Surfactants

What are the four main classifications of surfactants?

- anionic - cationic - zwitterionic - nonionic

What counterions are generally combined with polar groups of anionic surfactants and describe their solubility characteristics?

- sodium and potassium are used for water solubility - calcium and magnesium are used for oil solubility

What kind are the three main category or agents that surfactants are used as

- solubilizing agent - wetting agent - emulsifying agent

What happens to surface activity as the overall hydrophobicity increases?

- surface activity increased with an increase in overall hydrophobicity - surface activity decreases with an increase in overall hydrophilicity

What are the desired properties of wetting agents?

- surfactants used should be effective in reducing the surface tension of water at concentrations below the CMC - surfactants with sufficiently high HLB to ensure a relatively polar solid surface

What affect does increasing the polyoxyethylene chain have on aqueous solubility?

- the longer the poly(oxyethylene) chain, the higher the aqueous solubility (hydrophilic chain with lipophilic group allows for interior incorporation of a lipophilic drug)

What influences the extent of solubilization?

- volume of the core - lipophilicity of the solubilizate - temperature for nonionic micelles - the ethylene oxide chain length of surfactants

What happens to water-soluble drugs and lipid-soluble drugs with the use of liposomes?

- water-soluble drugs can be entrapped in the aqueous layers - lipid-soluble drugs can be solubilized within the hydrocarbon interiors of the bilayers

Describe the chemical structure of zwitterionic surfactants.

- zwitterions possess polar head groups which on ionization can impart both positive and negative charges - the positive charge is almost always carried by an ammonium group and the negative charge is often a carboxylate

What is the desired HLB value for surfactants that are used as a solubilizing agent?

15 - 18 a high HLB value is needed for use as a solubilizing agent

Describe the solubility relationship of water-insoluble drugs with surfactant concentration at CMC.

At CMC, the solubility of water-insoluble drugs increases linearly with the surfactant concentration.

What happens when electrolytes are added to ionic surfactants?

CMC decreases and micelle size increases due to the reduction in repulsions between charges head groups and therefore decreases the electrical work of micellization

What is the desired HLB value for surfactants that are used as a wetting agent?

HLB range of 7-9 has been recommended for wetting agents

physical adsorption

adsorbate is bound to the surface through weak van der Waals forces

How do adsorption of surfactants enable the formation of suspensions?

adsorption of surfactants on insoluble particles enables particles to be dispersed in the form of a suspension

In what state can amine-based compounds function as surfactants?

amine-based compounds only function as surfactants in the protonated state and become nonfunctional at high pH

What is an example of a cationic surfactant and what kind of agent is it?

benzalkonium chloride used as a preservative in the preparation of ophthalmic and nasal solutions

What is an example of a zwitterionic surfactant and what kind of agent is it?

cocamidopropyl betaine (CAPB) used in skin cleansers because of its thickening and foaming properties

interfacial tension

force per unit length existing at the interface between two immiscible liquid phases

surface tension

force per unit length that must be applied parallel to the surface as to counterbalance the net inward pull

Where does the solubilization of water-soluble compounds occur?

in the palisade layer of a nonionic micelle

chemical adsorption

involves stronger valence forces

What happens when liposomes are equilibrated with excess water?

lamellar phases of hydrophobic chains form vesicles that can entrap drug and have potential use as drug carriers

Why does entropy increase in micellar systems of solution?

loss of the ordered structure of the water molecules when the hydrophobic regions of the surfactant are removed

maximum additive concentration

maximum amount of solubilizate (incorporated substance) that can be incorporated at a fixed concentration of surfactant

Compare micellization of nonionic surfactants with their ionic surfactant counterparts.

nonionic surfactants have lower CMC values and form larger micelles than their ionic counterparts because micellization of nonionic surfactants does not involve electrical work

What is the desired HLB value for surfactants that are used as a emulsifying agent?

oil-in-water (O/W) emulsions: HLB values from 8 to 18 water-in-oil (W/O) emulsions: HLB values of 3 to 6

When do particles need a wetting agent?

particles are wetted if the liquid surface tension is reduced to below the solid's critical surface tension

Describe the characteristics of liposomes.

phospholipids and other surfactants having two hydrophobic chains tend to form lamellar phases

What is an example of a 100% hydrophilic molecule and what is its HLB value?

polyethylene glycol with a HLB value of 20

What is an example of a nonionic surfactant and what kind of agent is it?

polyoxyethylene sorbitan esters of fatty acids used to solubilize steroids in ophthalmic formulations

Describe solubilization

process whereby water-insoluble substances are brought into solution by incorporation into micelles

What functional group allows a molecule to exist as a zwitterionic over a wide pH range and how is this possible?

quaternary ammonium group allows the molecule to exist as a zwitterionic over a wide pH range because the quaternary ammonium group is permanently charged

What occurs as a result of the intrusion of surfactant molecules?

results in water molecules being replaced by the hydrophobic groups of the surfactants

What is an example of an anionic surfactant and what kind of agent is it?

sodium lauryl sulfate used as a wetting and emulsifying agent

What happens to surface activity as the hydrophobic chain length of a surfactant is increased?

surface activity increases as the hydrophobic chain length increases forming larger micelles, decreasing the CMC, because less surfactants are needed to form micelles

How is equilibrium established in association colloids?

surfactant molecules at the surface and those in bulk solution are in equilibrium because micelles are continuously breaking down and reforming

Describe the chemical structure of surfactants.

surfactants are amphiphiles that are generally classified according to the nature of their hydrophilic group

Increasing the amounts of surfactants in a formulation will lower surface tension until what is reached?

the addition of surfactants will continue to lower surface tension until the critical micelle concentration is reached

What happens when the forces of intermolecular attraction between the water and hydrophobic groups become less than those between water molecules?

the contracting power of the surface is reduced and so is the surface tension

Which phase must an emulsifier be more soluble in?

the emulsifier must be more soluble in the continuous phase than in the dispersed phase

Describe the relationship of solubility and adsorption.

the greater the solubility of a solute in a solvent, the lower the extent of adsorption on to the solid

Why is there no equilibrium formed between the dissolved surfactant and the bulk solution in insoluble monolayers?

the low water solubility of the surfactant

As the pH is increased, how does the net charge on zwitterionic molecules change?

the molecule will change from net cationic to zwitterionic and finally to net anionic as the pH is increased

Describe the correlation between particle size and adsorption.

the more finely divided or porous the solid, the greater its adsorption capacity

What type of interactions must be strong to produce a greater solubility?

the stronger the interactions between solute and solvent, the greater the solubility - the ability of a solute to form hydrogen bonds with the solvent enhances solubility

What occurs when critical micelle concentration is reached?

the surface layer is saturated with surfactant molecules and no further decrease in surface tension is possible, at this concentration surfactants will form micelles

What is the primary reason for micelle formation?

to attain the state of minimal free energy

How do surfactants work as a wetting agent?

wetting of solid particles involves a reduction of surface tension of the solvent (water) via interaction of the surfactant at the solid-liquid interface