Richard M. Pashley

Bio: Richard M. Pashley is an academic researcher from University of New South Wales. The author has contributed to research in topics: Aqueous solution & DLVO theory. The author has an hindex of 44, co-authored 154 publications receiving 13120 citations. Previous affiliations of Richard M. Pashley include Imperial College London & Australian Defence Force Academy.

Direct measurement of colloidal forces using an atomic force microscope

Abstract: THE forces between colloidal particles dominate the behaviour of a great variety of materials, including paints, paper, soil, clays and (in some circumstances) cells. Here we describe the use of the atomic force microscope to measure directly the force between a planar surface and an individual colloid particle. The particle, a silica sphere of radius 3.5 µm, was attached to the force sensor in the microscope and the force between the particle and the surface was measured in solutions of sodium chloride. The measurements are consistent with the double-layer theory1,2 of colloidal forces, although at very short distances there are deviations that may be attributed to hydration forces3–6 or surface roughness, and with previous studies on macroscopic systems4–6. Similar measurements should be possible for a wide range of the particulate and fibrous materials that are often encountered in industrial contexts, provided that they can be attached to the microscope probe.

The hydrophobic interaction is long range, decaying exponentially with distance

Abstract: The attractive interaction between organic nonpolar molecules, such as hydrocarbons, in water is unusually strong. This ‘hydrophobic interaction’1 is responsible for the very low solubility of hydrophobic molecules in water, and has a central role in micelle formation, biological membrane structure, and in determining the conformations of proteins2,3. It was once believed that because the interaction is so strong there is a ‘hydrophobic bond’ associated with it2,4; but it is now recognized that the interaction involves the configurational rearrangement of water molecules as two hydrophobic species come together5–9 and is therefore of longer range than a typical covalent bond. However, there has been no experimental information available concerning the distance dependence and effective range of this interaction. From measurements of the total force as a function of distance between two hydrophobic surfaces immersed in aqueous electrolyte solutions we have determined accurately the attractive component due to the hydrophobic interaction and found that the hydrophobic interaction has the same range as, but is about an order of magnitude stronger than, the van der Waals-dispersion force; and that in the range 0–10 nm it decays exponentially with distance with a decay length of ∼1 nm. The results can be roughly extrapolated to molecular interactions and show that the interaction free energy of two hydrophobic solute molecules of radius R (nm) in water at 21 °C is approximately given by ΔGH = −40R kJ mol−1, which is in agreement with previous estimates. However, the hydrophobic interaction is not due to a ‘hydrophobic bond’, and its long-range nature has obvious implications for the mechanism and rates of folding as well as the equilibrium conformations of proteins and other macromolecules.

Molecular layering of water at surfaces and origin of repulsive hydration forces

Abstract: The short-range forces between hydrophilic surfaces in water determine the behaviour of many diverse systems such as the stability of colloidal dispersions1,2 and soap films3, the swelling of clays4 and the interactions of biological membranes5–8 and macromolecules9. So far, all experimental measurements of these forces have indicated that they are repulsive and decay monotonically with distance out to separations of up to ∼6 nm. These forces, variously termed ‘structural’ or ‘hydration’ forces, arise from the energy needed to dehydrate interacting surfaces which contain ionic or polar species. Here we have measured, in some detail, the short-range hydration force between two molecularly smooth surfaces of mica containing hydrated potassium ions. We find that while the hydration force is overall repulsive it is not monotonic at separations ≲1.5 nm but exhibits oscillations, that is, alternating maxima and minima with a mean periodicity of 0.25 ± 0.03 nm, roughly the diameter of the water molecule. These results rationalize apparently irreconcilable observations on clay–water systems and go some way towards clarifying the origin and nature of hydration forces.

Calibration of atomic‐force microscope tips

Abstract: Images and force measurements taken by an atomic‐force microscope (AFM) depend greatly on the properties of the spring and tip used to probe the sample’s surface. In this article, we describe a simple, nondestructive procedure for measuring the force constant, resonant frequency, and quality factor of an AFM cantilever spring and the effective radius of curvature of an AFM tip. Our procedure uses the AFM itself and does not require additional equipment.

Biomaterial-centered infection: microbial adhesion versus tissue integration.

Abstract: Biomaterials are being used with increasing frequency for tissue substitution. Complex devices such as total joint replacements and the total artificial heart represent combinations of polymers and metal alloys for system and organ replacement. The major barriers to the extended use of these devices are the possibility of bacterial adhesion to biomaterials, which causes biomaterial-centered infection, and the lack of successful tissue integration or compatibility with biomaterial surfaces. Interactions of biomaterials with bacteria and tissue cells are directed not only by specific receptors and outer membrane molecules on the cell surface, but also by the atomic geometry and electronic state of the biomaterial surface. An understanding of these mechanisms is important to all fields of medicine and is derived from and relevant to studies in microbiology, biochemistry, and physics. Modifications to biomaterial surfaces at an atomic level will allow the programming of cell-to-substratum events, thereby diminishing infection by enhancing tissue compatibility or integration, or by directly inhibiting bacterial adhesion.

Direct measurement of colloidal forces using an atomic force microscope

Abstract: THE forces between colloidal particles dominate the behaviour of a great variety of materials, including paints, paper, soil, clays and (in some circumstances) cells. Here we describe the use of the atomic force microscope to measure directly the force between a planar surface and an individual colloid particle. The particle, a silica sphere of radius 3.5 µm, was attached to the force sensor in the microscope and the force between the particle and the surface was measured in solutions of sodium chloride. The measurements are consistent with the double-layer theory1,2 of colloidal forces, although at very short distances there are deviations that may be attributed to hydration forces3–6 or surface roughness, and with previous studies on macroscopic systems4–6. Similar measurements should be possible for a wide range of the particulate and fibrous materials that are often encountered in industrial contexts, provided that they can be attached to the microscope probe.

Water as an Active Constituent in Cell Biology

Abstract: When Szent-Gyorgyi called water the “matrix of life”,1 he was echoing an old sentiment. Paracelsus in the 16th century said that “water was the matrix of the world and of all its creatures.”2 But Paracelsus’s notion of a matrixsan active substance imbued with fecund, life-giving propertiess was quite different from the picture that, until very recently, molecular biologists have tended to hold of water’s role in the chemistry of life. Although acknowledging that liquid water has some unusual and important physical and chemical propertiessits potency as a solvent, its ability to form hydrogen bonds, its amphoteric naturesbiologists have regarded it essentially as the backdrop on which life’s molecular components are arrayed. It used to be common practice, for example, to perform computer simulations of biomolecules in a vacuum. Partly this was because the computational intensity of simulating a polypeptide chain was challenging even without accounting for solvent molecules too, but it also reflected the prevailing notion that water does little more than temper or moderate the basic physicochemical interactions responsible for molecular biology. What Gerstein and Levitt said 9 years ago remains true today: “When scientists publish models of biological molecules in journals, they usually draw their models in bright colors and place them against a plain, black background”.3 Curiously, this neglect of water as an active component of the cell went hand in hand with the assumption that life could not exist without it. That was basically an empirical conclusion derived from our experience of life on Earth: environments without liquid water cannot sustain life, and special strategies are needed to cope with situations in which, because of extremes of either heat or cold, the liquid is scarce.4-6 The recent confirmation that there is at least one world rich in organic molecules on which rivers and perhaps shallow seas or bogs are filled with nonaqueous fluidsthe liquid hydrocarbons of Titan7smight now bring some focus, even urgency, to the question of whether water is indeed a * E-mail: p.ball@nature.com. Philip Ball is a science writer and a consultant editor for Nature, where he worked as an editor for physical sciences for more than 10 years. He holds a Ph.D. in physics from the University of Bristol, where he worked on the statistical mechanics of phase transitions in the liquid state. His book H2O: A Biography of Water (Weidenfeld & Nicolson, 1999) was a survey of the current state of knowledge about the behavior of water in situations ranging from planetary geomorphology to cell biology. He frequently writes about aspects of water science for both the popular and the technical media.