ACTA PHYSICO-CHIMICA SINICAVolume 24 Issue 3 March 2008Online English edition of the Chinese language journal
Cite this article as Acta Phys -Chim Sin 2008 24(3) 411minus415
Received September 20 2007 Revised December 5 2007Corresponding author Email kabir7rediffmailcom Tel +91571-2703515The project was supported by the fund of the Council of Scientific and Industrial Research (CSIR) New Delhi India (9112 (0389) 2K7-EMR-I)
Copyright copy 2008 Chinese Chemical Society and College of Chemistry and Molecular Engineering Peking University Published by Elsevier BV All rights reservedChinese edition available online at wwwwhxbpkueducn
ARTICLE
Influence of Electrolytes on the Micellar Growth ofAmphiphilic Drug Chlorpromazine HydrochlorideMd Sayem Alam Kabir-ud-Din
Department of Chemistry Aligarh Muslim University Aligarh-202002 UP India
Abstract The effect of electrolytes on the micellar behavior of an amphiphilic drug chlorpromazine (CPZ) hydrochloride wasstudied using cloud point (CP) and dye solubilization techniques In the presence of KBr increase in pH led to decrease in the CP of50 mmolLminus1 drug solution (prepared in 10 mmolLminus1 sodium phosphate (SP) buffer) because of deprotonation of drug molecules athigh pH The visible absorbance increased (due to dye solubilization) with the increase in pH from 65 to 69 which indicatedmicellar growth At fixed pH (67) addition of inorganic salts (KF KCl and KBr) to drug solutions (50 mmolLminus1) caused anincrease in the CP as well as in the visible absorbance with effectiveness being in the order FminusltClminusltBrminus The results were discussedon the basis of counter-ion binding and their effect toward micellar growth Cations (co-ions) also led to an increase in the CP (andalso the visible absorbance) with their effectiveness order being Li+gtNa+gtK+ which was explained by considering cognizance oftheir hydrated radii Compared with anions their effect was small Increase in [CPZ] caused micellar growth and hence the CP aswell as the visible absorbance increased The overall behavior was discussed in terms of electrostatic interactions and micellargrowth
Key Words Phenothiazine drug Chlorpromazine hydrochloride Cloud point Dye solubilization Hofmeister series Micelles
Many pharmacologically active compounds are amphiphilicand hence undergo different kinds of association in a surfac-tant-like manner[1minus5] Despite the investigation of the micellarproperties of these drugs by different workers there is lack ofdata on the effect of temperature on their micellar solutionsWhen the temperature is increased to a particular value theclouding phenomenon is generally observed in nonionic sur-factant solutions[6] When the nonionic surfactant solution hasa temperature higher than the cloud point (CP) phase separa-tion occurs resulting in the formation of the micellar-richphase or coacervate and the micellar dilute phase The CP hasbeen found to be highly dependent on the presence of addi-tives[7minus11] Encouraged by our own findings of the dependenceof the CP in anionic surfactant solutions on the nature andconcentration of additives[12minus15] and hence its possible tuningwe have started exploring clouding behavior in solutions ofamphiphilic drugs
The dye solubilization is a well-known technique for
studying micellar behavior of surfactants amphiphilic blockcopolymers and amphiphilic drugs[16minus22] The underlying prin-ciple is the dependence of solubilization of otherwise water-insoluble dye on the state of aggregation of the amphiphile Asa result the absorbance value is nearly negligible prior to mi-cellization which then rises steeply due to micelle formationand solubilization of the dye therein
Chlorpromazine (CPZ) hydrochloride a phenothiazine withneuroleptic activity has shown a high capacity to interact withbiological membranes and is often regarded as model drug forthe investigation of interactions between drug and membranes(both biological and model)[123minus37] As the electrolyte concen-tration in membranes may vary the CP of a drug may be af-fected With this viewpoint CPZ (which shows clouding at375 degC in 50 mmolLminus1 CPZ at pH 67 in 10 mmolLminus1 phos-phate buffer[3839]) was employed herein as a model to studythe CP phenomenon in the drug solutions with additives Fur-thermore as the effect of Brminus was significant[39] most of the
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
systems of the present study contained KBr The objective ofthe current study was to extend the scope of our previous workby studying dye solubilization in order to get insight into themorphological aspects involved when a drug solution ap-proaches the CP The results of this study provide support tothe mechanism of increase in CP due to the growth of CPZmicelles
1 Experimental
CPZ hydrochloride (ge950 Fluka Switzerland) was usedas received The electrolytes lithium bromide (LiBr ge99 EMerck Germany) sodium bromide (NaBr ge999 BDHEngland) potassium fluoride (KF ge99 BDH England)potassium chloride (KCl ge998 BDH England) and potas-sium bromide (KBr ge99 Merck Germany) were of ana-lytical grade The components of buffer trisodium phosphate(TSP) dodecahydrate and sodium dihydrogen phosphate (SDP)monohydrate were also purchased from Merck
The water used was doubly distilled and deionized (spcond=1minus2 microScmminus1) SP buffer solution of 10 mmolLminus1 wasprepared from TSP (61 mmolLminus1) and SDP (39 mmolLminus1)and subsequently used throughout as solvent The pH of theCPZ solutions was measured with an ELICO pH meter (modelLI 120) All CPs were obtained by placing Pyrex glass tubes(containing the sample drug solutions) in a temperature-controlled bath the temperature was ramped at the rate of 01ordmCminminus1 near the CP and onset of clouding was noted byvisual inspection The temperature at the commencement ofclouding was taken as CP However the temperature wasoscillated slowly through the CP until the results werereproducible (plusmn05 ordmC)[12minus15] Dye solubilization experimentsfor the aqueous drug solutions (with or without electrolytes)were performed at room temperature The sample solutionswith Sudan III dye (equilibrated for 24 h) were filtered andthen the spectra were recorded using a UV-visible Spectronic-20D+spectrophotometer (Thermo Electron Corp MadisonWI USA) The surface tensions were measured using the ringdetachment method using a SD Hardson tensiometer(Kolkata India)
2 Results and discussion
The critical micelle concentration (cmc) of CPZ in purewater determined by surface tension method was found to beapproximately 17 mmolLminus1 The literature values reported bydifferent authors[3439] using variety of techniques and experi-mental conditions are scattered over a range of two orders ofmagnitude (10minus5 up to 10minus3 molLminus1) CPZ has an amino groupand because of its pKa=94[40] exists essentially in its chargedform (cationic) at the physiological pH (with a small fractionin neutral form) At low pH values the tertiary amine portionof the CPZ molecule becomes positively charged whereas athigh pH values it becomes neutral (see the structure Scheme
1) Another point worth mentioning is that no minima wereobtained in the surface tension vs lg[drug] plots[22] this indi-cates that despite 95 purity the drug sample is free of sur-face-active impurities
Previously we found that the CP decreased with the in-crease in pH (ranging from 55 to 69)[39] As the pH increasesthe drug molecules become progressively deprotonated andhence repulsion due to positive charge on the headgroupsdecreases This increases the compactness of micelles and de-creases the CP (whether or not an electrolyte is present)[39]
Fig1 shows the effect of KBr concentration on the CP ofCPZ solutions at different fixed pHs (65 67 69mdashall belowthe pKa value of the drug) At all the three pHs CP follows thesame trend (at any fixed [KBr] increase in pH causes a de-crease in the CP) As the pH is increased more and more drugmolecules become unionized reducing the inter- and intra-micellar repulsions These reduced repulsions may give rise tomore compact micelles and cause a decrease in the CP Pres-ence of Brminus (as KBr) causes a decrease in the surface area oc-cupied per CPZ head group (Ao) with a simultaneous increasein the Mitchell-Ninham parameter Rp (=VclcAo where Vc is thevolume of the hydrophobic part of the CPZ monomer and lc isits length)[41] of the CPZ monomer Therefore a decrease in Ao
results in an increase in Rp and in micellar growth The resultsof Fig1 favorably support this interpretation As the Brminus ionsare hydrated their presence (binding) near the micellar headis expected to make the micelles relatively wet compared with
Scheme 1 Molecular structure of chlorpromazine (CPZ)hydrochloride
Fig1 Effect of KBr concentration on the CP of 50 mmolLminus1
CPZ solutions prepared in 10 mmolLminus1 sodium phosphatebuffer solutions at different pHs
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
grown and compact micelles formed with the increase in pHThe increase in the CP with micellar growth is in accordancewith Kim and Shahprimes earlier findings[21] where large increaseswere recorded in amitriptyline solutions with the addition ofKBr
Fig2 shows the visible spectra of Sudan III in 50 mmolLminus1
CPZ in 10 mmolLminus1 sodium phosphate solutions at differentpHs in the wavelength range of 400minus600 nm The visible ab-sorbance increases with increase in pH from 65 to 69 whichindicates an increase in the dye (Sudan III) solubility resultingmainly from the enlarged micellar size[21] Thus the resultsclearly suggest that an increase in the visible absorbance withincreasing pH mainly results from the enlarged micellar sizeFig3 illustrates the visible spectra of Sudan III solubilized in50 mmolLminus1 CPZ in water containing different fixed KBrconcentrations In this case also the visible absorbance in-creases with increasing KBr concentration indicating an in-crease in the dye solubility The addition of the electrolyteraises the aggregation number of ionic micelles due to electro-static effects
Addition of electrolytes (KF KCl and KBr) at constant pH(67) causes an increase in the CP and the order being Brminus gtClminus gtFminus (Fig4) The order correlates well with their position inHofmeister series As halide ions carry a charge opposite to
the drug micelles they interact electrostatically with the mi-celles The degree of counter-ion binding is known to affectthe size and shape of the micelles[142] Fminus binding to cationicheadgroups is weak (as Fminus is highly hydrated crystal radius0136 nm and hydrated radius 0352 nm) and therefore withNaF addition slow changes occur in the micelle shapesize asa consequence of which the CP also increases slowly Br ionsbind strongly to the micelles and hence are more effective inincreasing the size of micelles However as Brminus is also hy-drated although less than Fminus (crystal radius 0195 nm andhydrated radius 0330 nm) it increases micelle hydration aswell as the CP The trend (Figs4 5 and see Fig3 also for ad-ditional support to the effectiveness of KBr addition) is similarto earlier findings[21] of large increase in the CP as well as theabsorbance intensity with the electrolyte addition
The influence of cationic co-ions (Li+ Na+ K+) on the CPof 50 mmolLminus1 CPZ solutions (Fig6) shows the increase ineffectiveness order of CP as Li+gtNa+gtK+ Bearing similarcharge as the micelles these ions would remain in the aqueoussolution and would not affect the micelles directly Being thesmallest in size Li+ is the most hydrated Therefore to removewater from micelles it needs much energy which leads to the
Fig2 Visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ prepared in 10 mmolLminus1 sodium phosphate buffersolutions at different pHs
Fig3 Visible spectra of Sudan III solubilized in 50 mmolLminus1 CPZin water containing different fixed amounts of KBr concentrations
Fig4 Effect of anionic counter-ions on the CP of 50 mmolLminus1
CPZ solutions prepared in 10 mmolLminus1 sodium phosphatebuffer (pH 67)
indicates precipitation occurring beyond [KBr]gt100 mmolLminus1 at room
temperature (which could be due to formation of nonmicellar phases)
Fig5 Visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ in water containing 100 mmolLminus1 KX
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
highest CP (Fig6) K+ being the largest in size and least hy-drated less energy would be required to remove water frommicelles which is demonstrated by the lowest CP observedwith KCl (Fig6) Obviously Na+ imparts an intermediate ef-fect
The visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ in water at 100 mmolLminus1 bromide salt are shown in Fig7The absorbance changes with the size of cations and fully en-dorses the above-discussed order of the cations toward the CPincrease of CPZ micellar solutions The above results also im-ply that the effect of cations (co-ions) is small compared withanions (counter-ions)
In view of the results described above the Brminus ion has beenfound to be an effective cloud-point booster for CPZ solutionsIts effect therefore was further explored for the drug solu-tions containing different fixed CPZ concentrations (50 75and 100 mmolLminus1) (Figs8 9) The increasing trend observedfor CP is the same for all the CPZ concentrations (Fig8) In-crease in [CPZ] in the presence of KBr increases the numberof micelles (which are charged) For a fixed [KBr] increase indrug concentration increases the number size and charge ofmicelles which increase both inter- and intra-micellar repul-sions causing an increase in the CP The corresponding visiblespectra of Sudan III solubilized in different fixed amounts of
CPZ concentration in water are shown in Fig9 The absorb-ance increases with increasing [CPZ] indicating increase inthe dye solubility as well as micellar growth which supportsthe above explanation (see Fig8)
3 Conclusions
We have performed the CP and dye solubilization meas-urements to investigate the influence of electrolytes on themicellar behavior of CPZ The CP of CPZ micellar solutionsdecreased with the increase in pH because of deprotonation ofthe drug molecules Dye (Sudan III) solubilization experi-ments performed with increasing pH indicated micellargrowth Addition of KX (X=F Cl Br) decreased the electricalrepulsion resulting in micellar growth and increase in the CPas well as visible absorbance of the dye with the order beingBrminusgtClminusgtFminus The binding effect of co-ions is in the order Li+ gtNa+gtK+ The effect of co-ions (cations) is small comparedwith the counter-ions (anions)
References
1 Attwood D Florence A T Surfactant systems LondonChapman amp Hall 1983
2 Attwood D Mosquera V Villar V P J Chem Soc Faraday
Trans 1 1989 85 3011
Fig6 Effect of cationic co-ions on the CP of 50 mmolLminus1
CPZ solutions prepared in 10 mmolLminus1 sodium phosphatebuffer (pH 67)
Fig7 Visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ in water containing 100 mmolLminus1 MBr
Fig8 Effect of KBr concentration on the CP of CPZ solutionscontaining different fixed amounts of the drug prepared in
10 mmolLminus1 sodium phosphate buffer (pH 67)
Fig9 Visible spectra of Sudan III solubilized in different fixedamounts of CPZ concentrations in water
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
3 Ruso J M Attwood D Taboada P Suarez M J SarmientoF Mosquera V J Chem Eng Data 1999 44 941
4 Taboada P Attwood D Garcia M Jones M N Ruso J MSamiento V J Colloid Interface Sci 2000 221 242
5 Attwood D Boitard E Dubes J P Tachoire H J Colloid
Interface Sci 2000 227 3566 Gu T Galera-Gomez P A Colloids Surf A 1995 104 3077 Schott M Colloids Surf 1984 11 518 Karlstrom G J Phys Chem 1985 89 49629 Myers D Surfactant science and technology 2nd ed New York
VCH 199210 Gu T Galera-Gomez P A Colloids Surf A 1999 147 36511 Shigeto K Olsson U Kuneida H Langmuir 2001 17 471712 Kumar S Sharma D Kabir-ud-Din Langmuir 2000 16 682113 Kumar S Sharma D Khan Z A Kabir-ud-Din Langmuir
2001 17 581314 Kumar S Sharma D Khan Z A Kabir-ud-Din Langmuir
2002 18 420515 Kumar S Sharma D Kabir-ud-Din Langmuir 2003 19 353916 Mitra S Dungan S R Colloids Surf B 2000 17 11717 Mata J Varade D Ghosh G Bahadur P Colloids Surf A
2004 245 6918 Mata J Varade D Bahadur P Thermochim Acta 2005 428
14719 Bhattarai N Bhattarai S R Khil M S Lee D R Kim H Y
Eur Polym J 2003 39 160320 Kelarakis A Mai S M Havredaki V Brett A Booth C
J Colloid Interface Sci 2004 275 43921 Kim E J Shah D O Langmuir 2002 18 1010522 Alam M S Naqvi A Z Kabir-ud-Din J Chem Eng Data
24 Kwant W O Seeman P Biochim Biophys Acta 1969 183530
25 Seeman P Pharmacol Rev 1972 24 58326 Cater B R Chapman D Hawes S M Saville J Biochim
Biophys Acta 1974 363 5427 Leterrier F Mendyk A Viret J Biochem Pharmacol 1976
25 246928 Lee A G Mol Pharmacol 1977 13 47429 Frenzel F Arnold K Nuhn P Biochim Biophys Acta 1978
507 18530 Romer J Bickel M H Biochem Pharmacol 1979 28 79931 Yamagushi T Watanabe S Kimoto E Biochim Biophys Acta
1985 820 15732 Guth P S Spirtes M A Int Rev Neurobiol 1964 7 23133 Sheetz M Singer S J Proc Natl Acad Sci USA 1974 71
445734 Conrad M J Singer S J Proc Natl Acad Sci USA 1979 76
520235 Paiement J J Pharm Pharmacol 1984 36 61436 Luxnat M Galla H J Biochim Biophys Acta 1986 856 27437 Bindford J S Rao M S Pollock V Malloy R C J Phys
Chem 1988 92 352238 Wajnberg E Tabak M Nussenzveig P A Lopes C M B
Louro S R W Biochim Biophys Acta 1988 944 18539 Alam M S Kumar S Naqvi A Z Kabir-ud-Din Colloids
Surf B 2006 53 6040 Kartzung B G Basic and clinical pharmacology 9th ed New
York McGraw Hill 200441 Mitchell D J Ninham B W Faraday Trans 2 1981 77 60142 Rosen M J Surfactants and interfacial phenomena New York
Wiley 1989
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
systems of the present study contained KBr The objective ofthe current study was to extend the scope of our previous workby studying dye solubilization in order to get insight into themorphological aspects involved when a drug solution ap-proaches the CP The results of this study provide support tothe mechanism of increase in CP due to the growth of CPZmicelles
1 Experimental
CPZ hydrochloride (ge950 Fluka Switzerland) was usedas received The electrolytes lithium bromide (LiBr ge99 EMerck Germany) sodium bromide (NaBr ge999 BDHEngland) potassium fluoride (KF ge99 BDH England)potassium chloride (KCl ge998 BDH England) and potas-sium bromide (KBr ge99 Merck Germany) were of ana-lytical grade The components of buffer trisodium phosphate(TSP) dodecahydrate and sodium dihydrogen phosphate (SDP)monohydrate were also purchased from Merck
The water used was doubly distilled and deionized (spcond=1minus2 microScmminus1) SP buffer solution of 10 mmolLminus1 wasprepared from TSP (61 mmolLminus1) and SDP (39 mmolLminus1)and subsequently used throughout as solvent The pH of theCPZ solutions was measured with an ELICO pH meter (modelLI 120) All CPs were obtained by placing Pyrex glass tubes(containing the sample drug solutions) in a temperature-controlled bath the temperature was ramped at the rate of 01ordmCminminus1 near the CP and onset of clouding was noted byvisual inspection The temperature at the commencement ofclouding was taken as CP However the temperature wasoscillated slowly through the CP until the results werereproducible (plusmn05 ordmC)[12minus15] Dye solubilization experimentsfor the aqueous drug solutions (with or without electrolytes)were performed at room temperature The sample solutionswith Sudan III dye (equilibrated for 24 h) were filtered andthen the spectra were recorded using a UV-visible Spectronic-20D+spectrophotometer (Thermo Electron Corp MadisonWI USA) The surface tensions were measured using the ringdetachment method using a SD Hardson tensiometer(Kolkata India)
2 Results and discussion
The critical micelle concentration (cmc) of CPZ in purewater determined by surface tension method was found to beapproximately 17 mmolLminus1 The literature values reported bydifferent authors[3439] using variety of techniques and experi-mental conditions are scattered over a range of two orders ofmagnitude (10minus5 up to 10minus3 molLminus1) CPZ has an amino groupand because of its pKa=94[40] exists essentially in its chargedform (cationic) at the physiological pH (with a small fractionin neutral form) At low pH values the tertiary amine portionof the CPZ molecule becomes positively charged whereas athigh pH values it becomes neutral (see the structure Scheme
1) Another point worth mentioning is that no minima wereobtained in the surface tension vs lg[drug] plots[22] this indi-cates that despite 95 purity the drug sample is free of sur-face-active impurities
Previously we found that the CP decreased with the in-crease in pH (ranging from 55 to 69)[39] As the pH increasesthe drug molecules become progressively deprotonated andhence repulsion due to positive charge on the headgroupsdecreases This increases the compactness of micelles and de-creases the CP (whether or not an electrolyte is present)[39]
Fig1 shows the effect of KBr concentration on the CP ofCPZ solutions at different fixed pHs (65 67 69mdashall belowthe pKa value of the drug) At all the three pHs CP follows thesame trend (at any fixed [KBr] increase in pH causes a de-crease in the CP) As the pH is increased more and more drugmolecules become unionized reducing the inter- and intra-micellar repulsions These reduced repulsions may give rise tomore compact micelles and cause a decrease in the CP Pres-ence of Brminus (as KBr) causes a decrease in the surface area oc-cupied per CPZ head group (Ao) with a simultaneous increasein the Mitchell-Ninham parameter Rp (=VclcAo where Vc is thevolume of the hydrophobic part of the CPZ monomer and lc isits length)[41] of the CPZ monomer Therefore a decrease in Ao
results in an increase in Rp and in micellar growth The resultsof Fig1 favorably support this interpretation As the Brminus ionsare hydrated their presence (binding) near the micellar headis expected to make the micelles relatively wet compared with
Scheme 1 Molecular structure of chlorpromazine (CPZ)hydrochloride
Fig1 Effect of KBr concentration on the CP of 50 mmolLminus1
CPZ solutions prepared in 10 mmolLminus1 sodium phosphatebuffer solutions at different pHs
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
grown and compact micelles formed with the increase in pHThe increase in the CP with micellar growth is in accordancewith Kim and Shahprimes earlier findings[21] where large increaseswere recorded in amitriptyline solutions with the addition ofKBr
Fig2 shows the visible spectra of Sudan III in 50 mmolLminus1
CPZ in 10 mmolLminus1 sodium phosphate solutions at differentpHs in the wavelength range of 400minus600 nm The visible ab-sorbance increases with increase in pH from 65 to 69 whichindicates an increase in the dye (Sudan III) solubility resultingmainly from the enlarged micellar size[21] Thus the resultsclearly suggest that an increase in the visible absorbance withincreasing pH mainly results from the enlarged micellar sizeFig3 illustrates the visible spectra of Sudan III solubilized in50 mmolLminus1 CPZ in water containing different fixed KBrconcentrations In this case also the visible absorbance in-creases with increasing KBr concentration indicating an in-crease in the dye solubility The addition of the electrolyteraises the aggregation number of ionic micelles due to electro-static effects
Addition of electrolytes (KF KCl and KBr) at constant pH(67) causes an increase in the CP and the order being Brminus gtClminus gtFminus (Fig4) The order correlates well with their position inHofmeister series As halide ions carry a charge opposite to
the drug micelles they interact electrostatically with the mi-celles The degree of counter-ion binding is known to affectthe size and shape of the micelles[142] Fminus binding to cationicheadgroups is weak (as Fminus is highly hydrated crystal radius0136 nm and hydrated radius 0352 nm) and therefore withNaF addition slow changes occur in the micelle shapesize asa consequence of which the CP also increases slowly Br ionsbind strongly to the micelles and hence are more effective inincreasing the size of micelles However as Brminus is also hy-drated although less than Fminus (crystal radius 0195 nm andhydrated radius 0330 nm) it increases micelle hydration aswell as the CP The trend (Figs4 5 and see Fig3 also for ad-ditional support to the effectiveness of KBr addition) is similarto earlier findings[21] of large increase in the CP as well as theabsorbance intensity with the electrolyte addition
The influence of cationic co-ions (Li+ Na+ K+) on the CPof 50 mmolLminus1 CPZ solutions (Fig6) shows the increase ineffectiveness order of CP as Li+gtNa+gtK+ Bearing similarcharge as the micelles these ions would remain in the aqueoussolution and would not affect the micelles directly Being thesmallest in size Li+ is the most hydrated Therefore to removewater from micelles it needs much energy which leads to the
Fig2 Visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ prepared in 10 mmolLminus1 sodium phosphate buffersolutions at different pHs
Fig3 Visible spectra of Sudan III solubilized in 50 mmolLminus1 CPZin water containing different fixed amounts of KBr concentrations
Fig4 Effect of anionic counter-ions on the CP of 50 mmolLminus1
CPZ solutions prepared in 10 mmolLminus1 sodium phosphatebuffer (pH 67)
indicates precipitation occurring beyond [KBr]gt100 mmolLminus1 at room
temperature (which could be due to formation of nonmicellar phases)
Fig5 Visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ in water containing 100 mmolLminus1 KX
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
highest CP (Fig6) K+ being the largest in size and least hy-drated less energy would be required to remove water frommicelles which is demonstrated by the lowest CP observedwith KCl (Fig6) Obviously Na+ imparts an intermediate ef-fect
The visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ in water at 100 mmolLminus1 bromide salt are shown in Fig7The absorbance changes with the size of cations and fully en-dorses the above-discussed order of the cations toward the CPincrease of CPZ micellar solutions The above results also im-ply that the effect of cations (co-ions) is small compared withanions (counter-ions)
In view of the results described above the Brminus ion has beenfound to be an effective cloud-point booster for CPZ solutionsIts effect therefore was further explored for the drug solu-tions containing different fixed CPZ concentrations (50 75and 100 mmolLminus1) (Figs8 9) The increasing trend observedfor CP is the same for all the CPZ concentrations (Fig8) In-crease in [CPZ] in the presence of KBr increases the numberof micelles (which are charged) For a fixed [KBr] increase indrug concentration increases the number size and charge ofmicelles which increase both inter- and intra-micellar repul-sions causing an increase in the CP The corresponding visiblespectra of Sudan III solubilized in different fixed amounts of
CPZ concentration in water are shown in Fig9 The absorb-ance increases with increasing [CPZ] indicating increase inthe dye solubility as well as micellar growth which supportsthe above explanation (see Fig8)
3 Conclusions
We have performed the CP and dye solubilization meas-urements to investigate the influence of electrolytes on themicellar behavior of CPZ The CP of CPZ micellar solutionsdecreased with the increase in pH because of deprotonation ofthe drug molecules Dye (Sudan III) solubilization experi-ments performed with increasing pH indicated micellargrowth Addition of KX (X=F Cl Br) decreased the electricalrepulsion resulting in micellar growth and increase in the CPas well as visible absorbance of the dye with the order beingBrminusgtClminusgtFminus The binding effect of co-ions is in the order Li+ gtNa+gtK+ The effect of co-ions (cations) is small comparedwith the counter-ions (anions)
References
1 Attwood D Florence A T Surfactant systems LondonChapman amp Hall 1983
2 Attwood D Mosquera V Villar V P J Chem Soc Faraday
Trans 1 1989 85 3011
Fig6 Effect of cationic co-ions on the CP of 50 mmolLminus1
CPZ solutions prepared in 10 mmolLminus1 sodium phosphatebuffer (pH 67)
Fig7 Visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ in water containing 100 mmolLminus1 MBr
Fig8 Effect of KBr concentration on the CP of CPZ solutionscontaining different fixed amounts of the drug prepared in
10 mmolLminus1 sodium phosphate buffer (pH 67)
Fig9 Visible spectra of Sudan III solubilized in different fixedamounts of CPZ concentrations in water
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
3 Ruso J M Attwood D Taboada P Suarez M J SarmientoF Mosquera V J Chem Eng Data 1999 44 941
4 Taboada P Attwood D Garcia M Jones M N Ruso J MSamiento V J Colloid Interface Sci 2000 221 242
5 Attwood D Boitard E Dubes J P Tachoire H J Colloid
Interface Sci 2000 227 3566 Gu T Galera-Gomez P A Colloids Surf A 1995 104 3077 Schott M Colloids Surf 1984 11 518 Karlstrom G J Phys Chem 1985 89 49629 Myers D Surfactant science and technology 2nd ed New York
VCH 199210 Gu T Galera-Gomez P A Colloids Surf A 1999 147 36511 Shigeto K Olsson U Kuneida H Langmuir 2001 17 471712 Kumar S Sharma D Kabir-ud-Din Langmuir 2000 16 682113 Kumar S Sharma D Khan Z A Kabir-ud-Din Langmuir
2001 17 581314 Kumar S Sharma D Khan Z A Kabir-ud-Din Langmuir
2002 18 420515 Kumar S Sharma D Kabir-ud-Din Langmuir 2003 19 353916 Mitra S Dungan S R Colloids Surf B 2000 17 11717 Mata J Varade D Ghosh G Bahadur P Colloids Surf A
2004 245 6918 Mata J Varade D Bahadur P Thermochim Acta 2005 428
14719 Bhattarai N Bhattarai S R Khil M S Lee D R Kim H Y
Eur Polym J 2003 39 160320 Kelarakis A Mai S M Havredaki V Brett A Booth C
J Colloid Interface Sci 2004 275 43921 Kim E J Shah D O Langmuir 2002 18 1010522 Alam M S Naqvi A Z Kabir-ud-Din J Chem Eng Data
24 Kwant W O Seeman P Biochim Biophys Acta 1969 183530
25 Seeman P Pharmacol Rev 1972 24 58326 Cater B R Chapman D Hawes S M Saville J Biochim
Biophys Acta 1974 363 5427 Leterrier F Mendyk A Viret J Biochem Pharmacol 1976
25 246928 Lee A G Mol Pharmacol 1977 13 47429 Frenzel F Arnold K Nuhn P Biochim Biophys Acta 1978
507 18530 Romer J Bickel M H Biochem Pharmacol 1979 28 79931 Yamagushi T Watanabe S Kimoto E Biochim Biophys Acta
1985 820 15732 Guth P S Spirtes M A Int Rev Neurobiol 1964 7 23133 Sheetz M Singer S J Proc Natl Acad Sci USA 1974 71
445734 Conrad M J Singer S J Proc Natl Acad Sci USA 1979 76
520235 Paiement J J Pharm Pharmacol 1984 36 61436 Luxnat M Galla H J Biochim Biophys Acta 1986 856 27437 Bindford J S Rao M S Pollock V Malloy R C J Phys
Chem 1988 92 352238 Wajnberg E Tabak M Nussenzveig P A Lopes C M B
Louro S R W Biochim Biophys Acta 1988 944 18539 Alam M S Kumar S Naqvi A Z Kabir-ud-Din Colloids
Surf B 2006 53 6040 Kartzung B G Basic and clinical pharmacology 9th ed New
York McGraw Hill 200441 Mitchell D J Ninham B W Faraday Trans 2 1981 77 60142 Rosen M J Surfactants and interfacial phenomena New York
Wiley 1989
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
grown and compact micelles formed with the increase in pHThe increase in the CP with micellar growth is in accordancewith Kim and Shahprimes earlier findings[21] where large increaseswere recorded in amitriptyline solutions with the addition ofKBr
Fig2 shows the visible spectra of Sudan III in 50 mmolLminus1
CPZ in 10 mmolLminus1 sodium phosphate solutions at differentpHs in the wavelength range of 400minus600 nm The visible ab-sorbance increases with increase in pH from 65 to 69 whichindicates an increase in the dye (Sudan III) solubility resultingmainly from the enlarged micellar size[21] Thus the resultsclearly suggest that an increase in the visible absorbance withincreasing pH mainly results from the enlarged micellar sizeFig3 illustrates the visible spectra of Sudan III solubilized in50 mmolLminus1 CPZ in water containing different fixed KBrconcentrations In this case also the visible absorbance in-creases with increasing KBr concentration indicating an in-crease in the dye solubility The addition of the electrolyteraises the aggregation number of ionic micelles due to electro-static effects
Addition of electrolytes (KF KCl and KBr) at constant pH(67) causes an increase in the CP and the order being Brminus gtClminus gtFminus (Fig4) The order correlates well with their position inHofmeister series As halide ions carry a charge opposite to
the drug micelles they interact electrostatically with the mi-celles The degree of counter-ion binding is known to affectthe size and shape of the micelles[142] Fminus binding to cationicheadgroups is weak (as Fminus is highly hydrated crystal radius0136 nm and hydrated radius 0352 nm) and therefore withNaF addition slow changes occur in the micelle shapesize asa consequence of which the CP also increases slowly Br ionsbind strongly to the micelles and hence are more effective inincreasing the size of micelles However as Brminus is also hy-drated although less than Fminus (crystal radius 0195 nm andhydrated radius 0330 nm) it increases micelle hydration aswell as the CP The trend (Figs4 5 and see Fig3 also for ad-ditional support to the effectiveness of KBr addition) is similarto earlier findings[21] of large increase in the CP as well as theabsorbance intensity with the electrolyte addition
The influence of cationic co-ions (Li+ Na+ K+) on the CPof 50 mmolLminus1 CPZ solutions (Fig6) shows the increase ineffectiveness order of CP as Li+gtNa+gtK+ Bearing similarcharge as the micelles these ions would remain in the aqueoussolution and would not affect the micelles directly Being thesmallest in size Li+ is the most hydrated Therefore to removewater from micelles it needs much energy which leads to the
Fig2 Visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ prepared in 10 mmolLminus1 sodium phosphate buffersolutions at different pHs
Fig3 Visible spectra of Sudan III solubilized in 50 mmolLminus1 CPZin water containing different fixed amounts of KBr concentrations
Fig4 Effect of anionic counter-ions on the CP of 50 mmolLminus1
CPZ solutions prepared in 10 mmolLminus1 sodium phosphatebuffer (pH 67)
indicates precipitation occurring beyond [KBr]gt100 mmolLminus1 at room
temperature (which could be due to formation of nonmicellar phases)
Fig5 Visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ in water containing 100 mmolLminus1 KX
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
highest CP (Fig6) K+ being the largest in size and least hy-drated less energy would be required to remove water frommicelles which is demonstrated by the lowest CP observedwith KCl (Fig6) Obviously Na+ imparts an intermediate ef-fect
The visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ in water at 100 mmolLminus1 bromide salt are shown in Fig7The absorbance changes with the size of cations and fully en-dorses the above-discussed order of the cations toward the CPincrease of CPZ micellar solutions The above results also im-ply that the effect of cations (co-ions) is small compared withanions (counter-ions)
In view of the results described above the Brminus ion has beenfound to be an effective cloud-point booster for CPZ solutionsIts effect therefore was further explored for the drug solu-tions containing different fixed CPZ concentrations (50 75and 100 mmolLminus1) (Figs8 9) The increasing trend observedfor CP is the same for all the CPZ concentrations (Fig8) In-crease in [CPZ] in the presence of KBr increases the numberof micelles (which are charged) For a fixed [KBr] increase indrug concentration increases the number size and charge ofmicelles which increase both inter- and intra-micellar repul-sions causing an increase in the CP The corresponding visiblespectra of Sudan III solubilized in different fixed amounts of
CPZ concentration in water are shown in Fig9 The absorb-ance increases with increasing [CPZ] indicating increase inthe dye solubility as well as micellar growth which supportsthe above explanation (see Fig8)
3 Conclusions
We have performed the CP and dye solubilization meas-urements to investigate the influence of electrolytes on themicellar behavior of CPZ The CP of CPZ micellar solutionsdecreased with the increase in pH because of deprotonation ofthe drug molecules Dye (Sudan III) solubilization experi-ments performed with increasing pH indicated micellargrowth Addition of KX (X=F Cl Br) decreased the electricalrepulsion resulting in micellar growth and increase in the CPas well as visible absorbance of the dye with the order beingBrminusgtClminusgtFminus The binding effect of co-ions is in the order Li+ gtNa+gtK+ The effect of co-ions (cations) is small comparedwith the counter-ions (anions)
References
1 Attwood D Florence A T Surfactant systems LondonChapman amp Hall 1983
2 Attwood D Mosquera V Villar V P J Chem Soc Faraday
Trans 1 1989 85 3011
Fig6 Effect of cationic co-ions on the CP of 50 mmolLminus1
CPZ solutions prepared in 10 mmolLminus1 sodium phosphatebuffer (pH 67)
Fig7 Visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ in water containing 100 mmolLminus1 MBr
Fig8 Effect of KBr concentration on the CP of CPZ solutionscontaining different fixed amounts of the drug prepared in
10 mmolLminus1 sodium phosphate buffer (pH 67)
Fig9 Visible spectra of Sudan III solubilized in different fixedamounts of CPZ concentrations in water
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
3 Ruso J M Attwood D Taboada P Suarez M J SarmientoF Mosquera V J Chem Eng Data 1999 44 941
4 Taboada P Attwood D Garcia M Jones M N Ruso J MSamiento V J Colloid Interface Sci 2000 221 242
5 Attwood D Boitard E Dubes J P Tachoire H J Colloid
Interface Sci 2000 227 3566 Gu T Galera-Gomez P A Colloids Surf A 1995 104 3077 Schott M Colloids Surf 1984 11 518 Karlstrom G J Phys Chem 1985 89 49629 Myers D Surfactant science and technology 2nd ed New York
VCH 199210 Gu T Galera-Gomez P A Colloids Surf A 1999 147 36511 Shigeto K Olsson U Kuneida H Langmuir 2001 17 471712 Kumar S Sharma D Kabir-ud-Din Langmuir 2000 16 682113 Kumar S Sharma D Khan Z A Kabir-ud-Din Langmuir
2001 17 581314 Kumar S Sharma D Khan Z A Kabir-ud-Din Langmuir
2002 18 420515 Kumar S Sharma D Kabir-ud-Din Langmuir 2003 19 353916 Mitra S Dungan S R Colloids Surf B 2000 17 11717 Mata J Varade D Ghosh G Bahadur P Colloids Surf A
2004 245 6918 Mata J Varade D Bahadur P Thermochim Acta 2005 428
14719 Bhattarai N Bhattarai S R Khil M S Lee D R Kim H Y
Eur Polym J 2003 39 160320 Kelarakis A Mai S M Havredaki V Brett A Booth C
J Colloid Interface Sci 2004 275 43921 Kim E J Shah D O Langmuir 2002 18 1010522 Alam M S Naqvi A Z Kabir-ud-Din J Chem Eng Data
24 Kwant W O Seeman P Biochim Biophys Acta 1969 183530
25 Seeman P Pharmacol Rev 1972 24 58326 Cater B R Chapman D Hawes S M Saville J Biochim
Biophys Acta 1974 363 5427 Leterrier F Mendyk A Viret J Biochem Pharmacol 1976
25 246928 Lee A G Mol Pharmacol 1977 13 47429 Frenzel F Arnold K Nuhn P Biochim Biophys Acta 1978
507 18530 Romer J Bickel M H Biochem Pharmacol 1979 28 79931 Yamagushi T Watanabe S Kimoto E Biochim Biophys Acta
1985 820 15732 Guth P S Spirtes M A Int Rev Neurobiol 1964 7 23133 Sheetz M Singer S J Proc Natl Acad Sci USA 1974 71
445734 Conrad M J Singer S J Proc Natl Acad Sci USA 1979 76
520235 Paiement J J Pharm Pharmacol 1984 36 61436 Luxnat M Galla H J Biochim Biophys Acta 1986 856 27437 Bindford J S Rao M S Pollock V Malloy R C J Phys
Chem 1988 92 352238 Wajnberg E Tabak M Nussenzveig P A Lopes C M B
Louro S R W Biochim Biophys Acta 1988 944 18539 Alam M S Kumar S Naqvi A Z Kabir-ud-Din Colloids
Surf B 2006 53 6040 Kartzung B G Basic and clinical pharmacology 9th ed New
York McGraw Hill 200441 Mitchell D J Ninham B W Faraday Trans 2 1981 77 60142 Rosen M J Surfactants and interfacial phenomena New York
Wiley 1989
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
highest CP (Fig6) K+ being the largest in size and least hy-drated less energy would be required to remove water frommicelles which is demonstrated by the lowest CP observedwith KCl (Fig6) Obviously Na+ imparts an intermediate ef-fect
The visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ in water at 100 mmolLminus1 bromide salt are shown in Fig7The absorbance changes with the size of cations and fully en-dorses the above-discussed order of the cations toward the CPincrease of CPZ micellar solutions The above results also im-ply that the effect of cations (co-ions) is small compared withanions (counter-ions)
In view of the results described above the Brminus ion has beenfound to be an effective cloud-point booster for CPZ solutionsIts effect therefore was further explored for the drug solu-tions containing different fixed CPZ concentrations (50 75and 100 mmolLminus1) (Figs8 9) The increasing trend observedfor CP is the same for all the CPZ concentrations (Fig8) In-crease in [CPZ] in the presence of KBr increases the numberof micelles (which are charged) For a fixed [KBr] increase indrug concentration increases the number size and charge ofmicelles which increase both inter- and intra-micellar repul-sions causing an increase in the CP The corresponding visiblespectra of Sudan III solubilized in different fixed amounts of
CPZ concentration in water are shown in Fig9 The absorb-ance increases with increasing [CPZ] indicating increase inthe dye solubility as well as micellar growth which supportsthe above explanation (see Fig8)
3 Conclusions
We have performed the CP and dye solubilization meas-urements to investigate the influence of electrolytes on themicellar behavior of CPZ The CP of CPZ micellar solutionsdecreased with the increase in pH because of deprotonation ofthe drug molecules Dye (Sudan III) solubilization experi-ments performed with increasing pH indicated micellargrowth Addition of KX (X=F Cl Br) decreased the electricalrepulsion resulting in micellar growth and increase in the CPas well as visible absorbance of the dye with the order beingBrminusgtClminusgtFminus The binding effect of co-ions is in the order Li+ gtNa+gtK+ The effect of co-ions (cations) is small comparedwith the counter-ions (anions)
References
1 Attwood D Florence A T Surfactant systems LondonChapman amp Hall 1983
2 Attwood D Mosquera V Villar V P J Chem Soc Faraday
Trans 1 1989 85 3011
Fig6 Effect of cationic co-ions on the CP of 50 mmolLminus1
CPZ solutions prepared in 10 mmolLminus1 sodium phosphatebuffer (pH 67)
Fig7 Visible spectra of Sudan III solubilized in 50 mmolLminus1
CPZ in water containing 100 mmolLminus1 MBr
Fig8 Effect of KBr concentration on the CP of CPZ solutionscontaining different fixed amounts of the drug prepared in
10 mmolLminus1 sodium phosphate buffer (pH 67)
Fig9 Visible spectra of Sudan III solubilized in different fixedamounts of CPZ concentrations in water
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
3 Ruso J M Attwood D Taboada P Suarez M J SarmientoF Mosquera V J Chem Eng Data 1999 44 941
4 Taboada P Attwood D Garcia M Jones M N Ruso J MSamiento V J Colloid Interface Sci 2000 221 242
5 Attwood D Boitard E Dubes J P Tachoire H J Colloid
Interface Sci 2000 227 3566 Gu T Galera-Gomez P A Colloids Surf A 1995 104 3077 Schott M Colloids Surf 1984 11 518 Karlstrom G J Phys Chem 1985 89 49629 Myers D Surfactant science and technology 2nd ed New York
VCH 199210 Gu T Galera-Gomez P A Colloids Surf A 1999 147 36511 Shigeto K Olsson U Kuneida H Langmuir 2001 17 471712 Kumar S Sharma D Kabir-ud-Din Langmuir 2000 16 682113 Kumar S Sharma D Khan Z A Kabir-ud-Din Langmuir
2001 17 581314 Kumar S Sharma D Khan Z A Kabir-ud-Din Langmuir
2002 18 420515 Kumar S Sharma D Kabir-ud-Din Langmuir 2003 19 353916 Mitra S Dungan S R Colloids Surf B 2000 17 11717 Mata J Varade D Ghosh G Bahadur P Colloids Surf A
2004 245 6918 Mata J Varade D Bahadur P Thermochim Acta 2005 428
14719 Bhattarai N Bhattarai S R Khil M S Lee D R Kim H Y
Eur Polym J 2003 39 160320 Kelarakis A Mai S M Havredaki V Brett A Booth C
J Colloid Interface Sci 2004 275 43921 Kim E J Shah D O Langmuir 2002 18 1010522 Alam M S Naqvi A Z Kabir-ud-Din J Chem Eng Data
24 Kwant W O Seeman P Biochim Biophys Acta 1969 183530
25 Seeman P Pharmacol Rev 1972 24 58326 Cater B R Chapman D Hawes S M Saville J Biochim
Biophys Acta 1974 363 5427 Leterrier F Mendyk A Viret J Biochem Pharmacol 1976
25 246928 Lee A G Mol Pharmacol 1977 13 47429 Frenzel F Arnold K Nuhn P Biochim Biophys Acta 1978
507 18530 Romer J Bickel M H Biochem Pharmacol 1979 28 79931 Yamagushi T Watanabe S Kimoto E Biochim Biophys Acta
1985 820 15732 Guth P S Spirtes M A Int Rev Neurobiol 1964 7 23133 Sheetz M Singer S J Proc Natl Acad Sci USA 1974 71
445734 Conrad M J Singer S J Proc Natl Acad Sci USA 1979 76
520235 Paiement J J Pharm Pharmacol 1984 36 61436 Luxnat M Galla H J Biochim Biophys Acta 1986 856 27437 Bindford J S Rao M S Pollock V Malloy R C J Phys
Chem 1988 92 352238 Wajnberg E Tabak M Nussenzveig P A Lopes C M B
Louro S R W Biochim Biophys Acta 1988 944 18539 Alam M S Kumar S Naqvi A Z Kabir-ud-Din Colloids
Surf B 2006 53 6040 Kartzung B G Basic and clinical pharmacology 9th ed New
York McGraw Hill 200441 Mitchell D J Ninham B W Faraday Trans 2 1981 77 60142 Rosen M J Surfactants and interfacial phenomena New York
Wiley 1989
Md Sayem Alam et al Acta Physico-Chimica Sinica 2008 24(3) 411minus415
3 Ruso J M Attwood D Taboada P Suarez M J SarmientoF Mosquera V J Chem Eng Data 1999 44 941
4 Taboada P Attwood D Garcia M Jones M N Ruso J MSamiento V J Colloid Interface Sci 2000 221 242
5 Attwood D Boitard E Dubes J P Tachoire H J Colloid
Interface Sci 2000 227 3566 Gu T Galera-Gomez P A Colloids Surf A 1995 104 3077 Schott M Colloids Surf 1984 11 518 Karlstrom G J Phys Chem 1985 89 49629 Myers D Surfactant science and technology 2nd ed New York
VCH 199210 Gu T Galera-Gomez P A Colloids Surf A 1999 147 36511 Shigeto K Olsson U Kuneida H Langmuir 2001 17 471712 Kumar S Sharma D Kabir-ud-Din Langmuir 2000 16 682113 Kumar S Sharma D Khan Z A Kabir-ud-Din Langmuir
2001 17 581314 Kumar S Sharma D Khan Z A Kabir-ud-Din Langmuir
2002 18 420515 Kumar S Sharma D Kabir-ud-Din Langmuir 2003 19 353916 Mitra S Dungan S R Colloids Surf B 2000 17 11717 Mata J Varade D Ghosh G Bahadur P Colloids Surf A
2004 245 6918 Mata J Varade D Bahadur P Thermochim Acta 2005 428
14719 Bhattarai N Bhattarai S R Khil M S Lee D R Kim H Y
Eur Polym J 2003 39 160320 Kelarakis A Mai S M Havredaki V Brett A Booth C
J Colloid Interface Sci 2004 275 43921 Kim E J Shah D O Langmuir 2002 18 1010522 Alam M S Naqvi A Z Kabir-ud-Din J Chem Eng Data
24 Kwant W O Seeman P Biochim Biophys Acta 1969 183530
25 Seeman P Pharmacol Rev 1972 24 58326 Cater B R Chapman D Hawes S M Saville J Biochim
Biophys Acta 1974 363 5427 Leterrier F Mendyk A Viret J Biochem Pharmacol 1976
25 246928 Lee A G Mol Pharmacol 1977 13 47429 Frenzel F Arnold K Nuhn P Biochim Biophys Acta 1978
507 18530 Romer J Bickel M H Biochem Pharmacol 1979 28 79931 Yamagushi T Watanabe S Kimoto E Biochim Biophys Acta
1985 820 15732 Guth P S Spirtes M A Int Rev Neurobiol 1964 7 23133 Sheetz M Singer S J Proc Natl Acad Sci USA 1974 71
445734 Conrad M J Singer S J Proc Natl Acad Sci USA 1979 76
520235 Paiement J J Pharm Pharmacol 1984 36 61436 Luxnat M Galla H J Biochim Biophys Acta 1986 856 27437 Bindford J S Rao M S Pollock V Malloy R C J Phys
Chem 1988 92 352238 Wajnberg E Tabak M Nussenzveig P A Lopes C M B
Louro S R W Biochim Biophys Acta 1988 944 18539 Alam M S Kumar S Naqvi A Z Kabir-ud-Din Colloids
Surf B 2006 53 6040 Kartzung B G Basic and clinical pharmacology 9th ed New
York McGraw Hill 200441 Mitchell D J Ninham B W Faraday Trans 2 1981 77 60142 Rosen M J Surfactants and interfacial phenomena New York
