Arthur W. Adamson

Adsorption and contact angle studies: I. Water on smooth carbon, linear polyethylene, and stearic acid-coated copper

Ellipsometrically determined adsorption isotherms are reported for water on two types of pyrolytic carbon, on polyethylene, and on stearic acid-coated copper, for relative pressures up to close to the saturation pressure, P0, and for various temperatures. Contact angle data for bulk water on the same solids are included; advancing angles of 60°–90° were found. The adsorbed film thickness reaches 40–80 A in the first two systems, but only a few angstroms in the second two; correspondingly, the surface pressures of P0, π0, are large in the first two cases and small in the second two. Large contact angle thus does not necessarily imply low π0. The data are fitted to a previously published potential-distortion model, which allows adsorption and contact angle behavior to be related.

A model for micellar emulsions

Abstract A model is presented whereby oil external micellar emulsions (microemulsions) are treated as systems of swollen aqueous micelles in which Laplace and osmotic pressures are balanced. The model accounts for several of the features of these systems, and, in particular, for the ability of a micellar emulsion phase to be in equilibrium with an external aqueous electrolyte phase. The importance of obtaining equilibrium distribution data is stressed.

Physical adsorption on heterogeneous surfaces

Abstract The two principal criteria used to evaluate the physical nature of the physically adsorbed state, namely, fit-to-model isotherm equations and comparison of entropy quantities, are examined for the situation of a heterogeneous surface. It is concluded that even for quite homogeneous surfaces, the possibility of some heterogeneity offers enough scope to erase the differences between adsorption isotherms derived from different models. A similar situation pertains in the case of adsorption entropies. An added problem is that surface heterogeneity imposes apparent entropy contributions which can in fact lead to erroneous conclusions if comparison is made to calculated entropies for adsorption on a homogeneous surface. It is suggested that the interaction energy distribution function is the dominant one for most surfaces and the most uniquely characterizable, so that emphasis on it rather than on an adsorption model should have preference. An experimental approach is suggested that may help decide whether for a given surface, the heterogeneities are present randomly or patch-wise.

Physical adsorption of vapors on ice I. Nitrogen

Abstract Ice powder prepared at 77°K. gives nitrogen adsorption isotherms indicating a fairly uniform and not highly polar surface. Annealing at −70°C. leads to adsorption behavior characteristic of a nonpolar surface such as Teflon, and surface inertness towards nitrogen reaches an extreme in the case of snow samples. The type of surface structure indicated by these results is discussed. Considered also is an explanation for the apparently heterogeneous behavior of water as an adsorbent as contrasted with the surface homogeneity of ice observed here.

Adsorption and contact angle studies: II. Water and organic substances on polished polytetrafluoroethylene

Abstract Ellipsometrically determined adsorption isotherms are reported for water, bromobenzene, nitro-methane, benzene, amyl, butyl, propyl, and ethyl alcohols, carbon tetrachloride, n -octane, and n -hexane on a polished polytetrafluoroethylene surface. These are nonwetting systems, and contact angles were also measured. In addition, isotherms were determined for two wetting systems, carbon tetrachloride on oxide-coated stainless steel and n -hexane on oxide-coated chromium-plated glass. For most of the nonwetting cases, the film pressure of the adsorbed film was not negligible, and should not not be omitted in semiempirical treatments of contact angle. The isotherms may be fitted by a previously proposed potential-distortion model, the choice of parameters also giving the observed contact angle. Alternatively, the isotherms are found to be segments of a single characteristic isotherm of the Polanyi type and thus obey a corresponding state principle. This characteristic isotherm for nonwetting systems does not fit the data for the two wetting cases, and the possibility is discussed that in the nonwetting cases the adsorbed state consists of bulk-like liquid in the form of micropatches or lenses rather than as a film of uniform thickness.

Application of the BET equation to heterogeneous surfaces

Abstract It is shown that a previously published procedure for obtaining site energy distributions can be applied to data which include multilayer adsorption, with the BET equation used as the local isotherm function. When applied to the adsorption of argon on rutile, the resulting distribution function is essentially the same as that obtained by Drain and Morrison using a nearly thermodynamic procedure. The BET model can thus be applied to adsorption on heterogeneous surfaces. On the other hand, the BET c value obtained from a simple fitting of the equation to the adsorption isotherm reflects only the average energy of those sites being filled in the region of the fit, and is low. A better single-parameter description of the adsorbent surface is given by the average site energy as obtained from the actual distribution function.

An adsorption model for contact angle and spreading

Abstract The adsorption isotherm for a vapor on a substrate must cross the saturation pressure line at a finite film thickness in the case of those systems for which liquid adsorbate rests on the substrate with a finite contact angle. It is suggested that such isotherms be identified as types VI and VII, depending on the manner of the final asymptotic approach to the P0 line at infinite film thickness. A semiempirical model is presented, capable of representing type VI and VII isotherms, including the unstable regions. The model allows calculation of contact angles and spreading coefficients from adsorption data alone, and such calculations are illustrated with the use of published data for various vapors adsorbed on glass and on liquid water as substrates. A general purpose of the paper is to emphasize the central role of structural modifications in interfacial regions in determining wetting behavior and to provide a framework for predicting what types of liquid and substrate properties should give very high works of adhesion and very low (even negative) ones.

Physical adsorption of vapor on ice: II. n-alkanes

Abstract Physical adsorption studies of simple nonpolar hydrocarbon vapors on ice are described. The isotherms obtained cover a temperature range of from −96°C to −30°C, or up to roughly the temperature where drastic sintering occurs. Results are compared to existing data for hydrocarbon adsorption on liquid water. At temperatures below that of about −35°C, adsorption behavior is characterized by low adsorption energies, with isosteric heats lower than the heat of condensation at low coverages and rising to that of the heat of condensation as coverage increases to multilayer formation. As the temperature is increased to −35°C and above, adsorption behavior dramatically changes to that similar to adsorption on liquid water, marked by abnormally high adsorption heats at low coverage which drop to the heat of condensation as coverage increases. The explanation proposed is that H 2 O molecules in the ice surface are actively involved in the adsorption process at the higher temperatures, in a similar manner to the case of hydrocarbon adsorption on liquid water.

Potential distortion model for contact angle and spreading. II. Temperature dependent effects

Abstract A previously proposed model [J. Colloid Interface Sci., 27, 180 (1968)] relating adsorption isotherms and contact angle or spreading coefficient is extended to provide for temperature dependence. If it is assumed that the adsorbed film, while liquid-like, differs from bulk liquid both in enthalpy and entropy, then a number of new relationships are generated, including the temperature variation of contact angle. The model as modified is applied to literature data on the n-octane Teflon and n-decane Teflon systems. One prediction resulting from this application is that there should be a critical temperature for spreading, that is, a temperature above which the contact angle is zero. This temperature is found to be between the normal boiling point and the critical temperature of the bulk liquid.

Physical adsorption of vapors on ice. IV. Carbon dioxide

Abstract Carbon dioxide adsorbs rapidly, reversibly, and very weakly on ice powder at 195°K up to P P 0 about 0.5. At this relative pressure, an onset of massive uptake sets in, the pressure drops to 0.25 P P 0 , and remains at this value for up to 30 nominal monolayers of adsorbed carbon dioxide. This last pressure, as well as its temperature dependence, is essentially the same as that for the ice-carbon dioxide clathrate equilibrium, and it therefore appears that the massive uptake is due to clathrate formation. The adsorption and desorption processes are slow for the ice-clathrate system, the desorption being found to obey first-order kinetics. Further, after prolonged desorption, a residual, unremovable amount of carbon dioxide is present, corresponding to about a monolayer; this may represent surface carbonic acid formation.