Viscosities of Anionic-Nonionic Mixed Surfactant Systems HIROTAKA
M A S A H I K O A B E , * ' t A N D K E I Z O OGINO*'q"
*Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278, Japan; and t Institute of Colloicl and Interface Science, Science University of Tokyo, 1-3, Shinjuku-ku, Tokyo 162, Japan Received September 12, 1988 J
The viscous property of anionic-nonionic mixed surfactant systems in aqueous solutions is described. The systems studied are sodium dodecyl sulfate (SDS)-alkyl poly(oxyethylene) ethers (CmPOE,; m = 12, 14, 16, and 18, n = 10, 20, 30, and 40). In the single system, the relative viscosity of the nonionic surfactant is larger than that of SDS and increases with increasing number of ethylene oxide. In mixed systems the relative viscosity shows maximum at the mixed molar ratio of SDS around 0.3, except for the SDS-Cj6POE4osystem. The mixed systems having longer alkyl and/or polyoxyethylenechain lengths in the nonionic surfactant have a larger relative viscosity at any mixed ratio. The system in which the mixed micelle forms easily has a positive deviation from the ideal value. The relative viscosities of the surfactants in 0.1 M NaC1 solution decrease linearly with an increase in the mixed molar ratio of SDS. It is considered that the relative viscositiesin the mixed solutions show a maximum due to the electroviscous effect of a mixed solution being larger than that of a single solution. © 1990AcademicPress,Inc. INTRODUCTION
Some papers ( 15, 16) have been published on the pure surfactant in aqueous solution with or without electrolyte. Moreover, the aggregation n u m b e r has been calculated by the use o f intrinsic viscosity (17). Regardless o f its great importance, few studies on the viscosity in the mixed solution have been carried out. A n analysis o f the mixing effect on the viscosity in the mixed surfactant solution has not been carried out. In this paper, we report the relative viscosity o f mixed surfactant systems: sodium dodecyl sulfate-alkyl p o l y ( o x y e t h y l e n e ) ethers (CmPOEn; CmH2m+IO(CzH40)nH). We discuss the mixing effect o f two surfactants and the electroviscous effect on the relative viscosity with or without electrolyte.
T h e f o r m a t i o n o f aggregate structures due to the mixture o f surface active agents is o f great theoretical and industrial interest. Since surfactants used in applications such as enhanced oil recovery and detergency are rarely single systems, understanding how surfactants interact in the mixed micelle is essential for the m a n y industrial applications ofsurfactants. It can be speculated that, in solutions containing mixtures o f surfactants, the tendency to f o r m aggregated structures would be substantially different f r o m that in solutions having only pure surfactants. In fact, some phen o m e n a which are not expected to take place in the single system occur in aqueous solutions containing mixtures o f surfactants ( 1 - 4 ) . M a n y recent papers have, therefore, been published on the mixed surfactant systems by m e a n s o f N M R ( 5, 6 ), ESR (7), fluorescence probing m e t h o d (8, 9), etc. M a n y papers on the t h e r m o d y n a m i c s o f mixed micelle form a t i o n have also been reported ( 1 0 - 1 4 ) . Viscosity is one o f the i m p o r t a n t physicochemical properties o f surfactant solutions.
EXPERIMENTAL Materials Anionic surfactant. SDS (C12H25OSO3Na) was purchased from T o k y o Kasei K o g y o Co., Ltd., Tokyo, Japan, m o r e than 99.5% pure. It was extracted with ether and recrystallized from ethanol. 69 0021-9797/90 $3.00
Journal of Colloid and Interface Science, Vol. 138, No. 1, August 1990
Copyright © 1990 by Academic Press, Inc. All rights of reproduction in any form reserved.
U C H I Y A M A , ABE, A N D O G I N O
Nonionic surfactant. CmPOEn (m = 12, 14, 16, and 18 (at n = 20); n = 10, 20, 30, and 40 (at m = 16 )) were supplied by Nihon Surfactant Industries Co., Ltd., Tokyo. These have a narrow molecular weight distribution. Their purities were ascertained by surface tension measurements; no surface tension value of any surfactant used in this study showed a minimum with increasing concentration. Inorganic electrolyte. Sodium chloride (NaC1) was purchased from Wako Pure Chemical Industries, Co., Ltd. Water used in this experiment was twice distilled and was deionized with an ion-exchange instrument (NANO Pure D- 1791 of Barnstead Co., Ltd., Boston, MA.): its resistance was about 18.0 M~2. cm and its pH was 6.7. Methods Preparation of mixed surfactant solutions.
SDS-C12POE20 SDS-C14POE20 ; SDS'C16POE20 ; SDS'C18POE20
© ; 1.20
Mole fraction of SDS
FIG. 1. Relationship between the relative viscosity and the mole fraction of SDS in the SDS-CmPOE20 mixed surfactant systems at 30°C. Total concentration is 1.0 × 10-2 M.
Into several 100-ml beakers, 25-ml portions of a given concentration of an SDS solution were placed, followed by the addition of a given concentration of C4POEn solution. The solutions were stirred for 2 h in a thermostated bath at 30°C in order to establish equilibria. Determination of relative viscosities. The relative viscosities of aqueous solutions of the surfactant systems were measured at 30 + 0.1 °C using a Ubbelohde-type viscometer (Kusano Scientific Co., Tokyo, Japan). The flow time of water, surfactant solution, or NaC1 solution was used in calculating the relative viscosity. The experimental errors of the relative viscosity were within 0.001.
At any mixed ratio, the relative viscosities of the mixed system increased with an increase in the alkyl chain length in CmPOE20. Figure 2 exhibits the relative viscosity against the mole fraction of SDS in the mixed system, SDS-C16POEn, which has a different polyoxyethylene chain length in C~6POEn. Both the C~6POEn single and the mixed surfactant systems increased with increasing polyoxyethylene chain lengths in C16POE,. Also, the relative viscosities of mixed surfactant solutions had a maximum around a mixed molar ratio of 0.3 except for the SDSC~6POE40 system, in which the relative viscosity was decreased with increasing mole fraction of SDS.
Figure 1 shows the relationship between the relative viscosity and the mole fraction of SDS in the SDS-CmPOE20 mixed surfactant system at 30°C. The relative viscosity of CmPOE20 was larger than that of SDS. In the CmPOE20 pure system, the effect of differences in alkyl chain length on the relative viscosity has not been recognized. The relative viscosities were increased with increasing mole fraction of SDS, then had a maximum in the vicinity of 0.3. Journal of Colloid and Interface Science, Vol. 138,No. 1, August1990
Effect of Chain Length on the Relative Viscosities in the Nonionic Single System As reported previously ( 19-21 ), as the alkyl chain length increased, the cmcs of nonionic surfactant decreased (C12POE20, 1.5 × 10 -4 M; C14POE20, 2.3 × 10 -5 M; C16POE20, 1.7 × 10 -5 M; CIsPOE2o, 1.0 X 10 -5 M). The cmcs of CmPOEn decreased with increasing polyoxyethylene chain length (Cx6POE10, 2.5
1.30 ; S'DS-C16POElo ; SDS-C16POE20 ~ ; SDS-C16POE30 "~; SDS'C~6POE4° ©
"~>, 1 . 2 0
1 ), and the relative viscosity is increased with an increase in the number of ethylene oxide (Fig. 2).
Relative Viscosities of the Mixed Surfactant System
As can be seen from Figs. 1 and 2, the relative viscosities of mixed surfactant systems a~ showed maximum at mixed molar ratio about 0.3 except that in the SDS-C16POE40 mixed n- 1.10 surfactant system. The relative viscosities of the mixed systems are larger for a nonionic surfactant with longer alkyl a n d / o r polyoxyethylene chain length than for those with short 1.00 I r chain length at any mixed molar ratio, as 0 0.5 1.0 shown in Figs. 1 and 2. Mole fraction of SDS We assume that, when the two surfactants FIG. 2. Relative viscosity against the m o l e fraction o f are mixed, no interaction between them affects SDS in SDS-CI6POEnmixed surfactant systemsat 30°C. Total concentration is 1.0 × l 0 -2 M. the relative viscosity. The ideal line can be drawn linearly between the relative viscosities × 10 -5 M; CI6POE20 , 1.7 M 10 -5 M; of two single surfactants (dashed line in the figures). Then, the deviations of the relative Cl6POE30, 1.2 × 10 -5 M; CI6POE40, 7.0 × 10 .6 M). All relative viscosities of pure viscosities in the mixed surfactant system from nonionic surfactant increased from around the ideal line are larger for a nonionic surfac10-4 with an increase in the concentration of tant with longer alkyl a n d / o r shorter polyCmPOE,. The concentration at which the rel- oxyethylene chain lengths than for one with ative viscosities increased were almost the shorter a n d / o r longer chain lengths. We have recently reported ( 19-21, 23, 24) same, which is why the effect of m o n o m e r on that the mixed micelle is formed more easily the relative viscosities could be negligible due by a nonionic surfactant including long alkyl to low critical micelle concentration. ( a n d / o r short polyoxyethylene) chains than As can be seen from Fig. 1, the relative visby one having short alkyl ( a n d / o r long polycosities of single nonionic systems which have oxyethylene) chains. In other words, the sysdifferent alkyl chain lengths were almost contem in which the mixed micelle forms easily stant. The relative viscosity of a single system has a larger deviation from the ideal line. We having a different polyoxyethylene chain can conclude that the mixed micelle formation length is, however, increased with an increase makes the viscosity larger. The maximum in in the number of ethylene oxide, as shown in the relative viscosity, therefore, does not seem Fig. 1. Most water molecules were mechanito be shown in the SDS-C16POE40 mixed syscally trapped in the polyoxyethylene chain in tems. the CmPOE20 molecule, from 5.2 to 10.5 water Examination of the Electroviscous Effect molecules per ethylene oxide unit, as the hyon the Relative Viscosity drophilic chain length (22) increased. We can consider that the viscosities are dependent on Next, we must consider why the relative the hydration o f water molecules around the viscosities showed maximums in the figure. hydrophilic portion of CmPOEn in the pure The critical micelle concentrations of the nonionic system. Therefore, no difference ap- SDS-CIsPOE20 mixed surfactant system are peared in the system which had a constant listed in Table 1. The cmcs of the mixed surpolyoxyethylene chain length in CmPOEn (Fig. faetant solution are much lower than that of O
Journal of Colloid and Interface Science, Vol. 138, No. 1, August 1990
UCHIYAMA, ABE, AND O G I N O
SDS and almost the same as that of pure nonionic surfactant at any mixed molar ratio. Therefore the number of micelles in the mixed surfactant solution is equal to that in the nonionic single surfactant solution. The descending slopes in the relative viscosities at more than mole fractions of SDS 0.3 may be caused not so much by a decreasing percentage of the surfactant mixture existing in the form of micelles but rather by the mixed micelles turning gradually more anionic or by mixed micelle properties. According to Ito's work, the intrinsic viscosity by Einstein's equation should be 2.5 (25). However the intrinsic viscosity of the usual particle is larger than 2.5 since a colloidal particle has a hydrated layer around it and has an electrical surface charge. That is, there are two effects on the viscosity: one is the hydration of colloidal particle surface, the other is electroviscous effect. We have measured the intrinsic viscosity of the mixed surfactant solution (mixed molar ratio 1:1 ) with electrolyte, which suppresses the electroviscous effect. The intrinsic viscosities of the mixed surfactant solution were still larger than the calculated values based on an anhydrous weight due to hydration of the micelle (26). Alexander and Johnson (27) have previously reported that the electroviscous effect can be neglected in the presence of an electrolyte above 0.1 M. Parker and Wasik (28) have also mentioned that the electroviscous effect decreased noticeably due to the addition of 0.1 M electrolyte. The relative viscosity of the
TABLE I The Critical Micelle Concentration of SDS-CIsPOE20 Mixed Surfactant Systems Xsos 0 0.2 0.4 0.6 0.8 1.0
cmc (M) 1.0 1.1 1.2 1.5 2.3 8.0
× × X × × ×
10-5 10-5 10 -5 10-s 10-5 10-3
Journal of Colloid and Interface Science, Vol. 138,No. 1, August1990
SDS-C 1sPOE20 System 1.20 (3 ; NaCI free soln. • ; 0.1 N NaCI soln.
Mole fraction of SDS FIG. 3. Relationship between the relative viscosity and the mole fraction of SDS with or without 0.1 M NaC1 in SDS-CI8POE20 mixed system at 30°C.
SDS-C18POE20 mixed system was measured in the presence of 0.1 M NaC1. The mixed solution, SDS-ClaPOE20, is the system in which the mixed micelle formed easily. Figure 3 shows the relative viscosities with or without 0.1 M NaC1 against the mole fraction of SDS in the SDS-ClaPOE2o mixed surfactant system. The relative viscosities in the presence of electrolyte decreased linearly with increasing mixed molar ratio of SDS. The relative viscosity of single nonionic surfactant was not changed by addition of electrolyte. In the SDS single solution, the relative viscosity was decreased slightly by adding NaC1. The electroviscous effect is decreased since the electrical charge of the micelle surface decreased. In the mixed surfactant systems, the electroviscous effect decreased appreciably due to the addition of NaC1. The relative viscosities are almost the same as the ideal line mentioned above. It is noted that the degree of ionic dissociation of the micelle decreased due to the addition of electrolyte. As a result, the repulsive force between head groups in the SDS pure micelle or the mixed micelle decreased: therefore, the cmc is decreased and the aggregation number is increased.
MIXED SURFACTANT SYSTEMS
The electroviscous effect on the viscosity decreased with the decrease in the surface charge of the micelle. As shown in Fig. 3, the relative viscosity was decreased remarkably by adding salt in the mixed surfactant systems, due to the reduction of electroviscous effect, even though the aggregation number of the mixed micelle increased. That is, we can consider that the deviations of relative viscosities from the ideal line are due to the electroviscous effect in the mixed micelle. Furthermore, it seems that the electroviscous effect is large in the system in which the mixed micelle forms easily. We have previously studied the degree of ionic dissociation of micelle (a) in SDSCmPOEn mixed surfactant solutions ( 19-21 ). These values are listed as follows: SDS, 0.21; SDS-C16POElo, 0.74; SDS-C16POE20, 0.64; SDS-CI6POE30, 0.54; SDS-C16POE40, 0.49; SDS-ClzPOE20, 0.56; SDS-C14POE2o, 0.55; SDS-C18POE20, 0.72. When nonionic surfactants are added into the SDS solution, the degree of ionic dissociation of the mixed micelle increased. Moreover, its value increased with an increase in the alkyl chain lengths and/or a decrease in the number of oxyethylene groups in the nonionic surfactant. Since the degree of ionic dissociation of the mixed micelle and the radius of the mixed micelle with the electric double layer increase as the alkyl chain lengths increase (and/or the number of oxyethylene groups decreases) in the nonionic surfactant, the electroviscous effect of mixed surfactant solutions becomes larger. Consequently, the relative viscosities in the mixed solutions showed a maximum due to the electroviscous effect of a mixed solution being larger than that of a single solution. The viscosity of the mixed surfactant solution can be reduced by the addition of salt for industrial applications.
The authors are greatly indebted to Mr. Hayami for his careful experimental work.
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