Richard M. Pashley

DLVO and hydration forces between mica surfaces in Li+, Na+, K+, and Cs+ electrolyte solutions: A correlation of double-layer and hydration forces with surface cation exchange properties

Abstract The forces between two molecularly smooth mica surfaces were measured over a range of concentrations in aqueous Li + , Na + , K + , and Cs + chloride solutions. Deviations from DLVO forces in the form of additional short-range repulsive “hydration” forces were observed only above some critical bulk concentration, which was different for each electrolyte. These observations are interpreted in terms of the corresponding ion-exchange properties at the mica surface. “Hydration” forces apparently arise when hydrated cations adsorbed on mica are prevented from desorbing as two interacting surfaces approach. Dehydration of the adsorbed cations leads to a repulsive hydration force. A simple and remarkably successful method of analysis of the charging mechanism at the mica surface suggests a novel approach to the determination of the hydrated radius of adsorbed cations. Incorporation of this charge regulation in the exact DLVO force calculation gives much better agreement with experimental results, justifying the precise validity of the DLVO theory in cases where hydration forces are absent. The regulation-force theory allows for a more exact analysis of the net hydration force. Using this approach an exponential “hydration” force of decay length 1.0 ± 0.2 nm was inferred for the interaction of mica surfaces fully covered with hydrated K + and Na + ions. From these results the interaction energy between two hydrated ions can also be estimated.

Hydration forces between mica surfaces in aqueous electrolyte solutions

Surface forces between molecularly smooth mica sheets were measured in Na+ and K+ aqueous salt solutions (at ∼10−3 M) both at 21°C and, for the case of Na+, at 65°C. Additional short-range repulsive forces were found to be the same at both temperatures. In very low concentrations of electrolyte solution (∼5 × 10−6M) and in hydrochloric acid solutions (up to 1.2 × 10−3M) these forces were completely absent-giving force curves in very good agreement with DLVO theory. Analysis of these force-interaction results appears to prove conclusively the existence of counterion hydration forces on mica. These forces apparently also prevent both bubble coalescence and coagulation of latex colloids at high concentrations of electrolyte. The total interaction between charged colloidal surfaces should involve, as apparently in the case of mica, both short-range repulsive forces due to bound partially hydrated ions and a longer range repulsion due to hydrated counterions in the compressed double-layer. These “hydration” forces should be present in many colloidal systems and be dominant in those with high negative charge densities and little possibility of hydrogen bonding to adjacent water layers. Surfaces consisting of ionic species (e.g., Zwitterionic lecithin bilayers) should give rise to repulsive forces due to the surface ion hydration effects only.

Measurement of the hydrophobic interaction between two hydrophobic surfaces in aqueous electrolyte solutions

Abstract The attractive force-law between two hydrophobic (hydrocarbon) surfaces in aqueous solutions has been derived from total force measurements on monolayer coated mica surfaces. This “hydrophobic interaction” is much stronger than the expected van der Waals interaction at distances below 8 nm and decays exponentially with distance. The results highlight the long-range nature of hydrophobic (solvation) forces and indicate that these forces may determine the magnitude of the energy barriers between interacting charged particles and hence the stability of hydrophobic colloids. The results are also consistent with previous experimental and theoretical estimates of the hydrophobic interaction between contacting nonpolar solute molecules in water.

DLVO and hydration forces between mica surfaces in Mg2+, Ca2+, Sr2+, and Ba2+ chloride solutions

Abstract The total force between curved sheets of muscovite mica was measured in a range of concentrations of Mg2+, Ca2+, Sr2+, and Ba2+ chloride solutions as a function of separation. In each case an increase in concentration caused a reduction in Debye length in close agreement with theory and, in addition, a reduction in double-layer potential consistent with weak adsorption of the divalent cation at the negatively charged mica surface. At concentrations above about 10−3 M the weak double-layer repulsion was completely overcome by van der Waals forces such that the total force was attractive at all distances, corresponding to “coagulation” of the mica surfaces. However, at higher concentrations (⩾1.0 M) the divalent cations become firmly bound to the interacting mica surfaces and then give rise to strong, short-range, repulsive forces which prevent coagulation in a primary minimum. These forces are qualitatively similar to those previously observed for alkali metal ions (Li+, Na+, K+, and Cs+ chloride solutions) and appear to be due to the residual hydration shells of the bound cations. The main, qualitative, difference between divalent and monovalent cations is that the former ions are more strongly hydrated and therefore do not easily shed their hydration layers in order to bind to the mica surface. Divalent ions bind and give rise to deviations from DLVO theory at higher concentrations than do monovalent cations.

Molecular layering of water in thin films between mica surfaces and its relation to hydration forces

Abstract The force as a function of distance between mica surfaces in aqueous KC1 solutions has been measured with particular attention given to the forces at separations below 2 nm. As previously reported, the forces in dilute electrolyte solutions are well described by the DLVO theory (i.e., repulsive double-layer forces and attractive van der Waals forces), but above a certain electrolyte concentration an additional short-range repulsive hydration force arises as hydrated cations adsorb to the mica surfaces. As more cations adsorb, the hydration force increases in both magnitude and range (attaining 4–5 nm). We now find that the repulsive hydration force is not purely monotonic, but has an oscillatory component superimposed on it which is particularly pronounced at separations below about 1 nm. The periodicity of the oscillations is 0.25 ± 0.03 nm, corresponding to the diameter of water molecules. The results are compared with those obtained using other, nonaqueous, liquids and with the crystalline swelling properties of clays. The finding that hydration forces are oscillatory at short-range carries implications for the theoretical understanding of hydration effects in general, and its significance for other systems is discussed.

A comparison of surface forces and interfacial properties of mica in purified surfactant solutions

Abstract The results of direct measurements of forces between two mica surfaces in aqueous CTAB solutions at concentrations in the range 10 −6 M to 4 × 10 −3 M (c.m.c. ∼ 10 −3 M) are reported. These results are correlated with adsorption, wetting and adhesion properties, and together provide an almost complete description of a surfactant/solid interfacial system. Double-layer forces are altered by the effect of surfactant adsorption on the surface potential, as well as by the effect of the bulk surfactant concentration. At a CTAB concentration of about 1/20 c.m.c. a packed monolayer adsorbs on mica, exposing a hydrophobic surface to the solution; near the c.m.c. a second monalayer adsorbs, exposing a hydrophilic (bilayer) surface. Mica surfaces “coagulate” in the region of monolayer adsorption but are stable both at higher and lower surfactant concentrations. The force between mica surfaces in CTAB solutions at the c.m.c., where a bilayer adsorbs on each surface, exhibits a force maximum and shows no deviations from DLVO theory down to bilayer separations of about 1.5 nm, below which additional repulsive forces were measured down to final bilayer collapse. These may be due either to hydration effects or to simple compression of the bilayers, though both effects are likely to be occurring in this narrow distance regime. Large deviations from DLVO theory occur in the hydrophobic monolayer adsorption (coagulation) regime where the attractive force appears to be at least three times larger than the expected van der Waals force. Wetting studies of surfactant solutions on mica indicate that only advancing contact angles yield the correct values for the surface and interfacial energies, as calculated from the Young equation.

Amphiphilic components of the murchison carbonaceous chondrite: Surface properties and membrane formation

We have investigated physicochemical properties of amphiphilic compounds in carbonaceous meteorites. The primary aim was to determine whether such materials represent plausible sources of lipid-like compounds that could have been involved as membrane components in primitive cells. Samples of the Murchison CM2 chondrite were extracted with chloroform-methanol, and the chloroform-soluble material was separated by two-dimensional thin layer chromatography. Fluorescnece, iodine stains and charring were used to identify major components on the plates. These were than scraped and eluted as specific fractions which were investigated by fluorescence and absorption spectra, surface chemical methods, gas chromatography-mass spectrometry, and electron microscopy. Fraction 5 was strongly fluorescent, and contained pyrene and fluoranthene, the major polycyclic aromatic hydrocarbons of the Murchison chondrite. This fraction was also present in extracts from the Murray and Mighei CM2 chondrites. Fraction 3 was surface active, forming apparent monomolecular films at air-water interfaces. Surface force measurements suggested that fraction 3 contained acidic groups. Fraction 1 was also surface active, and certain components could self-assemble into membranous vesicles which encapsulated polar solutes. The observations reported here demonstrate that organic compounds plausibly available on the primitive Earth through meteoritic infall are surface active, and have the ability to self-assemble into membranes.

A theoretical and experimental study of forces between charged mica surfaces in aqueous CaCl2 solutions

The interaction between two mica surfaces immersed in CaCl2 solutions has been directly measured. Accurate theoretical calculations, including the anisotropic hypernetted chain (HNC) theory used in this work, predict that at reasonably high surface charge densities the electrical double layer interactions in the presence of divalent counterions should be attractive at short surface separations (in the range 0.6–2 nm). Under most conditions investigated, the experimental results indicate that this indeed is the case. The attraction is a consequence of the correlation between the ions. In addition to the double layer interaction, in most cases the measured force contains an oscillatory contribution. At low CaCl2 concentrations and small surface separations, Ca2+ ions between the surfaces are exchanged for H3O+ ions, which decreases the oscillatory interaction and the ion‐correlation attraction. At high concentrations the force is dominated by a strong hydration repulsion, which is related to the adoration o...

The effect of cation valency on DLVO and hydration forces between macroscopic sheets of muscovite mica in relation to clay swelling

Abstract Direct measurements of the total force as a function of separation between basal plane surfaces of cleaved muscovite mica immersed in aqueous solutions containing mono-, di- and trivalent cations are reported. The cations studied, which have similar crystal radii of about 0.1 nm, alter the interaction both via surface adsorption and by adsorption in the diffuse double-layer. Surface ion adsorption determines the double-layer potential and, above some critical concentration, gives rise to strong repulsive pressures — apparently due to the hydration or hydrolysis properties of the adsorbed cation. The results presented in this study are compared with those obtained for the “crystalline” and double-layer swelling of other clay minerals.

Forces between bilayers of cetyltrimethylammonium bromide in micellar solutions

A direct force-measuring technique has been used to study the interaction forces between adsorbed CTAB (cetyltrimethylammonium bromide) bilayers at concentrations well above the CMC (critical micelle concentration). An analysis of these results based on the Poisson-Boltzmann equations leads to the conclusion that CTAB micelles and adsorbed bilayers are about 22(±4)% dissociated. The apparent agreement of bilayer and micellar ion binding parameters raises an important challenge for theories of double-layer interactions. In addition, the double-layer decay lengths observed in these micellar solutions appear to be due entirely to the dissociated bromide and free CTA+ ions, with no apparent contribution from charged micelles.