Volume 29, Issue 5, 01 November 1958
Index of content:
29(1958); http://dx.doi.org/10.1063/1.1744695View Description Hide Description
The coefficient of self‐diffusion for argon is measured from 68 to 291 atmos at 49.4°C. A radioactive tracer technique employing A37 is used and coefficients are measured in a cell in which the path of diffusion is through a plug of porous bronze. Results agree reasonably well with the Enskog dense gas theory calculated from p‐v‐T data.
29(1958); http://dx.doi.org/10.1063/1.1744696View Description Hide Description
It is shown that the growth behavior of potassium chloride crystals from aqueous solution is consistent with the Frank growth mechanism. The growth behavior is altered by the presence of a few parts per million of lead chloride. It is imputed that the rate of growth at unit steps is inhibited and that the rate of two‐dimensional nucleation is markedly increased by the presence of lead chloride. The effects are consistent with complete line adsorption of a complex lead containing ion at growth steps.
Nuclear Magnetic Resonance Absorption in Glass. I. Nuclear Quadrupole Effects in Boron Oxide, Soda‐Boric Oxide, and Borosilicate Glasses29(1958); http://dx.doi.org/10.1063/1.1744697View Description Hide Description
The nuclear magnetic resonance spectrum of B11 has been investigated in several glassymaterials containing boron oxide. A value of 2.76±0.05 Mc for the nuclear quadrupole coupling constant of B11 has been calculated from the second‐order quadrupole perturbation of the m = ½→—½ resonance line. The same quadrupole interaction has been observed in pure boron oxide, B2O3, soda‐boric oxide binary glasses, and two commercial borosilicate glasses. The quadrupole coupling constant is interpreted as arising from the bonding electrons in BO3 groups which comprise the glass network in boron oxide. Addition of oxygen ions to the glass network induces a change in the boron configuration from BO3 planar groups to BO4 tetrahedra resulting in a reduction in the quadrupole interaction. This effect has been observed in the Na2O–B2O3 glasses and in Corning ``Pyrex'' and 707 borosilicate glasses.
29(1958); http://dx.doi.org/10.1063/1.1744698View Description Hide Description
Calculated entropies of dimerization for a number of gaseous alkali metal halides, assuming the rhombic planar model for the dimer, fall in the range —31.0±1.5 entropy units. Entropies of dimerization deduced from experimental data for several species diverge appreciably from the calculated values. The differences can be attributed to uncertainties in the graphical determination of dimerization energies. Energies of dimerization recalculated by combining experimental values of ΔF° (dimerization) with theoretical values of ΔS° (dimerization) correlate well with the interionic separations in the dimers.
29(1958); http://dx.doi.org/10.1063/1.1744699View Description Hide Description
The formal theory of scattering of Gell‐Mann and Goldberger is used to obtain expressions for the differential cross sections describing collisions between two molecules with internal structure in terms of the asymptotic forms of the wave functions. The effect of the Pauli principle is considered explicitly.
29(1958); http://dx.doi.org/10.1063/1.1744644View Description Hide Description
The distribution of intensity in the perpendicular bands of allene has been examined, taking into account the effect of the Coriolis interaction between the degenerate levels v 9 and v 10. The results indicate that in all the perpendicular bands of allene and allene‐d 4 the R Q 0 branch lies on a line which divides the band into two equal (or very nearly equal) areas. This has been used as a criterion for a new rotational assignment of these bands and the rotational constantsA and ζ have been re‐evaluated. The present A values, together with Stoicheff's (reference 16) B values indicate that the CH bond length is 1.061 A and the HCH angle 116°.
29(1958); http://dx.doi.org/10.1063/1.1744645View Description Hide Description
The free electron model of Pauling is used to calculate the magnetic field around a benzene ring which is rotating rapidly about all axes in an external magnetic field. It is assumed that the π electrons precess in two circular paths, one on each side of the ring, equal in radius to the C–C distance in the benzene ring. The separation of these rings is taken as 1.28 A, which gives a calculated value for the nuclear magnetic resonance shielding value for benzene protons equal to the observed value. The field thus calculated is employed to predict shielding values for other aromatic compounds. Agreement with experiment is in general good, but there are some exceptions.
29(1958); http://dx.doi.org/10.1063/1.1744646View Description Hide Description
The sublimation of hexagonal SiC has been studied under equilibrium conditions. The predominant gaseous species above SiC are Si, SiC2, and Si2C. By combining the heat of formation of gaseous Si from solid SiC and the known standard heat of formation of SiC, a value of 113 kcal/g atom is obtained for the heat of sublimation of Si at 298°K. From the measured partial pressures, using estimated free energy functions, dissociation energies for Si2, Si3, SiC, SiC2, Si2C, Si2C2, Si2C3, and Si3C are calculated and compared with previously known dissociation energies for group IV B molecules.
Structural Isomer Discrimination between Cyanamides and Carbodiimides by Nitrogen Nuclear Magnetic Resonance29(1958); http://dx.doi.org/10.1063/1.1744647View Description Hide Description
29(1958); http://dx.doi.org/10.1063/1.1744648View Description Hide Description
Measurements of infrared spectra obtained with polarized radiation are reported for nylon, silk fibroin, feather rachis, stretched wool, and kangaroo tail tendon before and after reaction with deuterium oxide. In every case except tendon replacement of hydrogen by deuterium in the peptide linkage occurs preferentially in the less well oriented regions of the specimen. A residual absorption band between 4600 and 4650 cm—1 is evident in the partially deuterated protein specimens and it is suggested that this may be assigned to a combination of CH and CO stretching modes.
29(1958); http://dx.doi.org/10.1063/1.1744649View Description Hide Description
The absorption spectra of solutions of alkali halides and of halogen gases in alkali halide eutectic melts are described. The spectra of the halides are interpreted as charge transfers from the anion. The halogens are shown to dissolve in these solvents to form trihalide ions.
29(1958); http://dx.doi.org/10.1063/1.1744650View Description Hide Description
Thermal diffusion, or the Ludwig‐Soret effect, can be described by adding, to the isothermal flux equation, a second term proportional to the product of the temperature gradient and Q *, the heat of transport. The magnitude of Q * for vacancies in zinc, was determined by placing markers in a piece of pure zinc, which was then held in a temperature gradient near its melting point. No marker movement was observed in twenty days. Thus the net flux of vacancies was effectively zero so that the effect of the strong concentration gradient was canceled by the thermal‐diffusion force. This result is rationalized in a discussion of Q *. A brief analysis of Q * for solutes, other than vacancies, shows that in general Qi * will be approximated by the partial molar heat of solution of the solute, H̄i, where the pure solute is taken as the standard state. A survey of the available data for Q * shows that one obtains the experimentally observed sign for Q * in each case if one takes simply Qi *=—H̄i.
29(1958); http://dx.doi.org/10.1063/1.1744651View Description Hide Description
Field emission patterns of organic dye molecules such as phthalocyanine or flavanthrene show improved stability when the tip temperature is lowered. No prior contamination or buildup is necessary for the production of typical quadruplet or doublet patterns. Upon heating or ion bombardment the molecule patterns may disappear in several distinct steps of size and intensity. On the average 13 phthalocyanine or 2 flavanthrene molecules must strike the tip to produce one pattern. These observations suggest that the substances studied here evaporate and arrive at the tip as polymerized units consisting of two to six molecules. A mechanism for image formation is proposed.
29(1958); http://dx.doi.org/10.1063/1.1744653View Description Hide Description
The effect of impurities is inexplicable by the accepted theory of crystal growth. In this report a mechanism is presented which describes the specific action of crystal‐growth poisons.
By this model the step motion in growth is hindered and nucleation rate on an ideal surface is increased. To rationalize the effect of growth poisons it is necessary to postulate that complete monostep adsorption occurs at growth steps. In addition, the poison ion or molecule must present energetic and/or steric barriers to the addition of solute atoms so as to form an unpoisoned step by burying the adsorbed line of molecules.
29(1958); http://dx.doi.org/10.1063/1.1744654View Description Hide Description
An explanation based on kinetic considerations is developed for the failure of some pure liquids to crystallize. The important parameters are (1) α, the ratio of the molar liquid‐solidsurface tension to the heat of fusion ΔHf, (2) β, the ratio of the entropy of fusion to R, (3) ΔG′, the kinetic barrier to nucleation, and (4) ΔG″, the kinetic barrier to the growth of the finite crystal. In the absence of foreign nucleating agents sensible crystallization will not occur if αβ⅓≲0.9 and ΔG′≤0, or if ΔG′ or ΔG″≤30(ΔHf /β) = 30 RTm.
In nonreconstructive crystallization the molecules incorporated into the crystal are the same as those existing in the liquid. In this case ΔG′ and ΔG″ are likely to be the same and of the order of magnitude of the free energy of activation for fluid flow, ΔGf. Usually, and ΔGf ≪30 RTm, in nonreconstructive crystallization and the nucleation of crystals in clean liquids, is sensible at some undercooling. There are some organic liquids in which crystallization is nonreconstructive and that do not crystallize when clean. We have found that the ratio Tb/Tm of the normal boiling point to the melting point is abnormally large in these cases and near two or more for those compounds that clearly form glasses. Since β and ΔG′/RTm are both likely to increase with Tb/Tm, the difficulty of crystallization would increase with this ratio. It is probable that this correlation also holds for inorganic molecular liquids.
In reconstructive crystallization the molecular entities in the crystal differ from those which predominate in the liquid and bonds between atoms must be broken in order for nucleation to occur. Thus ΔG′ can be of the order of the bond energy, which often exceeds 30 RTm, and, excepting in network liquids, much greater than ΔGf.
Bonds between atoms also must be broken for the reconstructive growth of finite crystals. However, this process can be promoted by impurities and molecular fragments in ways not possible in homogeneous nucleation so that ΔG″ as well as ΔGf can be much less than ΔG′. The reconstructive growth of finite crystals at the expense of a different crystal phase can be enhanced similarly so that ΔG″ may be much less than ΔG′ in this case also.
29(1958); http://dx.doi.org/10.1063/1.1744655View Description Hide Description
The orientation distribution function for rigid colloidal particles immersed in Couette flow with an electric field parallel to the velocity gradient is derived in this study. Simple streaming birefringence experiments with variable or constant electric fields can be devised to test this theory which promises to yield a new method for measuring some geometrical and physical properties of colloidal particles.
Ion Exchange Kinetics. A Nonlinear Diffusion Problem. II. Particle Diffusion Controlled Exchange of Univalent and Bivalent Ions29(1958); http://dx.doi.org/10.1063/1.1744656View Description Hide Description
The differential equation derived previously which describes the particle diffusion controlled ion exchange between spherical beads of uniform size and a well‐stirred solution is solved numerically for the exchange of monovalent ions for bivalent ions, and of bivalent ions for monovalent ions. The approach is based on the Nernst‐Planck equations of ionic motion. Numerical results for six different mobility ratios are presented and discussed. They were obtained by use of a digital computer. An explicit equation approximating the numerical data is given.
29(1958); http://dx.doi.org/10.1063/1.1744657View Description Hide Description
Vapors effusing from Knudsen cells containing LiF, NaF, and LiF–NaF mixtures have been analyzed mass spectrometrically. The over‐all temperature range studied is between 900 and 1150°K. Monomers, dimers, and trimers of LiF and NaF are observed. The LiF system has the higher concentration of dimers and trimers. In the LiF–NaF system at 50 mole percent LiF the mixed dimer, LiNaF2, is the major polymeric vapor species. Thermochemical data for LiF(g), NaF(g), Li2F2(g), Na2F2(g) and Li3F3(g) have been obtained. For the reaction Li2F2(g)+Na2F2(g)=2LiNaF2(g), ΔH° is approximately zero.
29(1958); http://dx.doi.org/10.1063/1.1744658View Description Hide Description
A general perturbation theory is developed for the solution properties of systems with comparable intermolecular interactions. By employing our earlier general statistical mechanical theory of solutions as a starting point, the dominant contribution to the excess chemical potential is developed rigorously to first and second orders in the perturbation parameters. The very small first‐order terms are in agreement with those originally found by Longuet‐Higgins and the second‐order results may be exhibited in alternative forms. One of these, with use of a physically plausible approximation for the solute‐solvent radial distribution function, evaluated in the pure solvent, leads to formulas identical with those of the Prigogine‐Scott two‐fluid model. Alternatively, this distribution function may be expressed exactly in terms of the pair and triplet correlation function of the solvent. Numerical calculations on the latter, based on liquid argon as reference fluid, provide additional support for the successful two‐fluid model.