The Effective Rotary Power of the Fatty Esters of Cholesterol

Abstract

The pitch of two component mixtures of certain cholesteric liquid crystals is a strong function of chemical composition. We have found that over a wide range of materials the pitch of a mixture can be accurately represented by a weighted average of ingredients. If an effective rotary power,which we define as the inverse of pitch, is assigned to each constituent, the resultant pitch is just the inverse of the net effective rotary power of the mixture. In particular, if components with opposite intrinsic screw sense are mixed, there will exist one composition corresponding to no net rotation or infinite pitch. This technique was used to measure the effective rotary power of the fatty esters of cholesterol. Each ester was studied in mixtures with cholesteryl chloride. The results indicate a strong dependence on composition, with effective rotary power decreasing with decreasing aliphatic chain length. This technique provides a direct comparison of all esters at a common temperature. This is not possible in single component systems since there is no common cholesteric temperature and the strong pitch temperature dependence complicates studies. Although neither ingredient is liquid crystal at room temperature, the mixtures are liquid crystals over a wide compositional range and a modest extrapolation provides the single component data.