Sterically Stabilised Systems

Alternatively, the 0-condition may be reached by increasing the temperature. Here the interactions between the medium and the segments of the polymer decrease because of the decreasing density of the medium, and segment-segment interactions dominate when an upper flocculation temperature (u.f.t.) is reached. This is analogous to the lower consolutc behaviour of those polymer solutions which undergo a phase separation as the temperature is raised. 

A decrease in temperature may also lead to flocculation at a lower flocculation temperature (l.f.t.), partly because the Brownian motion can no longer maintain the dispersed state and partly 

Interaction between the polymer layers is not the only factor influencing flocculation. Thus changing the medium also affects the effective Hamaker constant, Tu. It is then Lhe increased attractive potential arising from an increase in A h that may determine the flocculation. In this case there may be little correlation with the 0-point. 

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The forces between sterically stabilised particles have been measured with a compression cell (see page 223)213, and have been shown (as expected in the light of the foregoing discussion) to be short-range, with a range comparable with twice the contour length of the lyophilic chains. For sterically stabilised systems the total interaction energy can be written as... 

The small droplets have much smaller van der Waals attraction, and flocculation is prevented. This is particularly the case with sterically stabilised systems. 

Figure 8.4 Energy-distance curves for sterically stabilised systems.

Figure 3.11 (a) Contribution to the repulsive free energy for sterically stabilised systems (i) hard surface, (ii)—(iv) decreasing density of the... 

Figure 3.12 Typical forms of the total-interaction free energy for (a) electrostatically stabilised systems [curves (i), (ii), and (iii) refer to increasing electrolyte concentration], (b) sterically stabilised systems [curves (i), (ii), and (iii) refer to constant density of polymer chains, but decreasing <5, arising from decreasing values of a].

Diphosphenes stabilised by bulky groups, e.g. (157, R = 2,4,6-tri-t-butylphenyl), can be isolated in high yield from the reactions of alkyl- or aryl-(trichlorogermyl)phosphines with an excess of the base DBU. The formation of less sterically crowded systems, e.g. (157, R = t-butyl), is also possible by this route,... 

Standard theories of steric stabilisation assume that the free energy of the system can be expressed as a sum of two terms, volume restriction and osmotic, which are mutually independent. 

In studying the stability of colloidal dispersions it is of considerable advantage if the particles concerned are monodisperse and spherical. For aqueous, charge-stabilised systems polymer latices have proved invaluable in this regard. With non-aqueous systems, steric stabilisation is usually required. In this case it... 

However, the degree of accumulation of liposomes in tumour tissue depends on their blood circulation time. In order to maximise the accumulation of liposomes, and thus boron, in the tumour, liposomes with long survival time in systemic circulation, should be employed. The liposomal circulation time may be dramatically increased by attachment of long flexible polyethylene glycol (PEG) chains to a proportion of the lipids in the liposomal membrane. Such sterically stabilised liposomes have been shown to circulate for several days, up to a week, without detection by the immune system. 

Stability of Disperse Systems Containing Adsorbed Nonionic Surfactants or Polymers Steric Stabilisation... 

The efficiency of steric stabilisation depends on both architecture and the physical properties of the stabilising molecule. Steric stabilisers should have an adsorbing anchor with a high affinity for the particles and/or be insoluble in the medium. The stabiliser should be soluble in the medium and highly solvated by its molecules. For aqueous or highly polar oil systems, the stabiliser block can be ionic or hydrophilic... 

Oupicky D, Ogris M, Howard KA, Dash PR, Ulhrich K, Seymour LW. Importance of lateral and steric stabilisation of polyelectrolyte gene delivery vectors for extended systemic circulation. Molecul Therap 2002 5 463—472. 

The jetness properties of dispersant D are also the best across the three chosen pigments, which indicates excellent compatibility of the steric stabilisation chain with the binder system and that particle stability is maintained in the dry film after removal of water. 

Inverse (or water-in-oil) emulsions (315, 401) are emulsions in which an aqueous phase is dispersed within a continuous organic phase. This system is essentially the inverse of a conventional emulsion, hence the name inverse emulsion. The organic phase is typically an inert hydrocarbon (such as mixed xylenes or low-odour kerosenes), and the aqueous phase contains a water-soluble monomer such as acrylamide (268). The aqueous phase may be dispersed as discrete droplets or as a bicontinuous phase (335), depending upon the formulation and conditions of the inverse emulsion. The hydrophilic-lipophilic balance (HLB) value of the stabiliser determines the form and stability of an inverse emulsion, with HLB values of less than 7 being appropriate for inverse emulsions. Steric stabilisers such as the Span , Tween , and Plutonic series of nonionic surfactants are usually used in preparing inverse emulsions. Inverse emulsions, suspensions, miniemulsions (199), and microemulsions have been prepared, primarily as a function of the stabiliser concentration. Commercial products produced by inverse emulsion polymerisation include polyacrylamide, a water-soluble polymer used extensively as a thickener. 

Unlike the Gj-h curve predicted by the DLVO theory (which shows two minima and one energy maximum), the Gj-h for systems that are sterically stabilised show only one minimum, G iin, followed by a sharp increase in Gt with decreasing h (when h, 23). The depth of the minimum depends on the Hamaker constant A, the particle radius R and adsorbed layer thickness 3 G iin increases with increasing... 

K. Kostarelos, T. F. Tadros, P. F. Luckham, Physical conjugation of (tri-) block copolymers to liposomes toward the construction of sterically stabilised vesicle systems, Langmuir, 1999, 15, 369-376. 

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