Hugo K. Christenson

Cavitation and the Interaction Between Macroscopic Hydrophobic Surfaces

The interaction in water of neutral hydrocarbon and fluorocarbon surfaces, prepared by Langmuir-Blodgett deposition of surfactant monolayers, has been investigated. The attraction between these hydrophobic surfaces can be measured at separations of 70 to 90 nanometers and thus is of considerably greater range than previously found. Spontaneous cavitation occurred as soon as the fluorocarbon surfaces were brought into contact but occurred between the hydrocarbon surfaces only after separation from contact. The very long range forces measured are a consequence of the metastability of water films between macroscopic hydrophobic surfaces. Thus the hydrophobic interaction between macroscopic surfaces may not be related to water structure in the same way that the hydrophobic effect between nonpolar molecules is related to water structure.

Confinement effects on freezing and melting

A review of experimental work on freezing and melting in confinement is presented. A range of systems, from metal oxide gels to porous glasses to novel nanoporous materials, is discussed. Features such as melting-point depression, hysteresis between freezing and melting, modifications to bulk solid structure and solid-solid transitions are reviewed for substances such as helium, organic fluids, water and metals. Recent work with well characterized assemblies of cylindrical pores like MCM-41 and graphitic microfibres with slit pores has suggested that the macroscopic picture of melting and freezing breaks down in pores of molecular dimensions. Applications of the surface force apparatus to the study of freezing and melting phenomena in confinement are discussed in some detail. This instrument is unique in allowing the study of conditions in a single pore, without the complications of pore blockage and connectivity effects. The results have confirmed the classical picture of melting-point depression in larger pores, and allowed the direct observation of capillary condensation of solid from vapour. Other results include the measurement of solvation forces across apparently fluid films below the bulk melting point and a solid-like response to shear of films above the bulk melting point. These somewhat contradictory findings highlight the difficulty of using bulk concepts to define the phase state of a substance confined to nanoscale pores.

Direct measurements of the force between hydrophobic surfaces in water

Direct measurements of the force between hydrophobic surfaces across aqueous solutions are reviewed. The results are presented according to the method of preparation of the hydrophobic surfaces. No single model appears to fit all published results, and an attempt is made to classify the measured interactions in three different categories. The large variation of the measured interaction, often within each class, depending on the type of hydrophobic surface is emphasized. (I) Stable hydrophobic surfaces show only a comparatively short-range interaction, although little quantitative data on this attraction have been published. (II) Many results showing very long-range attractive forces are most likely due to the presence of sub-microscopic bubbles on the hydrophobic surfaces. Such an interaction is typically measured between silica surfaces made hydrophobic by silylation. Between self-assembled thiol layers on gold surfaces very short-range attractive forces are possibly due to the presence or nucleation of bubbles. The reason for the apparent stability of these bubbles is not clear and warrants further investigation. (III) Results obtained with LB films of surfactants or lipids on mica appear to give rise to a different type of force that fits neither of these two categories. This force is an exponentially decaying attraction, often of considerable range. The force turns more attractive at smaller separations, and may at short range be similar to the interaction measured between stable hydrophobic surfaces. An apparently similar, exponential attraction is also found between mica surfaces bearing surfactants adsorbed from cyclohexane, between silylated, plasma-treated mica surfaces and between both mica and silica surfaces with surfactants adsorbed in situ. This type of force also occurs between some surfaces of relatively low hydrophobicity as well as between one such hydrophobic surface and a hydrophilic surface. No convincing model can explain this third type of interaction for all systems in which it has been observed. This review of work to date points to the importance of the morphology and structure of the hydrophobic surface, and how it may change during the interaction of two surfaces.

Structuring in liquid alkanes between solid surfaces: Force measurements and mean‐field theory

Measurements have been made of the solvation forces between mica surfaces in the even‐numbered n‐alkanes from hexane to hexadecane. In all cases the force law is qualitatively very similar, characterized by a decaying oscillatory function of distance, as occurs for simple isotropic liquids. The spacing between successive minima in the force does not increase with carbon number, and is comparable to the width of a linear alkane molecule rather than its length or any average diameter. This suggests that the alkanes have some tendency towards a parallel orientation near the mica surfaces. The measurements give no indication of any strong repulsive component expected from mean‐field theories of higher alkanes or polymers. The results of one such theory are presented, and the reasons for its failure to match the experimental data are discussed.

Experimental measurements of solvation forces in nonpolar liquids

Forces between molecularly smooth mica surfaces immersed in tetrachloromethane, benzene, and 2,2,4‐trimethylpentane have been measured. In tetrachloromethane and benzene the force law is an oscillatory function of the separation between the mica surfaces, with a periodicity equal to the mean molecular diameter and a measurable range of appoximately ten periods, or about 5 nm. Beyond this separation, the oscillations merge into a purely attractive ‘‘tail’’. In 2,2,4‐trimethylpentane the force law shows only short‐range oscillatory behavior, below 2 nm; at larger separations the force is everywhere attractive. The results are compared with measurements on cyclohexane, octamethylcyclotetrasiloxane and n‐octane, including some repeat measurements. When the distance scale is normalized by the mean periodicity of the force curve for each liquid, the results for the four fairly rigid molecules benzene, tetrachloromethane, cyclohexane, and octamethylcyclotetrasiloxane are very similar; minor differences possibly ...

Adhesion between surfaces in undersaturated vapors—a reexamination of the influence of meniscus curvature and surface forces

Abstract The pull-off force between molecularly smooth mica surfaces has been measured as a function of the relative vapor pressure of cyclohexane, n-hexane, and water. For the experiments with water both normal mica (with surface potassium ions) and ion-exchanged mica (covered with hydrogen ions) have been used. The results of an earlier study (L. R. Fisher and J. N. Israelachvili, Colloids Surf. 3, 303 (1981)) have been shown to lead to erroneous conclusions due to the use of a rolling and shearing spring. Here a nonrolling and virtually shear-free double cantilever spring has been used. At high relative vapor pressures (p/ps ≳ 0.7) the pull-off force for all vapors is dominated by the Laplace pressure in a capillary-condensed annulus. There is a small but noticeable contribution from a solid-solid interaction across the annulus, but surface deformations do not appear to affect the measured pull-off force. As the relative vapor pressure decreases from ∼0.7 to 0, the pull-off force varies smoothly and approaches its value in the “dry” state. There is no difference in behavior between water and the nonpolar liquids other than can be explained in terms of the solid-solid contribution to the adhesion. It is concluded that such measurements do not give any immediate information on the validity of the bulk surface tension for very high (r

Kinetics of the homogeneous freezing of water

Rates of homogeneous nucleation of ice in micrometre-sized water droplets are reported. Measurements were made using a new system in which droplets were supported on a hydrophobic substrate and their phase was monitored using optical microscopy as they were cooled at a controlled rate. Our nucleation rates are in agreement, given the quoted uncertainties, with the most recent literature data. However, the level of uncertainty in the rate of homogeneous freezing remains unacceptable given the importance of homogeneous nucleation to cloud formation in the Earths atmosphere. We go on to use the most recent thermodynamic data for cubic ice (the metastable phase thought to nucleate from supercooled water) to estimate the interfacial energy of the cubic ice-supercooled water interface. We estimate a value of 20.8 +/- 1.2 mJ m(-2) in the temperature range 234.9-236.7 K.

Dehydration and crystallization of amorphous calcium carbonate in solution and in air

The mechanisms by which amorphous intermediates transform into crystalline materials are poorly understood. Currently, attracting enormous interest is the crystallization of amorphous calcium carbonate, a key intermediary in synthetic, biological and environmental systems. Here we attempt to unify many contrasting and apparently contradictory studies by investigating this process in detail. We show that amorphous calcium carbonate can dehydrate before crystallizing, both in solution and in air, while thermal analyses and solid-state nuclear magnetic resonance measurements reveal that its water is present in distinct environments. Loss of the final water fraction—comprising less than 15% of the total—then triggers crystallization. The high activation energy of this step suggests that it occurs by partial dissolution/recrystallization, mediated by surface water, and the majority of the particle then crystallizes by a solid-state transformation. Such mechanisms are likely to be widespread in solid-state reactions and their characterization will facilitate greater control over these processes.

Device for measuring the force and separation between two surfaces down to molecular separations

We present an apparatus for measuring the force as a function of distance between surfaces at separations down to the order of molecular dimensions. The device is a simplified yet improved version of the surface force apparatus first developed by Israelachvili and Adams [J. Chem. Soc. Faraday Trans. 1, 74, 975 (1978)]. It gives the same measurement resolution and has the same possibilities of studying various phenomena in thin films such as friction, viscosity, adhesion, and phase transitions. It offers improved performance with regard to control of surface separation and increased versatility by virtue of variable chamber dimensions and geometry.

Measurement of forces due to structure in hydrocarbon liquids

Direct measurements of the force between two molecularly smooth mica sheets immersed in cyclohexane show not a monotonic van der Waals attraction, but an oscillatory function of distance, where the spacing between successive minima corresponds to the molecular diameter of cyclohexane. As surface separation increases the oscillations become less pronounced, and beyond 5 nm (typically seven or eight oscillations) they are no longer detected. These results accord with theoretical ideas on structural forces resulting from the inhomogeneous arrangement of molecules of the liquid near the solid surface. In n-octane the force law does not show the same pronounced oscillations, except at very small separations where repulsive barriers are found. These are attributed to the difficulty of removing the last layers of adsorbed molecules of the liquid from the mica surfaces, and they reduce the mice-mica adhesion significantly. Small amounts of water in the hydrocarbon liquids condense to form a bridge between the surfaces at small separations, causing a very strong adhesion between them. Some implications of these results for the stability of colloids in organic media are discussed.