675
ADSORPTION OF BRANCHED-CHAIN ALCOHOLS
Adsorption on Flat Surfaces. 111. Adsorption of Branched-Chain Alcohols by T. D. Blake* and W. H. Wade Department of Chemistry, Universitu of Texas at Austin, Austin, Texas
(Received September 27, 1971)
Publication costs assisted by the Robert A . Welch Foundation
Gravimetric measurements have been made of the vapor-phase adsorption of various branched-chain alcohols on the oxidized surface of aluminum foil. All resulting isotherms are Langmuirian up to at least 0.8 relative pressure, but then rise steeply. Tertiary alcohols show a subsequent step at two to three monolayers. In the case of 2-methyl-2-propanolJas the temperature is lowered below the melting point, multilayer adsorption diminishesand the step moves towards higher relative pressures. At temperatures just below the melting point, isotherms intersect the saturation vapor pressure axis, and nucleation of the bulk phase is sufficiently delayed for isotherm measurement above saturation. This delay is attributed to a large contact angle resulting from a structural dissimilarity between the adsorbed and bulk phases, If the alumina surface is first covered with 70% of a monolayer of pentanol, then although the total adsorption of tertiary alcohols is reduced, the general shape of their isotherms is unchanged. In general, the work provides further evidence that the high energy, heterogeneous character of alumina surfaces can be masked by a monolayer of a low-molecular-weight alcohol to give a low energy surfaceof considerableuniformity.
Introduction Previous work carried out in this lab~ratory,'-~ has demonstrated that the important autophobicity concept due to Zisman and coworker^^-^ may be applied to the vapor-phase adsorption of low-molecular-weight n-aliphatic alcohols on alumina. Evidently, monolayers of alcohols having hydrocarbon chains containing as few as 3 carbon atoms per molecule are sufficient to obscure the high energy character of this surface. Incomplete monolayers of longer chain alcohols are also effective. Thus, 70Oj, of a monolayer of pentanol reduces the surface pressure of decane from 36.2 to 7.1 dyn cm-l, and increases the contact angle from 0 to about 8 deg.3 These investigations have now been extended to include various branched-chain alcohols. As in the earlier papers in this s e r i e ~ ,the ~ ? substrate ~ chosen was oxidized aluminum foil. The use of a planar rather than a powdered adsorbent reveals certain features of the isotherms that might otherwise be obscured by subsaturation condensation. The adsorbates investigated were cyclohexanol, 2-propanol, and the tertiary alcohols 2-methyl-2-propano1, 2-methyl-2-butano1, and 3-methyl-3-pentanol. Hereinafter, these will be designated, respectively, cyclo-C6, iSO-Ca, tert-C4, tert-C5 and tert-C6. Straight chain alcohols will be referred to as Ca, C4,etc. Experimental Section Details of the gravimetric adsorption apparatus and the experimental technique used in this work have been given else~vhere.~,~ The foil sample (package B)2comprised a stack of 200 rectangular sheets of 5-pm aluminum foil having a total geometric area of 1.02 m2 and a roughness factor of 1.08. The sheets were held in place by a light silica framework. Prior to each run, the
sample was cleaned by overnight treatment with pure oxygen at 400", and then outgassed to Torr before being cooled to isotherm temperature and constant weight. If required, pretreatment with C5 was carried out as previously d e ~ c r i b e d . ~ The alcohols (99 mol % grade, supplied by Matheson Coleman and Bell) were exhaustively dried over molecular sieve, and outgassed and repeatedly distilled under vacuum before use. The arrangement used to maintain the sample within 0.02" of the quoted isotherm temperature is shown in Figure 1. Ambient room temperature was either 23 or 28 f 0.5". Saturation vapor pressures, pol (listed in Table I) were always determined with excess bulk alcohol present in the sample chamber since in most cases literature values were not considered sufficiently reliable. The presence of foreign vapors (typically air from solution) could be detected by partially evacuating the system and remeasuring po. Contact angles were measured using the sessile drop apparatus described previously.
+
Results and Discussion Up to p/po = 0.8, all adsorption isotherms (Figures 2-5) resemble those obtained with straight-chain alco-
* Address correspondence to this author at Kodak, Ltd., Wealdstone, Harrow, Middlesex, England, HA1 4TY. (1) J. Barto, J. L. Durham, V. F. Baston, and W. H. Wade, J . Colloid Interface Sci., 22,491 (1966). (2) T.D. Blake and W. H . Wade, J . Phys. Chem., 75, 1887 (1971). (3) T . D. Blake, J. L. Cayias, W. H. Wade and J. A. Zerdecki, J . Colloid Interface Sci., 37, XXX (1971). (4) E. F. Hare and W. A. Zisman, J . Phys. Chem., 59, 335 (1955). W. H . Fox, E. F. Hare, and W. A. Zisman, ibid., 59, 1097 (1955). (5) 0 . Levine and W. A . Zisman, ibid., 61, 1068 (1957). (6) W. A. Zisman, Advan. Chem. Ser., No. 43, 1 (1964), and papers cited therein. The J0UTna.l of Physical Chemistry, Vol. 76, No. 6, 1072
676
T. D. BLAKEAND W. H. WADE TO BALANCE
t
R
SUSPENSION FIBER
OJ 02
06
04
P/
08
I
p.
Figure 2. Adsorption of Cyclo-CE (0)and iso-ca (A)on aluminum foil at 20".
Figure 1. Details of the arrangement used to control sample temperature. For the 0" isotherm, the water bath was replaced by a larger vessel containing distilled water-ice slush.
Table I : Saturation Vapor Pressures (Torr) at T, "C oc
-------T, Alcohol
cyclo-Ce iso-C8 tert-Cc (liquid) tert-C:r (solid) tert-Cs tert-CE
0.00
7.77" 5.74
_-______-
13.30
20.00
19.4" 17.22
0.485 31.64 30.6" 28.28 11.74 4.61
26.60
46.6
...
a Indicates supercooled liquid (from data of Parkes and Barton, J.Amer. Chem. Soc., 50,24 (1928)).
hols C3 t o C5r1*2 being highly Langmuirian in character, possessing well-developed: "knees" and long linear regions. Adsorption on the bare surface was reversible above 8 = 0.65. However, the 0.35 8 remnant could not be removed even after prolonged evacuation a t Torr and was, presumably, dissociatively adsorbed. 1 ~ 2 The Langmuir molecular areas of all the alcohols so far investigated are listed in Table I1 together with measured contact angles and selected physical properties obtained from the literature. Assuming alignment roughly normal to the surface, the molecular areas follow a logical progression, with molecules of potentially similar cross-section having similar areas. Thus, The Journal of Physical Chemistry, Vol. 76, N o . 6 , 1972
OJ
02
04
06
08
P I Po
Figure 3. Adsorption of tert-Cs ).( aluminum foil a t 20'.
and tert-cs (A) on
w2,
iso-Ca and cyClO-c6 differ by only 1.1 whereas the introduction of two -CH2- groups in goin8 from tert-C4 to tert-Ca involves an area increase of 4.7 A2. Above p / p o = 0.8, all the isotherms rise steeply. CycbC6 adsorption exceeds five statistical monolayers at p/po = 0.97 and appears to approach the saturation vapor pressure axis asymptotically. This suggests a zero contact angle for the bulk alcohol, which is a solid at the experimental temperature (20'). iso-Ca also shows multilayer adsorpt,ion, and in this case, the contact angle was found to be less than 5 deg, but possibly nonzero. The fact that C32 is more autophobic than iso-Ca can be explained if it is assumed that the critical surface tensions of their monolayers are, respectively, lower and higher than their bulk surface tensions.
677
ADSORPTION OF BRANCHED-CHAIN ALCOHOLS
, Table 11: Molecular Areas, Contact Angles (e), Surface , Boiling and Melting Points (bp and mp) Tensions ( y ~ v ) and of the Alcohols so far Investigated Molecular -4, deg at 20°-&rea Advano- Receding
Alcohol
15.9 19.9 24.6 Ca 24.4 C4 24.8 cs 0yclO-c~ 26.7
C1
CZ
tert-Cc
25.4 32.4
tertCr tert-Cs
33.3 37.1
iso-c3
0 0 13 16 31
ing
0 0
(5
...
