AYAOKITAHARA AND KIJIRO KON-NO
3394
presumably 3-4 kcal mole-1,12*28 and:is in good agreement with the data of Baxendale.12
Conclusions There can be little doubt that the very fast reactions of Co(II1) complexes with eaQ- a,re diffusion controlled regardless of the source of esq-. The three oxidants employed in the low acidity aspect of this study exhibit a 104-fold variation in their specific rates with Ru(NH3)2+,I3but exhibit nearly identical reactivities
toward eSq-. This is just as expected if the latter reactions are diffusion controlled.8-10 From our study of high acidity solutions, we conclude that the (1.H) geminate pair is a relatively poor reducing agent for cobalt(II1)-ammine complexes. Acknowledgment. We wish to express our appreciation to Professor G. Stein for his many helpful comments and stimulating discussions. (28) J. K. Thomas, S. Gordon, and E. J. Hart, J . Phys. Chem., 68, 1524 (1964).
Mechanism of Solubilization of Water in Nonpolar Solutions of Oil-Soluble Surfactants: Effect of Electrolytes
by Ayao Kitahara and Kijiro Kon-no Tokyo College of Science, Kagurazaka, Shinjiku-ku, Tokyo, Japan
(Received January 17,1966)
Solubilization of aqueous electrolyte solutions into nonpolar solutions of oil-soluble surfactants was studied in order to clarify the mechanism of solubilization of water. The behavior of solubilization by nonionic surfactants was compared with that of an ionic one with relation to the difference in the micellar structure. Solubilization by the ionic surfactant decreased suddenly by the minor presence of every kind of electrolyte. Solubilization by nonionic surfactants was not influenced as much as that by ionic ones and depended on the kind of electrolyte, the order of decrease of solubilization being salts = base > acids. The mechanism of solubilization was interpreted by reference to the change of the behavior of an aqueous surfactant solution by the presence of electrolytes. The effect of salts on solubilization by nonionic surfactants was explained by the salting-out mechanism and the lyotropic effect of ions was reflected in the difference in the effect of different salts. The property of an acid as a proton donor was applied to the explanation of the effect.
Introduction Recent investigations have supported the existence of micelles of oil-soluble surfactants in nonpolar solvents.' It has been known that a micellar solution is
The Journal of Physical Chemistry
is necessary in order to clarify the micellar properties of oil-soluble surfactants. The study of solubilization of various aqueous elec-
3395
SOLUBILIZATION OF AQUEOUS ELECTROLYTE SOLUTIONS
trolytes by surfactant solutions in nonpolar solvents seems to be interesting and furnishes useful information about the micellar properties. However, none has yet appeared except the work of Aebi and Wiebush on solubilization of NaCl in wet nonpolar solutions of Aerosol OT.' It is hoped that the study will be carried out for both nonionic and ionic surfactants because of the difference in their micellar structure which has already been ~uggested.~ I n the present paper, solubilization of various aqueous electrolyte solutions, including salts, acids, and bases, is measured in nonionic and ionic surfactant solutions in order to clarify the mechanism of solubilization of water. This mechanism relates to the difference in the micellar structure.
continuous occurrence of vague cloudiness or tiny precipitates was sharper than the usual method of optical density measurement with a nephelometer. The optical density method was preferable for the benzene solutions because of the continuous occurrence of cloudiness. The experimental temperature was 25 f 1O .
r
a
/
Experimental Section Materials. Polyoxyethylene nonylphenol ethers and Aerosol OT (sodium dioctyl sulfosuccinate) were furnished from the laboratory of the Nippon Surfactant Co. The average numbers of ethylene oxide of three samples of nonionic surfactants used here were 7.5, 4.5, and 2, and designated as NP-7.5, NP-4.5, and NP-2, respectively. Purification of the NP series was carried out by a countercurrent distribution method6 using a solvent system of water-1-butanol for NP-7.5 and NP-4.5 and water-ethyl ether for NP-2. Traces of water, 1-butanol, and ethyl ether were not detected by gas chromatography for purified samples. Aerosol OT was dissolved into methanol and the precipitate produced was excluded with a fine glass filter followed by drying. Cyclohexane, benzene, and n-heptane used as solvents were purified by the usual method from GR grade reagents. It was ascertained by the Karl Fischer method that the water content in solvents was below 50 ppm. Each GR grade'reagent of sodium chloride, magnesium chloride, aluminum chloride, sodium sulfate, sodium hydroxide, perchloric acid, and hydrochloric acid was dissolved in water, run through an ion-exchange medium, followed by distillation with potassium permanganate to give solubilized electrolyte solutions. Method. The volume of a solubilized aqueous electrolyte solution was titrated with a microburet having a minimum scale of 0.005 ml. The limiting point of solubilization was determined by observing cloudiness by comparing a wet solution with an original dry solution through the light of a white fluorescent lamp for the measurement of solubilization in cyclohexane and n-heptane solutions. The equilibrium of the solubilization process was ascertained by prolonged observtL tion. The above comparative observation of the dis-
0.00 0.00
0.05
0.10
0.15
Equivalent concentration of surfactants.
Figure 1. The maximum amount of water and aqueous solutions solubilized us. molarity of an oil-soluble surfactant (NP-7.5) in cyclohexane: 0,water; 0, aqueous NaCl solution (0.3 M ) ; (3, aqueous NaOK solution (0.1 M); x, aqueous Na2SOd solution (0.05 M).
I
0.0 I I I I I I 0.0 0.2 0.4 0.6 Equivalent concentration of acids and a base, equiv/l.
Figure 2. The maximum solubilization of aqueous electrolyte solutions by cyclohexane solutions of surfactants us. the equivalent concentration of the electrolyte. Solubilization of aqueous solutions of HClOI (e),HCI (V), and NaOH (a) by NP-7.5 solutions. (2) S. R. Palit and V. Venkateswwarlu, Proc. Roy. Xoc. (London), A208. 642 (1951); M. B. Mathews and E. Hirschhom, J . Colloid Sci.,8,86(1953) ; A. Kitahara, BUU. C h m . SOC. Japan, 28,234 (1955). (3) A. Kitahara, J . Phys. Chem., 69,2788 (1965). (4) C. M. Aebi and J. R. Wiebush, J . Colloid Sci., 14, 161 (1959). (5) K. Nagaae and Y. Sakaguchi, Kogyo KagolMl Zasshi, 64, 635 (1961).
Volume 70,Number I 1
November 1966
3396
AYAOKITAHARA AND KIJIROKON-NO
NP-4.5
Aerosol OT HClOa ' HCl NaOH AlCls MgCL NaCl Benzene
0.05
B
NP-7.5
Results and Discussion The maximum solubilization of three aqueous electrolyte solutions is depicted in Figure 1 as a function of the concentration of the nonionic surfactant NP-7.5 in cyclohexane. The amount solubilized is seen to increase with increasing surfactant concentration in each case. Therefore, further experiments were carried out with a fixed concentration (0.15 M ) of each surfactant studied. The presence of electrolytes always decreased solubilization of aqueous solutions at the concentration of each surfactant studied. The effect was greater at higher electrolyte concentration except the behavior with HCIOl in benzene solutions of nonionic surfactants. The typical plots of maximum solubilization vs. the equivalent concentration of electrolytes in aqueous solutions solubilized are presented in Figures 2-4. In these figures, the solubilization of aqueous solutions of acids and a base by cyclohexane solutions of a nonThe Journd of Physical Chemistry
...
ionic surfactant, XP-7.5, that of salt solutions by the same surfactant solutions, and that of acid, base, and salt solutions by Aerosol OT in cyclohexane are shown, respectively. Solubilization curves were classified as types of A, B, and C. Type A is the case of solubilization of acid solutions by NP-7.5 solutions shown in Figure 2 in which curves decrease to level off at higher concentre tions. Type B is the case of a base in Figure 2, NaSOc in Figure 3, or Figure 4 in which curves decrease markedly and linearly to intercept the concentration axis at extrapolation. Type C is the intermediate case between A and B as seen in solubilization of salts (chlorides) solutions in Figure 3. The results of the solubilization are arranged in Table I taking the three types into consideration. The term solubility ratio in the table was used for A and C. This term is the ratio of the level value (for A) or the value at the equivalent concentration of 0.7
3397
SOLUBILIZATION OF AQUEOUS ELECTROLYTE SOLUTIONS
Solvent
Benzene
Surfactant
NP-4.5
NP-2
Aerosol OT n-Heptane
NP-4.5
Solubility ratio
Electrolyte
NonHClOi HC1 NaOH AlCls MgClz NaCl Na~S04 NonHClO4 HC1 NaOH AlCls MgCln NaCl NanSO, NonAll NonHC104 HCl NaOH NaCl
NP-2
NonHClO4 HC1 NaOH NaCl
(for C) of the maximum solubilization to that of pure water. The intercept used for B is the value extrapolated to the electrolyte concentration at which zero solubilization occurs. Comparison of the results for nonionic surfactants with the phenomena in solutions of Aersol OT shows that all of electrolytes studied decrease water solubilization by the anionic surfactant more drastically than they do solubilization by nonionics, all of the solubilization by Aersol OT showing type B in benzene as well as in Figure 4. The limiting amount of solubilization by nonionic surfactants depended largely on the nature of electrolytes as seen in Figures 2 and 3 or Table I. The differences seen for nonionic surfactants were marked between acids (type A) and a salt (NazSO4) or a base (type B) in cyclohexane and between acids (type A or C) and salts or a base (type B) in benzene or n-heptane. The difference in solubilization by anionic and nonionic surfactants is considered to be the result of the
(0.053)" 1.0 0.66
Intercept
...
... ...
Type
... A
...
0.15
C B
...
0.10
B
...
0.05
(0.03)" 1.0 0.5
...
... ...
B
...
...
0.10
A C B
...
0.10
B
...
0.05
(0.505)"
...
(0,035)" 0.7 0.57
...
0.05 .
.
I
... ...
B
... B
,,.
...
0.10
A C B
...
0.05
B
(0.02)" 0.75 0.65
...
...
...
...
A
C
...
0.10
B
...
0.05
B
difference in the structure of the micelle formed, especially in the loci of the micelle at which solubilization of water occurs. The salt effect on the solubilization of water by nonionic surfactants in cyclohexane appears in Figure 3 and Table I. That is, there are the minor differences among solubilization of NaCl, MgC12,and AlC1, aqueous solutions and the marked difference between that of NaCl and Na2S04 solutions. These differences correspond to those in lyotropic numbers among cations and between anions. This salt effect may be explained by the salting-out action for hydrogen bonding between the polar part (oxygen of either type) in the micelle and the water molecule solubilized. This salting-out mechanism was used to interpret changes in the cmc and the cloud point in aqueous solutions of nonionic surfactants by Schick6 and Maclay,' by (6) M.
J. Schick, J. Collaid Sci., 17, 801 (1962). (7) W. N. Maclay, ibid., 11, 272 (1956).
Volume 70.Number 11
November 1966
AYAOKITAHARA AND KIJIRO KON-NO
3398
. 0.3
3 5:
a
$
f 0.27
8
2
2 .r! 0.2
=e
1 n
3*
g
8
8, 0.02
I I
0.0 0.0
: I
!
I
I
I
I
I
I
0.6 Equivalent concentration of salts, equiv/l. 0.2
0.4
Figure 3. Solubilization (as in Figure 2) of aqueous solutions of AICla (a), MgClz (A), NaCl ( O ) , and NazSOc ( X ) by NP-7.5 solutions.
whom the similar phenomena relating to the salts mentioned above were observed. The change of solubilization with the kind of salts was hardly observed in benzene and n-heptane because of the minor solubilization. In nonionic surfactant solutions in all of the solvents studied, the presence of acids decreases solubilization much less than that of salts or a base does. This effect of acids is considered to be caused by the property of acids as proton donors suggested by Hsiao, et a1.8 Schick showed that the cloud point and the cmc of nonionic surfactants in aqueous solutions increase as HC1 's added' whereas they decrease by adding NaOH or Salk6 This behavior is in accord
The JOuTnd
of Physical Chemistry
0.00 0.00 0.05 0.10 Equivalent concentration of electrolytes, equiv/l.
Figure 4. Solubilization (as in Figure 2) of all of electrolyte aqueous solutions ( 0 )by Aerosol OT solutions.
with our observation that the uptake of water is scarcely affected by the presence of acids when compared with the effect of salts and a base and that the uptake is affected less by the stronger acid HCIOI than by the weaker HC1. In fact, an increase in solubilization in the presence of HC1 was actually observed in more concentrated nonionic surfactant solutions (0.5 mole/l.). Acknowledgment. The authors wish to thank the Nippon Surfactant Co. for furnishing the samples and Mr. N. Okima for taking part in the measurement of solubilization. (8) L. Hsirto, H.N. Dunning, and P. B. Lorenz, J . Phys. Chon., 60, 657 (1956).
