f
I
\\.
to6
5
-
Poly- DLalanine
b” I W
15
20
A
25
b residue
Figure 7 . Comparison between E , 7 vs. area curves of BSA and poly-DL-alanine monolayers a t the benzene-water interface: E (0)and ( 0 ) .
viscoelastic characteristics. This must be due to the part played by hydrogen bonding. Furthermore, we can see a difference between the viscoelastic behavior at the air-water and a t the benzene-watcr interfaces. The difference of the variation of the E and 7 values with area is graphically shown in Fig. 5. This confirms that the benzene phase strengthens the film structure. The benzene phase appears to have a similar effect as plasticizers have in lowering the second-order transition point of polymers, when the increase of the surface viscoelasticity is lcft out of consideration. BSA Monolayers at the Benzene-Water Interface. Thc viscoelastic behavior of BSA nionolaycrs a t the benzene-water boundary was found to bc conipletcly different from the behavior at thc air-water intcrfacc and to bear a resemblance to those of synthctic polgrner monolayers both a t the benzene-water and air-watcr interfaces. Thc BSA exhibited viscoelasticity a t areas larger than the close-packed area. The results are shown in Fig. 6, where the area is also cxpressed in
The Journal of Physical Chemistr?i
h.2,/rcsith~oon the assuiiiptiori of the iiiolcculat~weight arid thc number of rvsiducs of BSh bcing 66,00069,000 arid 388, respectivcly.19 It is noticeable that the values of l$arc highcr than those of B, ovcr all the areas in contrast to poly-nr,alanine. AIorcover, the aspect of viscoelasticity ~ s . area is distinctly different betwecn the BS.1 and thc poly-Dr,-alanine monolayers. This niay be ascribed to thc decrease in the rolc of hydrogen bonding or to incoinplcte unfolding in the 13SA monolayer. If the RSA molecule is completely unfolded, thc monolayer will show higher viscoelasticity than polg-Dr,-alaninc because of the greater length and the polar character of the side chains in I3SA. As shown in Fig. 7, wherc thc E and 9 values arc plotted against the area, the RSA shows smaller values. This fact indicates that the inolccules are extensively but not cornpletcly unfolded. It may, therefore, be concluded from these results that thc 1’3S.t molecule is unfolded with the aid of henxenc up to a partially extcndcd configuration in which some parts of the molecule move around freely, like synthetic polypcptidcs, and other parts are prevented from extending by the side chains and s-S linkages.
Acknowledgment. The author wishcs to thank Dr. Harry Sobotka for discussion and aid in the preparation of the manuscript.
Discussion 11. h I A N X l i E l > l E R (Southwest. Ilescarcth Institute, San Antonio). I)id you, i n your expcrirncnts, varry out any niessurenierita at, a variety of angles, so i i u to irivest.igetc the efSwt of shear stress on your filrns?
K. hlomnrrxh. So c1Tec.t of shear stress upon the viscoelastic#it y of nionolaycru was otmrvcd in the fcw cxpeririients carried out. IIowwer, sirive thc visroelasticit y rneasurernents were rnadc ovcr a nLirrow r:ingc of stresses, bccausc of the stiffness of the inonolaycru,sueh cffects variriot be positively rriled out.
I+‘. W A U E(Irnivcrsity of ‘l’cxas). I h i you ever observe any tirne-depcntierit etfevts i n your studies of the visc:oclastic-ity of monolayers? tiin(! dependences of the viscwIC. ~ I W I Y ) M ~ . RIAobserved , elnstivity whir11 varied somewhat ac.c,ording to the rrionolayer. After a h n i t 1 hr., however, the monolayers secrned to be s t ~ b l e . (19) G.
R. Tristrmn, Proteins, I A ,
181 (1963).
