The effects of straight-chain carboxylic acids on cathode overpotential during electrodeposition of copper were studied by galvanostatic methods at current densities up to 0.020 A-cm -2. No modification of the chargetransfer mechanism of copper deposition occurred. Overpotential increments caused by lengthening of the carbon chain can be correlated with the reduced concentrations of the carboxylic acids in accordance with Traube's rule. This can be rationalized in terms of the general relation between adsorbability and solubility of the additive. The general behavior is consistent with the blocking theory of additive function according to which overpotential increments are directly dependent on additive adsorbability. The fractional surface coverage calculated from overpotential increments appears to fit a Langmuirtype isotherm from which the free energies of adsorption of the carboxylic acids can be determined. The magnitudes of these free energies are consistent with physical adsorption forces. A relation was derived between the free energy of adsorption and the free energy contributions from the carboxyl group and the carbon chain. This shows that the longer the chain, the higher the adsorbability.Overpotential and adsorbed additives.--The ratedetermining step in electrodeposition varies with the overpotential region (1). The charge-transfer process is believed to be rate-determining at high overpotentials (2) and the accepted mechanism in the case of copper deposition is the two step charge transfer Cu 2+ + e~ Cu + slow Cu + -b e "-> Cu fastThe charge-transfer overpotential ~1 is related to the current density i by the Tafel equation (3) ~1 = a q-blni

The cathode overpotential and electrosorption effects of some normal monocarboxylic acids during electrodeposition of copper were studied galvanostatically at current densities up to 20 mA-cm -2. Equations were derived by means of which overpotential increments caused by the blocking action of adsorbed additive can be used to calculate the separate adsorption free-energy contributions of the carboxyl and methylene groups in the additive molecule, and also a coverage-dependent lateral interaction free energy arising primarily from dipole-dipole interaction between adjacent carboxyl groups in the adsorbed phase. The carboxyl group contribution was found to be --1570 cal mole -1 at zero coverage, while the methylene group contribution was --704 cal mole-' and independent of coverage.

The steady state polarization of 100 mv. in acid copper sulphate electrolyte, at 2 amp./dm.2, appears to consist of 45 to 50 mv. activation overpotential to deposit aquo–copper complexes, 20 to 25 mv. concentration polarization, and about 30 mv. polarization due to hydrogen ion interference. The presence of cystine in the electrolyte gave rise to polarization–time curves similar to those observed previously with gelatine. The increase of polarization caused by cystine appears to be due to an obstructive effect of adsorbed cystine (or its copper complex), together with an increase of concentration polarization. Cystine alone probably does not affect the activation overpotential. Addition of sufficient chloride virtually eliminated the polarization due to obstruction by cystine, possibly by acting as an electron bridge or by forming more readily dischargeable chloro–cystine–copper complexes. Chloride also eliminated the increment in concentration polarization caused by cystine. Attainment of a minimum total steady state polarization of about 40 mv. in the presence of cystine and chloride appeared to reflect an increase of surface, hence a decrease of true current density with time of deposition. The addition agent behavior of methionine was, in most respects, similar to that of cystine. The behavior of thiourea at low concentrations appeared to be complicated, but the effects of chloride were similar to those observed with gelatine.

Mercaptoacetic acid (MAA) in acid copper sulphate electrolyte decreased the cathode polarization throughout the course of electrolysis a t all concentrations LIP t o 20 mgm./liter. Addition of small amounts of chloride caused further depolarization. When present together with cystine, WIAA showed an independence of action. The depolarization caused by WIAA was ascribed to a decreased activation overpotential due t o substitution of a more readily dischargeable MAA-copper complex for the aquo-copper complex. The additional depolarization due to chloride was ascribed to formation of a still more readily dischargeable chloro-MAA-copper compIex. T h e depolarizatioil in both cases increased with time during prolonged electrolysis, apparently due to surface roughening with, presumably, a consequent increase in surface area and decrease in true current density.

Additives containing carboxyl and amino groups increase the overpotential of a copper cathode during electrolysis in an acid sulfate electrolyte, the amino group being more effective than the carboxyl. At a given additive concentration the overpotential increment apparently depends on the adsorptivity and size of the additive molecule. The adsorptivity in turn depends on the number and kind of polar functional groups, the structural relation between electron‐donating atoms, and the lattice dimensions of the copper deposit. It is suggested that the behavior of a chemisorbed additive can be understood in terms of two effects, (i) the complete blockage of growth sites by chemisorption bonds and (ii) the partial blockage of adjacent sites by the physical bulk of the adsorbed molecule.