Surface effects accompanying the L-alpha-to-L-alpha(+) transition of the amphiphile C12E4 in water as studied by neutron reflectivity

The interface of a concentrated solution of the amphiphile C12E4 in D2O aga inst a hydrophobic-coated silicon wafer has been studied near the congruent transition from the isotropic phase L-1 to the lamellar phase L-alpha in a temperature range from 20 to 44 degrees C by neutron reflectometry. The me asurements reveal that the L-alpha phase - which is stable only in a narrow temperature range (20.6-20.9 degrees C at the chosen composition of 29 wt- % C12E4 in D2O) - couples strongly to the solid substrate, i.e. adjacent la mellae exhibit strong lateral correlation and are oriented parallel to the surface of the substrate. As the temperature is increased the sample transf orms to a phase L-alpha(+) which is stable from 20.9 degrees C up to the hi ghest experimental temperature. Unlike L-alpha, the L-alpha(+) phase exhibi ts a broad Bragg peak in the reflectivity curve, indicating weak lateral co rrelation of adjacent lamellae. Furthermore, rocking scans reveal no signif icant coupling of the layers to the substrate surface. This finding is cons istent with a disordered multilayered structure of the L-alpha(+) phase sug gested by other authors.