STRUCTURE OF A DODECYLTRIMETHYLAMMONIUM BROMIDE LAYER AT THE AIR-WATER-INTERFACE DETERMINED BY NEUTRON REFLECTION - COMPARISON OF THE MONOLAYER STRUCTURE OF CATIONIC SURFACTANTS WITH DIFFERENT CHAIN LENGTHS

We have determined the structure of a monolayer of dodecyltrimethylamm onium bromide (C(12)TAB) adsorbed at the air/water interface at three surface concentrations corresponding to areas per molecule of 44, 48, and 60 Angstrom(2), using neutron reflection. We have used isotopic la beling to distinguish the hydrocarbon chain, head group, and solvent d istributions. We have also labeled the two halves of the hydrocarbon c hain to obtain information about the mean conformation of the chain. A t the highest surface concentration we find that the inner half of the chain (next to the head group) is oriented closer to the surface norm al than the outer half of the chain indicating that chain defects cont ribute to the chain structure. We also find that the surfactant layer is substantially rougher than that predicted from the simple capillary wave model of a pure liquid. We have compared the structure of the di fferent C(n)TABs with chain lengths varying from C-12 to C-18 at an ar ea per molecule of about 44 Angstrom(2) and from C-10 to C-16 at an ar ea per molecule close to 60 Angstrom(2). The most remarkable observati on about the lower area measurements is that the thickness of the laye r is the same for the four chain lengths even though the extended leng th of the chain increases by 50%. When account is taken of surface rou ghness, the intrinsic thickness of the chain region of the monolayer i s also constant and for C-18 is less than half the extended length. Co mparison of the thickness of the layer and the separation of the chain and head distributions indicates that the mean orientation of the out er half of the chain is further from the surface normal than is the in ner half of the chain, and this difference increases with chain length . At the higher area per molecule the C(10)TAB forms the thickest laye r and this is associated with the chains being closer to the surface n ormal than for the other C(n)TABs.