Stig Ljunggren

The lifetime of a colloid-sized gas bubble in water and the cause of the hydrophobic attraction

When considered to be open in the thermodynamic sense with respect to the enclosed gas, bubbles that are present in a liquid bulk phase are unstable in all respects and tend to dissolve. Conversely, full mechanical (and physicochemical) stability is guaranteed when a bubble is closed with respect to the gaseous component. By assuming the diffusion of dissolved gas molecules away from a spherical gas bubble to determine the shrinkage rate, we calculate the lifetime of a bubble as a function of its size. Gas bubbles in the colloidal size range, with radii between 10 and 100 nm, have surprisingly short lifetimes, between about 1 and 100 μs, whereas a nm-sized bubble can persist for months. Our results (which confirm and partwise extend the old calculations of Epstein and Plesset [5] [J. Chem. Phys., 18 (1950) 1505], indicate that the bridging bubble/cavity mechanism proposed earlier, can hardly provide the proper explanation of the long-ranged attraction force observed between hydrophobic surfaces immersed in water.

A phenomenological theory of long-range hydrophobic attraction forces based on a square-gradient variational approach

Through recent surface force measurements it has been convincingly demonstrated that strong and amazingly long-range, attractive interaction forces act between hydrophobic surfaces immersed in water. Upon separating two such surfaces from molecular contact a vapour/gas cavity normally forms. This is not the case, however, when gradually diminishing the surface separation. The hydrophobic attraction forces have been recorded in this latter, metastable regime.A mean-field theory based on a square-gradient assumption is presented in this paper which is shown to account reasonably well for the surface forces found experimentally for two cylindrically shaped, hydrophobic surfaces interacting in water. The order parameter/ variational approach taken is closely related conceptually to earlier theories of repulsive hydration forces by Marcelja et al. and Cevc et al. The present theory implies that rather minor, hydrogen-bond-propagated molecular ordering effects, in the contact layers of water molecules next to the hydrophobic surfaces and in the core of the thin water film, give rise to the attraction observed. However, it does not fully address the intriguing question as to how it comes about that the hydrophobic attraction forces extend over such a wide range as 70–90 nm. It merely points in the direction that surface-induced structural changes in the core of the this water film (so far not captured by molecular dynamics simulations) which demand minimal free-energy expense may generate an interaction of a long-range nature.

The molecular structure of N-benzylidene-aniline

Abstract The molecular structure of N-benzylidene-aniline has been studied experimentally by the gas electron diffraction method, and also by molecular mechanics calculations. Both approaches gave the same results for the most stable conformer of the free molecule. The phenyl ring bonded to the carbon end of the CN bond was found to be coplanar with this bond, while the other phenyl ring was extensively (ca. 52°) rotated about the NΦ bond.

Theory of curved interfaces and membranes: Mechanical and thermodynamical approaches

Abstract The mechanical and thermodynamical approaches to the theory of the general curved interfaces are presented and compared. In the mechanical approach a curved interface or membrane is characterized by the tensors of surface stresses and moments. They are connected by the surface balances of the linear and angular momentum. On the other hand, in the thermodynamical approach the surface is characterized by the scalar dilation and shear tensions as well as by the bending and torsion moments. In this review we investigate the problem about the relationships connecting the mechanical and thermodynamical approaches. We find that these two approaches are in a good agreement, that they are complementary to each other and represent the two parts of a self-consistent theory. The latter can be applied to any system where curved interfaces, thin films or membranes are present: microemulsions, lamellar and sponge phases, lipid vesicles and cell membranes, capillary waves at interfaces, undulation and peristaltic surface forces, lateral capillary forces between particles in thin liquid films, etc.

A novel approach to the mechanics and thermodynamics of spherical micelles

A detailed mechanical theory for a spherical micelle is presented, based upon the notion that a micelle has the nature of an interface throughout. Accordingly, we presume that the (ensemble average) pressure tensor is generally anisotropic in a micelle except at r= 0 (for symmetry reasons). Following the Gibbs scheme for treating curved interfaces, the surface tension is usually defined with reference to the surface of tension (s.o.t.). In the case of a micelle, however, this method is inconvenient since the location of the s.o.t. is highly variable and difficult to assess a priori. An alternative approach is developed whereby the hydrocarbon core volume is employed to define the hydrophobic dividing surface (h.d.s.) located at r=R, which separates the internal (i) core part of the micelle from the exterior (e) water solution phase. In order to achieve a complete thermodynamic description it is then necessary to introduce an additional intensive variable, the interaction (disjoining) pressure πm. At the h.d.s., the Laplace equation can be written on the modified form pi–pe= 2γe/R–πm where pi is the average tangential pressure within the hydrocarbon core and γe is the surface tension originating outside r=R. πm is normally attractive (< 0) and is due chiefly to the hydrocarbon-chain packing constraints and the effect of curvature on the electrostatic interactions.Surfactant micelles are very small systems for which fluctuations of the aggregation number, N, are important. Hence the surface thermodynamics of micelles is calculated, taking account of the general principles of the thermodynamics of small systems developed by Hill. On this basis we show that, at phase equilibrium, the average surfactant chemical potential in the micellar state, 〈µN〉, (but not each µN separately) is equal to the monomer chemical potential in the intermicellar solution and, moreover, that the average net work expended to form a micelle with surface area A is equal to 〈(γe+πmR)A/3〉. At aggregation equilibrium this work is counterbalanced by free-energy gains of entropic origin caused by the dispersion of the micelles in the solution and their size fluctuations.Our model calculations for SDS micelles show that a consistent, quantitative theory of micelle formation can be obtained by using (i) Tanfords expressions for the solubility of hydrocarbons in water, (ii) the Poisson–Boltzmann/cell-model treatment of the electrostatics of a uniformly charged sphere, (iii) the macroscopic excess free energy, 50 mJ m–2, to account for the hydrocarbon-core/water contact and (iv) the conformation free-energy function for hydrocarbon chains packed in a spherical aggregate computed by Gruen and de Lacey. The large and negative πm components caused by factors (ii) and (iv) strongly promote micelle formation. The average pressure level in the hydrocarbon core is predominantly determined by the Laplace term 2γe/R and may typically amount to several hundred atmospheres, in agreement with experiments on the solubility of gases in micellar solutions.

Microwave spectrum and planarity of p-fluorostyrene

Abstract The microwave spectrum of p -fluorostyrene has been studied in the frequency region 18.0–26.5 GHz. Rotational transitions in the ground and several vibrationally excited states have been identified. The molecule has been shown to be planar in the ground state. From intensity measurements, the frequency of the first torsional transition has been estimated to be 30 ± 15 cm −1 . The potential barrier to the internal rotation is briefly discussed.

Stereochemistry of copper- and nickel-catalyzed insertion of carbon dioxide into epoxides. A microwave study

Reaction of trans-1,2-dideuterioethene oxide (1) with carbon dioxide, using copper and nickel catalysts, and subsequent analysis of the product ethene carbonate-d2 (2) by microwave spectroscopy, shows that the copper-catalyzed reaction is stereo-specific (retention) whereas the nickel-catalyzed reaction is non-stereospecific.

Stereochemistry of the hydroxypalladation step in the wacker process

Stereospecific formation of threo-1,2-dideuterio-2-chloroethanol [threo-(3)] in the Wacker reaction of trans-1,2-dideuterioethene (2) indicates that the hydroxy-palladation step is a trans-process.

Microwave spectrum of p-chlorostyrene

Abstract The microwave spectrum of p -chlorostyrene has been studied in the frequency region 18.0–26.5 GHz. Rotational transitions of the ground state, the first two torsionally excited states and two other vibrational states of the 35 Cl isotopic species have been identified. The molecule was found to be planar in the ground state. From intensity measurements, the frequency of the first torsional transition has been estimated to be 35 ± 15 cm −1 .

Theory of spin relaxation in bicontinuous cubic liquid crystals

A theoretical framework is presented for the interpretation of spin relaxation data from bicontinuous cubic lyotropic phases described by periodic minimal surfaces. Specifically, the two irreducible time correlation functions (TCFs) that determine the contribution from surface diffusion to the observable spin relaxation rates are considered. Simple analytical results are obtained that relate the initial TCFs to the fourth‐rank orientational order parameter of the dividing interface and the initial decay of the TCFs to the average Gaussian curvature over the cubic unit cell. These exact results are used to construct single‐exponential approximations for the TCFs. Explicit calculations are reported for the three cubic triply periodic minimal surfaces of simplest topology, i.e., Schwarz’s D and P surfaces and Schoen’s gyroid surface, as well as for the corresponding parallel surfaces that have been used to model the dividing interface (the locus of surfactant headgroups) in bicontinuous cubic phases. The the...