colloid stability -...

JURUSAN TEKNOLOGI HASIL PERTANIAN Fakultas Pertanian Universitas Lampung

➡ Mengetahui mekanisme stabilitas colloid sangat penting untuk pemanfaatan colloid tersebut. ๏ Meningkatkan stabilitas (produk-produk colloid) ๏ Menurunkan/merusak stabilitas (pemisahan)

➡ Stabilitas colloid tergantung pada kesetimbangan daya tarik dan tolak antar partikel colloid.

➡ Gaya yang bekerja pada partikel colloid: ๏ Van der Waals force, ๏ Dispersion force, ๏ Electrical force, ๏ Universal repulsive force.

C O L L O I D S TA B I L I T Y

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➡ Van der Waals dan Dispersion Force: ๏ Daya yang menyebabkan partikel koloid menyatu/

bergabung (coagulation). ๏ Semakin dekat jarak partikel, semakin kuat. ๏ Menyebabkan pemisahan fase.

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Stabilisation of colloids¾A stable colloidal system is one in which the particles resist flocculation or aggregation and exhibits a long shelf-life.

¾Depends upon the balance of the repulsive and attractive forces that exist between particles as they approach one another.

¾If all the particles have a mutual repulsion then the dispersion will remain stable.

¾If the particles have little or no repulsive force then some instability mechanism will eventually take place e.g. flocculation, aggregation etc.

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➡ Electrical dan Universal Repulsive Force: ๏ Setiap partikel koloid mempunyai muatan (listrik)

permukaan yang sama. ๏ Terjadi daya tolak antar partikel. ๏ Koloid menjadi stabil.

It is worth noting that Albert Einstein'sanalysis of Brownian motion in 1901constituted the first proof of the moleculartheory of matter.

Brownian motion of colloidal particles

If you observe a single colloidal particle through the ultramicroscope, you will notice that it is continually jumping aroundin an irregular manner. These movements are known as Brownian motion. Scottish botanist Robert Brown discovered thiseffect in 1827 when observing pollen particles floating in water through a microscope. (Pollen particles are larger thancolloids, but they are still small enough to exhibit some Brownian motion.)

Brownian motion arises from collisions of the liquid molecules with the solidparticle. For large particles, the millions of collisions from different directionscancel out, so they remain stationary. The smaller the particle, the smaller thenumber of surrounding molecules able to collide with it, and the more likelywill random fluctuations occur in the number of collisions from different sides.

Simple statistics predicts that every once in a while, the imbalance in collisions from different directions will become greatenough to give the particle a real kick!

Electrical properties of colloids

In general, differences in electric potential exist between all phaseboundaries.

If you have studied electrochemisty, you will know that two dissimilar metals in contact exhibit a "contact potential", and thatsimilar potential differences exist between a metal and a solution in which it is immersed. But this principle extends well beyondordinary electrochemisty; there are small potential differences even at the water-glass interface in a drinking glass, and the water-air interface above it.

Colloids are no exception to this rule; there is always a difference in electric potential between the colloid "phase" and thatof the surrounding liquid. Even if the liquid consists of pure water, the polar H2O molecules at the colloid's surface arelikely to be predominantly oriented with either their oxygen (negative) or hydrogen (positive) ends facing the interface,depending on the electrical properties of the colloid particle itself.

Interfacial electrical potential differences can have a variety of origins:

Particles composed of ionic or ionizable substances usually have surface charges due to adsorption of an ion (usually an anion)from the solution, or to selective loss of one kinds of ion from the crystal surface. For example, Ag+ ions on the surface of asilver iodide crystal go into solution more readily than the Br- ions, leaving a negatively-charged surface.

The charges of amphiprotic groups such as those on the surfaces of metal oxides and hydroxides will vary with the pH of theaqueous medium. Thus a particle of a metal oxide M–O will become positive in acidic solution due to formation of M–OH+,while that of a sparingly soluble hydroxide M–OH will become negative at low pH as it changes to M–O–. Colloidal-sizedprotein molecule can behave in a similar manner owing to the behavior of amphiprotic carboxylate-, amino- and sulfhydrylgroups.

Non-ionic particles or droplets such as oils or latex will tend to selectively adsorb positive or negative ions present in solution,thus "coating themselves" with electrical charge.

In clays and other complex structures, isomorphous replacement of one ion by another having a different charge will leave a netelectric charge on the particle. Thus particles of kaolinite clay become negatively charged due to replacement of some of theSi4+ ions by Al3+.

Charged colloidal particles will attract an exess of oppositely-charged counter-ions to their vicinity from the bulk solution,forming a localized "cloud" of compensating charge aroundeach particle. The entire assembly is called an electric doublelayer. Electric double layers of one kind or another exist at allphase boundaries, but those associated with colloids areespecially important.

3 Stability of colloidal dispersionsWhat keeps the colloidal particles suspended in the dispersion medium? How can we force the particles to settle out? Theseare very important practical matters:

Colloidal products such as paints and many foods (e.g., milk) must remain in dispersed form if they are to be useful;

Other dispersions, often those formed as by-products of operations such as mining, water treatment, paper-manufacture, orcombusion are environmental nuisances. The only practical way of disposing of them is to separate the colloidal material fromthe much greater volume of the dispersion medium (most commonly water). Simple evaporation of the water is usually not apractical option; it is generally too slow, or too expensive if forced by heating.

A balance of forces

You will recall that weak attractive forces act between matter of all kinds. These are known generally as van der Waals anddispersion forces, and they only "take hold" at very close distances. Countering these is the universal repulsive force thatacts at even shorter distances, but is far stronger; it is the basic reason why two atoms cannot occupy the same space.

For very small atomic- and molecular-sized particles, another thing that keeps them apart is thermal motion. Thus when twomolecules in a gas collide, they do so with more than enough kinetic energy to overcome the weak attractive forces betweenthem. As the temperature of the gas is reduced, so is the collisional energy; below its boiling point, the attractive forcesdominate and the gas condenses into a liquid.

How electrical forces help keep colloids dispersed

When particles of colloidal dimension suspended in a liquid collide with each other, they do so with much smaller kineticenergies than is the case for gases, so in the absence of any compensating repulsion forces, we might expect van der Waalsor dispersion attractions to win out. This would quickly result in the growth of aggregates sufficiently large to exceedcolloidal size and to fall to the bottom of the container. This process is called coagulation.

So how do stable dispersions such as sols manage to survive? In the preceding section, wesaw that each particle with its double layer is more or less electrically neutral. However,when two particles approach each other, each one "sees" mainly the outer part [shown herein blue] of the double layer of the other. These will always have the same charge sign(which depends on the type of colloid and the nature of the medium), so there will be anelectrostatic repulsive force that opposes the dispersion force attractions.

Electrostatic (coulumbic) forces have a strong advantage in this respect because they act overmuch greater distances do van der Waals forces.

But as we will see further on, electrostatic repulsion can lose its effectiveness if the ionicconcentration of the medium is too great, or if the medium freezes. Under these conditions, there are other mechanisms thatcan stabilize colloidal dispersions.

How colloids interact with solvents

Colloids can be divided into two general classes according to how the particles interact with the dispersions medium (oftenreferred to as the "solvent").

Lyophilic colloids

In one class of colloids, called lyophilic ("solvent loving") colloids, the particles contain chemical groups that interactstrongly with the solvent, creating a sheath of solvent molecules that physically prevent the particles from coming together.

Ordinary gelatine is a common example of a lyophilic colloid. It is in fact hydrophilic, since it forms strong hydrogen bonds withwater. When you mix Jell-O or tapioca powder to make a gelatine dessert, the material takes up water and forms a stable colloidalgel.

Lyophilic (hydrophilic) colloids are very common in biological systems and in foods.

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Interaksi Partikel Koloid dengan Medium:

➡ Terdapat 2 (dua) jenis interaksi: 1. Hydrophilic (Lyophobic) Colloid

๏ Merupakan jenis koloid yang umum di alam ๏ Permukaan partikel koloid mempunyai polaritas

yang sama dengan air 2. Hydrophobic (Lyophilic) Colloid

๏ Banyak dĳumpai pada sistem koloid biologis dan bahan pangan

➡ Untuk meningkatkan stabilitas sistem koloid, ditambahkan emulsifier/surfaktan dan stabilizer. ๏ Bahan yang mempunyai dua sisi yang berbeda

polaritasnya.

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Mekanisme Stabilisasi Koloid

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Emulsifier/Surfactant

Stabilizer

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COLLOID MILLS DAN HOMOGINOZER

➡ Alat (mesin) yang digunakan untuk mengurangi ukuran partikel zat padat dalam cairan atau untuk mengurangi ukuran droplet dalam emulsi. ๏ Colloid Mills (untuk partikel padat) ๏ Homogenizer (untuk partikel cair/droplet)

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COLLOID MILLS ➡ Colloid mills termasuk dalam kategori rotor-stator

mixers. ➡ Terdiri dari dua disk, yang satu berputar sangat cepat

(2000-18000 rpm) dan satunya diam. ➡ Kecepatan putaran menghasilkan high level of

hydraulic shear, mengganggu system fluida, dan mengurangi ukuran partikel terdispersi yang pada akhinya meningkatkan stabilitas system koloid.

➡ Digunakan untuk dmencampur dan mengecilkan padatan dalam suspensi cair atau cairan yang tersuspensi dalam cairan lain.

Colloid Mills dan Homogenizer …………………

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Colloid Mills …………………

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Colloid Mills …………………

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HOMOGENIZERS ➡ Digunakan untuk mencampur dan mengecilkan ukuran

partikel terdispesi/teremulsi berbentuk cair (droplet) dan sekaligus menstabilkan system emulsi.

➡ Prinsipnya adalah partikel terdispesi berukuran besar dalam emulsi dilewatkan pada lubang sempit pada tekanan tinggi akan pecah menjadi partikel berukuran kecil yang memiliki tingkat keseragaman dan stabilitas yang lebih besar.

➡ Setiap homogenizer, terdapat dari: ๏ Pompa tekanan tinggi (500-500psi) ๏ Orifice (lubang kecil untuk keluar produk), ๏ Valve (katup untuk mengatur tekanan)

➡ Homogenizer terdiri dari Single Stage Homogenizer dan Double stage Homogenizer.

Colloid Mills dan Homogenizer …………………

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Homogenizer …………………


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Homogenizer …………………

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colloid stability -...

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