Nanomaterials continue to bring promising advances to science and technology. In concert have come calls for increased regulatory oversight to ensure their appropriate identification and evaluation, which has led to extensive discussions about nanomaterial definitions. Numerous nanomaterial definitions have been proposed by government, industry, and standards organizations. We conducted a comprehensive comparative assessment of existing nanomaterial definitions put forward by governments to highlight their similarities and differences. We found that the size limits used in different definitions were inconsistent, as were considerations of other elements, including agglomerates and aggregates, distributional thresholds, novel properties, and solubility. Other important differences included consideration of number size distributions versus weight distributions and natural versus intentionally-manufactured materials. Overall, the definitions we compared were not in alignment, which may lead to inconsistent identification and evaluation of nanomaterials and could have adverse impacts on commerce and public perceptions of nanotechnology. We recommend a set of considerations that future discussions of nanomaterial definitions should consider for describing materials and assessing their potential for health and environmental impacts using risk-based approaches within existing assessment frameworks. Our intent is to initiate a dialogue aimed at achieving greater clarity in identifying those nanomaterials that may require additional evaluation, not to propose a formal definition.

Advances in adding nanomaterials to various matrices have occurred in tandem with the identification of potential hazards associated with exposure to pure forms of nanomaterials. We searched multiple research publication databases and found that, relative to data generated on potential nanomaterial hazards or exposures, very little attention has focused on understanding the potential and conditions for release of nanomaterials from nanocomposites. However, as a prerequisite to exposure studying release is necessary to inform risk assessments. We identified fifty-four studies that specifically investigated the release of nanomaterials, and review them in the following release scenario groupings: machining, weathering, washing, contact and incineration. While all of the identified studies provided useful information, only half were controlled experiments. Based on these data, the debris released from solid, non-food nanocomposites contains in varying frequencies, a mixture of four types of debris. Most frequently identified are (1) particles of matrix alone, and slightly less often, the (2) matrix particles exhibit the nanomaterial partially or fully embedded; far less frequently is (3) the added nanomaterial entirely dissociated from the matrix identified: and most rare are (4) dissolved ionic forms of the added nanomaterial. The occurrence of specific debris types appeared to be dependent on the specific release scenario and environment. These data highlight that release from nanocomposites can take multiple forms and that additional research and guidance would be beneficial, allowing for more consistent characterization of the release potential of nanomaterials. In addition, these data support calls for method validation and standardization, as well as understanding how laboratory release scenarios relate to real-world conditions. Importantly, as risk is considered to be a function of the inherent hazards of a substance and the actual potential for exposure, data on nanomaterial release dynamics and debris composition from commercially relevant nanocomposites are a valuable starting point for consideration in fate and transport modeling, exposure assessment, and risk assessment frameworks for nanomaterials.

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Aggregation numbers (Nagg) and critical micelle concentrations (cmc) are reported for distearyldimethylammonium (DSDMA) hydroxide and chloride in aqueous solutions with and without added salt. The DSDMA hydroxide has a higher Nagg and cmc than either the chloride, bromide, or methyl sulfate DSDMA salts. The measured cmc's were 1.5 x M for DSDMA OH and 3.0 x lo-' M for DSDMA C1. The aggregation number for the DSDMA C1 is 63 (0.05 M NaC1) vs 6000 for the hydroxide at the same NaOH concentration. The DSDMA OH is highly soluble in water, forming spherical vesicles of size RH = 300 A, and does not flocculate upon addition of salt. The DSDMA OH vesicles revert to micelles with increasing surfactant concentration having an aggregation number of approximately 60. The same aggregation number for the hydroxide can be obtained by raising the salt concentration to 1.0-2.0 M NaOH, keeping the surfactant concentration constant. In the overlapping range with respect to DSDMA OH concentrations and salt, the appropriate hydrodynamic description is that of thread-like micelles, as determined from inelastic light scattering experiments. In dilute DSDMA OH and 0.01 M NaOH solutions, the first normal mode of the chains (TI) was isolated from the decay time spectrum as a function of measurement angle. The z1 value supports the free-draining Zimm model when compared to theoretical calculations. At semidilute surfactant concentrations and 0.8 M NaOH, or at higher surfactant concentrations and low salt concentrations (0.02 M; above which the solutions become viscoelastic), the decay time distribution is bimodal with two well-separated components on the inelastic time scale. The faster $-dependent component is due to cooperative motions of a transient network being formed through the interchain entanglements. The slow q-independent component is of large amplitude and reflects the disruptiodcoalescence kinetics of the micellar aggregates, which are characterized by a strong positive concentration dependence of the obtained relaxation rate. The relaxation time decreases with increasing concentration of salt and DSDMA OH. The results obtained for DSDMA OH, which are in contrast to DSDMA C1 and other halides, can be interpreted according to theories of solutions of long flexible polymer chains within the semidilute concentration range where the DSDMA OH polymer coils overlap by forming a network of mesh size 5 = 815 A. This has been found for DSDMA OH in the concentration range of c2 = (20-50) x lop3 M (7.44c2*-18.6~2*) or at low surfactant concentrations (2.0 x M, 7 . 4 4~" ) and salt concentrations cs = 0.55-0.75 M NaOH. Above DSDMA OH concentrations of 0.08 M the aggregation numbers decreased to 61, leaving micelles of hydrodynamic radius 25.0 A which are stable at high salt concentrations. The transition of DSDMA OH vesicles to long flexible entangled micelles to spherical micelles of diameter 50 8, is reversible by changing the surfactant or salt concentration. These results strongly support a model of long flexible entangled DSDMA OH mi...

A sensitive conductance method has been used to determine binary and ternary diffusion coefficients of distearyldimethylammonium chloride, bromide, and hydroxide (DSDMACl, DSDMABr, and DSDMAOH) in aqueous solutions in the absence and presence of salt. Experiments were conducted above and below the critical micelle concentration (CMC) at 25°C, and the concentration of salt, when present, was 0.015 M NaX (X ) Cl, OH, Br). The experiments reveal that diffusion of DSDMA chloride, bromide, and especially hydroxide induces concurrent flow of NaCl, NaBr, and NaOH, respectively. Also, diffusion of DSDMAX in the presence of NaCl drives countertransport of DSDMACl or DSDMAOH. At pH 7.5 and an ionic strength of 0.015 M, each mole of diffusing DSDMACl (N agg ) 72, R ) 0.15) cotransports 9.0 mol of NaCl. However, coupled flow of NaCl decreases as the solution pH approaches 5.2 and is almost zero at pH 4.5. The corresponding value for diffusing DSDMAOH at pH 7.5 is about 120 mol of NaCl. At pH 6.5 only 10 mol of NaCl are cotransported, and the value is almost zero at pH 5.5. However, at pH 4.5, diffusing DSDMAOH generates a counterflow of NaCl due to bound protons and a release of H 2 O (DSDMAOH + H + T DSDMA + + H 2 O), yielding a net positive charge; hence, the D 21 value is negative. The cross-diffusion coefficient D 21 of diffusing DSDMACl in the presence of 0.015 M (0.095-0.009) NaOH and 0.015 M NaCl yields a flux density of NaOH and NaCl, which is generated by the DSDMAX (X ) Cl + OH) gradient. A similar cross-diffusion coefficient, D 21 , is revealed when DSDMAOH, is used, instead of DSDMACl, in the presence of 0.015 M NaCl through induced coupled transport of salt. The coupled transport of NaCl or of NaCl and NaOH is primarily driven by the diffusion-induced electric field along the DSDMAOH and DSDMACl concentration gradients. For the DSDMAOH system, the direction and magnitude of the coupled flow of either NaCl or NaOH can be explained through the ionic mobility of DSDMA + and OH -, in addition to the charge and degree of dissociation (N agg ) 6400, R ) 0.33-0.35) at 25°C. The tracer and mutual diffusion coefficients for the DSDMAX system were compared with those from light-scattering measurements of polyelectrolyte solutions of DSDMAX.