Surfactant

The  word Surfactant is  coined from  the  expression  " surface  active  agent" . As the  phrase implies, a  surfactant  molecule possesses  surface activity,  a  property associated  with the  chemical structure of  the  molecule. The characteristic feature  of a  surfactant  molecule is  its two  ends attached by a covalent  bond  The  two  ends have diametrically opposed  polarities.  The non - polar  end  is  lyophilic (strongly attracted to  organic  molecules)  while  the  strongly polar  end  is  lyophobic  (having  little attraction  for  organic  molecules)  yet  strongly hydrophilic (water loving).  Duality of
polarity  causes the  molecule to align  itself  with respect  to the  polar  nature of  the surfaces  it  contacts.

Physical  Chemistry  of Surfactant  Solutions

When soap (sodium  oleate) molecules are  added  to pure  water  one  molecule at a  time,  the  first  few  molecules align  at the  air/water  interface and the  hydrocarbon tails  orient  toward  air.  The driving  force  for  this  alignment  is  the  non - polar  tails seeking  to associate  themselves with the  most non - polar interface it  can  find,  in this case air.  As additional  molecules  are  added,  they  too  will  align  at the  water/air interface until all of  the  surface  area is  completely packed.  As more molecules are added, they  are  forced  into the  bulk of  the  water, floating  about  as individual
molecules until  a  saturation  level  is  reached. At this  point,  called  the  Critical Micelle Concentration, soap  molecules  agglomerate  into  water soluble  clumps (Micelles), where  the  lyophobic  tails  are  associated  with themselves  and  the hydrophilic heads are  surrounded  by water  molecules.  This sequence of events are diagrammed  in figure  27.  The first  beaker represents close  packing  of surfactant molecules at the  surface.  The second beaker represents the  condition before micelle formation.  The third  beaker shows the  formation  of  micelles.

Orientation of Surfactant  Molecules in Water 

 

CLASSIFICATION  OF SURFACTANTS 

Surfactants are classified  according  to use,  to ionic  charge and  to chemical structure.

(A) By Use

1.Wetting Agents:  The ability of  a  liquid  to spread on a  smooth  solid  surface  is dependent  on the  polar nature of  the  solid and  the  surface  tension  of  the  liquid. More about  the  thermodynamic relationships will  be brought  up in the  chapter  on water and  oil repellents.  A  non - polar  solid surface  such as paraffin  wax or Teflon will  cause a  drop of  pure water  to bead- up and  not  spread.  Water containing surfactants on the other  hand will  easily  spread on paraffin surfaces  and  have lower  contact  angles on Teflon.  Surfactants used this  way are  called  wetting agents, or penetrating agents when used to wet out repellent fabrics.  This is  illustrated in figure. Penetration  of  fabrics  is  a  function  of  surface  wetting,  however  fabric assemblies have a  volume of  air entrapped  in the  void formed  spaces.  Before a  liquid can  move in,  the  air must move out. Penetrants facilitate this  process.

Spreading  of Water on Smooth Surfaces

 

2.Detergents: Detergents are  surfactants that help remove soils from solid surfaces. Over and above  reducing  water's  surface  tension,  detergents must adsorb  onto  the  soil's surface to  aid  in spontaneous  release.  Detergents must also keeps  the  soil  suspended to prevent  redeposition.

3.Emulsifying Agents: Emulsifying Agentsare  surfactants that convert water- insoluble  oils  into stable, aqueous suspensions.  The lyophilic part of  the  surfactant  molecule is absorbed by the oil droplet and the  lyophobic  head is  oriented  outward, surrounding the  droplet with a  hydrophilic sheath.  Ionic  surfactants add another  dimension  to  the  stability of emulsions, they set  up a  charge - charge  repulsion  field  which  adds to keeping the droplets  separated.

3.Dispersing  Agents:Dispersing  Agents function  in a  manner  similar  to emulsifying  agents.  The difference  is  that  solid particulate matter, rather than insoluble oils, is  dispersed.  The nature of the  lyophilic part  of  the surfactant  molecule must be such that it  adsorbs onto the  particle's  surface.  Surfactant molecules  must be  matched  with their intended use.

B. By Ionic  Charge
1.Anionic:  Those  that develop  a  negative charge on the  water  solubilizing  end.
2.Cationic: Those  that  develop  a  positive  charge on the  water  solubilizing  end.
3.Non -Ionic:  Those  that develop  no ionic  charge on the  water  solubilizing  end.
4.Amphoteric:  Those  that have both a positive  and  negative charged  group  on the  molecule.