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.
Soaps that form association colloids are effective drag-reducing agents in nonaqueous fluids, while simultaneously building the desired viscosity characteristics of the fluid. Such agents, while effective in a drag-reduction sense, are sensitive to trace amounts of water and catalyze the oxidative degradation of oils at elevated temperatures.Association colloids of soap type were the first agents observed to decrease the frictional drag of fluids in turbulent flow. During World War 11, Mysels and coworkers (Agoston et al., 1954) discovered that under turbulent flow conditions decreased pressure drops were found for gasoline slightly thickened with Napalm when compared with ordinary gasoline at the same flow rate. Napalm, the drag-reducing ingredient, is primarily an aluminum disoap corresponding to the approximate formula Al(OH)R?, where R refers to a mixture of commercial coconut, oleic, and naphthenic acids. While the mechanism of drag reduction is not yet fully understood, no chemical drag-reducing agents are yet known which do not possess good dispersibility in the subject solvent, high molecular weight, and extended molecular structure. Soluble, high molecular weight polymers having essentially linear molecular structures are good examples of drag-reducing agents in both aqueous and nonaqueous media. Oil-dispersible polar molecules of the soap type are considered also to conform to these requirements through molecular association of the soap "monomer" units to form extended linear structures of high molecular weight (Figure 1).While high molecular weight polymers of linear structure are much more efficient drag-reducing agents than the soap-type association colloids, association colloids possess the unique virtue of being able to reheal their dragreducing linear structures after passing through regions of intense shear. This ability to regenerate the dragreducing species makes association colloids emminently suitable for use in closed systems involving high shear pumping devices.In 1966 it was reported that the use of drag-reducing additives in hydraulic fluids might improve the efficiency of power transmission in hydraulic systems (Pruitt and Crawford. 1965). Soap-type additives might be used for such purposes. Prior research, however, showed that the physical properties of carboxylate soaps in organic liquids are strongly affected by polar impurities such as water To whom correspondence should be sent. and acids. Specifically, Singleterry (1954. 1956) in their studies of phenyl stearate soaps in benzene, found that the soaps of different metals were different in their responses to the addition of polar materials. The viscosities of lithium soap solutions remained high in the presence of small quantities of water. while the sodium and potassium soap solutions rapidly decreased in viscosity. Thus, the chains or aggregations of soap molecules, presumed to be the active species in reducing fluid friction in turbulent flow, would be more stable in the presence of water if they were lithium rather than sodium...
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