Rheology of the lamellar liquid-crystalline phase in polyethoxylated alcohol/water/heptane systems

Rheology o he lamella liquid-c ys alline phase in polye hoxyla ed alcohol/wa e /hep ane sys ems By F. Co dobés1*, J.M. F anco2 and C. Gallegos2 1 Depa amen o de Ingenie ía Química. Facul ad de Química. Uni e sidad de Se illa. c/P o . Ga cía González s/n. Se illa 41012, Spain. co [email protected] 2 Depa amen o de Ingenie ía Química. Facul ad de Ciencias Expe imen ales. Uni e sidad de Huel a. Campus Uni e si a io del Ca men, 21071, Huel a. Spain. [email p o ec ed], [email p o ec ed] RESUMEN Reología de la ase líquido-c is alina lamina en sis e- mas alcohol polie oxilado/agua/hep ano. En es e abajo se han es udiado las p opiedades eológicas de una ase líquido-c is alina lamina con enida en un sis ema al- cohol polie oxilado/agua/hep ano, median e ensayos iscoelás i- cos lineales, es aciona ios y ansi o ios. El e ec o de dis in as a iables como la composición de ensioac i o y hep ano sob e dichas p opiedades eológicas ha sido analizado. El espec o me- cánico ob enido de la ase lamina mues a en odos los casos una egión «pla eau» en el in e alo de ecuencias es udiado así como mayo es alo es de los pa áme os iscoelás icos pa a a- lo es in e medios de la concen ación de ensioac i o y hep ano, indica i o de un mayo desa ollo del en amado elás ico es uc u- al de dicha ase líquido-c is alina. Los ensayos de lujo es aciona- io y ansi o io pusie on de mani ies o la exis encia de es uc u as inducidas po cizalla. En las cu as de iscosidad se ob u ie on di e en es egiones de a iación po encial de la iscosidad en e a la elocidad de cizalla y, po an o, dis in os alo es de índices de lujo. Las modi icaciones de la mic oes uc u a de la ase líqui- do-c is alina lamina causadas po la cizalla, ue on con i madas median e el uso de mic oscopía de luz pola izada en un sis ema de cizalla que inco po a un mic oscopio óp ico. Las modi icacio- nes es uc u ales pa ecen es a muy in luenciadas po la eloci- dad de cizalla y, gene almen e, la aplicación de ele adas elocidades de cizalla p o oca el alineamien o de las bicapas de ensioac i o y la apa ición de la clásica ex u a de «oily s eaks» de la meso ase lamina . La p esencia de esículas se de ec a en sis emas con al o con enido en hep ano, como indica la ex u a de ec ada po mic oscopía de luz pola izada. PALABRAS-CLAVE: C is al líquido - Meso ase lamina - Reología - Viscoelas icidad - Viscosidad. SUMMARY Rheology o he lamella liquid-c ys alline phase in polye hoxyla ed alcohol/wa e /hep ane sys ems. Linea iscoelas ic es s as well as ansien and s eady low expe imen s we e ca ied ou on lamella liquid c ys alline samples o poly(oxye hylene) alcohol/wa e /hep ane sys ems. The e ec o su ac an and hep ane concen a ions on he heological p ope ies o he lamella mesophase was in es iga ed. The mechanical spec um inside he linea iscoelas ic egime shows, in all cases, a well-de eloped pla eau egion in he whole equency ange s udied. The alues o he dynamic unc ions we e highe o in e media e su ac an o hep ane concen a ions indica i e o a majo de elopmen o he elas ic ne wo k in he mid ange o exis ence o he lamella phase. T ansien and s eady low expe imen s poin ou a shea -induced e olu ion o he lamella mic os uc u e. Di e en powe law egions wi h di e en alues o he low index we e de ec ed in he iscosi y e sus shea a e plo s. These shea -induced s uc u al modi ica ions we e con i med by using pola izing mic oscopy in an op ical shea ing cell. S uc u al modi ica ions appea o be highly in luenced by shea a e. In gene al, applying ela i ely high cons an shea a es, he alignmen o he bilaye s ollowed by he appea ance o he «oily s eaks» s uc u e was obse ed. Appea ance o shea -induced esicles occu s a high hep ane con en , as indica es he ex u e o close-packed monodispe se sphe uli es de ec ed by pola izing mic oscopy. KEY-WORDS: Lamella mesophase - Liquid c ys al - Rheology - Viscoelas ici y - Viscosi y. 1. INTRODUCTION Aqueous and non-aqueous solu ions o nonionic su ac an s (Tiddy, 1980; Robles-Vásquez, 1993; Dimi o a, 1995; Co dobés, 1997) may o m di e en lyo opic liquid-c ys alline phases as a esul o in e ac ions among associa ion s uc u es o micella agg ega es. These phases p esen s a ic and dynamic p ope ies ela ed o he c ys alline (long- ange posi ional and o ien a ional o de , elas ici y,...) and liquid s a es ( iscosi y, concen a ion luc ua ions, e c.). Knowledge o he s uc u e and dynamic p ope ies o lyo opic phases is e y impo an om bo h undamen al and p ac ical poin s o iew, especially conside ing hei applica ions in he ields o de e gen s (Ba nes, 1980), cosme ics (Comelles, 1994), pha maceu icals (Mülle -Goyman, 1987; Engs öm, 1990), emulsions and colloidal sys ems (Pilpel, 1987; Riego, 1988; F ibe g, 1990), e c. The phase diag ams o polyoxye hylene nonionic su ac an s exhibi a succession o di e en lyo opic liquid-c ys alline phases (Mi chell, 1983; F anco, 1995; Mo os, 2001). The ype o liquid-c ys alline s uc u e o med depends on he cu a u e adop ed by he in e ace be ween he agg ega es and he su ounding sol en . Lamella , hexagonal and cubic phases a e usually shown in hese sys ems. The lamella lyo opic phase, Lα, appea s a a he high concen a ions when he in e ace ends o be plana , al hough su ac an wi h mo e hyd ophobic chains a o s hei appea ance a low concen a ions. This G asas y Acei es 96 Vol. 56. Fasc. 2 (2005), 96-105 phase consis o a simple one-dimensional c ys al composed o al e na ing laye s o wa e and su ac an bilaye s wi h polyc ys alline cha ac e is ics, i.e. comp ised o small, mic on-sized domains wi h in e ening g ain bounda ies (Kilpa ick, 1994). This pa icula s uc u e con e s o his phase heological p ope ies clea ly di e en om o he s. I is well known ha he lamella liquid c ys alline phase shows lowe alues o he heological unc ions han o he s lyo opic phases de ec ed in nonionic su ac an /wa e phase diag ams (Mo os, 2001; Muñoz, 1991). Se e al aspec s conce ning he s eady-s a e and ansien low beha io s o lamella liquid-c ys alline phase ha e been p e iously s udied by di e en au ho s (F anco, 1995; Gallegos, 1987; Lockwood, 1987; Paasch, 1989; Roux, 1994). In hese s udies, he impo ance o he appea ance o shea -induced s uc u al de ec s in he heological beha io o lamella mesophases has been poin ed ou . Fi s in es iga ions abou he e ec o shea on he s uc u e o a lamella phase was analyzed by Dia e al. (Dia , 1993) in a qua e na y sys em o anionic sodium dodecyl sul a e (SDS), pen anol, dodecane and b ine. As shown, a mul idomain Lα phase, which usually holds de ec s in he liquid-c ys alline s uc u e (Laüge , 1996), is aligned by shea in o a s a e o o ien a ed bilaye s pa allel o he wall o he Coue e cell. A a h eshold alue o shea a e cha ac e is ic o each sys em, he bilaye s b eak up and a e ans o med o esicles, also called lamella d ople s, sphe uli es, liposomes, o «onions» ( an de Pas, 1993; an de Linden, 1996). Fo shea a es highe han a c i ical alue, a wo-phase egion exis s in which ex ended bylaye s a e in equilib ium wi h esicles. A single esicle phase is eached by inc easing shea a e, which can exhibi a high deg ee o posi ional o de ing (Dia , 1995). Fo e en highe shea a es he esicles a e inally ans o med o a pe ec ly aligned lamella phase ha is supposed o be ee o de ec s. Mo e ecen ly, o he au ho s ha e in es iga ed he p esence o mul ilamella esicles (MLVs) in di e en su ac an sys ems (Weigel, 1996; Sie o, 1997). Escalan e e al. (Escalan e, 2000) ha e obse ed ha he ansi ion om a non-pe u bed lamella phase is i e e sible and, o a gi en sys em, he cha ac e is ic pa ame e o he ans o ma ion p ocess is he de o ma ion applied. Pa al e al. (Pa al, 2001) ha e ound ha he ansi ion o a esicle phase occu s a a c i ical s ess which inc eases wi h su ac an concen a ion. On he con a y, in some bina y sys ems wi h a ela i ely high su ac an concen a ion, his ansi ion o a MLV phase has no been obse ed (F anco, 1995; Neme h, 1998; Laüge , 1995), p obably due o an insu icien bending lexibili y o he memb anes (Vinches, 1994). On he o he hand, his o ically less a en ion has been paid o he linea iscoelas ic beha io o lamella liquid-c ys alline phases and only in he las yea s he iscoelas ici y o lamella phases has been ex ensi ely s udied. Kilpa ick e al. (Kilpa ick, 1994) ha e s udied he linea iscoelas ic p ope ies o di e en lyo opic phases p esen in he Cesium n-Te adecanoa e/wa e sys em; Robles-Vásquez e al. (Robles-Vásquez, 1993) ha e in es iga ed he linea iscoelas ic esponse o aqueous sys ems con aining a sodium sul osuccina e as su ac an ; and Dimi o a e al. (Dimi o a, 1995) ha e p o ided some linea iscoelas ic da a ela ed o phase ansi ions among di e en liquid-c ys alline s uc u es in he phase diag am o a polye hoxyla ed alcohol con aining 7 mol o e hylene oxyde. Finally, Jage -Leze e al. (Jage -Leze , 1998, 1999) ha e s udied he ole o he chemical na u e o he su ace in con ac wi h a lamella mesophase o unde s and i s heological beha io . In his sense, p e ious s udies (Co dobés, 1997) demons a ed ha mo e elas ic esponses can be ob ained by adding a linea chain hyd oca bon o a bina y hexagonal sys em. Thus, de elopmen o a pla eau egion in he equency dependence o he dynamic unc ions can be ob ained as hep ane concen a ion inc eases. All o hese s udies p o e he complex beha io o lamella phases in he linea iscoelas ic egime. The p esen esea ch is conce ned wi h he iscous and linea iscoelas ic beha io o he lamella mesophase in bo h bina y polye hoxyla ed alcohol/wa e sys ems and e na y sys ems, solubilizing a linea chain hyd oca bon. The phase diag am o polye hoxyla ed alcohol/wa e /hep ane has been p e iously s udied (Co dobés, 1997). This phase diag am shows a di ec hexagonal liquid-c ys alline egion be ween 40 and 50% (w/w) su ac an and a lamella liquid-c ys alline egion be ween 55 and 75% (w/w) solubilizing a maximum amoun o hep ane o 30% (w/w). The o e all objec i e o his wo k was o s udy he e ec ha su ac an and hep ane concen a ions exe on ansien and s eady lows and linea iscoelas ic esponse. In addi ion o his, he e ec o shea on he non-pe u bed lamella phase s uc u e was analyzed. 2. EXPERIMENTAL A poly(ehylene glycol) nonylphenyle he (NPFE-10) om Sigma (S . Louis, U.S.A.) was used as ecei ed. Lamella liquid-c ys alline sys ems we e p epa ed by mixing his nonionic su ac an wi h dis illed deionized wa e and n-hep ane o analy ical g ade, ob ained om Pan eac (Ba celone, Spain). Su ac an concen a ion anged om 60 o 75% (w/w) o bina y sys ems and 50 o 75% (w/w) o e na y sys ems con aining up o 30% (w/w) o hep ane solubilized, inside o he zone o exis ence o he Vol. 56. Fasc. 2 (2005) 97 lamella s uc u e in he phase diag am (Co dobés, 1997). Th ee di e en ypes o heological measu emen s we e pe o med in his wo k: 1) Oscilla o y es s we e ca ied ou in a con ol s ess heome e , RS-100 om Haake (Ka ls- uhe, Ge many) wi h cone and pla e geo- me y (60 mm, 4o). A anspa en bell wi h sa u a ed a mosphe e was placed a ound he measu ing de ice in o de o p e en chan- ges in composi ion due o hep ane o wa e e apo a ion du ing measu emen . All he samples had he same ecen pas his o y, and we e le o 20 minu es in he senso sys em be o e any measu emen . F equency sweep es s we e ca ied ou a cons an s ess inside he linea iscoelas ic egion in a equency ange o 10-2-102 ad/s. P e- iously, in o de o ob ain he linea iscoelas- ic ange o he samples s udied, s ain sweeps a a cons an equency o 1Hz we e pe o med, and he alues o he c i ical s ain, γc, which delimi he linea iscoelas ic egion, we e de e mined. 2) S eady-s a e low measu emen s we e pe o - med in con olled-shea a e heome e s RV100/CV100 o RV100/CV20N, depending on he sample iscosi y, om Haake (Ka ls u- he, Ge many), using a Mooney-Ewa geo- me y (ME15). S eady-s a e low cu es we e ob ained in a shea a e ange o 0.1-300 s-1. 3) T ansien low expe imen s we e ca ied ou in he abo e men ioned con olled-shea a e heome e s a di e en cons an shea a es be ween 0.5 and 100 s-1. All heological es s we e pe o med a 25oC and 2 days a e he sample was p epa ed. In p e ious s udies, i was poin ed ou ha heological p ope ies o samples was no in luenced by ime. A leas wo eplica es o each es we e done. Mic os uc u al obse a ions unde shea in an op ical shea ing cell (Linkham Scien i ic Ins umen s, U.K.) equipped wi h pola izing mic oscopy comple ed he heological s udy o he lamella phase. Samples we e collec ed in he shea ing cell and hen we e submi ed o he same ansien shea his o y inpu in he con olled-shea a e heome e s. 3. RESULTS AND DISCUSSION a) Linea iscoelas ici y . As is well-known, he applica ion o s ains lowe han he c i ical s ain which delimi s he linea iscoelas ic egion, γc, allows he s udy o he heological beha io o he non-pe u bed liquid-c ys alline s uc u e, while highe alues p oduce s uc u al modi ica ions. The alues o γc depend o he linea iscoelas ic unc ion selec ed o i s de e mina ion. In his wo k, he s o age and loss moduli we e selec ed, and he alues o γc we e calcula ed by he c osso e be ween wo lines: G’ o G’’=cons an (a low shea s esses) and G’ o G’’= (σn) (a highe shea s esses). Figu e 1A shows he e olu ion o he s o age and loss moduli wi h he applied s ain o selec ed lamella sys ems (5% hep ane), om he hexagonal liquid-c ys alline phase (55% NPFE-10) up o he iso opic su ac an - ich phase (75% NPFE-10) bounda ies. Figu e 1 B shows he linea iscoelas ic ange o sys ems con aining 60% NPFE-10 and di e en hep ane con en . As can be obse ed in Tables I and II, highe alues o γc we e ob ained om he s o age modulus excep o he bina y sys em. When highe s ains we e applied he e olu ion o bo h linea iscoelas ic unc ions was clea ly di e en . The alues o G’ le down con inuously when s ain inc eases, while he loss modulus shows an ini ial g ow h ollowed by a u he dec ease. Howe e , his ini ial inc ease is a unc ion o he equency selec ed o pe o m he s ain sweep. Thus, i has been e i ied ha he loss modulus con inuously dec eases as he s ain inc eases by applying a cons an equency o 10 Hz (Co dobés, 2000. This ac has been obse ed in o he s lamella phases s udied in he li e a u e (Al a o, 2000). As p e iously demons a ed (Robles-Vásquez, 1993; Robles-Vásquez, 1994) and shown in Figu es 1 and 2, lamella liquid-c ys alline phases a e cha ac e ized by a small linea iscoelas ic ange. The alues o he c i ical s ain, γc, which p oduces a ce ain deg ee o des uc ion in he liquid c ys alline s uc u e, did no p esen a signi ican dependence wi h su ac an con en , o a cons an hep ane concen a ion (Table I). On he o he hand, his c i ical s ain shows minimum alues a in e media e Figu e 1 S ain sweep es s o sys ems con aining 5% (w/w) hep ane (A) and 60% (w/w) NPFE-10 (B). 98 G asas y Acei es hep ane concen a ions o ixed su ac an con en s (Table II). These minimum alues mo e o highe hep ane con en as NPFE-10 concen a ion dec eases. Mo eo e , liquid-c ys alline sys ems wi h highe hep ane concen a ions show a mo e apid dec ease o he iscoelas ic unc ions in he non-linea egion, which can be a ibu ed o a highe s uc u al b eakdown. The linea iscoelas ic p ope ies o lamella phases we e de e mined by means o equency sweeps inside he linea iscoelas ic egion. As can be obse ed in Figu es 2 and 3, he equency dependence o G’ and G’’ was always simila . Thus, almos cons an alues o G’ and a clea minimum in G’’ can be de ec ed o mos sys ems. This mechanical spec um co esponds o he «pla eau» egion, which has been ela ed o he o ma ion o an elas ic s uc u al ne wo k due o he in e ac ions be ween liquid-c ys alline domains (Kilpa ick, 1994). Simila esul s ha e been ound in he li e a u e o sys ems wi h di e en ypes o su ac an s and hyd oca bons (Al a o, 2000; Jiménez, 2000; Neme h, 1998). As can be obse ed, lamella phases a e cha ac e ized by highe alues o s o age modulus in a wide ange o equencies. Thus, only he sys em con aining 50% (w/w) NPFE-10 and 5% (w/w) Table I Values o he c i ical s ain which delimi s he linea iscoelas ic egion o sys ems con aining 5% (w/w) hep ane. %(w/w) su ac an %(w/w) wa e γc in G’ γc in G’’ 55 40 0.30±0.03 0.15±0.03 60 35 0.19±0.02 0.07±0.02 65 30 0.30±0.03 0.10±0.03 70 25 0.27±0.02 0.04±0.02 75 20 0.44±0.04 0.08±0.04 Table II Values o he c i ical s ain which delimi s he linea iscoelas ic egion o sys ems con aining 60% (w/w) su ac an %(w/w) wa e %(w/w) hep ane γc in G’(%) γc in G’’(%) 40 0 0.29±0.03 0.42±0.03 35 5 0.19±0.02 0.07±0.02 30 10 0.14±0.03 0.07±0.03 25 15 0.30±0.04 0.23±0.04 20 20 0.35±0.03 0.09±0.03 15 25 1.11±0.05 0.20±0.05 Figu e 2 E olu ion o s o age and loss moduli as a unc ion o equency o sys ems con aining 5% (w/w) hep ane. Vol. 56. Fasc. 2 (2005) 99 hep ane, loca ed inside he hexagonal egion in he phase diag am, showed a c osso e poin be ween bo h iscoelas ic unc ions in he equency ange s udied. The e o e, he dynamic oscilla o y p ope ies o hexagonal phases a e cha ac e ized by he appea ance o a e minal egion a low equencies (Co dobés, 1997), which was no de ec ed in he lamella sys ems. As can be obse ed in Figu e 2, an inc ease in su ac an concen a ion leads o an inc ease in he alues o he linea iscoelas ic unc ions up o a maximum alue (i.e. 70% NPFE-10 o sys ems con aining 5% hep ane) and hen dec eases. This e ec can be clea ly obse ed in Table III in which he alues o he pla eau modulus, GoN, calcula ed as he G’ alue o which he loss angen was minimum (Co dobés, 1997), a e shown. These maximum alues we e ound a lowe su ac an concen a ions as hep ane con en inc eases, p obably due o he p oximi y o he hexagonal phase bounda ies (Co dobés, 2000). These esul s indica e a highe deg ee o de elopmen o he liquid-c ys alline s uc u e as a consequence o he solubiliza ion o hep ane (Kilpa ick, 1994). These maximum alues o GoN a in e media e su ac an con en coincide wi h minimum alues o loss angen . On he o he hand, he p oximi y o he phase ansi ion is cha ac e ized by minimum alues o he pla eau modulus, which dec ease by inc easing he amoun o hep ane solubilized. This beha io can be ela ed o a change in he ex u e o he lamella phase when i is obse ed in he pola iza ion mic oscope. Thus, Neme h e al. (Neme h, 1998) s udying a non-ionic su ac an /wa e sys em, obse ed a ypical ex u e o «Mal ese C osses» a su ac an concen a ions p e ious o he maximum alues in he iscoelas ic unc ions. A e wa ds, he oily s eaked- ype mic os uc u e was obse ed. This change o ex u e indica es a change in he o ien a ion o he domains (Pla z, 1995), which could be ela ed wi h he de ia ion ound in he end o he s o age modulus (see Table III). These obse a ions a e in good ag eemen wi h he indings o o he au ho s (Dimi o a, 1995; Thad os, 1995). Figu e 3 shows he e olu ion o he s o age and loss moduli as a unc ion o equency o a ixed NPFE-10 con en and di e en hep ane concen a ions. As can be obse ed, ini ially, he highe he solubiliza ion o hep ane, he highe he alues o he iscoelas ic unc ions. This occu s in a hep ane concen a ion o up o a ound 15%. S a ing om his poin , a decline occu s p obably due o he p oli e a ion o de ec s in he liquid-c ys alline s uc u e in he p oximi y o he phase ansi ion ha induces mino elas ic p ope ies. Mo eo e , he bina y sys em shows a highe endency o a c osso e poin be ween he Figu e 3 E olu ion o s o age and loss moduli as a unc ion o equency o sys ems con aining 60% (w/w) su ac an . %(w/w) su ac an %(w/w) hep ane G No (Pa) 25 55 ± 5 50 20 40 ± 3.5 5300 ± 10 10 550 ± 10 15 520 ± 15 20 380 ± 25 120 ± 5 55 30 110 ± 4 088 ± 5 5 1020 ± 6 10 1280 ± 20 15 950 ± 15 20 550 ± 10 60 25 185 ± 5 0570 ± 10 51350 ± 20 10 1270 ± 15 15 950 ± 12 65 20 400 ± 8 0670 ± 9 5 1520 ± 18 70 10 1350 ± 15 0800 ± 6 75 5 1200 ± 12 Table III Values o he pla eau modulus o lamella sys ems 100 G asas y Acei es s o age and loss moduli a low equencies, lowe de elopmen o he pla eau egion and highe alues o he slope o G’ s. ω. The pla eau modulus eaches a maximum alue o a concen a ion o 10% o hep ane indica i e o a majo de elopmen in he elas ic ne wo k. These maximum alues mo e o lowe hep ane con en as NPFE-10 concen a ion inc eases (Table III). b) Viscous low beha io . The low beha io o lamella liquid-c ys alline sys ems was de e mined in a ange o shea a es comp ized be ween 0.1 and 300 s-1 (Figu es 5, 6 and 7). The low p ope ies a e a he simila o all he sys ems s udied, al hough o highe and in e media e alues o shea a es, iscosi y cu es depend on composi ion. Up o 10 s-1, he e olu ion o appa en iscosi y as a unc ion o shea a e is cha ac e is ic o a shea hinning luid, showing, in gene al, a powe -law dec ease in iscosi y. Fo highe alues o shea a es, low cu es a e g ea ly dependen on a composi ion showing shea hinning, New onian o e en shea hickening beha io s. Thus, di e en egions o po en ial dependence o iscosi y can be obse ed when shea a e is inc eased up o 300 s-1. The expe imen al da a can be i ed o a powe law in each egion, de ined as: whe e n is he low index and k is he consis ency in- dex de ined o each po en ial egion. Two di e en egions o po en ial decline o iscosi y we e ound o bina y sys ems (Figu e 4), de ined by low indexes «n» and «m», espec i ely, «n» o alues o shea a e lowe han a c i ical shea a e, γ1, which delimi s he change in he powe law endency, and «m» o alues comp ized be ween γ1 and he maximum shea a e achie ed. Simila alues o «n» (a ound -0.6) we e ound o all he bina y sys ems s udied, excep o he sys em wi h he lowes con en in su ac an (60% NPFE-10), which p esen s a alue o «n» equal o –0.77. This sys em is close o he hexagonal phase ansi ion. In he second egion o po en ial dec ease, he same e ec was ound (see Table IV). This shea - hinning beha io has been ela ed o he o ma ion o o de ed bylaye s uc u es being aligned in he low di ec ion (Laun, 1992; Mon al o, 1998). Mo eo e , Meye e al. (Meye , 2000) ound di e en low egimes depending o shea a e o wo lyo opic and he mo opic sys ems using a con ol s ess heome e . These au ho s p opose ha he i s egime can be associa ed o he p esence o «oily s eaks» aligned in he low di ec ion and de ec ed by pola izing mic oscopy as elonga ed objec s. The second egime was cha ac e ized by he p esence o a shea -induced onion s a e. In his case, he iscosi y cu es can be ela ed o he exis ence o bylaye o «oily-s eaks» s uc u es aligned in he low di ec ion. The di e en alues o low indexes ound can be explained on he basis o he esis ance ha he bylaye s oppose o low. I mus be emphasized ha alues o «m» a e always lowe han «n» (Table IV), p obably due o he highe shea -induced alignmen o he bylaye s in he low di ec ion achie ed as shea a e inc eases. Mo eo e , he ex ension o he i s egion o po en ial decay o iscosi y, de ined by he alues o γ1, is highe o he sys ems wi h he highes su ac an con en . As a consequence, i can be deduced ha sys ems wi h lowe su ac an concen a ion and close o he ansi ion phase egion, a e mo e sensi i e o he o ma ion o s uc u es induced by shea low due o a highe amoun o de ec s in he liquid-c ys alline s uc u e (F anco, 1995). As p e iously men ioned, low p ope ies o lamella liquid-c ys alline sys ems a e s ongly dependen on shea a e and composi ion. Figu e 5 shows he iscous low cu es o selec ed sys ems (1) Figu e 4 S eady-s a e low cu es o bina y sys ems. Figu e 5 S eady-s a e low cu es o sys ems con aining 60% (w/w) su ac an . Vol. 56. Fasc. 2 (2005) 101 as a unc ion o hep ane con en . All he sys ems e alua ed show he same low beha io abo e men ioned. Howe e , he numbe o po en ial dec ease iscosi y egions depends on he abili y o he liquid-c ys alline phase o solubilize hep ane. Thus, sys ems wi h in e media e concen a ions o hyd oca bone (15 and 20% (w/w)) p esen only a egion o po en ial dec ease. Bo h sys ems p esen he highe ela i e elas ic p ope ies o his su ac an concen a ion showing minimum alues o he loss angen . The es o lamella liquid-c ys alline sys ems show he wo egions o iscosi y dec ease. A mo e complex beha io can be obse ed in Figu e 6 o sys ems con aining 50 and 55% (w/w) su ac an and maximum con en o hyd oca bone solubilized. Th ee di e en egimes can be obse ed. Fi s , a shea - hinning egion, a low shea a es, whe e he sample beha es as a pseudoplas ic luid, cha ac e ized by he o ma ion o o de ed bilaye s s uc u es being aligned in he low di ec ion (Laun, 1992; Mon al o, 1998). In he second egion, shea - hickening o New onian low egimes can be obse ed, depending on he composi ion, when a c i ical shea a e, γ1, is eached. This could be explained on he basis ha , a his poin , some o he bilaye s begin o close, esul ing in he o ma ion o esicles. The ansi ion be ween bo h low egimes is aken o be an indica ion o mic os uc u al changes. The hi d egime, abo e a c i ical shea a e, γ2, is cha ac e ized by ano he shea - hinning egion. Escalan e e al. (Escalan e, 2000) ela ed his shea - hinning beha io wi h he alignmen o new mul ilamella esicles o med du ing shea . Howe e , Roux e al. (Roux, 1994) iden i ied he las egime wi h a New onian beha io , ela ed o he up u e o he esicles and he o al alignmen o he bilaye s. In he second egime, o 55% NPFE-10, an inc ease in iscosi y was obse ed in he sys em con aining 25% (w/w) hep ane. This inc ease was no ound o a highe amoun o hep ane (30%), in which a New onian zone can be de ec ed. The same inc ease in iscosi y can be obse ed o sys ems con aining 30% hep ane bu only 55% NPFE-10 A simila beha io has been ound in a qua e na y sys em o he anionic sodium dodecylsul a e (SDS), pen anol, dodecane, and b ine (Roux, 1994). These au ho s obse ed ha a ansi ion o a lamella o a esicle phase could be discon inuous, when a jump in he iscosi y is p oduced (∆η ≠ 0), o con inuous, when a New onian egion is obse ed (∆η = 0). In his case, an inc ease in iscosi y alues akes place in sys ems wi h low alues o he su ac an /wa e a io, which can be ela ed o a highe appea ance o shea -induced esicles. Figu e 6 also p o ides in o ma ion on he possible e e sibili y o s uc u al changes occu ing in he sys em. The e ec o an upwa d/downwa d shea a e p og am o 30 minu es can be obse ed in Figu e 6. Thus, shea a e was inc eased up o 300 s-1 o 5 min, his maximum shea a e was main ained o 10 min and hen was dec eased o 15 min. The sys em wi h lowe su ac an /wa e a io (50% NPFE-10, 30% hep ane) clea ly shows i e e sibili y, as he signi ican highe alues o iscosi y ob ained by educing shea a e indica es (solid symbols) in compa ison o hose obse ed by inc easing shea a e (open symbols). This is due o he ac ha he esicles o med a e s able in ime Table IV Values o he low indexes o po en ial dec ease o iscosi y o bina y sys ems %(w/w) su ac an %(w/w) wa e n m 60 40 -0.77 -0.50 65 35 -0.57 -0.36 70 30 -0.63 -0.38 75 25 -0.60 -0.38 Figu e 6 S eady-s a e low cu es o selec ed sys ems con aining maxi- mum hep ane con en . Solid symbols co espond o low cu es ob ained a e an upwa d/downwa d shea a e p og am. 102 G asas y Acei es and he alues o iscosi y o he esicle phase a e highe han ha o he lamella phase o s acked bylaye s (Escalan e, 2000). Howe e , his beha io changes by inc easing he su ac an /wa e a io. Thus, he sys em wi h highe su ac an /wa e a io (55% NPFE-10, 30% hep ane) showed a dec ease in iscosi y alues, p obably due o he o al alignmen o he bylaye s in he low di ec ion and, consequen ly, he des uc ion o he esicles o med du ing he upwa d shea a e amp, as sugges ed by Pa al e al., (Pa al, 2001). The sys em wi h 55% NPFE-10 and 25% hep ane p esen s in e media e cha ac e is ics o he wo di e en beha io s p e iously desc ibed. c) Shea -induced s uc u al obse a ions. As p e iously shown, shea low causes an impo an e ec on he s uc u al modi ica ions o lamella liquid-c ys alline phases. In his sec ion shea -induced s uc u al changes a e s udied by means o combined heological and pola izing mic oscopy obse a ions. As p e iously desc ibed (F anco, 1995), he e olu ion o ansien iscosi y o simila sys ems consis s o an ini ial g ow h o iscosi y (s ess o e shoo ), due o he iscoelas ici y o he liquid-c ys alline phase, ollowed by a dec ease in iscosi y down o an equilib ium o s eady-s a e alue (Figu e 7). These esul s can be explained by he pa ial alignmen o he lamella bylaye s in he di ec ion o he low ield, and inally, o al alignmen o he bylaye s occu s, and consequen ly, an equilib ium iscosi y is eached. Howe e , o sys ems shea ed a in e media e alues o shea a e (be ween 2 and 10 s-1 ) a sligh inc ease in iscosi y (s ess unde shoo ) akes place ela ed o he de elopmen o mechanically induced de ec s in he s uc u e o he lamella phase (F anco, 1995; Pa al, 2001). As can be obse ed, his sligh inc ease in iscosi y s a s a lowe imes as shea a e inc eases and disappea s o he highes alues o shea a e (100 s-1). Figu e 8 shows he e olu ion o he lamella ex u es du ing a ansien es a wo-selec ed shea a es. Ini ially, applying a shea a e o 2 s-1 (Figu e 8A), he cha ac e is ic mosaic ex u e o he non-o ien ed lamella phase is de ec ed. The p oli e a ion o elonga ed s uc u al uni s in he low di ec ion is obse ed la e on. As can be obse ed, some ypical «Mal hesian c osses» a e always p esen in he shea ed lamella ma ix. The p esence o «Mal hesian c osses» has been ela ed o he exis ence o shea -induced s uc u es in lamella phases such as onions, esicles o sphe uli es (Pa al, 2001; Goldszal, 1995). Howe e , he appea ance o hese new s uc u es is no gene alized. As can be obse ed in Figu e 8A, a in e media e imes an incipien oily s eak ex u e appea s, and inally a o al alignmen o he bylaye s occu s. Op ical mic og aphs ob ained when he lamella phase is shea ed a 10 s-1 show he same sequence o liquid-c ys alline ex u es (Figu e 8B). Howe e , in his case, he mosaic ex u e was clea ly ollowed by he «oily s eaks» ex u e a in e media e imes and, inally, he o al alignmen o his ex u e was again obse ed. As can be seen in Figu e 9, he shea a e exe s an impo an in luence on he ex u e o he lamella phase. Thus, he di e en ex u es obse ed a he end o he expe imen s a e a he simila o un-p eshea ed samples when low alues o shea a e we e applied (i.e. 0.5 s-1), in spi e o he alignmen s o he lamella uni s. On he con a y, an inc ease in shea a e p omo es he de elopmen o he oily s eaked ex u e and inally, o highe alues o shea a es (i.e. 100 s-1), he o al alignmen o he lamella c ys al is ob ained (Figu e 9). As can be obse ed in Figu e 10, he ansien low beha io o a lamella e na y sys em wi h a ela i ely high hep ane concen a ion (30% (w/w)) is qui e di e en o ha p e iously desc ibed o bina y sys ems. Thus, an ini ial inc ease in iscosi y wi h ime, ollowed by a sligh dec ease o iscosi y was always obse ed. Ne e heless, a s ess unde shoo was no de ec ed in any case. This ansien shea Figu e 7 T ansien es s o a bina y sys em con aining 70% (w/w) su ac- an . Figu e 8 E olu ion o lamella ex u es wi h ime o a bina y sys em (70% (w/w) su ac an ) a di e en shea a es (A: 2 s-1, B: 10 s-1). Vol. 56. Fasc. 2 (2005) 103 low causes mic os uc u al changes in he liquid-c ys alline phase, which ha e been ela ed in he li e a u e o he g adual de elopmen o esicles o «onions» in he lamella ma ix (Roux, 1994; Escalan e, 2000). The p esence o hese new s uc u es has been de ec ed using se e al echniques, being mic oscopy (pola izing and elec on mic oscopy) he mos widely used. The s uc u al changes a e e lec ed in he op ical mic og aphs shown in Figu e 11. The ex u es de ec ed a ela i ely high shea a es, i.e. 40 s-1, in conco dance wi h o he s ound in he li e a u e (Dia , 1993; Meye , 2000; Mülle , 1999), seem o indica e he p esence o close-packed monodispe se sphe uli es showing a homogenous g anula ex u e. Fu he mo e, he ex u e o he sys em shea ed a 100 s-1 becomes mo e egula , which can be ela ed o lowe cha ac e is ic sizes o he s uc u al uni s (Mülle , 1999). Rheological da a and mic oscopy images used in his s udy seem o con i m he exis ence o esicles in he lamella shea -induced ex u e o sys ems o med by a non-ionic su ac an and a ela i ely highe con en o a linea hid oca bu e, bu do no p o ide in o ma ion abou esicles o ma ion. In o de o ob ain in o ma ion abou his p ocess, echniques such as small angle ligh and neu on sca e ing should be used. Se e al au ho s ha e epo ed di e en mechanisms o shea induced esicle o ma ion depending on ac o s such as su ac an s uc u e and concen a ion bu also on hei ini ial shea induced mo phology (Rich e ing, 2001). The e o e, a mo e de ailed in es iga ion o his p ocess is necessa y. 4. REFERENCES Al a o, M.C., Gue e o, A. and Muñoz, J.(2000) Dynamic Viscoelas ici y and Flow Beha io o a Polyoxye hylene Glycol Nonylphenyl E he / oluene/wa e Sys em. Langmui , 16, 4711-4719. Ba nes, H.A. (1980). De e gen s in Rheome y Indus ial Applica ions , p.33-110, K. Wal e s (Ed.)., John Wiley&Sons, New Yo k. Comelles, F., Caelles, J., Pascual, A. and Sánchez Leal, J. «18 h In e na ional I.F.S.C.C. Cong ess», Venice, 1994. Co dobés, F. and Gallegos, C. (2000) Viscoelas icidad lineal de la ase líquido-c is alina lamina de un sis ema ensioac i o no iónico/hid oca bu o/agua. G asas y Acei es , 53, 167-172. Co dobés, F., Muñoz, J. and Gallegos, C. (1997) Linea Viscoelas ici y o he Di ec Hexagonal Liquid C ys alline Phase o a Hep ane/Nonionic Su ac an /Wa e Sys em. J.Colloid In e ace Sci. , 187, 401-417. Dia , O., Roux, D. and Nalle , F. (1995) Laye ing e ec in a shea ed lyo opic lamella phase. Phys. Re . E , 51, 3296-3299. Dia , O., Roux, D. y Nalle , F. (1993) E ec o shea on a lyo opic lamella phase. J. Phys. II F ance , 3, 1427-1452. Dimi o a, G.T., Thad os, Th.F. and Luckman, P.F. (1995) In es iga ions o he phase changes o nonionic su ac an s using mic oscopy, di e en ial calo ime y, and heology. 1. Synpe onic A7, a C13/C15 alcohol wi h 7 mol o e hylene oxide. Langmui , 11, 1101-1111. Figu e 9 Lamella ex u es obse ed a he end o ansien expe imen s o a bina y sys em (70% (w/w) su ac an ). A: un-p eshea ed sample, B: shea ed a 0.5 s-1, C: shea ed a 5 s-1, D: shea ed a 10 s-1, E: shea ed a 100 s-1. Figu e 10 T ansien es s o a e na y sys em con aining 50% (w/w) su ac- an and 30% (w/w) hep ane. Figu e 11 Lamella ex u es obse ed a di e en shea a es (A) 40 s-1 and (B) 100 s-1) o a e na y sys em (50% (w/w) su ac an and 30% (w/w) hep ane). 104 G asas y Acei es