Emulsions

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Introduction

Emulsions are unbiquous ... and some of the most interesting and challenging are in foods:

Dickenson, Food Science.


Microfluidics are terrific ... where can we add the "magic" of emulsion science?

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What is an emulsifier?

An emulsifier is a type of surfactant typically used to keep emulsion (mixtures of immiscible fluids) well dispersed. Emulsifiers typically have a hydrophobic (water-hating) and a hydrophilic (water-liking) end. The emulsifiers will surround an oil (or other immiscible molecule) and form a protective layer so that the oil molecules cannot "clump" together. This action helps keeps the dispersed phase in small droplets and preserves the emulsion. An example of emulsifiers is oil dispersed into water. Emulsifiers are very effective in applications in which a high shear is required, as they can provide fast mechanical and hydraulic shear.

Emulsifiers are common in the food processing, pharmaceutical, cosmetic and toiletry, chemical, agricultural, pulp and paper, automotive, and adhesive and sealant industries. There are countless substances emulsifiers are used for, including mixing beverages, medicines, adhesives, petroleum products, and more.

While some applications require specific types of emulsifiers, others can be achieved by a variety of methods. Cost effectiveness and efficiency are often the most important considerations when choosing emulsifiers.

Lipophilic emulsifiers were introduced in the late 1950's and work with both a chemical and mechanical action. After the emulsifier has coated the surface of the object, mechanical action starts to remove some of the excess penetrant as the mixture drains from the part. During the emulsification time, the emulsifier diffuses into the remaining penetrant and the resulting mixture is easily removed with a water spray.

Hydrophilic emulsifiers also remove the excess penetrant with mechanical and chemical action but the action is different because no diffusion takes place. Hydrophilic emulsifiers are basically detergents that contain solvents and surfactants. The hydrophilic emulsifier breaks up the penetrant into small quantities and prevents these pieces from recombining or reattaching to the surface of the part. The mechanical action of the rinse water removes the displaced penetrant from the part and causes fresh remover to contact and lift newly exposed penetrant from the surface.


Terminology

Macroemulsions At least one immiscible liquid dispersed in another as drops whose diameters generally exceed 1000 nm. The stability by addition of surfactants and/or finely divided solids. Considered only kinetically stable.
Miniemulsions An emulsion with droplets between 100 and 1000 nm. Reportedly thermodynamically stable.
Microemulsions A thermodynamically stable, transparent solution of micelles swollen with solubilizate. Usually requires a surfactant and a cosurfactant (e.g. short chain alcohol).

Becher, P. Emulsions, theory and practice, 3rd ed.; Oxford University Press: New York; 2001.

Examples of mircoemulsions include cutting oils, oil/water mixtures used for dry cleaning, pesticides and many household cleaners. They make especially effective cleaning products because they often have low surface tensions permitting them to be easily removed from solid surfaces. Microemulsions may form in systems with Ternary phase diagrams. Such materials may exist in an oil phase, an aqueous phase and surfactant. These diagrams look like triangles and were previously discussed in the phase diagram lecture of this course. Microemulsions may form from slight mixing and do not require dramatic sheering to form.

There is currently much research going on to develop microemulsions for pharmaceutical delivery.

Macroemulsions

They are thermodynamically unstable and break apart when left for an elongated period of time. Surfactants or small particles can be used to stabilize them.

The main factors of destabilization for macroemulsions are: creaming (density difference causing a large concentration of drops at the bottom), flocculation (drops form aggregates of two or more drops as the move into the secondary minimum of the interaction curve), coalescence (2 drops combining), and ostwald ripening (diffusion of the molecules that causes small droplets to decrease in size whil large continue to grow).

These are shown in the image below:

Macro.jpg

Reference: http://www.hull.ac.uk/scg/paunov/paunov06536-9.pdf

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Emulsion processes

Dickenson?
A - Inversion C - Sedimentation E - Coalescence
B - Creaming D - Flocculation F - Ripening
  • A - Inversion: An emulsion becomes "inverted" when the dispersed and continuous phases become switched. An application of emulsion inversion is shipping of industrial chemicals; for example, in the paper making industry, chemicals called "retention aids" (that help keep the small particles in paper from flaking out) are often manufactured and shipped as emulsions. The reason is that the retention aids are long polymers, and are kept in water droplets in an oil solution. The emulsion is then reversed, allowing the polymers to expand/interweave and add strength to the paper. See the Mini-Encyclopedia of Papermaking Wet-End Chemistry for details.
  • B - Creaming: Creaming is when the dispersed phase floats to the top of the solution (due to gravity), the way cream will float to the top of fresh milk. For a typical oil-in-water emulsion, creaming can only occur if the oil droplets are smaller than ~1 micron (at which point brownian motion takes over). Therefore, one method to prevent creaming is to add surfactants that keep the droplets from combining and therefore small enough to remain dispersed.
  • C - Sedimentation: The physics of sedimentation is exactly the same as that of creaming, except sedimentation occurs when the dispersed phase is more dense than the continuous phase so it sinks to the bottom. The methods for preventing sedimentation are similar to those that prevent creaming.
  • D - Flocculation: Flocculation is when droplets of the dispersed phase begin to aggregate, but not still maintain a layer of the continuous phase between them (as opposed to coalescence or ripening). Studies have shown that saliva can induce significant flocculation, which may have a strong effect on the sensory perception of emulsified food and drinks, like milk. See this paper from the journal "Food Hydrocolloids" for one such study.
  • E - Coalescence: Coalescence is when the droplets of the dispersed phase combine into larger, individual droplets. This process can significantly enhance creaming/sedimentation (as can flocculation). Coalescence is therefore bad for something that should remain an emulsion like food or cosmetic products, but inducing coalescence is important when we wish to extract one of the phases and discard the other. For example, crude oil often is a water-in-oil emulsion, so the water must be coalesced out. A common technique is to use electric fields; see this paper for more details.
  • F - Ripening: Ostwald Ripening (or just ripening) is a type of coalescence, but it refers in particular to the process whereby large droplets consume smaller droplets to grow even larger. This effect is responsible for ice cream becoming crunchy if it is not frozen properly; ice cream should have small ice crystals for good flavor, but ripening can cause large crystals to grow at the expense of the smaller ones, resulting in an unpleasant texture.

The stability of emulsions is determined by a variety of factors:

Electrostatic stabilization at lower volume fractions
Steric stabilization at all volume fractions
Additional factors Temperature is important – solubility changes quickly.
Steric stabilization is enhanced by solubility in both phases:
EmulsionStericStabilization.png
Mixed emulsifiers (cosurfactants) are common. They can come from either phase.
EmulsionElectrostericStabilization.png


The creaming of emulsion can be shown by the variation in volume fraction at various heights and times as determined by measuring the speed of sound:

Morrison, Fig. 22.13


Emulsion inversion - As the concentration increases (A) the droplets get closer until they pinch off into smaller, opposite type of emulsion (B).
EmulsionInversion.png


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Making emulsions

Method of phase inversion e.g. Use a poor O/W emulsifier, go to high volume fractions, the emulsion inverts to smaller droplets of W/O
Phase-inversion-temperature method e.g. Heat and emulsify O/W 2-4o below the PIT, creates low s and small drops, cool to room temperature.
Solubilize vapor in micelles The energies driving the condensation, drive Ostwald ripening, therefore a formulation challenge.
Electric emulsification Charging the surface produces electrohydrodynamic instabilities.
Intermittent milling Surfactant adsorption is slow – waiting helps.


Intermittent milling


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Breaking emulsions

From: Menon, V.B.; Wasan, D.T. Demulsification, in Encyclopedia of emulsion technology; Becher, P., Ed.; Marcel Dekker: New York; 1985, Vol. 2; pp 1-75.

Creaming Especially with a centrifuge, taking advantage of temperature and salt.
Mechanical Sometime high shear; filtering through bed whose surfaces are wetted by internal phase; ultrafiltration; dialysis;
Thermal Most emulsion a less stable hot; At the PIT many are quite unstable; freeze-thaw.
Chemical Chemically change the emulsifier; mismatch of HLB, pH; replace with strong surfactant but not strong emulsifier; addition of other solvents.

Separation of crude oil is probably the most studied emulsion breaking process. Biodiesel is separated by adding a solvent to the mixture. Once enough solvent has been added to the mixture, it will destabilize and separate. Other patented methods for separating emulsions of organic compounds from aqueous solution include adding clay to adsorb organic material in an emulsion. The clay removes enough organic material to separate some of the aqueous solution from the organics. This may be mechanically removed. An iterative process of adding clay and removing water may be used until the emulsion is completely broken. Mechanical methods are used to breakup an emulsion of tank bottom sludge found in petroleum tanks into commercial grade oil, water and an aqueous mixture of non-oil based solids (sounds toxic, but they didn't mention in the patent how companies dispose of it).

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Bancroft's rule and the HLB

Bancroft's Rule:

The emulsifier stabilizes the emulsion type where the continuous phase is the medium in which it is most soluble.
A hydrophilic solute in an O/W emulsion.
The long tail on the surfactant is to represent the longer range interaction of a hydrophilic molecule through water.
A hydrophilic solute in a W/O emulsion.


The Hydrophile-Lipophile Schema:

Variation of type and amount of residual emulsion with HLB number of emulsifier at room temperature. Morrison, Fig. 22.5


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Phase inversion temperatures

The "type" of an emulsifier often changes with increasing temperature; typically from being water soluble, hence a high HLB number to being water insoluble, hence a low HLB number. Therefore the type (O/W or W/O) emulsion it stabilizes changes. The temperature of the transition is called the Phase Inversion Temperature or PIT.

www.bias-net.com/chimica/pdf/set_baglioni.pdf

The HLB and the PIT are often related:

HLBandPIT.png

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Interfacial properties

Morrison, Fig. 22.8

The rheology of O/W interfaces can be examined by measuring the diffusion of traced particles at the interface:

Wu and Dai, Langmuir, 23, 4324 – 4331, 2007.
For viscous liquids: <math>\left\langle \Delta r^{2}\left( \tau \right) \right\rangle =4D\tau \text{ where }D=\frac{k_{B}T}{4\pi \eta a}\,\!</math>

For elastic liquids: <math>\left\langle \Delta r^{2} \right\rangle =\frac{2k_{B}T}{3\pi a{G}'}\,\!</math>

The particles have to sit properly at the O/W interface.


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Multiple emulsions

Rosen, p. 313
(a) W/O/W double emulsion (b) O/W/O double emulsion
Consider, for either diagram: Each interface needs a different HLB value.

The curvature of each interface is different.

One the practical applications of multiple emulsions would be for use in drug delivery systems. The ability to make a double walled cell around a pharmaceutical drug would be break through for drug delivery systems. This goal remains far in the distance. Currently, multiple emulsions can be generated using two concentric capillary tubes. The volume of and phase of the emulsions may be adjusted by changing the flow rates out of the capillary tubes. Similar procedures permit the creation of emulsions containing multiple droplets. There is a project in the Weitz lab focused on creating multiple emulsions. There are many cool photos on this website: http://www.seas.harvard.edu/projects/weitzlab/alvaroweb/Triple%20and%20Multiple%20Emulsions.htm.

Double Emulsions as Templates for Vesicles

Water/Oil/Water (W/O/W) emulsions stabilized with phospholipid surfactants can be formed using the typical capillary methods for creating multiple emulsions in microfluidics. If a volatile organic compound is chosen as the oil phase for the emulsion, the phospholipids adsorbed to the two surfaces are brought into contact as the oil phase is a evaporated, and thus, a vesicle is formed. Vesicles are a promising application of double emulsions which may enable applications including drug delivery.

A phospholipid vesicle formed from a double emulsion template (Shum et al., Langmuir, 2008).

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Particles as emulsion stabilizers

Almost all particles are only partially wetted by either phase.

When particles are “adsorbed” at the surface, they are hard to remove – the emulsion stability is high, sometimes thousands of kT. Crude oil is a W/O emulsion and is old!!

Morrison, Fig. 22.3


Variation of the free energies of desorption (relative to kT) of a spherical particle 10 nm radius at a planar O/W interface of interfacial tension of 36 mN/m with a contact angle which the particle makes with the interface (measured through the oil phase) at 298 K.
Morrison, Fig. 22.4


The thermodynamics is rich.
Wu and Dai, Langmuir, 23, 4324 – 4331, 2007.
Wu and Dai, Langmuir, 23, 4324 – 4331, 2007.
Wu and Dai, Langmuir, 23, 4324 – 4331, 2007.
Figure 7. Sketch of a particle of radius a, which is bridging between the surfaces of a film from phase 2 formed between two drops of phase 1. h is the film thickness. <math>\sigma </math> is the contact angle. Figure 8. Definitions of phases, angles, and emulsions: By definition, the particles are initially dispersed in phase 2. The contact angle, <math>\sigma </math>, is always measured across phase 2. The emulsion 1-in-2 is a Bancroft-type emulsion, in which the particles are dispersed in the continuous phase. In contrast, the emulsion 2-in-1 is of anti-Bancroft type.


Hydrophilic silica stabilizing a wax/water emulsion:

J. Giermanska-Kahn,† V. Laine,† S. Arditty,† V. Schmitt,† and F. Leal-Calderon
Langmuir 2005, 21, 4316-4323
Figure 1. Microscopic image of a paraffin-in-water emulsion stabilized by CTAB alone. T ) 25 °C. Figure 3. Microscopic image of a paraffin-in-water emulsion stabilized by P2 particles. Inset: same image taken at T ) 25 °C under crossed polarizers, confirming the presence of crystals

in the droplets.

Texturas by Ferran Adria

In his recent visit to Harvard, world-famous chef and culinary innovator Ferran Adria gave several video demonstrations of his cutting-edge cooking techniques. He spoke of his desire to create a new language of food. Some chef in the past made the first salad, the first soup, and the first bread, and now there are thousands of varieties of each. Each of these examples could be considered new "words" that could create a new world of culinary possibilities. Over the past decades, he has been searching for fundamental advances in cooking that could be considered new letters of the culinary alphabet.

This new alphabet is exemplified by Albert and Ferran Adria's new line of Texturas, various chemicals that enable new culinary experiences. Below are some specific cases related to emulsions, including annotated recipes:

Receta lecite 01.gif
Lecite: Soy-based lecithen was discovered at the end of the 19th century and has now made its way into may of Ferran Adria's new creations, especially the airs. This particular form comes as a fine powder that is soluble in water, even at cool temperatures. As the website says, lecite is ideal for "converting juices and other watery materials into airs."

Frozen Parmesan air

  • 500g grated Parmesan
  • 450g water
  • 3g Lecite'
  1. Mix the Parmesan with the water and gradually heat to 80 °C. (how would this be different with other cheeses, which have different fat-protein ratios and have different water contents?)
  2. Steep for 30 minutes and strain. (can this duration be calculated from the principles in this class?)
  3. Add 1.3g of Lecite for every 250g of Parmesan solution obtained. (0.5% ratio by mass.)
  4. Use a hand-held mixer on the surface of the liquid, allow to stabilize for one minute and collect the air that has formed on top.
  5. Freeze the air in a container of choice.


Muelle.gif
Sucro: This form of sacarose (is this the same as sucrose?) is already widely used in Japan to create water-in-oil type emulsions. This form also comes in a water-soluble powder, which dissolves even at cool temperatures.

Olive oil spiral: For virgin olive oil caramel

  • 100 g of Isomalt
  • 25 g of glucose
  • 1.5 g of Sucro
  • 45 g of extra virgin olive oil
  • 1.5 g of Glice
  1. Mix the Isomalt, the glucose and Sucro and cook at 160° C (they will obtain the missing 5° C with their own heat (good time for a thermodynamics calculation)).
  2. As the caramel is cooking, dissolve Glice with the virgin olive oil at 50° C.
  3. When the caramel is at 160° C drizzle the oil and bind with a spatula.
  4. When the caramel has absorbed all the oil, spread out on sulphurised paper. (what is the purpose of the suplhur?)
  5. With this caramel we can make many different forms, such as the olive oil spiral.


Aceitunaa.gif
Glice: The mono- and di-glyceride emulsifier is only soluble in oil heated past 60 C.

Black olive emulsion

  • 50 g of black olive water
  • 1 leaf of gelatine, 2 g

(previously rehydrated in cold water)

  • 0.5 g of Sucro
  • 50 g of black olive grease
  • 0.5 g of Glice
  1. Dissolve the gelatine with 1/3 part of the black olive water at medium temperature and add the rest of the water.
  2. Add Sucro and blend with a turmix.
  3. At the same time, dissolve Glice with the black olive grease at a temperature of about 50° C. Continue to add the grease to the black olive water while binding with the turmix.
  4. Keep in the refrigerator for 2 h. When it has set, cut 10 pieces of 0.2 g each. This emulsion is served with the disc of mango.

Emulsions in Medicine

Nanoemulsions of soybean oil can be used as drug delivery systems (400-600nm in diameter). The drug treatment is physical rather than chemical which means instead of correcting chemical imbalances or using a chemical some cell does not like the nanoemulsion which uses surface tension to merge with bacteria cells and viruses. By merging with the pathogens it destroys their membranes and kills it. These soybean oil nano emulsions do not merge with the majority of human cells but are known to destroy sperm cells and blood cells, which bascially means this cannot be used intravenously.

"The most effective application of this type of nanoemulsion is for the disinfection of surfaces. Some types of nanoemulsions have been shown to effectively destroy HIV-1 and various tuberculosis pathogens, for example, on non-porous surfaces." [http://en.wikipedia.org/wiki/Emulsion ]

It is plausible that one day the emulsions can be tailored to target certain cells. Certain eating disorders, GI tract issues, cardiac conditions and eye problems already have treatments being developed. The references below are still in infant stages. [1] [2]

Also certain topical treatments are being improved upon by adding emulsions. (Wounds etc) [3]


Nanoemulsions

Nanoemulsions can be defined as oil-in-water (o/w) emulsions with mean droplet diameters ranging from 50 to 1000 nm. Usually, the average droplet size is between 100 and 500 nm. The terms sub-micron emulsion (SME) and mini-emulsion are used as synonyms. Emulsions which match this definition have been used in parenteral nutrition for a long time. Usually, SMEs contain 10 to 20 per cent oil stabilized with 0.5 to 2 per cent egg or soybean lecithin. A typical formulation is given in Table 1.

Table.jpg

The preparation of nanoemulsions requires high-pressure homogenization. The particles which are formed exhibit a liquid, lipophilic core separated from the surrounding aqueous phase by a monomolecular layer of phospholipids. The structure of such lecithin stabilized oil droplets can be compared to chylomicrons. Nanoemulsions therefore differ clearly from the liposomes, where a phospholipid bilayer separates an aqueous core from a hydrophilic external phase (see figures 1 and 2). If nanoemulsions are prepared with an excess of phospholipids, liposomes may occur concurrently.

Picture.jpg

What are the Benefits of Using Nanoemulsions in Skincare Products? Due to their lipohilic interior, nanoemulsions are more suitable for the transport of lipophilic compounds than liposomes. Similar to liposomes, they support the skin penetration of active ingredients and thus increase their concentration in the skin. Furthermore, nanoemulsions gain increasing interest due to their own bioactive effects. Nanoemulsions are able to favor the transport of suitable lipids into the skin. This may reduce the transepidermal water loss (TEWL), indicating that the barrier function of the skin is strengthened.

Nanoemulsions Do Not Cream - How This Can Benefit New Products? In addition, a special product feature is to be mentioned: nanoemulsions do not cream. This allows us to formulate liquid products which are sprayable and do not show a phase separation during storage. As an alternative to phospholipid-containing nanoemulsions, emulsifier-free o/w submicron emulsions may also be prepared.

Emulsions in Personal Care products and Pharmaceuticals

Creaming in personal care products (which are often oil/water emulsions) and pharmaceuticals is generally undesirable, due to the appearance and risk of coalescence. Thus, many steps are taken with personal care products to prevent creaming and other forms of deterioration that can occur with emulsions. Small particle size and homogenization can help prevent creaming in such products. Many emulsions are sensitive to light and oxidative decomposition, so they are often packaged in tightly sealed packages that do not allow light to pass through. Antioxidants are included to prevent oxidative decomposition. Fungi (more often than bacteria) can often contaminate emulsions, which is why fungistatic preservatives like methylparaben and propylparaben are included in most personal care products.Ansel's Pharmaceutical Dosage Forms and Drug Delivery Systems


Emulsion Polymerization

How are polymer colloids fabricated? How do they make colloids so monodisperse? One of the most common methods is emulsion polymerization.

The most common type involves an oil-in-water emulsion, where droplets of hydrophobic monomer are emulsified in water. The particles are kept stable in solution either with a surfactant or by steric stabilization (with water coluble polymers).

Smith and Ewart first developed a theory of how this process works (for surfactant stabilized emulsion polymerization):

1. "A monomer is dispersed or emulsified in a solution of surfactant and water forming relatively large droplets of monomer in water."

2. High concentration of surfactant creates micelles in the water.

3. Small amounts of monomer become soluble inside the micelles.

4. "A water-soluble initiator is introduced into the water phase where it reacts with monomer in the micelles... This is considered Smith-Ewart Interval 1."

5. "The total surface area of the micelles is much greater than the total surface area of the fewer, larger monomer droplets; therefore the initiator typically reacts in the micelle and not the monomer droplet."

6. "Monomer in the micelle quickly polymerizes and the growing chain terminates [due to lack of monomer]. At this point the monomer-swollen micelle has turned into a polymer particle. When both monomer droplets and polymer particles are present in the system, this is considered Smith-Ewart Interval 2."

7. "More monomer from the droplets diffuses [osmosis?] to the growing particle, where more initiators will eventually react."

8. "Eventually the free monomer droplets disappear and all remaining monomer is located in the particles. This is considered Smith-Ewart Interval 3."

"Depending on the particular product and monomer, additional monomer and initiator may be continuously and slowly added to maintain their levels in the system as the particles grow.

The final product is a dispersion of polymer particles in water. It can also be known as a polymer colloid, a latex, or commonly and inaccurately as an 'emulsion'. "

I imagine if you want to grow the particles to larger sizes, the micelles would have to shift shape to avoid over-swelling.

One can also have surfactant free emulsion polymerization:

"Smith-Ewart theory does not predict the specific polymerization behavior when the monomer is somewhat water-soluble, like methyl methacrylate or vinyl acetate. In these cases homogeneous nucleation occurs: particles are formed without the presence or need for surfactant micelles."

This generously borrowed from a wiki article: [[4]]

Emulsion Polymerization Cartoon.JPG


Keyword in references:

Droplet Microfluidics for Fabrication of Non-Spherical Particles (ktian)

Dynamic Pattern Formation in a Vesicle-Generating Microfluidic Device.

The Science of Chocolate: Interactive Activities on Phase Transitions, Emulsification, and Nucleation


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