The Foundations of Chemistry

HYDROPHILIC AND HYDROPHOBIC COLLOIDS

Colloids are classified as hydrophilic(“water loving”) or hydrophobic(“water hating”)
based on the surface characteristics of the dispersed particles.

Hydrophilic Colloids

Proteins such as the oxygen-carrier hemoglobin form hydrophilic sols when they are
suspended in saline aqueous body fluids such as blood plasma. Such proteins are macro-
molecules (giant molecules) that fold and twist in an aqueous environment so that polar
groups are exposed to the fluid, whereas nonpolar groups are encased (Figure 14-20).
Protoplasm and human cells are examples of gels,which are special types of sols in which
the solid particles (in this case mainly proteins and carbohydrates) join together in a semi-
rigid network structure that encloses the dispersing medium. Other examples of gels are
gelatin, jellies, and gelatinous precipitates such as Al(OH) 3 and Fe(OH) 3.

Hydrophobic Colloids

Hydrophobic colloids cannot exist in polar solvents without the presence of emulsifying
agents,or emulsifiers.These agents coat the particles of the dispersed phase to prevent
their coagulation into a separate phase. Milk and mayonnaise are examples of hydrophobic

14-18

Fe(OH) 3 is a gelatinous precipitate

(a gel).

14-18 Hydrophilic and Hydrophobic Colloids 579

Some edible colloids.

Figure 14-20 Examples of

hydrophilic groups at the surface of

a giant molecule (macromolecule)

that help keep the macromolecule

suspended in water.

Figure 14-19 Stabilization of a colloid (Fe 2 O 3 sol) by electrostatic forces. Each colloidal
particle of this red sol is a cluster of many formula units of hydrated Fe 2 O 3. Each attracts
positively charged Fe^3
ions to its surface. (Fe^3
ions fit readily into the crystal structure, so
they are preferentially adsorbed rather than the Clions.) Each particle is then surrounded
by a shell of positively charged ions, so the particles repel one another and cannot combine
to the extent necessary to cause actual precipitation. The suspended particles scatter light,
making the path of the light beam through the suspension visible.

[Fe 2 O 3 yH 2 O]x

[Fe 2 O 3 yH 2 O]x

Fe^3 


Fe^3 

Fe^3 

Fe^3 
 Fe 3 


Fe^3 

Fe^3 
 Fe^3 

Fe^3 

Fe^3 


H
 H

H
 H

Cl

Cl Cl Cl

Cl

Cl

Cl

Cl Cl



Cl

Cl

Cl Cl Cl

Cl

H

Hydrogen bond

N+

H

N

C

O

Protein