Article pubs.acs.org/jced
Thermophysical Properties of VX and RVX David E. Tevault,*,† Ann Brozena,† James H. Buchanan,† Patrice L. Abercrombie-Thomas,† and Leonard C. Buettner‡ †
Edgewood Chemical Biological Center, Aberdeen Proving Ground, Maryland 21010, United States EXCET, Inc., 8001 Braddock Road, Suite 303, Springfield, Virginia 22151, United States
‡
ABSTRACT: Selected thermophysical properties are reported for VX (Oethyl-S-[2(diisopropylamino)ethyl] methylphosphonothiolate) and its isomer, RVX (O-isobutyl-S-[2(diethylamino)ethyl] methylphosphonothiolate). Several properties have been reported previously for both compounds; the focus of the current work has been the measurement of additional properties and expansion of the experimental ranges for the existing data. This report consolidates and compares physical property data measured in our laboratory for both compounds. It is important to know the physical properties of these supertoxic materials accurately to understand quantitatively the threat posed by them during military conflict, perform testing of defensive equipment, and determine the necessity to perform decontamination procedures. Knowledge of the properties of these toxic materials facilitates the selection of lower-toxicity candidates to simulate their behavior during testing where the use of the toxic agent is inappropriate. The current work has employed standard American Society for Testing and Materials (ASTM) international methods, intact and modified, to measure physical properties of VX and RVX. Properties investigated in the present work include liquid density, viscosity, surface tension, flash point, vapor pressure, heats of vaporization, and volatility. We also report correlations determined from previously available and new data, where appropriate.
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INTRODUCTION This report documents new density, viscosity, surface tension, flash point, and vapor pressure data measured recently in our laboratory for two supertoxic chemicals, VX (O-ethyl-S-
Knowledge of the physical properties and toxicology of chemical warfare agents such as VX and RVX is required to assess the threat posed by these materials following their dissemination and to select appropriate surrogates for the testing of defensive equipment. Physical properties dictate practical considerations such as how much agent can be loaded into a fixed volume (density), how it will spread (viscosity), how it will break up when released at high speed (surface tension), and suitability for explosive release (flash point). Vapor pressure is a critical physical property for understanding the behavior of chemicals in the environment. Knowledge of the vapor pressure is important for a number of applications such as testing of defensive and emergency response equipment, removal by air filtration systems, and other applications. A recent report from our laboratory1 detailed methods and measured vapor pressure data for a series of organophosphorus nerve agent surrogates. Those methods are routinely used in our laboratory to measure the vapor pressure of highly toxic materials and were used in the current work for VX and RVX. The measurement of VX and RVX physical property data presents a significant challenge owing to the extreme toxicity of these materials and requires stringent handling protocols. In the present work, the major experimental difficulty was noted during the measurement of ambient-temperature vapor
Figure 1. Structures of VX (top) and RVX (bottom).
[2(diisopropylamino)ethyl] methylphosphonothiolate) and its isomer, RVX (O-isobutyl-S-[2(diethylamino)ethyl] methylphosphonothiolate), structures shown in Figure 1. In addition, data generated in our laboratory over the past several decades for these compounds are included. The vapor pressure correlations for VX and RVX have been used to infer heats of vaporization and volatility. Data measured in the earlier work are clearly identified in the accompanying discussion and tables. Selected data are used to develop correlations that allow accurate interpolation and limited extrapolation of the measured data. This article not subject to U.S. Copyright. Published 2012 by the American Chemical Society
Received: February 2, 2012 Accepted: May 23, 2012 Published: June 13, 2012 1970
dx.doi.org/10.1021/je200891j | J. Chem. Eng. Data 2012, 57, 1970−1977
Journal of Chemical & Engineering Data
Article
the current work at T = (238.15 to 323.15) K by following ASTM D 445, “Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and the Calculation of Dynamic Viscosity)”,5 using a series of Cannon−Manning semi-micro viscometers with recommended viscosity ranges between (7 and 1200) mPa·s, a Cannon CT-1000 constant temperature bath for ambient temperature data, and a Cannon TE-1000 constant temperature bath for subambient data. Viscosity is measured by determining the time required for a fixed volume of liquid to pass through a calibrated, glass, precision-bore, capillary viscometer. Kinematic viscosity values were converted to dynamic viscosity (μ) by multiplying the measured values by the calculated density at the experimental temperature. The surface tension (σ) of VX had been measured previously by Coulter et al.2 at T = (298.15 to 323.15) K using two different methods (a Cassel tensiometer and the capillary-rise method) on three different samples. New data have been measured for both VX and RVX at T = (273.15 to 323.15) K using a Kruss K12 tensiometer equipped with an external Lauda RM-6 circulating bath according to the Wilhelmy plate method.6−8 The surface tension was determined by measuring the force acting on a glass coverslip as it is withdrawn from the liquid. The flash point of VX had been previously determined by Coulter et al.2 using an open cup method.9 The flash points of both VX and RVX were determined in this work in accordance with ASTM D 6450, “Standard Test Method for Flash Point by Continuously Closed Cup (CCCFP) Tester”,10 using a Grabner FLPH Miniflash Tester. The continuously closed cup method defines the occurrence of a flash as the lowest temperature at which the ignited vapor over the liquid specimen exceeds a predefined pressure threshold. The vapor pressure of VX has been studied previously by several investigators. Savage and Fielder11 used Knudsen effusion at T = (325.15 to 373.35) K, Belkin and Brown12 used differential thermal analysis (DTA) at T = (385.65 to 510.15) K, and Rittfeldt13 used vapor saturation at T = (261.15 to 376.15) K. The latter work reported a Clausius−Clapeyron equation but did not publish the numerical values. The present VX work used a modified vapor saturation technique at T = (260.15 to 293.15) K, based on ASTM methodology,14 and has been described in detail previously in an in-house publication.15 The vapor pressure of RVX, published previously in-house,16 was measured at T = (263.15 to 291.25) K using the vapor saturation method and at T = (418.75 to 505.15) K using DTA in accordance with an ASTM method employing thermal analysis techniques.17 Rittfeldt13 also reported a Clausius− Clapeyron equation for RVX measured using the saturation method at T = (261.15 to 376.15) K. The modified ASTM gas saturation method was implemented for ambient-range vapor pressure measurements in combination with sample concentration and GC analysis using flame photometric detection (FPD). Both analytes were measured directly, that is, without conversion to their fluorinated analogues, to preclude the possibility of experimental error resulting from inadvertent conversion of impurities to the respective fluorinated analogues. This method extends the measurable vapor pressure range to below 10−6 Pa for phosphorus-containing species,18 corresponding to subambient temperatures for VX and RVX, which have normal boiling points approaching 570 K. These modifications have the distinct advantage of enabling accurate vapor pressure measure-
pressure data due to the low volatility of these materials and the presence of higher-volatility impurities. Modifications to the ASTM vapor saturation method have allowed us to overcome this challenge.
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EXPERIMENTAL SECTION VX and RVX are potent cholinesterase inhibitors and should only be handled in suitable facilities by skilled workers using extreme caution under appropriate engineering controls. Owing to the extreme toxicity of these materials and lack of commercial availability, all VX and RVX samples were synthesized and purified in-house. The purities of all samples and temperature ranges for all measurements are listed in Table 1. For the data measured by the authors, sample purity was assessed using nuclear magnetic resonance (NMR) spectroscopy or gas chromatography (GC). Table 1. Sample Purity and Temperature Ranges for Properties Reported Hereina property vapor pressure (vapor saturation) vapor pressure (Knudsen effusion) vapor pressure (DTA) density viscosity surface tension
temperature range/K
purity/(mole fraction)
VX 260.1 to 293.1
0.946
15
325.1 to 373.1
0.890
11
382.1 298.1 233.1 298.1 273.1 298.1 298.1 298.1
0.919 b 0.935 b 0.963 c d e 0.958
12 2 4 2 this work 2 2 2 this work
0.738
16
0.927 0.927 0.960 0.927 0.892 0.957
16 this this this this this
to to to to to to to to
510.1 323.1 253.1 323.1 323.1 323.1 323.1 323.1
flash point vapor pressure (vapor saturation) vapor pressure (DTA) density viscosity surface tension flash point
RVX 263.1 to 291.2 413.1 298.1 238.4 298.1 273.1
to to to to to
505.1 323.1 283.1 323.1 323.1
reference
work work work work work
a
Elemental analysis calculated for C11H26NO2PS: C, 49.41; H, 9.80; P, 11.58; N, 5.24; S, 11.99. bElemental analysis found: C, 49.8; H, 9.8; P, 11.53; N, 5.25; S, 11.97. cElemental analysis found: C, 49.5; H, 9.9; P, 11.68; N, 5.19; S, 12.2. dElemental analysis found: C, 49.3; H, 8.66; P, 11.40; N, 5.4; S, 12.18. eNo elemental analysis data available.
The VX density (ρ) has been measured previously by Coulter et al.2 using the dilatometer method at T = (298.15 to 323.15) K. New density data have been measured for RVX, also at T = (298.15 to 323.15) K using an Anton Paar model DMA 55 digital density meter in accordance with ASTM D 4052, “Standard Test Method for Density and Relative Density of Liquids by Digital Density Meter”,3 which is based on the vibrational frequency response of a quartz U-tube loaded with the analyte. Kinematic viscosity (ν) data for VX were measured previously using capillary viscometry by Fielder and Beck4 at T = (233.15 to 253.15) K and by Coulter et al.2 at T = (298.15 to 323.15) K. New RVX viscosity data have been measured in 1971
dx.doi.org/10.1021/je200891j | J. Chem. Eng. Data 2012, 57, 1970−1977
Journal of Chemical & Engineering Data
Article
Table 2. Experimental and Correlated Density for VX and RVX VXa ρexpt T/K 298.15 308.15 323.15 a
kg·m
ρcalc
−3
1008.3 1000.1 987.6
RVXb
kg·m
−3
ρexpt percent difference
1008.33 −0.003 1000.04 0.006 987.62 −0.002 ρVX = 1255.32 − 0.82842·T/K
c
kg·m
ρcalc
−3
1006.44 998.45 986.14
kg·m−3
percent differencec
1006.51 998.38 986.19 ρRVX = 1248.81 − 0.81268·T/K
−0.007 0.007 −0.005
Reference 2. bThis work (bold); the uncertainty of these measurements is 0.01 %. c100·(ρexpt − ρcalc)/ρcalc.
ments for impure low-volatility materials in the presence of higher-volatility impurities and were particularly valuable for RVX since the sample used in that work had a purity of 0.738 mole fraction. Prior to data collection, all test specimens were loaded into glass saturators and purged extensively using UHP dried nitrogen (dew point
