THE PREPARATION AND STABILITY OF COLLOIDAL METAL DISPERSIONS PREPARED BY A TWO-PHASE NON-AQUEOUS ROUTE

Stable suspensions of small metal Au, Ru, PI, Pd, Rh, Co and Nl particles dispersed in n-heptane and n-dodecane have been prepared using a novel two-phase system, Involving the formation of the particles In a methanolic phase and subsequent phase transfer of the panicles to the alkane medium. The dispersions consisted of small particles having diameters In the range of 8-30 nm (the gold sols were very polydlsperse having average diameters of ca.34 nm). The phase transfer of the particles and their subsequent colloid stability were effected by the presence of dissolved dispersant in the hydrocarbon phase (either Oloa 1200 or Hypermer LP 8). In the case of Oloa 1200, a widely-used polylsobutylene succinimide automotive engine dispersant, It Is proposed that the amlne groups adsorb strongly to the acidic surface o1 the particles, and the 70-carbon polyisobutylene chains extend Into the hydrocarbon medium sufficiently to maintain the separation of adjacent particles by steric and possibly also by electrical repulsion.     

Eddy diffusivities derived from a numerical model of the convective planetary boundary layer

Summary Functional forms for the vertical eddy diffusivityK _z (z) are sought that optimize the performance of theK-theory diffusion equation. The method developed to determine the optimal diffusivity is first tested by applying it to analytic solution of the diffusion equation in which the functional form of the diffusivity is known precisely. In all test cases performed, the technique yields the correctK _z profile regardless of the initial estimate ofK _z from which the technique’s search procedure begins. When applied to “observed” mean, cross-wind-integrated point source concentration fields derived from Lagrangian diffusion theory and data from a numerical turbulence model jointly, the technique yields optimal diffusivities that make the solution of the diffusion equation agree within ±20% of the “observed” values within the core of point source plumes. Expressed in terms of the convective-velocity scalew _* and the mixed-layer depthz _i , the optimal diffusivity has a quasiuniversal form for atmospheric stabilities in the rangez _i /L⪯-10 whereL is the Monin-Obukhov length. The optimal diffusivity is found to be strongly dependent on the source hight. TheK _z profiles derived for the two source heightsz _s∼-0.025z _i andz _s ∼-0.25z _i are of opposite shape, but they have comparable maximum values ofK _z ∼-0.25w _* z _i .

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Riassunto Si cercano forme funzionali per la diffusibilità turbolenta verticaleK _z (z) che rendano ottimale l’efficacia dell’equazione di diffusione della teoriaK. II metodo sviluppato per determinare la diffusibilità ottimale è dapprima controllato applicandolo a soluzioni analitiche dell’equazione di diffusione in cui la forma funzionale della diffusibilità è nota esattamente. In tutti i casi di prova la tecnica dà il profilo corretto diK _z qualunque sia la stima iniziale diK _z da cui prende inizio il procedimento di ricerca della tecnica. Quando è applicata contemporaneamente ai campi di concentrazione medi “osservati”, a sorgente puntiforme integrati sui venti trasversali, derivati dalla teoria di diffusione lagrangiana ed ai dati presi dal modello di turbolenza numerico, la tecnica dà diffusibilità ottimali per le quali la soluzione dell’equazione di diffusione è in buon accordo entro il ±20% dei valori “osservati” nel nocciolo di pennacchi della sorgente puntiforme. Espressa in termini della scala di velocità di convezionew _* e della profondità dello strato mistoz _i , la diffusibilità ottimale dipende fortemente dall’altezza della sorgente. I profili diK _z derivati per due altezze di sorgentez _s ∼-0.025z _s ez _s ∼-0.25z _i sono di forma opposta ma hanno valori massimi diK _z paragonabili,K _z ∼-0.25w _* z _i .

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Резюме Определяются функциональные выражения для вертикального вихревого козффициента диффузииK _z (Z), которые оптимизируют получение уравнения диффузии вK-теории. Развитый метод для определения оптимального коэффициента диффузии сначала проверяется на аналитических решениях уравнения диффузии, в которых функциональное выражение козффициента диффузии известно точно. Во всех исследованных случаях предложенный метод дает правильный профильK _z . Предложенный метод дает оптимальные козффициенты диффузии, при которых решение уравнения дуффузии согласуется, в пределах ±20%, с ≪наблюдаемыми≫ величинами. Выраженный через масштаб конвекционной скоростиw _* и глубину смешанного слояz _i оптимальный коэффициент диффузии имеет квази-универсальную форму для атмосферных устойчивостей в областиz _i /L≤10, гдеL есть длина Монина-Обухова. Обнаружено, что оптимальный козффнциент диффузии сильно зависит от высоты источника. ПрофилиK _z , выведенные для двух высот источниковz _i ≅0.025z _i иz _i ≅0.25z _i , имеют противоположную форму, но они имеют сравнимые максимальные значенияK _z ≅0.25w _* z _i .

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