Wetting Characteristics of Keratin Substrates

6 Wetting Characteristics of Keratin Substrates A. EL-SHIMI and E. D. GODDARD

Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0145.ch006

Lever Brothers Research Center, 45 River Road, Edgewater, N. J. 07020 The wettability of living human skin, bovine hoof keratin, and various synthetic polymers has been examined using two liquids, water and methylene iodide, and Wu's empirical approach to obtainγs and γs . The sum of these parameters agreed with reported values of γ , the critical surface ten sion, based on Zisman plots.γ ,values obtained using aque ous ethanol solutions were lower and showed little or no dependence on the type of solid surface. The results show that skin and keratin possess surfaces of low free energy comparable with that of polyethylene. Experimental values of contact angles at the hydrocarbon liquid/water/solid interface showed satisfactory agreement with the Wu equa tion for Teflon and poly(methyl methacrylate), but those ob served for nylon 11 and bovine hoof keratin (> 170°C in oil) were higher than predicted. d

p

c

c

A n i m p o r t a n t f u n c t i o n of a n i m a l s k i n is to act as a b a r r i e r to p r e v e n t w a t e r loss a n d e n t r y of f o r e i g n m a t e r i a l s . I n m a n a n d other m a m mals this b a r r i e r p r o p e r t y is c o n f e r r e d b y the outermost l a y e r k n o w n as the s t r a t u m c o r n e u m . A l t h o u g h i t is c o m p o s e d essentially of a f e w layers of cells w h i c h u n d e r g o extensive k e r a t i n i z a t i o n as they are f o r c e d u p ward

from

the

d e r m i s l a y e r , the b a r r i e r p r o p e r t i e s

c o r n e u m l a y e r far e x c e e d those of k e r a t i n .

of

this s t r a t u m

T h e efficiency of s t r a t u m

c o r n e u m as a b a r r i e r d e p e n d s o n the presence i n i t of l i p o i d a l m a t e r i a l . T h i s efficiency ( 1 ) is severely i m p a i r e d i f the s t r a t u m c o r n e u m is t r e a t e d w i t h c o m b i n a t i o n s of p o l a r , w a t e r m i s c i b l e l i q u i d s s u c h as m e t h a n o l a n d w a t e r i m m i s c i b l e l i p o p h i l i c solvents s u c h as hexane or c h l o r o f o r m . M u c h effort is e x p e n d e d c o s m e t i c a l l y to i m p r o v e the c o n d i t i o n of s k i n . T h i s g e n e r a l l y involves a p p l y i n g p r o d u c t s to restore i m p a i r e d s k i n to n o r m a l . normal skin.

O u r i n v e s t i g a t i o n examines the w e t t i n g characteristics of C l e a n s k i n i n as n a t u r a l a state as p o s s i b l e w a s

used.

A g g r e s s i v e solvent treatments w e r e a v o i d e d s i n c e s u c h t r e a t m e n t w o u l d 141 In Applied Chemistry at Protein Interfaces; Baier, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

142

APPLIED CHEMISTRY AT PROTEIN

INTERFACES

r e m o v e a c o n s i d e r a b l e p o r t i o n of s k i n l i p i d s a n d influence the w e t t i n g properties. F o r c o m p a r i s o n w e h a v e i n v e s t i g a t e d the w e t t i n g of a k e r a t i n substrate itself, n a m e l y b o v i n e hoof. S u c h a substrate has some s i m i l a r i t y to h u m a n h a i r a l t h o u g h f r e s h l y c l e a n e d h u m a n h a i r w i l l also i n t i m e a c q u i r e a l a y e r of l i p i d s b y m i g r a t i o n of s e b u m constituents f r o m the scalp. T o a l l o w m o r e c o m p l e t e e v a l u a t i o n of the properties of s k i n a n d k e r a t i n surfaces, the w e t t i n g b e h a v i o r of several s y n t h e t i c p o l y m e r s w e r e s t u d i e d . T h e experiments i n v o l v e w e t t i n g a l l these materials i n a i r a n d submerged i n water.


Experimental S u b s t r a t e s a n d M a t e r i a l s . F o r studies of contact angles o n s k i n , the area chosen s h o u l d be as s m o o t h as possible a n d free f r o m h a i r . F o l l o w i n g e a r l i e r p r a c t i c e ( 2 ), the area selected was the d o r s a l side of the i n d e x finger w i t h the s k i n r e n d e r e d taut b y b e n d i n g of the finger. P r i o r to contact a n g l e measurements, the f e m a l e subjects w a s h e d t h e i r h a n d s w i t h soap a n d r i n s e d t h e m u n d e r t a p w a t e r . T h e e x p e r i m e n t a l surface w a s a l l o w e d to a i r d r y for 15 m i n ; t h e other areas w e r e l i g h t l y p a d d e d w i t h p a p e r towels. T w o subjects w e r e chosen f r o m a large g r o u p as h a v i n g the smoothest s k i n i n t h e finger d o r s a l r e g i o n . Since the findings for b o t h subjects w e r e v e r y close, the d a t a for o n l y one are r e p o r t e d . T h e alcohols u s e d w e r e d i s t i l l e d c o m p o u n d s , a n d the w a t e r was d o u b l e d i s t i l l e d . A v a r i e t y of substrates was chosen w h i c h i n c l u d e s paraffin w a x ( G u l f O i l C o r p . , m.p. 5 5 - 5 6 ° C ) ; T e f l o n a n d n y l o n 11 ( D u P o n t ) ; poly (methyl methacrylate ) ( P M M A ) ( R o h m and H a a s ) ; b o v i n e h o o f k e r a t i n ( B H K ) ; a n d h u m a n s k i n . B o v i n e hooves w e r e o b t a i n e d f r e s h f r o m the slaughter house. T h e h a r d k e r a t i n was i s o l a t e d b y first b o i l i n g the hooves i n w a t e r a n d t h e n s c r a p i n g the u n w a n t e d components w i t h a s h a r p k n i f e . T h e d r i e d k e r a t i n pieces thus o b t a i n e d w e r e g e n e r a l l y i r r e g u l a r i n shape, a n d i t was necessary to m a c h i n e the k e r a t i n hoofs into some d e s i r a b l e g e o m e t r i c p l a n a r surface. I n this w o r k , p l a n a r discs 20 m m i n d i a m e t e r a n d not less t h a n 5 m m t h i c k w e r e u s e d . D i s c s t h i n n e r t h a n 5 m m t e n d e d to d e f o r m a n d thus lose t h e i r p l a n a r geometry. T h e discs w e r e p o l i s h e d u s i n g e m e r y p a p e r of s u c c e e d i n g l y finer g r a d e u n t i l a h i g h l y reflecting surface was o b t a i n e d . M i c r o s c o p i c e x a m i n a t i o n s h o w e d n o r e s i d u a l d e b r i s o n the surface. T e f l o n , n y l o n , a n d P M M A w e r e s u p p l i e d i n sheet f o r m . T h e s e solids w e r e w a s h e d i n detergent s o l u t i o n , r i n s e d t h o r o u g h l y i n d i s t i l l e d w a t e r , a n d s t o r e d i n a v a c u u m o v e n at r o o m t e m p e r a t u r e . Paraffin w a x discs w e r e o b t a i n e d b y d i p p i n g P M M A discs or glass slides i n m o l t e n w a x , t h e n r e m o v i n g a n d g e n t l y p r e s s i n g the discs or slides onto w a r m a l u m i n u m f o i l p l a c e d o n a p l a n a r , b l a c k granite d r e s s i n g p l a t e n o r m a l l y u s e d for gage b l o c k s . T h e resultant c o a t i n g was flat a n d s m o o t h . T h e surface of t h e s y n t h e t i c p o l y m e r s was c a r e f u l l y p r o t e c t e d a n d u s e d for w e t t i n g studies i n v i r g i n f o r m . F o r the o i l / w a t e r studies, discs of paraffin w a x a n d the other m a t e rials w e r e g l u e d to a glass r o d ca. 3 i n . i n l e n g t h . T h e j u n c t i o n of the

In Applied Chemistry at Protein Interfaces; Baier, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

6.

E L - S H i M i A N D GODDARD

Keratin

143

Substrates

glass r o d a n d the discs w a s c o a t e d w i t h paraffin w a x to ensure n o c o n t a m i n a t i o n of s u r r o u n d i n g l i q u i d w i t h g l u e ( E a s t m a n 910 a d h e s i v e ) . F o r c o n v e n i e n c e T e f l o n , n y l o n , a n d P M M A w e r e also s t u d i e d i n the f o r m of c y l i n d r i c a l rods of ca. 1-in. d i a m e t e r at the base a n d 4 i n . l o n g . T h e u p p e r 3-in. p o r t i o n was m a c h i n e d to a ^ % - i n . d i a m e t e r a n d w a s u s e d as a h a n d l e to fit i n a l a b o r a t o r y c l a m p . It w a s s e c u r e d i n a v e r t i c a l p o s i t i o n b y c l a m p i n g onto a l a b o r a t o r y jack a n d the b r o a d e r , l o w e r p o r t i o n of the c y l i n d e r s was u s e d for contact a n g l e measurements. T h i s set-up p r o v i d e s a c o n v e n i e n t w a y to m o v e the s o l i d c y l i n d e r v e r t i c a l l y into the o p t i c a l c e l l w h i l e t h e d r o p p i n g a s s e m b l y r e m a i n s stationary. A s m o o t h reflecting surface was o b t a i n e d b y p o l i s h i n g w i t h a suspension u s e d for p r e c i s i o n o p t i c a l e q u i p m e n t . Results o b t a i n e d s h o w e d agreem e n t b e t w e e n the d i s c a n d r o d forms of P M M A a n d T e f l o n , b u t for n y l o n the r o d surface was e v i d e n t l y s c r a t c h e d a n d was d i s c a r d e d . T h e h y d r o c a r b o n l i q u i d s w e r e v e r y p u r e s a m p l e s : C - (spectral g r a d e , M a t h e s o n , C o l e m a n , a n d B e l l ) , C - , C i - , C i - , a n d Cie-n-alkanes ( H u m p h r e y C h e m i c a l C o . ) , a n d c y c l o h e x a n e ( s p e c t r a l grade, E a s t m a n ) . T h e absence of i m p u r i t i e s was c o n f i r m e d b y G L C . T h e m i n e r a l o i l ( D r a k e o l 15 U . S . P . grade, P e n n s y l v a n i a R e f i n i n g C o . ) w a s f u r t h e r p u r i f i e d b y treating w i t h Fluorosil. T h e methylene iodide was a h i g h purity specimen ( F i s h e r Scientific C o . ) a n d was u s e d as r e c e i v e d . F o r the o i l / w a t e r studies, a l l the l i q u i d s w e r e e q u i l i b r a t e d w i t h d o u b l e d i s t i l l e d w a t e r for 24 hrs i n glass containers w i t h continuous a g i t a t i o n i n a r o t a t i n g m a c h i n e . A f t e r e q u i l i b r a t i o n , the l i q u i d s w e r e left to stand for a f e w hours a n d separated u s i n g a s e p a r a t i n g f u n n e l . T h e surface a n d i n t e r f a c i a l tensions of the systems i n v e s t i g a t e d w e r e m e a s u r e d at r o o m t e m p e r a t u r e u s i n g the W i l h e l m y p l a t e m e t h o d , a n d the results are s h o w n i n T a b l e I. Technique. T h e set-up consisted of a n o p t i c a l b e n c h w i t h t w o c a r riers. O n e c a r r i e r h e l d the s u p p o r t for t h e o p t i c a l c e l l , the h e i g h t of w h i c h c o u l d be adjusted b y a v e r t i c a l l y m o v a b l e m o u n t i n g shaft. T h e s e c o n d c a r r i e r , s u p p o r t i n g a l o w p o w e r m i c r o s c o p e fitted w i t h a g o n i o m e ter scale, w a s c a p a b l e of v e r t i c a l a n d h o r i z o n t a l m o v e m e n t s . T h i s d e s i g n facilitates m i c r o s c o p e m o v e m e n t i n three d i m e n s i o n s a n d eliminates the n e e d for a d j u s t i n g the n e e d l e p o s i t i o n w i t h respect to substrate a n d o p t i c a l p a t h . T h e l i q u i d d r o p s w e r e d i s p e n s e d w i t h the h e l p of a n u l t r a p r e c i s i o n m i c r o m e t e r s y r i n g e ( M a n o s t a t ) fitted w i t h a stainless steel n e e d l e w h i c h was p l a c e d i n a n o p t i c a l c e l l c o n t a i n i n g the s o l i d disc. T h e s y r i n g e was c l a m p e d o n a v i b r a t i o n - f r e e stand. I n a l l cases, the sessile


7

8

Table I.

c c Cio 7

8

Cl2 Ci6 cyclohexane ft-hexanol mineral oil

0

2

Surface and Interfacial Tensions (dynes/cm) of Mutually Saturated Liquids T l (water)

71/water

7water(l)

20.4 21.8 23.9 25.4 27.5 25.5 26.5 31

50.2 50.8 51.2 51.5 52.3 50.2 6.9 51.5

72 72 72 72 72 72 34.2 72


144

APPLIED

CHEMISTRY

AT PROTEIN

INTERFACES

d r o p m e t h o d was u s e d for the measurements w h i c h u s e d s t a n d a r d contact a n g l e g o n i o m e t e r e q u i p m e n t . A d v a n c i n g ( 0 ) a n d r e c e d i n g ( 0 ) angles w e r e m e a s u r e d w i t h the n e e d l e t i p i n t h e l i q u i d phase b y c h a n g i n g the v o l u m e of the d r o p u n t i l a m o v e m e n t of the contact l i n e was o b s e r v e d . T h i s d e v i c e a l l o w s the m e a s u r e m e n t of the contact angle at b o t h edges of the d r o p w i t h o u t a n y d i s t u r b a n c e s of the d r o p c o n f i g u r a t i o n . M e a s u r e ments w e r e c a r r i e d out at r o o m t e m p e r a t u r e (25.5 ± 1 . 5 ° C ) d u r i n g w i n t e r m o n t h s w h e n t h e a m b i e n t h u m i d i t y was l o w (.— 3 0 % ) (2). F o r measurements o n s k i n , the subject h e l d h e r finger a r o u n d a r u b b e r stopper to generate a s k i n surface as p l a n a r as possible. F o r measurements of contact angles o n solids s u b m e r g e d i n w a t e r , since a l l the h y d r o c a r b o n l i q u i d s i n v e s t i g a t e d w e r e l i g h t e r t h a n w a t e r , t h e c a p t i v e d r o p m e t h o d w a s u s e d . T h e substrate surface w a s n o r m a l l y l o w e r e d i n t o t h e aqueous phase w i t h the h e l p of the l a b o r a t o r y jack, a n d its d i s t a n c e f r o m the t i p of the n e e d l e w a s c o n t r o l l e d b y o b s e r v a t i o n t h r o u g h the m i c r o s c o p e . A d v a n c i n g a n d r e c e d i n g angles w e r e m e a s u r e d w i t h the n e e d l e t i p i n the o i l d r o p b y c h a n g i n g the v o l u m e of the d r o p . A l l measurements w e r e a g a i n c a r r i e d out at r o o m t e m p e r a t u r e . L i q u i d drops w e r e d e p o s i t e d o n at least five different locations o n the surface of a p a r t i c u l a r s o l i d as a c h e c k o n the r e p r o d u c i b i l i t y of t h e m e a s u r e d contact angle. A g r e e m e n t for t h e least r e p r o d u c i b l e surface, h o o f k e r a t i n , w a s ± 3 ° . F o r attenuated t o t a l reflectance ( A T R ) I R studies, a P e r k i n - E l m e r g r a t i n g I R s p e c t r o p h o t o m e t e r w a s u s e d w i t h a K R S - 5 plate.


A

R

Theoretical T h e F o w k e s (4)

e q u a t i o n for t h e i n t e r f a c i a l tension, y ,

between

12

t w o c o n t a c t i n g phases w h i c h interact b y d i s p e r s i o n forces o n l y , has the form 712

w h e r e ji

=

=

7i

+

7 2 - 2

V

surface tension of phase 1, y

=

2

7i 72 d

(1)

d

surface tension of p h a s e 2,

yi , J2 = the respective d i s p e r s i o n f o r c e c o m p o n e n t s of γι, d

y.

d

2

F o r the case w h e r e p o l a r force interactions are also i n v o l v e d , W u ( 5 ) has p r o p o s e d , b y u s i n g r e c i p r o c a l m e a n expressions f o r b o t h the d i s p e r sion a n d the p o l a r i n t e r a c t i o n s , that the i n t e r f a c i a l tension y

between

i2

t w o c o n t a c t i n g phases b e r e p r e s e n t e d as 4 7 /

712 = where γ / , y

p

2

7i +

72

7i

- 72

d

jt—â

+72

47i*>

- 72

/ x

P

9

χΓΤ—V

d

7i

p

+72

W

p

= the respective p o l a r f o r c e c o m p o n e n t s of γ

ί9

y. 2

T h e Y o u n g e q u a t i o n f o r a s o l i d / l i q u i d system has the f o r m yi cos0 =

js — Jsi

and combination w i t h E q u a t i o n 2 yields


(3)

6.

Keratin

E L - S H I M I A N D GODDARD

yi c o s 9 =

-

7, H

145

Substrates

, 7 s +yr d

Η

d

(4)

y +yi p

s

p

w h e r e s a n d Z, r e p l a c i n g 1 a n d 2 r e s p e c t i v e l y , d e n o t e s o l i d a n d l i q u i d . B y Zisman's (6)

definition, y

=

t

w e t t i n g , w h e n cos0 =

[y.

c

· yi

d

9

7s

d

=

· yi ~\

p

p

s

=

d

7c

, y

d

It is clear f r o m E q u a t i o n 5 that i f yi


the c r i t i c a l surface tension for

y,

1, a n d u n d e r these c o n d i t i o n s E q u a t i o n 4 becomes

y

d

s

a n d yf

+

=

y, p

s

then

7s

p

w h i c h represents a n expression for t h e t o t a l surface free energy of the solid. I n the s p e c i a l case w h e n i n t e r a c t i o n b e t w e e n s o l i d a n d l i q u i d at the p o i n t of c o m p l e t e

s p r e a d i n g is a t t r i b u t a b l e solely to d i s p e r s i o n

forces,

E q u a t i o n 5 becomes =

y

c

= ^4_ιζ 7*

=

y

/

( 6 )

+ 7 Γ

F o r the same c o n d i t i o n s , a s i m i l a r result emerges f r o m the

Fowkes-

Young equation yi cos0 =

-

yi +

2 V

O n the basis of E q u a t i o n 4, t h e values of y

d

s

(7)

y yi d

s

d

and y

p

s

for a s o l i d p o l y m e r

surface c a n be c a l c u l a t e d f r o m the contact angles o n i t of t w o l i q u i d s , the surface tensions of w h i c h h a v e b e e n defined i n terms of t h e respective c o n t r i b u t i o n of d i s p e r s i o n a n d p o l a r f o r c e components

(5).

I n this case,

E q u a t i o n 4 is r e a r r a n g e d to t w o simultaneous equations, a n d s o l v e d for y

d

and γ / . I n the case of a s o l i d i n contact w i t h t w o

(mutually

saturated)

l i q u i d s , p r e f e r e n t i a l w e t t i n g c a n b e p r e d i c t e d o n the basis of the contact angles of the i n d i v i d u a l l i q u i d s o n t h e g i v e n s o l i d i n a i r a n d a p p l i c a t i o n of Y o u n g ' s e q u a t i o n . w a t e r (w)

F o r e x a m p l e , consider the case of a n o i l ( o )

o n a l o w energy substrate (s),

s u c h that w e h a v e for

and the

a i r / w a t e r / s u b s t r a t e system y

s

—

y

s

cose „,

a n d for the a i r / o i l / s u b s t r a t e system


(8)

146


7s

—

7so

=

C0s9

To

INTERFACES

(9)

o

a n d for the o i l / w a t e r / s u b s t r a t e system y where y

(10)

— 7 s o = Ίow cosQow

8W

is the s o l i d surface tension a n d the other terms h a v e t h e i r u s u a l

s

significance. S u b s t i t u t i n g E q u a t i o n s 8 a n d 9 i n t o E q u a t i o n 10, w e o b t a i n the Bartell-Osterhof (7)

equation


y ow c o s 9

=

o w >

y ο cos9

— y

(11)

cosG

w

0

w

E q u a t i o n 11 a l l o w s contact angle p r e d i c t i o n i n systems c o n t a i n i n g t w o i m m i s c i b l e l i q u i d s a n d a s o l i d . S i n c e Y o u n g ' s e q u a t i o n is o n l y v a l i d i n cases w h e r e the contact angle is finite, E q u a t i o n 11 w o u l d not be expected to h o l d i n s u c h cases w h e r e the contact a n g l e θ equals zero i n a i r as is c o m m o n l y o b s e r v e d for l o w energy solids a n d n o n p o l a r l i q u i d s ( 8 ) . H o w e v e r , E q u a t i o n 11 c a n be v e r i f i e d u s i n g T e f l o n substrates since a l l c o m m o n n o n p o l a r l i q u i d s e x h i b i t a finite contact angle o n it. A less direct a p p r o a c h to p r e d i c t 0 , i n l i q u i d / l i q u i d / s o l i d systems, w h e r e θ = 0 i n air for one of the l i q u i d s , is to use a n e q u a t i o n l i k e F o w k e s ' ( 4 ) for i n t e r f a c i a l tension. T h u s w e w o u l d h a v e for the s o l i d / w a t e r system, OM

y.w

=

7s +

y

~

2 V

y -y

y ο

-

2

7 s

w

d

s

(12)

d

w

a n d for t h e s o l i d / o i l system, 7s

=

0

7s

+

V

- 7 o

d

(13)

d

S u b s t i t u t i n g E q u a t i o n s 12 a n d 13 i n t o 10 gives yw

eos&ow =

0

y

2 V

-

w

y y d

s

-

d

w

y

2 V~t7t7

+

0

(14)

w h e r e the y terms are the d i s p e r s i o n c o m p o n e n t of the surface tensions. y is u s u a l l y t a k e n as e q u a l to y for n o n p o l a r l i q u i d s . d

s

d

0

0

E q u a t i o n 14 allows us to p r e d i c t 6 w f r o m y v a l u e of ca. 22 e r g / c m a c c o r d i n g to F o w k e s (4). 0

and y

d

s

w h i c h has a

d

w

2

C o r r e s p o n d i n g l y , u s i n g the W u ( 5 ) expression w e w o u l d h a v e y ow C O S 0

OW

=

4y -y y + y d

y

s

s

,

4 7 s

y

s

d

d

* y · 7 100° (2, 8, 12).

T h e f o r m e r corresponds m o r e to s k i n i n a n o r m a l state a n d

t h e l a t t e r to s k i n that has b e e n s u b j e c t e d to severe c l e a n i n g , g e n e r a l l y i n v o l v i n g exposure to a d e l i p i d i z i n g solvent. Severe c l e a n i n g m a y affect the s t r u c t u r e of t h e outer l a y e r of t h e s k i n . S i n c e o u r interest w a s i n t h e Table I V . Comparison of Predicted ® w Values (degrees) in O i l / W a t e r / T e f l o n System (y — 17, y* = 1.7 dynes/cm) with Experimental D a t a 0

d

s

Equation u c c Cio 7

8

Cl2 Ci6 Cyclohexane Mineral oil n-Hexanol

0 0 14 14 19 9 20 0

Equation 15 29 30 33 35 37 32 38 0

Experimental (ΘΑ)

(ΘΒ)

42 40 41 40 38 42 37 48

32 31 32 33 34 33 33 40

Equation 11


25

— — — — — —

38

6.


Keratin

149

Substrates

n o r m a l state of the s k i n , the s k i n surface w a s p r e p a r e d w i t h b a r soap w ashing f o l l o w e d b y t h o r o u g h r i n s i n g u n d e r r u n n i n g t a p w a t e r . r

T o c h e c k the results, a s e c o n d p r o c e d u r e w a s u s e d to e l i m i n a t e the p o s s i b i l i t y of l i m e soap deposits. vigorously rubbed i n a 4 %

I n this p r o c e d u r e ,

the hands

were

n o n i o n i c detergent ( N e o d o l 4 5 - 1 1 E O , S h e l l )

s o l u t i o n i n d i s t i l l e d w a t e r f o l l o w e d b y extensive r i n s i n g w i t h

distilled

w a t e r . T h i s second t r e a t m e n t gave contact angles w h i c h a g r e e d w i t h i n e x p e r i m e n t a l error w i t h those

o b t a i n e d b y the s o a p / t a p

w a t e r rinse

procedure. Table V . Comparison of Predicted ® Values in O i l / W a t e r / P M M A System (γ* = 33, y * dynes/cm) with Experimental Data o w


8

(degrees) = 12

Experimental Equation c c

7 8

ClO

Cl2 Cj6 Cyclohexane Mineral oil n-Hexanol

14

Equation

6 9 8 9 12 0 9 0

15

78 77 76 76 76 76 75 180

(ΘΑ)

(ΘΕ)

94 92 94 93 92 95 93 47

80 75 78 77 75 79 78 35

T h e d a t a i n T a b l e I I I r e v e a l that whereas the d e r i v e d y

s

v a l u e of

h u m a n s k i n is v e r y close to that of hoof k e r a t i n , its p o l a r c o m p o n e n t

is

r e l a t i v e l y m u c h h i g h e r . T h i s reflects the presence of p o l a r l i p i d s i n t h e stratum c o r n e u m layer. It is clear, h o w e v e r , t h a t b o t h s k i n a n d k e r a t i n present l o w free e n e r g y surfaces c o m p a r a b l e i n w e t t i n g characteristics w i t h those of p o l y e t h y l e n e . L i t e r a t u r e values of the c r i t i c a l surface tension of s k i n r a n g e f r o m 26 to 27.5 d y n e s / c m ( 2 , 11, 12).

I n a l l these cases, aqueous solutions of

materials s u c h as acetone ( 2 , 1 3 ) or p r o p y l e n e g l y c o l (11) the y

c

w e r e u s e d for

d e t e r m i n a t i o n to o b t a i n a n a p p r o p r i a t e r a n g e of l i q u i d

tensions. D a n n ( 9 ) , h o w e v e r , has p o i n t e d out t h a t y

c

surface

values o b t a i n e d i n

this w a y w i l l b e less t h a n the v a l u e o b t a i n e d w i t h l i q u i d s , s u c h as h y d r o carbons, w h i c h d o not possess a y

p

component.

Correcting an experi

m e n t a l v a l u e of 27.5 d y n e s / c m f o r this effect, R o s e n b e r g et al. (12) t a i n e d 37.0 d y n e s / c m for y . c

A s suggested b y M u r p h y et al

(13)

ob

i n their

w o r k o n w e t t i n g b y aqueous a l c o h o l solutions, the a b o v e effects are p r o b a b l y l i n k e d to p r e f e r e n t i a l a d s o r p t i o n of the solute onto the s o l i d . I n the w o r k o n the w e t t i n g b e h a v i o r of e t h a n o l / w a t e r m i x t u r e s o n n y l o n 11, P M M A , b o v i n e h o o f k e r a t i n , a n d h u m a n s k i n , w e f o u n d


two

150


INTERFACES

Table V I . Comparison of Predicted ® tv Values in O i l / W a t e r / N y l o n System (γ* = 32, y* — 12 dynes/cm) with Experimental Data 0

14

Equation C C Cio

6 9 8 9 12 0 9 0

7

8

C12

C Cyclohexane Mineral oil n-Hexanol Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0145.ch006

M

Equation

15

78 77 76 76 76 76 75 180

i m p o r t a n t results: t h e d e r i v e d y

(27 ±

c

ΘΑ

Experimental >170 >170 >170 >170 >170 >170 >170 115

0.5) is i n d e p e n d e n t of the s o l i d

surface, a n d i t is c o n s i d e r a b l y l o w e r t h a n t h a t o b t a i n e d b y u s i n g n o n p o l a r liquids. These

findings

thus r e i n f o r c e those of M u r p h y et al. (13)

who

also f o u n d that t h e d e r i v e d y values w e r e r a t h e r insensitive to t h e w a t e r c

s o l u b l e a l k a n o l u s e d (10).

T h e y e x p l a i n this b y the a d s o r p t i o n of the

a l c o h o l m o l e c u l e s at t h e s o l i d surfaces w h i c h s u b s t a n t i a l l y affects t h e i r w e t t i n g properties (10, 13).

C a r e is o b v i o u s l y n e e d e d w h e n u s i n g a q u e

ous solutions as w e t t i n g l i q u i d s to d e r i v e values of the c r i t i c a l

surface

tension of w e t t i n g . T u r n i n g n o w to t h e s o l i d / w a t e r / o i l measurements, w e c o m p a r e t h e p r e d i c t e d θ ιν values a c c o r d i n g to E q u a t i o n s 14 a n d 15 w i t h e x p e r i m e n t a l 0

values i n T a b l e s I V - V I I for the substrates i n v e s t i g a t e d . I n the case of T e f l o n , w h e r e i t is possible to test E q u a t i o n 11, values a r e g i v e n f o r heptane a n d n-hexanol.

T h e d i s p e r s i o n a n d p o l a r c o m p o n e n t s of t h e

surface tension of water—n-hexanol, i.e. w a t e r s a t u r a t e d w i t h n - h e x a n o l , a n d h e x a n o l - w a t e r w e r e o b t a i n e d b y m e a s u r i n g t h e contact l i q u i d drops o n paraffin w a x ( y

s

d

angle of

= 25.5 d y n e s / c m ), w h i c h s e r v e d as a

Table VII. Comparison of Predicted ® Values in O i l / W a t e r / B o v i n e Hoof Keratin System (y = 3.28, y = 5.6 dynes/cm) with Experimental Data o w

d

s

p

s

Equation C C Cio, C Cie Cyclohexane Mineral oil n-Hexanol 7

8

1 2

6 9 8 9 12 0 9 0

14

Equation 54 54 53 53 53 51 51 136

15

Experimental >170 >170 >170 >170 >170 >170 > 170 >170


Θ

Α

6.


Keratin

151

Substrates

p r o b e f o r d i s p e r s i o n interactions, a n d t h e n a p p l y i n g t h e F o w k e s - Y o u n g equation .—

where y

(cose +

^ =

v

1)71

2 ^

is t h e surface energy of paraffin w a x , a n d yi is t h e d i s p e r s i o n

d

d

s

c o m p o n e n t of t h e l i q u i d surface tension. T h e values o b t a i n e d for n - h e x a n o l - w a t e r w e r e y

d

t

1.9 d y n e s / c m a n d f o r w a t e r - h e x a n o l , y

d

t

= 24.6 a n d y

p

t

=

= 22 a n d yf = 12.4 d y n e s / c m .

A s regards t h e contact angle results o b t a i n e d w i t h n - h e x a n o l , values o f 6 Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0145.ch006

0W

o b t a i n e d f r o m E q u a t i o n s 14 a n d 15 a n d b y e x p e r i m e n t s h o w a w i d e

disparity.

See T a b l e s I V - V I I .

F o r paraffin w a x , b o t h equations p r e d i c t

zero contact angle f o r n - h e x a n o l whereas t h e e x p e r i m e n t a l values are 45° (Θ ) a n d 3 7 ° C (θ ). Α

A s discussed earlier, a d s o r p t i o n of alcohols at

1{

the s o l i d / l i q u i d interface m a y affect t h e w e t t i n g b e h a v i o r of substrates. T h e s e effects are n o t a c c o u n t e d f o r b y the F o w k e s or W u treatment, a n d h e n c e i t is n o t u n e x p e c t e d t h a t these equations d o n o t correlate w i t h experimental θ

οηι

values for h e x a n o l / w a t e r .

A l t h o u g h E q u a t i o n s 14 a n d 15 are i n a d e q u a t e i n p r e d i c t i n g contact angles of h e x a n o l / w a t e r systems, E q u a t i o n 11 b a s e d solely o n t h e Y o u n g e q u a t i o n leads to m u c h better agreement i n t h e case w h e n i t w a s a p p l i e d , viz., f o r T e f l o n . C o n s i d e r i n g t h e results w i t h n o n p o l a r oils, w e find that b o t h E q u a tions 14 a n d 15 p r e d i c t zero contact angle of t h e h y d r o c a r b o n l i q u i d s o n paraffin w a x i n w a t e r . T h i s w a s c o n f i r m e d e x p e r i m e n t a l l y , w i t h e v i d e n c e that t h e h y d r o c a r b o n l i q u i d s h a d a c t u a l l y e t c h e d t h e paraffin w a x surface. T h e results for the T e f l o n surface are i n t e r e s t i n g i n that t h e e x p e r i m e n t a l values f o r θ ι are i n f a i r agreement w i t h W u ' s e q u a t i o n w h i c h , u n l i k e t h e 0 ν

F o w k e s e q u a t i o n , takes i n t o account n o n - d i s p e r s i o n force interactions. T h e correlation w i t h θ

η

for P M M A predict θ

οχο

is g e n e r a l l y better t h a n w i t h Θ · T h i s also h o l d s Α

( T a b l e V ) . A s a n e x a m p l e of t h e use of E q u a t i o n 11 to f o r a h y d r o c a r b o n l i q u i d , the values for h e p t a n e o n T e f l o n

are i n c l u d e d i n T a b l e I V . A g r e e m e n t w i t h e x p e r i m e n t w a s satisfactory, b u t w i t h t h e p r e d i c t e d v a l u e a g a i n i n better agreement w i t h θ . κ

T a b l e s V I a n d V I I s h o w t h e results f o r n y l o n 11 a n d b o v i n e h o o f k e r a t i n . I n b o t h cases t h e p r e d i c t e d values f o r θ

οιο

a c c o r d i n g to E q u a t i o n s

14 a n d 15 v a r y m a r k e d l y w i t h t h e e x p e r i m e n t a l θ , v a l u e s ; note t h e d i s ου

parity i n the predicted 0

OW

values a c c o r d i n g to E q u a t i o n s 14 a n d 15.

E x p e r i m e n t a l l y , t h e s i t u a t i o n w i t h n y l o n 11 a n d h o o f k e r a t i n w a s c l e a r l y v e r y different f r o m t h e other p o l y m e r s as n o contact b e t w e e n t h e h y d r o c a r b o n d r o p a n d substrate c o u l d b e established. T h e o b s e r v e d ancy between predicted θ

ονο

discrep

values a c c o r d i n g t o b o t h E q u a t i o n s 14 a n d 15


152


INTERFACES

a n d e x p e r i m e n t a l values is v e r y l i k e l y because of the t e n d e n c y of n y l o n a n d k e r a t i n w h e n s u b m e r g e d i n w a t e r to h y d r a t e , g i v i n g rise to stable w a t e r films o n the surface.

T h e thickness of s u c h aqueous

films

could

c o n c e i v a b l y e x t e n d to a f e w m o l e c u l a r layers. T h e s e films w o u l d t e n d to increase t h e a t t r a c t i o n b e t w e e n the aqueous phase a n d the s o l i d substrate at the expense of i n t e r a c t i o n w i t h the h y d r o c a r b o n l i q u i d . It w a s t h o u g h t that the film m a y b e s l o w d r a i n i n g a n d that i f t h e d r o p w e r e pressed against the substrate for a sufficient p e r i o d of t i m e , i t w o u l d b e possible to establish contact b e t w e e n the h y d r o c a r b o n l i q u i d a n d substrate. E v e n 1 h r was insufficient to o b t a i n a contact angle r e a d i n g , a n d t h e w h o l e d r o p d e t a c h e d i n t a c t u p o n recession a n d w i t h d r a w a l .


W e h a v e c o n d u c t e d some q u a l i t a t i v e experiments to g a i n s o m e m o r e i n f o r m a t i o n o n the b e h a v i o r of the h y d r a t i o n films o n n y l o n a n d k e r a t i n w h e n discs of the m a t e r i a l s are b r o u g h t i n t o a i r after b e i n g s u b m e r g e d for a p e r i o d of t i m e u n d e r w a t e r .

T h e i n i t i a l aqueous

l a y e r present

exhibits m a r k e d i n s t a b i l i t y a n d recedes i n t o discrete drops.

T h e discs

w i t h these aqueous d r o p s w e r e gently d r i e d either w i t h a n ashless p a p e r or b y b l o w i n g t h e

fluorocarbon

filter

F - 1 2 ( F a l c o n Safety P r o d u c t s )

across the surface for a f e w seconds. T h e discs a p p e a r e d d r y after this treatment. W h e n w a t e r d r o p s w e r e p l a c e d o n the surface a g a i n , the s u r faces w e r e not w e t t e d , a n d the aqueous drops e x h i b i t e d a contact angle c o m p a r a b l e w i t h those o b t a i n e d o n substrates d r i e d i n a v a c u u m for several days. It s e e m e d i n c o n c e i v a b l e t h a t s u c h gentle d r y i n g p r o c e d u r e s of h y d r a t e d surfaces c o u l d e l i m i n a t e a l l the associated surface w a t e r a n d r e n d e r it h y d r o p h o b i c a g a i n . A T R I R spectroscopy

revealed no water

b a n d for n y l o n , b u t k e r a t i n d i d e x h i b i t a w a t e r b a n d at 3 0 0 0 - 2 8 0 0 c m " . 1

T h e latter c o u l d h a v e b e e n t h e result of s q u e e z i n g w a t e r out of m i n u t e pores w h e n a p p l y i n g the pressure necessary to e s t a b l i s h a d e q u a t e contact b e t w e e n the substrate a n d the I R p l a t e . T o e x a m i n e the h y d r a t i o n of b o v i n e h o o f k e r a t i n f u r t h e r , specimens ( s u p p l i e d b y J . C l i f f o r d , N . G . P r y c e , a n d G . K . R e n n i e of U n i l e v e r R e s e a r c h , P o r t S u n l i g h t , E n g l a n d ) w e r e s t u d i e d b y t w o other t e c h n i q u e s , viz.,

p u l s e d N M R o n 6 - m m d i a m e t e r a n d 6 - m m t h i c k B H K discs, a n d

differential s c a n n i n g c a l o r i m e t r y ( D S C ) o n B H K p o w d e r .

T h e keratin

discs took u p 3 6 % of t h e i r w e i g h t i n w a t e r i n 25 hrs. T h e u p t a k e was l i n e a r w i t h the square root of t i m e , a n d the discs suffered less t h a n a 5 % c h a n g e i n l i n e a r d i m e n s i o n s d u r i n g this process.

A f t e r 5 hrs t h e u p t a k e

was — 1 4 % , a n d h a l f of this w a t e r , i.e. ~ 7 % , h a d a r e l a x a t i o n t i m e , T , 2

c h a r a c t e r i s t i c of b o u n d water. D S C o n k e r a t i n p o w d e r exposed to w a t e r s h o w e d that a n u p t a k e of — 2 0 0 % w a t e r o c c u r r e d i n 2 days, w i t h 3 6 % being bound.

T h i s p r o b a b l y represents the e q u i l i b r i u m v a l u e .

results c o n f i r m t h e h i g h l y h y d r a t a b l e n a t u r e of k e r a t i n .

These

W h i l e bound

w a t e r is n o t r e a d i l y lost o n exposure to l a b o r a t o r y atmosphere, i t is not


6.


Keratin

153

Substrates

possible t o m a k e a n y d e d u c t i o n s a b o u t w a t e r i n the surface l a y e r w h i c h w o u l d c o n t r o l the w e t t i n g b e h a v i o r of the specimens.

Further informa

t i o n is necessary to e l u c i d a t e the c o n d i t i o n o f the surface o f k e r a t i n after its r e m o v a l f r o m w a t e r . Studies of B l a k e a n d K i t c h e n e r (14) o n the s t a b i l i t y o f aqueous films o n m e t h y l a t e d s i l i c a s h o w t h a t t h i c k aqueous films are essentially m e t a stable a n d sensitive to e n v i r o n m e n t a l d i s t u r b a n c e s .

I f t h e thickness is

decreased to a c r i t i c a l v a l u e , the film becomes u n s t a b l e a n d breaks into discrete drops i n e q u i l i b r i u m w i t h a n a d s o r b e d film, p r e s u m a b l y

mono-

m o l e c u l a r . W e h a v e s h o w n that n y l o n a n d b o v i n e h o o f k e r a t i n are essen t i a l l y h y d r o p h o b i c i n air. I t is possible that a n analogous

mechanism


operates i n these cases, i.e., w a t e r films associated w i t h t h e surface a r e i n h e r e n t l y u n s t a b l e after e m e r s i o n a n d are easily r e m o v e d f r o m the sur face e v e n b y the m i l d w i p i n g p r o c e d u r e .

A n o t h e r possible

mechanism

to e x p l a i n the h y d r o p h i l i c - h y d r o p h o b i c t r a n s i t i o n i n a i r of h y d r a t -

(15)

a b l e l o w energy surfaces is r e l a t e d to the c o n f o r m a t i o n o f the p o l y m e r i c surface molecules w h i c h m i g h t e x t e n d into aqueous phase i n the f o r m of loops.

T h e c o n f i g u r a t i o n o f these p o l y p e p t i d e molecules

w h e n totally

i m m e r s e d i n w a t e r w o u l d thus differ f r o m that w h e n i n air. I n other w o r d s , w e postulate a degree of f l e x i b i l i t y o f the molecules i n the surface r e g i o n w h i c h c a n a i d t h e i r h y d r a t i o n b y f o r m a t i o n of n e w h y d r o g e n bonds.

T h e speed o f restoration of h y d r o p h o b i c properties i n air e m p h a

sizes the r e v e r s i b l e n a t u r e of a n y c o n f i g u r a t i o n a l changes. A n o t h e r i n t e r e s t i n g feature o f n y l o n a n d h o o f k e r a t i n surfaces is o b s e r v e d w h e n these substrates are treated w i t h m i n e r a l o i l i n a i r ( w h e r e θ = 0 ) , t h e n s u b m e r g e d i n w a t e r . T h e objective was to s t u d y the rate of o i l recession as a result o f the h y d r a t i o n process d e s c r i b e d above. I n the case o f n y l o n , 0 a t t a i n e d a m a x i m u m v a l u e o f > 170° i n the course R

of a f e w hours as expected.

H o w e v e r , h o o f k e r a t i n r e t a i n e d the o i l i n

the f o r m of a w e t t i n g d r o p (B ~ R

3 0 ° ) f o r a l o n g t i m e ( 4 8 h r s ) ; this is

a b o u t t h e same θ v a l u e o b t a i n e d o n T e f l o n . I t seems t h a t the d i s p e r s i o n i n t e r a c t i o n forces b e t w e e n o i l a n d k e r a t i n are strong e n o u g h that n o n d i s p e r s i o n i n t e r a c t i o n b e t w e e n the k e r a t i n a n d w a t e r is not sufficient t o d i s p l a c e t h e o i l at least w i t h i n t h e t i m e l i m i t of the e x p e r i m e n t . T h e p a r t i c u l a r b e h a v i o r o b s e r v e d is thus g o v e r n e d b y the h i s t o r y of exposure. T h i s m a y b e a specific feature o f n a t u r a l k e r a t i n surfaces, i.e., surface m o l e c u l e s are able t o a d o p t a n d r e t a i n a configuration c o m p a t i b l e w i t h their immediate environment.

Literature Cited 1. Onken, H. D., Moyer, C. Α., Arch. Dermatol. (1963) 87, 584. 2. Ginn, M. E., Noyes, C. M., Jungermann, E., J. Colloid Interface Sci. (1968) 26, 146.



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APPLIED CHEMISTRY AT PROTEIN INTERFACES

3. Alter, H., Cook, H., J. Colloid Interface Sci. (1969) 29, 439. 4. Fowkes, F. M., J. Phys. Chem. (1962) 66, 382. 5. Wu, S.,J.Polym. Sci. Part C (1971) 34, 19. 6. Zisman, W. Α., ADVAN. CHEM. SER. (1964) 43, 1. 7. Bartell, F. E., Osterhof, H. J., Colloid Symp. Monogr. (1927) 5, 113. 8. Adamson, A. W., Knichika, K., Shirley, F., Orem, M. J., J. Chem. Ed. (1968) 45, 702. 9. Dann, J. R., J. Colloid Interface Sci. (1970) 32, 302. 10. El-Shimi, Α., Goddard, E. D., J. Colloid Interface Sci. (1974) 48, 242. 11. Schott, H., J. Pharm. Sci. (1971) 60, 1893. 12. Rosenberg, Α., Williams, R., Cohen, G., J. Pharm. Sci. (1973) 62, 920. 13. Murphy, W. J., Roberts, M. W., Ross, R. H., J. Chem. Soc. Faraday I. (1972) 68, 1190. 14. Blake, T. D., Kitchener, J. Α., J. Chem. Soc. Faraday I. (1972) 1435. 15. Van den Tempel, M., personal communication. 16. Schonhorn, H., Encyclopedia of Polymer Science and Technology (1 13, 543. 17. Wu, S.,J.Phys. Chem. (1968) 72, 3332. RECEIVED

December 4, 1973.


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