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Thermodynamic properties of aqueous PEO–PPO–PEO micelles with added methylparaben determined by differential scanning calorimetry
Journal of Colloid and Interface Science 398 (2013) 270–272
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Short Communication
Thermodynamic properties of aqueous PEO–PPO–PEO micelles with added methylparaben determined by differential scanning calorimetry Andre Lamont Thompson, Brian James Love ⇑ Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA
a r t i c l e
i n f o
Article history: Received 27 October 2012 Accepted 31 January 2013 Available online 16 February 2013 Keywords: Micelle Methylparaben Perturbation Additive Compensation temperature Enthalpy PluronicÒ P105
a b s t r a c t DSC experiments were performed on aqueous solutions of PEO–PPO–PEO (P105) amphiphiles in the low concentration regime (0–1%) to resolve the critical micelle concentration (cmc) both neat and co-formulated with methylparaben (MP). Further work was done at 10% amphiphilic copolymer concentrations and co-formulated with MP to resolve the variations in enthalpy. The compensation temperature, Tcompensation, was determined from the analyses for neat P105 as 293.9 K; adding MP raises this to 328.43 K. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Considerable interest has evolved in packaging of organics within amphiphilic copolymer micelles in skin care, the life sciences, and colloid science [1]. The relative hydrophobicity, solubility, and isoelectric point can affect how organics are partitioned within micelles [2,3]. There are larger issues relating the size of organic flux to safe and effective dosing. But the rationale for understanding how ternary additives affect the micelle formation energetics is likely key to also resolving how formulated micelles perform as drug delivery vehicles. Research on drug-loaded micelles has included rheology [4,5], DSC [6–8], SAXS [4,6,9], (SANS) [10] and cell culture studies [11]. Sharma et al. studied in detail the structures evolving from adding organics as perturbations to the ordered structures forming from PEO–PPO–PEO based solutions as resolved by SAXS [6]. The presence of the ternary constituents can alter the phase structure, swell the micelle, and otherwise affect the energetics of micelle formation. Generally, the presence of additives lowers the micelle formation temperature and within the solubility limits of each constituent, adding more of it correlates with a larger response [6,10].
⇑ Corresponding author. Address: 2644 Bob and Betty Beyster Bldg, Ann Arbor, MI 48109-2136, USA. E-mail addresses: [email protected] (A.L. Thompson), [email protected] (B.J. Love). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.01.064
We have previously evaluated the enthalpy and structural changes in other amphiphiles such as PluronicÒ F127 micelles by several methods [5,9]. The most remarkable changes noted by time-resolved SAXS scanning through the micelle formation temperature, Tmicelle, was that neat F127 solutions formed micelles more abruptly suggesting a nucleation-based mechanism. Adding methylparaben, MP, softened the transition such that ordered micelle formation occurred at lower temperatures and the size of the enthalpy grew more systematically with changes in temperature. Smaller micelle formation endotherms were noted in the presence of the drug mimics. Combined, adding the ternary species to the PEO–PPO–PEO solution altered both the structural disorder depending on how it is partitioned and the micelle formation energetics. More hydrophobic species are more likely contained within the core, and more hydrophilic additives within the corona. The energetics of micelle formation has also included an analysis of the enthalpy–entropy compensation that was first described by Armstrong et al. [12]. DGmicelle is the free energy to take one mole of amphiphile dispersed in solution into the micelle phase and is given by [12]
DGmicelle ¼ RTmicelle lnðcmcÞ where Tmicelle is the T_ micelle formation temperature, and cmc is the critical micelle concentration. The enthalpy is the integral of each micelle formation endotherm. The entropy of micelle formation is extracted from rearranging the Gibbs Helmholtz equation to yield [12].
A.L. Thompson, B.J. Love / Journal of Colloid and Interface Science 398 (2013) 270–272
DSmicelle ¼
1 ðDHmicelle DGmicelle Þ T micelle
271
ð1Þ
Resolved at the micelle formation temperature at each concentration of amphiphile used. From the determinations of DSmicelle and DHmicelle, a direct plot can resolve both the slope, identified as the compensation temperature, Tcompensation, and the intercept (identified as DH0). The compensation temperature represents the driving force for solute–solvent interactions and has been identified for neat amphiphiles in solution [8,13] and ionic liquids such as the Gemini surfactants [1]. But resolving whether the ternary additive also affected the compensation temperature has not been done which we set out to evaluate using PluronicÒ P105 solutions and MP as a ternary additive. We studied this more directly using amphiphilic triblock copolymers more volumetrically balanced between the hydrophobic and hydrophilic elements.
Fig. 1A. DSC results at low P105 content: the critical micelle concentration (cmc) of neat PluronicÒ P105 was noted between 0.2 and 0.4% by DSC.
2. Materials and methods PEO–PPO–PEO (PluronicÒ P105: BASF Wyandotte MI) was obtained and used as received. Aqueous solutions of P105 were prepared according to ‘‘cold’’ processing methods [14] and formulated in both a low concentration regime to resolve the critical micelle concentration (cmc) and a higher concentration regime above 1% to probe the enthalpy of micellization. Methylparaben (Sigma–Aldrich) was added to aqueous solutions of varying P105. Two types of formulation protocols were created. One fixed the P105 content and varied the amount of MP. The other scheme used a fixed amount of MP and varied the P105 content. As dispersions were produced, aliquots were extracted by syringe and deposited into DSC pans and tested using a TA Instruments Q-2000 DSC. Tests were conducted under nitrogen purge while the temperature was ramped from 0 to 40 °C at 10 °C/min, typical of other heating rates measures for other amphiphilic copolymers. At least three replicates were evaluated per formulation. The onset and peak temperatures and the size of the endotherm were determined from the heat flow curve. As the P105 content was lowered, the micellization endotherm was smaller and at some concentration, no endotherm is eliminated indicative of concentrations below the critical micellization concentration, cmc. 3. Results and discussion The influence of amphiphilic copolymer content by DSC is noted in Table 1. The temperature of the peak in the micelle formation is shifted slightly to lower temperatures (2 °C) by adding more copolymer to solution. The enthalpy is also reduced with less P105 in solution since there are fewer micelles that can form in more diluted mixtures of amphiphile in solution above the cmc. With more dilution, there is some minor distortion in the DH peak shape and at low enough concentration; there is an absence of a micelle endotherm, shown in Fig. 1. The driving force is insufficient to form micelles below some threshold and that demarcation is noted as the cmc. Fig. 1 shows the concentration
Table 1 DSC results for 4–10%. P105 in H2O. Larger P105 concentrations show both a lower Tmicelle and a larger DH on a per gram basis. P105 concentrations (% wt/v)
DH (J/g)
Micelle formation temperature (°C)
4.0 6.0 8.0 10.0
2.667 3.667 4.499 5.29
20.54 19.20 17.97 17.42
Fig. 1B. Increasing the concentrations of P105 in water also increased the enthalpy (DH).
dependence; the endotherm is suppressed at 0.4% wt/v. The P105 cmc we resolved by DSC is slightly higher than that observed by Alexandridis et al. (0.3% wt/v) using UV–visible absorption spectroscopy at 25 °C [15,16]. For the larger thermodynamic analysis, we used the cmc determinations from Alexandridis et al. [15,16] for determining DS and DG although similar trends arise using the cmc resolved by DSC. Fig. 2 shows how adding MP to 10% P105 solutions with increasing amounts of MP affected DH, which changed from 350.8 to 318.2 kJ/mol when increasing amounts of MP was added up to 1%. The presence of MP modulates the interaction energy of the mixtures and reduces the size of both the micelle formation endotherm and its peak temperature. A similar suppression was noted by Bouchemal et al. who characterized 1,2 propanediol in F127 and noted a 20% smaller endotherm when added as much as 2.3% wt/v [7]. Kelarakis noted the near athermal micellization in other diblocks [17,18]. If ternary compounds raise structural disorder, the enthalpy contribution to micelle formation should be reduced. At higher concentrations relative to the P105 solution concentration, the presence of MP cannot suppress micelle formation. 80% of the original endotherm is still observable in 10% P105 with 1% MP. The larger question overall is how the relative impact of the ternary additive affects the thermodynamics. To address this
272
A.L. Thompson, B.J. Love / Journal of Colloid and Interface Science 398 (2013) 270–272
mal et al. who using 1,2-propanediol with F127 using Isothermal Titraction Microcalorimetry. Interestingly, by using separate temperature compensation slope determinations, we observe from their results a similar rise in the compensation temperature from 293.3 to 316.5 K when co-formulated with 1,2-propanediol. Their original analysis reported a mean Tcompensation of 298.1 K [7]. 4. Conclusions
Fig. 2. Increasing concentrations of MP in 10% P105 reduces both Tmicelle and DH.
We have probed how MP affects PEO–PPO–PEO micelle formation in two different concentration regimes. Using DSC we confirmed the cmc for P105 to be 0.4% wt/v at 26.3 °C, which is close to but slightly larger than 0.3% wt/v at 25 °C of Alexandridis et al. [15,16] Increasing PluronicÒ P105 from 4% to 10% reduced Tmicelle and raised the endotherm. We propose that the enthalpy–entropy compensation plot for neat and MP-loaded solutions of PluronicÒ P105 can resolve the perturbation in the micelle energetics and determine Tcompensation of 293.9 K for neat P105; when also loaded with MP, the modified Tcompensation is 328.43 K compared to Bouchemal’s Tcompensation of 293.3 K for neat F127 and 316.5 K when loaded with 1,2-propanediol [7]. The use of temperature compensation plots might resolve surfactant quality in formulated dispersions coerced into directed assembly. These results significantly help in understanding how organics mix within amphiphilic copolymer micelles and dispersions. Our findings present a rationale for understanding how the presence of a ternary additive, like MP, affects the thermodynamics of the micelle formation and is key to understanding how formulated micelles function as drug delivery vehicles. Acknowledgments We acknowledge the Rackham Summer Institute at UM for support and discussions with KA Juggernauth, NAK Meznarich, and KM Batzli during the project.
Fig. 3. The enthalpy–entropy (DH–DS) compensation plot for P105, solutions both neat (red with solid line) and co-formulated with MP (green dashed slope). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
question, extractions from the low concentration regime allow the determination of DG and direct measurements of the enthalpy of micellization. From these measurements, thermodynamic plots of DH–DS can be produced both in a neat state and in the presence of MP. Fig. 3 is the enthalpy–entropy compensation plot for neat P105 (red triangles) that yields a compensation temperature of 293.9 K. The compensation temperature determined for neat P105 is very similar to that resolved for the 8-series of PluronicÒ (e.g. F68, F98) of 291.2 K [8]. Adding MP to the mixture triggers a subtle rise in the compensation temperature from 293.9 K to 328.43 K (green squares) treated as a separate dataset. If the compensation temperature represents solute–solvent interactions, then the slopes of DH–DS curves with and without ternary additives should be different. It seems appropriate that adding a ternary species might influence the energetics of micelle formation. We compared our results to published work by Bouche-
References [1] G. Liu, D. Gu, H. Liu, W. Ding, Z. Li, J. Colloid Interface Sci. 358 (2011) 521. [2] F. Artzner, S. Geiger, A. Olivier, C. Allais, S. Finet, F. Agnely, Langmuir 23 (2007) 5085. [3] M.L. Veryries, G. Couarraze, S. Geiger, F. Agnely, L. Massias, B. Kunzli, F. Faurisson, B. Rouceix, Int. J. Pharma. 192 (1999) 183. [4] C. Chaibundit, N.M.P.S. Ricardo, C.A. Muryn, M.B. Madec, S.G. Yeates, C. Booth, J. Colloid Interface Sci. 351 (2010) 190. [5] N.A.K. Meznarich, B.J. Love, Macromolecules 44 (2011) 3548. [6] P.K. Sharma, M.J. Reilly, S.K. Bhatia, N. Sakhitab, J.D. Archambault, S.R. Bhatia, Colloids Surf., B 63 (2008) 229. [7] K. Bouchemal, F. Agnely, A. Koffi, G. Ponchel, J. Colloid Interface Sci. 338 (2009) 169. [8] H.W. Tsui, J.H. Wang, Y.H. Hsu, L.J. Chen, Colloid Polym. Sci. 288 (2010) 1687. [9] N.A.K. Meznarich, K.A. Juggernauth, K.M. Batzli, B.J. Love, Macromolecules 44 (2011) 7792. [10] B. Foster, T. Cosgrove, B. Hammouda, Langmuir 25 (2009) 6760. [11] C. Nagant, P.B. Savage, J.P. Delaye, Appl. Microbiol. 112 (2012) 1173. [12] J. Armstrong, B. Chowdhry, R. O’Brien, A. Beezer, J. Mitchell, S. Leharne, J. Phys. Chem. 99 (1995) 4590. [13] B. Naskar, S. Ghosh, S.P. Moulik, Langmuir 28 (2012) 7134. [14] I.R. Schmolka, J. Biomed. Mater. Res. 6 (1972) 571. [15] P. Alexandridis, J.F. Holzworth, T.A. Hatton, Macromolecules 27 (1994) 2414. [16] P. Alexandridis, T.A. Hatton, Colloids Surf., B 96 (1995) 1. [17] A. Kelarakis, V. Havredaki, C. Rekatas, C. Booth, Phys. Chem. Chem. Phys. 3 (2001) 5550. [18] A. Kelarakis, V. Havredaki, Macromolecules 31 (1998) 944.
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Short Communication
Thermodynamic properties of aqueous PEO–PPO–PEO micelles with added methylparaben determined by differential scanning calorimetry Andre Lamont Thompson, Brian James Love ⇑ Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA
a r t i c l e
i n f o
Article history: Received 27 October 2012 Accepted 31 January 2013 Available online 16 February 2013 Keywords: Micelle Methylparaben Perturbation Additive Compensation temperature Enthalpy PluronicÒ P105
a b s t r a c t DSC experiments were performed on aqueous solutions of PEO–PPO–PEO (P105) amphiphiles in the low concentration regime (0–1%) to resolve the critical micelle concentration (cmc) both neat and co-formulated with methylparaben (MP). Further work was done at 10% amphiphilic copolymer concentrations and co-formulated with MP to resolve the variations in enthalpy. The compensation temperature, Tcompensation, was determined from the analyses for neat P105 as 293.9 K; adding MP raises this to 328.43 K. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Considerable interest has evolved in packaging of organics within amphiphilic copolymer micelles in skin care, the life sciences, and colloid science [1]. The relative hydrophobicity, solubility, and isoelectric point can affect how organics are partitioned within micelles [2,3]. There are larger issues relating the size of organic flux to safe and effective dosing. But the rationale for understanding how ternary additives affect the micelle formation energetics is likely key to also resolving how formulated micelles perform as drug delivery vehicles. Research on drug-loaded micelles has included rheology [4,5], DSC [6–8], SAXS [4,6,9], (SANS) [10] and cell culture studies [11]. Sharma et al. studied in detail the structures evolving from adding organics as perturbations to the ordered structures forming from PEO–PPO–PEO based solutions as resolved by SAXS [6]. The presence of the ternary constituents can alter the phase structure, swell the micelle, and otherwise affect the energetics of micelle formation. Generally, the presence of additives lowers the micelle formation temperature and within the solubility limits of each constituent, adding more of it correlates with a larger response [6,10].
⇑ Corresponding author. Address: 2644 Bob and Betty Beyster Bldg, Ann Arbor, MI 48109-2136, USA. E-mail addresses: [email protected] (A.L. Thompson), [email protected] (B.J. Love). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.01.064
We have previously evaluated the enthalpy and structural changes in other amphiphiles such as PluronicÒ F127 micelles by several methods [5,9]. The most remarkable changes noted by time-resolved SAXS scanning through the micelle formation temperature, Tmicelle, was that neat F127 solutions formed micelles more abruptly suggesting a nucleation-based mechanism. Adding methylparaben, MP, softened the transition such that ordered micelle formation occurred at lower temperatures and the size of the enthalpy grew more systematically with changes in temperature. Smaller micelle formation endotherms were noted in the presence of the drug mimics. Combined, adding the ternary species to the PEO–PPO–PEO solution altered both the structural disorder depending on how it is partitioned and the micelle formation energetics. More hydrophobic species are more likely contained within the core, and more hydrophilic additives within the corona. The energetics of micelle formation has also included an analysis of the enthalpy–entropy compensation that was first described by Armstrong et al. [12]. DGmicelle is the free energy to take one mole of amphiphile dispersed in solution into the micelle phase and is given by [12]
DGmicelle ¼ RTmicelle lnðcmcÞ where Tmicelle is the T_ micelle formation temperature, and cmc is the critical micelle concentration. The enthalpy is the integral of each micelle formation endotherm. The entropy of micelle formation is extracted from rearranging the Gibbs Helmholtz equation to yield [12].
A.L. Thompson, B.J. Love / Journal of Colloid and Interface Science 398 (2013) 270–272
DSmicelle ¼
1 ðDHmicelle DGmicelle Þ T micelle
271
ð1Þ
Resolved at the micelle formation temperature at each concentration of amphiphile used. From the determinations of DSmicelle and DHmicelle, a direct plot can resolve both the slope, identified as the compensation temperature, Tcompensation, and the intercept (identified as DH0). The compensation temperature represents the driving force for solute–solvent interactions and has been identified for neat amphiphiles in solution [8,13] and ionic liquids such as the Gemini surfactants [1]. But resolving whether the ternary additive also affected the compensation temperature has not been done which we set out to evaluate using PluronicÒ P105 solutions and MP as a ternary additive. We studied this more directly using amphiphilic triblock copolymers more volumetrically balanced between the hydrophobic and hydrophilic elements.
Fig. 1A. DSC results at low P105 content: the critical micelle concentration (cmc) of neat PluronicÒ P105 was noted between 0.2 and 0.4% by DSC.
2. Materials and methods PEO–PPO–PEO (PluronicÒ P105: BASF Wyandotte MI) was obtained and used as received. Aqueous solutions of P105 were prepared according to ‘‘cold’’ processing methods [14] and formulated in both a low concentration regime to resolve the critical micelle concentration (cmc) and a higher concentration regime above 1% to probe the enthalpy of micellization. Methylparaben (Sigma–Aldrich) was added to aqueous solutions of varying P105. Two types of formulation protocols were created. One fixed the P105 content and varied the amount of MP. The other scheme used a fixed amount of MP and varied the P105 content. As dispersions were produced, aliquots were extracted by syringe and deposited into DSC pans and tested using a TA Instruments Q-2000 DSC. Tests were conducted under nitrogen purge while the temperature was ramped from 0 to 40 °C at 10 °C/min, typical of other heating rates measures for other amphiphilic copolymers. At least three replicates were evaluated per formulation. The onset and peak temperatures and the size of the endotherm were determined from the heat flow curve. As the P105 content was lowered, the micellization endotherm was smaller and at some concentration, no endotherm is eliminated indicative of concentrations below the critical micellization concentration, cmc. 3. Results and discussion The influence of amphiphilic copolymer content by DSC is noted in Table 1. The temperature of the peak in the micelle formation is shifted slightly to lower temperatures (2 °C) by adding more copolymer to solution. The enthalpy is also reduced with less P105 in solution since there are fewer micelles that can form in more diluted mixtures of amphiphile in solution above the cmc. With more dilution, there is some minor distortion in the DH peak shape and at low enough concentration; there is an absence of a micelle endotherm, shown in Fig. 1. The driving force is insufficient to form micelles below some threshold and that demarcation is noted as the cmc. Fig. 1 shows the concentration
Table 1 DSC results for 4–10%. P105 in H2O. Larger P105 concentrations show both a lower Tmicelle and a larger DH on a per gram basis. P105 concentrations (% wt/v)
DH (J/g)
Micelle formation temperature (°C)
4.0 6.0 8.0 10.0
2.667 3.667 4.499 5.29
20.54 19.20 17.97 17.42
Fig. 1B. Increasing the concentrations of P105 in water also increased the enthalpy (DH).
dependence; the endotherm is suppressed at 0.4% wt/v. The P105 cmc we resolved by DSC is slightly higher than that observed by Alexandridis et al. (0.3% wt/v) using UV–visible absorption spectroscopy at 25 °C [15,16]. For the larger thermodynamic analysis, we used the cmc determinations from Alexandridis et al. [15,16] for determining DS and DG although similar trends arise using the cmc resolved by DSC. Fig. 2 shows how adding MP to 10% P105 solutions with increasing amounts of MP affected DH, which changed from 350.8 to 318.2 kJ/mol when increasing amounts of MP was added up to 1%. The presence of MP modulates the interaction energy of the mixtures and reduces the size of both the micelle formation endotherm and its peak temperature. A similar suppression was noted by Bouchemal et al. who characterized 1,2 propanediol in F127 and noted a 20% smaller endotherm when added as much as 2.3% wt/v [7]. Kelarakis noted the near athermal micellization in other diblocks [17,18]. If ternary compounds raise structural disorder, the enthalpy contribution to micelle formation should be reduced. At higher concentrations relative to the P105 solution concentration, the presence of MP cannot suppress micelle formation. 80% of the original endotherm is still observable in 10% P105 with 1% MP. The larger question overall is how the relative impact of the ternary additive affects the thermodynamics. To address this
272
A.L. Thompson, B.J. Love / Journal of Colloid and Interface Science 398 (2013) 270–272
mal et al. who using 1,2-propanediol with F127 using Isothermal Titraction Microcalorimetry. Interestingly, by using separate temperature compensation slope determinations, we observe from their results a similar rise in the compensation temperature from 293.3 to 316.5 K when co-formulated with 1,2-propanediol. Their original analysis reported a mean Tcompensation of 298.1 K [7]. 4. Conclusions
Fig. 2. Increasing concentrations of MP in 10% P105 reduces both Tmicelle and DH.
We have probed how MP affects PEO–PPO–PEO micelle formation in two different concentration regimes. Using DSC we confirmed the cmc for P105 to be 0.4% wt/v at 26.3 °C, which is close to but slightly larger than 0.3% wt/v at 25 °C of Alexandridis et al. [15,16] Increasing PluronicÒ P105 from 4% to 10% reduced Tmicelle and raised the endotherm. We propose that the enthalpy–entropy compensation plot for neat and MP-loaded solutions of PluronicÒ P105 can resolve the perturbation in the micelle energetics and determine Tcompensation of 293.9 K for neat P105; when also loaded with MP, the modified Tcompensation is 328.43 K compared to Bouchemal’s Tcompensation of 293.3 K for neat F127 and 316.5 K when loaded with 1,2-propanediol [7]. The use of temperature compensation plots might resolve surfactant quality in formulated dispersions coerced into directed assembly. These results significantly help in understanding how organics mix within amphiphilic copolymer micelles and dispersions. Our findings present a rationale for understanding how the presence of a ternary additive, like MP, affects the thermodynamics of the micelle formation and is key to understanding how formulated micelles function as drug delivery vehicles. Acknowledgments We acknowledge the Rackham Summer Institute at UM for support and discussions with KA Juggernauth, NAK Meznarich, and KM Batzli during the project.
Fig. 3. The enthalpy–entropy (DH–DS) compensation plot for P105, solutions both neat (red with solid line) and co-formulated with MP (green dashed slope). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
question, extractions from the low concentration regime allow the determination of DG and direct measurements of the enthalpy of micellization. From these measurements, thermodynamic plots of DH–DS can be produced both in a neat state and in the presence of MP. Fig. 3 is the enthalpy–entropy compensation plot for neat P105 (red triangles) that yields a compensation temperature of 293.9 K. The compensation temperature determined for neat P105 is very similar to that resolved for the 8-series of PluronicÒ (e.g. F68, F98) of 291.2 K [8]. Adding MP to the mixture triggers a subtle rise in the compensation temperature from 293.9 K to 328.43 K (green squares) treated as a separate dataset. If the compensation temperature represents solute–solvent interactions, then the slopes of DH–DS curves with and without ternary additives should be different. It seems appropriate that adding a ternary species might influence the energetics of micelle formation. We compared our results to published work by Bouche-
References [1] G. Liu, D. Gu, H. Liu, W. Ding, Z. Li, J. Colloid Interface Sci. 358 (2011) 521. [2] F. Artzner, S. Geiger, A. Olivier, C. Allais, S. Finet, F. Agnely, Langmuir 23 (2007) 5085. [3] M.L. Veryries, G. Couarraze, S. Geiger, F. Agnely, L. Massias, B. Kunzli, F. Faurisson, B. Rouceix, Int. J. Pharma. 192 (1999) 183. [4] C. Chaibundit, N.M.P.S. Ricardo, C.A. Muryn, M.B. Madec, S.G. Yeates, C. Booth, J. Colloid Interface Sci. 351 (2010) 190. [5] N.A.K. Meznarich, B.J. Love, Macromolecules 44 (2011) 3548. [6] P.K. Sharma, M.J. Reilly, S.K. Bhatia, N. Sakhitab, J.D. Archambault, S.R. Bhatia, Colloids Surf., B 63 (2008) 229. [7] K. Bouchemal, F. Agnely, A. Koffi, G. Ponchel, J. Colloid Interface Sci. 338 (2009) 169. [8] H.W. Tsui, J.H. Wang, Y.H. Hsu, L.J. Chen, Colloid Polym. Sci. 288 (2010) 1687. [9] N.A.K. Meznarich, K.A. Juggernauth, K.M. Batzli, B.J. Love, Macromolecules 44 (2011) 7792. [10] B. Foster, T. Cosgrove, B. Hammouda, Langmuir 25 (2009) 6760. [11] C. Nagant, P.B. Savage, J.P. Delaye, Appl. Microbiol. 112 (2012) 1173. [12] J. Armstrong, B. Chowdhry, R. O’Brien, A. Beezer, J. Mitchell, S. Leharne, J. Phys. Chem. 99 (1995) 4590. [13] B. Naskar, S. Ghosh, S.P. Moulik, Langmuir 28 (2012) 7134. [14] I.R. Schmolka, J. Biomed. Mater. Res. 6 (1972) 571. [15] P. Alexandridis, J.F. Holzworth, T.A. Hatton, Macromolecules 27 (1994) 2414. [16] P. Alexandridis, T.A. Hatton, Colloids Surf., B 96 (1995) 1. [17] A. Kelarakis, V. Havredaki, C. Rekatas, C. Booth, Phys. Chem. Chem. Phys. 3 (2001) 5550. [18] A. Kelarakis, V. Havredaki, Macromolecules 31 (1998) 944.