Role of carboxyl groups in water absorption by keratin

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Abstract
The contribution of carboxyl groups to the water sorbing properties of keratin has been investigated over the complete water vapour pressure range. A series of partially methylated wools were studied: below 80% R.H. there is a reduction in water content linear with the reduction in the number of carboxyl groups, but at humidities above 80% R.H. extra water is absorbed because of disruption of the structural network under the acid conditions of treatment.Extrapolation of the plot of equilibrium water content vs. free carboxyl group content of partially methylated wool samples at a particular humidity. made at several humidity values, enables an isotherm to be deduced for wool containing no COO groups. The reduction of water content for wool of zero carboxyl group content, as a function of relative humidity, represents the isotherm attributable to the free carboxyl groups in keratin; this is a simple Langmuir isotherm at low humidities. The reduction of water content is proportionately greater at lower humidities—at 5% R.H., 36% of the total water and at 65% R.H., 22% of the total water absorbed by keratin is associated with free carboxyl groups. The number of water molecules associated with each carboxyl group is greater than previous literature values.

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