Synergism in Binary Mixtures of Surfactants at Various Interfaces

Chapter 21

Synergism in Binary Mixtures of Surfactants at Various Interfaces Milton J. Rosen

Downloaded by UNIV OF ARIZONA on August 8, 2012 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch021

Surfactant Research Institute, Brooklyn College, City University of New York, Brooklyn, NY 11210

Synergism in mixtures of surfactants depends upon the existence of attractive interactions between the different surfactants. The conditions for synergy, in mathematical terms, can be derived by use of the so-called β-parameters that measure surfactant interactions in mixed monolayers and mixed micelles. Various types of synergy, resulting from mixed monolayer formation at various interfaces and from mixed micelle formation in aqueous solution, can be distinguished. The mathematical conditions for each of these to exist at the aqueous solution/air, aqueous/liquid hydrocarbon, and aqueous/hydrophobic solid interfaces have been derived, together with the surfactant ratios and relevant properties at the points of maximum synergism. When surfactant-surfactant interactions are strong, e.g., in cationic-anionic systems, terms for the change in interfacial areas of the surfactants upon mixing must be included in the relevant equations. Negative synergism can occur when attractive surfactant interactions in the mixed system are weaker than in the individual surfactant systems. At the 59th Colloid and Surface Science Symposium held in Potsdam, New York, in 1985, I presented the work we had been doing in my laboratory in designing a quantitative framework for the study of synergism in binary mixtures of surfactants. That study was initiated to replace the trial-and-error method used in the past to find synergistic combinations of surfactants and to replace it with an understanding of the principles involved in synergy, an understanding that would permit the selection of surfactant pairs for optimum performance in a rational, scientific fashion. Since then, the search for and study of synergy in mixtures of materials has become even more important because of the difficulties involved in the introduction of new compounds, either for industrial or consumer use. Questions of toxicity, biodegradability, and general environmental impact must now be addressed whenever a new compound is suggested. As a result, in the search for improved performance and properties, a more feasible approach at the present time is often through the use of combinations of existing acceptable materials that show synergy, rather than through the design of new chemical structures. In this chapter, I will discuss what we have been doing in the area of synergism since 1985. 0097-6156/92/0501-0316S06.00/0 © 1992 American Chemical Society

In Mixed Surfactant Systems; Holland, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


21. ROSEN

317

Synergism in Binary Mixtures of Surfactants

Our investigations have moved in three directions: 1) we have extended our study of synergism, which originally was confined to the aqueous solution/air interface, to the aqueous solution/liquid hydrocarbon interface and to the aqueous solution/hydrophobic solid interface, 2) we have modified the basic equations that we have been using to account for the change in interfacial area of the surfactants upon mixing, and 3) we have investigated the phenomenon of negative synergism, which we have discovered in a number of systems. However, before discussing these developments, let me review some of the principles upon which our work is based. Synergism in binary mixture of surfactants is due to attractive interactions between the two types of surfactants present in the system. We measure the nature and strength of those interactions by the so-called β parameters, first suggested by Donn Rubingh, one of the organizers of this symposium, back in 1978 (I). The equations used by him to evaluate β parameters for mixed micelle formation are: M

2

M

M

M

(X ) ln(aC p/X C i) 1_ M

2

(l-X ) ln|l-a)C

M

=

M 1 2

/(l-X

M

M

N / I

)C

1

(1)

M 2

M

ln(a C , /X C ,) 2

Μ

β

=

(2) (

1_χΜ)2

Where X ^ is the mole fraction of surfactant 1 in the total surfactant in the mixed micelle; CX is the mole fraction of surfactant 1 in the total surfactant in the solution phase; C ^ j , C ^ i , and C ^ p are the critical micelle concentrations of individual surfactants 1 and 2 and their mixture at a given value of CX, respectively; is the interaction parameter for mixed micelle formation. We extended this treatment, which was developed for mixed micelle formation in aqueous solutions, to mixed monolayer formation at the aqueous solution/air interface (2). The relevant equations are: 2

0

X ln(aC, /XC ,) 2

= 1

(3)

2

(l-X) ln[l-a)Cp/(l-X)C° )] 2

ln(aC /XC°,) 12

(4) (1-X)

2


318

MIXED SURFACTANT SYSTEMS

where X is the mole fraction of surfactant 1 in the total surfactant in the mixed monolayer at the interface; C ° j , C°9, and C | are the solution phase molar concentrations of individual surfactants 1 and 2 and their mixture at a given value of 2

σ

CX, respectively, required to yield a given surface tension value; β

is the interaction

parameter for mixed monolayer formation. The experimental evaluation of β from the surface tension - concentration curves of the systems (3).

σ

is

Modification for Change in Interfacial Areas of the Surfactants upon


Mixing. The derivation of the basic equations is based upon the assumption that interfacial areas of the two interacting surfactant molecules do not change significantly when they are mixed with each other. This assumption is tenable when interaction between the two surfactants is weak or moderate, but is not valid when interaction is strong. For example, for mixtures of anionic with cationic surfactants, it has been shown by a number of investigators (4-7) that the molar interfacial areas of the two surfactants decrease markedly when mixed with each other, due to mutual neutralization of their charges. This change makes the value of the β parameter, in these cases, change significantly with change in the ratio of the two surfactants at the interface and in the mixed micelle or with change in the interfacial tension of the system. In such cases, terms must be added to the basic equations to account for the work involved in changing the molecular areas. The relevant equations then become (7) :• X

2

ln(aC12/C°]X) - γ ( Α ° | -Aj)/RT =

(1-X)

2

1

(5)

l n [ ( l - a ) C i / C ° ( l - X ) ] - Y(A° -A )/RT 2

2

2

2

0

I n i a C p / C ^ ) - Y(A°|-A|)/RT β°

=

(6)

(1-X)

2

Y = surface or interfacial tension of the system A°j = surface or interfacial area of surfactant 1 before mixing Aj = surface or interfacial area of surfactant 1 after mixing A ° = surface or interfactal area of surfactant 2 before mixing 2

A = 2

surface or interfacial area of surfactant 2 after mixing

The values of A | and A7 are obtained by assuming that their ratio after mixing is the same as their ratio before mixing, i.e., Al = A^l A2 o A

2


21. ROSEN

319


We have found that the use of these equations gives β values that do not change significantly with either the ratio of the two surfactants in the system or with the surface or interfacial tension of the system.

Synergism in surface tension reduction efficiency. The efficiency of surface tension reduction by a surfactant is measured by the solution phase concentration required to produce a given surface tension (reduction). Synergism in this respect is present in a Binary mixture of surfactants when a given surface tension (reduction) can be attained at a total mixed surfactant concentration lower than that required of either surfactant by itself.


We have shown (8) that the conditions for synergism in this respect are: 1. β

σ

must be negative.

2. I l n C ° / C ° 1 < I p ° I 1

2

where C°] and C°-> are the solution phase molar concentration of individual surfactants 1 and 2, respectively, required to attain a given surface tension (reduction). When the equations that include the change in area of the surfactants upon mixing are used, the conditions for synergism in surface tension reduction efficiency become (9) : 1. β® is negative. 2. I p

a

0

0

0

l > l l n ( C | / C ) Ι+Ιγ K A ! - A ) - ( A ° - A ) ] / R T I 2

1

2

2

The term Ιγ | ( A ° j - A | ) - ( A ° o - A9)|/RTI, however is small, generally less than 1, and always less than the increase in the absolute value of β° resulting from the use of the extended equation. Consequently, the condition for synergism in this respect will always be fulfilled when use of the simple equations shows that the conditions are met. At the point of maximum synergism, the mole fraction a * , of surfactant 1 in the solution phase equals its mole fraction in the mixed monolayer at the aqueous solution/air interface, and is given by the relationship: In ( C V C ^ + p

0

a * = x* = 2

β

σ

where X * is the mole fraction of surfactant 1 in the mixed monolayer at the point of maximum synergism in this respect. The minimum mixed surfactant concentration in the solution phase, C j j , required to attain a given surface tension (reduction) is 2

m

n


320


given by the expression:

2

In ( d y C ^ M

Ci2,min = C ° j exp

26" We have extended (10) this treatment to the aqueoiis solution-hydrocarbon interface and have determined the conditions for synergism in interfacial tension O R


m i x t (

reduction efficiency. The interaction parameter, P ^ L L ' ^ ^ monolayer formation at the liquid-liquid interface is determined from plots of interfacial tension vs. total surfactant concentration in the system at constant phase volume ratio and constant initial ratio of the two surfactants. When the conditions of either constant initial phase volume ratio ( Φ ) and constant partition coefficient (K) are met, or Κ φ « 1, the equations and the conditions for synergism in this respect, as derived by this treatment, are completely analogous to those obtained for the liquid-air interface: 1. Ρ

σ

LL

m

u

s

t

^

e

n e

a t

y e

& i -

C°2,t Where C ° j

t

and C°9 are the total system concentrations of individual surfactants 1 t

and 2, respectively, required to produce a given interfacial tension in the two-phase systems containing only the individual surfactants. Under these conditions, the mole fraction, a*, of surfactant 1 in the total surfactant in the system at the point of maximum synergism equals the mole fraction at the interface and is given by the expression: a

In ( C ° i y C ° 2 , t ) + p L L

a*=X*=

2β° L L The minimum total concentration of mixed surfactant in the system C j

2t m

j

n

to produce a given interfacial ternsion is given by the expression:

2-, 0

' P ° L L - I n (COi, /C 2,tV t

Cl2,t,min = C ° i ,

e

x

p

t 2

G

P LL

The next extension was to aqueous solution/hydrophobic solid interfaces, e.g., Parafilm, polyethylene, and Teflon. For a hydrophobic, low energy solid, the solid/vapor interfacial tension Y ^ y , may be considered to be constant with change in surfactant concentration in the


21. ROSEN

321


aqueous phase. From Young's equation: Y s v - YSL = Y L V

C O S 0

> s

surfactant solutions having the same Y L V C ° ®

(

=

constant Y $ L ) value can

therefore be used for C ° j , C ° 2 , and C p in the equations given above for X and β In order to evaluate Y $ L

a t t

n

e

σ

.

solid/liquid interface, then, both the surface tension of

the aqueous solution and its contact angle on the solid must be known. Alternatively,


the adhesion tension, γ ^ γ Cos Θ, can be measured directly by the Wilhelmy plate technique, when suitable plates of the solid can be obtained.

Table I. Effect of Interface on β— (α

Mixture

1

p

= 0.53; 25°C; Ή = 32 mN m' )

Interface

β

C S0 Na/C P

0.1MNaCI/air

-3.1

37.9

C S0 Na/C P

O.IMNaCI/Parafilm

-2.9

38.1

C S0 Na/C P

O.IMNaCi/Teflon

-2.5

42.0

12

12

12

3

8

3

8

3

8

2

C P • N-octyl-2-pyrrolidinone 8

It is interesting to see what happens to the value of β when air is replaced by a hydrophobic condensed phase. The data shown in Table I are typical. The replacement of air by a liquid alkane or a solid nonpolar solid in almost all the mixtures investigated results in a small decrease in the negative value of β air consists mainly of nonpolar

σ

. Since

7

gas, the similarities between the β * values against

air and against other nonpolar phases should not be surprising. The decrease in the σ

negative value of β is most probably due to the increase in average interfacial area of the surfactant., resulting in weaker interaction between them. In the case of liquid hydrocarbon phases, other work in our laboratory (10) has shown that the presence of the hydrocarbon phase increases the area/molecule of the individual surfactants, by themselves, at the interface. Table II lists some data for mixtures of N-octyl-2-pyrrolidinone (CgP) and sodium 1-dodecanesulfonate (CpSC^Na) in 0.1 M NaCl, a system that meets the conditions for and shows synergism in surface and interfacial tension reduction efficiency at a number of interfaces (11) . As can be seen from the data listed, calculated values for C p agree well with experimental values, even when some of the


322


experimental values of CX are not very close to CX* (the calculated mole fraction in the solution phase for optimum efficiency).

Table II. Synergism in γ Reduction Efficiency at Various Interfaces ( / / ) C P / C S 0 N a : 0.1M NaCl (aqu.); 25°C;H=32 mN irf


8

12

1

3

Ci°

C °

C

Χ1Θ3Μ

X103M

X103M

X103M

2

1

C

2

12fCa

icd.

Interface

α(α)

Air

0.53(0.46)

1.91

1.51

0.78

0.77

Hexadecane

0.23(0.30)

1.32

0.55

0.46

0.47

Teflon

0.53(0.45)

3.31

2.59

1.58

1.55

Parafilm

0.53(0.45)

1.93

1.41

0.81

0.79

CQP = N-octyl-2-pyrrolidinone Synergism in mixed micelle formation. Synergism in this respect is present when the critical micelle concentration of any mixture is lower than that of either individual surfactant by itself. We have found (8) that the conditions for synergism in this respect are: 1. β M must be negative. 2. l l n ( C

M 1

/C

M

M

2

Ι Ι η Μ

C°i cmc.C 7 - ± I C°2, .C i M

c r n c

c m c

M

c m c

where C ° ] , , C ° 2 , are the molar concentrations of individual surfactants 1 and 2, respectively, required to yield a surface tension value equal to that of any mixture of the two surfactants at its CMC. Extension of this treatment to the aqueous solution/liquid hydrocarbon and aqueous solution/hydrophilic solid interfaces yields analogous relationships. Some data are listed in Table IV.


324


Table IV. Synergism in Interfacial Tension Reduction Effectiveness at Various Interfaces (13) C B M G / C S 0 N a : H 0 , 25°C 12

12

3

2

caic


ν cmc M

νcmc *2

rmNnrd.) (mRirA

c

ν M2

m

c

y ·1 min 1

Interface

Q^L&rl

fmNrr4) imllriF !

Air

.028(.047)

32.8

39.0

n-Heptane

.051 (.042)

1.8

7.7

1.1

0.6

n-Hexadecane .051 (.047)

3.5

9.9

1.4

1.4

Isooctane

.051 (.042)

2.0

8.3

1.0

1.3

Heptamethylnonane

.051 (.048)

2.8

9.6

1.3

1.1

28

29.9

The next extension of our work was due to an experience I had during a lecture tour of the People's Republic of China several years ago. I was asked for an explanation of the well-known fact that soap decreases the foam of alkylbenzenesulfonate-based laundry detergents. I offered the commonly-accepted explanation that the Ca^ in the water formed insoluble Ca soap that acted as a foambreaker, only to learn that this foam reduction also occurs in distilled water! In an attempt to find the correct reason for this foam-reducing action, upon my return home we measured the β parameters for a number of long-chain carboxylate (i.e., soap)σ

alkylbenzenesulfonate mixed systems and obtained a most interesting result. The β and p val ues for long-chain carboxylate - alkylbenzenesulfonate mixtures are positive, not negative, in contrast to what we had found in all previous systems studied! This means that molecular interaction between the two surfactants when mixed is weaker than surfactant-surfactant interactions in the individual compounds by themselves. Going back to our basic equations, we found that this could produce negative synergism in the properties that we had previously investigated: surface or interfacial tension reduction, mixed micelle formation. For example, the CMC of the mixture of two surfactants might be greater than the CMC of either surfactant by itself. To date, only a few systems have shown positive β values, notably mixtures of hydrocarbon-chain and fluorocarbon-chain surfactants (14) . The only systems that we have discovered to date containing hydrocarbon-chain surfactants only are mixtures of long-chain carboxylates (soaps) and long^chain sulfonates. Under the proper conditions, these systems can show negative synergism. The conditions for negative synergism in the phenomena previously discussed are (12) : M

In surface of interfacial tension reduction efficiency: 1. β

must be positive.


21. ROSEN

325

Synergism in Binary Mixtures of Surfactants (For interfacial tension reduction, β positive). 2. β

σ

(or p °

L

or p

L

a

LL

0 Γ

β

LS

m u s t

be 0

L S

) must be greater than I ln (C^/C ^) I.

In mixed micelle formation: 1. p 2. p

M

M

(or p ^ L or p ^ s )

m u s t

1 x 5

(or p ^ or p \ s )

m u s t

b e

P S

o s i t i v e

-

r e a t e r t h a n

1l n

M

M

(C l/C 2> >·

In surface interfacial tension reduction effectiveness: 1. Ρ (or p ° positive.


σ

2. p

a

L

or P

L

- pM (or p

than iln ( C ° ] ,

c m c

a L

L

CT LS

) - p™

-ρ \

. C^/^,

0 1

υ

( o r

or p

p M ^ r p ^ g ) must be 0

a L

S

-

)

m u s t

be greater

™:^!.

σ

The condition that ρ - p ^ must be positive means that attractive interaction between the two different surfactants in the mixed micelle is greater than interaction in the mixed monolayer at the interface. As a result, upon micellization, surfactant is removed from the interface into the micelle, with a consequent reduction in surfactant concentration at the interface and increase in interfacial tension. This same phenomenon is the cause of the well-know "dip" in surface tension - concentration curves in the vicinity of the CMC of surfactants containing an impurity that is more surface-active. Upon micelle formation, the impurity is removed from the interface and solubilized into the micelles because of stronger interaction with the micellized surfactant than with the unmicellized surfactant at the interface, with a resulting increase in the surface tension of the system. We have found (15) that long-chain carboxylate- alkylbenzenesulfonate mixtures meet the conditions for and show negative synergism in surface tension reduction effectiveness, i.e., the surface tension of their mixtures at the CMC can be higher than that either the soap or the alkylbenzenesulfonate. We have also found (16) a good correlation between synergism in surface tension reduction effectiveness and initial foam height as measured by the Ross-Miles foaming technique. Systems that show synergism in surface tension reduction effectiveness also show synergism in foaming effectiveness (that is, the initial foam height of the mixture at a fixed surfactant concentration in the aqueous phase can be greater than that of either component by itself at the same surfactant concentration). On the other hand, when a system shows negative synergism in surface tension reduction effectiveness, then it also shows negative synergism in foaming effectiveness. The reduction in the foam of alkylbenzenesulfonate solutions by soap in the absence of Ca^, consequently, appears to be is due to the increase in the surface tension of the system, due to negative synergism in surface tension reduction effectiveness. Literature Cited 1. 2. 3.

Rubingh, D. In Solution Chemistry of Surfactants, Mittal, K.L., Ed.; Plenum: New York, NY, 1979, Vol 1; pp 337-354. Rosen, M.J.; Hua, X.Y. J. Colloid InterfaceSci..1982, 86, pp. 164. Rosen, M.J. Surfactants and Interfacial Phenomena; 2nd ed. John Wiley: New York, NY, 1989, pp 394-397.


326 4. 5.


6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.


Zhao, G.-X.; Chen, Y.-Z.; Ou, J.-G.; Tien, B.-S.; Huang, Z.-M. Acta ChimicaSinica 1980, 38, 409. Ding, H.-J.; Wu, X.-L.; Zhao, G.-X. Acta Chimica Sinica . 1985, 43, 603. Yang, W.-S.; Zhao, X.-G. Acta Chimica Sinica. 1985, 43, 705. Gu, B.; Rosen, M. J. Colloid InterfaceSci.1989, 129, 537. Hua, X.Y.; Rosen, M.J. J. Colloid InterfaceSci.1982, 90, 212. Rosen, M.J.; Gu, B. Colloids and Surf aces 1987, 23, 119. Rosen, M.J.; Murphy, D.S. J. Colloid Interface Sci. 1986, 110, 224. Rosen, M.J.; Gu,B.; Murphy, D.S.; Zhu, Z.H. J. Colloid Interface Sci. 1989, 129, 468. Hua, X.Y.; Rosen, M.J. J. Colloid Interface Sci. 1988, 125, 730. Rosen, M.J.; Murphy, D.S. J. Colloid Interface Sci. 1989, 129, 208. Zhao, G.-X.; Zhu, B.-Y. In Phenomena in Mixed Surfactant Systems, Scamehorn, J.F., Ed.; ACS Symp. Series 311; Amer. Chem. Soc., Washington, D.C., 1986, pp. 184-198. Rosen, M.J.; Zhu, Z.H. J. Colloid Interface Sci. 1989, 133, 473. Rosen, M.J.; Zhu, Z.H. J. Amer. Oil Chem. Soc. 1988, 65, 663.

RECEIVED January 22,

1992


Synergism in Binary Mixtures of Surfactants at Various Interfaces

Recommend Documents