490
VOTES
-800
N^-7OC
t
3
2-600
0 z
g -5oc W
I
z -400 LL 0
+ 2-300
-200
-- --
-100
B
0
0
TEFLON
DIPOLE
I
RNO, I
I
I
De Booera has calculated the field at a distance of 2.7 A. from the surface of NaCl to be 2.15 X lo5e.s.u. so the agreement in magnitude is excellent. In addition, De Boer has calculated from completely different experiments values for the fields on rutile of4.1 X 1O6e.s.u.and0.9*X105e.s.u.atsparseand approximately monolayer coverage, respectively. The values listed in Table I do not include the contributions due to dispersion forces. The heats of immersion in heptane clearly indicate a large difference in this contribution between Graphon and Teflon. Smaller differences occur between the polar solids. The positive slope found for Aerosil suggests that this material is weakly polar. However, water and nitrogen adsorption measurements show that the surface of this solid is predominantly hydrophobic; only about 25% of the surface will adsorb water up to about 0.3 relative pressure. The hydrophilic portion consists of silanol groups. Aerosil was included in this study to show that judgment of the nature and the over-all polarity of a solid on the basis of heat of immersion measurements can be misleading if other information is not available. Graphon and Teflon are both hydrophobic solids. It is noteworthy that Teflon has more polar sites per unit area than Graphon.2 These polar sites amount to 0.75 and 0.15% of the surfaces of Teflon and Graphon, respectively. The data in Table I suggest an absence of a field a t the Graphon and Teflon surface. This certainly is not so. Apparently, the experimental approach is not sensitive enough to detect the small electrostatic fields at the surface of these solids.’ Acknowledgment.-The authors greatly appreciate the support provided by the Office of Ordnance Research, U. S. Army.
*
-
,
ROH R C I I
Vol. 62
2 MOMENT.
3
4
Fig. 1.
function of the dipole moment5 of the wetting l i q ~ i d . Assuming ~ that the differences in the heat values were due primarily to E,,, the slope of the linear curve was taken as a measure of the average electrostatic field strength of the rutile surface. Similar determinations are possible for other solids. Heat of wetting values for rutile, CaF2, Aerosil, Graphon and Teflon immersed in hexane, butyl alcohol, butyl chloride and nitropropane are shown plotted in Fig. 1 as a function of the dipole moment of the wetting liquid. The average electrostatic field strength calculated from the slopes of the curves in Fig. 1 using equation 1 are shown in Table I, assuming a surface concentration of adsorbed liquid molecules of 5 X 1014molecules per cm.213and neglecting any differences in the packing of polar molecules in the surface of the solid. It is probably proper to place the lines for the rutile, calcium fluoride and Aerosil so that the intercepts fall above the points for the hydrocarbon as was done previously. The other wetting liquids contain polar groups so that the contributions of the induced dipole moments are no doubt more important in the latter cases. TABLEI CALCULATED AVERAGE FIELDSTRENGTHS FOR SOLID ADSORBENTS Solid
F X 10-6, e.s.u.
Rutile CaFz Aerosil Graphon Teflon
2.7 2.5 1.1 0 0
(5) R. J. W. Le Fevre, “Dipole Moments,” John Wiley and Sone, Ino., New York, N. Y., 1953, p. 133.
(6) J. H.De Boer, “Advances in Colloid Science,” Vol. 111, Inter1950,Vol. VIII, p. 102, 1956. science Publishers Ino., New York, N. Y., (7) J. H.De Boer, “Advances in Catalysis,” Vol. VIII, Academic Press Inc., New York, N. Y . , 1956,p. 36.
THE MOLECULAR WEIGHT AND VAPOR PRESSURE OF GASEOUS BORON
e
SUBOXIDE BY MILTOND. SCHEER Applied Research Operation, Flight Propulsion Laboratory Department, General Electric Company, Cincinnati, Ohio Received September 9, 19SY
The stability of a suboxide of boron has been known for some A polymer of composition (BO), has been prepared3 by the dehydration of sub-boric acid (H4B204). The preparation of this same suboxide from a B(s)-B2O3(1) mixture has been accomplished by Kanda and his eo-workers. These investigators heated such a mixture in oucuo in a tantalum crucible covered by a perforated lid. Condensation of macroscopic quantities of the polymeric (BO), upon a glass target was observed with the crucible a t temperatures as low as 1320°K. (1) H. Kahlenberg, Trans. A m . Eleelrochem. Soc., 47, 23 (1925). (2) E. Zintl, W. Moraweits and E. Gastinger, Z. anorg. allgem. Chem., 246, 8 (1940). (3) T. Wartik and E. Apple, J . A m . Chem. Soc., 7 7 , 8400 (1955). (4) F. A. Kanda, A. J. King, V. A. Russell and W. Katz, ibid., 78, 1609 (1966).
w
NOTES
April, 1958 Inghram, Porter and Chupka6 recently have made a mass spectrometric study, in the temperature range 1300 to 15OO"K., of the gaseous species effusing from an alumina Knudsen cell containing a B(s)-B203(1) mixture. The major ion peaks observed were B202+,B203+, B + and BO+. The presence of B + and BO+ was attributed to the dissociative ionization of B202(g) and B203(g), while BzOz+and B203+seemed to be formed by the simple ionization processes BzOz(g)
10-4
49 1 I
I
I
I
I
T-
I
+ e- -+ i3:02++ 2e- and
+ e - +Bi08+ + 2e-
Bz03(g)
Assuming that the relative ionization cross sections for Ag(g) : BzOz(g):B203(g)were as 1:2:2, Inghram, et al., calculated the partial pressures of B203(g) and B20z(g)from the known vapor pressure of Ag and the measured ion current ratios BL03+/Ag+and Bz02+/Ag+ a t an ionizing energy of about 120 e.v. The B2O2:BzO3pressure ratio, calculated in this fashion, was found to be about 7 to 1 in this temperature range. The pressure of B203 from the B(s)-Bz03(1) mixture was found to be about onefifth that measured by Speiser, Naiditch and Johnstone and about one-half that observed in this Laboratory7 for pure B203. Inghram, et al., tentatively attributed this decreased thermodynamic activity of the B203(1) to its depletion in a reaction with B(s) forming another unidentified condensed phase. From the slope of a log pB202 versus 1/T plot, a value of 94 kcal./mole was obtained for the heat of sublimation of BzOz(g) from the B(s)-B203(1)mixture. This present investigation is concerned with an independent method of measuring the pressure and gas phase constitution of the species evaporating from a B(s)-B2O3(1)mixture. The apparatus used is described in detail in a previous communication.7 I n brief, it consists of a device which can measure both the weight of, and force exerted by, t,he vapor evaporating from a condensed phase and effusing through two small circular orifices. From kinetic theory considerations, such measurements yield both the molecular weight and pressure of the effusing species, Experimental The only modification of the apparatus described previously7 is the substitution of a tantalum cell for the one made of platinum. The reason for this is the formation of a 1ow:melting (
