solvents II. Viscosity and Conductance of Trilaurylammonium Salts in Carbon Tetrachloride and Benzene G. Y. MARKOVITS 1, O. LEVY 1, AND A. S. K E R T E S ~ Department of Chemistry, The University of the Negev, Beer Sheva1, and Institute of Chemistry, The Hebrew University of Jerusalem, ~Israel Received February 2, 1973; accepted April 16, 1973 The viscosity and conductance of carbon tetrachloride and benzene solutions of trilaurylammonium chloride, bromide, nitrate, perchlorate, and tetrachloroferrate have been measured at 25 °, 30 °, and 35°C in the concentration range up to 0.15 or 0.2 molar. In some plots representing the physical property as a function of concentration a break is discernible which is usually attributed to the apparent critical micelle concentration. A comparison of the present results with vapor pressure lowering data on the same systems expressed in terms of aggregatesize distribution curves, reveals that the apparent cmc is associated with the formation of higher oligomers in dynamic equilibrium with monomeric and dimeric (or trimeric) species. The apparent heats of micellization estimated from the viscosity data at different temperatures, suggest that the enthalpy changes are those of the desruption of single dipole-dlpole bonds. The data support the mass-action law model of aggregation of surfactants in organic media.

Recent vapor-pressurelowering (osmometric) measurements (1) on hydrocarbon solutions of several secondary, tertiary, and quaternary long-chain alkylammonium salts strongly suggest that the process of aggregation of these cationic surface active agents does not follow the conventionally assumed pattern of mono m e r ~ micelle equilibrium, borrowed from the known behavior of these and similar surfactants in aqueous solutions. The experimental data have thus been interpreted in terms of a set of mass-action law equilibria of a stepwise formation of aggregates, reversibly formed from monomers by a progressive molecular association, represented as

S ~ $2 ~ Ss ~ S~-1 ~ S~

[1]

It has been suggested that such a stepwise aggregation process leads in fact to a dynamic equilibrium where the monomer and several

oligomers of different size coexist in solution. Obviously, the proposed mass-action law approach renders the concept of a critical micelle concentration as based on a monomer ~ micelle equilibrium, nonapplicable. As a matter of fact, the two concepts are mutually exclusive. In order to provide further evidence in support of the mass-action law concept outlined above, we have now measured the viscosity and conductance of benzene and carbon tetrachloride solutions of several trilaurylammonium salts and supplemented earlier (1, 2) osmometric data on these systems. These properties of surfactant solutions in organic media have been reviewed (3-7). As a rule, viscosity of such solution increases with the concentration of the surfactant. The consensus has been that at a concentration where aggregation starts, the critical micelle concentration, a rapid increase in viscosity is

424

Journal of Colloid anclInterJaceScience, Vol. 47, No. 2, May 1974

Copyright ~ 1974 by Academic Press, Inc. All rights of reproduction in any form reserved.

LAURYLAMMONIUM SALTS IN HYDROCARBONS to be expected. Conductance measurements on organic solutions of surfactants are few, and essentially limited to organic solvents of medium polarity and dielectric constant, such as alcohols, ketones, and some esters. In hydrocarbon solvents of low polarity and with no hydrophilic character, conductance is rather limited, though increases with the concentration of the surfactant. This phenomenon has been attributed to the fact that the bulk dielectric constant of the solution at high surfactant concentrations is significantly higher than that of the pure solvent, affecting thus the dissociation equilibrium. The limited conductance of such high-molecular-weight alkylammonium salts in nonpolar solvents is of course not surprising in view of the fact that these salts are usually associated to ion pairs at concentrations as low as l0 -5 M, to such an extent that the concentration of free ions seldom exceeds one ppm. EXPERIMENTAL The preparation and purification of the

tri-n-dodecylammonium salts have been described previously, along with some of their physical properties (2, 8). All salts are white and crystalline, except the tetrachloroferrate which is yellow, nonhygroscopic, and stable, and none contained either free acid or amine. For all measurements, the anhydrous salts were freshly dissolved in dry carbon tetrachloride (BDH product) or benzene (Mallinkrodt) of high analytical purity. Conductance was measured using either an Industrial Instruments conductivity bridge Model RC-I6B2 in a cell with a constant of 0.01 cm -1, or a LKB conductivity bridge Type 3216 B, or both. The viscosity measurement were carried out by means of an Ostwald viscosimeter (the average running time t, being about 200 sec), the temperature of the bath kept constant to =t=0.01°C. Four different solvents were used for calibration. The reported viscosities are the mean values of five runs, calculated by n = A p t - B(p/t), where the constants A and B are determined using

425

several different solvents. The densities of the solutions, p, were determined at least in duplicate with calibrated 63-ml pycnometers, using the above constant temperature bath. The vapor pressure lowering of the organic solutions was determined by a Mechrolab Osmometer, Model 301-A by a technique described elsewhere (1, 2, 16), RESULTS The viscosity measurements at 25°C of the solutions investigated are plotted in Fig. 1 as the fluidity, 1/7, versus the molar concentration. The fluidity curves at 30°C and 35°C are parallel to those at 25°C, the temperature coefficient of fluidity in the concentration range studied being 0.17 cp-1/°C, identical to that of the pure solvents. Table I list the concentrations (mole/liter) at which the break appears between two straight lines of different slopes, usually attributed to the apparent critical micelle concentration. No such break point is apparent in the bisulfate curve at any of the temperatures studied, and that for the perchlorate is probably less precise. The viscosity of solutions in either solvent is not far different from that of the pure solvent in the concentration range investigated, though it increases with the solute concentration. The viscosity is affected by the size of the anion, increasing, at higher salt concentrations in CC14, in the order C 1 Q - > B r > C1- > NO3- > HSO4-. The equivalent conductance of the solutions investigated are plotted in Fig. 2 as function of the total salt concentration in carbon tetrachloride. Table II compiles the apparent cmc values. There is a good agreement between the values as found by the two methods. Here, no cmc is apparent in the perchlorate and tetrachloroferrate systems, the latter not having been studied by viscosity measurements. The mean aggregation numbers =

(s.)

~(s.)/E 1

1

as obtained from osmometric measurements at Journal of Colloid and Interface Science, Vol. 47, No. 2, M a y 1974

426

MARKOVITS, LEVY AND KERTES

16

'~'~°~o~

^

14 12 •-

10

10 8 6 4

I

0,02

I

I

I

I

I

I

I

.04

.06

,08

.10

.12

.14

.16

R3NHXmole/lit

FIG. 1. Fluidity of trilaurylammonium salts in organic solvents as a function of concentration, 25°C. (1) bromide in benzene; (2)chloride in CC14; (3)bromide in CC14; (4)perchlorate in CCI~; (5)bisulfate in CCI4; (6) nitrate in CCI4. TABLE I ~BREAI( POINTS" IN TRILAURYLAMMONIUMSALTS: ORGANICSOLVENT SYSTEMS AS OBTAINED FROM VISCOSITY MEASUREMENTS

Salt (Cl~H25)~NHC1

(CnH2~)aNHBr

Solvent

t°C

Conc. range studied mole/lit

Apparent cmc mole/lit

CC14 CC14 CC14 CC14 CCI4 C61-I~

25 30 35 25 30 25 30 25 30 35 25 30 35 25 30 35

0.02 -0.10 0.02 -0.10 0.02 -0.10 0.005-0.10 0.005-0.10 0.005-0.20 0.005-0.20 0.01 -0.20 0.01 -0.20 0.01 -0.20 0.01 -0.25 0.01 -0.25 0.01 -0.25 0.01 -0.15 0.01 -0.15 0.01 -0.15

0.050 0.055 0.060 0.035 0.039 0.076 0.085 0.090 0.10 0.112 ---~0.023 N0.024 ~-~0.024

C6H6

(CnH~)3NHNO3

(C12H25)~NI-I2SO4

(CI~H~5)~NHC10~

CC14 CC14 CC14 CC14 CC14 CCh CC14 CC14 CC14

Journal of Colloid and Interface Science, Vol. 47, No. 2, May 1974

LAURYLAMMONIUM SALTS IN HYDROCARBONS

7

14

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X ,<

6

./

! / /

4

./

/

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RaNHX mole/lit FIG. 2. Conductance of trilaurylammonium salts in carbon tetrachloride as a function of concentration, 25°C. (l)chloride; (2)bromide; (3)perchlorate; (4)tetrachloroferrate. 25°C for trilaurylammonium bromide and bisulfate in carbon tetrachloride are plotted in Fig. 3 as a function of the total salt concentration, along with those for trilaurylammonium chloride (1) and nitrate (2) measured previously. Table I I I compiles the overall formation constants for the different aggregates, log ~n, calculated b y the method described elsewhere (1, 2). DISCUSSION

property versus concentration curves without necessarily analyzing the magnitude of the property. This is of course made possible by the appearance of sharp breaks between two

5

,

For most practical purposes of cmc determination in aqueous solutions, it is usually sufficient to concentrate on the breaks in the

3

f /'

.of

TABLE II '~BREAK POINTS" IN TRILAURYLAMMONIUM SALTS: CARBON TETRACHLORIDE SYSTEMS AS OBTAINED FROm CONDUCTANCE MEASUREMENTS Salt;

(C~2H25)~NHC1 (CI~H~)~NHBr (Cl~H25)3NHCIO4 (C12H~5)3NHFeCI4

t°C

25 25 30 25 25

Conc. range studied mole/lit

0.02-0.10 0.02~0.10 0.02-0.i0 0.02-0.15 0.01.0.10

Apparent cmc mole/lit

0.054 0.032 0.038 ---

1 L

I

I

I

I

I

0.02 .o4 .o6 .o8 .lo R~NHX m o l e / l i t

FIo. 3. Average aggregation number of trilaurylammonium salts in carbon tetrachloride as a function of concentration, 25°C. (1) chloride; (2) nitrate; (3) bromide; (4) bisulfate. Journal of Colloid and Interface Science, Vol. 47, No. 2, M a y 1974

428

MARKOVITS, LEVY AND KERTES TABLE III T H E OVERALL FORMATION CONSTANTS OF TRILAURYLAMMONIUM SALTS AS OBTAINED ~RO~ OSMOMETRIC MEASUREMENTS, 25oC Salt

(C12H25)3NHCI1 (C~H25)~NHBd (C12H~5)~NHHO~2 (C12H~)3NH~SO4

Solvent

Conc. range studied mole/lit

a,b,c

log fa

log fib

log/~

CCI4 C~H6 CC14 CC14

0.005-0.16 0.005-0.16 0.005-0.15 0.005-0.12

2,6 2,3 2,3,4 3,6,9

2.02 1.34 1.34 4.84

8.29 2.76 3.43 11.96

5.64 19.01

straight lines, one representing a given colligative property of the surfactant when in a monomolecular dispersion, and the other that of the completely aggregated micellar solute. This is in accordance with a monomer ~ micelle equilibrium. A smooth curvature of such plots rather than a sharp break in them appears to be much more characteristic to surfactants in organic solvents, regardless whether they are ionic or nonionic (7, 9-15). Such plot is, for example, that of the average aggregation number shown in Fig. 3. No break is apparent in any of the curves. The apparent discrepancy between some of the curves in Figs. 1 and 2 where a break is discernible, and those in Fig. 3, where none is apparent, is not necessarily surprising in view of the different physical properties measured (7). In an attempt to resolve this apparent discrepancy it is necessary to compute the relative concentration of the different oligomers using the formation constants from Table 3. The relative concentration of each oligomer is defined as =

n

.Es]-

1 n

=n(S,)/Y-~.n(S,)

[21

1

which is the fraction of the surfactant in the form of a particular aggregate (7), where/3, values are the overall formation constants of the particular aggregate (ill is unity by Journal of Colloid and Interface Science, Vol. 47, No. 2, M a y 1974

definition) = ESd/ES3-

= ks, k,, . . . k .

[-31

where k~ is the stepwise formation constant defined as

k, = [S,]/[S,-~XS]

[4]

The aggregate size distribution curves based on this calculation are plotted in Fig. 4 as a function of the total solute concentration. It is appropriate to discuss the shape of the fluidityand conductance plots (Figs. 1 and 2) as compared to that of the oligomer distribution curves (Fig. 4). For example, in the chloride system the apparent cmc at 0.05 -- 0.06 molar corresponds to the concentration where the fraction of the hexamer starts to increase; at the same time however, that of the intermediate oligomer, the dimer, starts to decrease. Thus, and this is the essence of the massaction law concept advocated here, at increasing suffactant concentration the higher oligomers are formed from the smaller aggregates which are predominant at low solute concentration, rather than from the monomers directly. This is a stepwise aggregation process represented by Eq. [-1"],The smooth curvature in Fig. 3 is a consequence of the stepwise aggregation: in the ~ values computed the variables enter in their appropriate statistical weight. Similar arguments hold for the other systems, except the bisulfate where no break is detected in viscosity measurements. One explanation of this discrepancy can be that the aggregation process in this system starts at very low concentrations (high formation

429

SALTS I N H Y D R O C A R B O N S

LAURYLAMMONIUM

1

2

0.6 l( 2

0.4

.¢6

-

" dt

0.2

.

K~

"

.

o( t

o(. 4 Kj

0.4 K2

O.2

|

0.04

!

.08

i

!

.12

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i

.08

!

.12

.16

R3NHX mole/lit FIG. 4. Degree of formation of oligomers (o~) of trilaurylammonium salts in organic solvent as a function of concentration, 25°C. (1) chloride in CC14; (2) nitrate in CC14; (3) bisulfate in CC14; (4) bromide in benzene.

constant, Table 3) and the break point, if existent at all, should be shifted to very low concentration of the salt. We wish to emphasize that, in the systems under consideration, the break points in viscosity or conductance versus concentration plots may be misleading in a sense that they do not necessarily correspond to the surfactant concentration at which a molecular dispersion gives place to a micellar dispersion. Thus, the fundamental requirement for the critical micelle concentration is not met. In view of the similarity in behavior of both anionic and cationic surfactants in hydrocarbon solution (7), we believe that the conclusions reached

here have a more general validity 1. Indeed, Fuoss and Accascina (18) have found in similar systems that the several slope changes appearing in conductance versus concentration plots are due to changes in the size of the molecular clusters formed. From the "break points" of the viscosity measurements at different temperatures we have estimated the apparent "heat of demicellization" using the thermodynamically rather unsound, but frequently employed, modification (3) of the v a n ' t Hoff isochore. The 1 Except perhaps in cases where ~ has a constant value over a wide concentration range of the solute (dinonylnaphthalene sulfonates (19). Journal of Colloid and Interface Science, Vol. 47, No, 2, May 1974

430

MARKOVITS, L E V Y AND KERTES

approximate AH values computed are all endothermic, ranging between 1.6 and 2.5 kcal/mole. While these values are comparable to those few reported in the literature (3-7), they seem to be too low for a process involving an interaction of several highly polar (7, 17) monomeric molecules in a non polar medium. Instead, they are of a magnitude corresponding to the formation or destruction of one single dipole-dipole bond (2). We invoke this argument again in support of the concept of a stepwise aggregation process. REFERENCES 1. KERTES, A. S., GUTMANN, H.~ LEVY, O., AND MAI~I(OVlTS, G. Y., Proceedings Vlth Intern. Conf. Surface Activity, Zurich, 1972 (Part I of this series). 2. KERTES, A. S., ANDMARKOVITS,G., J. Phys. Chem., 72, 4202 (1968); MAI~KOVI~SG., ANI) KE~TES, A. S., in "Solvent Extraction Chemistry," (D. Dyrssen, J. O. Liljenzin, and 7. Rydberg, Eds.), North-Holland, Amsterdam, 1967, p. 390. 3. SttINODA, K., in "Colloidal Surfactants," Ch. 1, Academic Press, New York, 1963. 4. FOWKES, F. M., in "Solvent Properties of Surfactant Solutions," (K. Shinoda, Ed.) M. Dekker, New York, 1967, p. 65.

Journal of Colloid and Interface Science. Vol. 47, No. 2, May 1974

5. BECIIER, P., #~ "Nonionic Surfactants," (M. J. Schick, Ed.) M. Dekker, New York, 1967, p. 511. 6. KITA~ARA, A v in "Cationic Surfactants," Ch. 8, (E. Yungermann and M. Dekker, Eds.), New York, 1970. 7. KEI~TES,A. S., AND GUTgANN, H., in "Surface and Colloidal Science," Vol. 8, (E. Matijevic, Ed.), Wiley-Interscience, New York, in press. 8. LEVY, O., ANDKERTES, A. S., J. Inorg. Nud. Chem. 31, 888 (1969). 9. KITRARA,A., Bull. Chem. Soe. Japan 30, 586 (1957). 10. Ross, S., AND OLIVIER, J. P., J. Phys. Chem. 63, 1671 (1959). 11. SINGEETERRY, C. 1~., J. Amer. Oil Chemists Soe. 32, 446 (1955). 12. REERINK, H., J. Coll. Sci. 20, 217 (1965). 13. DEBYE, P., AND COLE, H., J. Coll. Sci. 17, 220 (1962). 14. EEWORTItY,P. H., AND YIYSELS,K. J., J. Coll. Sci. 21, 331 (1966). 15. SHINOI)A,K., ANDHUTCmNSON, E., J. Phys. Chem. 66, 577 (1962). 16. KElZTES, A. S., LEVY, O., A N D MARKOVITS,G., J. Phys. Chem. 74, 3568 (1970). 17. LEVY, O., MA~KOVlTS, G., AND KERTES, A. S., J. Phys. Chem. 75, 542 (1971). 18. Fvoss, R. M., AND ACCASCINA,F., "Electolytic Conductance," Interscience, New York, 1959, p. 265. 19. KAUFMAN,S., ANDSINGLETEREY,C. R., J. Coll. Sci. 12, 465 (1957) ; LITTLE, R. C., ANDSINOLETEI~I~Y, C. R., J. Phys. Chem. 68, 3453 (1964).