Particles, hygroscopic growth

Chemical Properties of Aerosols. Surface chemical properties of aerosol particles can also be tailored to improve deaggregation [273]. Hygroscopic particles absorb water when inhaled into the humid airways [282-284], increasing particle size and density in the process, as well as creating the potential for capillary bridge formation between particles. Hygroscopic growth can be reduced by the use of hydrophobic additives [285] or compounds with low aqueous solubility [286,287]. 

United States that had been impacted by anthropogenic emissions had a hygroscopic growth factor of 1.7 0.1, compared to 2.7 0.4 for particles in marine... 

The interaction between molecules in the gas phase and the surface of particles and droplets in the atmosphere is central to important phenomena such as ozone depletion in polar areas, hygroscopic growth of particles, aging, acid rain, particle growth and cloud droplet formation. 

Berg OH, Swietlicki E, Krejci R (1998) Hygroscopic growth of aerosol particles in the marine boundary layer over the Pacific and Southern Oceans during the First Aerosol Characterization Experiment (ACE 1). J Geophys Res 103 16535-16545... 

The second reason for the difference between the deposition of stable aerosols and many therapeutic aerosols is hygroscopic growth and evaporation. For drugs to be effective, they must show an appreciable aqueous solubility. Solid drug particles may pick up water vapor and dissolve, and droplets of drug solutions can exchange water with the environment to equalize vapor pressure. The relative humidity profile in the respiratory tract depends on the ambient conditions and the breathing pattern [67]. 

Ferron GA, Kreyling WG, Haider B. Inhalation of salt aerosol particles—II. Growth and deposition due to hygroscopic growth. J Aerosol Sci 1988 19 611-631. 

Figure 1. Predicted hygroscopic growth factor of NaCl / (NH4)2S04 aerosol at 75% RH as a function of particle size in metres.

Evaporation is accompanied by local cooling and condensation by local warming. The addition of mas s to droplets and the conduction of latent heat away from the surface of the droplet or particle are, as has been pointed out by Pritchard (1987), the determining processes of particle growth. Under conditions of turbulent airflow, conduction of heat away from the surface of droplets or particles is enhanced and hygroscopic growth rates exceed those found under still conditions or under conditions of laminar airflow. The implications of this will be considered later. 

The effect of hygroscopic growth of particles was considered by the Task Force and is considered in detail below. 

This figure shows the mass deposition fraction plotted against the initial geometric particle size. Martonen et al (1985) argued that a critical particle size could be defined Dc. For hygroscopic particles initially smaller than this size, hygroscopic growth would be expected to reduce... 

Acid droplets absorb ammonia and become partially neutralized as they pass along the respiratory tract. Cocks and McElroy (1984) extended the work already discussed (Cocks and Fernando, 1982) on the hygroscopic growth of particles to model the growth and neutralization of acid aerosols. Before considering the results of the Cocks and McElroy (1984) study, two difficulties identified and dealt with by these authors should be examined ... 

For the results shown in Table 13, the model has included sufficient ammonia to bring about complete neutralization of the acid droplets in the air. As the initial relative humidity was high (99.5%), no hygroscopic growth of the particles occurred. Complete neutralization of 5 qin droplets would be expected in 3 s. The capacity of the ammonia present to neutralize all of the acid present should be contrasted with the extent of neutralization possible had the ammonia concentration been 50 pg m 3. 

The hygroscopic growth of pharmaceutical particles is usually less than that of sodium chloride particles. It is, however, not negligible, although it is often neglected in aerosol therapy. This was shown for a number of pharmaceutical aerosols. Particles were produced by nebulization of aqueous solutions of drugs, exposed to dry air and passed through a differential mobility analyzer for selec-... 

Figure 17 Hygroscopic growth of 0.7-pm sodium chloride particles in the respiratory tract of two volunteers as a function of the volnmetric depth to which the particles are carried with the tidal air as aerosol boluses. The sizes of the grown particles were determined with the respiratory aerosol probe (Fig. 4) in the expired air.

For pMDIs, in vitro studies do not take hygroscopic growth in the airways or the differences in plume geometry into account. The ability of the cascade im-pactor to predict the behavior of aerosolized drugs in vivo in a reliable way has been recently questioned. Indeed, it has been demonstrated that several generic pMDIs with fine particle fractions comparable to the original formulation (as measured in vitro) appeared to behave differently in vitro once attached to a large-volume spacer (3-5). 

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