Factors Influencing The Adsorption From Solutions

Bun-ichi Tamamushi

Nezu Chemieal Institute Musashi University, Nerimaku

TokyOj 176; Japan

1. INTRODUCTION

The adsorption from solutions generally takes place at any interface of liquid/gas, 1iquid/1iquid or liquid/solid. If we arbitrarily confine the phenomenon to the case of liquid/solid interface, we still deal with the system of at least three components, i.e. adsorptive (a single solute), adsorbent(so1 id) and so1vent(1iquid medium), where the interaction forces are not only adsorptive/ adsorbent, adsorptive/solvent, so 1vent/adsorbent, but are also adsorptive/adsorptive. The phenom­enon under consideration is therefore very compli­cated and difficult to analyse completely. For the study of the problem it is accordingly desirable to select certain conditions: namely, to select the combination of adsorptive, adsorbent and solvent of definite natures and to find out certain main factors which control experimental data. From such a point of view, some classical and some recent data of the adsorption from solutions at the liquid/solid interfaces will be reviewed in this paper.

2. REVIEW

Many classical works of the adsorption from solutions were carried out with the system: relatively simple inorganic or organic, ionic or non-ionic compounds as adsorptive, charcoal of various origins as adsorbent, and aqueous

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solutions as liquid medium. For example, in the study of the adsorption of some homologous organic compounds(carbonic acids, urethanes, etc.) from their aqueous solutions onto blood charcoal, a regularity in the order of adsorption amounts, as known TRAUBETs rule, was found, which indicates that the attraction forces between adsorptive molecules and adsorbent surface play an important role(l). It was however later found that this rule does not hold, and that the adsorption order is even reversed, when toluene instead of water as solvent and silica or alumina as adsorbent were applied(2). This indicates that the adsorption depends on the nature of solid surface and of the solvent medium. As well known, polar adsorbents such as alumina or silica, as well as organic solvents of different natures have been successfully applied in the chromatographic adsorption method(3).

In an early study of the present author on the adsorption of some isomeric bisubstituted benzenes (ortho-,meta-,para-nitroanilines,-nitrophenols) from benzene or methanol solutions onto charcoal or alumina, it was found that meta-compounds with greater dipole moments were more strongly adsorbed by polar adsorbent from non-polar benzene solutions than ortho-compounds with smaller dipole moments, while the former compounds (meta) were more weakly adsorbed by charcoal than the latter (ortho) from methanol so1utions (4) . Such experimental results suggested that the polarity of the adsorbent (solid surface) as well as the polarities of liquid medium and adsorptive molecules are important factors in controlling the adsorption from solutions. In another study of the present author on the adsorption of some ionic surfactants (long paraffin chain electrolytes); like dodecylammonium chloride, dodecylpyridinium bromide, sodium dodecyl sulphate, from their aqueous solutions onto various adsorb­ents; alumina, barium sulphate, calcium fluoride, carbon black, ion-exchange resins, it was found that the adsorption isotherms are generally S-shaped and that the second maxima of the adsorp­tion isotherms appear at equilibrium concentrations of the bulk solutions corresponding to the critical micelle concentrations of the respective surfactants(5). It was considered that the adsorption of these amphiphilic compounds would

FACTORS INFLUENCING ADSORPTION 81

take place in two steps; first, monomolecular adsorption of the LANGMUIR-type, and then, multi­layer adsorption of the BRUNAUER-EMMETT-TELLER-type. At the surface of polar adsorbent, like alumina, polar groups of surfactant molecules will be attracted to the active spots of the surface by the electrostatic attraction forces, forming oriented monomolecular layers and then bi-molecular layers by the VAN DER WAALS attraction forces between non-polar chains of surfactant molecules. It was also remarked that the adsorption of these surfactants on ionic resins took place in two steps; first due to the exchange reaction and then due to VAN DER WAALS adsorption. These experi­mental results suggested that there appears the role of the interaction forces among adsorptive molecules, in determining the whole process of the adsorption from solutions. Critical reviews on the problem of the adsorption of such surfactant mole­cules at solution/so1 id interfaces have been made in some details by the present author as well as by KIPLING in their monographs(6). K.EDA, former collaborator of the present author, in a series of papers published in Japanese(7), reported his studies on the adsorption and desorption processes of paraffin chain compounds at the so1ution/mercury interface by applying electric differential capac­ity method, which was particularly developed by GRAHAME(8). It was therefore suggested, from the characteristic curves of C(differential capacity)-E(applied potential),particulary, for paraffin chain anions like dodecyl sulphate ion, that they are adsorbed at the mercury/solution interface, forming bi-molecular layers by a similar mechanism as described above for the case of surfactant ions at the so1id/so1ution interface, provided that the solution concentrations are greater than the critical micelle concentrations of the surfactant i on s .

In a more recent work on the molecular alignment of some liquid crystals: N-methoxybenzylidene-p-buthylaniline(MBBA), a mixture of 4-ethoxy-4 f-buthylazoxybenzene with 4-ethoxy-4 1 -ethylbenzene (NP-5A), in the thin films of these compounds sandwiched between two optical glass or quartz plates, whose surfaces were previously covered by hexadecy1trimethylammonium bromide(HTAB) or

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lecithin, it was confirmed that their molecular alignment is homeotropic, being confirmed quali-tavely, by use of a polarization microscope, and confirmed semi-quantitative1y, by applying the resonance RAMAN-spectroscopy(9). It was therefore considered that the amphiphilic molecules of HTAB or lecithin will be adsorbed on the clean surface of glass or quartz by the polar-polar attraction forces, forming monomolecular layer, hydrophobic groups being directed outward, and then molecules of liquid crystals will be attracted to this mono­layer by the hydrophobic-hydrophobic attraction forces, and thus the homeotropic structure may be realized.

Those results cited above (5.,7.,9.) therefore suggest that the interaction forces between adsorptive/adsorptive play a role in controlling the whole process of the adsorption from solutions.

Finally, the results so far obtained recently on the adsorption of lecithin from solutions will be reported in the following. The systems studied were: I) adsorption of lecithin by alumina or silica from hexane solutions; II) adsorption of lecithin by alumina or silica from aqueous solu­tions. Materials used: Egg-1ecithin, the product of MERCK. Darmstadt alumina, the BROCKMANN T s chromato­graphic alumina of MERCK.,Silica, the special product of Kanto Chem. Co.,Tokyo; Hexane, extra pure sample of Kanto Chem. Co.,Tokyo; and water was twice distilled. The adsorbents: alumina and silica were washed with conductivity water till the con­ductivity of the washing water was unchanged and then dried carefully. The procedure of the adsorp­tion experiment applied was the usual one carried out at temperature 30 C. The analysis of lecithin in solutions was made by measuring the extinction coefficient of absorption maximum at 280 nm in hexane solutions and at 400 nm in aqueous solutions by applying HITACHI Model 100-10 spectrophotometer. The specific surface areas of the adsorbents were determined by the method of stearic acid adsorption from hexane solution with the result: 41sq. m/g for alumina, and 32.8sq. m/g for silica, respectively. These values are generally smaller than those determined by N^-adsorption method(for example, 78.8sq. m/g for the same- sample of alumina). In this study, however, the former values were

FACTORS INFLUENCING ADSORPTION 83

0 5.0 10.0 Equilibrium Concentration (mM)

Fig. 1. Adsorption isotherm for lecithin at alumina/hexane interface at 30°C.

CNI o

0 1.0 2.0 3.0 4.0 5.0 Equilibrium Concentration (mM)

Fig. 2. Adsorption isotherm for lecithin at silica/hexane interface at 30°C.

84 TAMAMUSHI

used, considering that the adsorption experiment was done also in solutions. Adsorption isotherms obtained are shown in Figures 1-2 for alumina-, silica-hexane solution systems; and in Figures 3-4 for alumina-, silica-aqueous solution systems, respectively. As noticed in Fig. 1 and 2, the adsorption isotherms obtained for hexane solutions are the type of LANGMUIR-isotherm, while those for aqueous solutions shown in Fig. 3 and 4, are the type of BRUNAUER-EMMETT-TELLER(BET)-isotherm. It is also noticed that the adsorption amount reaches saturation in the case of Fig. 1,2, and the first maximum point in the case of Fig. 3,4 at about 3-5 mM of the bulk solutions.

If we calculate the molecular areas of the adsorbed lecithin, by combining these maximum amounts of adsorption with the values of specific surface areas for alumina and silica above obtained we obtain the following approximate values for each case: 140sq. A for the system; alumina/hexane solution; 30sq. A for the system; silica/hexane solution; 1350sq. A for the system; alumina/aqueous solution; and 270sq. A for the system; silica/aque­ous solution; respectively. From the former two values for the adsorption from hexane solutions, it seems probable to assume the monomo1ecu1arity of the adsorption layer required by the LANGMUIR-equation, which actually fits the isotherms illus­trated in Fig. 1 and 2. The above obtained values of the molecular area for the system: alumina/ and silica/aqueous solutions are, however, too large for assuming the mutual interaction forces among adsorbed molecules to build up bi- or multi-layers at these interfaces. Very small amounts of adsorp­tion at those interfaces might be due to the adsorption of solvent (water) (10). In order to make a further discussion of the problem, it is necessary to have more accurate experimental data for those systems involving aqueous lecithin solutions. It is also desirable to develop a theory of adsorption which explains the BET-type adsorp­tion isotherms at the so1 id/solution interface. As well known, the BET-theory has been originally developed for the gas/solid interface, but not for the solution/so 1 id interface ( 11) .

FACTORS INFLUENCING ADSORPTION 85

0 10 20 30 40 50 60 7.0 Equilibrium Concentration (mM)

Fig. 3. Adsorption isotherm for lecithin at alumina/aqueous solution interface at 30°C.

Fig. 4. Adsorption isotherm for lecithin at si1ica/aqueous solution interface at 30°C.

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Acknowledgment, t

The author's thanks are due to Dr. N.Watanabe, MM. T.Ogawa and K.Onishi for their technical assistance in preparing this paper.

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