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Mixed adsorption of cationic and anionic surfactants from aqueous solution on silica gel
Colloids and Surfaces, 36 (1989) 353-358 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
353
Mixed Adsorption of Cationic and Anionic Surfactants from Aqueous Solution on Silica Gel ZHI HUANG, ZHULIN YAN and T I R E N GU
Laboratory of Colloid Chemistry, Department of Chemistry and Institute of Physical Chemistry, Peking University, Beijing (China) (Received 2 August 1988; accepted 6 January 1989)
ABSTRACT The adsorption of dodecyltrimethylammonium bromide (DTAB), dodecylpyridinium bromide (DPB), sodium dodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS) from their single aqueous solutions and DTAB-SDBS and DPB-SDS binary mixed solutions on silica gel at 25°C has been investigated. The results show that the individual cationic surfactants can be adsorbed strongly onto the silica gel, but no significant adsorption of anionic surfactants can be detected. However, in the mixed systems, the adsorption amounts of both the cationic and anionic surfactant ions are enhanced, and the excess adsorption of cationic surface-active ions is exactly equal to the adsorption of anionic surface-active ions. Thus, it is reasonable to suggest that the excess adsorption of cationic and anionic surface-active ions is in the form of ion-pairs.
INTRODUCTION
The adsorption of surfactants at solid/liquid interfaces is fundamentally important for many technical applications. Although the adsorption of single surfactants at solid/liquid interfaces has been investigated intensively, there have only been a few studies of mixed systems, in spite of their great importance [ 1-3 ]. In recent years, some investigations on the adsorption from mixed solutions of surfactants on solids have been made in our laboratory [4-8]. To our knowledge, however, only two papers (Miiller and Krempl [9] and Schwuger [10] ) on the adsorption of anionic-cationic mixed surfactant systems on solids have been published. Both of them showed that the adsorption of an anionic or a cationic surfactant was enhanced by the presence of an oppositely charged surfactant. However, only one of the surfactants was determined so it was difficult to explain the results convincingly. In the present paper, the adsorption of both components of mixed surfactant aqueous solutions ( D T A B - S D B S ) on silica gel have been determined and the results have been explained reasonably.
0166-6622/89/$03.50
© 1989 Elsevier Science Publishers B.V.
354 EXPERIMENTAL
Materials The silica gel used was prepared by a method described earlier [11-13], it was dried at 200 ° C for 4 h before use. The B E T surface area (nitrogen adsorption) was 370 m 2 g- 1, the average pore radius was 42 ~, and the pore volume of the silica gel was 0.89 ml g-1. The dodecyltrimethylammonium bromide (DTAB), dodecylpyridinium bromide (DPB), sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) were supplied by Beijing Chemical Co. DTAB, DPB and CTAB were purified by recrystallization from an acetone-ethanol solvent, several times, and SDS was recrystallized from ethanol. They showed no minimum in the surface tension curves and had critical micellization concentrations (c.m.c.) of 1.6-10 -2, 9.5.10 -3, 9.6" 10 -4 and 8.5.10 -3 mol dm -3 for DTAB, DPB, CTAB and SDS at 25 ° C, respectively. Sodium dodecylbenzenesulfonate (SDBS) was supplied by BDH Ltd and used without further treatment. It also showed no m i n i m u m in the surface tension curve and had a c.m.c, of 1.6.10 -3 mol dm -3 at 25°C. The water used throughout the experiments was prepared by distillation from an alkaline permanganate solution after being passed through an ionexchange column. The pH of the distilled water was 5.6. Methods The adsorptions were carried out at a constant feed molar ratio of cationic/ anionic surfactants or at a constant feed concentration of one component. The adsorption a m o u n t was obtained by measuring the concentration of surfaceactive ions in the solution before and after equilibrium with silica gel. 25 ml portions of solution were placed in contact with 0.1 g silica gel in vials, the vials were shaken for 6 h in an air-thermostat at 25.0 + 0.5 ° C. The concentration of DBS and DP + were determined by UV absorption at 232 and 257 nm, respectively, and those of DTA + and D S - were determined by the two-phase titration method [14,15] with bromophenol blue as the indicator, chloroform as the organic phase and SDS and CTAB as the titrants respectively. In DTABSDBS and D P B - S D S mixed systems, the concentrations of D B S - and DP + were determined by UV absorption. For the D T A B - S D B S system, the difference of concentration of cationic and anionic surfactant was determined by the two-phase titration method, then the concentration of DTA + was calculated from the concentration of D B S - and the concentration difference of cationic and anionic surfactants. For the D P B - S D S mixed system, only the concentration of DP + was measured and the accurate equilibrium concentration of D S - cannot be obtained by calculation.
355
The adsorption experiments were repeated at least two or three times. In general, the results were reproducible within 4%. It should be pointed out that these cationic-anionic surfactant mixed systems (the molar ratios of cationic/anionic surfactants were carefully chosen) are all clear. The UV-absorption density of the solutions before and after adsorption is reduced quantitatively with the dilution of the solutions. Thus, it is safe to say that the mixed systems used in this work are true solutions. RESULTS AND DISCUSSION
Adsorption from single component solutions Figure 1 shows the adsorption isotherms of DTA +, DP +, D S - and D B S from their single aqueous solutions on silica gel. It can be seen from Fig. 1 that the anionic surfactant ions are not adsorbed on silica gel. It is not surprising because the silica gel is charged negatively in the present experimental conditions (its isoelectric point is about p H 2 [ 17 ] ), and thus, it is unlikely to adsorb the negatively charged anionic surfactant ions. On the contrary, the cationic surfactant ions can be adsorbed strongly by the silica gel. We [16,17] have investigated the adsorption of cationic surfactants from aqueous solutions on silica gel carefully and have proposed an adsorption model. As compared with the results in the previous papers [16,17], it is obvious that the adsorption isotherms of cationic surfactants in the concentration ranges used in the present work are located in region II, the first adsorption plateau. Based on the model we suggested previously [16,17], the adsorption of cationic surfactants on silica gel results from electrostatic interactions in the very low concentration range and both electrostatic attraction and specific adsorption in the first plateau region.
E 4
%
o...o ~ . . _ _ , ,
_o-,L,,-o-- ~
"-----w~-
~2q
0
~ 0
~ ~
~
t 1
~.
C/10 -3 mol/dm
3
Fig. 1. Adsorption isotherms from single solutions on silica gel: (@) DTA+; ( X ) DP÷; ( • ) DBS-; ( 1 ) D S - .
356
Adsorption from mixed aqueous solutions on silica gel The adsorption isotherms of DTA ÷ and the corresponding adsorption of D B S - from D T A B - S D B S mixed solutions on silica gel are shown in Fig. 2, with a constant feed concentration of S D B S and Fig. 3 with a constant feed molar ratio of D T A B / S D B S . It can be seen from Figs 2 and 3 that the adsorption of DTA + is increased in the mixed systems and an apparent adsorption of D B S - occurs. The result is similar to that obtained by MLiller and Krempl [9] and Schwuger [10]. It is interesting to note that the excess adsorption amounts of both DTA + and D B S - in the mixed systems are equal to each other. In other words, the adsorption isotherms of DTA ÷ will coincide with that from the single component solution if the adsorption amount of D B S - is subtracted from the adsorption amount o f D T A + in the mixed systems. It should be noted that the adsorption of D B S - is a constant in the system with a constant feed concentration of SDBS (Fig. 2 ) and increases with its concentration in the system with a constant feed molar ratio of D T A B / S D B S (Fig. 3). This suggests that the adsorption of D B S - in the mixed system is not influenced by the concentration o f D T A + and mainly depends upon its own concentration if the feed molar ratio of D T A B / S D B S is much higher than 1. Figure 4 shows 6
6
~4
A__
%
__~
tl
.-* ......
~Ji~'" ~
A
~~
~_...O.&O/__O4___
....
..... • .
.
.
.
O
&-
o
0
~,
•
A
,
1 C/tO~3 mol/dm 3
t.
2
o
(
~---~-'~'
~/
1 C/lO-3mol/dm 3
Fig. 2. (left) Adsorption of DTA + and D B S - on silica gel from D T A B - S D B S mixed solutions with a constant feed concentration of SDBS. SDBS concentration of 1-10 - 4 tool dm-3: ( O ) adsorption of DTA +; (~) adsorption of D B S - ; ( • ) adsorption of DTA ÷ minus the adsorption o f D B S - . SDBS concentration of 0.4" 10 -4 mol dm-~: ( A ) adsorption of DTA+; ( A ) adsorption of D B S - ; ( • ) adsorption of D TA + minus the adsorption of DB S - . (The dotted line represents the adsorption isotherm of DTA + from single solution. ) Fig. 3. (right) Adsorption of DTA + and D B S - on silica gel from D T A B - S D B S mixed solutions with a constant feed molar ratio of D T A B - S D B S . Molar ratio of 10:1 D T A B - S D B S : ( A ) adsorption of DTA ÷; ( z~ ) adsorption of D B S - ; ( • ) adsorption of DTA + minus the adsorption of D B S - . Molar ratio of 20:1 D T A B - S D B S : (C)) adsorption of DTA+; (~) adsorption of D B S - ; ( • ) adsorption o f D T A + minus the adsorption of D B S - . Molar ratio of 1:10 D T A B - S D B S : ( [] ) adsorption of D B S - from mixed solution. (The dotted line represents the adsorption isotherm of DTA + from single solution. )
357 6
~4
E
-¢
°o
I
C/10 -3 moI/dm 3
Fig. 4. Adsorption of DP + from mixed DPB-SDS solution on silica gel at a feed molar ratio of 52:1 DPB-SDS. (The dotted line represents the adsorptionisothermof DP + fromsinglesolution.) the adsorption isotherm of DP + from D P B - S D S mixed aqueous solution on silica gel. The adsorption of DP + is also enhanced in a similar way to the systems mentioned above. Cationic surface-active ions will strongly interact with anionic surface-active ions through electrostatic attraction [ 18 ], when they are mixed in aqueous solution, and ion-pairs will be formed between the oppositely charged surfact a n t s [ 19 ]. This leads to a great increase in their surface activity [ 18 ] and will promote their adsorption at interfaces. Based on the adsorption model of cationic surfactant from aqueous solution on silica gel [16,17], the negatively charged positions of the silica gel surface (SiO-) can adsorb cationic surfact a n t ions and may not adsorb anionic surfactant ions. However, the anionic surface-active ions may be coadsorbed specifically with cationic surface-active ions as ion-pairs at the non-charged sites of silica gel through van der Waals interactions. This explains why the excess adsorption a m o u n t of the cationic surfactant is exactly equal to t h a t of the anionic surfactant. It should be noted t h a t the area per adsorbed molecule in the first adsorption plateau region is more t h a n 1000 A2. This means t h a t there is no significant lateral interaction between the neighbouring absorbed molecules. Thus, it is not surprising t h a t the adsorptions of cations and the ion-pairs are relatively independent of each other.
REFERENCES 1 G.D.Parfitt and C.H. Rochester (Eds), Adsorption from Solution at the Solid/Liquid Interface, AcademicPress, London, 1983. 2 J.F. Scamehorn,ACS Symp. Ser., 311 (1986} 1. 3 W. Rybinski and M.J. Schwuger, in M.J. Schick (Ed.), Nonionic Surfactants, Physical Chemistry,Marcel Dekker, New York, 1987, Chap. 2. 4 Y. Gao, C. Yue, S. Lu, W. Gu and T. Gu, J. ColloidInterface Sci., 100 (1984) 581. 5 Y. Gao, W. Gu, S. Lu, C. YueandT. Gu, Sci. Sin. Ser. B, 29 (1986)920.
358 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Z. Huang and T. Gu, Colloids Surfaces, 28 (1987) 159. Z. Huang, X. Li and T. Gu, Acta Scientiarum Naturalium Universitatis Pekinesis, in press. Y. Gao, Y. Zhang, Z. Huang and T. Gu, Acta Scientiarum Naturalium Universitatis Pekinesis, in press. H. Mtiller and E. Krempl, Tenside, 5 (1968) 333. M.J. Schwuger, Kolloid Z. Z. Polym., 243 (1971) 129. P. L i a n d T . Gu, Sci. Sin. (Eng. Ed.),22 (1979) 1284. M. Dai, Y. Gao, Z. Zhao and T. Gu, Chem. J. Chin. Univ., 2 (1981) 495. Z. Zhao and T. Gu, Chin. J. Catal., 5 (1984) 295. T. Barr, J. Oliver and W.V. Stubbings, J. Soc. Chem. Ind. London, 67 (1948) 45. Z. Huang, MS Thesis, Peking University, 1986. Y. Gao, J. Du and T. Gu, J. Chem. Soc. Faraday Trans. 1, 83 (1987) 2671. Z. Huang, C. Ma and T. Gu, Acta China. Sin. (Eng. Ed. ), in press. G.X. Zhao, Physical Chemistry of Surfactants, Peking University Press, Beijing, 1984, Chap. 5 (in Chinese). E. Tomlinson, S.S. Davis and G.I. Mukhayer, in K.L. Mittal (Ed.), Solution Chemistry of Surfactants, Vol. 1, Plenum Press, New York, 1979, p. 3.
353
Mixed Adsorption of Cationic and Anionic Surfactants from Aqueous Solution on Silica Gel ZHI HUANG, ZHULIN YAN and T I R E N GU
Laboratory of Colloid Chemistry, Department of Chemistry and Institute of Physical Chemistry, Peking University, Beijing (China) (Received 2 August 1988; accepted 6 January 1989)
ABSTRACT The adsorption of dodecyltrimethylammonium bromide (DTAB), dodecylpyridinium bromide (DPB), sodium dodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS) from their single aqueous solutions and DTAB-SDBS and DPB-SDS binary mixed solutions on silica gel at 25°C has been investigated. The results show that the individual cationic surfactants can be adsorbed strongly onto the silica gel, but no significant adsorption of anionic surfactants can be detected. However, in the mixed systems, the adsorption amounts of both the cationic and anionic surfactant ions are enhanced, and the excess adsorption of cationic surface-active ions is exactly equal to the adsorption of anionic surface-active ions. Thus, it is reasonable to suggest that the excess adsorption of cationic and anionic surface-active ions is in the form of ion-pairs.
INTRODUCTION
The adsorption of surfactants at solid/liquid interfaces is fundamentally important for many technical applications. Although the adsorption of single surfactants at solid/liquid interfaces has been investigated intensively, there have only been a few studies of mixed systems, in spite of their great importance [ 1-3 ]. In recent years, some investigations on the adsorption from mixed solutions of surfactants on solids have been made in our laboratory [4-8]. To our knowledge, however, only two papers (Miiller and Krempl [9] and Schwuger [10] ) on the adsorption of anionic-cationic mixed surfactant systems on solids have been published. Both of them showed that the adsorption of an anionic or a cationic surfactant was enhanced by the presence of an oppositely charged surfactant. However, only one of the surfactants was determined so it was difficult to explain the results convincingly. In the present paper, the adsorption of both components of mixed surfactant aqueous solutions ( D T A B - S D B S ) on silica gel have been determined and the results have been explained reasonably.
0166-6622/89/$03.50
© 1989 Elsevier Science Publishers B.V.
354 EXPERIMENTAL
Materials The silica gel used was prepared by a method described earlier [11-13], it was dried at 200 ° C for 4 h before use. The B E T surface area (nitrogen adsorption) was 370 m 2 g- 1, the average pore radius was 42 ~, and the pore volume of the silica gel was 0.89 ml g-1. The dodecyltrimethylammonium bromide (DTAB), dodecylpyridinium bromide (DPB), sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) were supplied by Beijing Chemical Co. DTAB, DPB and CTAB were purified by recrystallization from an acetone-ethanol solvent, several times, and SDS was recrystallized from ethanol. They showed no minimum in the surface tension curves and had critical micellization concentrations (c.m.c.) of 1.6-10 -2, 9.5.10 -3, 9.6" 10 -4 and 8.5.10 -3 mol dm -3 for DTAB, DPB, CTAB and SDS at 25 ° C, respectively. Sodium dodecylbenzenesulfonate (SDBS) was supplied by BDH Ltd and used without further treatment. It also showed no m i n i m u m in the surface tension curve and had a c.m.c, of 1.6.10 -3 mol dm -3 at 25°C. The water used throughout the experiments was prepared by distillation from an alkaline permanganate solution after being passed through an ionexchange column. The pH of the distilled water was 5.6. Methods The adsorptions were carried out at a constant feed molar ratio of cationic/ anionic surfactants or at a constant feed concentration of one component. The adsorption a m o u n t was obtained by measuring the concentration of surfaceactive ions in the solution before and after equilibrium with silica gel. 25 ml portions of solution were placed in contact with 0.1 g silica gel in vials, the vials were shaken for 6 h in an air-thermostat at 25.0 + 0.5 ° C. The concentration of DBS and DP + were determined by UV absorption at 232 and 257 nm, respectively, and those of DTA + and D S - were determined by the two-phase titration method [14,15] with bromophenol blue as the indicator, chloroform as the organic phase and SDS and CTAB as the titrants respectively. In DTABSDBS and D P B - S D S mixed systems, the concentrations of D B S - and DP + were determined by UV absorption. For the D T A B - S D B S system, the difference of concentration of cationic and anionic surfactant was determined by the two-phase titration method, then the concentration of DTA + was calculated from the concentration of D B S - and the concentration difference of cationic and anionic surfactants. For the D P B - S D S mixed system, only the concentration of DP + was measured and the accurate equilibrium concentration of D S - cannot be obtained by calculation.
355
The adsorption experiments were repeated at least two or three times. In general, the results were reproducible within 4%. It should be pointed out that these cationic-anionic surfactant mixed systems (the molar ratios of cationic/anionic surfactants were carefully chosen) are all clear. The UV-absorption density of the solutions before and after adsorption is reduced quantitatively with the dilution of the solutions. Thus, it is safe to say that the mixed systems used in this work are true solutions. RESULTS AND DISCUSSION
Adsorption from single component solutions Figure 1 shows the adsorption isotherms of DTA +, DP +, D S - and D B S from their single aqueous solutions on silica gel. It can be seen from Fig. 1 that the anionic surfactant ions are not adsorbed on silica gel. It is not surprising because the silica gel is charged negatively in the present experimental conditions (its isoelectric point is about p H 2 [ 17 ] ), and thus, it is unlikely to adsorb the negatively charged anionic surfactant ions. On the contrary, the cationic surfactant ions can be adsorbed strongly by the silica gel. We [16,17] have investigated the adsorption of cationic surfactants from aqueous solutions on silica gel carefully and have proposed an adsorption model. As compared with the results in the previous papers [16,17], it is obvious that the adsorption isotherms of cationic surfactants in the concentration ranges used in the present work are located in region II, the first adsorption plateau. Based on the model we suggested previously [16,17], the adsorption of cationic surfactants on silica gel results from electrostatic interactions in the very low concentration range and both electrostatic attraction and specific adsorption in the first plateau region.
E 4
%
o...o ~ . . _ _ , ,
_o-,L,,-o-- ~
"-----w~-
~2q
0
~ 0
~ ~
~
t 1
~.
C/10 -3 mol/dm
3
Fig. 1. Adsorption isotherms from single solutions on silica gel: (@) DTA+; ( X ) DP÷; ( • ) DBS-; ( 1 ) D S - .
356
Adsorption from mixed aqueous solutions on silica gel The adsorption isotherms of DTA ÷ and the corresponding adsorption of D B S - from D T A B - S D B S mixed solutions on silica gel are shown in Fig. 2, with a constant feed concentration of S D B S and Fig. 3 with a constant feed molar ratio of D T A B / S D B S . It can be seen from Figs 2 and 3 that the adsorption of DTA + is increased in the mixed systems and an apparent adsorption of D B S - occurs. The result is similar to that obtained by MLiller and Krempl [9] and Schwuger [10]. It is interesting to note that the excess adsorption amounts of both DTA + and D B S - in the mixed systems are equal to each other. In other words, the adsorption isotherms of DTA ÷ will coincide with that from the single component solution if the adsorption amount of D B S - is subtracted from the adsorption amount o f D T A + in the mixed systems. It should be noted that the adsorption of D B S - is a constant in the system with a constant feed concentration of SDBS (Fig. 2 ) and increases with its concentration in the system with a constant feed molar ratio of D T A B / S D B S (Fig. 3). This suggests that the adsorption of D B S - in the mixed system is not influenced by the concentration o f D T A + and mainly depends upon its own concentration if the feed molar ratio of D T A B / S D B S is much higher than 1. Figure 4 shows 6
6
~4
A__
%
__~
tl
.-* ......
~Ji~'" ~
A
~~
~_...O.&O/__O4___
....
..... • .
.
.
.
O
&-
o
0
~,
•
A
,
1 C/tO~3 mol/dm 3
t.
2
o
(
~---~-'~'
~/
1 C/lO-3mol/dm 3
Fig. 2. (left) Adsorption of DTA + and D B S - on silica gel from D T A B - S D B S mixed solutions with a constant feed concentration of SDBS. SDBS concentration of 1-10 - 4 tool dm-3: ( O ) adsorption of DTA +; (~) adsorption of D B S - ; ( • ) adsorption of DTA ÷ minus the adsorption o f D B S - . SDBS concentration of 0.4" 10 -4 mol dm-~: ( A ) adsorption of DTA+; ( A ) adsorption of D B S - ; ( • ) adsorption of D TA + minus the adsorption of DB S - . (The dotted line represents the adsorption isotherm of DTA + from single solution. ) Fig. 3. (right) Adsorption of DTA + and D B S - on silica gel from D T A B - S D B S mixed solutions with a constant feed molar ratio of D T A B - S D B S . Molar ratio of 10:1 D T A B - S D B S : ( A ) adsorption of DTA ÷; ( z~ ) adsorption of D B S - ; ( • ) adsorption of DTA + minus the adsorption of D B S - . Molar ratio of 20:1 D T A B - S D B S : (C)) adsorption of DTA+; (~) adsorption of D B S - ; ( • ) adsorption o f D T A + minus the adsorption of D B S - . Molar ratio of 1:10 D T A B - S D B S : ( [] ) adsorption of D B S - from mixed solution. (The dotted line represents the adsorption isotherm of DTA + from single solution. )
357 6
~4
E
-¢
°o
I
C/10 -3 moI/dm 3
Fig. 4. Adsorption of DP + from mixed DPB-SDS solution on silica gel at a feed molar ratio of 52:1 DPB-SDS. (The dotted line represents the adsorptionisothermof DP + fromsinglesolution.) the adsorption isotherm of DP + from D P B - S D S mixed aqueous solution on silica gel. The adsorption of DP + is also enhanced in a similar way to the systems mentioned above. Cationic surface-active ions will strongly interact with anionic surface-active ions through electrostatic attraction [ 18 ], when they are mixed in aqueous solution, and ion-pairs will be formed between the oppositely charged surfact a n t s [ 19 ]. This leads to a great increase in their surface activity [ 18 ] and will promote their adsorption at interfaces. Based on the adsorption model of cationic surfactant from aqueous solution on silica gel [16,17], the negatively charged positions of the silica gel surface (SiO-) can adsorb cationic surfact a n t ions and may not adsorb anionic surfactant ions. However, the anionic surface-active ions may be coadsorbed specifically with cationic surface-active ions as ion-pairs at the non-charged sites of silica gel through van der Waals interactions. This explains why the excess adsorption a m o u n t of the cationic surfactant is exactly equal to t h a t of the anionic surfactant. It should be noted t h a t the area per adsorbed molecule in the first adsorption plateau region is more t h a n 1000 A2. This means t h a t there is no significant lateral interaction between the neighbouring absorbed molecules. Thus, it is not surprising t h a t the adsorptions of cations and the ion-pairs are relatively independent of each other.
REFERENCES 1 G.D.Parfitt and C.H. Rochester (Eds), Adsorption from Solution at the Solid/Liquid Interface, AcademicPress, London, 1983. 2 J.F. Scamehorn,ACS Symp. Ser., 311 (1986} 1. 3 W. Rybinski and M.J. Schwuger, in M.J. Schick (Ed.), Nonionic Surfactants, Physical Chemistry,Marcel Dekker, New York, 1987, Chap. 2. 4 Y. Gao, C. Yue, S. Lu, W. Gu and T. Gu, J. ColloidInterface Sci., 100 (1984) 581. 5 Y. Gao, W. Gu, S. Lu, C. YueandT. Gu, Sci. Sin. Ser. B, 29 (1986)920.
358 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Z. Huang and T. Gu, Colloids Surfaces, 28 (1987) 159. Z. Huang, X. Li and T. Gu, Acta Scientiarum Naturalium Universitatis Pekinesis, in press. Y. Gao, Y. Zhang, Z. Huang and T. Gu, Acta Scientiarum Naturalium Universitatis Pekinesis, in press. H. Mtiller and E. Krempl, Tenside, 5 (1968) 333. M.J. Schwuger, Kolloid Z. Z. Polym., 243 (1971) 129. P. L i a n d T . Gu, Sci. Sin. (Eng. Ed.),22 (1979) 1284. M. Dai, Y. Gao, Z. Zhao and T. Gu, Chem. J. Chin. Univ., 2 (1981) 495. Z. Zhao and T. Gu, Chin. J. Catal., 5 (1984) 295. T. Barr, J. Oliver and W.V. Stubbings, J. Soc. Chem. Ind. London, 67 (1948) 45. Z. Huang, MS Thesis, Peking University, 1986. Y. Gao, J. Du and T. Gu, J. Chem. Soc. Faraday Trans. 1, 83 (1987) 2671. Z. Huang, C. Ma and T. Gu, Acta China. Sin. (Eng. Ed. ), in press. G.X. Zhao, Physical Chemistry of Surfactants, Peking University Press, Beijing, 1984, Chap. 5 (in Chinese). E. Tomlinson, S.S. Davis and G.I. Mukhayer, in K.L. Mittal (Ed.), Solution Chemistry of Surfactants, Vol. 1, Plenum Press, New York, 1979, p. 3.