J. Phys. Chem. 1983, 87, 2369-2373
Also plotted in Figure 1 are the results (as triangles) if the higher order electrostatic terms are omitted. The systematic departures are apparent, primarily for dilute solutions. The values are given in Table I11 of 'OH,L~, # H , L ~ , c ~ ,and the standard deviation of fit determined at each temperature via the least-squares procedure, and Figure 2 presents the temperature dependence of the results graphically. The error bars correspond to the standard deviations of the parameters as determined by the fitting procedure. Also included in Table I11 are values of the standard deviation and of OH,Le and #HhLa,C1determined without E O H , h and EO'Hb; inclusion of the higher order electrostatic terms is clearly indicated by the standard deviation values as well as by Figure 1. The mixing parameters are well represented by the equations = 0.281 + 0.0018(T - 298.15) #H,b,CI
= 0.006 - 0.0025(T - 298.15)
(14)
The temperature coefficients of these parameters are then
a
-('OH,La)
aT
a dT
-(#H,La,C1)
= 0.0018 f 0.0007 = -0.0025 f 0.0003
(15)
where, again, errors were determined via the least-squares fitting procedure. Most of the earlier measurements's21 for this system are (19) Randall, M.; Breckenridge, G . F. 1435.
J. Am.
Chem. SOC.1927, 49,
2389
not sufficiently precise to contribute to those aspects considered here. The most recent study, that of Khoo et al.,n reports data with precision comparable to the present study but only for 25 "C and only to I = 3 mol kg-'. They used slightly different pure-electrolyte parameters and there are small differences in the reported E values for pure HCl. In general, the results and conclusions of Khoo et al. for 25 "C are the same as those of this investigation. The activity coefficientsof HC1 and LaC13and the excess enthalpy were calculated by eq 3 and 9-11 and are presented in Table IV. The enthalpy of mixing is the difference between the next to last column (y = 0.5) and the average of the adjoining columns (y = 0, 1). These calculations are reasonably straighforward; hence, we have not made efforts to provide extensive tables.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work, and to the Director, Office of Energy Research, Office of Basic Energy Sciences, Division of Engineering, Mathematics, and Geosciences of the US.Department of Energy under Contract no. DE-AC03-76SF00098for additional support. We thank Kennan Buechter and Steven Faszholz for their assistance in the electrochemical cell measurements. Registry No. HCl, 7647-01-0;Lac&, 10099-58-8. (20) Lietzke, M. H.; Stoughton, R. W. J. Phys. Chem. 1967, 71, 662. (21) Lietzke, M. H.; Danford, M. D. J. Chem. Eng. Data 1972,17,459. (22) Khoo, K. H.; Lim, T.-K., Chan, C. Y. J.Solution Chem. 1981,10, 683.
Influence of the Physical Structure of Irradiated Starches on Their Electron Spin Resonance Spectra Kinetics Jacques J. Raffl' and Jean-Pierre L. Agnei Service de Radio-egronomie, CEN Cadarache, 13 115 Saint-Paul lez Durance, France (Received: February 19, 1982; In Final Form: September 9, 1982)
This study deals with the shape and kinetic changes of the ESR spectra of eight irradiated starches, from several hours to several months after y-irradiation. Whatever the origin and water content of the starches two major radicals or groups of radicals are observed. The kinetic law depends on the water content; two main zones are pointed out which are relative to the amorphous and crystalline parts of starches.
Introduction The chemical and toxicological studies that we carried out on y-radiolysis of starches'$ naturally lead us to inquire into the mechanism of radiolysis of these starches. Electron spin resonance (ESR) studies have been undertaken for a long time on the "polysaccharide radicals" induced by y radiation or chemical initiation. As the observed phenomena are very complex, the most recent works moved toward the use of low temperature^,^,^ hydrogen (1)
J. Raffi, L. Saint LBbe, and G . Berger, Food Preseru. Irradiat.,
Proc. Int. Symp., 1977, 1, 517 (1978). (2) J. Raffi, J. P. Agnel, C. ThiCry, C. FrCjaville, and L. Saint-LBbe, J. Agric. Food Chem., 29, 1227 (1981), and references cited therein. (3) G . Abaghian and A. Apresyan, Arm. Khim. Zh., 32, 850 (1979). 0022-365418312Q87-2369$Ql.5010
donor^,^ or sensitizers.6 These methods allow the study of the initial process (excited states, radical ions, primary radicals) but they yield only little information about the chemical structure of the long-lived radicals, i.e., those remaining after months of storage at room temperature; however, such radicals are the more interesting from a toxicological point of view,'P2 in order to ascertain the wholesomeness of irradiated food. Moreover the results (4) A. Plonka, J. Bednarek, and H. Zegota, Z. Naturforsch, B, 34,1525 (1979). (5) G . Abaghian, J.Magn. Reson. Relat. Phenom., R o c . Congr. AMPERE 20th, 173 (1979). (6) A. Merlin and J. P. Fouassier, Angew. Makromol. Chem., 86, 123 (1980).
0 1983 American Chemical Society
2370
Raffi and Agnel
The Journal of Physical Chemistry, Vol. 87,No. 13, 1983
w 2.8
3.9 5.1
I I I I I I
p I
6.0 II
I
10 Flgure 1. ESR spectra of some irradiated starches (20 kGy): (curve 1) “initial” AA‘ shape, example of dry haricot bean starch, right after irradiation under nitrogen (Xl); (curve 2) “final” BB‘ shape, same sample as before but 95 days later (X100); (curve 3) “intermediate” spectrum, example of bread wheat starch, right after irradiation under air (X20).
of ESR often contain some contradictions, as explained in the excellent report of Kochetkov et al.,’ and are difficult to compare because of the large range of applications.8 These contradictions may be due partly to use of spectrometers with poor resolution but mainly to the complexity of the physical structure of the polysaccharide^.^ Moreover the powder spectra are only poorly resolved as they are due to the overlapping of a lot of spectra relative to the random directions of radicals with regard to the magnetic field. So the aim of this work will be to determine the main parameters influencing the kinetic evolution of ESR spectra in order to choose sugars which will be the best models for starches; therefore, these sugars will be also studied in powder and monocrystal states and by spin trapping.’+12
Experimental Section Materials. We shall consider the following starches: maize (MN), waxymaize (WM), amylomaize or hylon (AM), manioc (M), breadwheat (B), potato (P), rice (R), and haricot bean (H). Different water contents are obtained by low-temperature-controlled dehydration under vacuum (freeze drier, Sogev, Type Sublimac RP12). Radiodextrins (RD) were extracted from irradiated maize starch by water.’P2 “Crystalline parts” isolated from maize (CMN), waxymaize (CWM), and potato (CP) starches were prepared by Dr. Tollier and Robin (INRA) by the use of spared hydrolysis in the heterogeneous phase.13 Sigma Corp. supplied the maltotriose (MT). All the products are usually kept at room temperature under constant atmosphere and water content. ~~~~~~
(7)N. Kochetkov, L. Kudrjashov, and M. Chlenov, in “Radiation Chemistry of Carbohydrates”, Pergamon Press, Oxford, 1979,especially Chapter 4. (8)A. Henderson and A. Rudin, J. Polym. Sci. 19, 1721 (1981). (9)A. Ahmed and W. Rapson, J.Polym. Sci., 10, 1945 (1972). (10)M. Kuwabara, Y.Lion, and P. Riesz, Znt.J.Radiat. Biol., 39,451 (1981). (11)P. Riesz and I. Rosenthal in “International Symposium on Spin Trapping and Nitroxyl Radical Chemistry”, University of Guelph, Ontario, Canada, July 12-18,1981. (12)A. Forrester in ‘International Symposium on Spin Trapping and Nitroxyl Radical Chemistry”, University of Guelph, Ontario, Canada, July, 12-18,1981. (13)J. P. Robin, C. Mercier, R. CharbonniBre, and A. Guilbot, Cereal Chem., 51, 389 (1974).
23 7.9 7 I I 1
10
20
1 1
Flgure 2. Kinetic evolution of BB’ line for maize starch with different water contents W ( % ) . The time tis measured in days and the radical concentration (R.) in arbitrary units (sum of Y , and Y,’ heights, calculated at constant amplification).
Irradiations. Unless otherwise stated, the irradiations are carried out under nitrogen, at room temperature, with a@ C o‘ source of 12 OOO Ci supplying a dose rate of 6.2 kGy h-l. Electron Spin Resonance. The spectra were recorded at room temperature on an ER 200 D 10 Bruker spectrometer. Results and Discussion General Change of E S R Spectra. The “initial” spectra, i.e., recorded right after irradiation, of the eight starches, studied in the dry state and under nitrogen, always show the so-called “AA’” shape (Figure 1, curve 1);the signal intensity decreases with time and its shape changes. The “final” spectra, recorded several months later (Figure 1, curve 2), display the “BB’” shape. Meanwhile we can find all the intermediate shapes (Figure 1, curve 3) between the two extremes: the shape change is faster if the irradiation is performed under oxygen or for increasing values of starch water content. The introduction of oxygen only changes suddenly the intensity and shape of the ESR signal but not the slope value of the later kinetics; the result is the same whether the irradiation is performed at low temperature (77 K) or at room temperature (storage temperature of the samples). In order to follow the quantitative changes in the spectra we have determined the different heights ”E”’of the positive and negative lobes (Figure 1)and recalculated them at constant amplification, then we have checked that YA and YA,, on the one hand, YB and YB, on the other hand, have parallel kinetic evolutions with time. However, the corresponding peak-to-peak widths AHAA, and A H B , are constant (the peak-to-peak width is defined here, according to Wertz and Bolton,14as the difference between the field values of the extrema of the negative and positive lobes of AA’ and BB’ lines). Thus we have considered the two lines AA’ and BB’ as corresponding to two radicals or groups of radicals with different patterns of change. We know that the approximate relative concentration of radicals [R.] may be obtained from the expression14 [Re] 0: WW2
(1)
(14)J. Wertz and J. Bolton in “ESR, Elementary Theory and Practical Applications”, McGraw-Hill, New York, 1972,p 34.
ESR Spectra of Irradiated Starches
The Journal of Physical Chemistry, Vol. 87, No. 13, 1983 2371
TABLE I: Average Values of g Factors and Peak-to-Peak Widths A H ( G )of Lines AA’ and BB’ line AA’
AH
g 2.0044 2.0049 2.0050 2.0049 2.0050 2.0048 2.0047 2.0048 2.0048 2.0047 2.0052 2.0059 d 2.0042
26.1 25.9 25.3 24.9 26.0 26.1 26.1 25.5 t
0.0006b
line BB’
studied producta 2.0054 2.0048 2.0050 2.0051 2.0051 2.0061 2.0055 2.0053
R P M B
H
25.7 k 0.5b 21.7 21.7 19.2 d 26.5
2.0052
MT
8.51 9.28 8.44 9.10 8.45 8.54 8.64 8.56 f
8.67 r 0.45b
0.0006b
‘
CMN CWM
CP RD
AH
g
MN AM WM
-
2.0 0 4 9 2.0O2gc 2,0026‘ 2.0051 2.005‘1~
-
8.32‘ 8.29‘ 8-16’ 8.5 gw
‘
Symbols defined in the Experimental Section. Average value of 30 experiments. Line only found after heating of the product a t 80-90 “C. Line not observed alone and, consequently, not measurable. e Poor accuracy due t o a weak signal-tonoise ratio.
Thus,we can follow for instance [ReAAj] by determination of the product Y u W € M ‘ ) 2and, as AHAAtis a constant, by the value of the height YAAtalone. These species AA’ and BB’ exhibit two signals which do not saturate for the same microwave power level; furthermore, the kinetic curves (in semilog scales) showing the evolution of BB’ are parallel, as the dose varies from 5 to 20 kGy but it is different for AA’. Hence, we will now consider irradiations of 20 kGy only, under nitrogen and a t room temperature. The kinetic evolution of AA’ cannot be observed after more than 3 weeks; indeed “A” becomes very weak with regard to “B”. For the same reason BB’ can only be followed several days after the irradiation. As can be seen in Figure 2, the graph showing the change of line BB’-and for AA’ as well-may be separated in four zones: in zone I the radical concentration decreases quickly according to complex kinetics; in zone I1 (from several days to 2 weeks) the kinetics seems to be of first order; in zone I11 (from 2 weeks to several months) there is another first-order law; in zone IV (after 6 months to 1year) the concentrations of BB’ radicals seems to be constant (AA’is not detectable in this final signal). Study of the BB’ line. The g factor and the peak-topeak width of the final line (Table I) are likely to be the same for the eight investigated starches; the accuracy is comparable to the one found for one starch, for instance, maize starch where AH = 8.5 f 0.4 G with 15 values of water contents from 2.2 to 15%. The limits of the zone I1 (Figure 2) are related to the water content W (the greater W the smaller the zone) and to the type of starch. The rate constant kII of the curve log [R’BB,]= log [R.,] - kII(BB’)t (4) is proportional to the water content k,(BB’)/ W = (31.6 f 7.4) X
(day)-’
but independent from the type of starch. On the other hand, the rate constant km of the following zone is independent of both the water content and the starch type: kIII(BB’) = (34 f 11) X (day)-’
Study of the AA‘ line. The g factor and the total peak-to-peak width of the AA’ line (Table I) are likely to be the same for the eight investigated starches.
I
25
5
lo
‘W,
Flgure 3. Relation between the water content W ( o / )of starches and the time t(days), deflned by Y , = YA’ after which the BB‘ form is predominant over AA‘ form ( b l o g scale). See Experimental Section for an explanation of the symbols.
Since the time range is smaller than for BB’, the accuracy of the kinetics is not as good; however, it can be noticed that the rate constant kII of the curve log [R’u’] = log [R*o]- kII(AA?t
(5)
is not proportional to the water content, but to its square. In other words, on a log-log scale, the most probable line for the function kIr(W) a straight line of gradient P = 1.9 f 0.6 This value, found in 26 experiments, becomes P M N = 2.0 f 0.1 if the experiments are only performed on maize starch. In the zone 111, the rate constant kIIr(AA’) is found independent of both the water content and the starch type. It is true even if the accuracy is not very good as A becomes quickly very weak with regard to B: kIII(AA’)
= (68 f 16) X
(day)-’
The Meaning of Kinetic Zones 11 and III. Obviously, the radicals or groups of radicals AA’ and BB’ have distinct chemical structures as the g value, the peak-to-peak width, and the kinetic laws for evolution of their ESR signals are different. Despite the fact that the type of starch does not
2372
The Journal of Physical Chemlstry, Vol. 87,No. 13, 1983
influence the values of the rate constants, it changes the shape of the spectrum: if we follow the predominance of BB’ with regard to AA’ by measuring the time t when YA = YB,the predominant part of the type of starch is obvious (Figure 3). However, this fact is unrelated to the range in zones I1 and 111. Thus we are lead to explain the origin of these zones by physical and not chemical reasons, as it was done elsewhere for acid hydrolysis of polyIn this hypothesis, the following applies. sa~charides.’~J”’~ Zone I1 should be due to the disappearance of AA’ and BB’ radicals induced in amorphous parts of starch: here the water velocity is important and the “local” concentration of water (or of its radiolysis products) is proportional to the average measure of the water content W. Consequently, kI1 is directly related to W . Zone I11 probably corresponds to the disappearance of these same radicals, but induced in crystalline parts of starch. These parts are considered neither to contain nor to absorb water.lbm This hypothesis put forth by Ahmed and Rapson in the case of celluloses is experimentally supported by works on deuteration exchange,” crystal structure determinations,l8 and acid hydr01ysis~~J~ of starches. The particular resistance of crystalline parts is well-known and, for example, there is a special experiment where potato starch, placed in contact with hydrochloric acid for 6 years, was found to be incompletely attacked!20 In order to ascertain this hypothesis, we have studied the kinetics of ESR signals from different sugars (MN, WM, and P crystallites, amylose, amylopectine, maltotriose, and radiodextrins) and found the following results: Crystallites as well as maltotriose, which is class under a crystalline state, show only one first-order law for AA’. In the case of crystallites the rate constant is very close to the values of knI found for starches (Table I). The AA’ shape is always predominant in the dry state, even 1year later; in order to get the BB’ shape we had to heat these products to about 80-90 “C. If g values are very similar, the peak-to-peak widths are generally smaller than for starch. Radiodextrins and commercial amylose also show only one first-order law but their rate constants are completely different from those found for starches, their structure being very degraded with regard to the one of native amylose. On the other hand, commercial amylopectin, which is the residue of acid hydrolysis of starch, shows two first-order kinetics, the rate constants of which are similar to the one of starches. If the shape of A‘ lobe is observed carefully, it is possible to distinguish two poorly resolved peaks Afl and Arz. Their relative importance and spacing depend on the starch type, the water content, and the considered kinetic zone (I1 or 111) but A’, and Af2may be respectively assigned to the contribution of the amylose and amylopectin parts of the starch (Figure 4).
Interpretation Many hypotheses have been put forth during the past 20 years, but the recent attempt at simulation and interpretation by Henderson and Rudin8 is probably the most interesting one. We have to mention that the aim of their ~~~~
~~
(15)J. P.Robin, Thesis A 0 12534,CNRS, Paris, 1976. (16)M. Nelson, J. Polym. Sci., 18,351 (1960). (17)S. Nara, H.Takeo, and T. Komiya, Staerke, 33, 329 (1981). (18)P. Zugenmaier and A. Sarko, Biopolymers, 75, 2121 (1976). (19)J. P. Robin, M. C. Tollier, and A. Guilbot, Food Preseru. Zrradiat., Proc. Int. Symp., 1977, 1, 529 (1978). (20)D.French, MTP Intern. Reu. Sci., Biochem., 5 , 267 (1975). (21)S. Gol’din, V. Sharpatyi, and S. Markevich, Dokl. Akad. Nauk SSSR, 201,133 (1971).
Raffi and Agnel I
12.0018
1.99371
Flgure 4. Shape of the A’ lobe of the ESR spectrum of irradiated starch: contribution of amylose and amylopectin parts: (curve 1) commercial amylopectin; (curve 2) native potato starch; (curve 3) commercial amylose.
work-radiation-grafting of polystyrene to starch-was different from ours. Consequently, irradiations were carried out at lower doses (1kGy in the dry state) on one particular starch (B) and the ESR spectra were recorded the same day. Thus they could not observe the BB’ form and they did not take it into account in the simulation of what we call the “initial” spectra. Nevertheless they assigned to AA‘ line a doublet structure related to the radical
.*.._
P
I
OH
We think that such a radical is the most probable as it may lead to a breaking of the glycosidic bond, which is the main event due to y-radiation as already checked by radiodepolymerizationl,z or acid hydr01ysis’~J~ studies. The BB’ line has been assignedz0to Roe2 radicals, resulting from action of gaseous oxygen traces on the radicals deriving from the break of the glycosidic link ....o ....o
These authors gave values similar to ours, i.e., AH = 8 f 1G, g, = 2.0074, and g,,= 2.0043, but other hypotheses6
are relative to structures which are independent of gaseous oxygen. Conclusion It would be interesting to do new simulation attempts8 taking into account the BB’ line. But we must emphasize that, as the powder ESR spectra are very poorly resolved, it is necessary to carry out experiments on radiolysis of simple sugars, not only in the powder state but also in the monocrystal form or by spin trapping techniques. However, a preliminary study of the shape and the kinetic change of ESR signals in irradiated starches was required to allow the choice of the type of sugars to be used as models for starch. In addition and from a toxicological
J. Phys. Chem. 1903, 87,2373-2376
point of view, we have to notice that the radical concentrations always are too small to induce any toxicity in starches; besides, these radicals are destroyed when starches are put in suspension in water. Acknowledgment. The authors are grateful to Dr. M.
2373
C. Tollier and J. P. Robin (INRA) for preparation of crystallites and structure discussions. We also thank Pr. P. Tordo (Universit6 de Marseille) for helpful ESR discussions. Registry No. Water, 7732-18-5.
Atomic Structure, Size, and Shape of Small Nickel Particles in Thin Films F. Vergand,' D. Fargues, Laboratoire de Chimie Physique (LA 176). Universl Pierre et Marie Curie, F-75231 Paris Cedex 05, France
D. Olivler, L. Bonnevlot, and M. Che Laboratoire de Chimie des Solides (ER 1331, Unversl Pierre et Marie Curie, F-75230 Paris Cedex 05, France (Received: May 7, 1982; In Final Form: December 14, 1982)
Structural properties of small nickel particles, embedded in aluminum or MgF2 films, are studied both by transmission high-energy electron diffraction (THEED) and by ferromagneticresonance (FMR). These methods lead to knowledge of the lattice parameter, found equal to the bulk one, of the size, in the range of 2-4 nm, and of the shape of the particles, plates parallel to the film. A magnetostriction effect is evidenced for the samples embedded in aluminum.
1. Introduction Studies on small metal particles have been widely developed during the last few years because of their interesting properties in various fields as catalysis, photographic processes, recording technology, etc. In particular, small nickel particles are often used as catalysts; knowledge of their electronic and structural properties is necessary to understand the chemisorptive processes and thus the catalytic mechanisms.' Small particles are generally supported or included in a matrix and an important question is to know what is the influence of the interfacial interactions on the particle properties. Having in mind a correlated study of the electronic structure and the structural arrangement of supported small nickel particles, we have chosen to study samples in the form of thin films which are convenient for both investigations. In order to vary the electronic interaction between the particles and the matrix, we have used two embedding materials having different structural and electronic characteristics: aluminum metal and magnesium fluoride. We describe here the preparation of the samples and the characterization of the particles: (i) their crystallographic structure by transmission high-energy electron diffraction (THEED), (ii) their shape by ferromagnetic resonance (FMR), (iii) their size by both methods. We also evidence the stress due to the matrix when it exists. The study of the electronic properties of the same samples is reported in separate paper^.^^^ 2. Sample Preparation The thin nickel films were prepared by vacuum deposition at room temperature in a residual pressure of Pa. They were embedded in matrices of evaporated alu(1) Cyrot-Lackmann,F. 'Growth and Properties of Metal Clusters"; Bourdon, J., Ed. Elsevier: Amsterdam, 1980; pp 241-54. (2) Vergand, F.; Fargues, D.; Belin, E.; Bonnelle, C. J.Phys. F. 1981, 11, 1887-93. (3) Fargues, D.; Vergand, F., to be submitted for publication.
0022-3654183/2087-2373$01.ti010
minum or magnesium fluoride to prevent nickel oxidation during the transfer into the diffractometer, the FMR spectrometer, or the X-ray spectrometer. The samples were obtained by depositing successively on a primary substrate the protection material, nickel, and the protection material again as summarized in Table I; thicknesses were controlled by a quartz oscillator microbalance.
3. Experimental Techniques Transmission high-energy diffraction patterns were performed at room temperature in a 50-keV Trub Tauber KD3 diffractometer. FMR spectra were recorded on a Varian spectrometer (Model CSE 109, X band) equipped with a 77-300 K variable-temperature accessory. The microwave power was kept constant at 10 mW. The intensities of the FMR lines were computed by using a double integration program. 4. THEED Results Substrates. Aluminum 15 nm thick films are polycrystalline. Electron diffraction line widths and electron microscopy analysis show that the mean crystal size is around 8 nm. The size distribution is f 3 nm wide. The high intensity of the (220) diffraction line evidences a partial texture of the sample, a large number of A1 crystals having the [loo] direction perpendicular to the plane of the film. MgFz films several tens of nanometers thick are polycrystalline with no texture; the mean particle size can be evaluated to 3 nm. Therefore, most nickel particles nucleate either on (100) faces of aluminum or on a variety of MgF2 faces. Nickel Particles. The presence of A1 or MgF2 lines renders difficult the analysis of electron diffraction patterns of nickel embedded in these materials. Nevertheless, in each case the main (hkl) maxima of nickel can be observed for the values of 41r sin 8 / A corres onding to the fccub bulk metal; we find a = 3.52 f 0.02 (Figure 1; see also Figure I in ref 2). Note that the interatomic distance of A1 is 13% greater than that of Ni. Hence, we can expect
g:
0 1983 American Chemical Society
