Langmuir 1991, 7, 885-888
885
Synergism in Mixtures Containing Zwitterionic Surfactants Milton J. Rosen Surfactant Research Institute, Brooklyn College, City University of New York, Brooklyn, New York 11210 Received May 29, 1990. In Final Form: October 3,1990 Synergism can exist in mixtures of two or more surfactants if they interact attractively with each other and meet various other conditions which are reviewed. Attractions between surfactants are dominated by any electrostatic interaction of their hydrophilic head groups. Zwitterionic surfactants with no net formal charge show only weak interaction with nonionic surfactants. When the zwitterion is capable of accepting or donating a proton to form a species with a net charge, then interaction with an oppositely charged surfactant may be strong enough to exhibit synergy, even at neutral pH. Long-chain amino acids, capable of both accepting and donating protons to form charged species, show more or less equal attractive interaction with both anionic and cationic surfactants; those such as betaines, that are capable only of accepting a proton, interact much more strongly with anionic than with cationic surfactants. Materials normally not considered zwitterionic, such as amine oxides, sulfoxides, and pyrrolidones, but capable of accepting a proton to form a cationic species, also show much stronger interaction with anionic than with cationic surfactants. The interfacial and colloid properties of a solution of two or more surfactants can be quite different from those of a solution of the individual components. When a solution of the mixed surfactants shows greater surface activity (e.g., lower surface tension) than that attainable with any of the individual surfactants of the mixture at the same concentration, then the mixture is said to exhibit synergism. Mixtures of two surfactants can exhibit synergy if the two surfactants attract each other sufficiently, either through mutual electrostatic attraction of oppositely charged hydrophilic groups or through van der Waals attraction of their hydrophobic groups. Interactions between surfactants can be measured by the so-called j3 parameters related to their activity coefficients in the mixture. These parameters have been evaluated for mixed micelle formation (OM) in aqueous solution' and for mixed monolayer formation a t the surface of an aqueous s o l ~ t i o (p"), n ~ ~at~the ~ interface between an aqueous solution and a second liquid phase4 (~"LL),and at the interface between an aqueous solution and a solid phase5 (~"Ls). The more negative the value of the j3 parameter, the greater the attractive interaction between the two surfactants compared to the reference states of the individual surfactants by themselves. The evaluation of the interaction parameters is based upon equations (1 and 2), derived from the thermodynamics of the system by Rubingh,' for mixed micelle formation (1- a) Cl,M = (1- XM)f,MC,M where a is the mole fraction of surfactant 1 in the total surfactant in the solution phase, CIM, CzM,and C l P are the critical micelle concentrations (cmc's) of individual surfactants 1and 2 and their mixture at a given value of a,respectively, XMis the mole fraction of surfactant 1in the total surfactant in the mixed micelle, and flMand fzM are activity coefficients of individual surfactants 1and 2, (1) Rubingh, D. N.In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. I, p 337. (2) Roeen, M. J.; Hua, X. Y. J . Colloid Interface Sci. 1982,86, 164. (3) Rosen, .M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley-Interscience: New York, 1989; (a) pp 398-401, (b)pp 394-397, (c)
respectively, in the mixed micelle. Using the regular solution approximations for the activity coefficients flM= exp[PM(1- xM)'] fZM
= e x p [ P (xMY]
(3)
(4)
he obtained
and
Equation 5 is solved numerically for XM, and substitution of XMin eq 6 yields the value of OM (the molecular interaction parameter for mixed micelle formation in aqueous solution). The other fundamental property of surfactant mixtures is mixed monolayer formation at the interfaces of the system. Rosen and Hua2 derived equations analogous to eqs 5 and 6 to calculate p", the molecular interaction parameter for mixed monolayer formation at the aqueous solution/air interface. The molar concentrations of the individual surfactants and their mixture at a given value of a required to yield a given surface tension are used in these equations instead of the critical micelle concentrations. In similar fashion, equations were derived by Rosen and Murphy4 to evaluate BOLL, using the surfactant molar concentrations required to yield a given interfacial tension, and by Rosen and Gu5 to evaluate p " ~ using , the concentrations required to yield a given adhesion tension. The experimental determination of p" and OM therefore only involves determining the surface tension-log concentration curves for each of the pure Components and for at least one mixture of them at a specific value of a.3b The conditions for synergism due to mixed micelle formation in aqueous media or due to mixed monolayer formation at various interfaces have been S~OWI+J to be determined solely by the value of the relevant j3 parameters and the properties of the individual surfactants by themselves. From these quantities, the ratio of the two surfactants at which maximum synergism occurs and the
p 406.
(4) Roeen,M. J.;Murphy,D.S.J.ColloidZnterfaceSci. 1986,110,224. (5) Rosen, M. J.; Gu, B. Colloids Surf. 1987, 23, 119.
(6) Hua, X. Y.; Rosen, M. J. J Colloid Interface Sci. 1982, 90,212. (7) Hua, X. Y.; Roeen, M. J. J . Colloid Interface Sci. 1988,125,730.
0 1991 American Chemical Society
Rosen
886 Langmuir, Vol. 7, No. 5, 1991
3 I 0
Figure 1. Plots of log C12 (the mixed surfactant molar concentration to yield a particular effect) versus a (the mole fraction of surfactant 1 in the total mixed surfactant in the system) illustrating: (A) synergism when 8" < 0, Iln C1O/C2O1 = 0; (B) synergism when 8" < 0,1@1 > Iln C1O/Cg01 > 0; (C)no synergism when 8" < 0,18"1 I(ln Clo/C20)l,where Cl0 and C2O are the molar concentrations of individual surfactants 1and 2, respectively, in the solution phase required to produce the same desired surface tension. Relationships analogous to eqs 7 and 8, above, have been derived6for the mole fractions of surfactants and minimum value of Clz at the point of maximum synergism in this respect. Analogous equations have also been derived for synergism in this respect a t the interface between an aqueous solution and a liquid hydrocarbon phase4 and at the interphase between an aqueous solution and a hydrophobic solid.5 2. Synergism in Surface Tension Reduction Effectiveness. This exists when the mixture of surfactants at its cmc reaches a lower surface tension than that obtained at the cmc of either component of the mixture by itself. This is illustrated in Figure 2B. The conditions for this to occur are7 1. 8" PM must be negative
-
2.
where Clo*cmcand CZO~cmc are the molar cmc's of pure sur-
Langmuir, Vol. 7, No. 5, 1991 887
Synergism in Surfactant Mixtures Table I. B Parameters of Some Surfactant Mixtures in Aqueous Media
8"
BM
-21.8
-25.5
-3.5
ref 8 9
-2.2
9
system
ClaHeaSO1--ClzHzaN+(CHa)sBr CizH26Sor--CizHza(oCz~)sOH
(0.1M NaCl) CirHasPyr+Cl--CizH~(OCzH4)eOH
C,,N+(Bz)(Me)CH,COO-
(0.1M NaCI) Cl~N+(Bz)(Me)CH~COO%lzH~(OCzHl)eOH 4.6 (pH = 5.8)
-0.9
C12N+H2CH,CH,COO-+ C,,SO;
-
HP
C12N+H2CH~CH2COOH~C12S0~ + OH- (9) (pH = 5.8; 0.1 M NaBr) 8"= -4.2;
-
C,,N+(Bz)(Me)CH,COOH~C,,SO~ + OH- (11) (pH = 5.8) j3" = 5.7; @" = 5.0
BM = -1.2
(pH = 5.8) @' = 1.3;
(pH = 5.8; 0.1 M NaBr) 8"= -4.8;
?k (12)
= 1.3
zwitterionic molecule. The presence of the oppositely charged surfactant ion is generally sufficient to produce the interaction, because of the mutual attraction of the two oppositely charged ions. Of course, a favorable pH in the solution will promote the interaction and an unfavorable one will inhibit it, as will favorable or unfavorable acidic or basic strengths in the functional groups (Table 11). Surfactant-urfactant interactions of this type have been used for the past few decades to improve the properties of surfactants, the most common example being the use of long-chain amine oxides to improve the foaming properties of anionic surfactants (eq 13). Although amine C,,N+(CH,),O-
+ C,,SO;
-
Ha0
C12N+(CH3)20H~C12SO~ + OH- (13) (5 X
M Na,CO,)
BM = -4.413
oxides are often classified as cationic surfactants, they may be considered zwitterionics, similar to betaines, since they contain a permanent poaitive charge on the nitrogen atom, and a negative charge on the oxygen atom that may be removed by the addition of a proton. In the presence of an anionic surfactant in aqueous solution, the amine oxide picks up a proton from the solution, even at neutral pH, and forms with the anionic surfactant an anioniccationic salt that is much more surface-active than either component14 of the salt. In similar fashion, dialkyl sulfoxides interact with anionic surfactants, by accepting a proton, to form an anionic-cationic salt, but interact much more weakly with cationic surfactants (eqs 14 and 15)lb,since there is no
-
Hz0
C12N+H2CH,CH,COO-+ C12Pyr+ C,,NHCH,CH,COO-~C,,Pyr+
Hz0
C,,N+(Bz)(Me)CH,COO- + C,,N(CH,);
10
factants 1and 2 required to yield a surface tension value equal to any mixture of the two surfactants at its cmc. Condition 1means that the attraction of the two surfactants for each other must be greater in the mixed monolayer at the surface than in the mixed micelle in the solution phase. It is apparent from the above discussion that the values of the relevant j3 parameter are crucial in determining whether a mixture of surfactants will exhibit synergy. Molecular interactions between surfactants, and consequently the values of the @ parameters, are usually dominated by any electrostatic forces that may exist between their hydrophilic groups. The values of j3 parameters are generally most negative (attractive) for mixtures of anionic surfactants with cationic surfactants (Table I). Consequently, zwitterionic surfactants that have no net charge would be expected to show only weak interaction with nonionic surfactants, and this is borne out by measurements of their j3 parameters (A @ value of zero indicates ideal mixing of the two surfactants). However, when the zwitterionic surfactant is capable of accepting or donating a proton, it can acquire a net + or - charge by interaction with the water used as a solvent, and interaction with an oppositely charged ionic surfactant may be strong enough to produce synergy. In the case of a zwitterionic surfactant capable of both accepting and donating a proton (an amphiprotic compound), significant attractive interaction (B < -3) of the zwitterionic surfactant may occur with both an anionic surfactant (eq 9) and a cationic surfactant (eq 10). On the other
-
Ha0
+ C,,SO;
Hz0
+ H30+ (10)
CloH2,S+(CH3)0-+ C,,SO; CloH21S+(CH3)OH~Cl~SO~ + OH- (14)
OM = -3.4
hand, when the zwitterionic compound is capable only of accepting a proton (and not of donating one), then its interation with an anionic surfactant will be significantly greater than its interaction with a cationic surfactant will be significantly greater than its interaction with a cationic surfactant. Consequently, the j3 values of betaineanionic mixtures are much more negative than those of betaine-cationic mixtures" (eqs 11 and 12). does not require a low or high pH, respectively, of the solution, or even the presence of strongly basic or acidic groups in the (8)Lucaeeen-Reyndere,E. H.; Lucaeeen,J.;Gilee, D. J. Colloidlnterface Sei. 1981,81, 150. (9) Rosen, M. J.; Zhao, F. J. Colloid Interface Sci. 1983,95,443. (10) Zhu, 9. Y.andRoeen, M. J. J. ColloidInterface Sci. 1984,99,436. (11) Rosen, M. J.; Zhu, 9. Y.J. Colloid Interface Sci. 1984,99,427.
(pH = 5.9) 0" = -4.3;
BM = -4.3
+
Hz0
ClOH2,S+(CH3)0-+ ClON(CH3),+
(15)
(pH = 5.9) @' = -0.6; ,BM= -0.5 possibility of forming a mutually neutralized salt from these two types of surfactants. The j3 values reflect this difference in interaction with anionics and cationics. (12) Roeen, M. J.; Zhu, Z. H. J. Am. Oil Chem. SOC.1988,65, 663. (13) Holland, P.M.; Rubingh, D. N. J. Phys. Chem. 1983,87, 1984. (14) Rosen, M. J.; Friedman, D.; Gross, M. J. Phys. Chem. 1964,68, 3219.
(15) Zhu, D.; Zhao, G.-X.; Wuli Huaxue Xuebao Acta Phys.-Chim. Sin. 1988, 4, 129.
Rosen
888 Langmuir, Vol. 7, No. 5, 1991 Table 11. Effect on @ Parameters of pH of Solution and Basicity of Anionic Group of Betaine system pH 8" BM ref C1zNt(Bz)(Me)CH&OO- + C12SOa5.0 -6.9 -5.4 11 C1ZN+(Bz)(Me)CHzCOO-+ ClzSO35.8 -5.7 -5.0 11 6.7 -4.9 -4.4 11 ClzN+(Bz)(Me)CHZCOO-+ C12SO311 CloNt(Bz)(Me)CH2CH~S03-+ C12S03- 6.6 -2.5 ClzNf(CH&CH2C00- + LAS 5.8 -3.8 -2.9 12 ClzNt(Bz)(Me)CH2C00- + C12SO39.3 -2.9 -1.7 12
Droves skein w e l t i n g
o.sg// 0 0 . 7 5 g//
0
20
-
x
1.0 g/.l
I
Even a nominally neutral structure such as a pyrrolidinone (usually called pyrrolidone) shows interaction similar to a zwitterionic surfactant in the presence of the proper type of ionic surfactant; in this case, an anionic surfactant (eq 16). T
6H
0 (T.I.S.= 0.1 M; pH 5.9)
pa= 3 . 1
The pyrrolidone structure is similar to the amine oxide structure in being capable of accepting a proton and acquiring a positive charge on the nitrogen atom. Consequently, it can form a salt with an anionic surfactant.16 As expected from its structure, its interaction with a cationic surfactant is much weaker (eq 171,since there is no mechanism for forming an ion pair with the cationic surfactant.
! I
++ H2O
+
C12W+
C8-N: (T.I.S.= 0.1 M; pH 5.9)
(17)
pa = -1.6
Mixtures that show synergism at the aqueous solution/ air interface generally also show synergism a t the aqueous solution/liquid and aqueous solution/solid interface, when the second condensed phase is hydrophobic, although the value of the relevant p parameter is generally somewhat less negative.415 Thus, with the proper zwitterionic and the proper ionic surfactant, we can expect to find synergy. The two surfactants must be selected so that they meet the conditions for synergy shown above. The larger the negative value of P, and the closer the two different surfactants are in their surface properties, the greater the probability of synergism in surface tension reduction efficiency. The larger the negative value of b', and the closer the critical micelle concentrations of the two surfactants, the smaller will be the cmc of the mixture. For synergism in surface tension reduction effectiveness, the attraction of the two surfactants in the mixed monolayer must be greater than in the mixed micelle. Some examples of synergism in both fundamental surface properties and in 'performance" properties by surfactant mixtures in which the molecular interactions described above occur are shown in Figures 3 and 4. The first is for a mixture of sodium dodecylbenzenesulfonate and N-octylpyrrolidinone, and shows synergism in wetting" by the Draves skein technique. This mixture shows (16) Rosen, M. J.;Gu, B.; Murphy, D. 5.;Zhu, Z. H. J. Colloid Interface Sci. 1989, 129, 468. (17) Zhu, Z. H.; Yang,D.; Roeen, M. J. J . Am. Oil Chem. Sac. 1989, 66, 998.
I
T /
I
I
0
IO
I
25
I
1
50 75 % CSP(w/w) in C 8 P - L A S Mixture
I
90
I
)O
Figure 3. N-Octylpyrrolidone (C&')-sodium linear dodecylbenzenesulfonate (LAS) mixtures showing synergism in wetting by the Draves skein technique. Reprinted with permission from ref 17. Copyright 1989 American Oil Chemists' Society. I
U
Figure 4. Sodium linear dodecylbenzenesulfonate-N-dodecylbetaine mixtures showing synergism in foaming (initial foam height, 60 "C, 0.25% total surfactant concentration, Ross-Miles technique): -, pH 5.8;- - -;p H 9.3. Reprinted with permission from ref 12. Copyright 1988 American Oil Chemists' Society.
synergism in surface tension reduction effectiveness (the mixture shows a lower surface tension than the individual materials at the cmc). This often correlates with synergism in wetting. Figure 4 shows foaming data for a mixture of sodium dodecylbenzenesulfonate and a betaine, which also shows synergism in surface tension reduction effectiveness. This often produces synergy in initial foam height1* by the Ross-Miles technique. Acknowledgment. This material is based upon work supported by the National Science Foundations (Grants CTS-8706859 and INT-8603193), and by Dow Chemical, Exxon Research and Engineering, GAF Corporation, and Shell Development. The assistance of Zhen Huo Zhu in determining some of the B parameters is greatly appreciated.
