Surface phenomena

• the boundary between two immiscible phases. When one of

  the phases is gas or vapour, the term surface is often applied.

interface

• It refers to molecules forming the interface. These

  molecules have different characteristics from those in the bulk.

Interfacial phase

• the force per unit length acting parallel

  to the surface or interface so as to counterbalance the net inward

  attraction.

• γL/V (dyne/cm or N/m); γL/L < γL/V 

• Cohesion – attraction between like molecules

• Adhesion – attraction between unlike molecules

Surface or Interfacial tension

• Formation of surface or interfacial tension

  • In the bulk of a liquid, molecules are subject to equal attraction in all

  directions.

  • At the surface or interface, molecules are subject to a net inward force of

  attraction.

surface phenomena

• the extra energy which is the molecules at the surface have compared to the bulk molecules

• works to increase the surface area

• Because liquids naturally resist an increase in surface area to lower their surface free energy,

• creating fine droplets in emulsions or suspensions requires inputting work

• Adding surfactants reduces surface tension, requiring less work/energy to disperse the liquid into tiny droplets.

• minimum amount of work required to bring molecules from the bulk liquid up to the surface in order to expand the surface by one unit of area

 Surface Free Energy

• In the bulk of a liquid, molecules are subject to equal attraction in all

  directions

• At the surface or interface, molecules are subject to a net inward force of

  attraction.

• unbalanced forces between molecules - molecules move closer (contract)

• surface molecules - higher free energy

• to bring the molecule from the bulk to the surface - work must be done

formation of a surface or interfacial tension

• the force per unit length acting parallel to the surface to counterbalance the net inward molecular attraction

• surface free energy change

  per unit area increase, directly related to the tendency of a liquid

  to decrease its surface area.

surface tension

• To keep the bubble from collapsing, the air inside the bubble has to push out harder than the air outside is pushing in.

• pressure in the bubble has to be greater inside compared to outside

• smaller the radius = higher pressure

• If a bubble shrinks slightly by distance $dr$, its surface area drops, which releases surface free energy.

• The internal pressure does work pushing back to resist that shrinking.

• pressure = force/surface area

pressure differences

• if you dip a tiny, narrow glass tube (a capillary tube) into a dish of liquid (like water), the liquid will naturally climb up the inside of the tube against gravity until it stops at a certain height

• this is because - liquid molecules stick to the glass walls (adhesion) and drag the liquid column upward. Surface tension is what keeps pulling the liquid surface up.

• As the liquid rises, the weight of the liquid column in the tube gets heavier and pulls downward due to gravity

• The liquid stops rising when the upward surface tension force exactly balances the downward weight of the liquid column.

• for liquid - air surface tension

• Heating up the liquid gives molecules more kinetic energy, weakening the intermolecular forces holding the surface together - decreasing surface tension

• used to determine surface tension

capillary rise method

• placing a clean platinum wire ring onto the surface of a liquid (or at the boundary between two unmixable liquids, like oil and water) and then slowly pulling it upward.

• lift the ring, a thin liquid film clings to it, pulling back down - measures the maximum upward force needed to completely snap/detach the ring away from the liquid.

• used for liquid-liquid surface tension and liquid-air surface tension

ring method

• a surface layer that is exactly one molecule thick.

• for solids, liquid, gas

monolayer

• molecules gathering or sticking to a boundary (surface).

adsorption

• added molecules can dissolve in the liquid

• Because they are soluble, the molecules constantly swap places—some move from the bulk liquid up to the surface, while others leave the surface and dissolve back into the liquid.

soluble monolayers

• added molecules cannot dissolve in the liquid.

• Since they hate being inside the liquid, they sit right at the top boundary and spread out across the water, forming a thin, stable film

• monolayers can transition to different phases (gas,liquid,condensed)

insoluble monolayers

• accumulation of added

  molecules at interface or surface.

• reduces surface free

  energy and surface tension.

adsorption

• Added molecules migrate away from

  the surface to bulk - increase surface free energy and surface

  tension.

negative adsorption

• penetration of one component

  throughout the body of a second. eg taking up water by a

  sponge.

absorption

• dual personality because they contain two distinct parts:

  • Hydrophilic Head (Blue Circle): Water-loving (polar).

  • Lipophilic Tail (Squiggly Line): Oil-loving / water-hating (non-polar

  • adsorb at interfaces – orientate themselves

    to keep the lipophilic group away from the aqueous environment

    and thereby achieve a minimum free energy state

  • adsorption layer at the surface or interface is one molecule

    thick - monolayer

  • surfactant to sit nicely at the interface, it needs a proper balance between its hydrophilic and lipophilic parts (HLB).

    • If it's too water-soluble, it just stays dissolved inside the water.

    • If it's too oil-soluble, it just sits inside the oil.

surfactants - adsorption of monolayer step 1

• add surfactant molecules, they go straight to the surface/interface first.

• non-polar tails replace some of the water molecules at the surface.

• attractive forces between water molecules and the non-polar tails are much weaker than the strong cohesive forces between two water molecules. This breaks up the strong inward pull, reducing the surface/interfacial tension.

• add more surfactant, the surface eventually gets completely packed full (saturated) with a monolayer of surfactant molecules.

• At this point, there is no more room left at the surface, and surface tension cannot drop any further.

adsorption of monolayers - step 2

• once surface is full, any extra surfactant added is forced down into the bulk liquid below.

• To protect their water-hating (lipophilic) tails from touching the surrounding water, the molecules clump together into spherical clusters called micelles.

• Structure:

  • Tails tuck inside the core (away from water).

  • Heads face outward towards the water.

  • Some surface-active drugs form small micelles in aqueous

    solution

micelle formation - step 3

• the exact tipping point where the surface becomes completely saturated (packed full) with surfactant molecules.

• pass the CMC, adding more surfactant will no longer change or drop the surface tension.

• Before CMC is reached, surfactant molecules are closely packed at the

  surface, which allows the calculation of the surface area occupied by each

  molecule (A)

critical miceelle concentration

• how many surfactant molecules sit at the surface per unit area

surface excess

• determine emulsion stability

• determine efficiency of wetting

• determine dimensions of molecules

application of the surface area occupied by each molecule

• insoluble amphiphiles - fatty acids

• polymeric materials - proteins and synthetic polymers.

insoluble substances that will form monolayers

• Dissolve the substance in a suitable volatile solvent and

  carefully inject the solution on to the surface to form a film one

  molecule thick.

• film-forming molecules remain on the surface, and they are not

  in equilibrium with the bulk - area can be determined

forming a insoluble monolayer

• studies monolayers

• Shallow Teflon-coated trough with a movable barrier at the top of the

  trough.

• filled with liquid and the surface impurities are swept off using the barrier.

• Tiny amount of film-forming material is injected onto surface.

• movable barrier is squeezed towards to the floating barrier and the

  surface (film) pressure (π) can be measured.

• equates to the difference in

  surface tension between the pure liquid and the

  coated liquid.

Lagmuir trough

• studying the size and shape of molecules adsorbed at the surfaces;

• revealing the physical state of the monolayer (eg solid, liquid or gaseous

film)

• measuring the strength of the film.

what the lagmuir trough identifies

• Difference in surface tension between pure liquid and the same liquid covered with an

  insoluble monolayer

surface pressure

• Molecules are far apart, moving around freely with minimal interaction.

• low surface pressure, large area

gaseous film

• Molecules are pushed closer together. Their hydrocarbon tails begin to interact and stand up slightly, but the film is still relatively flexible and mobile.

• intermediate pressure

liquid film

• Molecules are jammed tightly together in an upright, highly ordered array.

• Occupies minimum possible area per molecule.

• high pressure

• small area

condensed film

 Adsorption of constituents of drug products by polymers.

 Permeability of polymer and the effect of incorporation of plasticiser

• suitability of polymers as

enteric and film coatings for solid

dosage forms

• Change of monolayer density under

different conditions

application of monolayers and films

• Physical adsorption: weak Van der Waals’ forces

• Chemical adsorption (Chemisorption): strong valence

forces such as ionic interaction.

• Physical + chemical adsorption: - adsorption of toxins by

activated charcoal or attapulgite and kaolin

adsorption at the solid-liquid interface

• a = represents the absolute maximum amount of drug that can stick to the surface once every single binding spot is filled (forming a complete single layer, or monolayer).

• A higher value of a means the material is a better adsorbent because it has more space/capacity to trap the drug.

determining adsorption at the solid-liquid interface

• solubility of adsorbate - a drug prefers staying dissolved in liquid (high solubility), it won't want to leave the solvent to stick to a solid surface. Less soluble drugs adsorb much more readily.

• ph influencing adsorbate - Adsorption reaches its maximum when the drug molecule is completely unionised (uncharged), as uncharged species escape the aqueous phase more easily,

• max adsorption occurs in zwitterion state of amphioteric compounds

• solubility impacts more than ionisability

• higher surface area provides more binding sites.

• chemical attraction or ionic interactions between the surface and the solute determine binding strength.

• increase temp - decreases adsorption

Factors affecting adsorption at solid/liquid interface

• Separation (eg HPLC, TLC analysis)

   Purification (water or protein purification)

   Removal of toxins (eg use of activated charcoal for antidotal

  purposes)- (refer Florence and Attwood)

   Surface area is an important factor for its effectiveness.

   Chlorpheniramine, propoxyphene, acetylsalicylic acid are effectively

  adsorbed by activated charcoal.

   Highly ionised subtances of low MW – not well adsorbed

applications of adsorption

• reduce bioavaliability - aluminium hydroxide in Mylanta, or kaolin/attapulgite in anti-diarrhoeals, act as adsorbents.

• If taken at the same time as other medications (like antibiotics), the active drug sticks to these adsorbents in the gut and cannot be absorbed into the bloodstream.

• Insoluble ingredients inside a medicine (e.g., kaolin, bentonite) can trap the active drug right inside the bottle, reducing the amount available for absorption.

• When a drug or preservative is present in very small amounts, sticking to the plastic or glass wall of a container causes a huge drop in the active amount left in solution, some drugs can stick to specific plastics

adsorption issues in drug formulation

• Therapeutic proteins are extremely surface active - Adsorbed to many surfaces: bottles, syringes, filters

• maximum adsorption when -

• Charge of the surface is opposite to that of protein OR

   When the surface is extremely hydrophobic OR

   When the pH of the protein solution is equal or close to isoelectric point

protein adsorption

• Loss of bioactivity of proteins due to surface-induced protein denaturation by

  processes:

   Irreversible adsorption

   Surface-associated aggregation

   Precipitation of protein

issues of protein adsorption

• Smooth glass walls best to reduce adsorption or precipitation

   Avoid polystyrene or containers with silanyl or plasticizer coatings

   Block access of the protein to the surface by including surfactants

  containing long polyethylene oxide chains

   addition of polyols (glycerol and/or polyethylene glycol) to solubilize

  protein

   Use of surfactants (CHAPS, a non-denaturing zwitterionic detergent for

  protein) to reduce adsorption and aggregation

   Control pH to avoid isoelectric point

preventing protein adsorption

• structure is characterised by having

  two distinct regions: hydrophilic & hydrophobic moieties.

• hydrophilic - cationic, anionic , non-ionic or ampholytic

• Hydrophobic - Saturated or unsaturated hydrocarbon chains, or

  heterocyclic or aromatic ring systems

surfactants

• stability, compatibility; less irritant; less toxic.

advantages of non-ionic surfactant

• some drugs have hydrophilic and hydrophobic region on the same molecule

• these drug molecules have more complex ring structures and chains.

• drug's surface activity relies on the balance between its water-fearing sections and its water-loving sections

• Influences binding to proteins and other

  biological molecules, including receptors

surface active drugs

• depends on the balance between its hydrophilic and hydrophobic properties.

• Increase in hydrophilicity results in decreased surface activity

• Making the ethylene oxide chain longer adds more hydrophilic units. - increasing surface tension and cmc

surface activity

• describes the relationship between hydrocarbon chain

  length and surface/interfacial activity of surfactants

• Longer carbon tails CH2 make a surfactant more surface-active, meaning it lowers the surface tension of water much more efficiently., each CH3 makes it 3 times more active

• only need 1/3 as much concentration (molar amount) of the longer surfactant to get the exact same drop in surface tension.

Traube’s rule

• measures the relative balance between the "water-loving" (hydrophilic) and "oil-loving" (lipophilic) regions of a surfactant molecule.\

• low - hydrophobic

• high - hydrophilic

Hydrophile-Lipophile Balance (HLB)

• Solubilising: Helps dissolve insoluble drugs into liquid solutions (typically high HLB).

• Emulsifying: Blends oil and water together so they don't separate into layers.

• Wetting: Helps liquids spread evenly over solid surfaces instead of forming beads.

• Detergent: Helps trap and remove grease or oily residues.

surfactant functions based on HLB

• groups of water molecules bond together via hydrogen bonds to form temporary, ice-like structures (clusters).

• structured clusters continuously break apart and reform in fractions of a second, mixed in with loose, free water molecules.

• Non-polar (hydrophobic) molecules cannot form hydrogen bonds with water - hide inside these organized, ice-like water cages.

• ordering water molecules into rigid cages around non-polar tails takes energy (lowers entropy).

• minimize these restrictive water cages, non-polar tails push together away from water, which drives micelle formation.

Flickering Cluster theory of water

• For micelle formation to happen naturally (spontaneously), delta G must be negative.

• surfactant solutions, the increase in entropy - making delta G negative.

• Individual non-polar surfactant tails force surrounding water molecules into rigid, ordered "ice-like" cages.

• non-polar tails cluster together inside the center of a micelle, shielding each other from water.

• fewer water molecules are in contact with non-polar tails, the rigid ice-like water cages break down.

• reeing those water molecules creates a large increase in entropy/disorder, lowering delta g

energy changes in micelles

• form spontaneously

  because their formation is

  energetically favourable

• forming a micelle leads to a system of lower free energy.

• driven by releasing ordered, structured water molecules trapped around hydrophobic tails

• Micelles typically form spherical (or near-spherical) shapes.

• As surfactant concentration increases beyond CMC, micelle shapes shift into cylindrical or laminar (sheet-like) structures.

• dynamic equilibrium with individual surfactant molecules (monomers) in solution, constantly breaking apart and reforming

micelle formation

• aggregation number - The exact number of individual surfactant molecules that clump together to make a single micelle.

• smaller micelles - smaller aggregation number - Charged heads repel each other, preventing too many molecules from packing closely together.

• opposite-charged ions in solution) cluster near the head groups to help reduce this electrostatic repulsion.

• Larger Micelles (Higher Aggregation Number) - no electric charge repulsion between heads, more molecules can pack together.

• Long, highly hydrated hydrophilic chains (like polyoxyethylene) stretch outward into the surrounding water, surrounding the inner hydrophobic core.

micelle structure

• Increasing tail length lowers the CMC (micelles form more easily) and increases overall micellar size.

• Molecules with rigid aromatic/hetero-aromatic rings (e.g., purines, dyes, certain amphiphilic drugs) cluster via face-to-face stacking instead of true micelle formation, so they do not have a CMC.

structure of hydrophobic group

• Electrostatic repulsion between like-charged heads opposes packing, resulting in a higher CMC and smaller micelles (lower aggregation number).

• Experience no charge repulsion, leading to lower CMC values than ionic surfactants. However, making the hydrophilic chain (e.g., polyoxyethylene) longer increases the CMC and decreases micelle size.

nature of hydrophilic group

• Counter-ions with smaller hydration shells bind more easily to the micelle surface.

• closer binding shields head-group repulsion, allowing more surfactant molecules to pack together.

• cattions and anions increase micelle size

nature of counter ions

• Adding salts reduces head-group repulsion, which lowers the CMC and increases micellar size.

• Salts have little effect since non-ionic head groups carry no electric charge.

addition of electrolytes

• non-ionic surfactants - Heating initially increases micelle size and lowers the CMC. Further heating turns the solution cloudy at a specific temperature called the cloud point due to reversible phase separation.

• Ionic Surfactants: Temperature changes have a much smaller effect.

effect of temperature

• Water-insoluble or poorly soluble drugs are dissolved by hiding inside surfactant micelles.

• hydrophobic core of each micelle acts like a tiny pool of organic solvent, creating an ideal environment to trap non-polar drug

• only occurs when the surfactant concentration is above the Critical Micelle Concentration

• Surfactants used as solubilisers typically need a high Hydrophile-Lipophile Balance value (HLB 15–18).

• Once above the CMC, adding more surfactant creates more micelles, directly increasing the amount of drug that can be solubilised.

solubilisation

• Solubilised systems stay in dynamic equilibrium between free drug, drug incorporated inside micelles, and free surfactant monomers

equlibirum of solubilisation

• Drug solubility remains flat until surfactant concentration reaches the Critical Micelle Concentration (CMC). Once micelles form above CMC, drug solubility increases sharply in direct proportion to surfactant concentration.

solubility graph

• The maximum amount of a solute (solubilisate) that can fit into a micellar system at a fixed concentration of surfactant.

maximinum additive concentration

• Deep in the hydrophobic hydrocarbon core.

non-polar solubilisate

• Aligned between surfactant molecules with its polar head facing out and non-polar tail pointing in.

amphipathic solubilisate

• Positioned near the outer region of the hydrophobic core / close to head groups.

slightly polar solubilisate

• Trapped within the outer polyoxyethylene shell (palisade layer).

polar solubilisate

• When solubilisates are in the core, micellar size increases because of

  enlarged core & increased aggregation number

solubiltes

• CH2 chain length - Increasing the hydrocarbon chain length expands the inner hydrophobic core of the micelle.

• If the drug sits deep inside the micelle core, longer tails allow more drug molecules to fit inside each micelle (within limits).

• Ethylene Oxide Chain Length (Hydrophilic Head):

  • Smaller Micelles - Longer polyoxyethylene chains decrease the aggregation number, leading to smaller individual micelles.

  • More Micelles overall - Even though each smaller micelle holds fewer drug molecules, longer hydrophilic chains result in a greater total number of micelles forming.

  • Net Result: The total amount of drug solubilised per mole of surfactant overall actually increases

nature of surfactant - solublisation

• Raising the temperature generally increases the total amount of drug solubilised.

• Micellar Size Growth - some non-ionic surfactants, higher temperatures cause individual micellar sizes to increase, creating larger cores to trap drug molecules.

• Drug Solubility - Changing the temperature also directly alters the intrinsic solubility of the drug itself.

effect on temperature - solubilisation

• Ionisation Balance: Changing the pH shifts the ratio between the ionised (charged) and un-ionised (uncharged) forms of a drug.

• Water Solubility: Un-ionised drugs are less water-soluble, whereas ionised drugs dissolve better in water.

• Micellar Partitioning - pH dictates whether the drug prefers to stay dissolved in the surrounding aqueous phase (more ionised) or move into the micelle core (more un-ionised).

effect of pH - solublisation

• Conc. of surfactant must be maintained above CMC level.

• Caution with dilution – low CMC surfactant gives fewest problems on

dilution

• Biological influence of surfactants on drug absorption and bioavailability

• Non-ionic surfactants less toxic and lower CMC.

disadvantages of drug solubilisation

• Gathers at boundaries between liquid/vapour (L/V) and solid/liquid (S/L) to lower the interfacial tension.

• Lowers Contact Angle: Allows liquids to spread out flat over a solid surface instead of bead up.

• Replaces Trapped Air: Helps liquid displace trapped air on a solid's surface so the solid can be thoroughly wet.

• Pushes trapped air off hydrophobic powders (like sulfur or charcoal) so they mix evenly into liquid vehicles.

• Medical Materials: Helps liquids penetrate absorbent materials like cotton pads and wound dressings.

surfactants as wetting agents

• primary goal of a wetting agent is to lower the contact angle ($\theta$) between a liquid droplet and a solid surface.

• HLB Value: Wetting agents typically have a Hydrophile-Lipophile Balance (HLB) value of 7 to 9.

action of wetting agents

• angle formed where a liquid droplet meets a solid surface. It measures how easily a liquid spreads over a powder or surface

• Low Contact Angle The droplet flattens out, indicating high wettability.

• High Contact Angle The droplet beads up, indicating poor wettability.

• Lowering the solid-liquid and liquid-vapour tensions reduces contact angle allowing the liquid to wet the solid easily.

contact angle

• increases the dissolution rate of a solid drug by

   aiding penetration of fluid

   reducing the tendency of particle aggregation, effectively increasing

  surface area of solid exposed to fluid.

• wetting agent does not alter the solubility of a solid

wetting agents

• Trapping a drug inside a micelle shields it from degradation.

• The deeper a drug sits in the oily inner core, the better protected it is - benzocaine

• Protection Against Base Breakdown (Base-Catalysed Hydrolysis):

  • Anionic Micelles (Negative Charge): Repel negatively charged hydroxide ions keeping them away from the drug.

  • Cationic Micelles (Positive Charge): Bind and trap hydroxide ions at the outer surface, blocking them from reaching the drug inside.

• Self-Assembling Drugs: Some drugs form their own protective micelles; for instance, a micellar solution of penicillin G is 2.5times more stable than single molecules in solution.

stabilising drugs

• They are biologically active ingredients rather than completely inactive fillers.

• Impact on Bioavailability: They can increase, decrease, or have no effect on how much drug reaches systemic circulation.

• Effects on Drug Activity: Surfactants can directly alter a drug's pharmacological effect by:

  • Affecting drug-metabolising enzymes.

  • Influencing drug binding to target receptors.

surfactants and oral bioavailability

• Disruption of cell membranes increases permeability.

• Result: Enhances drug penetration and absorption across the GI barrier.

low surfactant conc - on bioavaliability

• Drug partitions between micelles and the aqueous phase.

• For soluble drugs: Inhibits absorption because the drug gets trapped inside micelles.

• For poorly soluble drugs: Increases dissolution and absorption rates by solubilising the drug.

surfactant conc above cmc level

• Decreases the chemical potential of the drug.

• Result: Decreases drug absorption overall.

very high conc of surfactant

• Surfactants influence drug absorption through more than one pathway at a time.

• Depends heavily on whether the concentration is above or below the Critical Micelle Concentration (CMC).

  • Nature of the Drug: Whether the drug is water-soluble or poorly soluble affects how it interacts with surfactant micelles.

  • Biological Membrane: The specific type of tissue/membrane being crossed changes permeability effects.

  • Direct Activity of Surfactants: Some surfactants have pharmacological or biological effects of their own (e.g., quaternary ammonium compounds)

surfactants and drug absorption

• consist of droplets of one phase (oil or water) dispersed

  throughout another immiscible phase (water or oil

• thermodynamically unstable - Large surface area due to small size and large number of liquid droplets.

• interfacial tension between inner (disperse) and outer (continuous) phases.

(Cohesive forces between molecules of each separate phase are greater

than the adhesive forces between the two liquids).

emulsions

• improve emulsion stability by creating a protective film layer at the interface between the oil and water phases.

• Typical agents include mixtures of surfactants, hydrophilic colloids, or finely divided solid particles.

• Surfactants:

  • Adsorb at oil/water (o/w) interfaces to form monomolecular films (single-molecule-thick layers).

• Hydrophilic Colloids:

  • Form multimolecular films around dispersed oil droplets in oil-in-water (o/w) emulsions.

• Finely Divided Solid Particles:

  • Insoluble particles adsorb at the liquid-liquid interface to create a protective particulate film surrounding the dispersed droplets.

emulsfying agents

• Reducing Interfacial Tension: Lowers surface free energy between phases.

• Electrostatic Repulsion: Charged agents cause droplets to push away from one another.

• Steric Stabilisation: Physical barriers around droplets stop them from getting close enough to combine.

• Increasing Viscosity: Thickens the liquid to slow down droplet movement and collision

other ways emulsfying agents stablises an emulsion

• Surface-active agents lower interfacial tension of

  two immiscible liquids; as a result, they facilitate the break-up of large

  droplets into small ones and maintain their dispersion.

surface tension theory

• Emulsifying agents adsorb at the interface between

  oil & water, surrounding the internal phase as a thin layer of film. The film,

  acting as a physical barrier, prevents the contact and coalescing of the

  dispersed phase.

Interfacial film theory

• Surfactant emulsifying agents form a

  monomolecular layer which curves around the droplet of the

  internal (disperse) phase of the emulsion. The emulsifying agents

  orientate themselves in a manner reflective of their solubility in the

  particular phase. If it is more water soluble, it would be embedded more

  deeply in water phase.

Oriented-wedge theory

• The external phase is the one in

  which the emulsifying agent is most soluble.

Bancroft’s Rule

• Surfactants reduce interfacial tension at the phase boundary.

• they form a continuous monolayer (interfacial film) on droplet surfaces, which is the primary mechanism preventing droplet coalescence.

• an ideal film is tough, flexible, closely packed, and rapidly reforms if disturbed.

• emulsion stability depends heavily on the film structure formed at the oil/water (o/w) interface.

• Emulsifiers are most commonly combined by pairing a hydrophilic emulsifier in the aqueous phase with a hydrophobic agent in the oil phase.

• This combination forms a complex condensed film at the interface.

surfactants - emulsfying agents

• Blending two non-ionic surfactants—one with a low HLB and one with a high HLB—allows you to achieve any target HLB value.

• Each oily ingredient has a specific "required HLB" value necessary to produce a stable (O/W)or (W/O) emulsion.

• Calculated Target: A composite required HLB is calculated for a mixture of oily ingredients to establish the target HLB of the overall surfactant system.

• Proportioning Emulsifiers: Once the required HLB is determined, it is used to calculate the exact proportions of two emulsifying agents needed for the cream.

blending of surfactants

• Exhibit little surface activity and do not significantly lower interfacial tension.

• Adsorb at the oil-in-water interface to form multi-layers or multi-molecular films, acting as a physical/mechanical barrier that prevents droplets from touching and coalescing.

• if ionic groups are present (e.g., in proteins and acacia), they provide electrostatic repulsion as an extra barrier against droplet aggregation.

• Due to their hydrophilic nature, they tend to promote the formation of o/w emulsions.

• Increase the overall viscosity of the continuous phase, further improving emulsion stability.

• Application Example: Liquid paraffin emulsions using acacia and methylcellulose mucilage as emulsifying agents.

Hydrophilic colloids as emulsifying agents

• Finely divided solid particles that are wetted to some degree by both oil and water can act as emulsifying agents

• Particles concentrate at the oil-water interface and form a physical particulate film around dispersed droplets to prevent coalescence (note: particles do not dissolve).

• Particles wetted preferentially by water form o/w (oil-in-water) emulsions.

• Particles wetted preferentially by oil form w/o (water-in-oil) emulsions.

Solid particles in emulsion stabilisation