Stability of Premicellar Aggregates in Water-in-Oil Microemulsion

3 Stability of Premicellar AggregatesinWater-in-Oil Microemulsion Systems 1

1

STIG E. FRIBERG , TONY D. FLAIM , and PATRICIA L. M. PLUMMER

2

1

Chemistry Department, University of Missouri at Rolla, Rolla, MO 65401 Physics Department, University of Missouri at Rolla, Rolla, MO 65401

Downloaded by UNIV OF ARIZONA on December 9, 2012 | http://pubs.acs.org Publication Date: March 27, 1985 | doi: 10.1021/bk-1985-0272.ch003

2

The stability of premicellar association structures in W/O microemulsion systems was calculated using the CNDO/2 method. The results revealed the importance of directed interaction of water molecules with the polar groups of the amphiphilic compounds. Even extremely strong hydrogen bonds such as those between ionized carboxylate and nonionized carboxylic groups could be intercalated by water molecules with energy conservation. The knowledge about microemulsions has reached an advanced state (1,2); especially so about the fundamentals for t h e i r s t a b i l i t y (3). However, some problems remain unsolved. One of them, which we have found i n t r i g u i n g , is the fact that the systems at the lowest water concentrations do no show the presence of inverse micelles; in f a c t , the W/0 microemulsions w i l l tolerate rather large amounts of water before any c o l l o i d a l association takes place. This fact was early pointed out by Shah (4) and the v a r i a t i o n of p a r t i c l e size with water content has been investigated using d i e l e c t r i c (5) methods, l i g h t scattering and electron microscopy (6). These results strongly indicate the size of the primary aggregates at low water concentrations not to be s i g n i f i c a n t l y d i f f e r e n t from the size of the s o l vent molecules. Our interest in this phenomenon is mainly the r o l e of the water molecules for the s t a b i l i t y of such aggregates; an i n t e r e s t i n g problem against the suggestion by Eicke (7) that small amounts of water are e s s e n t i a l f o r the s t a b i l i t y of inverse micelles of aerosol OT. In t h i s a r t i c l e we evaluate interactions in a system s t a b i l i z e d with an i o n i c surfactant and with a carboxylic acid as the cosurfactant. Such a system is distinguished from the common soap/alcohol s t a b i l i z e r combinations by the fact that the soap/acid system does not require a minimum water concentration to dissolve the soap.

0097-6156/85/0272-0033S06.00/0 © 1985 American Chemical Society

In Macro- and Microemulsions; Shah, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

34

M A C R O - A N D MICROEMULSIONS

Calculation Method In b r i e f , the CNDO (the acronym stands for complete neglect of d i f f e r e n t i a l overlap) approach is an a l l valence electron, s e l f consistent f i e l d c a l c u l a t i o n in which multicenter integrals have been neglected and some of the two electron integrals parameterized using atomic data. Slater type atomic o r b i t a l s are used as the basis 2s, 2 ρ , 2ρ , 2p for carbon and oxygen. In these calculations twoelectron intégrais are approximated as χ


(μν/λσ) = δ

μ

ν

δ

χ

(μμ/λλ) = γ

σ

μ

λ

where μ etc. stands f o r Slater Orbitals φ , ... centered on the n u c l e i . The electron interaction integrals are assumed to depend only on the atoms to which φ , φ^ belong, and not on the s p e c i f i c o r b i t a l s , e. g. V> - ΑΒ - / / Ύ

S

l

( r

12

) _ 1

S

B

( 2 )

d

T

l

d

V

Further

(y/v /v) = δ B

μ

ν V

a

b

where -V^ is the potential due ot the nucleus of charge inner s h e l l of atom Β and V

S

AB " B / l Z

(1

>

( r

lB

) _ 1

d

T

and the

l

In addition, the off-diagonal core matrix elements, Η , are set proportional to the overlap i n t e g r a l , S . ^ V

Η

μν

=

3

δ

AB μ ν

where 3 ^ is a parameter determined from atomic spectral data f o r Atoms A and B. The s p e c i f i c parameterization used is called CNDO/2. The computer codes used f o r these calculations are modifications of Dobash's program supplied by QCPE (3). The modifications p r i n c i p a l l y consisted of increased dimensions to handle the large systems, and a matrix extrapolation routine incorporated into the SCF portion of the program to enhance convergence. Results and Comments The basic unit to be studied was the sodium formate/formic acid com plex with two soap molecules and four acid molecules. This number of molecules in the soap/acid complex has been experimentally determined (9) for octanoic acid/sodium octanoate. In the present calculations, the shorter chains are used in order to save the labor of mapping the geometries. E a r l i e r calculations (10) have shown the



3. FRIBERG ET AL.

Stability of Premicellar Aggregates

35

contributions of the hydrocarbon chain electrons to the head group interactions to be n e g l i g i b l e and in addition, the role of the chains in mundus r e a l i t e r is only to provide a hydrophobic s h e l l f o r the head group structure. Since we postulate the geometry a p r i o r i from experimental data and since the hydrocarbon chains do not contribute to the head group interaction to a s i g n i f i c a n t degree, the formic acid/formate combination model is j u s t i f i e d . The 2/4 soap/acid molecular complex may be structured with two binding patterns (10). The acid carboxylic groups may be aligned h o r i z o n t a l l y , F i g . l a , or v e r t i c a l l y , F i g . l b . For our present evaluation, only the v e r t i c a l alignment w i l l be examined; it offers d i s t i n c t advantages for the i n c l u s i o n of water (11). When the water molecules are attached to the soap/acid association structure, s t r u c t u r a l changes may be expected. In the present investigation, three of these are examined. 1.

The water molecules are hydrogen bonded to the polar groups with no accompanying change of geometry of the association complex. 2. The two soap molecules are separated from each other along the horizontal axis b i s e c t i n g the carboxylic groups. The acid molecules r e t a i n their position r e l a t i v e to the soap molecule, F i g . 2. 3. The acid/soap hydrogen bond is broken and the -0 ...H..O. distance is increased. The experimental evidence at hand (12,14) shows a maximum of 14 water molecules to be attached to the 2/4 soap acid before a phase t r a n s i t i o n to a l i q u i d c r y s t a l l i n e structure occurs. The addition of water is accompanied by a l i n e a r reduction of the number of carboxylic acid/carboxylate hydrogen bonds (13,14). With this information at hand, an examination of the energy changes f o r a l l the alternatives 1-3 were considered useful in order to understand the energy foundation f o r the s o l u b i l i z a t i o n of water into a soap/acid complex. The energy needed to enhance the horizongtal distance between the ionized carboxylate group oxygens to a s u f f i c i e n t degree to enable water molecules to be inserted into the center of the structure was 25.6 Kcal/mole; the oxygen-oxygen distance now being 4.205 Â against 3.570 Â for the o r i g i n a l structure. This low value was obtained through repositioning of the carboxylic acid groups and by adjustment of the v e r t i c a l carboxylate group/sodium ion distance to i t s optimum value at 3.265 Â. This expanded structure allowed two water molecules to bind by two hydrogen bonds to the two ionized carboxylate groups and by a oxygen/metal ligand bond to the sodium ion. The p o s i t i o n of this water molecule is displayed as I^O^ in F i g . 3. Water molecule //2 was located symmetrically and has been omitted in the figure for reasons of c l a r i t y . These water molecules were maximally occupied in strong bonds which is r e f l e c t e d in their binding energy 37.9 Kcal/ mole water. It should be noted that the energy released by these two bonds more than compensates for the energy input to obtain the necessary expansion to accommodate the two water molecules.



M A C R O - A N D MICROEMULSIONS

F i g u r e 1. The 4:2 a c i d - s o a p dimer w i t h the a c i d s b r i d g i n g h o r i z o n t a l l y (a) and v e r t i c a l l y (b) a f t e r Bendiksen et a l . ( 1 0 ) . Key: Θ , a c i d ; ., soap.



FRIBERG E T A L .


F i g u r e 2. The 4:2 a c i d - s o a p dimer w i t h the a c i d s b r i d g i n g v e r t i c a l l y expanded to accommodate two water m o l e c u l e s in between the two main g r o u p s . A l and S l denote d i f f e r e n t p o s i t i o n s in the a c i d and soap m o l e c u l e s . Key: Θ , a c i d ; ., soap.

F i g u r e 3. The p o s i t i o n o f 14 water m o l e c u l e s added t o the expanded s o a p / a c i d a s s o c i a t i o n s t r u c t u r e s . Water m o l e c u l e 2 is the symmetric i d e n t i c a l t o #1, #4 the c o r r e s p o n d i n g p a i r t o #3 and so f o r t h . Key: o, water; Θ , a c i d ; ., soap.


38

MACRO- AND MICROEMULSIONS

In addition, water molecules marked 3 on F i g . 3 and i t s symmetric location 4 (not included) gave binding energies of 27.6 Kcal/mole water. These molecules may be bound to the unexpanded soap/acid association structure with similar energies involved. The bonding of these two water molecules obviously would provide s u f f i c i e n t i n i t i a l energy f o r the expansion of the soap/acid association complex to accommodate water molecules / / l and 2. Additional water molecules were added according to Figure 3, to a maximum of 14. The energies released are given in Table I with binding s i t e s according to F i g . l b .


Table I.

Association Energies and Binding Sites for Water 1-7

Water Molecule 1 2 3 4 5 6 7

Binding Site ( F i g . lb) S l , S3 S2, S4 S2, A4 S3, A l Sl, H 0 S4, H^O A3, A4 J

2

4

A

fl8iîtmJÎ§-l§!I?y -37.9 -37.9 -27.6 -27.6 -18.1 -18.1 -22.3

The numbers show an overwhelming s t a b i l i t y f o r i n c l u s i o n of the water molecules and encouraged the evaluation of energies involved in breaking the strong carboxylic acid/carboxylate hydrogen bonds. In the c a l c u l a t i o n the water molecules were added without breaking the carboxylate/carboxylic group hydrogen bonds, but experimental evidence (13,14) shows the hydrogen bonds to be reduced to one half of t h e i r o r i g i n a l number at the point of t r a n s i t i o n to a l i q u i d c r y s t a l l i n e phase and an evaluation of the energies Involved was considered useful. The carboxylate/carboxylic acid group hydrogen bond energies in the expanded structure t o t a l l e d 95.6 Kcal. Adding t h i s number to the expansion energy means an input of 121.2 Kcal/mole to the soap/acid complex in order to accommodate the water molecules. A comparison of this value with the energy released by 14 added water molecules is i l l u s t r a t i v e . Addition of the 14 molecules w i l l release an energy of 112.2 Kcal/mole [248 Kcal/mole (Table I) - 14 χ 9.7 (evaporation heat of water)] = 112.2 Kcal/mole. This number is s l i g h t l y lower than the value f o r the energy of a l l the hydrogen bonds to be broken, which is in good agreement with the experimental results showing one half of the hydrogen bonds to be disrupted. The energy needed f o r that to be accomplished plus the energy for the s t r u c t u r a l expansion amounts to 73.4 Kcal/mole; a value well below the 112.2 Kcal/mole released by the water bonding according to our present c a l c u l a t i o n s . So f a r , the results of the attempts to calculate the magnitude of these interactions are encouraging; the e f f o r t s w i l l be continued using more exact methods.


3.

FRIBERG ET AL.


39


Experimental results (12) showed a t r a n s i t i o n to a lamellar l i q u i d c r y s t a l for 14 added water molecules. Our calculations (to be reported at a l a t e r occasion) showed no discontinuity or any other i n d i c a t i o n of i n s t a b i l i t y of the soap/acid water complex for the sub sequent water molecules added in excess of 14. It appears reasonable to assume that the i s o t r o p i c l i q u i d / l i q u i d c r y s t a l t r a n s i t i o n does not depend on the energy levels of the polar group interactions. The phase t r a n s i t i o n probably depends on the hydrophobic/hydrophilic volume r a t i o and estimations according to Israelachvili/Ninham (15) approach may o f f e r a better potential for an understanding.

Literature Cited 1. Robb, I. D. (Ed.), "Microemulsions"; Plenum: New York, 1982, p. 259. 2. Friberg, S. E. and Venable, R. L., "Microemulsions", Encyclopedia of Emulsion Technology,; Becher, P., Ed.; Vol. 1, Chap. 4, pp. 287-336 , 1983. 3. Ruckenstein, E., J. Dispersion Science & Technol., 2, 1 (1981). 4. Shah, D. O. and Hamlin, R. Μ., Science , 1971, 171, 483 5. Clausse, M and Rayer, R., "Colloid and Interface Science II", Academic: New York, 1976, p. 217. 6. Sjöblom, E. and Friberg, S., J. Colloid Interface Sci., 1978, 67, 16. 7. Eicke, H. F. and Christen, Η., Helvetica Chemical Acta, 1978, 61, 2258. 8. Dobash, P. Α., QCPE, 1974, 10, 141. 9. Söderlund, G. and Friberg, S., Physik. Chem. Neue Folge, 1970, 70, 39. 10. Bendiksen, B., Friberg, S. E. and Plummer, P. L. M., J. Colloid Interface Sci., 1979, 72, 495. 11. Flaim, T., Thesis, University of Missouri at Rolla, Missouri, 1983. 12. Ekwall, P., "Advances in Liquid Crystals", Academic: New York, 1975, Vol. 1, p.1. 13. Friberg, S., Mandell, L. and Ekwall, P., Kolloid Z.u.Z Polymere 1969, 233, 955. 14. Bendiksen, Β., Thesis, University of Missouri at Rolla, Missouri, 1981. 15. Israelachvili, J., Mitchell, D. J. and Ninham, B. W., J. Chem. Soc. Faraday Trans II, 1976, 72, 1525. R E C E I V E D June 8,

1985


Stability of Premicellar Aggregates in Water-in-Oil Microemulsion

Recommend Documents