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VII. Emulsions and Emulsion Stabilizers

VII. Emulsions and Emulsion Stabilizers

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299



polar groups in the surface layer. At an oil-water interface the polar

group is in the water, the nonpolar in the oil. I n this way they overcome

the inherent incompatibility of these interfaces so that when the oil is

broken up into minute droplets there is little or no tendency for the

droplets to run together and form a Pepurat,e phase. I n other words, the

emulsion is stable.

Even in such stabilized emulsions the stability is a relative state; if

the oil particles are large enough, the force of gravity will cause them

to accumulate and eventually to coalesce. I n the case of light oils the

droplets cream out on top and form a layer over the water; in the case

of oils of high density the layer may form beneath the water. Two

factors are concerned in the long-time stability of such emulsions: (1)

the density of the oil, if i t is close to unity the tendency to separate is

less; (2) the droplet size is also concerned, if the droplets are so small

that they are violently agitated by thermal agitation they have less

tendency t o coalesce and form a separate layer. Homogenization of the

emulsion therefore is often of an advantage by reducing droplet size of

the oil below a certain threshold value so that they stay permanently

dispersed in a uniform concentrRtion throughout the mass of the dispersion medium.

VIII. SELECTIVITY

OF HERBICIDES

The various mechanisms of herbicidal selectivity have been considered in detail (Crafts, 194613). It suffices here t o enumerate the types

of selective herbicidal treatments and illustrate them with examples from

the field.

Selective herbicidal action may result from :

(1) Differential wetting, (2) morphological selectivity, (3) biochemical selectivity, (4) selective spray placement, ( 5 ) selective soil sterilization, (6) differential life span of plants.

Differential wetting is illustrated by Sinox application to kill mustards

and fiddleneck in flax. Leaves of the weeds are broad and readily

wetted; those of flax are small and difficult to wet. Peas are another

crop that illustrates this phenomenon.

Morphological selectivity results from differences in the location of

the vital tissues in plants. For exanple, in cereals, onions, and lilies the

growing points of the plants are located a t the base near or below the

ground level; they are also protected by the surrounding leaves. In

contrast, most broad-leaved weeds have exposed growing points which

can readily be wet by the spray. The selective action of herbicides containing iron or copper salts, sulfuric arid, or Sinox is due to a combination of these two mechanisms.



300



A. S. CRAFTS AND W. A. HARVEY



Biochemical selectivity depends upon inherent differences in the

makeup of plants with respect to their tolerance of certain poisonous

chemicals. When weeds in crops of the carrot family are sprayed with

Stoddard Solvent, all the plants are thoroughly wet; only the weeds die.

When mustard in a wheat crop is dusted with 2,4-D or given a low

volume spray treatment with an ester formulation of 2,4-D in oil, the

selectivity is biochemical; all plants are exposed to the chemical, only

the weeds die.

When a robust crop plant such as corn, milo, or sugar cane attains

a height of 12 to 18 inches, it has proved feasible to spray small weeds

around the base with a general-contact herbicide, the growing tissues of

the crop being protected from the spray by the bases of the older leaves.

B y using such sprays i t has been possible to bring a crop to maturity,

completely eliminating the frequent tillage commonly used (Crafts,

1948b). Under Home conditions this may be less expensive than the

common tillage practice; under some it is preferable because it minimizes

erosion and prevents the formation of cultivator sole. This latter is

important in the semi-arid west where such sole interferes with irrigation

and where impervious layers in t.he soil are not broken up by freezing

of the soil during the winter.

The same biochemical selectivity shown by the tops of certain crops

and weeds is reflected in their roots. For instance, the salts of 2,4-D in

the soil will kill mustards at concentrations t,hat cause little or no injury

to cereal crops. For this reason 2,4-D salts may be used either as preemergence or post-emergence treatments in small grains and corn, providing soil moisture conditions are right and dosage is carefully

controlled.

Certain perennial crops may be treated during their dormant period

or between cuttings. This is exemplified by alfalfa that may be sprayed

with a general contact herbicide to rid it of weeds during the winter or

early spring, or immediately after a cutting during the summer. This

is becoming common practice in the western states (Harvey and Riddle,

1946; Raynor, 1947).

The foregoing discussion presents a number of the types of herbicidal

action that are now being widely used in the field of chemical weed

control. The remainder of this paper will be devoted t o description of

the various chemicals and review of current literature concerning them.



IX. THE2,4-D HERBICIDES

The advent of 2,4-D has so rapidly and so profoundly changed the

practices of chemical weed control that the terms 2,4-D and herbicide

are considered synonymous by many. While a proper balance in em-



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301



phasis should be maintained with respect to the different herbicides

available, a material that has reached a production of around 20 million

lbs. by the fourth year of its use in the field deserves recognition. Particularly is this true when one realizes that in many cases one pound or

less of this chemical is used per acre. For the first time, chemical weed

control may be recommended as an economical substitute for tillage or

cropping practices. Selective weed control has assumed a major position

among pest-control practices.

Apparently 2,4-D was discovered independently in England and in

the United States during the war (Blackman, 1945; Kraus and Mitchell,

1947; Nutman et al., 1945; Slade et at., 1945). After its announcement

(Hamner and Tukey, 1944; Mitchell and Hamner, 1944), a wave of

interest spread across the land and thousands of field tests were made

by investigators and private individual farmers before adequate experience in its use had been gained.

Early experiments with 2,4-D and similar chemicals are reported by

the British workers, Slade et al., (1945), Nutman et al., (1945), and

Blackman (1945). Hamner and Tukey (1944) were the first to publish

experimental work in this country; Kraus and Mitchell (1947) later

described many experiments performed during the war and carried out

under restriction. These firmly established the herbicidal properties of

2,4-D and similar growth regulators. Subsequent work has been carefully reviewed by Akamine (1948). Readers are referred to this paper

where scientific references as well as government publications, popular

articles, and commercial reports are listed. Popular aspects are al3o

treated by Mitchell and Marth (1947) a n d by Avery et al. (1947).

2,4-Dichlorophenoxyacetic acid is a white crystalline substance

formed by reacting dichlorophenol with monochloroacetic acid (Wellman,

1948). It is only slightly soluble in water but may be readily dissolved

in alcohol and acetone (Anonymous, 1948j).

Early 2,4-D formulations involved the use of Carbowax (polyethyleneglycol) as an intermediary but it was soon realized that the

sodium and ammonium salts were soluble in water and convenient to use.

More recent formulations involve the amine salts, the principal advantage of these being that they are liquid a t high concentrations, which

makes for easy mixing by dilution. Some of these formulations contain

a quantity of the salt equivalent to four pounds or more of acid per

gallon.

The esters of 2,4-D are somewhat more effective than the salts since

they are less polar in nature. As these compounds lose in polarity,

however, they lose correspondingly in water solubility and most of the

formulations involve the ester or t.he ester in solution in oil, plus a large



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A. S. CRAFTS AND W. A. HARVEY



amount of emulsion stabilizer. Thus the material may be dissolved in

oil and applied as a very fine spray, or emulsified in water with application possible over a wide range of volume dosages.

Though appreciably more toxic on an acid equivalent basis (up to

twice), some esters have the drawback of being volatile and problems

of injury to adjoining sensitive crops have been more serious with ester

formulations than with the salts (Brown et al., 1948; Dunlap, 1948;

Pryor, 1948).

Because 2,4-D is available in such a variety of forms, workers have

agreed to recommend and use it on an acid equivalent basis. In order

to compute formulations, niolecular weights of the various 2,4-D compounds are necessary. Table I from Weeders Readers of November 1,

TABLE I

Concentrations and Weights of 2,4-D Materials



Substance



2,4-D Acid

2,4-D Ammonium Salt

2,4-D Sodium Salt (anhydrous)

2,4-D Sodium salt (monohydrate)

2,4-D Diethanolamine salt

2,4-D Triethanolamine salt

2,4-D Methyl ester

2,4-D Ethyl ester

2,4-D Isopropyl esters (2, same

weight)

10. 2,4-D Butyl ester (4, same weight)

11. 2,4-D Amy1 esters (about 15, all

with same weight)

12. 2,4-D Isopropanolamine salt

1.

2.

3.

4.

5.

6.

7.

8.

9.



Molecular

weights



Per cent

2,4-D



Units required to

contain 100 units

of 2,4-D acid



220

237

242

260

325

369

234

248



100

93

91

85

68

60

94

89



100

108

110

119

147

166

106

112



262

276



84

80



119

125



290

296



76

74



132



135



1947, gives these values. Structural formulas are given by Freed (1946).

From what has been said, it is apparent that 2,4-D is unique among

herbicides. Having growth-regulating properties (Synerholm and Zimmerman, 1945, 1947; Zimmerman and Hitchcock, 1942), it i8 effective

at dosage rates that are phenomenally low. Only the nit.ro- and chlorophenols approach 2,4-D in toxicity and while they have definite uses in

the field they do not approach 2,4-D in multiplicity of herbicidal characteristics.



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x. USES



OF



303



2,4-D



Because 2,4-D has hormone properties, it differs from the older

herbicides in its physiological action on plants. Upon coming in contact

with leaves or roots, it rapidly migrates to other parts of the plant; this

is the action that makes possible low-volume and dust treatments as

contrasted with most other herbicides that are required in much greater

volume t o effect sufficient cover. In spite of this remarkable quality,

2,4-D may be used in essentially the same way as the older contact

herbicides such as sodium arsenite, chlorate, and dinitro compounds.

Therefore, in what follows, where 2,4-D is being used as a contact herbicide it is so labelled even though it may move within the plant after its

application.

1. General Contact Spray

While not widely used as such, 2,4-D has found valuable use as a

general-contact killer. I n California it has been used to kill full grown

pigweeds, radish, and Chinese lettuce in ripe grain to facilitate harvest

by combining. It is being widely used in the Imperial Valley to kill

weeds on ditch banks. And, odd as it may seem, it has proved effective

in killing the leaves and stems of cotton preparatory to harvest with

mechanical pickers.

2. #elective Contact Spray



The outstanding application of 2,4-D has been as a selective spray

in cereal crops. Here the relatively low susceptibility of t8hegrass plants

is made use of in the control largely of broad-leaved plants. I n 1948,

many million acres of cereal crops have been treated (Anonymous,

194%). Application has been by ground sprayer and by airplane. Ester

and amine salt formulations have been predominant mainly because of

convenience; sodium and ammonium salts have been used in lesser quantities.

I n addition to the cereal crops, wheat, barley, and oats, corn has been

sprayed to the extent of over one million acres (Lee, 1948a, b; Stahler,

1948; Willard, 1948), rice t o several hundred thousand acres (Anonymous,

1948f; Brown, 1947a; Higgins, 1947; Tullis, 1948), sugar cane t o a like

extent (Brown, 1947b; Hance, 1948a, b; Hanson, 1948; Nolla, 1948; Shaw

et al., 1947; Van Overbeek and Velez, 1946a, b ; White and Mangual,

1948; White and Villafane, 1946), milo to many thousands, and flax has

been treated on a considarable acreage. Grass seed crops have been

profitably sprayed (Anonymous, 1947a, d ; Marth and Mitchell, 1946a ;

Marth et al., 1947), and many thousands of acres of pastures and range



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A. S. CRAFTS AND



W.



A. HARVEY



have been treated (Kephart and Evans, 1948; North Central Weed Control Conf., 1947; Savage and Costello, 1948; Savage et al., 1948). Lawns

and fairways have been successfully weeded (Ahlgren and Cox, 1947;

Marth and Mitchell, 1946a, b, 1947; Mitchell and Marth, 1947). Experimental work has been done on potatoes (Ennis et al., 1946; Smith e t

al., 1947), asparagus, strawberries (Carlson, 1947a), and a number of

other crops.

3. Translocated Spray

Probably the most remarkable characteristic of 2,4-D is the fact that

it is readily translocated in plants. Being relatively nonpolar, i t is

absorbed through the cuticle of leaves; the acid and ester forms are most

readily taken in. Arriving in the leaf, this chemical is apparently subject

to the same mechanisms of transport as auxin, namely polar transport

in parenchyma cells and rapid movement in the assimilate stream along

with organic foods. Literature on such transport is reviewed by Crafts

(1938, 1939a) and Crafts e t al. (1949).

Polar 2,4-D movement shows a positive geotropic pattern; taking

place through living parenchyma cells, it probably accounts for the complete destruction of the leaves and stems of annual weeds when application is by dust to localized areas or by low-volume application to a

relatively small portion of the total surface of the plant.

Transport of 2,4-D with the assimilate stream accounts for the movement of this toxicant deep into the root systems of perennial weeds. The

widespread and generally successful control of wild morning-glory, poison

oak, sage brush, and a host of other perennial weeds depends upon this

movement from the tops of the treated plants into the root systems

underground. Killing of the roots of wild morning-glory to depths of

15 or 20 feet has been noted on deep alluvial soils of California and other

states. The observation that the vertical tap root is often more completely killed than are lateral roots is probably a manifestation of polar

movement; geotropic movement in parenchyma cells would result in

accumulation in the cortex of the first few centimeters of a lateral and

the remainder of the root would not be killed.

Experiments by Mitchell and Brown (1946), Weaver and DeRose

(1946), and Rice (1948) have shown that 2,4-D transport takes place

most rapidly and effectively in plants in which photosynthesis and food

movement are going on. Evidently 2,4-D is carried along with foods

in the phloem of plants and attains a similar distribution in the roots.

It has been a matter of some confusion to plant physiologists that

2,4-D gives the most satisfactory root kills on wild morning-glory, hoary

cress, chicory, artichoke thistle, St. Johnswort, and similar perennial



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305



weeds if applied just before or during the early stages of blossoming,

whereas maximum translocation and storage of food is known to occur

somewhat later. The answer lies in the &ate of the roots at this time.

Van Overbeek (1947) has pointed out that 2,4-D accumulates in and

effectively kills meristematic tissues. During the earlier growth stages

the roots of perennials are still meristemat,ic and are using assimilated

foods in the production‘of new tissue. Later, the bulk of the assimilates

is accumulated in mature storage parenchyma, cells that can absorb

2,4-D (Mitchell e t al., 1947) but t,hat do not respond by rapid death as

does meristem. Hence, during the early blossoming period 2,4-D absorbed by leaves is rapidly taken to the meristems of roots and shoots,

later it is taken t o storage parenchyma where it has less killing action.

Although t.he translocation of 2,4-D into the roots is essential to the

killing of perennial weeds, a similar transport in annuals probably explains the extreme effectiveness of this chemical as a selective spray.

The control of many aquatic weeds such as Alisrna and Sagittaria species

in rice, nutgrass, t.ules, and cattails in wet lands and ditches, water

hyacinth, yellow primrose, and alligator weed in waterways, and a number of others illustrates further the wide herbicidal usefulness of 2,4-D.

These instances all involve translocation, though to a lesser extent than

in wild morning-glory.

The successful control of brushy and woody plants on pastures and

in right-of-way maintainance is of great credit t o 2,4-D (Anonymous,

1946; Ashbaugh, 1948; Barrons and Coulter, 1948; Hamner and Tukey,

1946; Savage and Costello, 1948; Savage et al., 1948). Successful treatment of such species during active growth undoubtedly results from

transport of the 2,4-D with the assimilate st,ream. The recent finding

that many such plants also respond to treatment with the esters of 2,4-D

in oil during the dormant period (Barrons and Coulter, 1948) indicates

some other mechanism of movement, possibly slow cell-to-cell transport

in living parenchyma.



4. Temporary Soil Sterilization

Although 2,4-D is outstanding in the uses noted above, probably its

unique role is that of a temporary soil “sterilant.” Such compounds do

not sterilize the soil in the sense that all biological life is killed. The

word “sterilant” is employed by weed men for compounds that prevent

germination of seeds or establishment of seedlings. I n contrast to arsenic,

borax, chlorate, and others of the older sterilants that are required in

amounts of hundreds of pounds per acre (Crafts, 1935, 1936; 1939c;

Crafts et al., 1941), 2,4-D performs many useful functions when applied

at rates of 1to 3 or 5 Ibs. per acre. This depends on two facts: (1) 2,4-D



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A. 8. CRAFTS AND W. A.' HARVEY



is not strongly fixed nor rapidly altered by contact with the soil (DeRose,

1946; Hanks, 1946; Kries, 1947; Taylor, 1947) ; (2) it is from 10 to 100

times as toxic to roots as to the tops of the plants. For these reasons it

will kill weed seedlings in concentrations of a few parts per million in the

soil. The same relatively high tolerance shown by the tops of grasa

plants is also present in the roots (Mitchell et al., 1947). Methoxone

(4 chloro-2 methyl phenoxyacetic acid) and 2,4-D have been tested by

the British workers (Nutman et al., 1945; Slade et al., 1945) and shown

to be effective when applied through the soil (see also DeRose, 1946).

I n contrast with resu1t.s in the eastern states and Britain, California

farmers have experienced deleterious effects from 2,4-D residues in the

soil (Crafts, 1946a, 1949). Studies have shown that decomposition of

2,4-D takes place most rapidly in warm, moist soils of acid reaction.

Seldom are all of these factors favorable for breakdown in California

soils a t any one time (DeRose, 1946; DeRose and Newman, 1947; Kries,

1947; Mitchell, 1948; Taylor, 1947).

Because 2,4-D breaks down too rapidly in soils to serve as a means

for controlling deep-rooted perennials by soil applications, its most logical

role is that of a selective herbicide. There are 3 general methods for

using 2,4-D as a selective temporary soil sterilant: (1) as a preplanting

treatment to provide a weed-free seedbed; (2) as a preemergence treatment to provide a weed-free soil medium, and (3) as a postemergence

treatment to eliminate weeds that have come up with the crop (Norman,

1948).

Where soil and climatic conditions favor rapid 2,4-D breakdown it

is possible that this chemical might be used as a general preplanting

treatment for all crops, including 2,4-D susceptible ones; however, since

a period of 4 to 6 weeks is usually required to free the soil of 2,4-D, the

most logical use would be as a selective treatment in tolerant crops. One

obvious advantage of the soil treatment method is the possibility of controlling grass seedlings that would not succumb to contact spray treatment (Anderson and Ahlgren, 1947; Mitchell and Brown, 1946).

Considering the use of 2,4-D as a soil treatment, there are 4 rather

distinct climatological conditions that might determine its effectiveness

(Crafts, 1948b): (1) no rainfall, that would result in failure because the

chemical would not be washed into.the soil around the roots of the weeds;

(2) very light rainfall or foggy drizzly weather, that might cause serious

injury because the chemical would be localized a t the soil surface in high

concentration; (3) moderate rainfall, that should be most favorable for

selective action, and (4) heavy rainfall or flood t.hat would leach the

chemical excessively and allow the weeds to survive. Results of trials

have shown that the soluble salts of 2,4-D are most effective during



WEED CONTROL



307



periods of light rainfall; 2,4-D acid may be used when heavy rainfall i s

anticipated (Anderson, 1948; Crafts, 1948b).

Preemergence treatment with 2,4-D has tremendous possibilities for

reducing the labor of tillage so commonly practiced in row crop agriculture. Not only will it lower costs and reduce drudgery, it will minimize

cultural operations that tend to break down soil colloids and render soils

impervious to water and air. Under some conditions it will appreciably

increase yields. It may also be used to control weeds in a mulch-covered

soil so that wind and water erosion are minimized.

6. Permanent Soil Sterilization



Because of its tendency to leach and to break down in soils, 2,4-D

offers little promise as a permanent soil sterilant. Possibly in desert

regions it might be used around power poles or on fences and around

structures. In agricultural areas the hazards from leaching into cropped

areas or of being blown on dust are too great. Permanent soil sterilization is apparently the least useful category into which 2,4-D may fall;

arsenic, chlorate, and borax would seem to be more logical materials to

use. However, 2,4-D may be useful to kill deep-rooted perennial weeds

growing in sterilized soil that are not killed by the shallow chemical

treatment of permanent soil sterilants.

XI. NITRO-AND CHLORO-SUBSTITUTED

PHENOLS

Sodium dinitro-ortho cresylate (Sinox) was introduced into the

United States in 1935 and soon became the most widely used selective

spray (Westgate and Raynor, 1940). Iron sulfate and sulfuric acid

sprays were displaced and selective weed killing was introduced in a

number of new crops such as flax, peas, onions, corn, alfalfa, lawns, etc.

Within a short period the use of acid salts to increase toxicity (activation) was introduced and thousands of acres of crops were sprayed

annually. Instead of the injury so often experienced with sulfuric acid,

Sinox proved beneficial to many crops and though occasional burning

of leaf tips occurred, in most instances the crops were actually stimulated.

This was particularly true of small grains the yields of which were often

significantly increased over and above those resulting from elimination

of weeds (Westgate and Raynor, 1940).

Following chemical studies the mechanism of Sinox activation was

described (Crafts and Reiber, 1945) and the suggestion was made that

the ammonium salt be used instead of the sodium salt. Further studies

along this line proved that the parent phenols in oils could be used as

general conttact sprays, that emulsions of such fortified oils were efficient

and inexpensive herbicides both for pre- and postemergence treatment.,



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A. S. CRAFTS AND W. A. HARVEY



and that pentachlorophenol and its salts could be used as direct substitutes for the dinitrocresols (Barrons, 1947; Crafts, 1945b, 1947b, 1948a,

b; Crafts and Reiber, 1945).

Because dinitrocresol proved more toxic than dinitropheno1, screening

tests were made on the ethyl, propyl, butyl, and amyl substituted phenols.

These proved that toxicity increased up to the butyl substitution, t,hat

the ortho compounds were more toxic than the meta or para compounds,

and that dinitro substitution was more effective than mononitro (Crafts,

1945b). With chloro substitutions on the phenol ring, toxicity increased

regularly to the penta compound.

At present the ammonium salt of dinitro secondary butyl phenol is

being used extensively as a selective herbicide (Dow Selective, Sinox W) .

Dinitro-secondary butyl phenol (Dow General) and amyl phenol (Sinox

General) are widely used to fortify oil sprays and to prepare fortified

oil emulsion sprays for general contact weed control, for potato top killing (Anonymous, 1947b) and as a selective spray in sugar cane (Crafts,

1948a, b; Crafts and Emanuelli, 1948; White and Mangual, 1948), corn,

milo, and similar crops (Crafts, 1948b) by selective placement of the

spray. Pentachlorophenol has recently been introduced as an oil fortifier

(Anonymous, 1948e; Crafts, 1947b, 1948a, b; Crafts and Emanuelli, 1948;

Crafts and Reiber, 1945, 1948; Hance, 1948a). Sodium pentachlorophenate is used in large quantities as a preemergence treatment and as a

selective soil sterilant in pineapples in Hawaii (Anonymous, 1948m).

There are still many opportunities for introduction of nitro-, chloro-,

and nitro-chloro substituted phenols as selective and general contact

herbicides as relatively few of such compounds have been tested as to

their special selectivities.

Combinations of nitro- and chlorophenol contact herbicides with 2,4-D

have proved very effective in certain situations where weeds susceptible

to both types of toxicants are present. Work on such combinations is

presented by White and Villafane (1946), Mangual (1948), White and

Mangual (1948), Crafts (1948a, b), Nolla (1948), and Hance (1948).



XII. OILS

Petroleum fractions have long been used as weed killers. At first

waste products such as acid sludge, waste engine oil, and similar materials

were used; later smudge pot oil, Edeleanu extract, and diesel fuel were

adopted (Robbins et al., 1942). More recently stove oil has been used

as a selective weed killer in crops of the carrot family (Crafts, 1947a;

Crafts and Reiber, 1944, Grigsby, 1946; Lachman, 1945; Raynor, 1943;

Sweet, 1945; Sweet e t al., 1944, 1945; Warren, 1946; and Warren and

Hanning, 19461, and since actual research on the herbicidal properties



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VII. Emulsions and Emulsion Stabilizers

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