COLLOID AND SURFACE CHEMISTRY

Physical Chemistry IV

Compiled by Bertha Yonata

FROM SEVERAL SOURCES

What are colloids?

• Any particle that has some linear dimension between 10-

9-10-6m is considered a colloid.

• The linear dimension is size range, rather than particle weight or the number of atoms in a particle.

• The properties of colloid system and surface phenomena of colloid system depend mainly on the specific surface area of colloid particle, little or lightly on the colloid structure and density of a particle.

Dispersions and Suspensions

•A solution is a stable,

homogeneous mixture.

•A dispersion (colloid) is

stable to gravity, but can be

separated by centrifugation

or filtration.

•A suspension contains

larger particles that will

settle over time.

Lyophobic Colloidal Systems

• Dispersions that are thermodynamically unstable and exist only during certain period of time (“kinetic stability”)

• For a lyophobic colloid there is always strong attractive (van der Waals) force between the particles so that if the particles get close to each other they become linked together (“coagulate”). The lyophobic colloids can be made to appear stable for some time only if there is a repulsion force that reduces the chance of the close contact of the particles as a result of Brownian movement. The lyophobic colloids are thermodynamically unstable and the to coagulation is merely a kinetic one.

Thermodynamic stability

Thermodynamic stability:

The Gibbs energy of the micellization of

surfactants molecule is less than zero.

So, the processing of the micellization of

surfactants molecule is spontaneous.

Examples of Lyophobic Colloids

• Emulsions

• Sol

• Foams and films

• Aerosols

– Mists (liquid in gas)

– Smokes ( very disperse solid in gas)

– Dusts and Powders (rough solid in gas)

– London smog (solid particles covered by condensed vapor in gas)

Spherical and cubic particles with

crystalline or amorphous microstructure

Model particles of different shapes with the same or different chemical compositions

Types of Colloidal Dispersions

Type Name Examples

Gas in liquid foam whipped cream, soapsuds

Gas in solid solid foam styrofoam, foam rubber, pumice

Liquid in gas liquid aerosol fog, clouds

Liquid in liquid emulsion milk, mayonnaise

Liquid in solid solid emulsion cheese, opals, jellies

Solid in gas solid aerosol smoke, dust in air

Solid in liquid sol India ink, gold sol

Solid in solid solid sol tire rubber, rubies

Properties of Colloids

Tyndall Effect in Dispersions

Light passing through a

solution (left) is not

scattered, but the larger

particles in a dispersion

(right) deflect some of the

light.

Brownian Motion

• If we observe colloidal system using ultra microscope, we will see the particles moving zigzag. The zigzag motion is called Brownian motion.

• The size of particle and the temperature influence Brownian Motion.

Colloidal Adsorption

if sol particles placed

into liquid or gas, the liquid or

gas particles will be accumulated

on the solid surface. This

phenomena is called adsorption.

On the other hand absorption is a

phenomena which all liquid or

gas particles are absorbed into

solid sol (not on the surface).

• Colloidal particles have huge surface areas in comparison to the volume of the same particles if they were aggregated into one large particle.

• Colloidal particles become electrically

charged when they adsorb ions on their

surfaces.

• This occurs because surface atoms or

ions attract and adsorb ions or polar

molecules from the dispersion medium.

1. Electrophoresis

2. Electro-osmosis

3. Streaming Potential

4. Sedimentation Potential

Explain the four electrokinetic phenomena

ELECTROKINETIC PHENOMENA

1. Elektroforesis

Oleh karena partikel sol bermuatan listrik, maka partikel ini akan bergerak dalam medan listrik. Pergerakan ini disebut elektroforesis. Untuk lebih jelas, mari kita lihat tabung berikut di samping.Pada gambar, terlihat bahwa partikel-partikel koloid bermuatan positif tersebut bergerak menuju elektrode dengan muatan berlawanan, yaitu elektrode negatif. Jika sistem koloid bermuatan negatif, maka partikel itu akan menuju elektrode positif.

2. Elektroosmotik: gerak partikel koloid bermuatan melalui membran semipermeabel oleh pengaruh medan listrik.

3. Potensial aliran: partikel koloid dipaksa bergerak melalui pori membran (>< elektroosmotik).

4. Potensial sedimentasi: terjadi perbedaan potensial antara bagian atas dan bawah dimana suatu koloid bermuatan mengendap (>< elektroforesis)

Muatan Koloid Sol

Semua partikel koloid pasti mempunyai muatan sejenis (positif atau negatif). Oleh karena muatannya sejenis, maka terdapat gaya tolak menolak antar partikel koloid. Hal ini mengakibatkan partikel-partikel tersebut tidak mau bergabung sehingga memberikan kestabilan pada sistem koloid. Namun demikian, system koloid secara keseluruhan bersifat netral karena partikel-partikel koloid yang bermuatan ini akan menarik ion-ion dengan muatan berlawanan dalam medium pendispersinya

Koagulasi

Jika partikel-partikel koloid tersebut bersifat netral, maka akan terjadi penggumpalan dan pengendapan karena pengaruh gravitasi. Proses penggumpalan dan pengendapan ini disebut koagulasi.

Koloid pelindung

Sistem koloid di mana partikel terdispersinya mempunyai daya adsorpsi relatif besar disebut koloid liofil yang bersifat lebih stabil. Sedangkan jika partikel terdispersinya mempunyai gaya absorpsi yang cukup kecil, maka disebut koloid liofob yang bersifat kurang stabil. Yang berfungsi sebagai koloid pelindung ialah koloid liofil.

Sol liofob/ hidrofob mudah terkoagulasi dengan sedikit penambahan elektrolit, tetapi menjadi lebih stabil jika ditambahkan koloid pelindung yaitu koloid liofil.

References

Principles of Colloid and Surface Chemistry BY: Paul C. Hiemenz and Raj Rajagopalan

Duncan, J.S.2000. Colloid and Surface Chemistry (Ebook edition). Butter Worths.