Adsorption from binary solutions on silanized silica gel

Colloids and Surfaces, 47 (1990) 167-177 Elsevier Science Publishers B.V., Amsterdam -

167 Printed in The Netherlands

Adsorption from Binary Solutions on Silanized Silica Gel J. GOWOREK Institute of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin (Poland) (Received 24 January 1989; accepted 30 October 1989)

ABSTRACT The adsorption process of polar substances from binary mixtures with benzene and n-heptane on nonmodified and chemically modified silica gels was compared. Liquid chromatography column packings, based on silica gel, with bonded dimethyl or octyl groups were used as the modified adsorbents. The excess adsorption isotherms were measured under static conditions. The capacity of the surface layers was calculated by using the Dubinin-Radushkevich type of adsorption isotherm equation. It was found that the capacity of the surface layer of silanized silica gel for the same mixture is about half the capacity of a nonmodified silica gel. The hydroxyl groups and methyl or octyl groups of the surface interact with polar and nonpolar segments of the adsorbate molecules, respectively. This causes a specific orientation of the molecules on silanized silica gel which is different from that on the fully hydroxylated silica gel. The correlation of the extent of adsorption from the liquid phase and the concentration of the polar and nonpolar species on the silica gel surface is discussed.

INTRODUCTION

Studies of the adsorption phenomena of liquid mixtures on solid surfaces under static conditions offer important information which may be used to explain the retention mechanism of solutes in chromatography. The adsorption process is especially complex on chemically modified silica gels used in reversed-phase liquid chromatography. The presence of nonpolar species on their surface influences the selectivity of adsorption of monofunctional organic compounds and their orientation in the adsorbed phase. The presence of silanols causes specific adsorption of polar substances in a way similar to adsorption on nonmodified silica gels. In the case of modifiers with longer hydrocarbon chains the separation mechanism involves both adsorption and partition. It follows from some early work that adsorption phenomena play the main role [l-3]. The influence of the alkyl chain length of a chemically bonded phase on the retention mechanism in RPLC wa discussed by Berendsen and de Galan [ 41. 0166-6622/90/$03.50

0 1990 Elsevier Science Publishers B.V.

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On the other hand, the measurements of adsorption from binary liquid mixtures permit the characterization of the heterogeneity of the adsorbent surface. Binary liquid mixtures containing components of differential polarity were widely used in investigations of adsorbents containing different types of functional groups on the surface. These investigations have been devoted to various types of adsorbents, e.g. active carbons [ 5-81 and soil minerals [g-14]. In the present paper we report investigations concerning the adsorption of n-butanol, t-butanol and butyl acetate from binary mixtures with benzene and n-heptane on silanized and nonmodified silica gel. The aim of these investigations was to check how the form and concentration of the surface hydrophobic species bonded on silica surface influences the extent of adsorption from the liquid phase and the structure of the surface phase. MATERIALS AND METHODS

The adsorption process for the following binary mixtures was investigated: (1) n-butanol+ benzene, (2) n-butanol +n-heptane, (3) t-butanol+ benzene, (4) t-butanol+n-heptane, (5) butyl acetate+benzene and (6) butyl acetate + n-heptane. Adsorbent Two reversed-phase adsorbents were used. The first was a home-made silanized silica gel of the RP-2 type. Silanization was carried out by using dimethylchlorosilane on 0.1-0.2 mm Si-100 silica gel particles. The specific surface area of the obtained adsorbent was determined by the BET method using nitrogen adsorption data at 78 K. The nitrogen adsorption isotherms were measured using automated apparatus (Sorptomatic 1800 from Carlo Erba, Italy ) . Assuming that the surface area occupied by a nitrogen molecule is 0.162 nm’ the specific surface area of the silica was calculated as 290 m2 g-l. Commercially obtained LiChrosorb RP-8 (Merck, F.R.G.) was used as the second adsorbent. The specific surface area of this adsorbent was calculated as 285 m2 g -‘. The silicas were dried before adsorption at 160°C for 12 h. Solvents Benzene, n-heptane and butyl acetate from Reachim (U.S.S.R. ), and n-butanol and t-butanol from Fluka (F.R.G ), all puriss grade, were dried over silica gel and were then used without further purification. The mixtures were prepared gravimetrically for each experimental point.

169

Measurement of adsorption isotherms The excess adsorption isotherms were determined at 298 K using a static method. For each composition two flasks containing the same mixture were prepared. A known quantity of the adsorbent ( z 4 g) was added to a weighed amount of the mixture ( w 8 g) in one of the two flasks. The system consisting of the mixture and the adsorbent was shaken for 8 h. The temperature was thermostatically controlled to 298 2 0.1 K. After equilibration the supernatant solution was centrifuged and then analyzed by gas/liquid chromatography using a gas chromatograph (type GChF 18.3, Chromatron, G.D.R. ). The initial mixtures were used to calibrate the detector. The adsorption isotherms for nonmodified silica gel were taken from the literature [ 15-171. The determination of the surface silunols The determination of silanol groups on the surface of bonded silica was performed by using high-performance liquid chromatographic instrumentation. The method is based on the complete exchange of the silanol protons with deuterium [ 181. The same method served the determination of surface silanols on the nonbonded, fully hydroxylated silica gel which was used in our earlier experiments [15-171. The appropriate concentration of hydroxyl groups for the nonmodified silica gel is 7.85 pm01 me2 and is 3.4 and 3.3 pmol m-’ for the RP-2 and RP-8 silicas, respectively. RESULTS AND DISCUSSION

For a binary mixture containing two components 1+ 2, the surface excess of the preferentially adsorbed component can be calculated from the relationship n;/a, =r$“’ = no (xi -x:)/ma,

(1)

where no is the total number of moles of both components in contact with m g of the adsorbent, xi and xi are the mole fractions of the first component in the initial and equilibrium solutions, respectively, and a, is the specific surface area of the adsorbent. The excess adsorption isotherms Fin) = f (xi) for the n-butanol+ benzene and n-butanol + n-heptane systems on silanized silica gel are presented in Figs 1 (a) and (b) (filled circles). The open circles represent the excess adsorption isotherms for the same systems on nonmodified silica gel (a, = 300 m2 g- ’ ) . It can be seen that adsorption of butanol on silanized silica gel from both solvents is markedly less than on nonmodified silica gel. The surface layer capacities of

Fig. 1. Excess adsorption isotherms of n-butanol from n-heptane (a) and benzene (b) on silanized (filled circles) and nonmodified (open circles) silica gel at 298 K. The dashed lines represent the adsorption isotherms calculated from Eqn (16). TABLE 1 Parameters of Eqn (4) for the adsorption systems investigated Mixture

Adsorbent

ns (pm01 m-‘)

B

k

n-Butanol+ n-heptane n-Butanol+ n-heptane n-Butanol + benzene n-Butanol + benzene t-Butanol + n-heptane t-Butanol+ n-heptane t-Butanol + benzene t-Butanol + benzene Butyl acetate + n-heptane Butyl acetate + n-heptane Butyl acetate + benzene Butyl acetate +benzene n-Butanol + benzene n-Butanol + n-heptane

SiOz SiOzC2 SiOz SiOx-C2 SiOz SiOz-C2 SiOz SiOz-C2 SiOz SiOz-C2 SiOz SiOzC2 SiOz-C8 SiOz-03

13.0 6.7 12.0 5.9 10.6 5.5 10.3 7.6 6.7 3.7 7.2 4.1 6.5 6.5

0.062 0.008 0.021 0.016 0.007 0.012 0.011 0.025 0.011 0.014 0.013 0.029 0.013 0.008

399 327 53 93 74 159 122 125 61 183 817 82 1.0.10’ 3.1.105

the investigated systems were calculated using the Dubinin-Radushkevich type equation describing the adsorption process from a liquid phase on energetically heterogeneous solid surfaces. According to this equation the mole fraction of the first component in the surface phase may be expressed as follows [ 191: x; =exp[ -B(ln

k/~)~]

(2)

171

where x=x\/xL, B is a parameter connected with the heterogeneity of the adsorbent surface, and k is a constant characteristic of a given system and related to the difference of adsorption energies of both components. Equation (2) can be written in the following linear form: (-In

xS,)‘j2= B’121n k-B’121n x

Taking into account that n; =ns(x; -xi) [ -ln(n;/ns+x:)]‘/2=B1/21n

k-B”21nx

(3)

we obtain (4)

where ns is the capacity of the surface layer. The parameters of Eqn (4) characterizing the investigated systems are summarized in Table 1. In most cases values of ns for silanized silica gel are about half the values calculated for nonmodified silica gel. The results are rather surprising if we consider that the specific surface areas of both adsorbents are very similar. It seems that the modified fraction of the silica gel surface does not take part in the adsorption process from the liquid phase. In this connection we will consider the adsorption model in which no preferential adsorption of any liquid component occurs on the hydrophobized part of the surface. This means that the composition of the surface phase is the same as in the bulk solution. However, adsorption on the hydroxylated part of the silica is identical to that on the nonmodified silica gel. Considering separately both parts of the silanized silica gel surface we can write for component 1: nL.il

= n”l,0H + nl,c2

(5)

where n;,,ii, n;,oH and n:,c2 are the numbers of moles (per m”) of component 1 in the surface phase for silanized silica gel and its fractions covered by the silanol and methyl groups, respectively. For the assumed model of the surface phase we have pa)

l,C2

=nS1,C2(1-xX:)-n;,c2x: =O

(6)

which means that n”l,c2In %c2 =X:/(1-X:)

(7)

The surface fraction covered by methyl groups, yc2, may be expressed by the real number of moles of both components in the adsorbed phase as follows: 3~~2 =

bG,c2aY

+n8.c24)lt

(8)

where a: is the area occupied by one molecule of component in the surface phase, and t is the number of the adsorbed layers. Combining Eqns (7) and (8) we obtain n”l,c2=yo2tl]a’;

+aW-xW41

(9)

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It is reasonable to assume that the surface fraction of the silanized silica gel covered by silanols, yen, is proportional to the number of these groups on th*e surfaces. Thus, the value you may be calculated on the basis of the surface silanol concentrations for silanized and nonmodified silicas: YOH=NoH/noH

(10)

where non and Non are the numbers of silanol groups on nonmodified and silanized silica gel, respectively. Similarly we have ye2 = (l-&-dInoH

(11)

On the basis of Eqns (5)) (9) and (10) under the assumptions involved earlier, we obtain

(12) where ni,9il is the number of moles of component 1 adsorbed per unit area on nonmodified silica gel. Defining n i,+ilsimilarly and taking into account that nLm -ri%m , +ni,x:

(13)

and (14) and (15) we have:

ri;h

=NoH/noHrill,h,

(16)

The dashed lines in Fig. 1 represent the excess adsorption isotherms calculating according to Eqn (16) for n-butanol systems. These curves are in good agreement with the experimental isotherms for silanized silica gel, which indicates the correctness of the assumed model of the surface phase. On the methylated part of silica surface preferential adsorption of any component of the bulk solution does not occur; however, the hydroxylated part covers the adsorbed phase with multilayer character [ 171.In the case of benzene solutions this surface structure possesses a mixed character; in the case of n-heptane solutions the surface phase contains only alcohol molecules [ 201. As was discussed earlier, a similar mixed and multilayer structure of the surface phase is characteristic of the adsorption process of esters from benzene on silica gel [ 161.In Fig. 2 (b), for illustrative purposes, the excess adsorption isotherms of butyl acetate + benzene mixtures on both adsorbents are compared. As in the case of n-butanol solutions, good agreement between the ex-

173 6.0 % dr Z P I_ c IL20

0 0

0.5

1 x1

1.0

0.5

Fig. 2. (a) Excess adsorption isotherms of t-butanol from benzene solutions on silanized (filled circles) and nonmodified (open circles) silica gels at 298 K. Dashed line represents the adsorption isotherm calculated from Eqn (16). Dotted line shows the dependence of T@z against 5:. (b) Excess adsorption isotherm of butyl acetate from benzene solutions on silanized and nonmodified silica gels at 298 K. Labelling as in Fig. 1.

perimental isotherms on silanized silica gel and theoretical results from Eqn (16) is observed. The decrease of adsorption of the polar component and the surface layer capacity on hydrophobized silica surface may be explained by the specific structure of the surface phase. All the systems discussed above were characterized by the presence in the molecule of the preferentially adsorbed component a relatively large alkyl radical which may be solvated by benzene molecules. It may be assumed that both n-butanol and butyl acetate are adsorbed on silanized silica gel in the form of complexes in the same way as on nonmodified silica gel. However, owing to the random localisation of silanols on the modified surface they are packed differently. In this connection there exists a possibility of screening some methyl groups by hydrocarbon parts of these complexes. The formation of this type of surface complex in the case of the adsorption of esters from hydrocarbons was discussed earlier by Mills and Hockey [21231. Also, our earlier investigations concerning the adsorption of esters [ 161, ketones [24] and alcohols [20] confirm the existence of complexes between the polar component and benzene on the silica gel surface. However, the screening effect may also occur in the case when only a polar component is adsorbed on silanols. The functional group is adsorbed specifically on the silanol group. However, the hydrocarbon chain of the same molecule is adsorbed on the hydrophobic centre (the methyl group). One can say that on a nonmodified silica gel hydrocarbon chains of alcohols or esters are oriented towards the bulk solution,

174

whereas in the case of silanized silica the radical is oriented parallel to the surface. It is worth noting that the composition of the surface layer is determined mainly by the number of surface silanols, but the orientation of the adsorbate molecules is determined by the topography of the surface species. The similarity between the experimental and theoretical curves in Figs 1 and 2 (b ) is the result of the domination of specific interactions in the adsorption process. However, the configuration of adsorbed molecules and/or their complexes with benzene is not the same on both adsorbents. The above hypothesis may be verified by a similar analysis of the adsorption data for t-butanol+ benzene and t-butanol+ n-heptane systems. Because of steric effects the association of the alcohol molecules in the surface phase is impossible. Thus its adsorption is monolayer [ 151. Physically adsorbed molecules of t-butanol interact mainly with silanols, so that at relatively low equilibrium concentrations of this alcohol in the bulk solution we obtain a fully methylated adsorbent surface, in part physically and in part chemically due to silanization. The adsorption isotherms for t-butanol+ benzene mixtures are presented in Fig. 2 (a). The experimental and theoretical curves are quite different. The experimental excess adsorption isotherm at a higher equilibrium concentration of alcohol possesses an azeotropic point, and the values of the surface excess on silanized silica gel are much smaller than calculated from Eqn (16). The assumption for this system that r$‘& = 0 makes the calculated isotherm too high. On the other hand, a decrease in the surface excesses of alcohol is connected with the preferential adsorption of benzene on the methylated surface. Similar behaviour is shown by the t-butanol+n-heptane mixture. If we express a total surface excess of component 1 on a silanized silica gel as a sum: G;l =r&

+ri”dz

(17)

then, for a given Non value the surface excess of component 2 (in our case benzene or n-heptane) may be calculated from the relationship rLr_d,= W,

+No&o,

ri:L

(16)

The dotted line in Fig. 2 (a) represents the excess adsorption isotherm of benzene for the t-butanol+ benzene system. The section of this isotherm, corresponding to small equilibrium concentrations of alcohol, leads to an unreal shape which indicates a higher adsorption of benzene on silanized silica gel in comparison to nonmodified silica gel. Over the whole concentration range of solutions the assumptions of Eqn (16) are in this case not fulfilled. The results presented confirm the possibility of a change in the surface character of the adsorbent by the monolayer of physically adsorbed molecules [ 251. A similar character of the adsorption process may be expected in the case of

175

Fig. 3. Excess adsorption isotherms of n-butanol from n-heptane and benzene solutions on nonmodified silica gel (curves 1 and 1’ ) and LiChrosorb RP-8 (curves 2 and 2’ ) . Dashed line represents excess adsorption isotherm calculated from Eqn (16) for n-butanol + benzene system. Dotted line shows the dependence of r&$s ag ainst xi for the n-butanol+benzene system.

silicas with the chemically bonded phase containing longer hydrocarbon chains on the surface. In Fig. 3 the adsorption of n-butanol from benzene and n-heptane on LiChrosorb RP-8 are presented. The experimental curves for both solvents possess an azeotropic point. The increase of the competition of the components to surface silanols in the case of benzene solutions in comparison to n-heptane solutions shifts the azeotropic point slightly in the direction of higher equilibrium concentrations of alcohol in the bulk solution. In the case of these systems the adsorption of hydrocarbon plays the main role. For illustrative purposes in Fig. 3 a comparison of the theoretical and experimental isotherms is presented, although the difference between the experimental isotherms for silanized and nonmodified silica gel is clearly visible. The comparison may be meaningful only at very small concentrations of alcohol, for which the adsorption of alcohol is positive. In the same concentration range on nonmodified silica gel a mixed character of the surface phase has been recognised [ 171.At lower concentrations of alcohol the formation of mixed alcohol-hydrocarbon complexes is more probable than the formation of self-associated clusters. The dotted line in Fig. 3 represents the adsorption isotherm of benzene on the modified fraction of the silica gel surface calculated from Eqn (18)) C$j8 =f
176

nonmodified silica gel. As expected, the assumptions of Eqn [ 161 are in this case not fulfilled over the whole concentration range. The surface layer capacities calculated on the basis of Eqn (2) are about two times smaller than on nonmodified silica gel in the case of LiChrosorb RP-8. The interpretation of the adsorption data in terms of molecular surfaces of the adsorbates is very difficult because the orientation of the molecules in the surface phase is unknown. On the other hand, the specific surface area in the case of modified RP materials is not precisely defined, especially for long hydrocarbon chains of the modifier. Assuming that one molecule of butanol blocks one silanol group on the surface the full monolayer coverage of alcohol should be 3.3 pm01 m -‘. From Table 1 it follows that the surface layer capacity is 6.5 pm01 rnd2. The higher ns value may be related to the surface phase model in which first alcohol molecules on silanols and then molecules of benzene or nheptane are adsorbed forming the second layer. This means, under the assumption of equality of molecular area occupied by adsorbate molecules, that the composition of the surface layer in relation to both components is 1: 1. This model shows some analogies with the surface structure for n-octanol-hydrocarbon-rutile system discussed in Refs [ 261 and [ 271. However, in our case the localisation of the azeotropic point of the adsorption isotherms at xi z 0.2-0.3 indicates a higher concentration of hydrocarbon in the surface phase, x;/x; =x:/x’ 1 x 3 : 1. Thus one can say that hydrocarbons preferentially adsorbed on the nonpolar bonded phase or between CS chains decrease the penetration of alcohol molecules onto surface silanols [lo]. The considerable adsorption of benzene and n-heptane on LiChrosorb RP-8 is a result of the presence on the surface of the nonpolar phase and its extent controls the level of hydrocarbon adsorption on nonmodified silica gel, which is connected with the solvation effects of alkyl radicals of polar adsorbates. Although the above results give important information on the adsorption process on chemically modified silica gel, they are still insufficient for a full description of the structure of the surface phase. Therefore this problem requires further experimental studies.

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