The Adsorption of Hydrophobic Monolayers of Carboxylic Acids

Abstract

The adsorption technique reported reviously has been successfully extended to the study of monolayers physically adsorbed onto platinum from aqueous soyutions of a variety of types of mono-and dicarboxylic acids. The effects have been investigated of varying the molal concentration of or anic acid over a 10,000-fold range. The condition for maximum adsorption, as evidenced by peak hydrophobic behavior$B,), has been found to lie between pH 4 and 6. Both the shape of the 8 VB. pH curve and the positions of the points of inflection were in ood agreement with the results predicted by the theoretical treatment of Merker and Zisman'; it was therefore concluded t f a t the pH effects observed were due primarily to the comy t i t i v e adsorption between the molecular and ionic species in solution rather than to micelle formation or association. rogressive increases in solute concentration for a given acid have been found to result in the asymptotic approach of e, to --_. a characteristic, limiting value, Bum.A new and convenient "thermal-gradient method" using-the pure molten compound has been found by which adsorbed, close-Dacked monolayers can be prepared and isolated on metal in tke absence of solvent. It is more.eenera1 than the oleophobic methods desciibed in pre6ois reports, since it has been found applicable to all amphipathic comcounds tried, whether straight-chain, branched or cyclic. The equivalence of the contact angle (Bwlt) obtained from films made from the molten polar compounds with that (61, ) of the films adsorbed from aqueous and non-aqueous solutions has been demonstrated.The way in which the contact angle characteristic of each polar compound (Oli, or Omel%) varies with the nature of. the acid and with homology has been studied and a correlation of the results with molecular orientation, structure and packing has been attempted. 76Cyclohexylpropionic 9 1 5 . 0.. 65Tetradecanedioic 14 124.5-125.5. . 76Aromatic monoacids u,w-Alkane dicarboxylic acids n-Alkyl succinic acids Octylsuccinio 12 88-89 7 1 69 73 Octadecylsuccinic 22 104-106 77 72 79 Decylsuccinic 14 92.5-95 72 71 76 a Contact angle measurements made at 20.0 i 0 . 1 ' .Measurements of the hydrophobic contact angle (0) on films isolated from aqueous solutions were made, as before, with drops of the generating solutions at the same concentration and pH at which the individual films were formed. Since conditions of solution equilibrium obtained, dissolution of the film into the superjacent drop was avoided. Changes in p H were effected by the dropwise addition of N = Total number of carbon atoms per molecule.

Flotation and adsorption investigations of the systemn-dodecanoic acid/mercury at different pH values

“…pH range corresponding to the observed maximum flotation yield of mercury agrees well with the maximum surface activity of higher fatty acids found from contact angle measurements of their films on platinum at different pH values [27,28]. The high hydrophobity of films is according to the authors primarily connected with the adsorption of undissociated acid molecules.…”

“…The formation of such associates has been confirmed by the investigations of the mercury/solution [40,42] and solution/air interfaces [19,20,23,27,28,43,44] as well as by the results of the equilibria in solutions [21,24,25].…”

Acetylenic alcohols as inhibitors of iron acid corrosion. Improvement of the inhibition efficiency of a class of substances based on their action mechanism

“…Such an improvement may depend on different causes: stronger Van der Waals interactions between the chains, increase in the thickness of the polymolecular layer [7] or increased hydrophobic characteristics of the film [42,43]. Yang [20] emphasised that the longer the alkynol saturated hydrocarbon chain, the faster the polymer film growth rate, and the higher the maximum surface coverage.…”

Surface activity and mercury flotation in solutions of n-dodecanoic acid and n-dodecylamine

“…Surface tension measurements of LA and DAC solutions [12][13][14] and contact angle observations for LA [15,16] suggest that both substances adsorb strongly at this pH at other interfaces as well.…”