Anionic surfactants with methylviologen or cupric ions as divalent cationic gegenion: Solubility and micelle formation

Anionic Surfactants with Methylviologen or Cupric Ions as Divalent Cationic Gegenion: Solubility and Micelle Formation Aqueous solubility and critical micelle concentration (CMC) of anionic surfactants whose cationic gegenion is N,N'-dimethyl-4,4'-dipyridinium (MV 2*) or cupric (Cu 2+) ion were measured, and the effect of chemical structure of divalent gegenions on solubility, CMC, and Krafft point was made clear, where much attention was paid to the water of crystallization of surfactant solids. The concept of Krafft point (or Krafft region) was also discussed from solubility and CMC changes with temperature.

INTRODUCTION

EXPERIMENTAL

In the preceding paper (1), the authors discussed dissolution, micellization, and solubilization in aqueous solution from the viewpoint of degrees of freedom based on the phase rule and reached the conclusion that the mass action model for micelle formation can perfectly elucidate the above phenomena of surfactant solutions. Furthermore, they defined the Krafft point as the temperature at which surfactant solubility as monomer becomes high enough to start micellization notably. Hence, the definition suggests two ways to decrease the Krafft point; one is to increase aqueous solubility of surfactant and the other is to decrease its CMC value. In general, gegenions of anionic surfactants have been alkaline or alkaline-earth cationic metal ions whose electrical charge locates in very small space of the metal ions. Then, the electrical potential of metallic ions becomes rather high and the crystalline state of the surfactants seems to be energetically stabilized by Coulombic interaction. This leads to smaller aqueous solubility of the surfactants, resuiting in higher Krafft point. On the other hand, the crystalline state of ionic surfactants whose gegenions have a diffuse charge like N,N'-dimethyl-4,4'-dipyridinium(II) ion (methylviologen ion) seems to be quite different from those which have gegenions of localized charge like Cu 2÷. Water of crystallization is, of course, another factor influencing the solubility. Methylviologen ion has been extensively used as a redox relay for light-initiated electron transfer reaction (2, 3) in micellar system, and such cationic surfactants as viologens substituted by a long alkyl chain have been published too (4). The binding of MV 2÷ to anionic detergent micelles has been also investigated (5). In spite of its widespread use, the solution properties of anionic surfactants whose gegenion is methylviologen ion itself have not been studied yet. This work is, then, intended to make clear the effect of chemical structure of divalent cationic gegenion on solubility, CMC, and Krafft point of anionic surfactants, by using MV 2+ and Cu 2+ as their gegenions.

Preparation of surfactants. Copper(II) dodecyl sulfate (Cu(C12H25SO4)2 or Cu(DS)2) and sulfonate (Cu(CI2H25SO3)2 or Cu(DSO)2) were prepared from respective sodium salts and purified by the standard procedure (6). N,N'-Dimethyl-4,4'-dipyridinium(II) (methylviologen) dodecyl sulfate (MV(CI2H25SO4)2 or MV(DS)2) and sulfonate (MV(CIxH25803) 2 or MV(DSO)2) were synthesized as follows. Sodium dodecyl sulfate and sulfonate were converted to the corresponding silver salts by double decomposition with AgNO3 in aqueous suspension. The silver ions were then exchanged with methylviologen ions by

a

2,0

D~

\

b

T "" 1,5

~ 1,0 e_

0,5

p

o,,

I

o'.8 i',2 I.G 2'.o /4

I

2.8

CONCENTRATION /mmol Kg -I

FIG. 1. Specific conductance vs concentration relation at 25°C: (a) MV(DSO)2.2H20; (b) Cu(DS)2.4H20; (c) MV(DS)2. 285

0021-9797/84 $3.00 Journal of Colloid and Interface Science, Vol. 101, No, 1, S e p t e m b e r 1984

Copyright © 1984 by A c a d e m i c Press, Inc. All rights o f reproduction in a n y f o r m reserved.

286

NOTES

14 I

12

10 I-

8

I-

,,=, (_) z 0 u

6

270

280

290

i

I

I

I

300

310

320

330

TEMPERATURE /K RG. 2. Solubility (©) and CMC (e) change with temperature: (a) MV(DSO)2 • 2H20; (b) Cu(DS)2 • 4H20; (c) MV(DS)2; (d) Cu(DSO)2.2H20.

introducing excess methylviologen dichloride, and the precipitated AgC1 was removed by centrifugation. MV(DS)2 and MV(DSO)2 thus prepared were purified by recrystallization twice from the aqueous solution with excess MV 2+ and twice from water. The purity of these surfactants was checked by an elemental analysis and the differential thermal analysis technique (7, 8). From the elemental analysis three of them were found to have water of crystallization: Cu(DS)2 • 4H20, Cu(DSO)2 • 2H20, and MV(DSOh. 2H20, while MV(DS)2 has no water of crystallization at 35°C. Solubility measurement. A suspension of recrystallized surfactant solid obtained by cooling the aqueous solution below the Krafft point was used in situ for the solubility measurement. The apparatus and the method employed were the same as those for the previous solubilization measurement (9, 10). Mechanical agitation was run for more than 1 hr for the system to reach a perfect equilibrium. The temperature was controlled within +0.02°C. Methylviologen ion has a maximum absorption band at 255 rim. The band was used for the determination of surfactant concentration, where concentration for the absorbance measurement were brought to below their CMC. Solubility of cupric surfactants was measured by the conductometric method. CMC determination. The critical micelle concentration (CMC) were determined by the usual conductivity method as the concentration at an intersection of two fines obtained by plotting the specific conductance against concentration. Journal of Colloid and Inte6Cace Science, Vol. 101, No. 1, September 1984

RESULTS AND DISCUSSION Plots of specific conductance against concentration at 25°C are shown in Fig. 1 for the three surfactants whose -4,0 (I

b

a

-5,0 c J

g -6,0 m o

g -7,0 J

-8,0

3.00

[

I

3,25

3,50

3.75

103/TEMP, / x - I

FIG. 3. Plots of logarithm of solubility against 1/T: (a) MV(DSO)2 • 2H20; (b) Cu(DS)2 • 4H20; (c) MV(DS)2; (d) Cu(DSO)2 • 2H20.

287

NOTES TABLE I Effect of Gegenions (MV 2+ and Cu 2÷) on CMC, Krafft Point, and Thermodynamical Parameters for Dissolution in Water CMC (mmole Kg-t)

Surfactant

MV(DSO)2.2H20 Cu(DSO)2.2H20 MV(DS)2 Cu(DS)2.4H20

1.29 1.67 0.55 1.17

Krafft point (°C)

Ah ° (kJ mole-I)

10.4 53.5 22.9 23.0

75 127 99 60

(25°C) (54°C) (25°C) (25°C)

Krafft point is less than 25°C. It is notable that the CMC of MV(DS)2 is less than half the CMC's of others. From the slope of the conductance above CMC, the degree of MV 2+ dissociation is thought to be less from MV(DS)2 than from MV(DSO)2 micelles, which might lead to smaller CMC value of MV(DS)2. This point will be discussed more in detail in a coming paper together with the effect of alkyl-chain length on solubility and micelle formation of MV 2÷ salts of alkyl-sulfonates (11). The changes of CMC with temperature are also plotted in Fig. 2. The change is less temperature-dependent compared with monovalent metal ion salts (12). The solubility changes with temperature are also plotted in Fig. 2. The change can make it possible to determine the Krafft point together with the CMC change with temperature. In this case the system is divariant from the phase rule (1), and the temperature can absolutely specify the system of solution, the solubility of the surfactants of course, at atmospheric pressure not only below the Krafft point but also above it. On the other hand, CMC was defined as the point corresponding to the maximum change in gradient in an ideal property-concentration relationship (13). This definition has been accepted and adopted in most cases, although the CMC rests on a definition. In other words CMC depends upon the property of the solution examined and, therefore, should be defined as the narrow concentration range (14). Thus, it becomes impossible to define the Krafft point as the single point as the intersection between solubility and CMC changes with temperature. In this sense, it seems correct that the Krafft point is not a single point but a diffuse region which might be called the Krafft region) Below the Krafft region, a singly dispersed Cu 2+ or MV z+ alkylsulfate or alkylsulfonate can be assumed to dissociate perfectly (15, 16), where Cc~2+ or CMV2. = S (solubility).

[ 1]

In Fig. 3 are shown the plots of logarithm of solubility. From the plots the Krafft point can be roughly determined as temperature at the intersection of two straight lines

The new definition recommended by IUPAC.

Ag ° (kJ mole -j)

49 58 58 51

As ° (J mole -I)

(9°C) (32°C) (1 I°C) (10°C)

92 226 144 32

(9°C) (32°C) (11 °C) (10°C)

below and above the Krafft temperature. The thermodynamical parameters for dissolution are then calculated by using the equations (17) Ag ° = - 3 R T In S =

e,l

e

1

o~s

/2gegenion-1- 2t~u~m.t o. - us.~tact~.t,

[2]

where tt ° and #° are standard chemical potential at infinite dilution and at pure state, respectively, and the superscripts 1 and s refer to liquid and solid phases, respectively, and Ah ° = - 3 R [d In S/d(1/T)le

[3]

As ° = (Ah ° - Ag°)I T.

[4]

The results are given in Table I. At lower temperatures solubility increases in the order Cu(DSOh. 2H20 < MV(DS)2 < Cu(DS)2.4H20 < MV(DSO)2.2H20, while the Krafft point increases in the order MV(DSO)2.2H20 < MV(DS)2 ~ Cu(DS)2.4H20 < Cu(DSO)2- 2H20. The lower Krafft point of MV(DS)2 is clearly due to its relatively low CMC value, judging from its smaller solubility. The following general conclusions can be derived from the above orders and &h ° and As ° values in Table I: (i) crystalline state with gegenion of diffuse charge is less stable and then leads to higher solubility of ionic surfactant (MV(DSO)2.2H20 and Cu(DSOh. 2H20). and (ii) surfactant solid with more water of crystallization is easier to dissolve with less enthalpy and entropy change of dissolution (Cu(DSh • 4H20), and (iii) the Ah ° values in Table I are much less than twice Ah ° of the corresponding surfactant with Na + gegenion (&h ° = 79 kJ mole -~ for NaDSO ( 11 )), which means that the surfactant ion with monovalent gegenion is energetically more stable in crystalline state. At any rate it is evident that the crystalline state o f surfactant has a crucial factor on the solubility and its related phenomenon such as the Krafft point (or Krafft region). Journal of Colloid and Interface Science, Vol. 101, No. 1, September 1984

288

NOTES ACKNOWLEDGMENTS

This work was partly supported by Grant-in-Aid for Scientific Research from the Minisitry of Education, No. 58118006. Technical help of Miss Chikako Akine is gratefully acknowledged.

REFERENCES 1. Moroi, Y., Sugii, R., and Matuura, R., J. Colloid Interface Sci. 98, 184 (1984). 2. Krasnovskii, A. A., Nikandrov, V. V., Brin, G. P., Gogotov, I. N., and Oshchepkov, V. P., Dokl. Akad. Nauk SSSR 225, 711 (1975). 3. Kiwi, J., and Gratzel, M., Nature (London) 281, 657 (1979); J. Amer. Chem. Soc. 101, 2714 (1979). 4. Krieg, M., Pileni, M.-P., Braun, A. M., and Gratzel, M., Z Colloid Interface Sci. 83, 209 (1981). 5. Bonilha, J. B. S., Foreman, T. K., and Whitten, D. G , J. Amer. Chem. Soc. 104, 4215 (1982). 6. Moroi, Y., Oyama, T., and Matuura, R., J. Colloid Interface Sci. 60, 103 (1977). 7. Moroi, Y., Motomura, K., and Matuura, R., Bull. Chem. Soc. Jpn. 44, 2078 (1971). 8. Moroi, Y., Motomura, K., and Matuura, R., Bull. Chem. Soc. Jpn. 45, 2697 (1972). 9. Moroi, Y., Sato, K., and Matuura, R., J. Phys. Chem. 86, 2463 (1982).

Journal of Colloidand InterfaceScience,Vol. 101,No. 1, September1984

10. Moroi, Y., Norma, H., and Matuura, R., J. Phys. Chem. 87, 872 (1983). I 1. Moroi, Y., Ikeda, N., Sugii, R., and Matuura, R., unpublished work. 12. Moroi, Y., Nishikido, N., Uehara, H., and Matuura, R., J. Colloid Interface Sci. 50, 254 (1975). 13. Phillips, J. N., Trans. Faraday Soc. 51, 561 (1955). 14. Mukerjee, P., and Mysels, K. J., Natl. Stand. Ref Data Ser. (U. S. Natl. Bur. Stand.) No. 36 (1971). 15. Satake, I., Tahara, T., and Matuura, R., J. Colloid Interface Sci. 42, 319 (1969). 16. Sasaki, T., Hattod, M., Sasaki, J., and Nukina, K., Bull. Chem. Soc. Jpn. 48, 1397 (1975). 17. Saito, M., Moroi, Y., and Matuura, R., J. Colloid Interface Sci. 88, 578 (1982). YOSHIKIYO MOROI2 NORIHIRO IKEDA RYOHEI MATUURA Department of Chem&try Faculty of Science Kyushu University 33 Higashi-ku, Fukuoka 812 Japan Received December 20, 1983

2 To whom all correspondence should be addressed.