Chemistry · Ch 12 — Surface Chemistry

Classification Based on Type of Particles of the Dispersed Phase, Multimolecular, Macromolecular and Associated Colloids

12.4.3

Classification Based on Type of Particles of the Dispersed Phase, Multimolecular, Macromolecular and Associated Colloids

By the nature of the dispersed-phase particles, colloids are of three kinds.

(i) Multimolecular colloids

On dissolving, a large number of atoms or small molecules aggregate into particles in the colloidal range (1–1000 nm). For example, a gold sol contains particles of many atoms, and a sulphur sol contains particles built from a thousand or more $S_8$ molecules.

(ii) Macromolecular colloids

Here a single macromolecule is itself of colloidal size in a suitable solvent. Such systems are very stable and resemble true solutions. Natural examples: starch, cellulose, proteins, enzymes; man-made: polythene, nylon, polystyrene, synthetic rubber.

(iii) Associated colloids (micelles)

Some substances behave as normal strong electrolytes at low concentration but form colloidal aggregates called micelles at higher concentration. Micelle formation requires a temperature above the Kraft temperature ( $T_k$ ) and a concentration above the critical micelle concentration (CMC). On dilution the aggregates revert to individual ions. Soaps and synthetic detergents are examples; for soaps the CMC is about $10^{-4}$ to $10^{-3}\ \text{mol L}^{-1}$ . These particles carry both lyophilic and lyophobic parts, and a micelle may hold 100 molecules or more.

Mechanism of micelle formation

Soap is the sodium or potassium salt of a higher fatty acid, $RCOO^-Na^+$ (e.g. sodium stearate $CH_3(CH_2)_{16}COO^-Na^+$ ). In water it dissociates into $RCOO^-$ and $Na^+$ . Each $RCOO^-$ ion has two very different parts (Fig. 5.5):

a long hydrocarbon chain $R$ , a non-polar, hydrophobic "tail", and
a $COO^-$ group, a polar, hydrophilic "head."

At low concentration these ions sit at the water surface with the $COO^-$ heads in the water and the tails sticking out (Fig. 5.6a). At the CMC the ions are drawn into the bulk and clump into a spherical ionic micelle — tails pointing to the centre, $COO^-$ heads outward (Fig. 5.6b) — containing as many as 100 ions. Detergents such as sodium lauryl sulphate, $CH_3(CH_2)_{11}SO_4^-Na^+$ , form micelles the same way, with $-SO_4^-$ as the polar head.

Cleansing action of soaps …

Figure 5.5Hydrophobic and hydrophilic parts of stearate ion

What this figure shows. A three-part vertical diagram of the stearate ion. TOP panel: a full structural formula of sodium stearate drawn as a zig-zag skeletal chain. Along the chain, alternating carbon labels are written above and below the vertices — starting at the left with 'CH3' then repeating 'CH2' groups (upper row 'CH2' and lower row 'CH2' alternating along the entire zig-zag). At the far right end the chain terminates in a carboxylate group: a carbon 'C' with a double-bonded 'O' drawn above it (O with '||' to C) and single-bonded to 'O' below carrying a negative charge, next to which is 'Na' with a plus charge (the sodium counter-ion). Below this structure is the label 'Sodium stearate (C17H35COO^- Na^+)'. MIDDLE panel: the same zig-zag hydrocarbon chain (CH3 at left, alternating CH2 labels above and below) but now the right-end carboxylate group is enclosed inside a large pink shaded circle (dotted outline) showing 'O' double-bonded ('||') to 'C' bonded to 'O^-'. A bracket underneath the long chain is labelled 'Hydrophobic tail / Stearate ion'; the pink circle at the right is labelled 'Hydrophilic head'. BOTTOM panel: a simplified schematic — a plain zig-zag line (the tail) with a solid pink fi …

Figure 5.6(a) Arrangement of stearate ions on the surface of water at low concentrations of soap (b) Arrangement of stearate ions inside the bulk of water (ionic micelle) at critical micelle concentrations of soap

What this figure shows. Two-panel figure. Panel (a): a rectangular beaker/container filled with water (shown as a light blue hatched/dashed region for the water body). At the top surface of the water sit several stearate ions drawn as small circles (the polar 'head', each marked with a minus charge) resting at the water surface with wavy hydrocarbon-chain 'tails' sticking up out of the water into the air above. A leader line on the left labels one of these units 'Stearate ion' and another leader line labels the blue hatched region 'Water'. Caption marker '(a)' below. Panel (b): a circular ionic micelle — a ring/sphere with many stearate ions arranged radially. Each ion has its wavy hydrocarbon tail pointing inward toward the centre of the sphere and its carboxylate head 'COO^-' pointing outward around the circumference. Multiple 'COO^-' / 'OOC^-' labels are written around the outside of the circle at various angular positions (top, sides, bottom). A leader line at the top right labels the whole structure 'Ionic micelle'. Caption marker '(b)' below. …

Figure 5.7(a) Grease on cloth (b) Stearate ions arranging around the grease droplet and (c) Grease droplet surrounded by stearate ions (micelle formed)

What this figure shows. Three small panels showing progressive cleansing action, drawn left to right. Panel (a): a low mound/hump shape (a plain curved bump on a horizontal baseline) representing a grease droplet sitting on cloth. Marker '(a)'. Panel (b): the same grease mound now with several stearate ions beginning to arrange around it — small charged circle heads on the outside with wavy tails buried into the grease mound (a few ions shown attaching around the hump). Marker '(b)'. Panel (c): a complete spherical micelle — a circle representing the grease droplet fully surrounded by stearate ions arranged radially, with wavy hydrocarbon tails pointing inward into the grease core and charged (minus) circular heads pointing outward all around the sphere like bristles. Marker '(c)'. …

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