NOTES
4125
-
cellent conformity to a single plot of log (&/At) t was always obtained. Activation Energy ( E a ) . Rate constants were measured at different temperatures for each of two acidities; E , was found from the linear plots of (5 log IC) against 2'-' (Table I).
+
Table I
T,"K lo%, sec-l
374'6 382'4 388'7
372'4 382'1 389'2
9 3 . 3 7 0 HiSOi
84.0% H2S04
32.9
34.8
15.2
E,, kcal
38.8
77.5
1.50
5.38
11.2
mole-'
Eflect of Acidity. Rate constants were measured at 100" for each of seven acid strengths; the results are shown in Figure 1. Stoichiometric concentration of sulfuric acid ( c H +) was calculated using standard density data.s (8) "International Critical Tables," Vol. 3, McGraw-Hill Publishing Co., Inc., New Tork, N. Y . , 1928, p 57.
The Critical Surface Tension of Sapphire'
by J. G. Eberhart Sandia Laboratory, Albuquerque, New Mezico (Receixed April 81, 1967)
Zisman2 has shown that if a single solid is wet by a homologous series of liquids, then the cosine of the contact angle, cos 0, is a linear function of the surface tension of the liquid, ULV. Zisman calls the value of ULV corresponding to complete wetting (e = 0) the critical surface tension of wetting, uc, and has shown that uc is a property of the solid surface only. This linear relationship has been demonstrated primarily for low surface tension organic liquids on low surface tension solids such as polymers or metals covered by monolayers of long chain organic acids. The correlation also appears to be valid for liquids and solids with intermediate surface tensions, as was demonstrated by Olsen and Osteraas3 in their consideration of existing data on the wetting of glass by low melting point metals. In order to test Zisman's relationship for the wetting of high surface tension solids by high surface tension liquids, the studies of a number of authors on the wettability of sapphire by liquid transition metals4ss and the
surface tension of liquid transition metals6J are examined here. Armstrong, Chaklader, and Clarke4 measured the contact angle of pure liquid Xi and its alloys with Ti, Cr, and Zr, under vacuum, at 1500", on a solid sapphire surface approximately coincident to the (1012) plane. The alloys with Ti and Cr were explored over compositions ranging from pure Ni to 9% Ti and to 19% Cr. I n both alloys the contact angle decreased through an inflection point (at -1.5% Ti and 8% Cr) and then leveled off to a constant terminal value with increasing Ti or Cr composition. Additional experiments and calculations made by Armstrong, e t al., showed that Ti or Cr was preferentially adsorbed at the solid-liquid interface. Because of the strong preferential adsorption, it is assumed that the terminal value of 0 is the value 0 would have for pure Ti or Cr at that temperature. Only one composition of the Zr alloy was studied (10% Zr) and this was also assumed to be the value of e for pure Zr. Thus from the data of Armstrong, et al., values of 108,85,83, and 75" are extracted as representing the contact angle of the pure liquids Xi, Ti, Cr, and Zr, respectively, on Al203 at 1500". More recently, Ritter and Burton5 published additional measurements of the contact angle of pure Ni and its alloys with Ti and Cr on a (1012) sapphire surface at 1500". Assuming again that 0 for the highest alloy concentration is characteristic of the solute only, values of 111.3 and 90" are found for Yi and Cr, respectively, under vacuum. The Ti alloys were not, studied under vacuum, but since the terminal Cr contact angle showed no atmospheric dependence, the Ar atmosphere value of 84" is employed for Ti. The surface tension of the above pure liquids has been determined by numerous authors. The values selected here are 1924 dynes/cm6 for Ni at 1550" and 1650,1700, and 1480 dynes/cm7 for liquid Ti, Cr, and Zr, respectively, at their melting points. These particular values are among the highest reported, which suggests high sample purit,y. Assuming a typical temperature (1) This work was supported by the U. S. Atomic Energy Commission. (2) (a) W. A. Zisman, Ind. Eng. Chem., 10, 19 (1963); (b) W. A. Zisman, Advances in Chemistry Series, No. 43, R. F. Gould, Ed., American Chemical Society, Washington, D. C., 1964, pp 1-51. (3) D. A. Olsen and A. J. Osteraas, J . Phys. Chcm., 68, 2730 (1964). (4) W. M.Armstrong, A. C. D. Chaklader, and J. F. Clarke, J . Am. Ceram. SOC.,45, 115 (1962); and J. F. Clarke, M S c . Thesis, University of British Columbia, Canada, 1959. (5) J. E. Ritter, Jr., and M. 9. Burton, Trans. Met. Soc. A I M E , 239, 21 (1967). (6) P. Kozakevitch and G . Urbain, J. Iron Steel Inst., 186, 167 (1957). (7) B . C. Allen, Trans. Met. SOC.A I M E , 227, 1175 (1963); 230, 1357 (1964).
Volume 71, Number 1% November 1967
NOTES
4126
r = 1050
500
1000
j,
100 dynes/cm.
1500
2000
2500
qv, dynes/om
Figure 1. Cosine of the contact angle us. surface tension for liquid transition metal sessile drops on solid sapphire a t 1500' under vacuum: 0,Armstrong, Chaklader, and Clarke;4 A, Ritter and Burton.6
coefficient of duLv/dT = -0.1 dyne/cm deg, the surface tension values of the liquids at 1500" are then estimated as approximately 1930, 1680, 1790, and 1515 dynes/cm for Ni, Ti, Cr, and Zr, respectively. I n Figure 1 the wetting and surface tension data considered above are plotted as prescribed by Zisman. The points do define the usual linear relationship between cos 6 and uLv with the negative slope which has always been found characteristic of this correlation. The least-squares intercept of the linear curve at 6 = 0 gives a critical surface tension for A1203 at 1500" of uc = 1050 f 100 dynes/cm. The slope of the curve is A = -0.00143 cm/dyne. With these two parameters A(uLV - uc), and the Zisman equation, cos 6 = 1 the contact angle of other liquid transition metals on sapphire can be estimated. The view is held by some authors8pgthat the critical surface tension, act is equal to the solid-vapor interfacial tension, usv, ie., the solid-vacuum surface tension, use, minus the spreading pressure of any adsorbed layer present, a. Olsen and Osteraas3 found excellent agreement between uc and us0 - x in their analysis of glass wettability. Thus although this interpretation of uc has been criticized,1° it is nonetheless of interest to compare the above value of uc with the surface tension of solid A1203. The surface tension of sintered solid A1203has been determined by Kingery" to be ugv = 905 dynes em at 1850". Assuming a temperature coefficient of dusv/dT = -0.1 dyne/cm deg the surface tension is estimated as 940 dynes/cm at 1500". This value is in reasonably good agreement with the above critical surface tension Of ac = 1050 * 100 dynes/cm. The liquid bransition metal-sapphire system considered here differs in several ways from the organic systems in which almost all previous Zisman-type analyses have been done. First, in liquid metal wetting, it is
+
The Journal of Physical Chemistry
difficult at present to identify the analog of Zisman's requirement that the liquids be members of a homologous series. In the absence of data on a greater variety of liquid metals on sapphire, it would seem reasonable to restrict tentatively the above correlation to transition metals. Second, although steady contact angles were obtained in both the wetting studies emp l ~ y e d ,evidence ~ , ~ of chemical reaction was found at the solid-liquid interfaces in the form of the oxide of the liquid metal. Caution must always be exercised in dealing with wetting in reacting systems; for example, the validity of Young's equation is doubtful. However, Zisman's equation is empirical in nature and its applicability in reacting systems can only be decided by experimental test. The above linear correlation is preliminary evidence that the equation can be applied to some reacting systems. (8) E. Wolfram, Kolloid Z., 182,75 (1962). (9) V. R. Gray, Chem. Ind. (London), 969 (1965). (10) AM.C. Phillips and A. C. Riddiford, Z. Physik. C h a . , 47, 17 (1965). (11) W.D.Kingery, J . A m . Ceram. SOC.,37, 42 (1954).
Helix Formation of Poly-L-lysine Thiocyanate in Aqueous Solutions
by D. Puett, A. Ciferri, Chemstrand Research Center, Inc., Durham, North Carolina 27708
Estella Bianchi,la Istituto di Chimica Industriale dell' Univeraita' Genova, Genova, Italy
and Jan Hermans, Jr.Ib Department of Biochemistry, University of North Carolina, Chapel Hill, North Carolina (Received M a y 17, 1967)
Aqueous solutions of poly-L-lysine serve as a model for basic proteins, since, particularly under alkaline conditions, this polypeptide may form &-helicesand can exist in a /3 Furthermore, the polyelectrolyte effects occurring a t low pH allow one to (1) (a) Supported in part by the National Research Council of Italy (Centro Virus Vegetali). (b) Research Career Awardee of the U. S. public Health Service (Grant GM-22015). Supported in part by a research grant from the National Institutes of Health. (2) (a) K. Rosenheck and P. Doty, PTOC. Nail. Acad. S C ~ .U, . s., 47, 1775 (1961);(b) P. K.Sarkar and P. Doty, ibid., 55, 981 (1966). (3) B.Davidson, N. Tooney, and G. D. Fasman, B w c h a . Bwphys. Res. Commun., 23, 156 (1966).
