Dispersal barriers

Function in Formula Emollient, pigment disperser, barrier protection, lubricant Use Concentration 1-5% in creams and lotions 1-5% in pigmented products 0.1-0.5% in anti-perspirants 1-5% in deodorant sticks, roll-ons... 

Genetic differentiation of Terebratella sanguinea in the New Zealand fiords a dispersal barrier in the marine environment ... 

Mayr, E. (1942) Systematics and the Origin of Species. Columbia University Press, New York. McCartney, M.A., Keller, G., and Lessios, H.A. (2000) Dispersal barriers in tropical oceans and speciation in Atlantic and eastern Pacific sea urchins of the genus Echinometra. Molecular Ecology, 9 1391-1400. 

Laminae of clay and clay drapes act as vertical or horizontal baffles or barriers to fluid flow and pressure communication. Dispersed days occupy pore space-which in a clean sand would be available for hydrocarbons. They may also obstruct pore throats, thus impeding fluid flow. Reservoir evaluation, is often complicated by the presence of clays. This is particularly true for the estimation of hydrocarbon saturation. 

A 25% dispersion of NaH crystals ia oil is obtained. The commercial product, after filtration, is a 60% dispersion of NaH crystals (5—50 p.m). The oil dispersions can be handled quite safely because the oil phase provides a barrier to air and moisture, whereas the unprotected crystals react vigorously. Traces of unreacted sodium metal give the product a gray color. 

Figure 4c also describes the spontaneous polymerisation ofpara- s.yX en.e diradicals on the surface of soHd particles dispersed in a gas phase that contains this reactive monomer (16) (see XylylenePOLYMERS). The poly -xylylene) polymer produced forms a continuous capsule sheU that is highly impermeable to transport of many penetrants including water. This is an expensive encapsulation process, but it has produced capsules with impressive barrier properties. This process is a Type B encapsulation process, but is included here for the sake of completeness. 

Paraffin wax vapor barriers are used in water rinse removers that can disperse the wax without coating the substrate. In soak tank applications, water is sometimes doated on top of an ad-solvent, neutral pH, nonwater rinse remover to prevent evaporation. Flotation devices that cover the exposed surface area may be used with other formulas. 

Diblock copolymers consist of one sequence of anchor segments and a second sequence of backbone segments. The relative lengths of the two sequences can be controlled to provide a wide variety of adsorption and barrier characteristics. Typical commercial dispersants may use alkane... 

As the particles in a coUoidal dispersion diffuse, they coUide with one another. In the simplest case, every coUision between two particles results in the formation of one agglomerated particle,ie, there is no energy barrier to agglomeration. Applying Smoluchowski s theory to this system, the half-life, ie, the time for the number of particles to become halved, is expressed as foUows, where Tj is the viscosity of the medium, k Boltzmann s constant T temperature and A/q is the initial number of particles. 

For dispersions at moderate concentrate, ca 10 particles per cm, has a value of the order of a few seconds. This expression assumes there is no barrier to coUision and every coUision is effective. For stable dispersions to exist, an energy barrier W is assumed that prevents coUision. In this case, the expression... 

If W is 15—20 / T, the half-life is several days and the dispersion is reasonably stable. The problem of colloidal stabiUty thus becomes a problem of the energy barrier, IF, that lessens the unifying coUisions. 

The stabilization of an emulsion iavolves slowiag the destabilization, primarily the flocculation process. This may be achieved ia two principal manners by reduciag the mobiHty of droplets through enhanced viscosity or by inserting an energy barrier between them (see also Dispersants Flocculating agents). 

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