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8.4.2 Cellulose derivatives

Cellulose itself is virtually insoluble in water, but aqueous solubility can be conferred by partial methylation or carboxymethylation.

Ethylcellulose is an ethyl ether of cellulose containing 44-51% of ethoxyl groups. It is insoluble in water but soluble in chloroform and in alcohol. It is possible to form water-soluble grades with a lower degree of substitution.

Methylcellulose samples are prepared by heterogeneous reaction which is usually controlled to allow substitution of, on average, about one-half of the hydroxyl groups. This leads to a product in which the methylated groups are not evenly distributed throughout the chains; rather, there are regions of high density of substitution (as in structure IX) which are hydrophobic in nature, and regions of low density of substitution which are hydrophilic in nature.

Methylcellulose is thus a methyl ether of cellulose containing about 29% of methoxyl groups; it is slowly soluble in water. A 2%

Me O

Me O

Structure IX Highly methylated region of methylcellulose chain solution of methylcellulose 4500 has a gel point of about 50°C. High concentrations of electrolytes salt out the macromolecules and increase their viscosity; eventually precipitation may occur. Low-viscosity grades are used as emulsifiers for liquid paraffin and other mineral oils. High-viscosity grades are used as thickening agents for medicated jellies and as dispersing and thickening agents in suspensions.

Since methylcelluloses are poorly soluble in cold water, preliminary use of hot water ensures wetting of all portions of the particle prior to solution in cold water. The water-soluble methylcelluloses possess the property of thermal gelation; that is, they gel on heating while the natural gums gel on cooling. Methylcellulose exists in solution as long thread-like molecules hydrated by water molecules. On heating, the water of solvation tends to be lost; the 'lubricating' action of the hydration layer is also lost and the molecules lock together in a gel. Gelation is reversible on cooling. Variation in the alkyl or hydroxy-lalkyl substitution can be a means of controlling the gel points (Table 8.4). As the methoxyl content is lowered, the temperature of gelation increases and water solubility decreases. Unlike the ionic celluloses, the non-ionic alkylcelluloses possess surface activity. As the methoxyl content is reduced, the surface and interfacial activities are also reduced, reflecting the importance of the hydrophobic moiety in determining surface activity.

Structure IX Highly methylated region of methylcellulose chain

Table 8.4 Gel point and surface activity of cellulose derivatives"

Derivative

Percentage

Percentage

Gel pointb

Surface tensionc

Interfacial tensiond

0 0

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