surfactants, micelles, solubilisation Flashcards
what is an interface
boundary between 2 phases
categorise interfaces
liquid liquid = emulsion
liquid vapour = suspension/emulsion
solid vapour = tablet
solid liquid = suspension
what is surface tension
force acting over surface of a liquid perpendicular to the force
what causes surface tension/why is there an even distribution in bulk
uneven distribution of forces lead to a net inward force that drives the molecule to decrease the surface area of the system
what minimises the overall free energy
spontaneous contraction of the surface
what is the surface free energy of a liquid defined as
work needed to expand the surface by an area of 1m^2
what do surfactants do
lower surface tension of an interface, increase stability of high surface area pharmaceutical formulations, also called surface active agents and amphiphiles
structure of a surfactant
2 regions, hydrophilic head and hydrophobic tail , can be saturated or unsaturated hydrocarbon, hydrophilic region can be anionic/cationic/non ionic
difference between a diagram of a ionic and non ionic surfactant
hydrophilic head of ionic is a circle, non ionic is a squiggly line
example of anionic surfactants
sodium dodecyl sulphate, sodium dodecanoate
example of cationic surfactants
cetrimide
example of non-ionic surfactants
polysorbate, cetomacrogol 1000
describe the orientation of surfactants at interfaces
hydrophilic head in the water and hydrophobic tail outside, minimises free energy of system, more surfactants together decreases free energy
toxicity of surfactants
ability to penetrate and disrupt biological membranes, can change fluidity/permeability of membrane affecting active transport, some surfactants not cleared rapidly, some cause hypersensitivity reactions
difference between ionic and non ionic surfactants
ionic- small hydrophilic group with intense electrostatic field, powerful surfactant, toxic for in vivo due to membrane disruption, pH dependent ionisation, incompatible with other ingredients
non ionic- large hydrophobic group, less surface active, less toxic, can be used in injections, not pH sensitive, compatible
what are micelles
tiny aggregates
what does micelle formation allow
allows more surfactant molecules to enter liquid phase by wrapping the hydrophobic region in an oil core, this is a colloid
what is a colloid
mixture of small particles suspended in another medium, contains aggregates/particles of molecules surrounded by solvent molecules
difference between solution and colloid
colloid contains aggregates surrounded by solvent molecules, in solution every molecule is separate
when do micelles start to form
critical micelle concentration (CMC)
describe structure of micelle
sphere of ionic surfactant molecules (o~~) with the o facing outwards and ~ inwards, charged, attracts counterions in electrolyte solution
what is critical micelle concentration
concentration of surfactant where micelles start to form
what happens to the properties of the system at the critical micelle concentration
surface tension stops decreasing and turbidity (clarity) increases
structure of non ionic micelles
palisade layer- hydrophilic layer, circle of ~~~~
what does the palisade layer do in non ionic micelles
trap water molecules due to hydrogen bonding
how does the structure of hydrophobic region affect cmc and micelle size
increase hydrocarbon chain decreases cmc
log[cmc]=A-Bm
A and B= homologous series of surfactants
m= number of carbons in a chain
how does the nature of hydrophilic region affect cmc and micelle size
non ionic have lower cmc than ionic, non ionic can form large non spherical structures, no repulsion in non ionics so less surfactant needed for micelle formation, ionic has electrostatic repulsion that stops surfactant molecules from entering micelle
how does the nature of counter ions affect cmc and micelle size in ionic surfactants
as concentration of counter ions increases, neutralisation of head group charge is improved so more charged molecules can pack together, weakly hydrated counter ions lose their water molecule more easily so neutralise easier, counter ions reduce the reulsion
how does temperature affect cmc and micelle size
small effect on ionic surfactants, major effect on non ionic bc interaction of hydrophilic group and water is temperature dependent
temp increase= cmc decreases then increases
=micelle size increase
factors that affect critical micelle concentration and micelle size
structure of hydrophobic region- increasing length of hydrocarbon chain decreases the cmc
log[cmc]=A-Bm
A and B= homologous series of surfactants
m= number of carbons in a chain
nature of hydrophilic group- non ionic have lower cmc than ionic, non ionic can form large non spherical structure as theres no repulsion so less surfactant needed
nature of counter ion- (only for ionic) counter ion conc increases neutralisation of head group so charge is improved so more charged molecules can pack together, counter ion reduces repulsion
what is solubilisation
use of surfactants/micelles/other agents to enhance dissolution
