Mixed Surfactant Systems - American Chemical Society

Chapter 28

Effects of Structure on the Properties of Pseudononionic Complexes of Anionic and Cationic Surfactants Ammanuel Mehreteab

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Colgate-Palmolive Company, Corporate Technology Center, 909 River Road, Piscataway, NJ 08855-1343 Soluble anionic/cationic surfactant complexes can be formed from anionic and cationic surfactants either or both of which have hydrophilic group in addition to their charged heads. These complexes exhibit properties that are more similar to nonionic surfactants than to their ionic components. For example they exhibit cloud point phenomena and have low critical micelle concentration. In addition, the area of the hydrophilic group was much less than the sum of the areas of the anionic and cationic surfactant components. Surfactants are unique as a class of compounds because they are soluble both in organic solvents and water. Their solubility in hydrocarbon solvents is due to their hydrophobic chain. Their solubility in water is due to the polarity and/or charge of their head group. The size of die hydrophilic group determines the degree of solubility of nonionic surfactants. Anionic and cationic surfactants are soluble due to their negative and positive charges respectively. When anionic and cationic surfactants are mixed the charges are neutralized and, consequently, the solubility is diminished. The resulting complex precipitates (Figure 1). Because of insolubility there had not been many studies of mixtures of anionic and cationic surfactants. Recently, however, some anionic/cationic surfactant salts have been studied in detail. For example, the precipitation phase boundaries for sodium alkyl sufate/dodecylpyridium chloride were measured over a wide range of surfactant concentrations as a function of pH, temperature, and anionic surfactant alkyl chain length (1). The surface concentrations and molecule interactions in anionic-cationic mixed monolayers at various interfaces were studied (2). The surface activity and micellization were studied for systems of different hydrophobic chain length symmetry (3,4). Anionic/cationic surfactant complexes, though very surface active are rarely used as surfactants because of their low solubility. Recently, however, we have introduced soluble anionic/cationic surfactant complexes which are effective and efficient surfactant systems (5). We called these complexes pseudo-nonionic because they behaved more like nonionic surfactants than ionic surfactants. The criterion for 0097-6156/92/0501-0402S06.00/0 © 1992 American Chemical Society

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

28.

Pseudononionic Complexes of Surfactants

MEHRETEAB

403

preparing these complexes was also given, which is, "if either the anionic surfactant or the cationic surfactant or both have hydrophilic group in addition to their charged head groups, the resulting neutralized complex would be water soluble like nonionic surfactants if the additional hydrophilic group is large enough" (Figure 2). The additional hydropilic group can be any charged group or noncharged polar group. In this paper, previously reported results of surface and interfacial tensions and cloud point phenomena (5) and new investigations of additional properties of the complexes such as head group area and dynamic properties will be given.


Materials And Methods Materials. The structure and abbreviations of the surfactants used in this study are shown in Table I. These surfactants can be classified as anionic and cationic surfactants. And within each class they can be categorized as ethoxylated and nonethoxylated. Table I. Surfactant Structure and Abbreviation Structure CH (CH ) 3

2

1 3

Abbreviation STS

S0 Na 4

AEOS

CH (CH ) (OC H ) S0 Na 3

CH

CH

3

3

2

m

2

4

n

4

(CH ) N - ( C H ) B r 2

m

3

3

(CH ) N - ( C H ) C 1 2

m

3

2

m=ll

LTAB

m=13

MTAB (TTAB)

m=15

CTAB

m=ll-15

VAR

m+n=2

EQC(2ÈO)

m+n=5

EQC(5EO)

m+n=15

EQC(15EO)

m+n=2

EQ18(2EO)

CH,

à '

x+l=coco (C H 0) H 2

CH 3 (CH

2

4

^ -N-CH

m

3

CI

(C H 0) H 2

x+l=18

4

n

m+n=5

EQ18(5EO)

m+n=15

EQ18(15EO)


MIXED SURFACTANT SYSTEMS

404

Surfactant

Solubility due to: - charge


+ charge Large hydrophilic group Cationic surfactant

Anionic Surfactant

Insoluble Complex (charges neutralized) Figure 1. Surfactants and solubility.

Figure 2. Pseudo-nonionic anionic/cationic complexes.



28.

MEHRETEAB

405


The anionic surfactants used were sodium tetradecyl sulfate (STS) from Eastman Kodak (Rochester, NY), alkylpolyethoxy(~9EO) sulfate (AEOS) i.e Alfonic 1214-65, with a carbon chain length of 12 to 14 and 65% degree of ethoxylation, from Vista Chemical Co., and an alkylphosphate ester (APE) i.e EMPHOS PS 236 from Witco Chemical Co. (Perth Amboy, NJ) which is a mixture of mono- and diester phosphate of hydroxy-terminated alkoxide condensate. The nonethoxylated cationic surfactants used were lauryltrimethylammonium bromide (LTAB), myristoyltrimethylammonium bromide (MTAB also abbreviated TTAB for tetradecyltrimethylammonium bromide) from Sigma Chemical Co.(St. Louis, MO), cetyltrimethylarnmonium bromide (CTAB) from Aldrich Chemical Co. (Milwaukee, WI) and Variquat 50MC from Sherex Chemical Co. (Dublin, OH). Variquat 50MC is composed of 50% of alkyl(50% C, , 40% C , 10% C ) dimethylbenzylammonium chloride, 7.5% isopropyl alcohol and 42.5% H20. Ethoxylated cationic surfactants used were Ethoquad 18/12, Ethoquad 18/15, Ethoquad 18/20, Ethoquad 18/25, (methylbis(x-hydroxyethyl)octadecylammonium chloride where χ = 2, 5, 10 and 15 respectively) and Ethoquad C/12, and Ethoquad C/25 (methylbis(x-hydroxyethyl)cocoammonium chloride where χ = 2 and 15 respectively). These ethoxylated cationic surfactants were obtained from Akzo Chemie America (ARMAK Chemicals) as approximately 95% solutions. All the above surfactants were used as supplied from the companies without further purification. It is to be understood, therefore, the quantitative results are subject to error based on the lack of purity. However, we do not believe the error substantially changes the interpretation of the results. 4

12

16

Method. Cloud point temperature and equilibrium and dynamic surface and interfacial tensions were measured as follows: Cloud Point Temperature. Solutions containing both anionic and cationic surfactant were placed in vials and were heated slowly in a water bath while monitoring their temperature. The temperature at which the solutions turned cloudy were recorded as their cloud point temperatures. Surface Tension. Several single and mixed surfactant solutions were prepared and their equilibrium and dynamic surface tensions and interfacial tensions were measured. Equilibrium surface tension was measured using Kruss Digital-Tensiometer K10T. Dynamic surface tension was measured using a modified SensaDyne Bubble Tensiometer using only the smaller orifice. The bubble rate was monitored using an oscilloscope connected to the pressure transducer of the instrument. Interfacial Tension was measured using the spinning drop tensiometer, hexadecane was the oil phase. Results And Discussion Formation of Soluble Complexes. When a neutral TTAB solution is added to an already acidic APE solution , the pH decreased (figure 3) indicating the replacement of the proton associated with the APE by TTAB resulting in the formation of APE/TTAB complex as shown in the equation below:


406


+

CH (CH ) (OC H ) -0-P-OR 3

2

m

2

4

CH (CH ) N-(CH >

m

3

OH (APE)

9

CH (CH ) (OC H ) -0-P.OR 3

2

m

2

4

13

3

3

H

C H (CH ) N^(CH ) 2

13

•

m

γ; 3

2

(TTAB)

3

3


(APE/TTAB)

If indeed a complex containing oxyethylene groups is formed, then we reasoned it should exhibit cloud point phenomena like ethoxylated nonionic surfactants. This is found to be so as shown in figure 4 (APE/TTAB at different pH's). pH affects the of dissociation of APE and, therefore, the neutraliztion point with TTAB. The decrease in pH of an APE solution when TTAB is added to it and the exhibition of cloud point phenomena for mixtures of APE/TTAB is an indication that pseudo-nonionic complexes have been formed. Cloud point phenomena is exhibited only by the complexes and not the surfactant components. Figures 5 and 6 show cloud point temperature vs. anionic mole fraction for two systems of anionic/cationic solutions where in one case the additional hydrophilic group is carried by the anionic surfactant and in the other case it is carried by the cationic surfactant respectively. The exhibition of cloud point phenomena of anionic/cationic surfactant solutions of compositions of around 1:1 mole ratio is another indication that a pseudo-nonionic complex is formed. Any composition that deviates from the 1:1 mole ratio can be assumed to be a mixture of the pseudo-nonionic complex and the ionic surfactant in excess. The micelles of such mixtures are charged and result in higher cloud point temperature. This is similar to nonionic surfactants where addition of ionic surfactants raised their cloud point temperature (6). The variables that affect cloud point values and other properties of the pseudo-nonionic complexes were studied with the results reported below. Cloud Point Temperature. As with nonionic surfactants the cloud point temperature of the pseudo-nonionic surfactant complexes are dependent on total surfactant concentration and structure. Effect of Surfactant Concentration. The cloud point temperature of the anionic/cationic surfactant mixtures depended on the total surfactant concentration and the relative concentrations of the anionic and cationic surfactants. Solutions with excess anionic surfactant showed one minimum in their cloud point temperature vs. total surfactant concentration (Figure 7). The cloud point temperature of the minimum increased with increase in the anionic surfactant mole fraction. Solutions with excess cationic surfactant showed two minima (Figure 8). The cloud point temperature of the minima remained fairly constant. However, the maximum between the minima increased with increase of the cationic mole fraction.


MEHRETEAB


407


28.

Figure 4. Effect of pH on cloud point temperature of APE/TTAB solutions. (Reproduced with permissionfromref. 5. Copyright 1988 Academic Press, Inc.)


408



100

"g

ο ϋ

20 -

ο

I

0

.

.

0.2

,

.

.

.

0.4 0.6 AEOS Mole Fraction

.

.

0.8

.

1

1

Figure 5. Cloud point temperature vs. AEOS mole fraction of AEOS/TTAB solutions. (Reproduced with permissionfromref. 5. Copyright 1988 Academic Press, Inc.) 100

0.4 0.6 STS Mole Fraction

Figure 6. Cloud point temperature vs STS molefractionof sodium tetradecyl sulfate/ethoxylated quat.



28. MEHRETEAB


Figure 7. AEOS/TTAB + AEOS. Cloud point temperature vs. concentration. (Reproduced with permission from ref. 5. Copyright 1988 Academic Press, Inc.)

Figure 8. AEOS/TTAB + TTAB. Cloud point temperature vs. concentration. (Reproduced with permission from ref. 5. Copyright 1988 Academic Press, Inc.)


409



410

Effect of Structure. As with nonionic surfactants, the cloud point temperature of the pseudo-nonionic complexes decreased with increase in the hydrophobicity of their surfactant components. The cloud point temperature of mixtures of AEOS and alkyltrimethylammonium bromides of different chain length is shown figure 9. The cloud point temperature decreased with approximately 10 degrees centigrade for every increase of methylene group. The cloud point temperature of the anionic and cationic mixtures increased with increase in the hydrophilicity of the surfactant components. The cloud point temperature of mixtures of AEOS and two ethoxylated cationic surfactants with 3 and 5 ethylene groups is shown in figure 10. The cloud point temperature increased by more than 60 degrees Centigrade for an increase of two oxyethylene groups. These results are similar to those of ethoxylated nonionic surfactants whereby their cloud point decreases with increase in carbon chain length and their cloud point increases with increase in number of oxyethylene groups. Equilibrium Surface and Interfacial Tensions. Equilibrium surface tension measurements showed that the pseudo-nonionic complexes are more efficient and effective than either of their ionic surfactant components, i.e. they have lower critical micelle concentration and lower attainable surface tension. The results are shown in figure 11. The interfacial tension between hexadecane and solutions of AEOS/TTAB is shown in figure 12. It was observed that solutions with AEOS molefractionsof around 0.5 have interfacial tensions which are 10 to 30 fold smaller than those of their cationic (TTAB) and anionic (AEOS) surfactant components. There is no doubt that the pseudo-nonionic surfactant complexes, though soluble, are more surface active than their components. Dynamic Surface Tension. Dynamic surface tension is found to be important to the flash foam and other properties of surfactants. Recently, a thorough theoretical investigation was made on the dynamic surface properties of mixed anionic-cationic surfactant solutions (7). The dynamic surface tension and the surface adsorption kinetics of the aqueous solutions of some anionic-cationic surfactant mixtures have been studied (8) using the oscillating jet method. In our study we used the maximum bubble pressure method to measure the dynamic surface tension of anionic and cationic surfactants and their mixtures at different bubble rates. As shown in figure 13, the dynamic surface tension of the pseudo-nonionic complex was found to be much lower than either of its surfactant components. It is observed that dynamic surface tension depends on the bubble rate and the nature of the surfactant. The dependency of dynamic surface tension on the property of the surfactant, specifically static equilibrium surface tension and diffusion coeficient and the bubble rate is given by the equation below (9):

where γ and y are the dynamic and static surface tensions respectively, n=l for nonionic surfactants and n=2 for ionic surfactants, R is gas constant, Τ is temperature, e


28.

MEHRETEAB


^100 h


S

LTAB/AEOS

Q

TTAB/AEOS

d 60

CTAB/AEOS VAR/AEOS

β

. 1» ϋ

c 40 δ Q_

"§ 20 o

ϋ 0.2

0.4 0.6 Cationic Mole Fraction

0.8

Figure 9. Effect of hydrophobic group on cloud point temperature.

100

0.2 0.4 0.6 0.8 Cationic Surfactant Mole Fraction

Figure 10. Effect of hyrophilic group on cloud point temperature.


411

412



80

20

'

'

1E-07

'

1

ι ι ι mill

1E-06

ι ι ι mill

ι ι ι mill

ι ι ιmal

1 ι 111 m l

L

1E-05 1E-04 1E-03 1E-02 1E-01 Surfactant concentration (Molar)

Figure 11. AEOS, TTAB and AEOS/TTAB solutions. Surface tension vs. concentration. (Reproduced with permissionfromref. 5. Copyright 1988 Academic Press, Inc.)

8

TTAB Mole Fraction

Figure 12. AEOS/TTAB. Interfacial Tension vs. TTAB mole fraction. (Reproduced with permissionfromref. 5. Copyright 1988 Academic Press, Inc.)


28. MEHRETEAB


413

ε 60

S EQC(15E0) 5,55

1 50 2 45

re°—&

„

J

B

ν·....

AE0S/EQC(15E0)

•2 40

-AE0S/EQC(2E0)

D CO

§ 35 Downloaded by UNIV OF ARIZONA on August 9, 2012 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch028

EQC(2E0) AEOS

ι

(0

10

£30

ι

I

20 30 Time/drop (seconds)

40

Figure 13. Dynamic surface tension of 0.02M total surfactant solutions.

Γ is adsorption, C is concentration, D is diffusion coefficient and t is time between bubbles. m

A plot of γ vs t" is linear as shown in figure 14 below. Molecular area. The surface tensions, γ , of several cationic surfactants and their complexes with AEOS were measured as a function of their concentrations, C. Using Gibbs equation (where n=l for nonionic and n=2 for ionic surfactants) die surface excess concentration, Γ, was calculated:

r=

1 (-EL.) 2.303n/?r^31ogC J,

From the surface excess concentration, Γ , the area, a', of their hydrophilic group was obtained using the following equation: 16

NT

where Ν is Avogadro's number. The pseudo-nonionic complexes were found to have a much lower area than the sum of the head areas of their anionic and cationic surfactant components. Figure 15 shows the head area for the surfactants studied. The reduction in areas per mole in mixed anionic/cationic surfactants at the air-solution interface was previously observed for nonethoxylated systems. For example, a decyltrimethylammonium/decyl sulphate complex gave an area which is about 70% of that corresponding to unmixed components (10,11).


414


60


.—-a"'

m l^-'ÂEOS ^••èk EQC(2EO) —Θ—

--Α-EQC(15EO) AEOS/EQC(2EO) 30

0.5

AEOS/EQC(15EO) 1 1.5 2 2.5 SQRT(No. of drops/sec)

Figure 14. Dynamic surface tension of 0.02M total surfactant solutions.

Figure 15. Effect of hydrophobe and hydrophile size on molecular area at interface.


28.

MEHRETEAB


415


Conclusion Anionic-cationic surfactant complexes, often named catanionic, are usually too insoluble to be used as surfactants in aqueous solutions. Recently, however, we have introduced soluble anionic-cationic surfactant complexes, which in many ways behave like nonionic surfactants, thus named pseudo-nonionic surfactant complexes. Like nonionic surfactants they exhibit cloud point phenomena unlike their ionic surfactant components. Factors that affect the cloud point temperature of nonionic surfactants, such as structure and concentration, influence the cloud point temperature of pseudo-nonionic complexes. Pseudo-nonionic complexes are more effective and efficient than their ionic surfactant components as shown by their equilibrium and dynamic surface tensions and interfacial tensions. They pack at the interface more than their ionic components. Since, pseudo-nonionic complexes show their own characteristics, they can be treated as sepearate classes of surfactants distinct from ionic and nonionic surfactants. Acknowledgment I wish to thank Dr. F. J. Loprest for many helpful discussions. Literature Cited (1) Amante, J. C.; Scamehorn, J. F.; Harwell, J. H. J. Colloid Interface Sci. 1991, 144, 243. (2) Gu, B.; Rosen M. J. J. Colloid Interface Sci. 1989, 129, 537. (3) Yu, Z.; Zhao, G. J. Colloid Interface Sci. 1989, 130, 414. (4) Yu, Z.; Zhao, G. J. Colloid Interface Sci. 1989, 130, 421.

(5) (5) (6) (7) (8)

Mehreteab, Α.; Loprest, F.J. J. Colloid Interface Sci. 1988, 125, 602. Mehreteab, Α.; Loprest, F.J. J. Colloid Interface Sci. 1988, 125, 602. Nakama, Y.; Harusawa, F.; Murotani, I. J. Amer. Oil Chem.Soc.1990, 67, 717. Joos, P.; Hunsel,J.;Bleys, G. J. Phys. Chem. 1986, 90, 3386. Zhang, L.; Zhao, G. J. Colloid Interface Sci. 1988, 127, 353.

(9)) Hansen, R. J. Phys. Chem. 1960, 64, 637. (10) Holland, P. M. In Phenomena in Mixed Surfactant Systems; Scamehorn J. F.,

Ed.; ACS Symposium 311; Amer. Chem. Soc: Washington, DC, 1986; p. 102. (11) Corkill, J. M.; Goodman, J. F.; Ogden, C. P.; Tate, J. R. Proc. T. Soc. London, Ser. A 1963, 273, 84. RECEIVED

March 9, 1992


Mixed Surfactant Systems - American Chemical Society

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