SURA XVII EL VIAJE NOCTURNO
SURA XVIII LA CAVERNA
1. B. Arkels, Chemtec 7,766(1977).
2. J.R Hollaham and A. T. B ell, Eds., “Techniques and applications of plasma chemistry”, Wiley, New York, 1974.
3. H.V. Boegin , Ed, “Plasma Science and Technology”, Connell University Press, N.Y 1982.
4. H.Yasuda , J. Macromolecular Sei. Chem. A10(3), 15(1966). 5. H. Yasuda , “Plasma Polymerization”, Academic Press N.Y. (1985).
6. P.C. Herder, P.M. Claesson and C. E. H erder/. Colloid Interface Sei. 119, 155 (1987).
7. H. K. Christenson and C. E. Blom, / . Chem. Phys. 86,419(1987). 8. H. K. Christenson, J. Phys. Chem. 90,4(1988).
9. R. M. Pashley, J. Colloid Interface Sei. 83,531(1981).
10. H. K. Christenson and P. M. Claesson, Science 390,239(1988). 11. P. M. Claesson and H. K. Christenson, J. Phys. Chem. 92,1650(1988). 12. H. K Christenson, P. M. Claesson, J. Berg and P.C. Herder, / . Phys. Chem.
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6. Novel Surfaces: General considerations.
E ach of th e different su rface m odification te ch n iq u e s employed in th e preceding c h a p te rs h a s som e advantages and disadvantages. The choice of surface m odification technique is, of c o u rs e , larg ely d e p e n d e n t on th e s u rfa c e p ro p e rtie s required b u t th e lim itations of th e tech n iq u e em ployed need to be considered.
6.1 Adsorption modification.
C h ap ter 2 clearly d em o n strates form ation of m ultilayers w hen a relatively sm all in terb ilay er a d h e sio n is p re se n t. The forces are extrem ely difficult to in te rp re t w hen m ultilayers are p rese n t, th e plane of origin of th e charge is u n k n o w n an d so a c c u ra te d e term in atio n of th e degree of ion b in d in g is also im possible.
The ad h e sio n betw een h e a d g ro u p s ca n also produce o th e r p ro b lem s. If th e a d h e sio n is stro n g e r th a n th e force h o ld in g th e s u r fa c ta n t m o lecu les in th e b ila y e r th e n , on s e p a ra tin g th e s u rfa c e s from c o n ta c t, th e b ila y e r c a n be disrupted. This can also lead to m ultilayer form ation 1.
It is im possible to control th e a re a p er m olecule w hen b ila y e rs or m o n o lay ers a re p re p a re d by a d s o rp tio n from so lu tio n . T he h y d ro p h o b ic in te ra c tio n m e a s u re d b etw een m onolayers of CTAB adsorbed from solution is m u ch sh o rter in ra n g e a n d s tre n g th w h en c o m p ared w ith th e in te ra c tio n m e asu red betw een Langm uir-B lodgett films 2 an d th is m ay be due to the form ation of a w eak additional layer.
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A dsorption modification h a s th e advantage th a t su rfaces c a n be p re p a re d from w a te r so lu b le s u r f a c ta n ts , th e e x p e rim e n ts a re e a sy to p e rfo rm a n d a g re a t d e a l of inform ation a b o u t the behaviour of su rfa c ta n ts in so lu tio n can be obtained.
6.2 Deposition modification.
T he LB te c h n iq u e h a s a d is tin c t a d v a n ta g e over a d so rp tio n m odification in th a t th e a re a p er m olecule c a n be controlled, b u t th is area is lim ited by th e tra n s fe r ratio of th e d ep o sitio n a n d collapse p re s s u re of th e film a t th e a ir-w a te r interface. The films them selves ca n be r a th e r u n sta b le . Loss of m aterial from th e film to b u lk p h a se o ccu rs a t ra te d e p e n d e n t on th e so lu b ility of the m olecules a n d if th e re is a d h e sio n betw een th e su rfa c ta n t headgroups m ultilayers tend to form after the surfaces have been brought into contact 1.
V acuum evaporation can be u se d to deposit a wide variety of s u b s ta n c e s onto su rfa ce s. The forces b etw een one m ica surface an d a m etal surface have been succesfully m easu red . The m ain experim ental com plication is su rface ro u g h n ess.
6.3 Moulding.
A film formed on mica by ad sorption or deposition can, in som e c irc u m s ta n c e s , be rem oved from th e m ica s h e e t by peeling or allowing th e film to “float off* in a su ita b le solvent. T his m odification process h a s th e ad v an tag e th a t th e su rfa c e s form ed ca n be relatively thick, an d for polym er films it m ay be
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possible to com pletely replace th e m ica sh eets. It is conceivable th a t th is tech n iq u e could be u sed to form su rfaces from alm ost an y m a terial, C h a p te r 3 d e m o n stra te s th e ap p lica tio n of th is technique to producing m etal and polym er surfaces.
6.4 Covalent modification.
Mica, w hich is norm ally chem ically inert, can be activated to w ard re a c tio n w ith ch lo ro silan e s by tre a tm e n t w ith a low te m p e ra tu re p la sm a. P lasm a tre a te d m ica is u n s ta b le in w ater b u t, tre a tm e n t w ith silan es creates m ore stab le su rfaces. The sta b ility of th e sila n a te d su rfa c e s d e p e n d s on th e sila n a tin g agent. T re atm en t w ith trim eth y lch lo ro silan e c re a te s a surface w hich degrades by hydrolysis of the su rface reacted silane. With a fluorocarbon silane hydrophobic su rfaces can p rep ared w hich m a in ta in (unlike m ica coated w ith a L angm uir-B lodgett layer) a high co n tact angle, in d e p en d en t of th e sa lt co n cen tratio n of the te s t droplet. It sh o u ld be possible to u se th e p la sm a technique to produce su rfaces w ith a variety of chem ical com positions and p ro p erties. For in sta n c e , polym er film s m ay be coated on the su rface or reactive g roups introduced. F u rth e r, m ica treated w ith a w ate r v a p o u r p la sm a is reactive to w ard s a large n u m b e r of d ifferen t c h lo ro sila n e s a n d it s h o u ld be p o ssib le to exploit ex istin g co u p lin g a g e n t c h e m istry to p ro d u c e fu n ctio n alized s u rfa c e s , for exam ple, p o lym ers or even p ro te in s could be covalently attac h ed to m ica an d th e forces studied.
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R e feren c e s.
1. P. M. Claesson and J. M. Berg Thin Solid Films, in press.