!i~lolds a - firp.ula.ve · the first part deals with milk and the dairy industry, which involve...

ELSEVIER A: Physicochemical and Engineering Aspects 91(1994) 57-77 Colloids and Surfaces

!i~LolDs SURFACES A

Some applications of emulsions

M. Chappat

COLAS, 7, Place Renk Clair, 92653 Boulogne Billancourt Cedex, France

Received 20 November 1993; accepted 5 July 1994

Abstract

This overview describes some applications of emulsions in industry. The first part deals with milk and the dairy

industry, which involve complex emulsion systems stabilized by natural surfactants. This is followed by a description of emulsions in the food industry in which complex recipes are required and storage stability becomes an important issue. The third application of emulsions is in pharmacy and is concerned with providing suitable systems for injecting insoluble drugs. Other systems include blood substitutes and emulsions used in chemotherapy. Cosmetic emulsions have been by far the most commonly used systems for many centuries. The interaction of emulsions with skin is briefly described. Another application of emulsions is in agriculture, where active substances for treating plants and soils have often been formulated as emulsions. The last part of this overview deals with the application of bitumen emulsions in road construction techniques.

Keywords: Emulsions; Industrial applications

1. Introduction

As defined in several papers in this conference, emulsions are formed from two immiscible liquids:

one constitutes the droplets which are dispersed in the other liquid which is referred to as the continuous phase (Fig. 1). In some applications, this

I I I I I I I I I 000000000

Water in Oil (W/O)

or “oil” Oil in Water (O/W)

01 “aqueous”

Fig. 1. Schematic representation of both O/W and W/O emulsions.

dispersion must remain perfectly stable and homo- geneous over a certain period of time. In many cases, the emulsion may be used as a concentrate

and the required period of stability depends to a large extent on the application. However, in many

other applications, dilution of the concentrate is necessary. In other words, the concentrated emul-

sion is dispersed in another liquid and the system may be further homogenised by mechanical means.

In some cases, dilution of the concentrated emulsion may be spontaneous. This dilution procedure is used in many medical and pharmaceutical pre-

parations, cosmetics, petroleum products, road construction and maintenance techniques, and agriculture. Since the active ingredient is contained in the emulsion droplets it is necessary in some applications to “break” the emulsion in order to release the active material. This is particularly the case with many pharmaceutical emulsions which may require “breaking” at the site of action.

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58 M. ChappatlColloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 57-77

Barnes [l] and Tadros [Z] showed the importance of the following factors on emulsion properties and stability: the relative volume of the dispersed phase, i.e. the volume fraction, and the average size of the droplets, the viscosity of each phase, the nature and concentration of the emulsifier. These parameters also control the rheology of the emulsion, which is important in many applications.

In this overview, I shall endeavour to examine different classes of emulsions, highlighting the importance of the above-mentioned factors according to their application demands. The examples that will be described demonstrate the wide application of emulsions in many industrial products.

2. Milk and dairy products

Milk is perhaps one of the most stable emulsions produced in nature. It is not well understood why milk is produced in nature as an emulsion. The exact mechanism of how the fat droplets in milk are stabilised is also not sufficiently understood although as we shall see later there are some acceptable concepts of how these droplets remain stable.

Milk is a low concentration natural oil-in-water (O/W) emulsion (4-5%). Its viscosity at room temperature is near that of water (1 mPa s). The emulsifier content is in the region of l-3%. The emulsifiers are phospholipids (lecithin), although proteins also play the role of an emulsifier. The properties of the milk depend on temperature. Milk is not stable over a long period of time; this is, however, due to fermentation rather than to the fact that it is an emulsion. The milk properties may be changed as a result of mechanical action. It can cream, and phase changes may occur at certain temperatures where phase inversion may become more favourable.

Fig. 2 shows a schematic representation of milk, which is an emulsion consisting of animal fat dispersed in water. The proportion of water is around 87.5%. The fatty substances are essentially triglycerides. The fat droplets, whose size ranges from 0.1 to 10 pm (average of 3-6 pm), are sur- rounded by a complex double membrane. A study

CAPILLARY SYSTEM

Fig. 2. Schematic representation of

globules; M, micelles; V, vacuoles.

milk emulsion: G; fat

of the interface shows that it is formed by an assembly of molecules which become more and more polar as one moves away from the lipid centre. A schematic representation of the fat globule “membrane” in its initial state (“fresh milk”) is shown in Fig. 3.

The composition of the membrane can be determined using centrifugation techniques. It consists mainly of phospholipids and cholesterol, glycoproteins, enzymes (alkaline phosphatide, oxydide xanthine or aldehyde reductide), various metals and elements.

The membrane’s inner layer is made up of globular proteins and phospholipids, as is the cell membrane. This membrane has “solid-like” properties; it is essentially a viscoelastic membrane.

With a high level of enzyme activity, the outer layer is quickly destroyed. It is separated from the inner layer by a cytoplasmic layer which is several nanometres thick. When the outer membrane has been broken down, it is replaced by casein micelles

M. ChappatlColloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 57-77

FAT

CELLULAR

CYTOPLASMA

LIPASE

CASEIN

MICELL WITH LIPASE e COFACTOR

Fig. 3. Diagram of the fat globule membrane, initial state in fresh milk.

I ENDOPIASMIC GoLG’

RETICULUM

MEMBRANE UNIT

\ CASEIN MICELLS RIBOSOMES

which are naturally present in the aqueous phase. These micelles increase the milk viscosity.

From the beginning of time, man has tried to transform milk, essentially to extract and preserve the fatty substances. Several dairy products are produced from milk, of which cream, butter and cheese are probably the most common examples. A schematic representation of the essential components in these products is given in Fig. 4. The main differences between these products are the water content and the nature of the emulsion. The water content is 87.5% in milk; 54% in cream, 45% in cheese, and 16% in butter (inverse emulsion).

Cream is the product which results from milk separation into two layers by sedimentation; it is the part which is rich in fatty substances. Butter is an inverse emulsion obtained by churning. Firstly, churning mixes in air bubbles and creates a foam. With time, the air bubbles compress the fat droplets, destroying their “protective” membrane. By phase inversion, the fat then becomes a solution

59

to form butter. This operation is most efficient when carried out at low temperatures.

A summary of the main properties and composition of milk, butter and margarine is given in Table 1. Fig. 5 shows an optical micrograph of the water droplet size distribution in a margarine emulsion (Fig. 5(a)) and the finished margarine (Fig. 5(b)). This illustrates the reduction of the water droplet size distribution obtained in the pro- cessing of margarine.

3. Emulsions in the food industry

In addition to the above-mentioned milk products, many emulsions are present in our daily diet, such as sauces, dressings and sausages, and many ready-to-use products.

In all these emulsions, it is true that the emulsifiers are quite easily identifiable and show very little variation. They are mostly monoglycerides (or fatty

60 M. Chappat/CoNoids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 57-77

MlLK(COW)

CREAM

BUTTER

CHEESE

rdr$extract of n&+~ I fat milk

t 9

water 45%

Fig. 4. Analysis of milk products.

Table 1

The properties and typical composition of milk, butter and margarine

Type Aspect Storage

stability

Components Emulsifiers Average

size

Viscosity

Milk O/W White fluid Infinite at Fat Phospholipids 2-5 pm 1mPas

low (triglycerides glycoproteins +

temperature 3-4%) chlolesterol + enzymes

Butter and margarine O/W Yellow mixture Infinite at Fat Phospholipids 2-5 pm 500 mPa s at

normal (triglycerides glycoproteins + 20°C;

temperature 1%) cholesterol + 100 mPa s at

enzymes 60°C

Lecithin

(several %o)

acid esters) produced from triglycerides, the fat

globules in milk, or their by-products such as lactic, acetic or citric acid esters, obtained from non-ionic animal fat (lard, tallow) or vegetable fat. These emulsifiers are soluble in oil and insoluble in water. They can also act as foaming agents, rising agents for pastry, or agents ensuring oleophi- lit quality in pasta. Generally a mixture of several different emulsifiers is used as it has been proven

that this is more efficient than using one emulsi-

fier only. As is the case with other emulsions, a sufficiently

high concentration of emulsifier is used to reduce coalescence between the droplets. The diameters of the latter increase with time until they reach a stable level at 3-6 pm. This is illustrated in Fig. 6 which shows the variation of droplet size with time. Fig. 7 shows a histogram of an olive oil in

II 100 urn

M. ChappatjColloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 57-77

Fig. 5. Water droplet size distribution in (a) margarine (b) finished margarine.

61

water emulsion stabilized with lecithin and con- taining 0.152 g dme3 hydroxyethyl cellulose (HEC) after storage for 10 months at 10°C.

It has been observed that emulsion stability is increased by adding sucrose or polymers (for example HEC), or even salt. The concentration level of these ingredients is usually of the order of several per cent. Proteins also play an active role in this emulsification process as well as in the long term stability of many food emulsions.

Emulsions are easy to manufacture because they can be obtained by simply mixing the ingredients together using a high speed stirrer, as is usually the case for making mayonnaise.

It should be mentioned, however, that the above considerations are not sufficient since the products which are to be sold must remain stable for many months at different temperatures. Lowering the temperature during emulsification helps in making

the emulsion (it is well known that to make good mayonnaise, one has to place the mixing bowl in cold water).

Apart from stability considerations, food products which are to be ingested must be of “good” quality. Besides the product’s physicochemical qualities, food emulsions need to satisfy the cus- tomers’ taste, which points to considering the oleophilic qualities of sauces and dressings. Fat is smooth and it gives the right consistency for appre- ciating the taste of food. Finally, uncooked sauces and dressings must be liquid at body temperature, i.e. 37°C. These considerations necessitate the use of more complex mixtures in which additives are added to the emulsifiers. These additives such as sugars, polymers, and salts will not only modify the emulsifier properties but also produce addi- tional modifications to the consistency of the final food emulsion. It has been claimed that the taste


Fig. 6. Average volume diameter of oil droplets as a function of time for lecithin-stabilized O/W emulsions stored at 10°C at various

oil volume fractions.

-_ 0.108 I I I I I I

60 120 180

Time/ days

of many food emulsions is determined by its rheology which is controlled by the proper choice of the emulsifiers and the thickeners.

Emulsified sauces and dressings are usually O/W emulsions, the most typical of which is mayonnaise. The oil content of these sauces may vary. Three classes may be distinguished: category A, 80% oil (mayonnaise); category B, 50-80% oil, sauces derived from mayonnaise; category C, salad dressings with 15-50% fat content (vinaigrette).

These emulsions are complex mixtures and the continuous aqueous phase comprises a great variety of components: proteins, polysaccharides, sugars, acidifiers, minerals, etc. Generally speaking, the presence of egg phospholipids and even hydro- colloids in the aqueous phase is enough to stabilise some of these emulsions, especially mayonnaise. Synthetic emulsifiers are often incorporated for economic or dietary reasons in order to limit the amount of eggs used. The emulsifier is thus

adsorbed at the oil/water interface and its main role is to stabilise the emulsion.

The above-mentioned emulsifiers produce a film around the droplets, which prevents emulsion coalescence as a result of its interfacial properties, i.e. elasticity and viscosity [3]. Owing to the complex nature of the emulsifiers, using the simple hydrophilic lipophilic balance (HLB) is not sufficient for emulsifier selection. In addition, emulsifier mesomorphic characteristics (which produce a liquid crystalline phase at the O/W interface) play a large role in stabilizing the emulsion against coalescence. The efficiency of liquid crytalline phases in stabilising emulsions can be attributed to their rheological characteristics (high interfacial elasticity and viscosity) [a].

Certain sausages and cooked pork meat products (pates etc.) may be considered as emulsions since they have an oil phase and an aqueous continuous phase. The emulsifiers are mostly pro-

M. ChappallColloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 57-77

25

20-i

1 ; s I

D(3,2)=220pm

D(v,05)= 2.58pm

DistributionRange=1.12

Specific Surface = 2.725 m2/g

L

63

Diameter /pm

Fig. 7. Size distribution of an olive oil in water emulsion (0.1 volume fraction) stabilized by 2% soy lecithin with 0.152 g 1-l HEC (M, = 950 000) and stored at 10°C for 10 months.

teins with a concentration of l-2% to which caseins are also added.

The size of the droplets should be kept small. It is well known that the smaller the size, the better the resistance against microbiological attacks such as moulding.

Another important emulsion in the food industry is ice cream. This is partially frozen foam, 40-50% of which is composed of air. The continuous phase contains dissolved or dispersed materials: sugar, proteins, stabilisers, and fatty substances in an O/W emulsified form. Several of these milk proteins are linked to the emulsion’s fat globules. Two steps in the manufacturing process may be distinguished. The first step is the formation of the “mixture”. The main aim of this process is to try to split up the fat globules as much as possible and disperse them with air bubbles to achieve proper expansion. Homogenisation is usually carried out at 80°C. The second step is freezing of the “mixture” at - 5°C. During this process, demulsification and crystallisation of the ice cream and fatty substances occur. Crystallisation plays an important role in the texture of the end product. Lowering the temperature facilitates protein

adsorption, in this case casein, at the surface of the fat globules, thus stabilising the cream and limiting demulsification. The emulsifiers are monoglycerides and digylcerides of fatty acid at a concentration of less than 3 g kg-‘.

A summary of the composition and properties of some food emulsions is given in Table 2.

4. Emulsions in pharmacy

As carriers of active materials, emulsions are especially useful in pharmaceutical applications. An active substance can only be efficient if it is able to reach its precise target in a proper condition and in the appropriate form, without producing any side effects. Another example can be cited during a surgical operation in which nutritive materials or blood substitutes are introduced into the human body. Three main ways to introduce active material may be considered: orally (absorp- tion), parentally (intravenous or intramuscular) and cutaneously (ointments). Here I shall mainly concentrate on the parental means of application.

Many water-soluble drugs may be injected either

64 M. ChappatlCoNoids Surfaces A: Physicochem. Eng. Aspects 91 ( 1994) 57-77

Table 2 Summary of composition and properties of some food emulsions

Advantages Type Storage Components Emulsifiers Average Viscosity

of stability size emulsions (months) (pm)

Sauces

Mayonnaise

Meat

Practical dilution oleophilic

O/W 6 Fat (vegetable oil)

o/w 6 Fat (vegetable oil)

Fatty acid esters 5-10

5-10

100 mPa s at 20°C

100 mPa s at 20°C

o/w 6 Fat (animal fat) Phospholipids >lOO 400 mPa s before cooking

as aqueous solutions or as water-in-oil (W/O) emulsions. However, experiments have shown that transportation by fatty emulsions was much more efficient. In fact, an aqueous dilution is digested in the stomach whereas a fatty emulsion is not [4]. This is illustrated in Fig. 8 which shows the effect of replacing an aqueous solution with a W/O emulsion for ‘311-labelled ragweed emulsion.

The components that make up an emulsion must be very carefully selected in order to avoid risks. The oil must be either paraffinic or vegetable. Only non-ionic emulsifiers are used and these are limited to lecithin or other phospholipids and to fatty alcohols. These emulsifiers usually have a low

IO

10 I I I I

0 5 10 15 20

Time / days

Fig. 8. Adjuvant effect of emulsions on the rate of disappearance of radioactivity from the site of application of ‘351-labelled ragweed pollen extract: W, saline; 0, W/O emulsion.

molecular weight. The size of the droplets must be small for a number of reasons, one of the most important being that the droplets must not clog up the vessels during their transport, which could otherwise lead to blockage. Large-size droplets may also increase the risk of toxicity. The average droplet size used in pharmaceutical applications is much smaller than that in the food industry; an average droplet size of the order of 1 urn or less is usually the case (the largest droplet should always be less than 5 urn).

O/W emulsions are sometimes used for intramuscular injections and W/O emulsions are fre- quently used for intravenous injections. O/W emulsions are often used for chemotherapy treatments for cancer. The emulsions used must have a low viscosity, usually in the region of 1 mPa s, i.e. very close to the viscosity of water.

Pharmaceutical emulsions are stored for long periods of time and they should remain stable for a period of 1-2 years. They must also remain sterile; they are normally kept at 3-4°C (i.e. in a refrigerator). On application the emulsion may have to break at its site of action under body temperature conditions (37°C) in order to liberate its active components. Investigations have shown that many pharmaceutical emulsions may become unstable at high temperatures (around 40°C) and this may cause problems of toxicity as a result of the increase in the droplet size. The emulsifier content in many of these pharmaceutical emulsions is usually a few per cent. High pressure homogenisers are normally used to produce these

M. ChappatlColloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 57-77 65

fine emulsions. Table 3 gives the composition of some examples of commercially available lipid emulsions. The composition of the vegetable oils in these lipid emulsions is given by the charts in Fig. 9.

Other examples of pharmaceutical emulsions are those used as blood substitutes. For those the droplet size is even smaller, being in the range 0.1-0.2 urn (perfluorocarbons). Other systems include radioplaques, vaccinates, anaesthetics (barbiturates).

The efficiency of pharmaceutical emulsions depends on the properties of the emulsifier. The latter should control the length of time needed to dissolve the active ingredient in the organism. In this respect, it may be necessary to modify the properties of the emulsifier by incorporating an additive. In some cases, using an emulsion enables

Table 3

Some commercially available lipid emulsions

one to reduce the amount of active medicine applied. Some examples of pharmaceutical emulsions that are used in parental use are given in Table 4. Table 5 gives an example of an emulsion used as a blood substitute.

A summary of the compositions and properties of some pharmaceutical emulsions is given in Table 6.

In some pharmaceutical applications, multiple emulsions (an emulsion of an emulsion) have been proposed as a means of sustained release. These multiple emulsions are produced by emulsifying a W/O emulsion in water (to produce W/O/W emulsion) or an O/W emulsion in oil (to produce O/W/O emulsion).

These systems enable one to formulate more than one active ingredient. For example, with a

Trade name Oil phase Emulsifier Other

components

Intralipid

(Kabi-Vitrum)

Lipofundin S

(Braun)

Lipofundin

(Braun)

Liposyn

(Abbott)

Travemulsion

(Travenol)

Soybean 10% or 20%

Soybean 10% or 20%

Cottonseed 10%

Safflower 10% (and 20%)

Soybean 10% or 20%

Egg lecithin 1.2%

Soybean lecithin 0.75% or 1.2%

Soybean lecithin 0.75%

Egg lecithin 1.2%

Egg lecithin 1.2%

Glycerol 2.5%

Xylitol 5.0%

Sorbitol 5.0%

Glycerol 2.5%

Glycerol 2.5%

Linoleic

\ OleicK’” /

Others (2.7%)

Fig. 9. Composition of vegetable oils: a, safflower; b, soybean.


Table 4

Medical and pharmaceutical examples of emulsions

1. Hexobarbital

Soy oil

Ethanol

Egg phosphatide

Mryj 52

Water

3.75 g

1Og

25 g

fg 0.5 g

to 100 ml

2. Phenylbutazone

Soy oil

Monoglyceride acetyl

Glycerol

Pluronic F68

Water

2g

fOg

10 g 2.5 g

0.5 g

to 100 ml

3. Diazepam

Soy oil

Monoglyceride acetyl

Egg phosphatide

Glycerol

Water

0.5 g

f5g 5.0 g

1.2g

2.5 g

to 100 ml

Table 5

Composition of an emulsion used as a blood substitute

Substance

Perfluorocarbon

Pluronic F68

Hydroxyethyl starch

Glucose

Potassium chloride

Magnesium chloride

Monobasic sodium phosphate

Sodium chloride

Calcium chloride

Sodium carbonate

water

Quantity

20 ml

2.2 g

3.0 g

100 mg

32.0 mg

7.0 mg

9.6 mg

54.0 mg

18.0 mg

to pH 7.44

to 100 ml

W/O/W multiple emulsion, two water-soluble active ingredients may be formulated with one in the internal water droplets and one in the external aqueous continuous phase. When taken orally the first active ingredient in the outside continuous phase will be digested in the stomach. The oil droplets that now makes a W/O emulsion will pass to another location to release the second ingredient in the water phase at the site of action.

Several other applications are common for

multiple emulsions, for example, in treatment of toxic uraemia and intoxication. It should also be

M. ChappatlColloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) S-77 61

mentioned that multiple emulsions are used in other fields such as removal of phenol in residual water (using O/W/O multiple emulsions) and to refine hydrocarbons (using W/O/W multiple emulsions). Multiple emulsions have also been used in cosmetic applications [ 51.

The emulsifiers used for the preparation of multiple emulsions are generally non-ionic in nature. With pharmaceutical systems, emulsifiers that are approved by the food and drug administra- tion can only be used. This may impose a limit on their stability. With cosmetic multiple emulsions the choice of surfactants is also limited. This is not the case with industrial applications for which most surfactants can be used. The droplet size in the internal phase of the multiple emulsion droplet is usually of the order of 1 pm. However, the size of the multiple emulsion droplet can be in the region of 5-20 pm. A schematic representation of a multiple emulsion is given in Fig. 10. An example of a formula for a W/O/W multiple emulsion that is used as a night cream is given in Table 7.

5. Cosmetic emulsions

Since antiquity, emulsions have been used in cosmetics. Galen (130-200 AD), mentioned “ceratum refrigerans”, an ancester of O/W emulsions. Then came the 13th and the 14th centuries

0 @,O @ C “o”

0

0

OO

o &I 0

o 8ooo 0

0

0

0

c3

\ @(& @ Fig. 10. Schematic representation of a multiple emulsion.

Table I

Example of a formula for a W/O/W multiple emulsion

Primary emulsion W/O %

Beeswax 10.0

Microcrystalline wax 10.0

Vaseline 15.0

Lanolin 5.0

Perhydrosqualane 15.0

Olive oil 12.0

Sorbitan monooleate 4.0

Sorbitan monostearate 3.0

Sucrose 1.5

Scent 1.0

Water 23.5

Multiple emulsion W/O

Primary emulsion 78.0

Sucrose ester 2.0

Preservatives q.s.

Water 2.0

and “Doctrina Decorationis” by Henri de Mondeville and Guy de Chauliac. In the 17th and 18th century, people only used tallow, beeswax, olive oil, lard and essential oils. Then factory- manufactured soap, almond paste ointments, cocoa butter and vanilla cleansing creams made their appearance. Olive oil was replaced by sweet almond oil and borax and spermaceti were discovered. To guarantee efficiency, safety and a long-lasting product, natural or synthetic raw materials that make up cosmetic emulsions must be pure, stable and inoffensive.

A summary of the main components of cosmetic emulsions is given in Tables 8 and 9.

Cosmetic emulsions are applied through skin contact. In contrast to a pharmaceutical ointment’s active ingredients, which must penetrate deep into the skin either to diffuse throughout the body or to act locally, the action of cosmetic emulsions is limited to the immediate surface of the skin, i.e. the epidermis. To understand the action of cosmetic emulsions we must first consider the structure of the skin. The latter is a complex living organism that contains between 10% (at the surface) and 65% (deeper layers) water and is very plastic. Skin is made up of three layers which are as follows from the outer layer inward: the epidermis

68 M. ChappatjCoNoids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 57-77

Table 8 Main components of cosmetic emulsions

Origin Vegetable Animal Mineral

Lipophilic components Oils

Waxes

Surfactants

Almonds Peanuts Castor oil Copra Cotton Carnauba candelilla Cocoa butter Lecithin

Squalene Mink

Lanolin Beeswax Spermaceti Egg lecithin Cholesterol

Vaseline Paraffin

Paraffin wax Microcrystalline wax

Hydrophilic components Arabic gum Dragon gum Guar gum

Colloid silicates (clay)

(0.1 mm), the dermis (0.5-2 mm), the hydrodermis (5-40 mm); the skin’s main role is to protect the internal organs. This is illustrated in Figs. 11 and 12. Penetration through this thick layer is still not very well understood. It is thought to take place by infiltration through the hair shafts, between the intercellular spaces or by dissolution or emulsification by cutaneous lipids (sebum). For these reasons, O/W emulsions have been widely used since they were thought to enhance penetration.

The skin’s characteristics determine the properties which a cosmetic emulsion should have. Firstly the emulsion should remain stable for a long period of time over a wide range of temperatures. The emulsion should give a pleasant feeling of application. The emulsion should have the right consistency (rheology) to achieve this pleasant feeling. It also should be suitable for various types of skin (greasy or dry). Epidermic reactions and intoxic- ations must be carefully avoided. In other words, the emulsion should not cause any skin irritation. For these reasons, selection of emulsifiers for cosmetic applications should be carefully considered and a compromise is sometimes reached between adequate shelf-life and suitability of application. The nature and the content of the emulsifiers must also be carefully considered. Generally speaking, O/W emulsions are preferable since they are easier to manufacture and they have better oleophilic

properties. Various other parameters must also be controlled such as the water content, the final consistency, the final pH and the nature of the additives (perfume, colouring agents).

In most cosmetic emulsions, non-ionic surfactants are commonly used. Sometimes, anionic and amphoteric surfactants can be used but rarely cationics are allowed owing to their skin irritation properties. The oil content in cosmetic emulsions varies between 20% and 40%. These oil droplets usually contain the fragrances. Fluid or semifluid emulsions only contain 5% oil and they are mostly described as lotions. The pH is usually adjusted to 7 (i.e. neutral). The emulsifier content is rather high, reaching a value of 10% in some cases.

A summary of the composition of some cosmetic emulsions and some of their properties are given in Table 10.

6. Emulsions in agriculture

Emulsions are used in agriculture to formulate active substances for the treatment of plants and soil. In this section, I shall only discuss plant treatments through leaves, i.e. pesticides and herbi- cides. As with the case of human or animal organ- isms, the active ingredient must cross a material barrier, i.e. the leaf, to penetrate into a living


Table 9

Main surfactants of cosmetic emulsions

Lipophilic components

Fatty acids

Fatty alcohols

Fatty esters

Silicones

Lanolin and wool grease

Emulsifiers

Anionic

Non-ionic

Cationic

Amphoteric

Hydrophilic components

Polyalcohols

Stearic, palmitic, lauric

Myristyl, cetyl, stearyl,

isostearyl, isocetyl..

Glycerol

Ethylene-di-ethylene, triethyne

glycol

Propylene glycol

Isopropyl alcohol

Ethylhexyl

Alkyl sulphates, alkyl

sulphonates

Alkyarylsulphonates

Soaps from alkali or amino

metals

Ethoxylic fatty alcohol

Ethoxylic fatty acid

Ethoxylic fatty esters

Ethoxyhc alkylphenol

Polyglycol fatty esters

Ethoxylic polyoxypropylene

Sorbitan fatty acids

Saccharose esters

Quaternary ammonium salts

Betainic by-products

Glycerol, sorbitol, propylene

glycol

Polyglycerols

Poly(ethylene glycol)

Cellulose by-products

Alginates, carraghenates

Polyvinyhc

Polyacrylates

organism and carry out its function as a pesticide or herbicide. In some cases, the active ingredient may remain on the leaf surface for protection as in the case for skin treatment. The method of penetration through the leaf will be considered below.

Before describing the process of penetration of the chemical, it is necessary to say a few words about photosynthesis. Photosynthesis takes place in leaves, more precisely in the limb. Water and dissolved minerals absorbed by the roots are drawn

up through the wood vessels to the leaves where they are combined with carbon from carbon diox- ide present in the air. Sugar (saccharose) is formed which is then transformed into cellulose, the build- ing block of wood. Fig. 13 shows a schematic representation of the cross-section of the limb.

The exchange of gases occurs on the inner side of the leaf through orifices called stomatas. Located on the lower side of the leaf, these openings also permit water exchange and transpiration. The outer side of the leaf is covered by the cuticle, a rather impervious material which prevents evaporation. These stomatas which are between 2 and 20 urn are located where they will encounter most of the water. Under drought conditions, they are protected by hairs. The protective film of the cuticle is also covered by a waxy layer consisting of paraffinic and fatty hydroxy acid polymers. This is in contrast to the skin whose epidermis ends with a layer of keratin cells. One may envisage an analogy between the skin and the limb in terms of the action of an emulsion; however, this is definitely not the case.

As carriers of active ingredients, emulsions must be able to spread out upon contact with the leaf to liberate the active materials, and then penetrate through the lower stomatas or through the upper intercellular zones after crossing the waxy cuticle. In the middle areas, at the heart of the limb, they may enter the plant cells. The easiest pathway is through the lower epidermis or the hidden side of the leaves. Active ingredients may also serve as external protection, in which case they will not penetrate but will simply stay as deposits on the leaf surface. In either case, the active ingredient will be diluted and care should be taken to ensure that this emulsion is not toxic. In addition to quick spreading, once in contact with the surface, the emulsion must break quickly to avoid dripping, being washed off, or evaporating. It should be mentioned, however, that the interaction between an emulsion and the hydrophobic leaf surface is far from understood. In the complex situation of treating plants and soil, the mode of action of the chemicals and the role of the formulation are also difficult to study particularly when one considers the complex atmospheric conditions under which the treatment is applied.

M. ChappatjCoNoids Surfaces A: Physieochem. Eng. Aspects 91 (1994) 57-77

0,25 mm

Comeus layer

Granulous layer

Malplghi mucous

Basal layer

Basal dermal membrane

I I \ \ Papilla dermis

E \,

d”

Captllafy

CROSS SECllON OF SKIN

Fig. 11. Schematic cross-section of skin.

Melssner corpuscle -L Z$ ‘I

Ruffini corpuscle _

LO5 mm

Comeus layer Mucous layer Dermis papllla

a- PaplIla loops vessels

Fig. 12. Schematic cross-section of skin.

L SubpapIlla network

Ascending vessels

Arterial networK

A schematic representation of the transfer of a lo-100 m from the point of impact may be used. herbicide that is applied by spraying is shown Conditions such as humidity, temperature and in Fig. 14. velocity of the impacting droplets are not easily

For application of emulsion to the leaves of a controlled. One can only apply the chemical within plant or a tree, it must be sprayed either from the certain threshold limits. Under spraying condi- ground or from an aeroplane. Thus a distance of tions, water evaporation will occur and this


Table 10 Summary of the composition and properties of some cosmetic emulsions

Advantages of emulsions

Type Aspect Storage Components Emulsifiers Average Viscosity PH stability size

Spreading power, dilution, psychology

o/w Rich, white Long (2-3 Vegetable or Lecithin or 0.1-2 urn 100-200 mPa s 7

w/o years), hot animal oil alcohol or or cold (20-40%), fatty acid

fatty acids or esters (several alcohol, %) hydrocarbons

UPPER EPIDERMIS

LOWER EPIDERMIS

1. Culicle

2. Epidmna

3. Paliis cells

4. Chlomptants

5. Paliasadr tissue

6. Vaeuok

7. Nuelwa

0. Nowe

9. Lacum parenetrqna

10. slorm

Fig. 13. Leaf: cross-section of the limb.

enhances breaking of emulsions. This process is further influenced by the wind speed. To avoid these disturbances the size of the spread droplets should be adequately controlled. The role of the emulsion droplet size distribution may also play a role in this process. To reduce the process of spray drift, one usually uses large spray droplets.

For many practical applications, pesticides are formulated as emulsifiable concentrates. In this case, the active ingredient is dissolved in an oil to which one or two surfactants are added. When this concentrate is diluted in the spray tank it spontane- ously emulsifies producing an O/W emulsion. The stability of the emulsion produced is influenced by the concentration and type of electrolyte in the water as well as the temperature. This is illustrated in Fig. 15 which shows the relative emulsion stability at various temperatures in the presence of calcium carbonate [ 63.

Emulsifiers are selected to produce optimum

stability. These generally consist of a mixture of an anionic surfactant such as calcium dodecyl benzenesulphonate and a non-ionic surfactant such as an alkyl phenol ethoxylate. In some cases, phosphate surfactants are added to make the emulsion compatible with fertilizers used in agriculture. The optimum emulsifier content in the final emulsion is between 0.5% and 1% and the droplets are in the region of l-3 urn.

A summary of the composition of a typical emulsifiable concentrate is given in Table 11.

7. Bitumen emulsion in road construction techniques

Bitumen emulsions are used to carry an active material, mainly bitumen, for road application while avoiding solvents. Bitumen is used to bind aggregates together to make up the layers of

12 M. ChappatlCoNoids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 57~ 77

DUCT FOR SLIGHTLY LIPOPHILIC SUBSTAINCES (dalapon piclorame)

HERBICIDE ON THE LEAF

!_I

UTICLJLE

PALISADE

PARENCHYMA

NERVE

LACUNA

PARENCHYMA

LOWER EPIDERMIS

P LEAF

(C.S.)

1

STOMA DUCT C.S. : Cross section 000 : Duct for slightly lipophilic substance: 000 : Duct for lipophilic substances

Fig. 14. Transfer of herbicide applied by spraying.

pavement. Emphasis is placed on emulsion stability from manufacturing to application on the

mix before the final compacted application under

aggregates and on quick separation from water traffic conditions. Adhesion takes place if the

after contact with the materials. This separation bitumen properly wets the aggregate in dry or

can be either total or partial depending on wet weather. Cationic emulsions have less wetting power than do anionic emulsions as the emulsifi-

whether or not the aggregates will be stored as a ers are less hydrophilic.


77°F 28°C

100°F 42°C

1000 500 0 500 1000 1500 Na as ppm CaC03 ppm CaC03

I I Water Hardness

I 1

Fig. 15. Stability of emulsions as a function of water hardness.

Table 11

Composition and properties of a typical emulsifiable concentrate used in agriculture

Advantages of

emulsions Type Storage

stability


size

Viscosity PH

Dilution, environment o/w Several days,

from -5 to

+30ac

Vegetable or

animal oil

( 120%)

Non-ionic,

anionic

(0.25-l%),

esters, soya

amino,

calcium

sulphonate

l-3 urn 1mPasat 7

20°C

Depending on whether the aggregates are calcar- bution. Under high pressure (0.1 MPa) a mobile eous or basaltic (electropositive surface) or siliceous spreader that projects a fine film of emulsion on (electronegative surface), proper adhesion is the road surface to be covered is used. In both obtained with cationic or anionic emulsions in the cases, the variable weather conditions such as first case and only with cationic solutions in the humidity, temperature and wind velocity must be second. This is why anionic emulsions have not taken into account. Clearly one should not work been commonly used. Siliceous aggregates are used in the rain. The breaking of the emulsion on the when possible and adhesion is better in the pH chipping surface can be induced using another range 3-5. agent.

A schematic representation of the bitumen emulsion application is shown in Table 12.

The emulsions are applied simply by spraying on aggregates under either low or high pressure conditions. Under low pressure (0.05 MPa) a mixer is used to ensure a proper liquid-aggregate distri-

The bitumen emulsion is manufactured from the following ingredients: bitumen (modified or unmodified) heated to a temperature of 130-150°C to obtain an optimum viscosity of 200 mPa s - emulsifying additives at room temperature (20” C) for liquid amines or at a higher temperature (about

14 M. ChappatlCoNoids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 57-77

Material Cationic Emulsion

Electropositive material

(calcium , basalt) neutralizing reaction

‘I +++ +++++ BREAKING + + ++ V + + 0 forming of insoluble + + ++ amine carbonate + + V +++++ ++ ADHESION

Table 12 Schematic representation of bitumen emulsion application

-_- 1 0 _-

-- _-

Electronegative materials attraction (silex, quartz, granite)

BRETKING

V forming of insoluble

amine silicate V

ADHESION

60°C) for thick amines - acid at room temper- the emulsion a viscosity range between 50 and ature (20°C) - cold water (lo-20°C) or hot 500 mPa s. A summary of the bitumen emulsion water (about 40°C). composition is given in Table 13.

Both temperature and water hardness play an important role in the quality of the emulsion produced. The bitumen emulsion must remain stable during storage in insulated, heated tanks which may be equipped with a mixing device. To maintain stability, one should keep the pH low (3-5).

8. Other applications of emulsions

For the cationic emulsions used, a bitumen with a fixed degree of acidity is selected. The role of the bitumen’s quality and composition on the emulsion stability is far from well understood. Hard water should be avoided, especially when preparing emulsions with polymers. The emulsifiers used are usually synthetic amine chlorhydrates which vary in composition according to the final applications. They have a very big effect on the droplet size distribution of the emulsion. This is illustrated in Fig. 16 which shows the droplet size distribution of 60% cationic bitumen emulsion prepared using three different emulsifiers. A typical histogram of the bitumen emulsion is shown in Fig. 17. In some cases, binders may be added and this will have an effect on the viscosity of the emulsion as illustrated in Fig. 18.

Several other examples where emulsions are used may be listed: paints, photographic films, paper coatings, lubrication, petroleum extraction, etc. In most of these systems or processes, toxicity may not be a problem, although in recent years several environmental constraints (such as use of volatile organic solvents) have been imposed. Paint emulsions have some common ground with bitumen whereby the oil droplets should strongly adhere to the substrate to be painted. In addition, storage stability must be adequate since such paints may not be used directly after preparation. The same principles also apply to other emulsions such as those used in photographic films and paper coatings (where a latex emulsion is used to coat the paper fibres).

9. Conclusions

Generally speaking, the bitumen emulsion concentration ranges from 60% to 70%, giving

In every application I have considered, emulsions are very environmentally friendly. They make

Anionic Emulsion

attraction

V

BREAKING

V

forming of insoluble

calcium soap

V

ADHESION

no neutralizing

no attraction


100 -

E .m

P .E 50 -

m I:

x?

B 3

2

3

8

1 2 3 4 5 6 78910 20 D/pm

Fig. 16. Droplet size distribution of 60% bitumen cationic emulsions prepared using three different emulsifiers (A, B and mixture BE).

III 6’8 12 16 24

I I , I 32 48 64 96 100

Diameter/pm

Fig. 17. Typical size histogram of a bitumen emulsion.


01 I I I I I I

40 50 60

Binder Content

Fig. 18. Effect of binder content on bitumen emulsion viscosity.

it possible to avoid using solvents, and the emulsifiers used, which are added in small quantities, are generally harmless. In many cases an emulsion form is selected because it is more fluid than the active matter and thus it spreads more easily. Moreover, homogeneity of an emulsion enables smaller amounts of active elements to be more evenly spread.

The production conditions are, generally speak-

ing, very simple and in some cases the emulsion is obtained by chemical action with little demand for mechanical energy to obtain shearing. However, with many emulsions, such as bitumen, mechanical energy is required to produce the system.

The storage conditions of emulsions must be carefully controlled to maintain their stability. For example, with pharmaceutical emulsions the systems must be stored at low temperature (4°C) to avoid bacterial growth and to give the emulsion a shelf-life of l-2 years. Often emulsions such as bitumen have to be stored at much higher temperatures (80’ C) but only for much shorter periods (l-2 months).

In many cases, such as cosmetic or agrochemical emulsions, the storage temperatures could vary over a wide range (- 5 to + 50’ C) and the formula- tions need to remain stable over a period of l-2 years. In addition, the consistency (rheology) of cosmetic and food emulsions needs to be controlled for aesthetic reasons (e.g. taste with food emulsions). Breaking of an emulsion upon application is essential in many cases. With bitumen emulsions the droplets need to coalesce at the chippings to provide good coating. This is also the case with many agrochemical emulsions which need to break on the leaf surface to allow spreading and adhesion to occur. The same applies to many cosmetic emulsion creams where the oil droplets need to spread on the skin to form a protective film. With many medical emulsions the droplets have to break at the site of action to release the active ingredient.

O/W emulsions are most common in industrial applications than W/O emulsions and in most cases non-ionic emulsifiers are preferred. This is not the case with bitumen emulsions where cationic emulsifiers are required.

Table 13

Summary of road bitumen emulsion characteristics

Advantages of

emulsions Type Storage

stability


size

Viscosity PH

Dilution, environment o/w From several

hours at 60°C to

several days

at 20°C

Bitumen

(60-70%)

Amine chlorohydrates

(2%)

4-6 urn 20-100 mPa s 3-5

at 25°C

M. ChappatlColloids Surfaces A: Physicochem. Eng. Aspects 91 (1994) 57-77 II

The average droplet size of most emulsions is usually of the order of a few microns and this is achieved in most cases by the use of high speed stirrers. In some cases, such as in pharmaceutical emulsions, submicron emulsions need to be prepared and this required application of homogenisers and/or ultrasonics.

The emulsifier concentration is usually a few per cent, except for cosmetic emulsions where higher concentrations may be used.

A number of technical problems still require a great deal of research in the future. For example, the selection of emulsifiers (which at present is carried out by trial and error) needs to be rational- ised using basic concepts. Also, the problem of emulsion breakdown (coalescence) needs to be addressed at a fundamental level. Then breakdown of emulsions upon application needs to be controlled and this also requires fundamental under- standing of the interactions between the droplets and the substrate. It should be stressed that

exchange of ideas between manufacturers in various fields is far from being achieved and every industry sets its own rules and concepts. A unified approach to emulsion formation and stability is required in the future.

References

[l] H. Barnes, Colloids Surfaces A: Physicochem. Eng. Aspects,

91 (1994) 89-95.

[2] Th.F. Tadros, Colloids Surfaces A: Physicochem. Eng.

Aspects, 91 (1994) 39-55.

[3] Th.F. Tadros and B. Vincent, in P. Becher (Ed.),

Encyclopedia of Emulsion Technology, Vol. 1, Marcel

Dekker, New York, 1983.

[4] S.S. Davis, J. Hadgraft and K. Palin, in P. Becher (Ed.),

Encyclopedia of Emulsion Technology, Vol. 2, Marcel

Dekker, New York, 1985.

[ 51 Th.F. Tadros, Int. J. Cosmet. Sci., 14 (1992) 93.

[6] D.Z. Becher, in P. Becher (Ed.), Encyclopedia of Emulsion

Technology, Vol. 2, Marcel Dekker, New York, 1985.

!i~lolds a - firp.ula.ve · the first part deals with milk and the dairy industry, which involve...

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