C O L L O I D A L A S S O C I A T I O N A N D B I O L O G I C A L A C T I V I T Y O F SOME RELATED QUATERNARY AMMONIUM S A L T S l

Sydney Ross, C. E. Kwartler 2 and John Hays Bailey

Rensselaer Polytechnic Institute, Troy, New York, Winthrop Stearns, Inc., and Sterling-Winthrop Research Institute, Rensselaer, New York

Received January 2, 1958

ABSTRACT

Two series of cationic surface-active agents, benzyldimethylalkylammonium chlorides and 4-nitrobenzyldimethylalkylammonium chlorides, where the alkyl group varies from a 6-carbon-atom straight chain to an 18-carbon-atom straight chain, have been prepared. The critical concentration for the formation of micelles has been measured by the dye method for each member of each series. As has been discovered in other cases of long-chain compounds (9, 10), here too there exists a linear relation between the number of carbon atoms in the alkyl group and the logarithm of the critical micelle concentration.

A third series of cationic surface-active agents has been prepared, similar to the series above save that the variation in structure is obtained by varying the nature or position of the substituent in the benzyl group, while the alkyt group is lauryl throughout. The critical mieelle concentration of each member of the series has been measured by the dye method. The critical micelle concentration thus obtained varies With the polarity of the benzyl portion of the molecule. Measures of this polarity are provided by the boiling points, melting points, dielectric constants, or dipole moments of the toluene compounds that correspond to the benzyl portions of the quaternary ammonium salts. I t is found that the critical micelle concentrations of the quaternary ammonium salts are proportional to the square of the dipole moment of these corresponding substituted toluenes.

Correlations with microbiological activities of these quaternary salts have been attempted.

INTRODUCTION

T h e re la t ion be tween molecu la r s t r uc tu r e and cr i t ica l micel le concen t ra - t ion ( C M C ) has been i nves t i ga t ed for a few t y p e s of compounds , such as o r d i n a r y po t a s s ium soaps and su l fa ted f a t t y alcohols. These p rev ious s tudies have es tab l i shed a loga r i thmic re la t ion be tween the C M C and the n u m b e r of ca rbon a t o m s in t he a l ipha t i c -cha in por t ion of t he molecule. The p resen t work has to do wi th some re la ted q u a t e r n a r y a m m o n i u m sal ts

i Presented in part before the Division of Industrial and Engineering Chemistry at the 122nd Meeting of the American Chemical Society, Atlantic City, N. J., Sep- tember 17, 1952.

Present address: Gamma Chemical Co., Great Meadows, N. J.

385

386 ROSS, KWARTLER AND BAILEY

containing an aliphatic group, a benzyl group, and two methyl groups, and the effect on the CMC of variations in the length of the aliphatic-chain group and of substituents in the benzyl group. The microbiological activi- ties of these compounds have also been measured for comparison with their tendency toward formation of colloidal micelles. A similar comparison has been undertaken by Celia et al. (1) for a series of quaternary ammonium salts closely related to those of the present investigation. The conclusions of the earlier work have been verified and extended.

EXPERIMENTAL

(a) Materials

One of the groups of compounds studied was a series of 4-nitrobenzyl- dimethylalkylammonium chlorides. In this series the alkyl group was varied from a normal 6-carbon straight chain to a normal 18-carbon straight chain. The general method of preparation of this series of compounds occurs in two steps:

1. Preparation of Alklydimethylamines. This preparation in general pro- ceeded through the reaction of the corresponding alkyl chloride with dimethylamine. This reaction was carried out in a steel autoclave, using an excess of dimethylamine. The tertiary amines were separated by the usual physical means and were then carefully purified, before they were used for the preparation of quaternary ammonium compounds. Our experience indi- cated that this purification of these intermediates was essential to the preparation of pure quaternary ammonium compounds. Each of the tertiary amines was purified and analyzed prior to their utilization as intermediates.

2. Reaction with p-Nitrobenzyl Chloride. The analytically pure tertiary amines were then caused to react with purified p-nitrobenzyl chloride. This reaction was carried out in conventional solvents such as acetone, ether, isopropyl alcohol, benzyl alcohol, and others. Under the proper operating conditions, the quaternary ammonium compounds frequently set- tled out as crystalline solids. The further purification of these quaternary ammonium compounds was accomplished by crystallization from such solvents as isopropyl alcohol, acetone, benzene, petroleum ether, or ethyl acetate. All the compounds so prepared have been purified to constant melting point.

The second group of compounds was a series of substituted dodeeyl- dimethylamine derivatives. In this series of compounds, various substituted benzyl chlorides have been caused to react with purified dodecyldimethyl- amine. The general method of preparation of this series of compounds occurs in two steps:

1. The Preparation of Dodecyldimethylamine. This preparation was ac- complished by the action of dodecyl chloride with dimethylamine under

COLLOIDAL ASSOCIATION AND BIOLOGICAL ACTIVITY 387

pressure in a steel autoclave. This compound was also prepared by the action of formaldehyde and formic acid on dodecylamine. In both methods of preparation, great care was taken to prepare unequivocally pure dodecyl- dimethylamine. I t was only after vigorous physical and chemical purifica- tion was effected, that the tertiary amine was considered suitable for use in quaternary ammonium compound formation.

2. Purification. The various substituted benzyl chlorides were all purified prior to their use. The 2-chloro, 4-chloro, 2,4-dichloro, and 3,4-dichloro- benzyl chlorides were carefully purified by fractional distillation. The 4-nitrobenzyl chloride and 2-hydroxy-5-nitrobenzyl chloride were crys- tallized prior to use. The 3,4-dimethoxy and 3,4-methylenedioxybenzyl chlorides were crystallized. The quaternization reactions were carried out as described above. In this series, too, extreme care was necessary and extensive purification resorted to in order to obtain pure crystalline com- pounds.

All of the compounds described in this paper gave satisfactory analyses for nitrogen and halogen. Formulas, analyses, and CMC values in moles per liter for the series of salts are given in Table I.

(b ) Bacteriological Technique

The minimum killing concentration was determined as follows. Various dilutions of the quaternary ammonium compound in water were prepared, and 5-ml. portions of each were brought to 20°C. in a water bath. A 22-26- hr. broth culture of the test organism was transferred to a sterile 2-oz. serum bottle containing a shallow layer of sterile 3-mm. glass beads, stop- pered with a sterile rubber stopper, and shaken in a clinical shaking ap- paratus for 2 min. After reaching 20°C., 0.5 ml. of culture was added to each of the tubes containing the diluted quaternary compound, and the contents were mixed by gentle twirling of the tubes in the bath. At the end of 5, 10, and 15 rain. exposure to the action of the quaternary am- monium compound, one standard 4-mm. loopful of the mixture was re- moved to culture tubes containing 10 ml. of nutrient broth (peptone 1%, meat extract 0.5 %, NaC1 0.5 %) and these tubes were incubated at 37°C. for 48 hr. That concentration killing the test organism in 10 min., but not in 5 min., was considered the minimum killing concentration.

The test organisms employed, Micrococcus pyogenes var. aureus 209 and Salmonella typhosa Hopkins, were killed in 10 rain., but not in five, by a 1: 60 to 1: 65 and 1 : 80 to 1 : 90 dilution of phenol.

The minimum killing concentrations reported are the average of at least three determinations and, inasmuch as they were obtained in the course of routine phenol-coefficient determinations, are subject to the inaccuracies and limitations of that procedure.

388 Ross, KWARTLER AND BAILEY

(c) Titration for CMC Values

The CIVIC was measured by the me thod of Corr in and H a r k i n s (2) using the dye sodium 2 ,6-dichlorobenzeneindophenol ( E a s t m a n No. 3463). The

t i t r a t ion in every case was cont inued un t i l the original red color of the acidified dye solut ion was jus t reached.

TABLE I

Formula, Analyses and Critical Micelle Concentration of Certain Substituted Benzyl- dimethylalkylammonium Chlorides

Alkyl

C 6H13 CsH17 CloH21 C12H25 C14H29

C~sH37 C8H17 C10H~ C12H~s C14H23 CI~-I~3 C13H37 Ci2H25 C~:H~5 C12H2~ C~2H25 C~H2s C12H2~ C12H~3

:Ring substitution J F i

None None None None None

N o n e None 4-Nitro 4-Nitro 4-Nitro 4-Nitro 4-Nitro 4-Nitro 2-Chloro 2,4-Dichloro 4-Chloro 2-Hydroxy-5-nitro 3,4-Dichloro 3,4-Methylenedioxy 3,4-Dimethoxy

CMC

moles~1.

4.34 X 10 -z

6.1 X 10 -3 2.8 X 10 -3 3.7 X 10 -t 4.4 X 10 -a 7.1 X 10 -3 5.7 X lO -~ 2.3 >( 10 -2 3.6 X 10 -3 5.1 X 10 -4 1.3 X 10 -4 2.9 X 10 -5 0.28 X 10 -3 0.37 X 10 -3 0.42 X 10 -3 0.69 X 10 -3 1.1 X 10 -s 3.8 X 10 -3 3.9 X 10 -3

Elementary analyses,

Nitrogen Chlorine

Calcd.

5.48 4.95 4.50 4.12 3.82 3.54 3.32 4.95 7.88 7.28 6.78 6.36 5.98 3.74 3.44 3.74 6.98 3.44 3.65 3.50

I Found I

5.37 4.86 4.52 4.05 3.89 3.51 3.64 4.86 ~ 7.99 7.18 6.76 6.05 5.80 3.70 3.49 3.62 7.23

[ 3.42 I

3.61 3.40

Calcd. Found

13.90 13.35 12.51 12.13 11.40 11.08 10.43 10.05 9.65 9.58 8.98 8.95 8.39 8.14

12.51 12.13 10.00 10.06 9.25 9.06 8.61 8.63 8.06 8.31 7.43 7.50 9.50 9.47 8.69 8.74 9.50 9.42 8.85 8.82 8.69 8.85 9.26 9.06 8.88 8.61

By TiCI3 method.

R E S U L T S

The results for the C M C in moles per l i ter of bo th series of q u a t e r n a r y a m m o n i u m salts are shown in Fig. 1. I n this figure the logari thm of the C M C is p lot ted versus the n u m b e r of carbon a toms in the alkyl group. For the 4 -n i t robenzy ld ime thy la lky l ammonium chlorides, the results are fair ly well expressed by a single s t ra ight line. For the benzy ld ime thy la lky lam- m o n i u m chlorides, two s t ra ight lines, in tersect ing a t the po in t t ha t cor- responds to 12 carbon a toms in the alkyl chain, are required to describe the results.

C O L L O I D A L A S S O C I A T I O N A N D B I O L O G I C A L A C T I V I ~ Y 389

In Figs. 2 and 3 each of the graphs of Fig. 1 is compared with the cor- responding antibacterial activities of the compounds. These microbiological activities, expressed as the minimum killing concentration in moles per liter required to kill 0.5 ml. of a 24-26-hr. culture of M. pyogenes vat. aureus or S. typhosa in 10 min., are sufficiently close for both organisms to warrant only a single curve through the experimental points. The position of this curve may, of course, be displaced upward or downward because of the merely relative character of the method o:[ comparing antibacterial activities.

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NUMBER OF CARBON ATOMS IN ALKYL CHAIN

FzG. 1. Logarithm of critical micelle concentrations of 4-nitrobenzyldimethyl- alkylammonium chlorides (upper curve) and benzyldimethylalkylammonium chlo- rides (lower curve) vs. number of carbon atoms in alkyl chain.

Figures 4 and 5 show a series of curves, similar to the lower curves of Figs. 2 and 3, for the logarithm of the minimum killing concentration in moles per liter versus the number of carbon atoms in the alkyl group. These curves are for compounds in which various substituents are present in the benzyl group. The figures il lustrate results obtained using an unsub- sti tuted be nz y l group, a 2-chlorobenzyl, a 4-chlorobenzyl, and a 3,4- dichlorobenzyl group. Figure 4 shows the antibacterial action for M. pyogenes vat. aureus and Fig. 5 the antibacterial action for S. typhosa. These curves represent a selection from a larger body of data obtained on a variety of quaternary ammonium salts that differed both in the length of

-LO 4 - NITROBENZYLOIMETHYLALKYLAMMONIUM CHLORIDE

-2.0

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390 ROSS, K W A R T L E R A N D B A I L E Y

KILLING CONCN, FOR S. TYPHOSA

KILLING CONCN. FOR M. PYOGENESVAR. AUREUS ¢ ¢

CRITICAL MICELLE CONCN. 0 ~ 0

I I I I ,,, 6 8 I 0 12 14. I~$ 18

NUMBER OF CARBON ATOMS IN ALKYL GROUP

Fro. 2. Comparison of critical micelle concentrations of a series of 4-nitrobenzyl- dimethylalkylammonium chlorides and their minimum killing concentrations for S. typhosa and M. pyogenes var. aureus, for varying number of carbon atoms in the alkyl group.

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KILLING CONCN. FOR M. PYGGENES VAR. AUREUS ~

KILLING CONCN. FOR. S. "I{¥PHOSA, ~ Z~.

6 8 I0 12 14 I 18

NUMBER OF CARBON ATOMS IN ALKYL GROUP

FIG. 3. Comparison of critical micelle concentrations of a series of benzyldimethyl- alkylammonium chlorides and their minimum killing concentrations for S. typhosa and M. pyogenes var. aureus, for varying number of carbon atoms in the alkyl group.

the chain and in the nature of the substituent in the benzyl group. Table II presents in extenso the results of which Figs. 4 and 5 are representative selections.

COLLOIDAL ASSOCIATION AND BIOLOGICAL .&CTIVITY 391

DISCUSSION

(a) Molecular Structure and Critical Micelle Concentration

The:dye method of obtaining the~CMC of colloidal electrolytes is always subject~to the uncertainty that the presence of the dye may induce the formation, of micelles at lower concentrations than they would occur in the absence of the dye. This uncertainty can be resolved only by comparison

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N U M B E R OF CARBON A T O M S IN ALKYL GROUP

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of the results of the dye method with those of an independent method, such as that of electrical conductance or refractive index. A wide discrepancy exists between the values of the CMC for benzyldimethyldodecylam- monium chloride in this paper (by the dye method) and that given by Cella et al. (1), who used electrical conductivity. The values are 0.0028 and 0.008] mole/1., respectively. The lower value by the dye method

392 ROSS, K W A R T L E R A N D B A I L E Y

suggests the influence of the dye in the formation of the micelle. The con- clusions of the present paper are therefore restricted to comparisons of the compounds here reported, each measured with the same conditions, and cannot be extended to any other substance whose CMC is measured by a different method.

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NUMBER OF CARBON ATOMS IN ALKYL GROUP

FIG. 5. Relationship between chain length and antibacterial action of certain benzyl- dimethylalkylammonium chloride homologs for B. typhosa.

Figures 1 and 2 show the proportionality between the log CMC and the number of carbon atoms in the alkyl group. The 4-nitrobenzyl series gives a single straight line, but the unsubstituted benzyl series gives two straight lines, one below 12 carbon atoms in the alkyl group and the other above 12 carbon atoms. The presence of two straight lines for the same series can be interpreted as evidence for two distinct types of micelle, one for short carbon chains and the other for longer carbon chains. The slope of the line has been used by Debye (3) as the basis for a calculation of the decrease

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394 ROSS~ KWARTLER AND BAILEY

in free energy per mole per carbon atom in the formation of a miceUe. The two types of micelle may therefore have different free energies of micelliza- tion, much less below 12 carbon atoms than it is above 12 carbon atoms.

One can measure the slope of these lines and express the results in the same form as was done by Harkins (4), leading to the following statements:

1. For the 4-nitrobenzyl series of these quaternary ammonium salts, a reduction by unity in the number of carbon atoms in the alkyl group multiplies the CMC by a factor of 2.17.

2. For the unsubstituted benzyl series of these quaternary ammonium salts, a reduction by unity in the number of carbon atoms below twelve in the alkyl group multiplies the CMC by a factor of 1.63.

3. For the unsubstituted benzyl series of these quaternary ammonium salts, a reduction by unity in the number of carbon atoms above twelve in the alkyl group multiplies the CMC by a factor of 2.69.

It may be significant that the average of the two factors, 1.63 and 2.69, for the unsubstituted benzyl series is very close to the single value of 2.17 for the 4-nitrobenzyl series.

The factor 2 has been quoted by Harkins for micelles of pure anionic soaps with 7-14 carbon atoms in the chain. From this Debye (3) has calcu- lated the corresponding decrease in free energy in the formation of a soap micelle as 1280 cal./mole/carbon atom.

It has been reported by Hess, Philippoff and Kiessig (5), by Scott and Tartar (6), by Hartley (7), by Stauff (8), and quoted by Harkins (4) that the relation between critical micelle concentration and hydrocarbon chain length for a homologous series of long-chain electrolytes is given by

log CMC = ]c3N -~ k4 1

or, in the equivalent form used by Harkins

(CMC)~/(CMC)N, = B exp(N' - N) 2

where N is equal to the number of carbon atoms in the paraffin chain and k3,/~4, and B are constants. These equations are valid only over a limited range of hydrocarbon chain lengths. The results shown in Fig. 1 add the present quaternary ammonium salts to the growing list of homologs whose behavior can be thus described.

A theoretical model that leads to the equations given above has been developed by Corrin (9). His development is based essentially on the sup- position that at the CMC the diffusing tendency of a single molecule inside the micelle is balanced by the gain in surface free energy that would ensue were the molecule removed completely from the micelle. The effect of substituent polar groups in the benzyl ring is to reduce the gain in surface free energy on the separation of the micelle into single molecules. The equilibrium that is represented by the concept of CMC is therefore dis-

COLLOIDAL ASSOCIATION AND BIOLOGICAL ACTIVITY 395

placed toward a system of lesser diffusing tendency, i.e., a more concen- trated solution. From the viewpoint of molecular structure, the presence of a polar substituent lessens the total hydrophobic character of the molecule and so reduces its tendency to form micelles. The CMC occurs, therefore, at a higher concentration.

The above reasoning leads us to look for some property that measures the degree of polarity of the benzyl portion of the molecule. Measures of this polarity are provided by the boiling points, melting points, dielectric constants, or dipole moments of the substituted toluene compounds that correspond to the benzyl portions of the quaternary ammonium salts. For the compounds studied, these data are collected in Table III.

For a number of these substituted salts, all with 12 carbon atoms in the alkyl group, it was found that the CMC increases as the square of the dipole moment of the corresponding substituted toluene compound (Fig. 6). I t is readily seen that variations in the substituent group may do more than merely change the polarity of the total molecule, and the specific action of different chemical groups may account for irregularities when the attempt is made to fit them by the same simple expression. Nevertheless, it is of in- terest that the 4-nitro substituent should fit so closely to the same relation that describes the monochloro and dichloro substituents. At least for the five compounds shown in Fig. 6, chemical specificity of the substituent group is less significant than the over-all physical polarity of the molecule. The noteworthy exception to the simple relation expressed in Fig. 6 is displayed by the quaternary salt with no substitution in the ring.

The relation depicted in Fig. 6 is expressed by the equation:

CMC = klt~ 2 ~- k2 3

where ]ci and k2 are constants and ~ is the dipole moment of the substituted toluene compound that corresponds to the benzyl portion of the quaternary ammonium salt. The term in t, 2 is suggestive of a force of interaction between adjacent dipoles, and the trend of the experimental results is in accord with the concept that the greater the force of repulsion between the polar heads of the molecules, the less is the tendency to form micelles.

(b) Molecular Structure and Antibacterial Effect

Figures 4 and 5 for M. pyogenes var. aureus and S. typhosa respectively, have many features in common. They both show the usual optimum anti- bacterial effect at between 12 and 14 carbon atoms in the alkyl group (10); they both show increasing antibacterial effect as more polar substitu- ents are placed in the benzyl group, as long as the alkyl chain is shorter than its optimum value, and they both show a reversal of this trend, namely a decreasing antibacterial effect for more polar substitution in the benzyl group, when the alkyl chain is longer than its optimum value. A rationale

396 Ross, KWARTLER AND BAILEY

T A B L E I I I Critical Micelle Concentration of Substituted Benzyldimethyllaurylammonium Chloride

Compared with Physical Properties of the Corresponding Substituted Toluene Compounds

Substitution in benzene ring

CtI2 I

CH~

I C1

CH~ J

0 I C1

CH~ I

N 0 s _ _ ~ - O H

CH2 I

I C1

Quaternary ammonium salt

CMC X 10 a

moles~liter 0.28

0.37

0.42

0.69 358

Corresponding substituted totuenes

1.1

Melting point Boiling point

°K. °K. 238 432

486

280 435

478

i Dipole moment

1.39

1.78

1.74

2.68

C O L L O I D A L A S S O C I A T I O N A N D B I O L O G I C A L A C T I V I T Y

TABLE III--Conctuded

397

Substitution in benzene ring

CH~ I

CH2

O-CH~

CH~ i

~ O C H +

t OCH3

CH~ ]

NO~

Quaternary ammonium sall

CMC X 103

moles~liter

2.43

3.8

3.9

3.6

Corresponding substituted toluenes

Melting point

425

Boiling point

°If, °K.

178 384

471

403

511 324

Dipole moment

0.40

4.40

of this behavior is attempted, based on the colloidal and other physical properties of the substances.

The first of the three characteristic features, i.e., the presence of an optimum antibacterial effect as the length of the alkyl group is varied, is the result of conflicting trends. As the alkyl chain is lengthened, the whole group becomes more hydrophobic. The increasing hydrophobic character of the alkyl group promotes at least two different results. The group itself, and by its action the whole molecule, is increasingly drawn to correspond-

398 Ross, K W A R T L E R & N D B A I L E Y

ing hydrophobic interfaces, of which two are closely available. One is the hydrophobic portion of the surrounding molecules and the other is the hydrophobic portion of the bacterial surface. Only the latter attraction conduces to antibacterial action. As the chain length increases, however, the tendency of the molecules to cling together as micelles grows exponen- tially. The single molecules are increasingly removed from the solution by incorporation in the micelles, presumably at a greater rate than the rate

w z

O b-

w p-

Y,

ca. O

p- z w

0

hi .J 0 a.

n.-"' ~ 2 ,4 - DICHLOROTOLUENE

= / - 4 - GHLOROTOLUENE o / o

/ u 2- CHLOROTOLUENE o3

I = q TOLUEI~ E

I 2 5

GRITIGAL MIGELLE CONCENTRATION OF QUARTERNARY

AMMONIUM SALT IN MOLES/LITER X lO 3 "

FIG. 6. Relationship between the critical micelle concentration of a series of ring- substituted benzyldimethylalkylammonium chlorides and the square of the dipole moment of the corresponding substituted toluene compound.

of increase of adsorption on a hydrophobic bacterial surface, until the original advantage conferred to the antibacterial properties by lengthening the chain is lost by a concomitant and more active tendency toward asso- elation.

The second characteristic feature of the antibacterial effect is the im- provement that results from polar substitution in the benzene ring, as long as the alkyl group is shorter than its optimum length. The nature of this effect can be traced in Figs. 4 and 5, but Figs. 7 and 8 give a more striking

C O L L O I D A L A S S O C I A T I O N A N D B I O L O G I C A L A C T I V I T Y 399

demonstration. In these figures the logarithm of the minimum killing con- centration is plotted against the logarithm of the dipole moment of the substituted toluene compound that corresponds to the benzyl portion of the quaternary ammonium salt. Toluene itself and its monochloro and di- chloro derivatives are included in these figures. The compound that proved exceptional here was not toluene, as happened in Fig. 6, but 4-nitrotoluene,

10"

"6 E

z 0

Q: I..- Z I.l.I ¢,,D Z 0 c.)

Z ,_J ,,_l

o .3

io"

io:

~0"~

10-5

0 8 - C A R B O N ALKYL GROUP

• IO 'CARBON ALKYL GROUP

Z~ 12- CARBON A__LKYL GROUP

f f I I I

~ 0 . 4 t O . 2 0 -0 .2 -0 .4

LOG. DIPOLE MOMENT

FIG. 7. Relationship between the antibacterial action for M . pyogenes var. aureus

of a series of ring-substituted benzyldimethylalkylammonium chlorides and the dipole moments of the corresponding substituted toluene compounds.

which lies at the opposite extreme of the range of dipole moment values. Figures 7 and 8 also differ from Fig. 6 in the nature of the mathematical function of the dipole moment.

Let us attend for the moment to the similarities between the two types of diagram. We might conclude that by increasing the polar character of the head of the molecule the tendency to form micelles is reduced, and that

400 ROSS~ KWARTLER AND BAILEY

t h i s in turn affects the antibacterial action by releasing single molecules to be sorbed by the hydrophobic portions of the bacterial surface. This conclusion makes no claim to be more than a partial explanation. There are unquestionably specific chemical effects that have not been taken into account, as shown, for example, by the unsubstituted benzyl derivative forming fewer micelles than would be predicted by Fig. 6, and by the 4-

io

io';

E

Z o io -3 b-

f~ b- Z W

Z O O

Z . J j iO "~

io-S

0 8- CARBON ALKYL GROUP • I0- CARBON ALKYL GROUP

z~ 2 "CARBON ALKYL GROUP

i i i I I

"1- 0.4 + 0 2 0 - 0 . 2 - 0 . 4

LOG. DIPOLE MOMENT

Fie. 8. Relationship between the antibacterial action for S. typhosa of a series of ring-substituted benzyldimethylalkylammonium chlorides and the dipole moments of the corresponding substituted toluene compounds.

nitrobenzyl derivative having less antibacterial action than would be pre- dicted by Figs. 7 and 8.

The third characteristic feature of the antibacterial effect occurs with the long alkyl chains, where the extreme degree of micellization has al- ready begun to deprive the substance of its antibacterial action. With these compounds, an increase in the polarity of the benzyl group still

COLLOIDAL ASSOCIATION AND BIOLOGICAL ACTIVITY '~01

further reduces the antibacterial action. The effect is slight, less pronounced than the reverse tendency noted above for the shorter alkyl chains, but quite definitely present. For the longer chains the effect of polar substit- uents in the ring on the behavior of the whole molecule must become in- creasingly less. This is shown in Figs. 7 and 8 by the progressively smaller slopes of the lines as the alkyl chain is lengthened. The cause of the third effect is sought, therefore, not directly in the polarity of the group but in another property that the group acquires with increasing polarity, namely, increasing effective bulk. I t is postulated that the long alkyl chains now occupy a relatively large area of the bacterial surface and may prevent, by virtue of their size, other molecules from taking up adjacent adsorption sites. This condition is aggravated by having a bulky benzyl group attached to the nitrogen atom. The polarity of the group is considered to affect the total behavior less significantly than does its effective bulk.

ACKNOWLEDGMENTS

The authors acknowledge the assistance of Mr. E. I. White of the staff of the Sterling-Winthrop Research Insti tute, who helped with the determination of the minimum killing concentrations, and the following students of Rensselaer Polytech- nic Institute, who helped with the determination of the CMC: J. R. Bianchine, P. B. Brock, J. W. Dean, Miss Grace Panza, and A. S. Slowe.

The authors are indebted to Dr. F. C. Naehod, who kindly gave of his time and energy for discussion of the work as it progressed.

REFERENCES

1. CELLA, J. A., EGGENBERGER, D. N., NOEL, D. R., HARRIMAN, L. A., AND HAR- WOOD, H. J., J. Am. Chem. Soc. 74, 2061 (1952).

2. CORRIN, M. L., AND I~ARKINS, W. D., J. Am. Chem. Soc. 69,679 (1947). 3. DEBYE, P., J. Phys. & Colloid Chem. 53, 1 (1949). 4. HARKINS, W. D., J. Am. Chem. Soc. 69, 1428 (1947). 5. HEss, K., PHILIPPOFF, W., AND KIESSIO, N., Kolloid-Z. 88, 40 (1939). 6. SCOTT, A. B., AND TARTAR, H. V., J. Am. Chem. Soc. 65,692 (1943). 7. HARTLEr, G. S., Kolloid-Z. 88, 22 (1939). 8. STAUFF, J., Z. physik. Chem. 183A, 55 (1938). 9. CORRIN, M. L., J. Colloid Sci. 3, 333 (1948).

10. MOILLIET, J. L., AND COLLIE, B., Surface Activity, pp. 287 e~ seq. Spon Ltd., London, 1951.