Chapter 21
Chain Interactions in Thin Oil Films Stabilized by Mixed Surfactants 1
1
1
Mark S. Aston , Carlier J. Bowden, Thelma M. Herrington , and Tharwat F. Tadros Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 12, 2016 | http://pubs.acs.org Publication Date: December 30, 1989 | doi: 10.1021/bk-1989-0384.ch021
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1Department of Chemistry, University of Reading, Reading RG6 2AD, United Kingdom ICI Plant Protection Division, Jealotts Hill Research Center Station, Bracknell, Berks RG12 6EY, United Kingdom 2
The thickness of horizontal films of n-decane sandwiched between two water (or aqueous electrolyte) droplets has been determined by a light reflectance technique. The films were stabilised by three surfactants: an XYX block copolymer of poly(ethyleneoxide) and poly(12-hydroxystearic) acid; soya bean lecithin; 'Arlacel 83' (sorbitan sesquioleate). Results obtained for two and three component mixtures of the surfactants were compared with those for the single surfactants. The results showed that, provided sufficient polymer is present in the film, the thickness is determined by the longest oleophilic chain, namely the poly(12-hydroxystearic) acid. Polymers have long been exploited to s t a b i l i s e emulsions; f o r example, n a t u r a l l y occurring macromolecules such as gums and proteins are used i n the food and pharmaceutical industries (1,2). In milk the f a t globules are s t a b i l i s e d against coalescence by adsorbed proteins. The remarkable s t a b i l i t y of these emulsions towards coalescence can be attributed to their mechanical properties, namely the v i s c o s i t y and e l a s t i c i t y of the i n t e r f a c i a l f i l m (3). Biswas and Haydon (4) systematically investigated the rheological c h a r a c t e r i s t i c s of various proteins at the oil/water interface. No s i g n i f i c a n t s t a b i l i s a t i o n occurred i n the case of non-viscoelastic f i l m s . However, the presence of v i s c o e l a s t i c i t y alone i s not s u f f i c i e n t to confer s t a b i l i t y . For example, i t was found that the highly v i s c o e l a s t i c f i l m of bovine serum albumin could not s t a b i l i s e a w a t e r - i n - o i l emulsion. They concluded that the requirements f o r s t a b i l i t y to coalescence are the presence of a f i l m of high v i s c o s i t y and of appreciable thickness. I t i s also necessary that the p r i n c i p a l contribution to the f i l m thickness should be located on the continuous - phase side of the i n t e r f a c e . 'Thick* adsorbed layers of nonionic polymers impart emulsion s t a b i l i t y through s t e r i c i n t e r a c t i o n s . A nonionic polymer chain i n an aqueous environment i s 0097-6156/89/0384-0338$06.00/0 « 1989 American Chemical Society
El-Nokaly; Polymer Association Structures ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 12, 2016 | http://pubs.acs.org Publication Date: December 30, 1989 | doi: 10.1021/bk-1989-0384.ch021
21. ASTON ET AL.
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Chain Interactions in Thin Oil Films
i n s e n s i t i v e to the presence of e l e c t r o l y t e s . I t has been shown that the best s t e r i c s t a b i l i s e r s are amphipathic block or graft copolymers (5). In t h i s work block copolymers of the type poly(12-hydroxystearic acid)-poly(ethyleneoxide)-poly(12-hydroxystearic a c i d ) , i . e . XYX, are used to s t a b i l i s e a w a t e r - i n - o i l emulsion where the aqueous phase i s concentrated ammonium n i t r a t e . Sorbitan sesquioleate (Arlacel 83) and soya bean l e c i t h i n are also added to aid emulsion s t a b i l i t y . To understand the behaviour of the i n t e r f a c i a l region i t i s necessary to examine the physicochemical properties of the i n t e r f a c e . A t y p i c a l study would involve i n t e r f a c i a l tension, i n t e r f a c i a l rheology and f i l m thickness and s t a b i l i t y measurements. Also surfactant packing at the interface w i l l play an important role i n determining f i l m s t a b i l i t y and t h i s can be assessed using spread monolayer ( i . e . Langmuir trough) techniques. A l l of these measurements have been c a r r i e d out as well as some neutron scattering studies. In t h i s paper, f i l m thickness and s t a b i l i t y measurements are presented for a water/n-decane/water f i l m s t a b i l i s e d with one of the XYX copolymers, i n combination with sorbitan sesquioleate and soya bean l e c i t h i n . Results of studies on the morphologies of the neat XYX polymers are also b r i e f l y described. The surfactants are dissolved i n the o i l phase and i t i s t h e i r presence i n the thin o i l f i l m between water droplets which prevents coalescence and hence breaking of the emulsion i n practice. The surfactants orient with t h e i r hydrophilic groups i n the aqueous phase and hydrocarbon t a i l s i n the o i l phase, giving r i s e to a s t e r i c a l l y s t a b i l i s e d system. In the case of the XYX block copolymer, the Y group i s hydrophilic and must orient i t s e l f at the i n t e r f a c e , whilst the two o l e o p h i l i c X groups extend i n t o the o i l . I t should be noted that e l e c t r o s t a t i c interactions are n e g l i g i b l e i n these systems, i n contrast to aqueous systems where they play a very important r o l e . Materials and Methods As noted above block copolymers had a poly(ethyleneoxide) head group Y and t a i l s X of poly(12-hydroxystearic) acid. The polymers are formed i n a one-step polymerisation from 12-hydroxystearic acid and poly(ethyleneoxide) polymers of various molar masses. The values of the number and mass average molar masses shown i n Table 1 were obtained by GPC. Vapour pressure osmometer measurements gave the Table I . Molar Mass of the Block Copolymers Polymer
M n / g mol"
1
Mm/g mol" 7000
1
PEO/g mol"
1
1500
61
3500
B2
7000
4000
B3
5000
1500 + 4000
El-Nokaly; Polymer Association Structures ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
340
POLYMER ASSOCIATION STRUCTURES 1
number average molar mass of B l as 3543±30 g mol" and t h i s was the value used i n a l l c a l c u l a t i o n s ; the number average molar mass was considered to be the most relevant to the study of monolayers where the number of molecules i n the interface i s the important f a c t o r . A l l three block copolymers are waxy yellow s o l i d s at 20°C soluble i n a l i p h a t i c hydrocarbons. B l i s a wide spectrum water-ino i l emulsifier with an HLB of 6. They are a l l anisotropic, transmitting l i g h t between crossed polars and a l l give p o s i t i v e u n i a x i a l conoscopic figures t y p i c a l of the lamellar phase. The phase diagram of B l with n-decane i s shown i n figure 1. Pure B l changes to an i s o t r o p i c l i q u i d at 33.2°C. The enthalpy of t r a n s i t i o n i s 147 kJ mol* consistent with a gel to i s o t r o p i c l i q u i d phase change. We are interested i n the narrow i s o t r o p i c region up to 2 wt% of B l at 25°C. Low angle X-ray d i f f r a c t i o n results on B l gave a lamellar interplanar spacing of 17.3 nm; t h i s i s equivalent to two chains of poly(12hydroxystearic) acid, each containing f i v e monomer units, placed end to end. An electron micrograph of pure B l i s shown i n figure 2; t h i s was obtained by freeze-fracturing the polymer and shadowing by platinum/carbon at a known angle. From the electron micrograph a s t r a t i f i e d morphology i s c l e a r l y v i s i b l e , with a repeat unit of the same order of magnitude as the X-ray d i f f r a c t i o n data. B l and B2 were heated on the hot stage of a p o l a r i s i n g microscope to the i s o t r o p i c phase and allowed to cool; they both showed the mosaic textures shown i n figures 3 and 4. B2 forms a more d e f i n i t e mosaic texture and i s more highly biréfringent. B3 which has the Y block of poly(ethyleneoxide) intermediate i n s i z e to that of B l and B2 shows behaviour i n between that of B l and B2 under the microscope. The magnification i n both these photomicrographs i s only times ten, so that there are much larger areas between d i s c l i n a t i o n s than for a t y p i c a l monomeric lamellar l i q u i d c r y s t a l . For the f i l m s t a b i l i t y studies three surfactants were used s i n g l y and as mixtures: B l , sorbitan sesquioleate and soya bean l e c i t h i n . The surfactant was dissolved i n n-decane and the thickness of the o i l f i l m between two water droplets was determined by a l i g h t reflectance technique. The basic c e l l which was used to form the films i s shown i n figure 5. Film thickness measurements were c a r r i e d out at 25+0.5°C; temperature control was achieved using a container through which water c i r c u l a t e d from a thermostat bath. Approximately equal-sized droplets of the aqueous phase were produced at the upper and lower o r i f i c e s by adjusting the Rotaflow taps. Using adjusting screws, i t was possible to accurately a l i g n the upper o r i f i c e above the lower one at a separation distance that was approximately equal to the o r i f i c e diameter, so that on contact the droplets would be approximately hemispherical. In order to prevent the droplets from wetting the glass and spreading around the o r i f i c e s , the immediate surrounding areas were made hydrophobic by treatment with a 2% s o l u t i o n of dimethyldichlorosilane i n carbon t e t r a c h l o r i d e . The o p t i c a l set-up f c r determining f i l m thickness i s shown i n figure 6. Light from a 1 mW helium-neon laser passed through the lower o r i f i c e of the c e l l (not shown) and was reflected back at both the lower and upper f i l m interfaces where r e f r a c t i v e index boundaries occurred. The net r e f l e c t e d l i g h t passed back through the lower o r i f i c e again onto the telescope system. The angle θ was kept small (
