Activity of surface active substances determined from their surface excess

Abstract

Commonly, the surface excess is determined from surface tension measurements via the Gibbs equation. This equation relates the activity (chemical potential), the surface excess, and the surface tension. When knowing two out of the three quantities, the third one can be calculated. Unfortunately, in the case of surface active components the concentration is in most cases too low to determine the activity from a measurable change in the bulk properties and thus assumptions are made about the activity coefficients. However, if the surface excess is measured directly and the surface tension is known, the activity can be determined making use of the Gibbs equation. The surface excess is the quantity of a surfactant solution which changes most strongly with the concentration. Thus it is obvious that this procedure should be used to determine activity coefficients of surfactants. One of the few techniques for determining the surface excess directly is neutral impact collision ion scattering spectroscopy (NICISS). With NICISS concentration depth profiles can be measured in the surface near region with a depth resolution of a few angstro¨ms. The surface excess and the activities are investigated here for the system tetrabutylphosphonium bromide (Bu 4 PBr) dissolved in the polar solvent formamide.

Angle resolved ion scattering spectroscopy at surfaces of pure liquids: topography and orientation of molecules

“…NICISS has so far been used to determine concentration depth profiles at a single angle of incidence of the projectiles, the angle close to the surface normal. 8,9 Here we implicitly assumed that the influence of the topography on the concentration depth profiles may be neglected. This assumption is of course true e.g.…”

“…Experimental methods used to determine concentration depth profiles are neutron reflectivity (NR) 1,2 and X-ray reflectivity, 3 angle resolved photoelectron spectroscopy (ARXPS) [4][5][6] and neutral impact collision ion scattering spectroscopy (NICISS). [7][8][9] The concentration depth profiles determined with these methods yield information about the orientation of the molecules. Another method used for this purpose is recoiling spectroscopy.…”

Angle resolved ion scattering spectroscopy reveals the local topography around atoms in a liquid surface

“…The procedure to determine concentration depth profiles from the NICIS spectra is described in detail in ref. 19.…”

“…NICISS uses the inelastic loss of energy of projectiles with a kinetic energy of some keV to determine concentration depth profiles of elements at liquid surfaces with a depth resolution of some a˚ngstro¨ms. [17][18][19] Projectiles moving through matter lose energy due to the low angle scattering and electronic excitations (stopping power). The energy loss can be converted into depth information.…”

“…16 Neutral impact collision ion scattering spectra (NICISS) is used in our group to determine concentration depth profiles of the elements. [17][18][19] Up to now we have tacitly assumed for the evaluation of the data, that the top surface layer and the adjacent layers are laterally homogeneous. Here we show that the angular mode of NICISS (ARISS) can be used to reveal the local topography around atoms in a liquid surface, and so to gain information about the lateral molecular structure of a liquid surface.…”