Micelles
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- Created by: LBCW0502
- Created on: 30-11-17 17:44
What is a surfactant molecule? (3)
An amphiphilic/ amphipathic molecule consisting of a hydrophilic and hydrophobic region. A molecule that accumulates are interfaces. A molecule that aggregates/self-assembles
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What are the uses of surfactants? (6)
Detergent/agrochemical/pesticide/paint/paper/food/pharmaceutical industry. Creams/suspensions/emulsions. Drug delivery (micelles/vesicles). Insoluble drugs solubilised using micelles. Drugs. Biological molecules (e.g. BSA)
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What are the three classes of surfactants?
Ionic (- surfactant and + counterion or + surfactant and - counterion), non-ionic (no charge) and zwitterionic (charge depends on pH)
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Which classification applies to the surfactant, sodium dodecyl/lauryl sulfate?
Anionic
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Which classification applies to the surfactant, cetylpyridinium chloride?
Cationic
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Which classification applies to the surfactant, dodecyl polyoxyethylene glycol?
Non-ionic
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Which classification applies to the surfactants, lecithin and dodecyl sulphobetaine?
Zwitterionic
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Give four examples of naturally occurring surfactants
Bile salts, lecithin, synovial fluid and lung surfactant
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What does the critical packing parameter (CPP) indicate?
Determines the shape of the micelle forming depending on the volume and length of the hydrophobic tail and the cross-sectional area of the hydrophilic head
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What shape is produced when CPP < 1/3?
Surfactants form a cone-shape, leading to the formation of micelles
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What shape is produced when CPP ~ 1?
Surfactants form a cylindrical shape (volume of tail increases), leading to the formation of a bilayer
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What shape is produced when CPP > 1?
(Bulky hydrophobic tails). Formation of reverse micelles
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Between which CPP range does a vesicle form?
1/2 < p < 1
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Between which CPP range does a worm micelle form?
1/3 < p < 1/2
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Give 7 examples of aggregate shapes formed by surfactants
Surfactant molecules, spherical micelles, rod-shaped micelles, hexagonal phase, lamellar phase, reverse hexagonal phase, reverse micelles
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What are micelles?
Colloidal spherical or near-spherical nanoparticles composed of 50-100 surfactant molecules (micelles are association colloids). Loose aggregates of a number of monomers (surfactant molecules) into a single particle. Dynamic structure (break/reform)
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What is the size of a micelle?
~5-10 nm (colloidal size range of ~10 nm - 1 micrometer). Diameter of ionic micelle is ~2-5 nm. Diameter of non-ionic micelle is ~10 nm
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What are colloids?
Micelles dispersed in another medium e.g. oil droplets in water, dispersion of one phase into another
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Is the case of individual monomers entering or leaving micelles a rapid or slow process?
A rapid process
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Is the case of micelles constantly breaking down and reforming a rapid or slow process?
A slow process (things constantly moving, not static, presence of thermal energy)
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What is the critical micelle concentration (CMC)?
The concentration at which micelles are formed (concentration of monomers remains the same at the CMC)
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What is osmotic pressure?
The pressure required to prevent the flow of water across a semi- permeable membrane from a region of lower solute concentration into a region of higher solute concentration
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Describe the original evidence for micelle formation
Osmotic pressure (depends on number of molecules present in water, for a given weight of material OP is inversely proportional to molecular weight). OP decrease, M increase. Linear increase. CMC - lower increase, micelles formed
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Describe the evidence for ionic surfactants
Amount of current carried depends on size and charge of molecule (small -singly charge/most current, large - multi-charged/least current). Expect increase conductivity with increase in ions. CMC, decrease in conductivity (ions bind to micelles)
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What are the five factors which provide evidence for micelle formation?
Surface tension, osmotic pressure, molecular conductivity, drug solubilisation, light scattering (micelles scatter more light)
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What is the general law of mass action?
Equilibrium constant (Km) = [C] / [A].[B] (equilibrium between micelles and surfactants in solution)
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What is the association-dissociation equilibrium (for non-ionic surfactants)?
Km = [M] / [D]^n (n is the aggregation number/number of monomers or surfactants making up the micelle)
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What happens to the concentration of monomers and non-ionic surfactants below the CMC?
Concentration of monomers increases as surfactant concentration increases
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What happens to the concentration of monomers and non-ionic surfactants below the CMC?
Concentration of monomers virtually constant; concentration of micelles increases as surfactant concentration increases
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What is the association-dissociation equilibrium (for ionic surfactants)?
Km = [M^p+] / [D+]^n . [X-]^n-p
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What does the association-dissociation equilibrium (for ionic surfactants) take into account?
Counterions present - stabilises head group of surfactant to produce neutral solution (complementary). Micelle more + charged. Counterions distributed insolution but no counterion for every surfactant present
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What happens to the concentration of monomers and ionic surfactants below the CMC?
Concentration of monomers increases
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What happens to the concentration of monomers and ionic surfactants above the CMC?
Concentration of micelles increases, slight increase in counterions, constant concentration for monomers (85% of counterions associated with micelles)
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What is the main thermodynamic origin of micelle formation (in water)?
Chemical reactions (here micelle formation) always go to the lowest energy state
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When considering energy changes in micelle formation, you need to consider the interaction with water and what? (2)
Hydrophobic tail (HC chains), hydrophilic head groups
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Describe the structure of water
Extensive H bonding structure. Lattice with temporary 3D flickering structures. Dynamic. Some water molecules are mobile/independent. Cluster formation
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Describe the interaction of water molecules and hydrocarbon chains
No possible interaction. Immediate contact. Limit ability to form H bonds. Water cage around HC chain. More ordered/lose entropy. Increase in free energy (unfavourable process)
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What is the hydrophobic effect?
HC chains cannot interact with water, water 'squeezes' chains out. Not a repulsive force but a driving force for micellisation. HC stabilised by head groups. HC chains shielded in core/regain entropy (increase entropy/decrease free energy)
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How significant is the change in enthalpy of micelle formation?
10% of overall free energy change
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How does the entropy change arise?
Loss of water structure around HC chain, increase internal freedom of HC chain inside micelle core, loss in high surface tension between HC and water in micellisation (HC in micelle core)
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Describe the interaction of water and hydrophilic head groups
Water molecules interact with surfactant head groups - compensate the loss of interaction with other water molecules e.g. polymer head groups such as polyoxyethylene glycol can form hydrogen bonds with water, ionised head groups, CO2- can be hydrated
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What stops micelles from growing and growing (ionic surfactants)?
Repulsion from charges in hydrophilic head groups (too close contact). Cannot insert more surfactants in micelles. No efficient packing (hole in micelle/water. Unfavourable entropy
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What stops micelles from growing and growing (non-ionic surfactants)?
Steric effects (but weaker repulsions of hydrophilic head groups)
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Nomenclature for surfactants
Learn concept
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Describe opposing micelle formation (2)
Hydrophobic effect is the driving force for aggregation. Repulsive force of head group is an opposing group for aggregation
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Is the aggregation number of ionic surfactants greater or less than the aggregation number of non-ionic surfactants?
Less
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Is the micelle size of ionic surfactant greater or less than the micelle size of the non-ionic surfactant?
Less
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The CMC of ionic surfactant is greater or less than the CMC of the non-ionic surfactant?
Greater (>>)
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How does the CMC affect the size of the micelles?
The lower the CMC, the larger the size of the micelles
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Describe the principle of opposing forces
Hydrophobic effect is the driving force for micellisation. Repulsive force between head groups is opposing formation of micelles
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What do the two opposing forces determine?
Aggregation number (size of micelle) and concentration at which aggregates form (CMC)
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What happens if there is an increase in hydrophilicity? (4)
Increase driving force for micellisation, increase aggregation number, increase micelle size, decrease CMC (applies to non-ionic surfactants)
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What happens if there is an increase in hydrophobicity? (4)
Increase repulsive force for micellisation, decrease aggregation number, decrease micelle size, increase CMC (applies to ionic surfactants)
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Which factors affect micelle formation? (6)
Altering hydrophobic chain length, altering hydrophilic head group, temperature, electrolyte, nature of counterion and pH
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How does altering the hydrophobic chain length affect ionic surfactants?
Increasing the number of Cs in HC chain increases micelle size and decreases CMC by 1/2 for each additional C (linear, Log CMC = A - Bm)
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How does altering the hydrophobic chain length affect non-ionic surfactants?
Increasing the number of Cs in HC chain increases micelle size and decreases CMC by 1/10 for each additional 2Cs
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When altering the hydrophobic chain length, why does the CMC increases for ionic and non-ionic surfactants?
Longer HCs, more structured water around hydrophobic region, greater driving force for micellisation, micelles form at lower CMC, larger micelles due to larger aggregation numbers
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How does decreasing the size of a micelle (made up of non-ionic surfactants) affect CMC and size of hydrophilic heads group?
Decrease size, increase CMC, increase size of hydrophilic head groups
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How does temperature affect ionic surfactants?
(Little effect) As temperature increases, Brownian motion increases, water structure decreases. Increase temperature, decrease size of micelle, increase CMC
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How does temperature affect non-ionic surfactants?
(Large effect) Increase temperature, increase size of micelles, decrease CMC (cloud point)
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What is screening?
The addition of extra counterions reduces the effective charge on surfactant head group
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How do electrolytes affect ionic surfactants?
Decrease CMC, increase micelle size, drive sphere-to-rod transformation/decrease SA head group/increase CPP
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How to electrolytes affect non-ionic surfactants?
Weaker effect due to uncharged head group/effect depends on nature of salt. Hofmeister (lyotropic) series: classification of ions based on their ability to salt out or salt in proteins
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How does the nature of the counterion affect ionic surfactants?
Cation - decrease CMC/increase micelle size. Anion - decrease CMC/increase micelle size (organic counterions have lower CMCs/higher aggregation numbers compared to inorganic counterions)
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How does pH affect ionic surfactants?
pH decrease, ionisation of head group increases, micelle size increases until it comes out of solution (head group unionised). Unionisation - increase size/decrease CMC
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How does pH affect non-ionic surfactants?
Little or no effect (hydrophilic head is not charged)
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What is micellar solubilisation?
A material insoluble in water can increase its 'apparent' solubility by being incorporated into a micelle (dissolve in hydrophobic region). A way for formulating poorly soluble drugs in aqueous solution
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What are the uses of micellar solubilisation?
Used for patients such as children and the elderly. Administration of insoluble drugs into the eye. IV administration (dosage form as a liquid)
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What is the general definition of solubilisation?
Process of preparation of a thermodynamically stable, isotropic solution of a substance (insoluble or sparingy soluble in a given solvent) by incorporation of additional amphiphilic component (affinity for soluble/insoluble components, equilibrium)
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What is a solubilisate?
The material to be solubilised e.g. drug
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Why is the general definition for solubilisation quite broad?
Doesn't state: concentration of amphiphile, whether it is a surfactant, nature of solubilisate (e.g. polar/non-polar), mechanism of inclusion
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What does the definition of solubilisation include?
Co-micellisation and hydrotropy (there are different methods of solubilisation/shouldn't be confused with micellar solubilisation)
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What is co-micellisation?
Large increase in solubility that a slightly soluble material undergoes when added to a solution containing a sub CMC concentration of a surfactant (aggregation of two compounds)
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What is hydrotropy?
Increase in solubility of an insoluble solute by a third substance which is not highly surface active - does not form micelles at low concentrations (e.g. type II solutes)
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What is a hydrotrope?
An amphiphile (not a surfactant), which contains a hydrophobic region and a hydrophilic region
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Give examples of hydrotropes
Salicylic acid, caffeine (not widely used/exert pharmacological effect)
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Describe a graph showing drug solubilisation against surfactant concentration?
Below the CMC, as the concentration of surfactant increases, drug solubilisation doesn't increase. Above CMC, as surfactant concentration increases, drug solubilisation increases linearly (micellisation or co-micellisation)
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How can the concentration of a drug in solution be measured?
Using UV absorbance
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What are the possible sites of solubilisation inside a micelle?
Hydrophobic core (non-polar), core/mantle region (intermediate polarity), head group (mantle/shell/corona) region (polar), adsorption onto surface of micelles. Applies to ionic micelles only
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Where is the site of solubilisation inside a micelle for a non-polar solute e.g. octanol?
Core of micelle
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Where is the site of solubilisation inside a micelle for a semi-polar solute e.g. octanediol?
Orientated in micelle with polar group either buried (deep) or near surface (shallow) depending upon the size and nature of the hydrophobic region
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Where is the site of solubilisation inside a micelle for a polar solute?
In the mantle region (non-ionic micelles) –or adsorbed on the surface (ionic micelles)
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What is the partition coeffient?
An expression for the distribution of a solute between two phases. P = C0 (non-aqueous) / Cw (aqueous). Expressed as log P (greater value/higher lipid solubility). E.g. octanol
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What is the effect of having large surfactants on the stability?
Large surfactants, very hydrophobic core, large micelles, lower CMC, more stable
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What is the assumption about solubilisation?
The larger the micelle, the more drug solubilised (vague)
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What are the factors that affect solubilisation?
Nature of surfactant (hydrophilic head group/hydrophobic tail), nature are solubilisate, temperature and additives (generic factors)
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How does altering the hydrophobic chain length affect an ionic surfactant?
Longer the hydrocarbon chain, the larger the micelle and the greater the solubilisation (up to ~ C16). Introduction of a cis double bond reduces both micelle size and solubilisation
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How does altering the hydrophobic chain length affect a non-ionic surfactant?
Longer the hydrocarbon chain, the larger the micelle and the greater the solubilisation. Introduction of a cis double bond reduces both micelle size and solubilisation
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Why does a cis bond reduce micelle size and solubilisation?
Change in shape, lack of rotation, less optimal packing of chains in micelles (steric hindrance), smaller hydrophobic core
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How does altering the hydrophilic group affect an ionic surfactant?
Can be significant, anionic surfactants > cationic surfactants, problems if solubilisate has a charge, variable/depends on hydrophilic groups
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How does altering the hydrophilic group affect a non-ionic surfactant?
Increasing the length of the polyoxyethylene chains leads to smaller micelles and reduced solubilisation (but also depends where the solubilisate is located)
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How does the nature of the solubilisate affect solubilisation?
Simple homologous series, the longer the alkyl chain, the lower the solubilisation. Complex effects/depends on nature of groups. Lipophilic - high partition coefficient, hydrophilic - low partition coefficient
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How does temperature affect solubilisation?
Complex factor. Affects micelle size and solubility of solubilisate. Ionic micelle - expect: increase in solubilisation with temperature, decrease micelle size, drug solubility in micelle decreases, drug solubility in water increases overall
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How do additives (electrolytes) affect ionic surfactants?
Can alter solubilisation due to changes in micelle size often solubilisation increases due to an increase in micelle size. Ion interacts with head group. Reduce charge interaction. Less repulsion. More surfactants in micelle. Large micelle
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How do additives affect non-ionic surfactants?
Little effect (salting in/out effect)
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How do additives (non-electrolytes) affect ionic and non-ionic surfactants?
E.g. ethanol often has a significant effect on solubilisation Ethanol reduces micelle size and therefore solubilisation. Some additives may themselves be solubilised e.g. preservatives and flavourings
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What are the applications of micelle solubilisation?
Phenolic compounds (soap), iodophors (sterilisation), drugs (vitamins/oils), polymeric micelles (cancer drugs)
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Other cards in this set
Card 2
Front
What are the uses of surfactants? (6)
Back
Detergent/agrochemical/pesticide/paint/paper/food/pharmaceutical industry. Creams/suspensions/emulsions. Drug delivery (micelles/vesicles). Insoluble drugs solubilised using micelles. Drugs. Biological molecules (e.g. BSA)
Card 3
Front
What are the three classes of surfactants?
Back
Card 4
Front
Which classification applies to the surfactant, sodium dodecyl/lauryl sulfate?
Back
Card 5
Front
Which classification applies to the surfactant, cetylpyridinium chloride?
Back
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