Nov., 1952
POTER'TIALB OF
ADSORPTI~R'-D.ES~RPTIOR' CAPACITY
proposed graphite oxide structhre which, because of the relative disorder of the oxide lamellae, would be difficult to confirm by X-ray measurements. Although sheets of graphite oxide appear folded in electron micrographs, it is probable that, when suspended in a suitable liquid, they are extended
PRAXS
935
and flat. The possibility that the folding just referred to was due entirely to the method of mounting specimens and not to structural weakness in the oxide lamellae is discounted because the same method of mounting did not produce similar effects in graphite lamellae.
APPLICAATZONOF THE CATHODE-RAY OSCILLOSCOPE T O POLAROGR4PHIC PHENOMENA. 111. POTENTIALS O F ADSORPTIONDESORPTION CAPACITY PEAKS AND SURFACE CHARGE DENSITY RELATIONSHIPS EXHIBITED BY ALCOHOLS AT AQUEOUS SALINE SOLUTION-MERCUR'I' INTERFACES BY J. WESTLOVE LAND'^ A N D PHILIPJ. ELVIR'G'~ The Pennsylvania Slate College, State College, Pennsylvania Received August 28, 1961
Differential capacity oscillograms for the saturated solutions of n-octyl and n-heptyl alcohols show four capacity peaks on each of t8hecat,hodic and anodic sweeps ratjher than the espected two peaks, indicating double film format,ion. The patterns for a sat*uratedn-hesyl alcohol solution are characterized by two and sonietimea three capacity peaks on the charging and discharging curves. A saturated solution of n-amyl alcohol gives only two adsorptioii-desorl)tion capacity peaks in each trace, indicating the formation of a mono film layer. The lower the molecular weight of the alcohol, the greater is the potential span between capacity peaks for the saturated solutions. For undegassed supersaturated solutions of the four normal alcohols, only two peaks per branch were observed. This picture reverts to that of the saturated solution upon degassing. The potentials of the reversible capacit'y peaks are given for both saturated and supersaturated solutions. In general, the desorption processes were found to proceed a t the same or a t a faster rate than the adsorpt,ion process aa indicated by the sharpness of the corresponding capacity peak heights. Calculations of surface charge density according t80the capacity peak potentials for the saturated solutions indicate that adsorption processes depend directly on the surface charge existing at the mercury surface.
In previous papers2 experimental arrangements were described which were capable of producing differential capacity and surface charge density patterns on the face of an oscilloscope. The application of these techniques to the observation of film formation a t the dropping mercury electrode, D.M.E., was indicated. The present paper is concerned with the results obtained from a systematic study based on oscilloscopic observation of the adsorption-desorption phenomena exhibited by various alcohols a t the D.M.E., ie., a t the interface between the mercury and aqueous salt solutions. Heretofore, the study of the effect of non-electrolytes on the surface properties of a mercury electrode have been carried out for the most part by the determination of the surface tension which exists between a mercury drop surface and an electrolyte solution to which has been added some organic material. The usual procedure is to determine the surface tension over a selected applied voltage range and to plot the values of surface tension as a function of the applied voltage to give what is commonly called a n electrocapillary curve. Gouy3f4published the most extensive surface tension data available for electrolytic solutions, both with and without the addition of non-electrolytes, The shape of the electrocapillary curve for a pure
electrolyte is parabolic in form with a maximum coming a t the potential of zero charge on the surface of the mercury. In the presence of slightly soluble organic compounds the maximum is either flattened or shifted to a potential different from that of the original electrocapillary maximum. F r ~ r n l t i n likewise, ,~ found that the presence of adsorbable organic substances in solution modified the shape of the electrocapillary curve from that of the pure solution. Frumltin, et aL16determined the influence of capronic (caproic?) acid and phenol on the surface tension of a mercury-sodium sulfate solution boundary by a capillary-electrometer method and found that in the neighborhood of the saturated solutions the two organic compounds are adsorbed as multilayers. Alternating current measurements by Prosburnin and Frumkin7 of the capacity a t a mercury surface in contact with a solution of sodium sulfate and octyl alcohol, showed abrupt increases in the capacity of the electrical double layer a t potentials a t which the adsorption and desorption processes of the alcohol took place, and a decreased capacity a t intermediate potentials. Grahames studied the capacity and resistance a t a mercury electrode in contact with solutions of potassium nitrate, sodium chloride and hydrochloric acid, all saturated with octyl alcohol, as a function
(1) (a) S u n Oil Co.; Norwood, Pennsylvania. (b) University of Michigan, Ann Arbor, Michigan. (2) (a) J. W, Loveland a n d P. J. Elving, THISJOURNAL, 56, 250 (1952); (b) J. W. Loveland a n d P. J. Elving, ibid., 56, 255 (I9ij2). (3) G. Gouy, Ann. c h i m . phya., [SI 8 , 291 (1906). (4) G. Gouy, ibid., [SI 9,75 (1906).
( 5 ) A. Frurnkin, 2. P h y s i k , 35, 792 (1926). (6) A. Frurnkin, A. Gorodetskaya a n d P. Chugunov, Acta Physi-
cochim. U.R.S.S., 1 , 12 (1934); C. A . , 29, 2040 (1935). (7) A. Proskurnin a n d A. Frunikin, Trans. F a r a d a y Soc., 31, 110
(1935). (8) D.C. Graharne, J . Am. Chem. Soc., 6 8 , 301 (104F).
of frequency and potential. Capacity peaks were observed at potentials corresponding to the adsorption and desorption processes of t.he alcohol at. t,he mercury surface. Similar efkcts were produced on the differential capacity between mercury and an aqueous sodium sulfate solution by the addition of n-heptyl alcohoLg Oscilloscopic observations of charging current curves were used hy I3arclay and Ihttler'nfor sturlying the adsorption effect,s of t-amyl alcoliol a t a mercnry electrode. The curves showed iliscontinuities over the potential range i n which the alcohol was adsorbed. Brdirka" has discussed the arlsorp1,ion 1,heory in view of anomalous polarogrums obtained with certain reversible orgtlnic oxidutiori-redui:t,ioti systems. The polarographic wave is shiftcd to more negative potentials if the oxidized form is adsorbed, and to more positive potentials if t h e reduced form is adsorbed. Current-time oscillograms were used to study adsorption cffeck For diffusion controlled processes, the currenttime patterns showed a '/G order parabola, while f i r adsorption processes definite maxima distorted t,he parahola. have determined the influence of
slightly soluble organic compounds on the polarographic diffusion currents of electroactive metals and t,lieir iints. Invariahly, t.he measured diffusion current is decreased at potentials over which adsorption takes place. When the applied potential is sufficiently negative that desorption occurs, the diffusion current becomes normal. As the concentration of the surface-active species is decreased, the diffitsion currents increase toward their limiting value it1 "pure" solut,ion, ie., in the ahsenm of snrface-active material. Thus far, the majorily of data concerning elect,rocapillary-act,ive phenomena at a polarized merriiry surface has beeti ol)tained as a result of the measnremetit, of either surface t.ension or rapacity at definite applied pot,entials. The dat,a available is too meager to make any correlations regarding the general behavior of a given series of adsorl)ahle compounds. To this end, the present authors have studied a series of alcohols under various condit.ions a t the D.M.E. by the oscillographic techniques described previously.1,2 There are several advantages in wing the oscilloscopic approach to the study of film formations. The method is rapid; a complete capacity or surface charge density spectrum may be obtained in a fraction of a second. The entire history for a single drop of mercury may he observed. Oscillograms for a given set of conditions are reprodueihle. Both charging and discharging curves are produced, the comparison of which serves to indicate t,he reversildity of the adsorption-desorption processes. The influence of film formation on the oxidation-reduction patterns of electroact,ive species can be studied. The method has the disadvanhge of not being able to diflerentiate hetween polarization resistance and time-lag. In the present work, resistance has been calculated on the assumption that time-lag, as shown by Grahame,gis negligible. If this assumption is invalid, the resistance valnes will he in error. Nevertheless, whet,lrer resistance or time-lag or the sum of the two is measured, the calculated capacit,y valnes would he the same. Method of Measurement
Photographic Technique.--Photographs or t h r diKercntial oaphcity and surirtee charge density (S.C.D.) oncillograms for the D.M.fi;. in contact wi1,h an electrolyk soluE ll.7. I I U,,~ . nod. v. , Fig. l.-DiKerential capacity relation for 1 N KRr solu- tion containing alcohol were oht,ained in a manner andogous tion saturated with n-(:al~,:,V1l;E, = -0.202 Y., E . = to that outlined previously.'.* For all alcohol nolutions in-1.017 v.; cxpo~uretinit:, 14.2 seconds; mass per second, vestigated, t.lie eapaeit,y p a k s oi the adaorpbion and denorp0.2Q1 mg.; senaitivity rraistsrice for curve, 12 ahrnii; for tion proeesws on the charging cy& (eathodie-top eurve) occurred a t B more negat,ive potent,ial than thost! lor t,he ralilmtion, 1.50, 300 ohms. corresponding I C ~ P I R I : pmrrsn on 1 . l ~discharging eyelc (anodic-bottom curve) (l
