Jordan G. Petrov

Three-phase contact line motion in the deposition of spread monolayers

Abstract A monolayer of a water-insoluble surfactant can be transferred with some maximum rate vmax from a water surface to the surface of a solid being taken out. This rate was measured by observing the motion of the three-phase contact line (detecting fluorescence quenching or change of light reflection by water entrainment). The rate vmax was found to be determined by the reactivity between the monolayer head groups and the solid surface. vmax is particularly large in the case of opposite sign of the charges of the immobile groups on the solid surface and in the monolayer. In the case of equal sign vmax is strongly increased by the presence of divalent counterions in the solution. The hydration of the head groups decreases vmax, and therefore optimal transfer is found when the charged head groups of the monolayer are laterally separated from each other by uncharged head groups. Particularly large values of vmax are observed when the monolayer head groups and the solid surface interact by zipper-like hydrogen bonding between undissociated carboxyl groups.

Neutron activation analysis of metal ions in Langmuir—Blodgett multilayers of arachidic acid

Abstract A combination of the Langmuir-Blodgett technique and the neutron activation method of analysis is used to determine the stoichiometry of the interaction between fatty acid monolayers and metal ions dissolved in the subsolution. Arachidic acid and Cd 2+ and Ba 2+ are considered. The results show the specificity of the reaction RCOOHMe 2+ . The experimental data are used to calculate the stability constants of arachidic acid bivalent metal soaps according to the method proposed by Matsubara et al.

On the existence of a maximum speed of wetting

Abstract The motion of the three-phase contact line upon entrainment or drainage of glycerol and glycerol—water films on hydrophobic substrates is studied. It is established that the relative speed of the perimeter of wetting with respect to the solid surface remains constant under these conditions, regardless of the changes in film thickness and length due to variations in withdrawal velocity and time, respectively. This speed is equal to the largest speed of the contact line under steady-state kinetics of wetting and depends only on the material characteristics of the system. The results obtained present a new and direct experimental confirmation of the existence of a maximum speed of wetting as postulated by Blake and Ruschak.

Structure of the Langmuir monolayers with fluorinated ethyl amide and ethyl ester polar heads creating dipole potentials of opposite sign

This study experimentally checks our previous hypothesis (Petrov, J. G.; Polymeropoulos, E. E.; Moehwald, H. Langmuir 2007, 23, 2623) that different conformations of the fluorinated heads of RCONHCH(2)CF(3) and RCOOCH(2)CF(3) monolayers cause the opposite signs and the striking difference of 1.480 V between their surface potentials Delta V. In situ X-ray diffraction at grazing incidence (GIXD) shows that both monolayers form orthorhombic lattices with closely packed chains tilted to the next-nearest neighbors in the RCONHCH(2)CF(3) film and upright in the RCOOCH(2)CF(3) monolayer. The packing of the chains in the plane perpendicular to them, which excludes the effect of the tilt, shows the same distance between the next-nearest neighbors, but significantly closer nearest neighbors in the RCONHCH(2)CF(3) film. This difference implies a specific anisotropic attraction between the adjacent amide heads. IR reflection absorption spectroscopy (IRRAS) shows that the -CONHCH(2)CF(3) heads have trans conformation and participate in H-bonds forming a -NH...O=C- lateral network. We speculate that such structure hinders the energetically optimal orientation of the hydrophobic -CH(2)CF(3) terminals toward air, so that the (delta+)C-(F (delta-))(3) dipoles at the monolayer/water boundary yield a strong positive contribution to Delta V. In contrast, most of the unbounded by H-bonds -COOCH(2)CF(3) heads statistically orient their hydrophobic (delta+)C-(F (delta-))(3) dipoles toward air, yielding a negative average dipole moment at the monolayer/water boundary and negative surface dipole potential.

Dependence of the maximum speed of wetting maximum speed of wetting on the interactions in the three-phase contact zone

Abstract The contact-line motion upon creation of Y—Langmuir—Blodgett multilayers was studied under conditions of liquid-film entrainment. Arachidic acid and octadecylamine were used to model respectively negatively and positively charged liquid/gas and liquid/solid interfaces. The existence of a maximum speed of the three-phase contact line in these systems was proved (as earlier for glycerol—water mixtures and hydrophobic substrates). The effects of counter-ion type and concentration and pH on V max were also studied. The results are discussed on the basis of the electrical characteristics and hydration of the interfaces interacting in the three-phase contact zone.

Influence of geometry of the three-phase system on the maximum speed of wetting

Abstract A study of the effect of solid surface curvature K, on the maximum speed of wetting Vmax shows no correlation between these quantities. This finding allows the data on Vmax obtained earlier for model flat walls, to be transferred to systems of more complex geometry. It also confirms the role of the material properties of the system in the contact line velocity.

Interaction of octadecylamine monolayers with phosphate counterions

Abstract By means of neutron activation analysis of octadecylamine multilayers deposited from phosphate subsolutions at different pH and salt concentrations, the interaction of the monolayers with the phosphate anions is studied. It is found that between pH 4.5 and 9.5 it is mainly the divalent form HPO 4 2− which interacts with the octadecylammonium cations. The stability constant of the octadecylammonium hydrogenphosphate is estimated by applying the Scatchard-Hildebrand theory to evaluate the activity coefficients of the monolayer components.

Why fluorination of the polar heads reverses the positive sign of the dipole potential of Langmuir monolayers: a vibrational sum frequency spectroscopic study.

Natural nonionic amphiphiles forming monolayers, bilayers, micelles, or biomembranes create a positive dipole potential at the boundary with water. In a series of papers we have reported on Langmuir monolayers with CF3 terminals of the polar heads, which show a negative surface dipole potential ΔV (Petrov , J. G.; Andreeva, T. D.; Kurt, D. K.; Möhwald, H. J. Phys. Chem. B 2005, 109, 14102). Here we use vibrational sum frequency spectroscopy (SF) to study the origin of the opposite ΔV signs of Langmuir films of CH3(CH2)20COCH2CH3 (ethyl ether, EE) and CH3(CH2)20COCH2CF3 (fluorinated ethyl ether, FEE). The vibrational sum frequency spectra are recorded at the same film density of the S-phase of the EE and FEE monolayers and analyzed in the spectral regions of OH, COC, CH3, and CF3 stretching vibrations because these functional groups could be responsible for the different dipole potentials. We compare the rearrangement of the pure water surface by EE and FEE monolayers and the conformations of EE and FEE polar heads. The analysis is performed according to the three-capacitor model of the dipole potential of Langmuir monolayers (Demchak, R. T.; Fort, T., Jr. J. Colloid Interface Sci. 1974, 46, 191). The results show that reversal of the ΔV sign caused by fluorination of the polar heads originates from the upward-oriented CF3 terminals of the FEE heads, whose negative normal dipole moment component determines the negative dipole potential of the FEE monolayer.