Surface modification*

4 3

A m ajor aim of th is w ork is to pro d u ce novel su rfaces su itab le for investigation w ith th e surface force technique. There are four possible strateg ies w hich ca n be u se d to produce new surfaces.

i) A dsorption m odification ii) D eposition m odification iii) M oulding

iv) Covalent m odification

The first two tech n iq u es have been extensively u se d for creating su rfaces form ed from su rfa c ta n ts. B oth m onolayers an d bilayers have b e e n form ed on m ica by a d s o rp tio n from so lu tio n or L a n g m u ir-B lo d g e tt d e p o s itio n . T he effect of in te rb ila y e r a d h e s io n o n th e a d s o r p ti o n of N -(l-h y d ro x y e th y l)N - m ethyldidodecylam m onium brom ide is d isc u sse d in C h ap ter 1. The L angm uir-B lodgett tech n iq u e w as u se d to p rep are bilayers containing charged glycolipids in C h ap ter 2. V acuum deposition an d a new surface m odification technique, “m oulding”, are u sed to produce silver su rfaces an d the forces betw een one silver and one m ica surface are reported in C h ap ter 3. Mica is n o t reactive to fu n ctio n alizatio n w ith th e com m on chlorosilanes. C h a p ter 4 d escrib es a cold p la sm a tech n iq u e w hich allows reactive m ica su rfaces to be produced.

4 4

2. The effect of interbilayer adhesion on the forces

between bilayers o f N-(l-hydroxyethyl)N-methyl-

didodecylammonium bromide

To d a te b ilay er su rfa c e s p re p a re d by a d so rp tio n from solution of su rfa c ta n ts to m ica have n o t show n a strong adhesion w hen b ro u g h t into bilayer contact. A serio u s com plication can be im agined w hen th e re is a m inim um in free energy w ith the bilayers in contact. If th ere is no force b a rrie r th e re is nothing to h a lt th e a d so rp tio n p ro cess a n d m ultiple layers m ay form. Even in th e p resen ce of a stro n g force b a rrie r, for in sta n c e a double-layer force, it is u n c le a r w h at effect interbilayer adhesion w ould have on th e a d so rp tio n p ro cess. N -(l-hydroxyethyl)N - m ethyldidodecylam m onium brom ide (HEMDAB) b e a rs a hydroxyl group close to th e head group (stru ctu re show n schem atically in fig. 7) a n d th is g ro u p ca n form h y drogen b o n d s w ith w ater, a p p ro p ria te c o u n te rio n s or even itself. T here is a s yet no correlation betw een ion binding an d th e chem ical n a tu re of the b ilay er su rfa ce . Previously, th e degree of b in d in g h a s been correlated w ith th e degree of h y d ratio n of th e anion, b ase d only on its size 1.

The ch em ical n a tu re of su rfa c e g ro u p s m u s t play an im p o rta n t role in ion binding a n d th e effect of a hydroxyl group in th e HEMDAB m olecule c a n be d eterm in ed by co m p ariso n w ith th e sim p ler q u a te rn a ry am m o n iu m brom ide s u rfa c ta n ts . M e asu rem e n ts w ith s u rfa c ta n ts b ea rin g different h e a d groups m ay also provide in fo rm atio n on th e p o ssib le o cc u rren c e of h y d ra tio n forces b etw een b ila y ers of q u a te rn a ry am m o n iu m su rfa c ta n ts. The hydroxide group in HEMDAB is in a sim ilar

4 5

a rra n g e m e n t to th a t expected w ith p en tan o l ad so rb ed in the d ih e x a d e c y ld im e th y la m m o n iu m a c e ta te (DHDAA) s u r f a c ta n t layer^ an d th is provides a n ideal o pportunity to investigate the effect of su rfa c e -b o u n d hydroxyl g ro u p s on th e in te rb ila y e r forces. We th u s have a possible model for th e action of pentanol in the bilayers of the sim pler su rfa cta n ts.

2.1 Results.

2.1.1 Bromide Bilayers.

All HEMDAB (obtained from Sogo Ltd) so lu tio n s w ere allow ed to e q u ilib ra te for a t le a s t 2 4 h rs p rio r to u se . The s u rfa c ta n t w as ad so rb ed on th e m ica b a s a l p lan e by filling the a p p a ra tu s w ith w ater an d th e n injecting th e su rfa c ta n t solution to give th e c o n c e n tra tio n req u ired . A lternatively, a d ro p let of w ater w as placed betw een the su rfaces an d th e a p p a ra tu s th e n filled w ith a solution of th e desired concentration.

W ithin 20 m in u te s of addition of p reeq u ilib rated 1 0 '4 M s u rfa c ta n t so lu tio n (with a w ate r droplet betw een th e surfaces) th e m e a su re d in te ra c tio n show s th e p rese n ce of a long-range r e p u ls io n a n d th e s u rfa c e s ju m p in from a s e p a r a tio n corresponding to th e adsorption of a bilayer on each surface (5.3 nm) (All m e a s u re m e n ts are m ad e rela tiv e to th e ad h esiv e c o n ta c t m in im u m of th e m ica su rfa c e s in p u re w ater). They come to re s t a t a sep aratio n one h alf of th is (2.7 nm) an d show very stro n g a d h e sio n , c h a ra c te ris tic of h y d ro p h o b ic c o n ta c t 9,10(400 m N /m ). On su b se q u e n t ap p ro ach after se p aratio n the m easu red in teractio n is often strongly repulsive an d indicative of

4 6

th e ad so rp tio n of m ultilayers of s u rfa c ta n t (several ste p s, each corresponding to the th ic k n ess of a single bilayer are observed).

If a n e q u ilib ra tio n tim e of 24 h rs is allow ed before m e a su re m e n ts are m ade th e in te ra c tio n a p p e a rs as in Fig. 1. T h e se r e s u lts a re very re p ro d u c ib le from m e a s u re m e n t to m e a s u re m e n t (the r e s u lts of th re e d ifferen t force r u n s are show n in th e figure) an d are fitted quite well by DLVO-theory u sin g th e n o n -lin ear Poisson-B oltzm ann eq u atio n a t d istan ce s > 9 n m (solid b la c k line). At d is ta n c e s le ss th a n 9 nm th e m e asu red curve show s an additional repulsion n o t predicted by DLVO-theory (see Fig. 2) followed by a n a ttra ctio n w hich causes th e s u rfa c e s to ju m p in to b ila y er c o n ta c t (5.3 nm) from a se p a ra tio n (8nm relative to m ica-m ica c o n ta c t in w ater) m u ch g reater th a n expected from van der W aals theory. The m easu red ju m p d istan ce relative to bilayer co n tact is 2.7 nm (equal to the bilayer thickness) as opposed to th e theoretically expected value of 0.7 nm . T he m e a su re d a d h e sio n b etw een th e a d so rb e d bilayers is 14.5 m N /m .

If the a p p a ra tu s is drain ed an d th e n refilled w ith w ater to give a s u rfa c ta n t c o n c e n tra tio n of approxim ately 10 '6 M the o u te r lay er of th e b ilay er w e a k e n s w ith tim e a n d can be s q u a s h e d out, as observed initially before e q u ilib ratio n . The s u rfa c e s ag ain com e to a n extrem ely ad h esiv e, h y drophobic co n tact. The m e asu red force is show n in Fig. 3 an d as can be seen th e surface charge h a s decreased considerably com pared to th e situ a tio n a t 10-4 M. The Debye length, however, h a s only in c re a se d very slig h tly a n d is m u c h s h o rte r th a n w ould be expected on b a sis of th e su rfa c ta n t concentration. F urtherm ore, a t s e p a ra tio n s below 10 nm th e m e asu red in te ra c tio n is more

10 M