3738 Langmuir 199410, 3738-3742

Quantification of Synergistic Interaction between Different Surfactants Using a Generalized Frumkin-Damaskin Adsorption Isotherm

R. Wustneck,*pt R. Miller,$ J. Kriwanek,$ and H.-R. Holzbauero

Department of Solid State Physics-Interfacial Dynamics, University of Potsdam, Rudower Chaussee 5, 12489 Berlin, Germany, Max Planck Institute of Colloid and Interface Science, Rudower Chaussee 5, 12489 Berlin, Germany, and Institute of Applied Chemistry, Rudower

Chaussee 5, 12489 Berlin, Germany

Received February 23, 1994. In Final Form: June 1, 1994@

On the basis of a generalized Frumkin-Damaskin isotherm, synergistic interactions between mixed surfactants adsorbed at fluid interfaces are discussed in terms of a mutual interaction parameter. By use of experimental a-log c curves of sodium dodecyl sulfate, oxethylated decanol(5EO)) and two homologous (N-n-alkyl-NJV-dimethy1ammonio)acetic acid bromides (at pH = 7 predominantly betains), the adsorption parameters of these surfactants were determined, including their self-interaction parameters. By use of these adsorption parameters, an additional interaction parameter was determined, which characterizes the interaction between two different surfactants. "his additional interaction parameter is small for mixtures of sodium dodecyl sulfate and the nonionic surfactant. It becomes considerable for mixtures of sodium dodecyl sulfate and (N-n-alkyl-NJV-dimethy1ammonio)acetic acid bromides, which indicates an additional synergistic attraction between the interacting molecules in the adsorption layer. The additional interaction parameters depend remarkably on the mixing ratio and correlate to the depression of the critical micelle concentration of these mixtures.

Introduction

Nonionics and zwitterionic surfactants like betaines are often applied in combination with anionic surfactant~l-~ as these mixtures exhibit properties which differ from those of the single components. Therefore, surface properties of relevant systems were in~es t iga t ed~ ,~ and approaches were used to quantify the interaction in such mixed systems.* Nonionics and anionics show competitive adsorption behavior in mixtures, accompanied by weak interaction between the surfactant^.^ Zwitterionic sur- factants and anionics however may form complexes in aqueous s o l u t i ~ n s . ~ ~ ~ J ~ J ~ As such complexes are more surface active than the single components, they may influence the adsorption behavior decisively. Tajima et a1.6 report a formation of 1: l complexes of 34dodecyl- ammoni0)propionate

H I I

R-NCCH2-CH2-CH2-COO-

H

with sodium alkyl sulfates (SDS), whereas the a-log c curves of the 1:l mixtures are shifted by 1 order of magnitude to lower surfactant concentrations in com- parison to the higher surface active component. For mixtures of the betaine-type

I

CH3

and SDS, the shift of the a-log c curves was found to be

t University of Potsdam. * Max Planck Institute of Colloid and Interface Science.

@ Abstract published inAdvance ACSAbstracts, August 1,1994. (1) Ploog, V. Seifen, Ole, Fette, Wachse 1982, 108, 373. (2) Miyazawa, K.; Ogawa, M.; Mitsui, T. Znt. J. Cosmet. Sci. 1984,

Institute of Applied Chemistry.

6, 33.

considerably ~ m a l l e r . ~ That may be explained by a smaller interaction as a result of the deteriorated accessibility of the quaternary ammonium atom for an electrostatic complex formation. The maximum shift was found at mixtures containing 60% betaine.

When the interaction in such systems is quanti6ed12 usually any complex formation is ignored. That may be reasonable when properties of the bulk phase are studied because the concentration of such complexes is consider- ably small.7 It can fail completely when interfacial properties are characterized, as these complexes may be the most surface active species in the system.

In a previous paper13 it was shown that the interaction between different homologs of (N-n-alkyl-NJV-dimethyl- ammonio)acetic acid bromides (at pH = 7 predominantly betaines14) can be characterized by using the generalized Frumkin-Damaskin isotherm in terms of an additional mutual interaction parameter. The same approach will be used here to characterize the interaction between homologous (N-n-alkyl-N,iV-dimethy1ammonio)acetic acid bromides at pH = 7 and SDS. As concluded by the results of Iwasaki et al.,' the interaction in this system should be small. A proper description of the adsorption behavior of

(3) Ernst, R.; Miller, E. J. In Amphoteric surfactants; Bluestein, B. R., Hilton, C. L., Eds.; Surfactant Sci. Ser. 12; Marcel Dekker: New YorkBasel, 1982.

(4)Aoki, Y.; Gomi, T.; Tokiwa, F. J. Jpn. Oil Chem. SOC. 1974,23, 737. (5) Tsujii, K.; Okahashi, K; Takeuchi, T. J . Phys. Chem. 1982,86,

1437. (6) Tajima, K.; Nakamura, A.; Tsutsui, T. Bull. Chem. Soc.Jpn. 1979,

52, 2060. (7) Iwasaki, T.; Ogawa, M.; Esumi, K.; Meguro, K. Langmuir 1991,

7, 30. (8) Rosen, M. J.; Zhu, B. Y. J. Colloid Znterfuce Sci. 1984,99, 427. (9) Rosen, M. J. ACS Symp. Ser. 1986, No. 311, 144. (10) Kolp, D. G.; Laughlin, R. G.; Krause, F. P.; Zimmerer, R. E. J .

(11) Rosen, M. J.; Friedman, D.; Gross, M. J . Phys. Chem. 1964,68,

(12) Holland, P. M.; Rubingh, D. N. J. Phys. Chem. 1983,87,1984. (13) Wiistneck, R.; Miller, R.; Kriwanek, J. Colloids Surf. 1993,81,

Phys. Chem. 1963, 67, 51.

3219.

1. (14) Wiistneck, R.; Kriwanek, J.; Herbst, M.; Wasow, G.; Haage, K.

Colloids Surf. 1992, 66, 1.

0743-7463/94/2410-3738$04.50/0 0 1994 American Chemical Society

Synergistic Interactions between Mixed Surfactants

these mixtures is possible only if the adsorption behavior can be described with respect to competitive adsorption only. Any adsorption of surface active complexes, which govern the adsorption behavior, is excluded to avoid problems with the theory as well as data interpretation. Normally the parameters of the adsorption isotherm of such a complex cannot be achieved independently. In addition, the presence of a third surface active component in the mixture would lead to further interaction param- eters which cannot be determined by measuring the adsorption isotherm of the mixture only. In the present case it is justified to use the generalized Frumkin- Damaskin isotherm without any problems for the char- acterization of interactions between anionic and nonionic surfactants in adsorption layers.

In the present paper we will report experimental data and surface interaction parameters calculated for mixed systems of oxethylated decanol(5EO) and SDS as well as of (N-n-tetradecyl- and N-n-hexadecyl-N,N-dimethyl- ammonio)acetic acid bromides and SDS at pH = 7. The results show that the interaction of these systems can be sufficiently quantified by using the generalized Frumkin- Damaskin approach.

The Generalized Frumkin-Damaskin Isotherm

The Frumkin-Damaskin isotherm15J7 takes into ac- count different values of the area per molecule for a solvent molecule To" and an adsorbed surfactant molecule Tl", an adsorption energy parameter bi, and one interaction parameter ai for the interaction between molecules i in the adsorption layer. These parameters can be determined by fitting the isotherm to the a-log c data of the single components of a mixture.

The generalized Frumkin-Damaskin isotherm16 for two coadsorbing components contains two sets of specific adsorption parameters each: Ti", ai, and bi as well as an additional interaction parameter a3. The parameter a3 characterizes the surface interaction between the two surfactants 1 and 2 in the adsorption layer:

exp(-2nla,8, - 2n,a38,) (1) 81 b,c, = n,(l - 8, -

and

exp(-2n,a28, - 2n,a38,) (2) 6, b,c, = n,(l - 8, -

were c1 and c2 are the concentrations of the surfactant in the mixture, Bi is the surface coverage of component i, and n, represents a coefficent taking into account the dis- placement of water molecules from the interface by adsorbed surfactant molecules, ni = To"/Ti".17 A value of 9.8 A2/molecule18 for the area occupied by one water molecule is used.

The integration of Gibb's equation using this isotherm leads to a relation between surface tension a and surface coverage Oil9

(15) Frumkin, A. N. 2. Phys. Chem. 1925,116, 466. (16) Damaskin, B. B.; Frumkin, A. N.; Borovaja, N. A. Elektrochimija

(17) Damaskin, B. B.; Frumkin, A. N.; Djatkina, S. L. Izu.ANSSSR.

(18) Fowkes, F. M. J . Phys. Chem. 1962,66, 385. (19) Damaskin, B. B. Elektrochimija 1969, 5, 346.

1972, 6, 807.

Ser. Chin. 1967, 2171.

Langmuir, Vol. 10, No. 10, 1994 3739

whereas

A = nlRITl" = n$lT," = RlT," (4)

When setting nl = n2 and a1 = a2 = a3 = 0, the Langmuir- type isotherm for surfactant mixtures results. By use of the adsorption parameters, the standard free energy of adsorption and the partial molar free energy of surface mixing can be calculated.20

The interaction parameters ai usually vary between 0 and 2.5 for attracting molecules in the case ni = 1.

In absence of any interaction between unlike molecules (a3 = 0) the generalized Frumkin-Damaskin isotherm should describe the adsorption behavior using only the two sets of parameters: Ti", ai, and bi; i = 1, 2. The parameter a3 can be determined by a fitting procedure minimizing the differences between the predicted isotherm and the experimental a-log c curve. In the special case a1 = a2 no additional (synergistic) interaction exists. Note, that in this case a1 = a2 = a3, because the interaction between like and unlike molecules is the same. A synergism exists if there is a difference between the calculated value of a3 and some kind of averaged value between a1 and a2.13 As a good approximation, ai was weighted by cibi - Bi. This leads to an average value aav, defined by

- a,b, + xa,b, aav- b ,+xb2 ( 5 )

with x = cz/c1, where c1 is the concentration of the higher surface active component of the mixture.

Synergism is defined by

a, = a3 - aav (6)

The case a, > 0 represents additional attraction.

Materials and Methods

The synthesis of the (N-n-alkyl-Nfl-dimethy1ammonio)acetic acid bromides was described in ref 14. The adsorption behavior ofthese surfactants can be characterized by a Frumkin isotherm; i.e. ni = 1.14

To prepare oxethylated decanol primer, decanol, purified by distillation, was oxethylated in the presence of 0.5 wt % NaOH (catalyst) at temperature 413 K. The degree ofoxethylation was controlled by increasing weight of reaction products. The average degree of oxethylation was about 5E0 units per molecule (GO- EO& Sodium dodecyl sulfate used was synthesized as described in ref 21.

Surface tension measurements were carried out using the ring method of Du Nouy as described in ref 14. The temperature of measurements was 283 K. The pH values of the (N-n-alkyl- Nfl-dimethy1ammonio)acetic acid bromide solutions were ad- justed using NaOH, as the pH values of the surfactant solutions were acidic (cf. ref 14). After adjustment of the pH a constant

(20) Schmaucks, G.; Sonnek, G.; Wiistneck, R.; Herbst, M.; Ramm,

(21) Wiistneck, R.; Miller, R.; Czichocki, G. Tenside, Surfactants, M. Langmuir 1992,8,1724.

Deterg. 1992,29, 4.

3740 Langmuir, Vol. 10, No. 10, 1994 Wustneck et al.

Table 1. Adsorption Parameters Determined by Using the Frumkin-Damaskin Isotherm of Sodium Dodecyl Sulfate, Oxethylated Decanol (5EO), UV-n-Tetra- and hexadecyl-N,iV-dimethy1a"onio)acetic Acid Bromides and Standard

Deviation, pH = '7 substances Ubi (IWcm3) Ti 10-lo (IWcm2) ai s (mN/m)

SDS, 0.01 Wdm3 NaBr (5.13 f 0.08) x 7.90 f 0.05 0.67 f 0.02 0.15 0.01 IWdm3 NaCl (5.15 f 0.08) x

C10E05 (1.70 f 0.03) x BHB 14 (2.35 f 0.04) x BHB 16 (5.78 f 0.09) 10-9

0 8 0 , I

-8 -7 -6 -5 -4 -3 -2 -1 Wh31

Figure 1. a-log c plot of oxethylated decanol(5EO) + SDS and their mixtures in 0.01 M NaCl aqueous solutions. c is the total surfactant concentration.

ionic strength of 0.05 M NaBr was fmed. Measurements for the system C10E05 + SDS were carried out in 0.01 M NaCl solutions.

Results and Discussion By use of the Frumkin-Damaskin isotherm, the

adsorption parameters of the single components of the mixtures were determined. The results are summarized in Table 1. a-logc curves were determined in 0.01 Wdm3 NaCl solutions for C10E05 and in 0.01 M/dm3 NaBr solutions for BHB 14 and BHB 16. The a-log c isotherm of SDS was determined in both 0.01 Wdm3 NaCl and 0.01 Wdm3 NaBr aqueous solutions. The differences between the two isotherms however are negligible. s is the standard deviation resulting from the difference between the measured and calculated a values. The standard devia- tions show that the a-log c plots (Figure 4 and 5) of the surfactants can be described sufficiently well by the Frumkin-Damaskin isotherm.

Using the standard deviation s, which reflects the average deviation between the experimental data and the isotherm approximated, the confidential intervals of the adsorption parameters were determined for a confidential level of 95%. A procedure of error propagation was described elsewhere.22

Figure 1 shows the a-logc plots ofoxethylated decanol, SDS, and their mixtures. The surface activities of the single components differ more than 2 orders of magnitude. Curve 1 indicates a slight minimum in the range of the critical micelle concentration (cmc). That is not unusual for oxethylated surfactants, as these surfactants usually contain small amounts of species with different degrees of oxethylation, which slightly differ in their surface activity. The maximum depression of the surface tension at cmc is higher for the oxethylated decanol than for SDS. The maximum depression realized by the mixtures obviously varies between these two levels, i.e. the maxi- mum surface tension depression of SDS and the oxethyl- ated decanol, respectively. Also the mixtures show a slight minimum in the a-log c curve close to the cmc, caused by the presence of the oxethylated decanol. Furthermore,

(22) Fiedler, H.; Wustneck, R.; Weiland, B.; Miller, R.; Haage, K. Langmuir, in press.

7.89 f 0.05 0.68 f 0.02 0.14 4.01 f 0.02 0.33 f 0.01 0.18 4.78 f 0.03 0.61 f 0.02 0.20 4.47 f 0.02 0.80 f 0.03 0.33

Table 2. Interaction Parameters Calculated for the Mixtures of Oxethylated Decanol + SDS and the

Standard Deviation s mixing S ratio x aav a3 a, (mN/m)

0.5 0.3301 f 0.010 0.8875 f 0.072 0.5574 f 0.073 0.40 1 0.3301 f 0.010 0.9875 z t 0.053 0.6574 f 0.054 0.31 10 0.3311 * 0.010 0.9575 & 0.102 0.6263 f 0.102 0.45 50 0.3357 f 0.010 0.8688 f 0.124 0.5331 f 0.124 0.61 100 0.3412 f 0.010 0.8594 f 0.173 0.5182 f 0.173 0.83 500 0.3796 f 0.009 0.7500 f 0.169 0.3704 f 0.169 0.83

the slopes of the a-log c plots of the single components differ. The slope is much steeper for SDS. Even at x = 500 (curve 7) the slope of the mixture indicates the influence of the surfactant with higher surface activity.

Table 2 summarizes the interaction parameters deter- mined by using eqs 1, 2, 3, and 6. The interaction parameters a3 slightly but significantly differ from the calculated values aav. Therefore, a small additional interaction is indicated by as, which depends on the mixing ratio and exceeds little the interaction determined for mixtures of the homologs BHB 12 and BHB 16.13 A small interaction between SDS and C10E05 is in good agreement with findings of synergism based on the theory of regular ~ o l u t i o n s . ~ J ~ ~ ~ ~

The confidential intervals (95%) ofaav were determined by using the standard deviations of the adsorption parameters of the single components and the Gauss distribution law.

The confidential intervals of a3 were estimated using the same procedure as used to determine the standard deviations of the single components, i.e. the isotherm was reconstructed using the parameters determined, al, bl, rl", a2, b2, Tz", and a3. The simulated curve consists of 20 pairs a,/c,, in which the al values are altered randomly in the range of f2s. s is the standard deviation achieved in the experiments (last column ofTable 2). The standard deviation ofaa is determined on the basis of 100 simulation runs.

Figure 2 gives the dependence ofthe relative adsorption, log Oi, on the total surfactant concentration, log c, for two mixing ratios. Regarding the difference of surface activity, the mixing ratio, and the concentration, a saturation level is reached for every component of the mixture. At x = 500 the relative adsorption of the less surface active compo- nent, SDS, exceeds the adsorption of the higher surface active component at higher concentration, i.e. to create a mixture 1:l at the interface, the concentration of SDS in the bulk phase should exceed the concentration of by more than 2 orders of magnitude and the total surfactant concentration in the bulk should be sufficiently high. Ageneral idea of the influence of different adsorption parameters on the adsorption of the single components of a mixture is given in ref 13.

The picture found for the mixtures of the (N-n-alkyl- NJV-dimethy1ammonio)acetic acid bromides and SDS differs from the former case. Figures 3-5 show different a-log c plots for mixtures of BHB 14 + SDS and BHB 16

(23) Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1982,90,212.

Synergistic Interactions between Mixed Surfactants

J m I

-2 I 1

-5 -4 -3 -2 b g c [Wdd

Figure 2. log @-log c plot of oxethylated decanol + SDS in 0.01 M NaCl aqueous solutions for the mixing ratios x = 100 and 500, respectively: 81, relative adsorption ofthe oxethylated decanol; 82, relative adsorption of SDS.

I

-8 -1 -6 -5 -4 -3 -2

b O c IWd4

Figure 3. a-log c plot of BHB 14 + SDS mixtures in 0.01 M NaBr aqueous solutions: c, total surfactant concentration; x = c&; c1, BHB 14 concentration; c2, SDS concentration.

" Q n " -.I I

V

-1 -6 -5 -4 -3 -2 -1 I W d d

Figure 4. a-log c plot of BHB 14 + SDS and their mixtures in 0.01 M NaBr aqueous solutions: c, total surfactant concen- tration; x = cz/c1; c1, BHB 14 concentration; c2, SDS concentra- tion.

+ SDS. The maximum reduction of surface tension is larger for the mixtures than for the single components, whereas the cmc's of some mixtures are lower than the cmc of the component of higher surface activity. Such results are well-known and in agreement with the findings of other authors7 and are usually explained by association of surfactants induced by electrostatic attraction which is more favorable between the cationic section of the betaine and the sulfate group in the mixed system than in solutions of the single components. Furthermore the minimum of surface tension in the vicinity of the cmc of some mixtures is reported in refs 8 and 24 and well

60 :

50 :

40 :

30 :

Langmuir, Vol. 10, No. 10, 1994 3741

F ( 1-0.5

a-1 I

-2 -1

log c (Wdd]

-8 -7 -6 -5 -4 -3

Figure 6. a-log c plot of BHB 16 + SDS and their mixtures in 0.01 M NaBr aqueous solutions: c, total surfactant concen- tration;x = cdc1; c1, BHB 16 concentration; c2 SDS concentration.

Mixtures of BHB 14 + SDS and the Standard Deviation s Table 3. Interaction Parameters Calculated for the

mixing ratio x aav a 3 as s (mN/m)

0.75 0.610 f 0.020 2.431 f 0.193 1.821 f 0.194 0.85 1 0.610 f 0.020 2.581 f 0.093 1.971 f 0.095 0.54 1.5 0.610 f 0.020 2.669 f 0.047 2.059 f 0.051 0.23 4 0.611 f 0.020 2.184 f 0.170 1.573 f 0.171 0.81 8 0.612 f 0.019 1.800 & 0.168 1.188 f 0.169 0.83 9 0.612 f 0.019 1.906 f 0.102 1.294 f 0.104 0.48

15 0.614 f 0.019 1.838 f 0.114 1.224 f 0.116 0.58 50 0.621 f 0.017 1.550 f 0.058 0.929 f 0.060 0.38

100 0.629 f 0.015 1.537 f 0.057 0.908 f 0.059 0.36 500 0.652 f 0.015 1.469 f 0.053 0.817 f 0.055 0.32

Table 4. Interaction Parameters Calculated for the Mixtures of BHB 16 + SDS and the Standard Deviation 8

mixing s (mN/m) ratio x aav a 3 as

0.5 0.800 f 0.030 2.463 f 0.027 1.663 f 0.040 0.23 1 0.800 f 0.030 2.869 f 0.092 2.069 f 0.097 0.46 4 0.799 f 0.030 2.544 f 0.087 1.745 f 0.092 0.41

10 0.798 f 0.030 2.263 f 0.073 1.464 f 0.079 0.33 1000 0.731 f 0.018 0.900 f 0.082 0.169 f 0.084 0.47

interpreted for mixtures in ref 25. It indicates the presence of a component of higher surface activity than the main component. Provided there is no complex formation, i.e. a formation of a component higher surface active than BHB 14, the minimum in the a-log c curves should disappear a t sufficiently high concentrations of BHB 14. This is exactly what can be seen in the figures.

Tables 3 and 4 summarize the interaction parameters calculated for the systems BHB 14 + SDS and BHB 16 + SDS, respectively. The standard deviations which result from the differences between the experimental points of the a-logc plots ofthe mixtures and the best fit calculated by using the adsorption parameters of the single com- ponents and eqs 1-3 are larger than those usually achieved by fitting the a-log c plots of the single components. Nevertheless the approximation describes the adsorption behavior sufficiently well. The interaction parameters are generally larger than those found for the system C10E05 + SDS. Because of the big differences between the surface activities of the components, the parameter aav is almost independent of the mixing ratio, with the exception of the range of high SDS excess. As a, =. a3, a real additional attractive effect is evident, which is larger for the BHB 16 + SDS system. Here, the interaction parameters a3 and a, show a pronounced dependence on the mixing ratio. The largest effects are found at mixing ratios of x = 1 and x = 1.5.

(24) Abe, M.; Kato, L; Ogino, K. J . Colloid Interface Sci. 1989,127, 328. (25) Rusanov, A. I. Adv. Colloid Interface Sei. 1993,45, 1.

3742 Langmuir, Vol. 10, No. 10, 1994 Wiistneck et al.

* l 1

1 I 0

Figure 6. Dependencies of a, and log cmc on the molar ratio for the systems BHB 16 + SDS and BHB 14 + SDS, pH = 7, 0.01 M/dm3 NaBr.

It should be noticed that the limiting values for a3 are given by lime,-I a3 = a1 and limsT1 a3 = a2 and aay varies between them. The physical meaning of the interaction parameter in the Frumkin equation is to express a deviation from the ideal state by a parameter which takes into account the interaction between identical molecules. For the generalized equation a3 expresses the interaction between nonidentical molecules. This parameter however should be constant, independent of the mixing ratio. The pronounced dependence of the interaction parameter a3 on the mixing ratio means that this parameter contains also a structural information. The deviation between the adsorption isotherm measured and predicted for the case aay is the largest at a definite mixing ratio, i.e. conditions, which produce the requirement for an optimal interaction. These conditions obviously occur in vicinity of x = 1.

Figure 6 gives the dependencies of the cmc's as well as the interaction parameters a, on the molar ratio x . This figure shows the largest depression of the cmc at a molar ratio between 0.5 and 0.6. This result is in good agreement with the findings of Iwasaki et al.,'who found a maximum depression of the cmc at a molar ratio of 0.6. Taking into account the standard deviations ofthe 0-logc plots, which were used to calculate the cmc values, an accurate determination of the molar ratio of the largest effect is not possible. Furthermore, the region of maximum cmc depression corresponds to the largest values of a,. The value a, is zero at cd(c1 + CZ) = 0 and cd(c1 + CZ) = 1, respectively. Even at cd(c1+ c2) = 0.95, a, > 0; i.e. there is still an pronounced additional attraction. When comparing the effects found for the systems BHB 14 + SDS and BHB 16 + SDS, it can be seen that the effect of cmc depression is more pronounced for the system BHB 16 + SDS.

-2 I I I I I I

-6 -5 -4 -5 -4 -3 -2

log c Fl/dn'?l Figure 7. log &log c plot of BHB 14 + SDS in 0.01 M NaBr aqueous solutions for the mixing ratios 2 = 1,9,100, and 500, respectively: 81, relative adsorption of BHB 14; 82; reltaive adsorption of SDS.

Figure 7 shows log @-log c plots for systems BHB 14 + SDS for different mixing ratios. With increasing contents of SDS the relative adsorption of SDS increases and exceeds the adsorption of BHB 14 at a mixing ratio x = 500.

Conclusions

The influence of interaction between surfactants in mixtures on the adsorption properties can be quantified by a mutual interaction parameter in the Frumkin- Damaskin isotherm. In the case of oxethylated decanol and sodium dodecyl sulfate only a small additional interaction has been found. In the case of homologous (N-n-alkyl-N&-dimethy1ammonio)acetic acid bromides at pH = 7, where these surfactants are predominantly betains, and sodium dodecyl sulfate a synergistic attrac- tion is found. The adsorption behavior of these mixtures can be described on basis of competing adsorption of the two surfactants in the mixture, while the formation of additional surface active complexes by these surfactants can be neglected. The interaction parameter depends on the mixing ratio, whereas this dependence correlates with the depression of the cmc of these mixtures. Therefore, it is assumed that the interaction parameter contains not only information about intermolecular interaction but also information about optimal conditions for interaction at the interface.

Acknowledgment. The project was sponsored gener- ously by the Senate of Berlin (ERP-2659).