Head-group variations and monolayer structures of diol derivatives

Monolayers of 5 chemically modified diols varying the head-group (nonadecane-1,2-diol (C1), hexadecyl-propane-1,3-diol (C2), hexadecyl-oxy-propane-1,2-diol (C3), hexadecyl-oxy-butane-1,2-diol (C4), hexadecanoyl-oxy-propane-1,2-diol (C5)) have been investigated by grazing incidence x-ray diffraction at 20 °C and at different lateral pressures. C1 and C5 exhibit a centred-rectangular lattice with NN (nearest neighbour) tilt and NN distortion directions. In the case of C1 on increasing the lateral pressure the distortion changes to NNN (next-nearest neighbour direction) without a change in tilt direction (NN). This behaviour could not be observed for the other compounds. C3 and C4 display a phase transition from a phase of NN tilt and NN distortion to a phase of NNN tilt and NNN distortion. At the transition pressure, which depends on the head group structure, both the tilt and distortion azimuths are changed. C2 forms very unstable films and is packed in an oblique lattice. The dependence of the tilt angle on the lateral pressure and the dependence of the signed distortion on the tilt magnitude, which shows a grouping behaviour of the diols, seem to imply that the NN-tilted rectangular phases of C3-C5 differ from that of C1.