PAT MACALUSO
F oster D . Snell, In c., B rooklyn , N. Y.
Synergy, or nonadditive nonideal behavior in mixtures and polycomponent systems, is common in all branches of chemistry. Solvents, catalysts, emulsions, surface-active agents, and plastics offer many examples of its practical utilization. Synergic mechanisms can be classified gener
ally as involving stepwise action, direct interaction, or complementary functioning o f the components. Self
association, packing effects, dissymmetry, and interaction of components appear to be among the factors underlying all truly synergic phenomena as distinguished from com
bined, stepwise processes which are only apparently syner
gic. The extent of synergy is not easily determined quantitatively in most systems, and erroneous assump
tions regarding ideal behavior have led to some confusion.
Valid measures depend on the use of significant and addi
tive quantities. Awareness of the possibilities o f synergy can greatly aid the work of the research chemist having at his disposal many gradations o f chain length, polarity, and particle size. The possibility of synergy should also be considered in the drawing up o f patents involving poly
functional reactants, classes o f materials, and m ulti
stepped processes.
S
YN E R G Y — the force that can make 2 + 2 = 5— is an effective agent in chemistry. While this term is better known to the medical and pharmaceutical professions, the chemist has encountered the principle under such designations as catalyst promoters, activators, latent solvents, builders, and the like.The use of sesame oil in pyrethrum and rotenone insecticides is an illustration (8). A millimolar solution of pyrethrins in re
fined kerosene (roughly one third the concentration of satisfactory household fly sprays) causes only 8 % mortality among test flies;
by the addition of 5 % by volume of sesame oil, mortality is in
creased to 88%. Sesame oil alone, either pure or diluted with kerosene, exhibits no toxic action on houseflies.
The dictionary defines synergism or synergy as the “ coopera
tive action of discrete agencies such that the total effect is greater than the sum of the two effects taken separately” . The question arises: Can anything of value to the chemist be learned from so general a phenomenon as synergy? D o the many different sys
tems displaying synergy pokkess any common factors in their mechanisms which can guide the industrial chemist in his work?
Consideration of a number of seemingly unrelated examples should lead to at least a few general conclusions.
S O LV E N TS
When synthetic solvents first made their appearance, it was observed that many of them were inferior to old-type “ natural”
or crude solvents. For example, methylacetone made from the distillation of wood is actually a mixed solvent which consists of methanol and acetone, with some esters and higher ketones (SI).
This mixture has certain desirable properties not obtainable by any of its component ingredients when used separately. For this reason “ synthetic methylacetone” made to simulate it, is not a chemical individual. Similarly amyl acetate manufactured from fusel oil has superior solvent properties over synthetic amyl acetate which lacks the impurities of the other.
The term “ mix solvency” has been coined for synergy in solvents for resins (S6) and for soaps (£7). The process is pic
tured as one of statistical solvation by means of van der Waals forces or hydrogen bonding of different portions of the solute molecule by the different components of the solvent mixture.
The efficiency of the solvents is increased in the presence of higher concentrations of solute since there are more attachments per
unit time. On this basis Palit has been able to account for the behavior of resins in certain mixtures of nonsolvents where solu
bility is improved by increasing the amount of solute as expressed by his rule of resinous or lyophilic solubility. The turbidity and precipitation which are known to occur upon the dilution of many resin solutions (18) are explained on the same basis.
S U R F A C E -A C T IV E A G E N T S
The most common case of synergy among surface-active agents is the building of soaps and synthetic detergents by salts which themselves display little or no surface activity. Many of the relatively low-cost sodium alkyl aryl sulfonates, as marketed, contain large amounts of sodium sulfate formed during neutral
ization. This so-called inert material greatly increases the sur
face-active properties of the sulfonate. Alone it does not have such a good balance of solubilizing against hydrocarbon groups under the usual conditions of use.
The addition of salts in suitable amounts to solutions of ionic surface-active agents promotes micelle formation and reduction of surface tension (1). The first appears to play an important role, not clearly understood at present, in deflocculating, dispers
ing. and solubilizing action (SO) ; the second, or rather a corre
sponding reduction of interfacial tension, is important in wetting and emulsification. An explanation (S3) of the reduction in surface tension is that the added salt reduces the extent to which the charged heads of the sorbed surface-active ions repel one another. This results in a closer packing which makes possible an increase in the amount of surface-active material in the sorbed layer.
Advantage has been taken of this effect in the marketing of wetting agents which have practically no surface activity in water alone but are very effective in concentrated salt solutions.
Dibutyl sodium sulfosuccinate (1) is extremely soluble in water.
A 0.1% solution has a surface tension of 67.5 dynes per cm. A 0.1% solution in 20% sodium sulfate, however, has a surface ten
sion of only 37.6 dynes per cm. This wetting agent is soluble even in a 75% solution of zinc chloride. Similarly, a polymer of about 90 molecules of ethylene oxide is very soluble in water and has little effect on surface tension, but in a high concentra
tion of any strong electrolyte it shows substantial surface tension reduction.
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I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y An important technical application of synergy in surface-active
agents is the so-called solutizer process (5) for sweetening gasoline.
The mercaptans are removed from gasolines by extraction with a solution of a solutizer, such as potassium isobutyrate in concen
trated aqueous potassium hydroxide. Under these conditions micelles of the otherwise nonsurface-active potassium isobutyrate are formed, and the mercaptans are apparently dissolved within these micelles. The mechanism of solution in micelles has been studied (22).
Synergic lowering of the surface tension of surface-active agents can also occur as a result of their direct interaction. Thus capronic acid and o-aminobenzoic acid together lower the surface tension of their mutual solutions more than could be expected from a summation of their separate effects (24). An addition product which possesses greater surface activity than either of the constituents is presumably formed in this case.
E M U L SIO N S A ND E M U L S IF IE R S
Emulsions containing mixtures of emulsifying agents for which enhanced stabilizing properties are claimed have been the subject of a number of patents (S). A mixture of two or more emulsifying agents selected from four different classes (S3) have a mutual stabilizing action such that the mixed emulsifiers are more effective than the separate emulsifiers. Synergy was also observed in the increased viscosity of solutions containing these mixed emulsifiers. A specific instance of increased stability is an emulsion prepared by the use of a mixture of lecithin and methylcellulose, which is much more stable than emulsions obtainable with either lecithin or methylcellulose alone (25).
Instead of increasing viscosity in an emulsion, a synergic effect may be just the opposite. For instance, in concentrated emul
sions and suspensions, viscosity is sometimes reduced by the use of mixed particle sizes in the disperse phase. Thus the produc
tion of a “ pourable” bituminous emulsion of high bitumen content by the creation of both large and small dispersed particles in the same system has been reported (34, 35). Two 60% asphalt dis
persions, alike except for particle size, were prepared. One had an average particle size of 2 to 3 microns, the other of less than 1 micron. Mixtures of these two emulsions possessed viscosities considerably lower than those of the separate emulsions. The decrease in viscosity is ascribed to the packing effect of the large and small particles. This applies only to concentrated disper
sions where some state of packing exists.
Particle-size distribution is no doubt a more important factor than particle size itself in regulating the viscosity of concentrated disperse systems (35). The principle of maximum close packing, recognized and applied in making concrete (32), is thus carried over into the field of emulsions.
Two possible mechanisms for synergy in emulsifiers are sug
gested. The first is an improvement in molecular balance as a result of interaction of the component emulsifiers, which might cause an increase in the toughness and stability of the interfacial sorption film. The second is the possibility of closer packing of mixed emulsifier molecules in the interfacial film, leading to in
creased stability of the film and a lower interfacial tension. The influence of molecular interaction and packing of emulsifiers on emulsion stability has been studied (10, 14, 31). Increased packing in the sorbed layer can occur as a result of differences in the chain lengths, of improved balance of polar and nonpolar groups in the molecules, or of a decrease within limits of the solubility of the emulsifiers in the mixture.
C A T A L Y S IS
A promoter has been defined as a substance which increases the catalytic activity to an extent exceeding many times the additive effect of the mixture. Catalytic promotion is thus characterized by a much greater yield at the same temperature or by the same yield at a substantially lower temperature. In
synergy as applied to catalysis, a distinction can be made be
tween a promoter which is not a catalyst and one which is a catalyst in its own right. Examples of promoter action in catalysis are well known and have been ably discussed elsewhere by workers in that field (4).
Synergic catalysts need not be mixtures in the ordinary sense;
they may be intramolecular mixtures. Thus a catalyst prepared from the mixed heteropoly acid, silicomolybdotungstic (H«SiMo»- W ,0 « ), gives considerably higher conversions in the vapor-phase oxidation of naphthalene to phthalic anhydride (6) than the catalysts prepared from either of the parent acids, silicomolybdic (HiSiMouOio) orsilicotungstic (HjSiWuO«).
M IS C E L L A N E O U S E X A M P L E S
Many varied applications of synergy are in use today. One example is in weed control (IS). Sodium chlorate sprays to con
trol weeds among sugar cane are harmful to both weeds and cane crop at effective concentrations. By the addition of spreaders and activators such as pentachlorophenate, spray concentrations can be reduced to a point at which weeds are destroyed without affecting the sugar cane.
In the field of plastics, it has been shown that styrene-methyl methacrylate copolymers-are stronger than either polymer alone (7). Definite compressive strength maxima occur at 30 and 50%
styrene. This may be regarded as an intramolecular effect, with the synergy probably a result of the nonuniformity or dis
symmetry introduced into the polymer chains by copolymeriza
tion.
Addition of a few per cent of halogenated aromatic hydro
carbons raises the breakdown potential of saturated hydrocarbons by 30-40% whereas greater additions lower it (29). This is probably a case of true interaction between the components.
The synergic action produced by the joint use of caustic soda and sodium peroxide in the modern fur-carroting process (36) re
sults from combining two steps, hydrolysis and oxidation, into one.
Examples of natural materials which are synergic mixtures are carnauba wax and shellac. Although synthetic wax esters such as cetyl stearate, myricyl palmitate, and the like are known, they are not used commercially as waxes. Indeed, pure wax esters have had notoriously poor success in commercial applications despite many attempts in this direction. Carnauba wax, toward whose duplication most synthetic waxes are aimed, is a mixture of wax esters and uncombined wax alcohols. The synergy operate ing here has been approached with some success in the manufac
ture of synthetic waxes by oxidation of hydrocarbons to a mixture of wax acids and alcohols, which are then partially condensed to esters, leaving a certain amount of uncombined acids and alcohols (.11).
Similarly, shellac (12) is a mixture composed largely of the com
plex esters of several different polyhydroxy acids together with a proportion of uncombined acids. The individual components are devoid of film-forming characteristics.
S Y N E R G Y IN T H E O R E T IC A L C H E M IS T R Y
Although theoretical methods for dealing with pure substances and ideal systems are well developed, the opposite is true in the case of nonideal systems and synergic mixtures where empirical methods are the rule. But the development of such methods for dealing with the admittedly more complex, nonideal mixtures has lagged far behind the accumulation of data and the demands of technology. This may be largely the result of a lack of uniform methods in the evaluation and presentation of data, the use of arbitrary assumptions as to what constitutes ideal behavior in complex systems, and a failure to appreciate the presence and significance of synergy in many cases.
The latter point is illustrated by the work of Michaelis and Timgnez-Diaz (23) on ionic synergy. They point out that the
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 1073 various rules of Hardy, of Schultze, and of Freundlich, describing
the coagulating power of electrolytes for colloids, were hindered from quantitative development by their failure to recognize that in aqueous solution one has to deal with hydrogen and hydroxyl ions, as well as the ions of the electrolytes. The coagulation of Congo Rubin sols was studied with sodium and potassium salts at various pH values, and it was shown that the coagulation power was a function of both pH and p i (where I indicates alkali metal ion concentration), both ions acting synergically. On the basis of this study they formulated a quantitative “ law of ionic syner
gism" which enables the true coagulating powers of the individual ions to be determined. The observation (£8) that the effect of salts of polyvalent ions in reducing the interfacial tension of surface-active agents is analogous to the Schultze-Hardy rule for the coagulation of colloids, is very suggestive in this connec
tion. It is possible that this phenomenon may yield to quantita
tive treatment according to the “ law of ionic synergism” or along similar lines.
A U T O S Y N E R G Y
For the sake of completeness the possibility of autosynergy should be considered. Pure substances which, upon dilution in an inert medium, are nonideal in their behavior may be regarded as autosynergic. Mixtures of the lower fatty alcohols and acids with hydrocarbons are typical, the polar molecular aggregates dissociating upon dilution with the nonpolar hydrocarbons.
The increased rate of tanning of leather obtained upon dissolving formaldehyde in nonpolar solvents has been ascribed (80) to dissociation of the formaldehyde polymers in such solvents.
The solution of nitrocellulose in mixtures of alcohol and ether may also be regarded as a case of autosynergy according to the mechanism suggested by Barr and Bircumshaw (£). Alcohol alone, which is highly associated, is a poor solvent, but mixture with ether (a nonsolvent) is believed to lessen the degree of aggregation of the alcohol and thereby improve its solvent powers for nitrocellulose.
R O L E O F D IS S Y M M E T R Y
A large role in synergic phenomena is apparently played by the factor of dissymmetry. Molecular dissymmetry in catalysts and plastics has been cited, and the effects of mixed particle sizes, representing a type of dissymmetry, in emulsions and fillers have been noted. The influence of mixed molecular sizes and shapes on the stability of Borption layers of emulsifying agents has also been suggested.
Mixtures of sodium palmitate and sodium laurate in water illustrate the influence of dissymmetry in mutual solubilization (19). Mixed micelles of smaller size, less orientation, and greater solubility than those of the pure soaps are attributed to the occurrence of the shorter laurate molecules among the longer homologs. This results in micelles of nonuniform internal structure.
The transformation of a symmetrical into a nonsymmetrical sys
tem produces changes in polarity, orientation, packing effect, etc., depending on the type of system involved, and is reflected in changes in various properties such as viscosity, elasticity, melting point, fiber strength, and the like. The organic chemist dealing with polyfunctional molecules and the formulator having at his disposal many grades of the same material differing as to chain length or particle size may consider with advantage the synergic possibilities of employing mixed reactants or mixed grades.
P A T E N T S
The word “ synergy” appears in the title of at least one patent (9), on sesame oil as a synergist or activator for pyrethrum in
secticides. The use of mixtures to obtain enhanced or synergic effects, if so demonstrated and claimed, may be classed as new, novel, and unexpected and be patentable, even though the use of
the separate components may be the subject of previous patents.
In a patent (16) covering the use of aliphatic amino derivatives or their salts as plasticizers for hydrophilic organic colloids, the statement that a mixture of amines is more effective than a single amine may be important. Because it is unpredictable, the possi
bility of synergy should always be considered in drawing up patents involving classes of materials, polyfunctional reactants, multistepped processes, etc.
Q U A N T IT A T IV E D E T E R M IN A T IO N
T o determine quantitatively the presence or absence and the degree of synergy in various systems is by no means so simple as might appear from the definition. The choice of a suitable measure or norm, which if exceeded indicates the occurrence of Bynergy, is not difficult from the practical point of view where evidence of improved performance, lower cost, and time saving is sufficient. The difficulty arises when a measure is sought such that deviations from it will be quantitatively significant— for example, in stoichiometrically establishing molecular association or in making valid comparisons as to the extent of synergic action among different systems.
Many of the mixed systems for which synergy has been claimed are actually dispersions or solutions of the active agents in a medium which is assumed to be completely inert. Insecticides, antiseptics, tanning agents, and surface-active agents are typical examples. In such cases the significant properties vary with the concentration, and it may be difficult to determine just what the
“ sum of the two effects taken independently” really is because of such complicating factors as dilution effects or autosynergy and differences in synergic mechanisms.
In the case of a mixture of two substances with or without an inert diluent, the sum of the independent effects, or the norm against which the actual behavior of the system is compared, may be represented by the expression,
P|Cl ~j- PiCi
C l + C l
where Ci and th represent the concentration of each component;
Pi and P i represent the value of the property of each pure com
ponent, taken at a concentration of c( + Ci when the property in question is extensive.
This is often considered a true measure of ideal or nonsynergic behavior, particularly in binary liquid mixtures. However it has not always been appreciated that the validity of this measure depends entirely upon the use of significant and additive quanti
ties. N ot all property scales or measures of concentration can be validly used in this measure. A binary liquid system is usually
ties. N ot all property scales or measures of concentration can be validly used in this measure. A binary liquid system is usually