George L Gaines

The molecular weight dependence of polymer surface tension

Abstract The surface tensions of a series of poly(isobutylenes) in the molecular weight range 400–3000 have been determined at 24°C. These results, together with surface tension values from the literature for poly(dimethyl siloxanes) and three series of different pure chain-molecule homologues, are found to exhibit a linear dependence on (molecular weight) −2 2 . A simple free-volume argument seems to be consistent with this empirical observation.

SOME OBSERVATIONS ON MONOLAYERS OF CARBON-14 LABELED STEARIC ACID

Abstract Films of radiostearic acid have been deposited on a variety of solids (Cu, Al, Pt, Cr, mica, glass), both by transfer from water surfaces and by adsorption from cetane and nitromethane solutions. The concentration of stearic acid in these films has been assessed radiometrically, and some observations on removal of the film by benzene rinsing have been made. At high surface pressures (>17 dynes/cm.), stearic acid monolayers on water are transferred to solids at the same concentration which exists on the water. At lower pressure (10 dynes/cm.), lower concentrations are found after transfer. This may be related to the discontinuous structure of the film at this pressure. Adsorption of more than a single monolayer (based on the geometrical area of the surface) occurs with Cu and Al, probably because of soap formation. With inert substrates (Pt, Cr, mica), adsorption leads to only a fraction of a close-packed monolayer in times up to several hours. These results are discussed in terms of mechanisms of film deposition. Their importance in the Wilhelmy method for surface pressure measurement is emphasized.

Contact angles during monolayer deposition

Abstract Preliminary results are reported in a study of the contact angles and adhesional energy changes during the transfer of monolayers from the air-water interface to solid (hydrophobic glass) supports. The technique used is the vertical-plate method of Neumann, as modified for this application by Clint and Walker. In building up cadmium arachidate layers, it is found that the hydrophobic (immersion) adhesion energy depends slightly on pH and concomitant extent of soap formation in the film, suggesting that polar group interactions, while small, are not negligible. (The narrow pH range of condensation for cadmium arachidate requires the use of a stable buffer system such as acetate, rather than bicarbonate, for adequate pH control.) The acute contact angles observed during the withdrawal step in layer buildup change slowly after motion is stopped. While no quantitative interpretation is yet possible, it is suggested that this may reflect water and ion redistribution in the interfacial region; such redistribution would also contribute previously ignored surface excess terms to the interfacial energies involved.

Monolayers of porphyrin esters: Spectral disturbances and molecular interactions

Abstract Monolayers at the air-water interface provide a model of the environment in the transition zone between the aqueous and nonaqueous components of biological membranes. As a preface to the study of mixed porphyrin systems, we have examined monolayers of several metal-free porphyrin esters, having hydrophilic groups varied in number and position on the periphery of the ring. With all the materials studied, regardless of the number and distribution of polar groups, the films formed are very rigid, with the porphyrin planes oriented vertically at the water surface. There are pronounced spectral disturbances with respect to solution in organic solvents, red shifts of as much as 70 millimicrons being observed. There are also distinct isomer effects. All results are consistent with very strong plane-plane interactions in the films at the air-water interface. The spectral disturbances are discussed in terms of these interactions.

Surface tension of homologous series of liquids

Abstract Literature data for the surface tensions of several homologous series of alkane derivatives, including compounds of the types R-X, X-R-X and R-X-R where R is an n-alkyl residue, as well as the interfacial tensions against water of n-alkanes, alkyl benzenes and dimethylsiloxanes, have been collected. It was found that in nearly all cases the surface or interfacial tensions are linear in M − 2 3 , where M is the molecular weight. The extrapolated infinite-molecular-weight surface or interfacial tensions (i.e., at M − 2 3 = 0 ) are nearly the same for all the alkane derivatives. This functional dependency is that which we had previously found for the surface tension of several series of polymer liquids.

Immiscibility and interfacial tension between polymer liquids: Dependence on molecular weight

Abstract The interfacial tensions between pairs of fluids from homologous series of polyisobutylenes, polydimethyl siloxanes, perfluoroalkanes and n-alkanes have been measured. The results are consistent with a 2 3 a power molecular weight dependence for each series. This leads to a criterion for miscibility (where the interfacial tension vanishes) which reproduces the sometimes strong anisotropy of molecular weight dependence which is observed experimentally. A few contact angles, yielding molecular weight-dependent adhesion tensions, and oil-water interfacial tensions, have also been measured and lend some additional support to the generality of the 2 3 power relationship.

The interfacial tension between n-alkanes and poly(ethylene glycols)

Abstract Interfacial tensions and surface tensions of the equilibrated phases, and mutual solubilities, have been measured at room temperature for liquid pairs of n-alkanes and poly(ethylene glycols); the alkanes ranged from heptane to hexadecane, and the glycols had molecular weights from 106 (diethylene glycol) to 400. The interfacial tensions were found to fit a linear dependence on (molecular weight)− 2 3 for both components. It was also found, however, that the alkanes substantially reduce the surface tensions of the glycols (the glycols are practically insoluble in, and do not alter the surface tensions of, the alkanes), and for all systems examined, Antonows rule is closely approximated. This permits prediction of the molecular-weight dependence of the sum (interfacial-tension + surface-tension lowering) from the properties of the respective pure liquids.

Acceleration of hydrolysis of bisphenol A polycarbonate by hindered amines

Abstract The hydrolysis of bisphenol A polycarbonate is accelerated when a small amount of any of a number of hindered amine light stabilizers is incorporated in the resin and it is exposed to elevated temperature and humidity. The degree of this base-catalyzed hydrolysis is markedly affected by the structure of the light stabilizer. Secondary amines appear to be more effective catalysts than the corresponding tertiary amines, and two amide derivatives exhibit only very slightly accelerated hydrolysis when compared with additive-free polycarbonate.

The thermodynamic equation of state for insoluble monolayers: III. Mixed monolayers

Abstract The thermodynamic equation of state for mixed insoluble monolayers has been integrated to evaluate the free energy of mixing in terms of the activity coefficients of the subphase (water) in the surface layer. The resulting expressions will permit prediction of free energies of mixing when molecular models which include interactions between surfactants and subphase are developed. Their implications for selecting limits of integration in the experimental determination of free energies of mixing from experimental Π— A data are also noted.

Interface curvature corrections for film balance measurements

Donnees relatives aux mesures quantitatives de couches monomoleculaires insolubles avec une balance a film