Autophobicity

C. Liquid-Surface Interactions Surface Changes and Autophobicity... 

When a surface-active agent is present in a liquid droplet, it can adsorb to the surface, lower the surface energy, and cause the liquid contact angle to increase. This phenomenon, known as autophobicity, was postulated by Zisman many years ago [78, 79]. Autophobicity is quite striking in wetting films on clean... 

The observation of liquid drops on top of a film of the same substance is often referred to as pseudo-partial [ 1 ] or autophobic wetting. This behavior is not surprising in this... 

Fig. 11 a, b. Autophobic wetting of mica by carbosilane dendrimer with OH end groups [73]... 

Another explanation has been offered for the observed maximum in spreading area with concentration, based on the concept of autophobicity, i.e., the existence of an adsorbed layer of surfactant at the liquid-solid interface that is not wetted by its own kind [44]. However, the short time scale of the spreading ( 2 seconds in the high concentration regime) requires that this autophobic layer is laid down in this short period, which is unlikely considering the kinetic limitations on reorientation of molecules [50]. 

The dendrimer droplets were used as a tool to examine surface heterogeneity on the nanoscale [319]. Figure 32 shows two pairs of SFM images of the droplets deposited on mica (a,b) and a copolymer film (c,d), respectively. On mica, the hydroxyl terminated dendrimer showed the autophobic or pseudo-partial wetting behaviour as the droplets sit on a thin film of the same material [312,320]. 

Fig. 32. Amplitude (a,c) and phase (b,d) SFM micrographs demonstrate autophobic wetting of mica (a,b) and semifluorinated copolymer (c,d) by carbosilane dendrimer with hydroxyl end groups. Fluid droplets with a contact angle of about 8.7 degrees in (a) and 18.5 degrees in (c) were determined after 24 h equilibration at room temperature from cross sectional profiles recorded along the reference lines indicated on the (a) and (b) respectively. Reproduced from [319]...

Fig. 34. SFM amplitude images of the oligo(hexafluoropropene) substituted PMA films on mica after annealing at room temperature for 50 h [324]. Depending on the number ol HFPO units, polymers exhibit either a autophobic wetting behaviour for HFP05 or b stable coverage with a 10 nm thick film for HFP03...

A sample of non-porous a-Al203 (of a(BET) = 2.7 m2 g 1) was used in an important investigation by Barto et al. (1966) of the adsorption of a series of n-aliphatic alcohols. The isotherms, which are displayed in Figure 10.15, show very clearly the effect of autophobicity . Thus, the increase in hydrocarbon chain length from Cl to... 

In a further study of autophobicity by Blake and Wade (1971), adsoiption isotherms were determined for water vapour and the first five n-aliphatic alcohols on the surface of oxidized aluminium foil. The results were very similar to those obtained for a-Al203. A Type II water isotherm was obtained, the initial steep slope being indicative of a high affinity of adsorption. The residual uptake of water was... 

Parfitt and his co-workers pointed out (Day, Parfitt and Peacock, 1971) that there is an important difference between ethanol and isopropanol in their interactions with rutile. Whereas ethanol can displace water and undergo dissociative chemisorption to form the surface ethoxide, isopropanol is more readily adsorbed in the molecular form. This is consistent with the hydrophobic nature of ethanol-treated Ti02 and the autophobic nature of the ethanol monolayer. The latter effect is manifested in the form of a Type I isotherm, which is remarkably similar to that given by ethanol on alumina (see Figure 10.15). 

Standard computer hard disk surfaces (substrates used in this experiment were industrial hard disk surfaces that have been sputter-coated with nominally 100 A of hydrogenated amorphous carbon) can be coated by dip coating with nm thin films of perfluoropolyethers, such as Fomblin ZDOL, from solutions in perfluorohexane. For films thicker than several nm dewetting occurs spontaneously over a period of days to weeks due to the autophobic effect [43]. 

Therefore, the inverse gas chromatography method allows us to determine the model of long-chain aliphatic alcohol distribution on the surface of porous silica gel if the amount of alcohol on adsorbent surface is equal or exceeds the monolayer capacity, then the monolayer is composed of alcohol molecules oriented their polar moieties to the adsorbent surface. The monolayer, in a solid-condensed state, is stable up to 81°C. At this temperature the monolayer transfers into a liquid-expanded state. The threedimensional excess of alcohol, because the autophobicity phenomenon, does not wet the monolayer surface. 

Microwriting is another simple technique to fabricate patterned layers. In this method a solution of alkanethiol is dispensed through the capillary on the surface, and this capillary is used to write the patterns on the surface. Because of the autophobicity of alkanethiols, no excess of the deposition solution remains on the treated surface. Features down to 10 pm could be prepared508. 

The effect of temperature on the oleophobic layers was studied by Bigelow, Glass and Zisman [18], Figure 10-5 summarizes some significant aspects of their observations. Figure 10-5a is a plot of the relation between C, the concentration of the solute, and T, the critical temperature above which the adsorbed layer is no longer autophobic to the solution on retraction, for solutions of n-octadecanol, n-octadecanoic acid. 

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