Electrical Conductivities of Shock-compressed Solutions of KI in Organic Solvents B Y SEFTON D. HAMANN* AND MAX LtNTON CSIRO Applied Chemistry Laboratories, G.P.O. Box 433 1, Melbourne, Victoria 3001, Australia Received 14th February, 1978 We have measured the electrical conductivities of solutions of KI in methanol, formamide, N-methylformamide (NMF), N,N-dimethylformamide (DMF), dimethylsulphoxide (DMSO) and acetone, under compression by shock waves with pressures of the order of 10GPa. The results indicate that acetone and NMF become more viscous in the shocked state and the DMF and DMSO become very much more so. The shock-wave conductivities of formamide, NMF, DMF and DMSO are consistent with static conductivities measured at much lower pressures. Transparency experiments showed no evidence of freezing in DMF and acetone at shock pressures between 1 and 10GPa.We have previously described measurements of the electrical conductivities of water 9 and aqueous solutions of strong electrolyte^,^ compressed by explosively driven shock waves. The initial conditions of pressure, temperature and density in the shocked states were usually in the ranges 5-13 GPa (1 GPa = lo4 bar E 9869 atm), 500-1 100 K and 1.48-1.73 g ~ m - ~ , respectively. Rather surprisingly, the results showed that the mobilities of dissolved ions (other than H+ and OH-) are little affected by shock compression. By inference, the viscosity of water is also little affe~ted,~ and we suggested 39 that the reason is that, although the pressure jump in a shock front tends to raise the vis~osity,~ the accompanying temperature jump has an opposite and compensating effect.The aim of the present work has been to see whether a similar compensation occurs in nm-aqueous solutions. To that end, we have measured the shock-wave conductivities of both the pure solvents and of solutions of a strong electrolyte, KI, in methanol, formamide, N-methylformamide (NMF), N,N-dimethylformamide (DMF), dimethylsulphoxide (DMSO) and acetone. For comparison, we have also measured the influence of lower static pressures on the conductivities of KI in formamide, NMF, DMF and DMSO at 25, 35 and 45°C. In addition, we have examined the transparency of shocked acetone and DMF, in the hope of establishing whether or not the liquids undergo partial freezing in the s hock-compressed state.EXPERIMENTAL The solvents were purified by standard methods and dried over molecular sieves. The technique employed in the shock wave conductivity measurements was the same as that used earlier for aqueous solution^.^ The conductivity cells were of the type designated E,2 with the thickness I of the electrolyte layer adjusted to 1 mm. Before shock compres- sion, the solutions were at ambient temperature and pressure, In the transparency experiments, shock waves in the bulk liquids were viewed transversely by back illumination from a long-duration argon flash source, in the geometry described 2742S . D . HAMANN A N D M. LINTON 2743 by David and Ewald.' They were photographed by a high-speed cine camera, which took a total of 60 frames at 0 .2 7 ~ s intervals and which followed the shock waves from their launching until they had travelled about 6 cm. The static conductivity measurements were made in a temperature-controlled Teflon cell 8 g and the change in cell constant under pressure was calculated from the known compressibility of Teflon. lo- RESULTS SHOCK CONDUCTIVITIES A few representative oscillograms are shown in fig. 1-5, where the numbering and lettering of the traces are the same as previ~usly.~ The conductivities 1c of the solution in the pre-shocked (A-B) and shocked (B-F) states were calculated from the traces in the manner described earlier,3 allowance being made for the effect of the shock compression of the cell on its cell constant. 4 2ps I+ time FIG. 1.-Oscillogram for formamide at a shock pressure of 9.6 GPa.FIG. 2.-Oscillogram for a 0.1 mol dm-3 solution of formamide in water at a shock pressure of 9.6 GPa. 1-872744 SHOCK CONDUCTIVITIES The pressure of the shocked liquids was not measured directly, but can be assumed to have been close to the corresponding shock-wave pressure in the surrounding polyethylene cell : the two would certainly have become equal after a few reverberations in the thin liquid layer, that is, after about 0.5 p s . The value of the pressure quoted in this paper is that in the polyethylene cell at the time when the wave reached the top of the liquid layer.2 The temperature of the shocked liquids is unknown, but was probably in the range 500-1500°C. Y +[ 2PS I+- time FIG, 3.-Oscillogram for a 0.1 mol dm-3 solution of KI in formamide at a shock pressure of 7.3 GPa.-4 2E.Ls I+ time FIG. 4.-Oscillogram for a 0.1 mol dm-3 solution of KI in DMF at a shock pressure of 7.3 GPa. METHANOL The shock conductivity of pure methanol was examined some years ago by David and Hamann,I2 who found that it behaves rather like the conductivity of water,'. in that K rises steeply from a low initial value to a relatively very high value immediately behind the front of a strong shock wave. For instance, a 10 GPa wave causes K toS . D. HAMANN A N D M. LINTON 2745 jump from less than to about 0-l cm-l. David and Hamann attributed this jump to an enhancement, at the high pressure and temperature of the shock wave, of the self-ionization of methanol : The conductivity subsequently decays in the tail of the wave as the pressure drops and the ions recombine : in the particular geometry of the experiments, the duration of the conductivity pulse at half-peak height is about 0.5 p.The present measurements have been made on two solutions of KI in methanol at concentrations of 0.1 and 0.8 mol dm-3 * and at 11.6 and 7.3 GPa, respectively. 2CH30H f. CH3OH; + CH30-. (1) 0 2 1 1 -4 2P.s I+ time FIG. 5.-Oscillogram for a 0.06 mol dm-3 solution of KI in acetone at a shock pressure of 7.3 GPa . The oscillograms show both short initial pulses ( ~ 0 . 5 p s ) due to the temporary self-ionization of methanol and later, protracted, periods ( N" 5 ps) of increased conductivity attributable to the dissolved KI. The oscillograms at the two pressures resemble those for aqueous solutions of KC1, shown in fig.3 and 2 of ref. (3), respec- tively, and can be interpreted similarly. FORMAMIDE We examined the behaviour of pure forrnamide at four different shock pressures. The oscillogram in fig. 1, for 9.6 GPa, shows that strongly shocked formamide develops quite a high conductivity. The con 'uctivity IC is very low initially (A-B), but when the shock wave arrives there is a fast rise to C, where K = 0.045 R-l cm-I, followed by a slower and protracted rise to E, where K = 0.167 0-l cm-I. Corres- ponding values measured in the other three experiments at lower pressures are listed in table 1. We interpret this behaviour as follows. First, the fast initial rise has the character- istics of conductivity arising from self-ionization of the liquid, enhanced by the high pressure and temperature in the shock wave.David and Hamann l2 observed similar sharp conductivity pulses in other shocked organic liquids in cases where their autoprotolysis constants were > 10-1 mo12 dm-6 at normal temperature and * The concentrations quoted in this paper are those of the pre-shodced solutions at 25°C and atmospheric pressure.2746 SHOCK CONDUCTIVITIES pressure. The autoprotolysis constant of formamide l3 is 1.6 x 1O-l' mo12 dm-', and presumably relates to the reaction 2HCONH, + HCONH; +HCONH-. (2) Second, the subsequent protracted rise of conductivity from C to E indicates the formation of new stable ionic species which, unlike the very reactive ions formed by reaction (2), survive the release of pressure and temperature in the decaying tail of the shock wave.The whole behaviour between C and F, and after F, is in fact remarkably like the behaviour of a shocked solution of KCl in water [see fig. 4 of ref. (3)]. We suggest that these new ions are ammonium and formate, produced by thermal- and pressure-induced decomposition of forinamide : (3) This suggestion is supported by some recovery experiments in which we found that formamide that had been recovered after shock compression to about 10 GPa smelt strongly of ammonia and gave a mass spectrum containing peaks for formic acid, which, in the presence of excess ammonia, must have existed in solution as ammonium formate. We also found that an analogous reaction occurs when a solution of forma- mide in water is shock-compressed : (4) This reaction (4) is simply the reverse of the well-known thermal decomposition of ammonium formate into formamide and water at low pressures.The oscillogram in fig. 2 for an aqueous solution of formamide shows a sharp pulse C, arising from enhanced autoprotolysis of the water,3 followed by a sustained residual conductivity E-F due to the ammonium formate. A similarly shocked aqueous solution of urea, H2NCONHz, gives no residual conductivity. 2HCONH2 + NHZ + HCO? + HCN. HCONH, + H20 -+ NH: + HCOT. TABLE 1 .-ELECTRICAL CONDUCTIVITY OF SHOCK-COMPRESSED FORMAMIDE conductivity1 shock pressure/ sd-1 cm-1 at: GPa Ca Ea 9.6 0.045 0.167 8.4 0.008 0.027 7.9 0.002 0.007 7.3 O.OWb 0.002 a The points C and E are indicated on the oscillogram in fig.1. b This conductivity was below the limit of measurement, 0.0005 i 2 - I cm-'. We carried out two experiments on solutions of KI in formamide, one at a con- centration of 0.1 mol dm-3 and the other at 0.3 mol dm-3, both at a shock pressure of 7.3 GPa, which is low enough to prevent the formamide conductivity (0-0.002 R-' cm-I, see table 1) from masking the KI conductivity (0.003-0.013 0-l cm-l). Fig. 3 shows the results for the 0.1 mol dm-3 solution. It will be seen that the arrival of the shock wave causes a temporary drop in current between B and D, but, when correction is made for the change of cell constant caused by compression of the liquid, the conductivity is almost unchanged in that region. Subsequently the conductivity increases to E as the liquid expands and remains irreversibly heated.3 The behaviour of the 0.3 rnol dm-3 solution is very similar.N-METHYLFORMAMIDE (NMF) 0-l cm-l) at a shock pressure of 9.6 GPa. Unlike formamide, pure NMF gives no detectable conductivity (i.e. K < 0.0005S . D . HAMANN AND M . LINTON 2747 A 0.1 mol dm-3 solution of KI in NMF, shocked to 7.3 GPa, gives an oscillogram very much like that for the formamide solution in fig. 3. The conductivity is reduced by a factor of 0.9 immediately behind the shock front but rises to twice its initial value in the subsequent expansion phase. N,WD I MET H Y L F o RM A M I D E ( D M F) Pure DMF also gives no detectable conductivity in 7.3 or 9.6 GPa shock waves. Fig. 4 shows the oscillogram for a 0.1 mol dm-3 solution of KI in DMF com- pressed by a 7.3 GPa shock wave.The initial conductivity A-B drops to " zero " (i.e. IC < 0.0005 C2-l cm-I) immediately behind the shock front, €3-D, but later rises slowly to slightly more than its original value, at E. TABLE 2 .-RELATIVE CONDUCTIVITIES OF SOLUTIONS OF KI UNDER STATIC COMPRESSION solvent temp./"(= KO/R-~ cm-la formamide 25 0.002 35 35 45 NMF 25 0.003 86 35 45 DMF 25 0.005 21 35 45 DMSO 25 0.003 02 35 45 KlKOQ pressurelGPa 0.000 0.100 0.200 1.000 0.739 0.568 1.390 1.054 0.806 1.690 1.312 1.038 1.000 0.715 0.528 1.181 0.858 0.642 1.350 0.997 0.764 1.OOO 0.704 0.518 1.155 0.833 0.628 1.298 0.960 0.729 1.205 0.758 1.401 0.914 0.636 1.000 b Throughout, the concentration of KI was 0.1 rnol dm-3 at 25°C and atmospheric pressure a K~ denotes the conductivity at 25°C and atmospheric pressure (= 0.0001 GPa).b Measurements for DMSO solutions were limited by freezing of the solvent at the higher pressures at 25 and 35°C. DIMETHY L s u L PHOXIDE ( DMS 0) A shock wave of 7.3 GPa produces no detectable conductivity in pure DMSO. However, a shock of the same pressure reduces the conductivity of a 0.1 mol dm-3 solution of KI in DMSO to " zero ", initially, and gives an oscillogram very similar to the one on fig. 4. ACETONE Although pure acetone develops a slight conductivity, rather slowly, in 10 GPa shock waves l4 (the conductivity may arise from decomposition products),1s it has no measurable conductivity at 7.3 GPa. Fig. 5 shows an oscillogram for a 0.06mol dm-3 solution of KI in acetone, compressed by a 7.3 GPa shock wave. The initial effect of the shock is to reduce the conductivity, at B-D, to about half its value in the pre-shocked state A-B.TRANSPARENCY EXPERIMENTS The transparency experiments were carried out on DMF and acetone, and followed the shock waves from their launching, at initial pressures of 13 and 11 GPa respec- tively, until they had travelled 6 cm into the liquids and their pressures had dropped to2748 SHOCK CONDUCTIVITIES ~1 GPa. At no stage was there any sign of opacity or partial opacity behind the shock fronts. STATIC CONDUCTIVITIES The results of the static measurenients are listed in table 2 in the form of the ratio of the measured conductivity IC to that of the same solution at 25°C and atmospheric pressure I C ~ . They all relate to solutions having a concentration of 0.1 mol dm- of KI at 25°C and atmospheric pressure.DISCUSSION In this work we are interested principally in the conductivities K , of shocked solutions of strong electrolytes in regions immediately behind the fronts of strong shock waves, that is, in the regions B-D in the diagrams. The behaviour we have observed there can be summarized as follows : (i) for solutions in water,3 methanol and formamide, K, is close to the conductivity K~ of the preshocked solution at normal temperature and pressure, (ii) for solutions in NMF, K , is about 0.9rio, (iii) for solutions in acetone, IC, is about O . ~ K ~ , (iv) for solutions in DMF and DMSO, K , is too small to measure and is certainly less than one tenth of I C ~ . In general, the shock-induced changes in 7c could arise either from changes in the mobilities of the K+ and I- ions or, if the KI were not fully dissociated, from changes in their concentrations.Earlier conductivity measurements at normal temperature and pressure have shown that KI is completely dissociated in methanol,16 forma- mide,17 NMF,I* DMF 1 9 9 2o and DMS0,21 but is partly associated as ion pairs in acetone.22 There are theoretical and experimental reasons for believing that dissociated salts remain dissociated under shock compression and ion pairs tend to dissociate, and it follows that the changes in IC for the first five solvents reflect changes in ionic mobility, whereas the change for acetone probably includes a contribution from enhanced dissociation. On that basis we interpret (i) as indicating that the mobilities of dissolved ions in water, methanol and formamide are not much affected by the shock conditions, while (ii) indicates that the mobilities in NMF are measurably reduced and (iv) shows that the mobilities in DMF and DMSO are very much reduced.In acetone, the mobilities probably drop by more than the 50 % decrease shown by the conductivity (iii). Since the transparency experiments gave no signs of partial freezing in shocked DMF and acetone, we can presume that the reduction of ionic mobilities in these solvents (and probably also in NMF and DMSO) arises from increases in their vi~cosities.~ For these liquids, the tendency of the high shock pressure to raise the viscosity evidently outweighs the tendency of the high temperature to lower it.Finally, for comparison with the above results for strong shock waves, we can use the static data in table 2 to estimate the response of some of the solutions to weak shock compression. From the p-v-T behaviour 23 and specific heat of DMF, we calculate that a 0.2 GPa shock wave would raise the temperature by 22.7"C, so that I C , / K ~ would be close to the value K ] K ~ = 0.764 at 45°C and 0.2 GPa. The corres- ponding values of IC/IC, for the other solvents in table 2 (whose p-v-T behaviour has not been measured, but is probably similar to that of DMF) decrease in the order formamide > NMF > DMF > DMSO. This is the order in which the strong shock conductivities I C , / K ~ decrease (except that the very low values of I C , / K ~ for DMF and DMSO are indistinguishable) and so the static and shock-wave results are consistent, to that extent, in spite of the large difference in their pressures.S .D . HAMANN AND M . LINTON 2749 We are indebted to W. Connick, D. J. Pinson and M. Wolfson of the Materials Research Laboratories, Australian Department of Defence, for taking the shock- wave photographs, and to IS. G. Carey for help with experimental work. H. G. David and S . 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