METHODS OF MEASUIUNG THE P O ~ Y OF SURFACES. 2793 CCCLXXXV.-A Comparison of Methods of Measuring the Polarity* of Surfaces. By NEIL K. ADAM ROBERT S. MORBEILL and RONALD G. W. NORRISH. “ POLBRITY ” has been recently measured by Norrish (J. 1923, 123 3006) the measure being the catalytic activity of the surface for the combination of ethylene and bromine. Another mewure of polarity is the attraction of a surface for water or the work W , required to separate the surface from water in contact with it; W is related to the angle of contact 8 of water with the solid surface by the relation T being the surface energy of water (Adam and Jessop this vol., p. 1863). Yet another indication of polarity is the objectionable property which some oil varnishes possess of condensing water on the varnished surface in a moist atmosphere and ‘blooming.’ Varnished surfaces which bloom easily may be considered more polar than non-blooming surfaces.This paper is an attempt to elucidate the nature of the “ polarity ” by comparing the results of the different methods of measurement. Glass stearic acid para& wax two non-blooming varnishes, A and B and one blooming varnish C were compared. The non-blooming varnish film was composed of a hard copal resin incorporated in linoxyn containing lead and manganese catalysfs. The blooming varnish film contained a rosin ester in tung oil “ oxyn,” with a manganese catalyst. Both for angle of contact and for catalysis measurements the varnish was coated on glass and allowed to dry in a current of air (5 litres per hour) at 40-55” until constant in weight.The volatile thinners of the varnishes were expelled and the drying oils oxidised as completely as possible. In the tables k is the initial velocity coefficient k’ gives the catalytic effect in terms of k expressed as percentage of the value for glass 8 and W (ergs per sq. em.) are defined above and W’ is the attraction for water as percentage of the attraction of water that “polar” is applied to substances (and to groups therein) which tend to dissolve in water and “ non-polar ” to those which dissolve in hydrocarbon solvents. This broad distinction of groups into two classes is a moat useful working hypothesis for many investigations and a more definite under-standing of its nature and causes is much needed.The term “ polarity ” is here used in this sense leaving entirely open the question whether there exists any resemblance to physical objects of unsymmetrical field of force such as W = T(l + cos 6), * The terms “ polar ” and “ non-polar ” have become general in the bar-qets.-N. K. A 2794 ADAM MORRELL AND NORRISH A COMPARISON OF for itself. The angle of contact method will not detect attractions for water greater than that of water for itself attractions equal to and greater than this giving zero angle. Glass has an attraction for water at least as great as that of one water surface for another. Catalytic action measured on dry ethylene and bromine. Surface. k. k' . e. W. W'. Glass .................. 0.05 1 100 0" not less than 100 Varnish .A ............0-048 94 95 66.6 45.5 Varnish C ............ 0-112 220 60 109.5 75 PtwaEin wax ......... 0.003 6 105 54 37 Stearic acid ......... 0-086 168 100 60-5 41 146 Caiulytic action measured on moist ethylene and chlorine. Glass .................. 9 . 7 ~ lo-' 100 0 146 100 Varnish B ............ 4.0 ) 41 95 66-6 45.5 Varnish C ............ 11 ,) 113 65 104 71 P a r a n wax ......... 0.01 )) 0.1 105 54 37 The angle of contact measurements were made as described by Adam and Jessop and are accurate to about 5". The velocity coefficients for the ethylene-bromine reaction were determined as described by Norrish fairly good constancy being obtained for the first five minutes of reaction. Ethylene and chlorine in the moist condition were employed for the second series as it has been found - possible thus to obtain more reproducible results than with dry ethylene and bromine; but owing to the higher pressure used there was appreciable attack of the varnish surfaces by the chlorine, which impaired the constancy of the velocity ' constants.' Details of the ethylene-chlorine method w-iil be published shortly.The values calculated as bimolecular coefficients are given for the first two minutes of reaction and afford a comparison of catalytic activity adequate €or the present purpose. The figures for reaction rates are of course not comparable between the two series. The angle of contact measurements were taken on the varnishes within a minute or two of immersion in the water. If allowed to soak these surfaces gave an angle lower by 10" or in some cases 20° the effect of soaking being more marked the greater the angle of contact.Evidently the attractions for water parallel the veiling properties of the varnishes. It appears that if the attraction is more than about 70% of that of water for itself the varnish veils; if less than 46 to 50% it does not veil. We attempted also to grade varnishes in respect of their veiling properties by angle of contact measure-ments but owing to the effect of soaking in water these could not be made sufficiently accurately to distinguish varnishes with METHODS OF MEASURINQ THE POLARITY OF SURFACES. 2795 slight tendency to veil from non-veiling varnishes. The varnishes which were found to be more polar by the angle of contact method were also more polar by the catalytic meamre.There is however considerable disagreement between the polarities of the surfaces of different kinds as measured by the catalytic method and by angle of contact. Glass has less catalytic power than either stearic acid or the veihg varnishes but very much greater attraction for water; and stearic acid which haa only a very slightly greater attraction for water than paraffin wax, has enormously greater catalytic activity. It would of c o r n , be possible to ascribe the difference to the “ polarity ” which c a m attraction for water being of a different kind from that which confers catalytic activity; but such an ad hoc assumption would be particularly unreasonable as it is not improbable that water itself is concerned in the catalysis.Adam and Jessop concluded that the evidence of angle of contact measurements on long-chain compounds pointed to the polar groups in stearic acid being buried in the interior. Hence it appears that in the catalytic measure-ments the reacting gases penetrated the surfaces to a short distance. The nature of the oxidised varnish surfaces is unknown we con-sider it possible that ethylenic linkings are present. In this way it would be possible for the stearic acid and varnish surfaces to present more catalytically active groups than a glass surface, which must be presumed practically impermeable to the gases. By penetrating the solid to a depth of only the length of a few molecules it would be possible for the gases to reach more polar groups than &re to be found on an equal area of glass.If this is the correct explanation it involves the assumption that the depth of the ‘ surface ’ is much greater for the catalysis than for the angle of contact measurements. Varnishes differ in their absorption of, and permeability to water and no doubt to gases also ; but we have not data enough to attempt a prediction of catalytic activity for Merent varnished surfaces taking this factor into account. This factor of permeability requires further investigation; in Adam and Jessop’s paper some evidence was given that there is more depth involved in the case of crystal flakes of the long-chain amine hydrochlorides than with the other long-chain aliphatic substances used. Nevertheless we feel that qualitatively it supplies an adequate reason for the difference in behaviour of Merent ‘ surfaces ’ to reagents. Para& wax is not a catalyst because no matter how far the g- may diffuse into the interior no polar groups are encountered. [Received October 9th 1925.