April, 1955
AUTOPHOBIC LIQUIDSAND PROPERTIES OF THEIR ADSORBBD FILMS
several a ~ t h o r smake ~ ~ ,a ~value ~ of 1.14 g./ml. quite probable for the density of micellized NaLS. If the micelle is assumed to be only NaLS, the corresponding molecular weights vary from 31,500 in water to 38,500 in 0.1 M sodium chloride as shown in Table I and by the open diamonds in Fig. 4. The corresponding molecular weights determined on other portions of the same sample by light scattering4 are shown in the table and by the solid line in Fig. 4. They are lower by some 25%. This corresponds to f/f0 values of 1.13to 1.07 and indicates either hydration or dissymmetry of the micelle or both. An axial ratio of about 3.5 to 2.5 would give proper molecular weights. We prefer, however, to assume that hydration accounts for the whole effect because it is to be expected that the hydrophilic heads and adsorbed gegenions will be hydrated and that the micelle will be rough due to the presence of the polar headss and thus entrain some water within the roughness. Furthermore, the radii of micelles here considered do not exceed the length (23 b.)of the extended LS- chain so that dissymmetry is not required on structural grounds. If a hydration layer averaging one molecule of HzO, i.e., 1.5 b.,is assumed to surround the NaLS, the unhydrated molecular weight is lowered to 23,000 (24) K. Hess, W. Phillipoff and H. Kiesig, Koll. Zeil., 8 8 , 40 (1939); N. V. Hakala, Thesis, Wisconsin, 1943.
4
x
104
-
0 -
0
3
2 1 x 104 -
335
1
0.01 0.1 N a + concn. ( c cmc), mole/l. Fig. 4.-Molecular weights of micelles of lauryl sulfate in water and salt solution: solid line, light scattering values4; O,, from diffusion assuming no hydration; from diffusion with monomolecular hydration.
+
+,
to 32,300 as shown in Table I1 and by the black diamonds of Fig. 4. The agreement with light scattering (which determines, of course, the unhydrated weight) is probably as good as could be expected. Acknowledgment.-We are indebted to the Fulbright Foundation for travel grants which permitted D. S. and R . J. W. to come to Los Angeles, and to the Office of Naval Research for supporting this work through Project ONR-356-254 for which this forms the Fourth Technical Report.
AUTOPHOBIC LIQUIDS AND THE PROPERTIES OF THEIR ADSORBED FILMS BY E. F. HAREAND W. A, ZISMAN Naval Research Laboratory, Washington,D. C . Received September 9, 19.54
A class of polar liquids has been found having the remarkable property of being unable to spread on their own adsorbed films. Many branched as well as unbranched polar compounds exhibit these properties on high energy surfaces as diverse as platinum, or-alumina and fused silica. A rectilinear relation exists between the cosine of the contact angle of each liquid and the surface tension. It is demonstrated that the “au%ophobic”property of such liquids is caused by the adsorption of an oriented monolayer of the liquid. When the resulting low energ surface has a critical surface tension below the surface tension of the liquid, non-spreading or “autophobicity” results. Audies of the effect of varying the baking temperature of CYalumina and silica on the contact angles of autophobic liquids revealed that considerable surface dehydration of or-alumina and vitreous silica surfaces takes place a t 200’. The effect of such dehydration was to decrease the contact angles in every instance, the effect being greatest on or-alumina. It is concluded that the acids and alcohols studied were able to adsorb on silica and a-alumina through hydrogen bonding to the adsorbed water layer.
Introduction In a recent report1 a secies of experiments were described which demonstrated that the low wettability of adsorbed monolayers is not restricted t o those comprising adsorbed, straight chain, amphipathic molecules but can be caused by adsorbed molecules of a number of other structural types. I n another studyZmany pure liquids including a wide variety of esters, ethers and hydrocarbons have been shown to exhibit finite and reproducible contact angles on high energy surfaces such as brass, steel, silica and a-alumina (sapphire). It was concluded from an examination of the spreading mechanisms and materials present that when an ester is nonspreading the behavior must be caused by the for(1) H. W. Fox,E. F. Hare and W. A. Zisman, J . Colloid Sci., 8,194 (1953). (2) H.W.Fox,E.F. Hare and W. A. Zisman, to be published.
mation of an adsorbed film of acid or alcohol which is released by hydrolysis in situ, ie., hydrolysis during adsorption at the liquid-solid interface. Furthermore, such non-spreading could be caused by adsorption of branched chain polar compounds like 2-ethylhexanoic acid and derivatives. This paper reports the results of an investigation of the wetting and spreading behavior on high energy surfaces of pure, lower-boiling acids, amines and alcohols (both branched and unbranched). I t is shown that many of these compounds have the property of being “autophobic” or unable to spread on their own adsorbed films. Also it is proved that acids and alcohols which would be released by hydrolysis “in situ” of many of the esters discussed in reference 2 are capable of forming monolayers which would be imperfectly wetted by the parent ester.
E. F. HAREAND W. A. ZISMAN
336
Materials and Procedures.-The solids used in this investigat,ion included platinum, 18-8 stainless steel, drill rod steel, fused silica and synthetic sapphire (a-Al20,). The polishing and cleaning techniques used to prepare the surfaces are described elsewhere.2 In the footnotes to the tables are listed the sources of the polar liquids used. Where it was found necessary to redistil the liquid this has been indicated, and the boiling point and refractive index of the "cut" used are given. The stearic acid was an exceptionally pure compound (m p. 69.5') obtained from the Hormel Foundation; there was no unsaturation as shown by a zero iodine number. The source and purity of the various liquids used to characterize the wettability of the ndsorhed monolayers have been described in previous papers in this series.3-6 Each liquid was percolated through appropriate adsorbents immediately before use. The contact angles measured were those of the liquid slowly advancing over the surface. All measurements were made in a constant temperature room a t 20' and 50y0 relative humidity with great care t,aken to exclude organic vapors. The surface tensions of the alcohols at 20' were measured by the Sugden maximum bubble pressure method with the usual corrections.6 The surface tensions of the acids cited from tthe literature were chosen from those values determined by methods independent of the contact angle. The importance of this last restriction is conclusively demonstrated by the results of the present
VOl. 59
study. The surface tensions used in this report are therefore considered the most reliable values available. A check of the surface tension of n-octanoic acid by maximum bubble pressure agreed with the literature value used in Table 11.
Experimental Results Early in this investigation it was noted that the contact angle between polar liquids and the surface of a-alumina varied erratically from one surface cleaning to the next. Since the variation was as much as twenty degrees and appeared random, chemical deterioration of the surfaces due to the action of the nitric-sulfuric acid cleaning solution was considered unlikely. Because such hydrophilic surfaces as a-alumina and silica were presumed t o be readily hydrated, and because the acid cleaning operation was followed by rinsing the surface in distilled water and drying it a t 120" for varying lengths of time up to 16 hours, it was suspected that uncontrolled dehydration of these surfaces was causing the difficulty. To test this theory the a-alumina and silica surfaces, after being cleaned as described previously, TABLE I were heated at 200" for 2.5 hours in a grease-free AUTOPHOBICCONTACT ANGLESOF POLAR LIQUIDSON HIGH- desiccator containing anhydrous CaC12. They ENERGY SURFACES AT 20' then were cooled in the desiccator t o 20", and the 18.8 ._ contact angles of drops of pure octanol-1 or nYLVO stainDrill octanoic acid were measured within a minute after Platiless rod Liquid Source num steel steel Si01 A k , exposure t o the room air (R.H. = 50%). Octanol-1 Alcohols and n-octanoic acid exhibited contact angles on the Octanol-1 a 27.8 42O 35' 35O 42' 43" a-alumina of only 6 and 5", respectively. These Octanol-2 b 26.7 29O 14O .. 30° 2G0 liquids had been found earlier to have contact 2-Ethylhexanol-1 E 26.7 20°
