Physicochemical Study on the Adsorption Properties of Asbestos. 1

The adsorption of organic molecules from solutions onto asbestos fibers was investigated by means of the computer-aided analysis of the electron param...
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J. Phys. Chem. 1996, 100, 265-273

265

Physicochemical Study on the Adsorption Properties of Asbestos. 1. EPR Study on the Adsorption of Organic Radicals M. Francesca Ottaviani* and Filippo Venturi

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Department of Chemistry, UniVersity of Florence, Via G. Capponi 9, Firenze, Italy ReceiVed: June 6, 1995; In Final Form: October 4, 1995X

The adsorption of organic molecules from solutions onto asbestos fibers was investigated by means of the computer-aided analysis of the electron paramagnetic resonance (EPR) spectra of neutral and charged nitroxides, in the absence and in the presence of a hydrophobic chain attached to the nitroxide group. Four different asbestos fibers were used for the present study, namely, chrysotile, belonging to the serpentine group, and anthophyllite, amosite, and crocidolite, belonging to the amphibole group. Neutral or negatively charged molecules, namely, the “chain-free” nitroxides 4-hydroxy-Tempo and Tempyo-, were scarcely ( CHRYS > AMOS > ANTHO. The spectrum obtained by CROX samples was not reported in Figure 2, since the signal was very broad and at low intensity, due to the spin-spin interactions of the radicals with the paramagnetic metal ions in the CROX structure. The spectrum obtained from dry-CHRYS-CAT1 showed the superposition of two contributions: (a) three partially broadened lines that arise from the radicals in the hydration layers of CHRYS but are still affected by the field of the surface paramagnetic ions; (b) the broad signal drawn as a dashed line, superimposed on the experimental pattern. The latter signal showed not only an increase in line width with respect to the other spectral component but also a decrease of mobility (τc, could not be evaluated due to the large line broadening), as indicated by the variation of the relative heights of the three hyperfine lines. It was assumed that a portion of the CAT1 solution was adsorbed into the central cylindrical cavities of the fibers,20 whose surfaces are constituted mainly of silica groups that interact well with a positively charged probe.5a Also the spectrum of dry-ANTHO-CAT1 (Figure 2) was constituted by two contributions: (a) the narrow three-line signal arising from the radicals in solution (since the dry samples were dried up to reach the same appearance as the untreated fibers, we suppose that small water pools remained in the fiber

Ottaviani and Venturi

Figure 2. EPR experimental spectra (293 K, full lines) of unadsorbed solution of CAT1; dry-CHRYS-CAT1 (dashed line, broad component); dry-ANTHO-CAT1 (dashed line, computed slow motion component); and dry-AMOS-CAT1.

structure); (b) the slow motion signal (τc ) 5 × 10-9 s, evaluated by computation) due to the radicals that interacted with the surface sites. This contribution is shown as a dashed line superimposed on the dry-ANTHO-CAT1 signal in Figure 2. The poorly broadened three-line signal, which is shown in Figure 2 for the sample dry-AMOS-CAT1, indicates that a small portion of CAT1 solution remained trapped at the surface/ solution interface, slightly affected by the surface potential. (3) Adsorption of Doxylstearic Acids (DXSA). Doxilstearic acids (5-DXSA, 12-DXSA, 16-DXSA, Figure 1b) were useful probes for the study of both hydrophilic and hydrophobic interactions at the asbestos surface and the eventual competition between these interactions. Cooperative interactions were also expected on the basis of the results of Chandar and co-workers.21 These authors verified the capability of self-aggregation of n-DXSA at the surface of aluminas. On the other side, Berkeiser22 has demonstrated, by means of infrared spectroscopy, the affinity of CHRYS fibers toward the stearic acid. The presence of a second polar group, the doxyl moiety, at various distances from the carboxylic group of the stearic acid provided a further “anchoring point” with the active sites of the asbestos surfaces. The extent of adsorption, measured in percentage as the decrease in signal intensity from the bulk solution to the supernatant solution, provided the following evidence. (a) The different asbestos fibers adsorbed the radicals in the trend CHRYS . ANTHO g CROX > AMOS. For instance, about 80% of the radicals were adsorbed by the CHRYS surface, whereas 35% was the adsorbed amount onto ANTHO. The adsorption was almost negligible for AMOS. These results therefore support the conclusion of Berkeiser22 about the good interacting ability of the stearic acid with the CHRYS surface. (b) A higher adsorption of the radicals onto the surface was achieved from ethanol solutions with respect to chloroform

Adsorption Properties of Asbestos

J. Phys. Chem., Vol. 100, No. 1, 1996 269

Figure 3. EPR experimental spectra (293 K, full lines) of dryCHRYS-5-DXSA-EtOH (dashed line, computed exchange-narrowed component); wet-ANTHO-5-DXSA-EtOH (dashed line, computed spectrum); partially dried-ANTHO-5-DXSA-EtOH (dashed line, computed spectrum); and dry-ANTHO-5-DXSA-EtOH.

solutions. Two contemporaneous effects were responsible for this behavior: (i) a larger solvation of the polar surface by ethanol with respect to chloroform; (ii) a larger affinity of the radical molecules toward chloroform with respect to ethanol and, therefore, a preferential interaction of the radicals with CHCl3 with respect to the interaction with the surface polar sites. (c) The dilution of the solution with the solvents scarcely affected the extent of adsorption, whereas the addition of stearic acid decreased the adsorption in a functional way with the increase in the [SA]/[DXSA] ratio. More precisely, SA and DXSA competed for the interaction with the asbestos surface, but no preferential adsorption of SA or DXSA onto the fibers

was found. For instance, the adsorbed amount of 5-DXSA become almost negligible at [SA]/[5-DXSA] ≈ 50. (d) The extent of adsorption followed the trend 16-DXSA > 12-DXSA > 5-DXSA. Two different effects might account for this behavior: (i) the polarity of the radicals follows the same trend; the more polar the substance, the better it interacts with a polar surface; (ii) a larger distance between the two polar groups of the radical facilitated the “anchoring” of these groups at the surface; therefore 16-DXSA interacted with the asbestos surface in the “arc” conformation. 5-DXSA in EtOH Adsorbed onto Asbestos. Figure 3 shows representative EPR experimental spectra (full lines) and some computed spectra (dashed lines) obtained from the asbestos fibers after adsorption of 5-DXSA from ethanol solutions. The parameters used for the spectral computation are listed in Table 2. The spectrum of dry-CHRYS-5-DXSA-EtOH sample clearly showed the superposition of the three narrow line component from unadsorbed solution, termed signal F, and a broad unresolved component, termed signal M, arising from a large number of radicals (about 95%) that were forming aggregates at the surface. The small fraction of radicals responsible for signal F was probably localized in the cylindrical cavities of the CHRYS structure (see Figure 1), in which the solution remained trapped in spite of the mechanical drying. The spectra from wet samples obtained after adsorption of 5-DXSA onto the amphiboles were not informative, since signal F totally covered signal M. An exception was the ANTHO5-DXSA-EtOH samples. Their behavior is described in Figure 3, which shows the spectra at three stages of sample drying. 5-DXSA adsorbed onto wet ANTHO was localized in the solvating layers at the ANTHO surface, and therefore, its mobility was partially hindered in the interaction of the 5-DXSA polar groups with the surface polar groups. However, the stearic acid chain also favored this interaction, since the neutral nitroxide radicals without a hydrophobic chain (4-hydroxyTempo) showed mainly fast motion signals. Partial drying of the ANTHO samples led to the formation of 5-DXSA aggregates at the surface (ωex ) 1 × 107 Hz, and 1/T2,0 ) 8 G, reported in Table 2). The increase in the Ai components with respect to those reported for DXSA in pure EtOH reflects the increased polarity of the radical environment,24 due to the interaction with the polar asbestos surface. A broad pattern, not really informative, was obtained at the end of the drying procedure. The dry fibers after adsorption of a mixed solution of 0.1 mM 5-DXSA and 5 mM SA in ethanol show a better resolution

TABLE 2: Parameters Used for Computation of the EPR Spectra of DXSA Adsorbed onto Asbestos Fibers sample 5DXSA EtOH dry CHRYS 5DXSA EtOH dry CHRYS 5DXSA + SA EtOH wet ANTHO 5DXSA EtOH partially dried ANTHO 5DXSA EtOH dry ANTHO 5DXSA + SA EtOH 5DXSA CHCl3 wet CHRYS 5DXSA CHCl3 dry CHRYS 5DXSA CHCl3 dry ANTHO 5DXSA CHCl3 dry AMOS 5DXSA CHCl3 dry CHRYS 16DXSA EtOH dry CROX 16DXSA EtOH wet ANTHO 12DXSA EtOH a

type of signala

Axx; Ayy; Azz (G)

gxx; gyy; gzz

τcb (s)

F F M F F F, M F F F M M M M M M F (slow)

6.5; 6.5; 34 6.5; 6.5; 34 6.5; 6.5; 34 6.5; 6.5; 34 6.5; 7.0; 36 6.5; 7.0; 36 6.5; 7.0; 36 6.5; 6.0; 32 6.5; 7.0; 34.5 6.5; 7.0; 34.5 6.5; 7.0; 34.5 6.5; 7.0; 34.5 6.5; 7.0; 34.5 6.5; 7.0; 36 6.5; 7.0; 36 6.5; 7.0; 36

2.0086; 2.0060; 2.0027 2.0086; 2.0060; 2.0027 2.0086; 2.0060; 2.0027 2.0086; 2.0060; 2.0027 2.0086; 2.0060; 2.0027 2.0086; 2.0060; 2.0027 2.0086; 2.0060; 2.0027 2.0088; 2.0061; 2.0027 2.0088; 2.0061; 2.0027 2.0088; 2.0061; 2.0027 2.0088; 2.0061; 2.0027 2.0088; 2.0061; 2.0027 2.0088; 2.0061; 2.0027 2.0086; 2.0060; 2.0027 2.0086; 2.0060; 2.0027 2.0086; 2.0060; 2.0027

1 × 10-11 2 × 10-10 (2 × 10-10) 3.5 × 10-9 2 × 10-9 3 × 10-9 2.5 × 10-9 1 × 10-11 1 × 10-10 (1 × 10-10) (2 × 10-9) (6-8) × 10-9 (6-8) × 10-9 (4 × 10-9) (4 × 10-9) 1 × 10-8

ωex (Hz)

1 × 108 1 × 107

1 × 108 2.3 × 108 1 × 108 1 × 108 3.5 × 108 3.5 × 108

M ) single line exchange signal; F ) spectrum of radical monomers. b In parentheses the low-accuracy values.

1/T2,0 (G)

%

1 2 15 3.5 2 8 4 1 2 10 5 15 15 3 3 2

100 5 95 100 100 100 100 100 5 95 100 100 100 98 90 93

270 J. Phys. Chem., Vol. 100, No. 1, 1996

Figure 4. EPR experimental spectra (293 K, full lines) of wetCHRYS-5-DXSA-CHCl3; dry-CHRYS-5-DXSA-CHCl3 (dashed line, computed spectrum); dry-ANTHO-5-DXSA-CHCl3 (dashed line, computed spectrum); dry-AMOS-5-DXSA-CHCl3; and dry-CROX5-DXSA-CHCl3.

of the EPR spectra (not shown), in spite of their low intensity, which allows a more correct evaluation of the correlation time for motion. For instance, the computation provides a reliable correlation time for motion for dry-CHRYS-5-DXSA, that is, τc ) 3.5 × 10-9 s, and for the dry ANTHO sample (τc ) 2.5 × 10-9 s). 5-DXSA in CHCl3 Adsorbed Onto Asbestos. Figure 4 reports some examples of experimental EPR spectra (293 K, full lines) and computed spectra (dashed lines) obtained for 5-DXSA adsorbed onto asbestos fibers from CHCl3 solutions. The parameters used for computation are listed in Table 2. The spectrum of wet-CHRYS-5-DXSA-CHCl3 is similar (see the parameters reported in Table 2) to that of dry-CHRYS-5DXSA-EtOH (Figure 3), probably because of the high volatility of chloroform. The spectra from dry samples reported in Figure 4 indicate, on the basis of the exchange narrowing of the signals, a larger local concentration of adsorbed 5-DXSA for CHRYS with respect to the other fibers. As found for the spectra obtained after adsorption from EtOH solutions, the spectra of the samples obtained by using CHCl3 solutions show an increase in the Ai components after adsorption of the radicals at the asbestos surface. The accuracy in the correlation time for motion is still low, but it is noteworthy that only quite a large value of τc (τc ≈ (6-8) × 10-9 s) could reproduce the “distorted” line shape of the signals M shown in Figure 4. The equivalence in the spin-spin effect for ANTHO and AMOS samples, in spite of the larger number of radicals adsorbed by the ANTHO samples, evolved from the interacting site distribution at the surface of these fibers. We propose that the mean distance among the interacting sites at the ANTHO surface is larger than the mean distance at the AMOS surface. Finally,

Ottaviani and Venturi

Figure 5. EPR experimental spectra (293 K, full lines) of dryCHRYS-5-,12-,16-DXSA-EtOH (dashed line, computed exchange narrowed component); wet-ANTHO-5-,12-DXSA-EtOH (dashed line, computed slow motion); and dry-CROX-16-DXSA-EtOH.

CROX samples show mostly broad and low-intensity signals, because of the spin-spin interaction between the paramagnetic ions at the surface and the radicals. 12-DXSA and 16-DXSA in EtOH Adsorbed onto Asbestos (Comparison with 5-DXSA). In line with the intensity variations of the spectra from unadsorbed solutions to supernatant solutions, the local concentration of adsorbed radicals increased in the sequence 5-DXSA < 12-DXSA < 16-DXSA. This is shown in Figure 5 for some representative cases, that is, dry-CHRYS5-,12-,16-DXSA-EtOH, wet-ANTHO-5-,12-DXSA-EtOH, and dry-CROX-16-DXSA-EtOH. The experimental spectra (293 K) are the full lines, whereas the computed spectra are the dashed lines. The parameters used for computation are listed in Table 2. The analysis of these spectra is summarized as follows. (1) The exchange frequency increased from dry-CHRYS5-DXSA-EtOH to dry-CHRYS-16-DXSA-EtOH. (2) A slow motion component contributed to the signal of wet-ANTHO-12-DXSA-EtOH. Therefore, the interaction of 12-DXSA with the ANTHO surface was much stronger than the interaction of 5-DXSA with the same fiber (τc ) 2.5 × 10-9 s). It is again noteworthy that the distribution of the interacting sites at the ANTHO surface prevented spin-spin interactions, and the absence of Heisenberg exchange effects provided deeper information on the mechanism of adsorption of ANTHO with respect to the other fibers. (3) The spectrum of dry-CROX-16-DXSA-EtOH (Figure 5) was one of the few spectra that could be recorded from CROX fibers, in spite of the presence of interacting paramagnetic metal ions, because of the high concentration of adsorbed radicals. The line shape of this spectrum is similar to the shape of the spectrum of dry-CHRYS-16-DXSA-EtOH. This

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J. Phys. Chem., Vol. 100, No. 1, 1996 271

Figure 6. EPR experimental spectra (293 K, full lines) of dryCHRYS-CAT10 (dashed line, computed exchange narrowed component; dotted line, computed spectrum) and dry-ANTHO-CAT10 (dashed line, computed spectrum).

similarity indicates a comparable interacting effect between CHRYS and CROX surfaces toward 16-DXSA. (4) Adsorption of Positively Charged Nitroxides with an Attached Carbon Chain (CAT10 and CAT16). The DXSA probes showed a preferential adsorption toward the Chrysotile, providing information on the cooperative interaction of the radicals with the CHRYS surface. On the other side, the results about CAT1 indicate a preferential interaction with the surface of the amphiboles. Therefore, the use of CAT radicals with a hydrophobic chain attached to the CAT group could provide deeper information on the hydrophilic and hydrophobic interacting sites of the amphiboles. CAT10, with the hydrophobic chain of 10 C atoms, and CAT16, with the hydrophobic chain of 16 C atoms, were selected with different purposes: CAT10 was not aggregated in solution (the concentration was well below the cmc) and then provided information on hydrophilic and hydrophobic interactions; CAT16 gave micelles in solution (the

starting concentration was above the cmc of the surfactant), and therefore the radicals competed between the aggregation in solution and the aggregation at the asbestos surface. CAT10. The adsorbed amounts, evaluated on the basis of the intensity variation from the unadsorbed solution and the supernatant solution after equilibration with the asbestos fibers, indicate that the affinity of the asbestos surface toward CAT10 follows the trend ANTHO > AMOS > CHRYS > CROX. With the exception of CROX, the asbestos fibers show increasing adsorption of the radicals when the hydrophobic chain was attached to the CAT group. Again, ANTHO fibers show a marked selectivity in the interaction with the positively charged radicals, whereas their adsorption onto CROX fibers was almost negligible. Figure 6 shows the EPR experimental (full lines) and computed (dashed-dotted lines) spectra obtained from dry CHRYS and ANTHO after adsorption of water solutions of CAT10 (the spectrum of the AMOS sample, not reported, was equivalent to the spectrum of the CHRYS sample, but lower in intensity; the spectrum of CROX fibers was not recordable). Table 3 reports the parameters used for the computation of the spectra. It is of interest to compare the spectrum of CAT10 adsorbed onto CHRYS with the corresponding spectrum of CAT1 adsorbed onto the same fiber (Figure 2). The effect of the hydrocarbon chain attached to the CAT group was mainly to favor the cooperative interaction of the radicals at the CHRYS surface, that is, the aggregation of the radical chains at the surface. The dry-ANTHO-CAT10 sample showed a similar effect due to the presence of the hydrocarbon chain, but the spin-spin interactions were weaker (lower ωex) with respect to the other fibers, in spite of the larger adsorption, due to the low packing of the CAT10 aggregates at the surface. CAT16. Figure 7 shows, as representative examples, the EPR experimental spectra of CAT16 solutions at concentrations of 2.5 mM (the dashed line is the computed signal) and 1.7 mM and the spectra obtained from supernatant CAT16 solutions, at 2.5 mM concentration, after adsorption onto CHRYS and ANTHO fibers. The parameters used for computation are listed in Table 3. The line shape clearly shows the superposition of two contributions: signal F, due to free radicals in solutions, and signal M, due to micellized radicals. The high local concentration of radicals due to the close packing of the surfactants in the micelles is responsible for the large value of the Heisenberg exchange frequency. The experimental spectra of nonadsorbed and supernatant solutions were computed by adding the two computed signals, F and M, at different relative amounts. The radicals were therefore in equilibrium between their micelles and the bulk solution, and this equilibrium was perturbed by the addition of

TABLE 3: Parameters Used for Computation of the EPR Spectra of CAT10 and CAT16 Adsorbed onto Asbestos Fibers sample dry CHRYS CAT10 dry ANTHO CAT10 CAT16 2.5 mM wet CHRYS CAT16 2.5 mM wet CHRYS CAT16 1.7 mM dry CHRYS CAT16 1 mM dry ANTHO CAT16 0.5 mM a

type of signala

Axx; Ayy; Azz (G)

gxx; gyy; gzz

τcb (s)

F M M F M F M F M M F M

6.8; 8.2; 35.4 6.8; 8.2; 35.4 6.8; 8.2; 35.4 6.8; 8.2; 35.4 6.8; 8.2; 35.4 6.8; 8.2; 35.4 6.8; 8.2; 35.4 6.8; 8.2; 35.4 6.8; 8.2; 35.4 6.8; 8.2; 35.4 6.8; 8.2; 35.4 6.8; 8.2; 35.4

2.0088; 2.0072; 2.0035 2.0088; 2.0072; 2.0035 2.0088; 2.0072; 2.0035 2.0088; 2.0072; 2.0035 2.0088; 2.0072; 2.0035 2.0088; 2.0072; 2.0035 2.0088; 2.0072; 2.0035 2.0088; 2.0072; 2.0035 2.0088; 2.0072; 2.0035 2.0088; 2.0072; 2.0035 2.0088; 2.0072; 2.0035 2.0088; 2.0072; 2.0035

5 × 10-10 (2 × 10-9) 6 × 10-9 5 × 10-11 (3 × 10-9) 5 × 10-11 6 × 10-9 5 × 10-11 6 × 10-9 (5-7) × 10-9 5 × 10-11 (3 × 10-9)

ωex (Hz) 2 × 108 3 × 107 9 × 108 6 × 108 5.2 × 108 3 × 108 1 × 108

M ) single line exchange signal; F ) spectrum of radical monomers. b In parentheses the low-accuracy values.

1/T2,0 (G)

%

2.5 15 12 1 4 1.5 3 1.5 3 3 1.5 15

2 98 99 46 54 19 81 22 78 100 5 95

272 J. Phys. Chem., Vol. 100, No. 1, 1996

Figure 7. EPR experimental spectra (333 K, full lines) of CAT16 solutions at concentrations of 2.5 mM (dashed line, computed spectrum) and 1.7 mM and the spectra obtained from supernatant CAT16 solutions, at 2.5 mM concentration, after adsorption onto CHRYS and ANTHO fibers.

the asbestos fibers. The analysis of the relative amounts of signal M and signal F for unadsorbed and supernatant solutions provides the following information. (a) The interacting ability of the asbestos surface toward CAT16 followed the trend ANTHO . CHRYS > AMOS > CROX. Unexpectedly, CHRYS adsorbed more CAT16 than AMOS and CROX. The affinity of the CHRYS surface toward CAT16, in spite of the positive charges of both the surface and the radical, may be accounted for by the energy gain due to the cooperative interaction of the radical chains. The formation of surface aggregates was favored by various surface properties, like the curvature and the distance between the interacting sites. The adsorbed amounts were therefore larger if compared to CAT10. In turn, the ANTHO surface was selective in the adsorption of CAT16. (b) Micelles were preferentially adsorbed at the asbestos surface with respect to the monomers. For instance, signal M was not recordable from supernatant solutions after adsorption of CAT16 onto ANTHO fibers at the CAT16 concentrations used for this study. (c) The relative amount of micelles in solution increased with the increase in CAT16 concentration. However, the “disappearance” of micelles from the solution, caused by the adsorption at the asbestos surface, was more effective at the higher CAT16 concentration. Figure 8 shows some significant examples of the EPR experimental spectra (full lines, recorded at 333 K), together with the computed signals (dashed lines), obtained from CAT16 solutions at different concentrations adsorbed onto the asbestos fibers. The two components, three resolved lines and the exchange-narrowed signal, were again termed signal F and

Ottaviani and Venturi

Figure 8. EPR experimental (333 K, full lines) and computed (dashed lines) spectra of wet-CHRYS-CAT16-2.5 mM, wet-CHRYS-CAT161.7 mM, dry-CHRYS-CAT16-1.0 mM, and dry-ANTHO-CAT160.5 mM.

signal M, respectively. The parameters used for computation were reported in Table 3. The analysis of the data listed in Table 3 provided the following information. (1) The mobility of the radicals adsorbed at the CHRYS surface was partially hindered due to the cooperative interaction of the surfactants with the asbestos surface. (2) The aggregates at the asbestos surface were less packed when compared with the micellar aggregates in solution. Different effects may be responsible for this finding: (i) the distance between the surface interacting sites; (ii) the curvature of the surface; (iii) the irregularities and defects at the surface. (3) The dilution of the radicals in solution corresponds to the dilution of the radicals adsorbed at the asbestos surface. Therefore, the packing of the aggregates was lower at lower concentrations. It is noteworthy that signal M was not recordable from unadsorbed solutions of CAT16 at 0.5 mM concentration, whereas it became the main contribution for CAT16asbestos samples at the same concentration. The concentration at which the aggregates were formed at the asbestos surface (“critical aggregate concentration” ) cac) was therefore smaller than the cmc of CAT16 in solution. The shift in the cmc has already been found for surfactants in the presence of surfaces.21 (4) The drying of the asbestos surface removed a portion of radicals from the surface. As a consequence, the packing of CAT16 in the aggregates at the surface was lower for the dry samples. (5) In spite of the preferential adsorption of micellar aggregates at the ANTHO surface, the spin-spin interactions were weaker when compared to the other fibers. This effect strongly favors the hypothesis that the interacting sites were far enough from one another to ensure a low packing of the surface aggregates.

Adsorption Properties of Asbestos Conclusions The computer-aided analysis of the EPR spectra is revealed to be a powerful tool for investigation of the adsorption process of various organic molecules onto asbestos fibers. The neutral and charged organic radicals Tempol, Tempyo-, and CAT1 were scarcely adsorbed at the asbestos surfaces. The interaction between these molecules and the asbestos surface was mainly electrostatic. For instance, the positively charged nitroxide CAT1 interacted with the negatively charged anthophyllite surface, giving a slow motion signal (τc ) 5 × 10-9 s). The paramagnetic ions at the surface were responsible for the very broad and low-intensity signals recorded for crocidolite samples. The adsorption was strongly enhanced on the condition that a hydrophobic carbon chain was attached to the radical group. About 80% of the doxylstearic acid (DXSA) molecules in solution were adsorbed at the surface of chrysotile, whereas 35% was the adsorbed amount onto anthophyllite. The adsorption and the strength of interaction increased with both the increase of the distance between the COOH and the doxyl groups (from 5-DXSA to 16-DXSA) and the increase in solvent polarity (from chloroform to ethanol). The presence of hydrophobic chains at high local concentration at the asbestos/water interface gave rise to a cooperative interaction of the radicals at the asbestos surface. The larger adsorption onto the chrysotile surface with respect to the other fibers resulted in a larger local concentration of adsorbed DXSA. On the contrary, the mean distance among the interacting sites in the anthophyllite surface was larger with respect to the other asbestos fibers. Therefore, the local concentration of the radicals adsorbed at the anthophyllite surface was lower when compared to the other fibers. This also held for CAT10 and CAT16 aggregates at the anthophyllite/ water interface. The decrease in the spin-spin interaction increased the accuracy in the determination of the correlation time for motion, indicating a quenching of mobility for the CAT10 radicals adsorbed at the anthophyllite surface (τc ) 6 × 10-9 s). The adsorption of CAT16 onto asbestos from micellar solutions was again favored to anthophyllite fibers. In any case, the aggregates at the asbestos surface were less packed when compared with the micellar aggregates in solution. Furthermore, the concentration at which the aggregates were formed at the asbestos surface was smaller than the cmc of CAT16 in solution. This indicates that the asbestos surface favored the aggregation process of the surfactants, leading to the formation of hemimicelles at the surface itself. In summary the chrysotile surface was very reactive toward neutral or negatively charged molecules, mainly in the presence of the hydrophobic chain attached to the polar group, which led to the formation of surface aggregates. The vicinity of the interacting sites at the surface favored a high local concentration of the radicals adsorbed by chrysotile fibers. On the contrary, anthophyllite fibers were revealed to be very selective in the adsorption of positively charged molecules. In this case too, the presence of hydrophobic chains attached to the positively charged moiety strongly enhanced the adsorption process. However, the cooperative interaction gave rise to low packed surface aggregates, due to the rather large distance among the

J. Phys. Chem., Vol. 100, No. 1, 1996 273 interacting sites at the surface. Amosite fibers showed almost negligible adsorption of the organic molecules used for the present study. Finally, the adsorption ability of crocidolite was comparable to the adsorption of anthophyllite, but the EPR spectra of crocidolite samples were very broad and at the limit of detectability due to the presence of paramagnetic ions localized at the interacting sites of the asbestos surface. Acknowledgment. Thanks are due to the Italian Ministero Universita` e Ricerca Scientifica e Tecnologica (MURST) and the Italian Consiglio Nazionale delle Ricerche (CNR) for financial support. The authors are particularly grateful to Prof. Bice Fubini, University of Turin, Italy, for the precious advice and very helpful discussions on the results reported in this paper. References and Notes (1) Pinnavaia, T. J. In AdVanced Chemical Methods for Soil and Clay Mineral Research; Stucki, J. W., Banwart, W. L., Eds.; Reidel: Dordrecht, 1980; p 391. (2) Mc Bride, M. B. Clays Clay Miner. 1979, 27, 97. (3) Bassetti, V.; Burlamacchi, L.; Martini, G. J. Am. Chem. Soc. 1979, 101, 5471. (4) (a) Ottaviani, M. F.; Martini, G. J. Phys. Chem. 1980, 84, 2310. (b) Martini, G.; Ottaviani, M. F. J. Phys. Chem. 1981, 85, 1922. (5) (a) Romanelli, M.; Ottaviani, M. F.; Martini, G. J. Colloid Interface Sci. 1983, 96, 373. (b) Martini, G.; Ottaviani, M. F.; Romanelli, M. J. Colloid Interface Sci. 1987, 115, 87. (6) Martini, G.; Ottaviani, M. F.; Pedocchi, L.; Ristori, S. Macromolecules 1989, 22, 1743. (7) Ristori, S.; Ottaviani, M. F.; Martini, G. Langmuir 1991, 7, 755. (8) Fubini, B. In Fiber Toxicology; Academic: New York, 1993; p 229. (9) (a) Selikoff, I. J.; Hammond, E. C.; Churg, J. JAMA, J. Am. Med. Assoc. 1968, 204, 106. (b) Shabad, L. M.; Pylev, L. N.; Krivosheera, L. W.; Kulagina, T. F.; Nemenko, B. M. J. Natl. Cancer Inst. 1974, 52, 1175. (c) Selikoff, I. S.; Lee, D. H. L. In Asbestos and Diseases; Academic: New York, 1978. (10) (a) Hodgson, A. A. R. Inst. Chem. Lecture Ser. 1965, 4, 1. (b) Timbrell, V. In Pneumoconiosis: Proc. Intl. Conf. Johannesburg 1969; Oxford University Press: Cape Town, 1970; p 25. (c) Light, W. G.; Wei, E. T. EnViron. Res. 1977, 13, 135. (11) Ottaviani, M. F.; Turro, N. J.; Jockush, J.; Tomalia, D. A. J. Am. Chem. Soc., in press. (12) Blackmore, E. S.; Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 1988, 84 (2), 1115. (13) Schneider, D. J.; Freed, J. H. In Spin Labeling: Theory and Applications; Vol. III, Biological Magnetic Resonance; Berliner, L. J., Reuben, J., Eds.; Plenum Press: New York, 1989; Vol. 8, p 1. (14) Harington, J. S.; Allison, A. C.; Badami, D. V. AdV. Pharmacol. Chemother. 1975, 12, 291. (15) Bonneau, L.; Pezerat, H. J. Chim. Phys. 1983, 80, 275. (16) Tsay, F. D.; Manatt, S. L.; Chan, S. I. Chem. Phys. Lett. 1972, 17, 223. (17) (a) Kivelson, D. J. Chem. Phys. 1957, 27, 1087. (b) Plachy, W.; Kivelson, D. J. Chem. Phys. 1967, 47, 3312. (18) Sackmann, E.; Tra¨uble, T. J. Am. Chem. Soc. 1972, 94, 4482, 4492, 4499. (19) (a) Aizawa, M.; Komatsu, T.; Nakagawa, T. Bull. Chem. Soc. Jpn. 1979, 52, 980. (b) Ibid. 1980, 53, 975. (20) (a) Yada, K. Acta Crystallogr. 1967, 23, 704. (b) Ibid. 1971, 27A, 659. (21) Chandar, P.; Samasundaran, P.; Waterman, K. L.; Turro, N. J. J. Chem. Phys. 1987, 91, 148. (22) Berkheiser, V. E. Clays Clay Miner. 1982, 30, 91. (23) Bonosi, F.; Gabrielli, G.; Martini, G.; Ottaviani, M. F. Langmuir 1989, 5, 1037. (24) (a) Janzen, E. G. Top. Stereochem. 1971, 6, 117. (b) Ottaviani, M. F.; Martini, G.; Nuti, L. Magn. Reson. Chem. 1987, 25, 897.

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