ELECTRICAL THEORY OF ADSORPTION

W. HARRISON, M.Sc., F.I.C. The paper by Mukherjee dealing with the development, of

Langmuir’s theory of adsorption is of considerable interest and im- portance and leads to some new ideas.

Recent ~7ork has shown that all atoms consist of positively charged nuclei associated with or surrounded by negatively charged electrons Crystals have besn shown to consist not of molecules bound together by some iinknown chemical force, but of atoms held together b y electrical forces. While there are examples such as that of alumina crystals. in which atoniic groupings which might be considered a s molecules are arranged in a regular manner, the distribution of electrons cannot bo the same in any such grouping isolated from the crystal as witbin the crystal itself. Within the crystal the electrons are arranged so as to bind the atoms together in three dimensions. At the surface of the crystal, the electrons must be arranged to hold the atoms together in two dimensions, hence they are able to rearrange themselves for the accommodation of other atoms not necessarily the same as those already present. The formation of crystals with alternate layers of ammonia, alum and chromium potash alum by successivc- growth in solutions of the two compounds illustrates the above point, J t is thus clear that a new surface mag be formed by contact of a crystal with a solution.

There seems no reason to limit this argument to crystals here the atonis are oriented and the electric forces distributed so as to give strong combination. It will apply, and perhaps more correctly, to non-crystalline substances where the irregular orientation of at oms allows greater freedom for the rearrangement of electric forces.

Physicists having proceeded so far with the electrical explanation of chemical phenomena it appears to the writer unnecessary to go back t o chemistry for the explanation of an electrical phenomenon.

It is not necessary, for instance, to assume that the electric charge at the surface of an adsorbent is due to chemical combination of the adsorbent with the ion. It is, of course, known that for most sus- pensoids the charge is of the same sign as that on the ion the substance has in common with the stabilising electrolyte, but stahilisation also occurs where there is no common ion, in which case the charge depends on the nature of the adsorbent and of the ions present. The colloidal metals form examples of this kind.

It is clear that the electrical equilibrium tetween two substances of different degrees of ionic or electronic saturation must necessarily be different from the equilibrium of electric charges a t either of the two surfaces when not in contact, hence when contact occurs there will be a redistribution of electrical forces resulting in a difference of electrical potential.

In a solution electrical charges are always associated with ions, hence the above argument in no way modifies the equations of Mukherjee .

Mukherjee is of the opinion that the collisions of ions due to osmotic forces are comparatively negligible, and that the ions of the same gi-p as the colloid have very little influence. It is, of course, obrious

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MR. W. HARRISON 117

that when the charge becomes neutralised the osmotic forces do come into play. The probability that a particular point on the contact surface becomes neutralised is the probability that an ion of opposite sign becomes fixed. This is also the probability that osmotic forces Come into play ah that particular point. The magnitude of this probability 1 - e--W/KT is by no means small as will be seen from Table 2 of Mukherjee's paper, moreover it increases with the valency of either or both of the ions. In general, the effect of polyvalent ions is greater than that of monovalent ions, hence it is to be expected that a polyvalent ion of the same sign will increase the electric charge.' This was found to be the case by the writer when making experiment8 with cotton.

The electric charge had the following values in N/1000 solutions of salts :--

NaCl Na,SO, Na,PO, NaOH 0183 * 0219 0240 * 0306

The anomalous effect of NaOH is undoubtedly due to the great rriobility of OH ions.

The equation 13A of Mukherjee applies very well to some experi- ments made by the writer in 1912 (Journ. SOC. Dyers and Cols., 2'4, 279, 1911 ; 34,91,1918).

Cotton--Hydrochloric acid solution Calculated empirical

Goncentra- Observed Calculated equation. - tion. E.M.F. 13A. E=*0081 log C-sO157. -- P

Volts. 1 x 10-4 * 0168 * 0168

2 . 5 x 10-4 -0138 -0136 5 x 10-4 -0112 -0110 1 x 10-3 * 0085 0085

2 . 5 x lo-' * 0055 * 0059 5 x 10-3 - 0028 - 0044 1 x lo-% 0016 0032

0167 0135 0110 0086 0053

* 0029 00006

Maximum 0220 With cotton and aluminium sulphate the results calculated from

concentration were quite out of relation with the observed results, but when calculated from conductivity by equation 13, substituting 2 for n2 and 3 for n,,, the results were much nearer to those observed.

Cotton--Aluminium-Sulphate Solution Cttlc . Empirical.

Concen- Conduc- Observed Calculated E= -0064 log K tration. tivity . E.M.F. 13. + *0219.

1 x 10-4 1-44 x 10-5 -0092 -0092 (92) *0091 2 x 10-4 2.37 x 10-5 -0078 *W72 (68) ' 0077

1 x 10 3 9 . 8 x 10-5 -0036 -0030 (24) - 0037 5 x l a - 4 5.4 x 10-5 *0055 -0045 (35) * 0066

2 x 10 3 16.2 x 10-5 ~0025 -0020 (15) 0024

-5 x 32.3 x 10-6 --.0020 and +.OOlS 1 x 56.5 x 10-5 --0075and +.0043.

values between

The values in brackets are calculated from 13 C.

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118 ELECTRICAL THEORY OF ADBORPTTON

The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.

The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.

This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.

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