DTA Study of Water in Porous Glass - ACS Publications - American

9 DTA Study of Water in Porous Glass CHAUR-SUN LING and W. DROST-HANSEN

Downloaded by MIT on April 16, 2013 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0008.ch009

Laboratory for Water Research, Department of Chemistry, University of Miami, Coral Gables, Fla. 33124

Introduction The properties of water near many interfaces are notably different from the properties of bulk water (or bulk aqueous solutions). Examples of unusual properties in interfacial behavior have been reviewed by various authors (1-4) and a few specific examples will be reviewed briefly in this paper together with some recent calorimetric measurements on water near silica surfaces. The problem of the structure of bulk water remains unsolved; several excellent reviews of water structure have been presented recently (5-7). In view of the gap in our fundamental knowledge about water, it is hardly surprising that far less is known about the structure of water near interfaces. Nonetheless, some information is beginning to accumulate, suggesting various possible features of vicinal water structuring (3, 8-11). Specifically, it appears that more than one type of structure may be stabilized preferentially near an interface over a certain temperature interval, while other types of structures may prevail in other temperature intervals. Thus, relatively abrupt changes in properties of vicinal water have frequently been observed near the following temperature ranges: 14-16°; 29-32°; 44-46°; and 59-62°C. A review of the thermal anomalies and specifically, the role of this phenomenon in biological systems is given in (12). The occurrence of no less than four different transition regions suggests that there exists at least five different types of structures which may be stabilized adjacent to various interfaces. For this reason, it has been suggested previously (3, 9, 13) that the anomalies represent higher-order phase transitions in vicinally ordered structures. The purpose of the present study was to test this suggestion through a calorimetric study of the properties of water adjacent to silica surfaces. Before proceeding, attention is called to the fact that water - especially in the solid state - may indeed occur in a 129 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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Zeitschrift fur Physikalische Chemie

Figure 1. Highly schematized drawing of disjoining pressure apparatus by Peschel (17). B, Balance beam; M, magnet; C, coil; A - A ' , optically polished quartz plates, submersed in liquid under study.

Naturwissenschaften

Figure 2. Arrhenius plot of viscosity of water between quartz plates (18) for various plate separations. 0> 30 nm; A, 50 nm; V, 70 nm; {j, 90 nm.

In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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l a r g e v a r i e t y of s t r u c t u r e s . Thus, at l e a s t e i g h t d i f f e r e n t , s t a b l e h i g h - p r e s s u r e p o l y m o r p h s e x i s t i n a d d i t i o n t o an i m p r e s s i v e a r r a y o f p o l y h e d r a l c l a t h r a t e s ( s e e p a r t i c u l a r l y 12_ pp. 55-60). I t w o u l d h a r d l y be s u r p r i s i n g i f somewhat s i m i l a r s t r u c t u r e d e l e m e n t s m i g h t e x i s t a d j a c e n t t o an i n t e r f a c e a n d b e t r a n s f o r m e d , a t s p e c i f i c c r i t i c a l t e m p e r a t u r e s , f r o m one t y p e o f s t r u c t u r e to another. The i m p o r t a n t q u e s t i o n h e r e i s w h e t h e r the changes r e p r e s e n t h i g h e r - o r d e r phase t r a n s i t i o n s o r f i r s t order phase t r a n s i t i o n s . O b v i o u s l y , a change f r o m , s a y , I c e I t o I c e I I , i s a f i r s t - o r d e r phase t r a n s i t i o n , c h a r a c t e r i z e d by a latent heat. I f , instead, v i c i n a l l y stabilized structures t r a n s f o r m f r o m one s t r u c t u r e t o a n o t h e r w i t h o u t a s p e c i f i c l a t e n t h e a t , t h e t r a n s i t i o n s w i l l be s e c o n d o r h i g h e r - o r d e r ( s e e 1 4 - 1 6 ) .


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M e a s u r e m e n t s a r e r e p o r t e d i n t h i s p a p e r on t h e t h e r m a l e f f e c t s o b s e r v e d i n a DTA c a l o r i m e t r i c s t u d y o f w a t e r a d j a c e n t to s i l i c a surfaces. The o c c u r r e n c e o f s t r u c t u r a l c h a n g e s a t d i s c r e t e (and r e l a t i v e l y i n v a r i a n t ) t e m p e r a t u r e s a r e r e a s o n a b l y w e l l c o n f i r m e d by t h e o b s e r v e d changes i n s l o p e s and b a s e l i n e s h i f t s o f t h e thermograms. I n a number o f c a s e s , s m a l l e n d o t h e r m i c p e a k s w e r e o b s e r v e d i n t h e thermograms ( p r i m a r i l y s e e n d u r i n g h e a t i n g ) , s u g g e s t i n g the o c c u r r e n c e of a l a t e n t heat of transition. However, t h e s e peaks w e r e by no means i n v a r i a b l y o b t a i n e d (and i n a few c a s e s , a p p a r e n t e x o t h e r m i c p e a k s w e r e , i n f a c t , encountered). The u t i l i t y o f a c a l o r i m e t r y a p p r o a c h t o the problem of the n a t u r e of the t h e r m a l t r a n s i t i o n s i n v i c i n a l water has been c l e a r l y demonstrated i n t h i s s t u d y , but the e x p e r i m e n t a l d i f f i c u l t i e s which were encountered p r e v e n t any f i r m c o n c l u s i o n s t o b e r e a c h e d r e g a r d i n g t h e s p e c i f i c n a t u r e of the t r a n s i t i o n s , i . e . , whether t r a n s i t i o n s a r e of f i r s t or h i g h e r - o r d e r . Background Before describing some p e r t i n e n t e a r l i e r w i l l be r e v i e w e d .

the r e s u l t s of the p r e s e n t i n v e s t i g a t i o n , r e s u l t s from s t u d i e s of v i c i n a l water

P e s c h e l ( 1 7 ) a n d P e s c h e l and A d l f i n g e r ( 1 8 , J.9) h a v e s t u d i e d the p r o p e r t i e s of l i q u i d s a t i n t e r f a c e s u s i n g a h i g h l y s e n s i t i v e e l e c t r i c b a l a n c e f o r m e a s u r i n g f o r c e s and d i s p l a c e ments o f two e x t r e m e l y c a r e f u l l y p o l i s h e d q u a r t z p l a c e s imm e r s e d i n t h e l i q u i d t o be s t u d i e d . F i g u r e 1 shows a h i g h l y s c h e m a t i z e d d r a w i n g o f t h e d e v i c e u s e d by P e s c h e l and A d l f i n g e r . With t h i s instrument, i t i s p o s s i b l e to determine the e f f e c t i v e , a v e r a g e v i s c o s i t y o f t h e w a t e r b e t w e e n t h e p l a t e s , as t h e ( e x c e e d i n g l y smooth) t o p p l a t e " s e t t l e s " o n t o t h e b o t t o m plate. O b v i o u s l y , t h e r a t e o f s e t t l i n g w i l l d e p e n d on t h e v i s c o s i t y o f t h e l i q u i d b e t w e e n t h e two p l a t e s . (Note t h a t the t o p p l a t e i s s l i g h t l y c u r v e d ; r a d i u s o f c u r v a t u r e , one m e t e r ; w h i l e the bottom p l a t e i s o p t i c a l l y f l a t ) . The r e s u l t s o f s u c h measurements a r e shown i n F i g u r e 2. Here the l o g a r i t h m o f the


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Figure 3. Disjoining pressure for various plate separations: a) 50 nm; b) 30 nm; and c) 10 nm

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Figure 4. Distance (h ) for which the disjoining pressure is 10 dynes/cm as a function of temperature +

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(apparent) v i s c o s i t y i s p l o t t e d as a f u n c t i o n of temperature f o r v a r i o u s p l a t e s e p a r a t i o n s . Note that d i s t i n c t maxima and minima are observed f o r a l l p l a t e separations s t u d i e d . This unusual Arrhenius p l o t i s one of the most astounding pieces of evidence f o r the existence of abrupt changes i n s t r u c t u r e of aqueous i n t e r f a c i a l l a y e r s , as discussed i n the I n t r o d u c t i o n . The apparatus has a l s o been used by P e s c h e l and A d l f i n g e r to measure the d i s j o i n i n g pressure between the two p l a t e s (19). Some r e s u l t s from t h i s study are shown i n Figure 3. The ob served d i s j o i n i n g pressure i s seen t o go through very sharp maxima at a number of temperatures, namely, near 13, 32, 45 and 61°C ( f o r a p l a t e s e p a r a t i o n of 50 nm). From the d i s j o i n i n g pressure data, Peschel and A d l f i n g e r c a l c u l a t e d the distance at which the d i s j o i n i n g pressure a t t a i n e d an a r b i t r a r i l y se l e c t e d value ( 1 0 dynes per square centimeter). The r e s u l t i s shown i n Figure 4, from which i t i s seen that a t the tempera tures o f the thermal anomalies, the d i s t a n c e at which the d i s j o i n i n g pressure reaches t h i s p a r t i c u l a r v a l u e , approaches 0.1 x H T cm ( O . l ^ m ) . The s t u d i e s by P e s c h e l and A d l f i n g e r have added s i g n i f i cant evidence f o r the e x i s t e n c e of h i g h l y anomalous tempera ture-dependent p r o p e r t i e s of water near a quartz s u r f a c e . Note a l s o that the d i s t a n c e over which the e f f e c t s are observed i s of the order of 0.1^//m - a distance which i s extremely l a r g e compared t o " c l a s s i c a l views" regarding s t r u c t u r a l e f f e c t s near an i n t e r f a c e (such as i m p l i e d i n statements about "adsorbed water", "surface m o d i f i e d water", "non-solvent water", e t c . ) . Distances of the order of 300 water molecule diameters are ob v i o u s l y of s i g n a l importance t o such d i v e r s e aspects o f aqueous surface and c o l l o i d chemistry as n u c l e a t i o n r a t e s , membrane phenomena, and c e l l p h y s i o l o g y , to mention but a few. The measurements reported by P e s c h e l and A d l f i n g e r were c a r r i e d out only w i t h pure water. However, thermal anomalies near q u a r t z i n t e r f a c e s have a l s o been observed f o r both pure water and aqueous s o l u t i o n s of e l e c t r o l y t e s by K e r r and Drost-Hansen (20). The instrument constructed by K e r r i s a long, t h i n - w a l l e d c a p i l l a r y , v i b r a t i n g i n an evacuated g l a s s c y l i n d e r . This device was o r i g i n a l l y developed by Thurn (21) for measurements of v i s c o s i t i e s (and more r e c e n t l y used f o r measurements of d e n s i t i e s ( 2 2 ) J The method was employed by F o r s l i n d (23) t o study the r h e o l o g i c a l p r o p e r t i e s of water. H i s r e s u l t s are shown i n Figure 5; the ordinate i n t h i s graph i s the h a l f - l i f e of the v i b r a t i o n times (t-^iy) Î a b s c i s s a i s the tem perature. I t i s seen that a notable maximum i n the h a l f - l i f e o f the v i b r a t i o n s occurs near 30°C. The measurements by K e r r have confirmed the r e s u l t s ob t a i n e d by F o r s l i n d . Figure 6 shows a t y p i c a l run obtained by K e r r , demonstrating an abrupt increase i n the h a l f - l i f e o f the v i b r a t i o n s near 30°C. S i m i l a r r e s u l t s have a l s o been obtained w i t h sodium c h l o r i d e s o l u t i o n s i n v a r i o u s concentrations (0.01; 4

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Figure 6.

Half-life (T ),in seconds, of vibrations of water-filled, "hairpin" capillary as a function of temperature ( 20) 1/2



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0.05: 0.2,molar). However, f o r a 2 molar s o l u t i o n , the thermal anomalies disappeared. Figure 7 shows the r e s i s t a n c e of the c a p i l l a r y ( i n the v i b r a t i n g h a i r p i n c a p i l l a r y viscometer) as a f u n c t i o n of tempera t u r e . I t i s seen that near 30°C the temperature c o e f f i c i e n t of the r e s i s t a n c e changes s i g n . The change i n the r e s i s t a n c e curve i s remarkably abrupt, s i m i l a r to the changes i n the h a l f - l i f e of the v i b r a t i o n s . In a separate paper (10), one of us (W. D r o s t Hansen) has speculated on v a r i o u s molecular models to account f o r the r e s u l t s obtained i n t h i s study and has compared these f i n d i n g s to other s t u d i e s of temperature-dependent s u r f a c e p r o perties . As a f i n a l example of thermal anomalies, some recent data by Wiggins (24) w i l l be reviewed. Wiggins measured the adsorp t i o n of equimolar s o l u t i o n s of sodium and potassium s a l t s on s i l i c a g e l . Equimolar s o l u t i o n s (0.075) moij-ar) o f e i t h e r so dium c h l o r i d e and potassium c h l o r i d e , or Na and Κ i o d i d e , or Na and Κ s u l f a t e were e q u i l i b r a t e d w i t h Davison s i l i c a g e l , type 950. Samples were "incubated" (with shaking) a t v a r i o u s temperatures o v e r n i g h t ; a f t e r e q u i l i b r a t i o n the supernatant l i q u i d was decanted from the s i l i c a g e l . Analyses were then made, u s i n g a flame photometer, of the c o n c e n t r a t i o n s of the Na and Κ i n the supernatant l i q u i d and the l i q u i d contained i n the pores of the s i l i c a g e l . For each i o n (sodium and potassium), a p a r t i t i o n c o e f f i c i e n t , A , was defined by the e x p r e s s i o n s : +

[κΊο A

N a

+

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+

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where the s u b s c r i p t s i and o, r e s p e c t i v e l y , r e f e r to pore s o l u t i o n and b u l k s o l u t i o n . Using the two p a r t i t i o n c o e f f i c i e n t s , a s e l e c t i v i t y c o e f f i c i e n t , K|L was formed as the r a t i o of the two p a r t i t i o n c o e f f i c i e n t s (Equation 3 ) .

F i g u r e 8 shows the temperature v a r i a t i o n of the s e l e c t i v i t y c o e f f i c i e n t f o r the three s a l t combinations s t u d i e d . I t i s obvious that h i g h l y anomalous temperature dependencies e x i s t . For each p a i r of s a l t s , maxima are obtained near 15, 30, 45 and 60°C, i n amazingly good agreement w i t h the tempera tures proposed f o r thermal anomalies almost 20 years ago by one of us (W. Drost-Hansen) (13) and the temperatures r e p o r t e d by Peschel and A d l f i n g e r (19). The curves shown i n F i g u r e 8 are a l s o seen to be almost superimposable ( w i t h i n experimental


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Biophysical Journal

Figure 8. Selectivity coefficient, K for potassium-sodium ion distribution near silica gel surface as a function of temperature ( 24 ) K

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e r r o r s ) f o r the c h l o r i d e s and i o d i d e s and q u a l i t a t i v e l y s i m i l a r r e s u l t s are found f o r s u l f a t e s . This degree of consistency i n the three s e t s of data, together w i t h the r a t h e r s m a l l d e v i a t i o n s f o r each p o i n t , makes i t u n l i k e l y i n the extreme that the observed temperature dependence i s caused by some random e x p e r i mental e r r o r . Note a l s o that the values f o r the s e l e c t i v i t y c o e f f i c i e n t are a l l greater than one - no doubt a p o i n t of c r u c i a l importance i n c e l l u l a r b i o l o g y where p r e v i o u s l y " a c t i v e t r a n s p o r t " was i n v a r i a b l y invoked to e x p l a i n the unusual d i s t r i b u t i o n of potassium and sodium.


Experimental Procedures A. M a t e r i a l s and Procedures. The s i l i c a used i n the pre sent study was obtained i n the form of s p h e r i c a l , porous beads of a s i l i c a t e g l a s s w i t h a network of i n t e r c o n n e c t i n g pores ("Bioglas"). The m a t e r i a l i s f r e q u e n t l y used f o r g e l permeation chromatography; v a r i o u s p r o p e r t i e s of the " B i o g l a s " porous ma t e r i a l has been reported by Cooper, £t JUL. (26). Samples were a v a i l a b l e w i t h average pore diameters of 20, 100, 200 and 250 nm. The water used was obtained from a M i l l i p o r e Super-Q i o n ex change system. Several methods were employed f o r studying the e f f e c t s of the p r o x i m i t y of the water t o the s i l i c a s u r f a c e . A p a r t i c u l a r l y u s e f u l approach c o n s i s t e d i n comparing two samples w i t h i d e n t i c a l amounts of water and i d e n t i c a l amounts of porous g l a s s , but d i f f e r i n g i n pore diameters. This method w i l l be r e f e r r e d to as the "double d i f f e r e n t i a l method". The various techniques used i n these experiments (to en hance any p o s s i b l e e f f e c t of the water near i n t e r f a c e s ) are summarized below. I.a) Equal amounts of porous g l a s s (of i d e n t i c a l l y same pore diameter) were weighed out i n aluminum l i n e r s f o r the c a l o r i m e t e r cups, but d i f f e r e n t amounts of water were added ( u s u a l l y 4 m i c r o l i t e r t o one cup and 5 m i c r o l i t e r to the other cup) on top of the dry m a t e r i a l s (by means of a m i c r o p i p e t ) . I.b) Samples of i d e n t i c a l amounts and type of porous g l a s s were placed i n d e s s i c a t o r s w i t h d i f f e r e n t r e l a t i v e humi d i t i e s ( f o r at l e a s t two days). A f t e r an e q u i l i b r i u m s t a t e was reached (determined by successive weighings), samples of i d e n t i c a l pore diameters were s e l e c t e d and compared w i t h samples from d e s s i c a t o r s w i t h d i f f e r e n t r e l a t i v e h u m i d i t i e s . I. c) To the samples taken from the d e s s i c a t o r s w i t h d i f f e r e n t r e l a t i v e h u m i d i t i e s , an a d d i t i o n a l amount of water ( u s u a l l y 3 m i c r o l i t e r ) was added to both sample and reference material. I I . The double d i f f e r e n t i a l method used employed i d e n t i c a l amounts of porous Vycor m a t e r i a l , but w i t h d i f f e r e n t average pore diameters f o r sample and references. Normally the reference would be the l a r g e s t diameter m a t e r i a l ( u s u a l l y 250 nm). I n


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Figure 10. Thermogram (Run #208) of water in porous glass beads. Pore diameter: 20 nm. Reference: 5 μΐ water; sample, 4 μΙ water; solid, 10 mg in each cup.



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these cases, the f o l l o w i n g separate c o n d i t i o n s were u t i l i z e d : I I . a ) I d e n t i c a l amounts of water were added to the samples ( u s u a l l y 5 m i c r o l i t e r s ) . This gives a s t r a i g h t f o r w a r d "double d i f f e r e n t i a l " run. II.b) Samples were p l a c e d i n d e s s i c a t o r s w i t h the same r e l a t i v e humidity f o r a t l e a s t two days. A f t e r an e q u i l i b r i u m s t a t e had been reached, d i f f e r e n t average pore diameter samples were s e l e c t e d . This gives a "double d i f f e r e n t i a l " s e t of data w i t h v a r y i n g s m a l l amounts of (mostly adsorbed) water (due t o adsorption and, i n p a r t , c a p i l l a r y condensation). I I . c ) Samples were used w i t h d i f f e r e n t average pore d i a meters, maintained i n a d e s s i c a t o r a t i d e n t i c a l r e l a t i v e , humi d i t i e s . To these samples an a d d i t i o n a l amount ( u s u a l l y 5 micro l i t e r ) of water was added t o the sample and r e f e r e n c e m a t e r i a l . B. Instrumentation. A DuPont, Model 900, DTA system was used w i t h a c a l o r i m e t e r c e l l (DuPont, Model 900350). Various h e a t i n g r a t e s and s e n s i t i v i t i e s have been employed. Generally, the h e a t i n g r a t e s were of the order o f 2° per minute ( r e l a t i v e l y slow); the s e n s i t i v i t i e s were u s u a l l y (0.1 or) 0.2° per i n c h (near maximum s e n s i t i v i t y ) . One p a r t i c u l a r experimental d i f f i c u l t y has made q u a n t i t a t i v e estimates d i f f i c u l t o r f r e q u e n t l y i m p o s s i b l e , namely, s l i g h t asymmetric e v a p o r a t i o n from the samples. To minimize the evaporation, some runs were made using a " l i q u i d - l i d " : a p p r o x i mately 5 m i c r o l i t e r of dodecane was c a r e f u l l y p i p e t t e d i n t o each cup on top of the porous g l a s s plus water samples. The a d d i t i o n of dodecane i s not expected to a f f e c t the o v e r a l l thermal p r o p e r t i e s , although experimentally the s i t u a t i o n has been compli cated by the i n t r o d u c t i o n o f both a s i l i c a / d o d e c a n e and water/ dodecane i n t e r f a c e . However, comparisons of experiments w i t h and without the organic l i q u i d s t r o n g l y suggest that the e f f e c t of the dodecane i s merely the ( p a r t i a l ) suppression o f evapora t i o n e f f e c t s , as intended. R e s u l t s and D i s c u s s i o n The t y p i c a l thermograms are shown i n Figures 9 and 10. Ex perimental d e t a i l s and summary remarks of f i n d i n g s from represen t a t i v e runs are l i s t e d i n Table I . Some a d d i t i o n a l comments are a p p r o p r i a t e : i n runs i n which d i f f e r e n t amounts of water have been used, i t i s to be expected that a d e f i n i t e b a s e l i n e slope should be observed; however, the " b a s e l i n e " i s not s t r a i g h t , but curved. This i s probably due to d i f f e r e n t r a t e s of evapora t i o n of water from sample and reference cups; such evaporation may p o s s i b l y r e s u l t i n an exponential c o n t r i b u t i o n t o the base line. (The i n i t i a l p a r t of the c u r v e , approximately over a 3-5° i n t e r v a l , r e f l e c t s an i n s t a b i l i t y c h a r a c t e r i s t i c of the be ginning o f each thermogram, and t h i s p a r t of each curve was d i s regarded) .



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216

218 145

/(I

Lil

4

20

20

208

5 5

5

5

5

al

4

20

20

184

Cd

Yd

til

id

id

95% RH + 3 Kl

52% RH + 3,til

20

20

289

95% RH + 3 Ul

52% RH + 3/(1

20

20

Ref.

Sample

Loading

299

Run #

Pore diameter (nm) Sample Ref.

0.1 0.2

0.1

2

2 2

0.1

2

0.1

0.1

2

0.5

0.1

°C/in.

Sensitivity

2

Heating Rate °C/min.

Table I . Data from Thermograms; Water i n Porous Glass Beads.


abrupt o f f s e t i n baseline change i n s l o p e offset i n baseline; change i n s l o p e s i . change i n s l o p e . offset i n baseline; change i n s l o p e change i n s l o p e large offset i n b a s e l i n e ( o r exo thermic p e a k ? ) ; change i n s l o p e

43 58 47

59 52 63

41

32

43

s i . endothermic peak; change i n slope s i . endothermic peak; change i n slope s i . endothermic peak; change i n slope

48

Temp. °C

Features


98% RH

250

100

306

95% RH

100

262

pX JUX

//X

20

20

261

5

95% RH

20 20

148 151

250

5 jul

1UÔ

20

162

250

98% RH

95% RH

95% RH

2

2

2

2 2

2

5 jui 3 /tl 3 'JUX

2

2

Heating Rate °C/min.

/a

5

/u\

5

5/ c l

3 3

20

150

250

Ref.

Sample

Loading

250 250

20

146

Run #

Pore diameter (nm) Sample Ref.

0.1

0.5

0.5

0.2 0.2

0.2

0.2

0.2

°C/in.

47;50

43-47 49

62

65

63

Temp. °C

45

large offset i n baseline large offset i n baseline large offset i n baseline offset i n baseline abrupt o f f s e t i n baseline abrupt changes i n slope abrupt changes i n slope i n f l e c t i o n point

Features

46;49

(Continued)

Sensitivity

Table I . Data from Thermograms; Water i n Porous Glass Beads.



142

ADSORPTION

AT

INTERFACES

The occurrence of s m a l l (apparent) endothermic peaks, and p a r t i c u l a r l y changes i n s l o p e are c h a r a c t e r i s t i c of the r e s u l t s p e r s i s t e n t l y obtained i n the present study. Because of non l i n e a r b a s e l i n e s l o p e , some of the data from these experiments have been r e p l o t t e d on a l o g a r i t h m i c scale, log(Z\T)vs. Τ . No t h e o r e t i c a l j u s t i f i c a t i o n can be made f o r t h i s procedure. How ever, i f indeed the curvature i s r e l a t e d to an e x p o n e n t i a l term i n temperature, i t i s reasonable that t a k i n g l o g a r i t h m of AT may remove some of the i n c u r v a t i o n . This has been observed i n many ins tances. Sometimes very abrupt decreases occurred i n ΔΊ near 70° to 80°C; t h i s e f f e c t i s probably an a r t i f a c t due to r a p i d , asymmetric e v a p o r a t i o n . In a l l , more than 350 runs have been made. The r e s u l t s of some of these are summarized i n the v a r i o u s t a b l e s . I n Table I I a r e l i s t e d the temperatures at which changes i n s l o p e and/or o f f s e t s i n b a s e l i n e and/or endothermic peaks have been observed. The m a t e r i a l used i n a l l of these runs had an average pore diameter of 20 nm. Table I I I shows s i m i l a r r e s u l t s obtained on i d e n t i c a l amounts of porous g l a s s w i t h an average pore diameter of 100 nm. A g a i n , the temperatures l i s t e d are those at which thermal anoma l i e s were observed, such as changes i n s l o p e , o f f s e t i n base l i n e , or endothermic peaks. Table IV shows s i m i l a r data f o r m a t e r i a l w i t h a 250 nm average pore diameter. Table V l i s t s r e s u l t s obtained by the double d i f f e r e n t i a l method, using 250 nm average pore diameter m a t e r i a l as r e f e r ence and 20 nm m a t e r i a l as sample. Table VI i s s i m i l a r to Table V, but f o r 100 nm v s . 20 nm. Table V I I i s s i m i l a r to t a b l e V, but f o r 250 nm vs. 100 nm. Table V I I I summarizes the r e s u l t s from the combined runs shown i n Tables I through V. The average temperatures of t r a n s i t i o n s f o r " r e g i o n s " I I , I I I and IV agree w e l l w i t h the tempera tures p r e v i o u s l y proposed f o r the t r a n s i t i o n s i n v i c i n a l water by one of us (W. Drost-Hansen). An anomaly near 74°C (Region V) appears not t o have been reported b e f o r e . Note that a number of anomalies were observed at temperatures where thermal anomalies have not p r e v i o u s l y been reported. The o r i g i n of these apparent anomalies i s by no means c l e a r , but i t i s c o n c e i v a b l e that the "spurious anomalies" (at temperatures other than those g e n e r a l l y encountered) may be r e l a t e d to the anomalies shown i n the study by P e s c h e l and A d l f i n g e r (19) f o r very t h i n f i l m s of water (10 nm p l a t e s e p a r a t i o n ) . I t i s unfortunate that l a r g e , and f r e q u e n t l y asymmetric, evaporation e f f e c t s are encountered, as these appear to i n f l u ence s i g n i f i c a n t l y a l l the thermograms obtained. However, i n s p i t e of the obvious experimental d i f f i c u l t i e s , i t i s c e r t a i n from these measurements that r a t h e r abrupt changes occur i n the thermal p r o p e r t i e s of the w a t e r / s i l i c a systems. The observed



378 379

95% RH (reference) 278 52% RH (sample) 288 289 (3 jjl i n both) 299 300 321 322 323 337

5 JjCl (reference) 4 JUl (sample)

7

Run Number 5 ul (reference) 177 4 ill (sample) 180 183 184 208 210 211 340 295 298 5 ici ( r e f e r e n c e ) 179 4 jUI (sample) 212

System

31 -

-

-

k

k

k

k

47 43 k

k

s

43

42

s

k

k

59 -

-

-

-

s

k

k

k

Region I I Region I I I Region IV 27-34°C 42-48°C 57-64°C 33^ 6p 57 32 42 47 64 43 57 33 -

-

-

-

-

Region I 12-18°C -

k

-

75

s

71.5

74

k

Region V 71-77°C -

k

k

56 67 50

38

-

s

s

s

k

49s

4 1 ^ 57k 56

5 6

53s

53k

Others (°C) -

Table I I . Data from Thermograms o f I d e n t i c a l Amount o f Porous Glass M a t e r i a l (with an average pore diameter o f 20 nm i n r e f e r e n c e and sample h o l d e r s ) . Various Water Contents.



/Τ

(SontÎnS)

Footnote:

k: s:

S

n

1

r

e

f

r

R e

e

£

n

e

l o n

c

a

n

d

S II 27-34°C ^ 34k

e

P l e

64

" s

7

7 1

71.5

7 7

s

C

Revion V i_; " ° 3 7

Others Cfi) *

Various Water Contents

Region IV 57-fiA°n

holders).

Region III 42-48°C

s a m

small endothermic (or, rarely, exothermic) peak, change in slope or offset of baseline.

100% RH (reference) 340 98% RH (sample)

± &

8 ±

±

^ °? 12-18°C T ^

™

Data from Thermograms of Identical Amount of Porous Glass Material (with an average

Number 98% RH (reference) 292 78% RH (sample) 293

S y S t a n

Table II.


In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975. s

s

28

310

335 328

100% RH (reference) 95% RH (sample)


279

95% RH (reference) 52% RH (sample) (3 Ml i n both)

0

s

s

s

46 45

42

2

s

s

46 42 4 s

2

4 s

s

s

s

Region I I Region I I I Revion IV 27-34 Ç 42-48°C 57-64°C 59 62 45

33

245 246 272


Region I 12-18°C

294 297

220

10 ul (reference) 9 '/
Run Number 216 217 237 296 235 74

s

Region V 71-77°C

s

s

s

s

52 38

79

70

Others (°C)

Data from Thermograms of I d e n t i c a l Amount o f Porous Vycor M a t e r i a l (with an average pore diameter of 100 nm i n sample and r e f e r e n c e cup). Various Water Contents.

5 / 1 (reference) 4 / ' l (sample)

System

Table I I I .



190 301

273

309 325 133

5 /

DTA Study of Water in Porous Glass - ACS Publications - American

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