Nanocarriers of Solid Lipid from Micelles of Amino Acids Surfactants

Article pubs.acs.org/Langmuir

Nanocarriers of Solid Lipid from Micelles of Amino Acids Surfactants Coated with Polymer Nanoparticles S. Angayarkanny,† Geetha Baskar,*,† and A. B. Mandal‡ †

Industrial Chemistry Laboratory, and ‡Chemical Laboratory, Council of Scientific and Industrial Research (CSIR) - Central Leather Research Laboratory (CLRI), Adyar, Chennai 600 020, India S Supporting Information *

ABSTRACT: Polymer nanoparticle coated micelle assemblies of lauryl ester of tyrosine (LET) act as potential nanocarriers for the model solid lipid stearyl alcohol. The coating is afforded by a simple methodology of heterophase polymerization reaction of styrene or the mixture of styrene and butyl acrylate at a mole ratio of 0.8:0.2 in the presence of 200 mM LET in water. On the contrary, the polymer nanoparticles produced under similar conditions in the presence of a structurally similar surfactant, lauryl ester of phenyl alanine (LEP), failed to act as nanocarrier. The micelle templates of LET and LEP favored polymerization under controlled conditions as observed from the near monodisperse distribution of molecular weight and size of the polymers. The particle size distribution of poly(styrene) (PS) and poly(styrene-co-butyl acryalte) (PS-co-PBA) nanoparticles from LET was smaller at 24 and 20 nm in comparison to those from LEP. The encapsulation efficiency of polymer nanoparticles from LET surfactant is explained on the basis of difference in the coating of micelle assemblies, which we believe must be arising due to difference in the solubilization site of the monomers in the surfactant micelles before polymerization reaction. The solubilization of the model monomer, benzene at different regions, varying between shell and core of LET and LEP micelles is established from 1H nuclear magnetic resonance spectra. The evidence for the coating of micelle assemblies from surface tension measurements and the encapsulation of stearyl alcohol in the polymer nanoparticle dispersions from LET drawn from transmission electron microscopy, differential scanning calorimetry, and thermogravimetric analysis is discussed.

1. INTRODUCTION The strategy of engineering self-organized assemblies and different phase structures of amphiphiles with polymer nanoparticles is a promising approach for the design of high performing nanocarriers. For this, polyelectrolytes or polymers of different chemical architectures are employed as demonstrated recently for example in the liposomes vehicles.1,2 The potential nanocarriers from smart micelles of polypeptides have been recently reported.3 The polymer nanoparticles coated vehicles from micelles, vesicles, or emulsion phases of amphiphiles are considered as high performing nanocarriers for the functional materials. They contribute to enhancement in the encapsulation and targeted delivery of the functional materials.4−6 They receive an increasing attention due to wide applications in multivarious industries. In fact, they have brought about revolutions in conventional industries such as coatings, plastics, rubber, leather, and cosmetics.7−9 Polymer nanoparticles are recognized as promising materials especially in the context of tuning important properties such as hydrophilic lipophilic balance (HLB) and glass transition temperature (Tg).10 The innovations in the design of nanocarriers are demanded in the context of conferring extra ordinary output from the functional materials.7,11,12 © XXXX American Chemical Society

The industrially significant poly(styrene) nanoparticles have been recently established to bring about more compaction of DNA structures in the presence of CTAB, a cationic surfactant.13 In this study, we explore the scope for innovative utilization of polymer nanoparticle coated micelle assemblies of surfactants that are employed in the emulsion polymerization reaction, as a potential nanocarrier. The synthesis of polymer nanoparticles employing natural surfactants derived from proteins, sugar, fatty acids, and lipids in the emulsion polymerization has been explored.14−28 The micelle assemblies in water of surfactants initially provide an ideal medium for the solubilization of oil soluble monomers and are subsequently coated with polymers after the reaction, which of course is determined by chemical architecture of surfactants and the nature of the polymer. Here, surfactants and polymer particles can be considered to compliment in situ functionalization. For this study, amino acids surfactants have been chosen in view of several advantages like biocompatibility, non toxicity, and side chain functional groups as detailed in the review of Pinazo et Received: February 15, 2013 Revised: May 14, 2013

A

dx.doi.org/10.1021/la400605v | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

with Zeiss Libra 120 on thin film of diluted dispersions, stained with uranyl acetate, and deposited on copper grid. Particle size analysis and TEM measurements were performed after diluting the dispersions to about 0.1 wt % polymer. Optical microscopy (OM) and scanning electron microscopy (SEM) studies were performed on thin film, formed by dip coating. In SEM experiments, the samples were transferred on a SEM stub followed by gold coating to minimize charge. The total surface area was calculated applying eq 1

al.29 The monomers, styrene and butyl acrylate, have been chosen in view of distinct difference in the chemical structures and physicochemical characteristics. The custom designed new amino acids surfactants, lauryl esters of tyrosine (LET) and phenyl alanine (LEP), have been chosen. It was shown that LET and LEP exhibit chemical architecture and polarity dependent conformations at the interface and influence the emulsification properties.30,31 The critical dependence of emulsion characteristics on the nature of oil soluble monomers and the surfactant varying between LEP and LET, implications in the coating of micelle assemblies of LET and LEP with the respective polymer nanoparticles, and encapsulation of a solid lipid have been addressed. The significance of solid lipid nanoparticles especially due to bio compatibility and nontoxic characteristics has been addressed by Melissa et al.32 For this study, stearyl alcohol has been chosen as a model compound of solid lipid. This is the first report on nanocarrires from polymer coated micelles of the aromatic surfactant for encapsulation of stearyl alcohol. This study opens up new avenues for the eco friendly approach in the design of nanocarriers for solid lipid nanoparticles, an emerging area in the field of nanotechnology.33

total surface area = Np[4πr 2]

(1)

where r is the radius of the particle and Np is the total number of particles, computed from the volume of the particles. It is to be mentioned that the spherical model is applied for the computation of area and volume of the particles. Surface tension measurements were performed on latex (polymer nanoparticle dispersions), solutions of LET and LEP in water at different concentrations in order to draw information on the coating of micelles with polymer nanoparticles. The measurements were performed using GBX 3S tensiometer where Wilhelmy platinum plate was used as a probe at temperature of 28 ± 0.1 °C. The reported values are the average of at least three measurements and represent the equilibrium values. Thermal characteristics of polymer nanoparticles in absence and presence of stearyl alcohol were measured using differential scanning calorimetery (DSC) and thermogravimetric analysis (TGA). DSC measurements were performed with a TA calorimeter (model DSC Q200) in the temperature range −80 to +150 °C at a rate of 10 °C/min. DSC measurements for the polymer nanoparticles dispersions were carried out on 20 μL, in sealed Tzero hermitic pan at a heating rate of 1 °C/min. TGA measurements were performed with a TA calorimeter (model TGA Q50), in the temperature range of 30−800 °C at a rate of 5 °C/min., with sample size of 5 mg. The results of DSC and TGA were analyzed using Universal 2000 software. 2.4. Contact Angle Measurements for Assessing Surface Characteristics. Contact angle measurements were performed using contact angle meter (model: HO-1AD-CAM01), Holmace opro-mechatronics Pvt Ltd. ‘Image J’ software was used for the analysis. In these measurements, glass slide was extensively cleaned and treated with chromic acid. Contact angle was measured on the thin film of polymer nanoparticle dispersions before and after encapsulation of stearyl alcohol. For this, the polymer dispersion was deposited on the slide by dip coating method, after dilution of dispersions with water to arrive 10 wt % active matter.

2. MATERIALS AND METHODS 2.1. Materials. The lauryl esters of tyrosine (LET) and phenylalanine (LEP) were synthesized according to the method reported elsewhere.34,35 Styrene 98% and butyl acrylate (BA) 98% pure were obtained from Aldrich chemicals, USA. Potassium persulfate (99%) and sodium dodecylsulfate (SDS) (99%) were from SD. Fine Chemicals Ltd., India. D2O, CDCl3, and 3-(trimethylsilyl)-1-propanesulfonic acid sodium salt (DSS) were from Aldrich Chemicals, USA and used as received. Milli-Q water with specific conductivity (κ) of about 5 μS from the Millipore system was used throughout the experiment. 2.2. Preparation of Polymer Nanoparticles. Polymeric nanoparticles were prepared by the photpolymerisation reaction of the degassed emulsions of styrene, and mixtures of styrene and BA at mole ratio of 0.8:0.2 in water, produced in the presence of 200 mM LET or LEP solutions. The reaction was performed with radiation of λ = 365 nm using Heber multilamp photo reactor, model HML-compact-LP-MP-812, 8 W medium-pressure UV lamp under inert atmosphere, at 30 °C in presence of 0.1% K2S2O8. From several preliminary experiments the duration of the polymerization reaction for the complete conversion was arrived as 12 h. The polymer was isolated and percentage yield calculated as detailed in the recent report.31 Control experiments were performed employing the anionic surfactant, sodium dodecyl sulfate (SDS). In the encapsulation experiments, appropriate amounts of molten stearyl alcohol was added to preheated polymer nanoparticle dispersions of polystyrene (PS) and PS-co-PBA, in gentle stream at a temperature of about 60 °C with continuous stirring. The emulsions were taken for detailed investigations after cooling to ambient temperature. 2.3. Characterization of Polymer Nanopartilces Dispersions. The particle size of polymeric nanoparticle dispersions was measured using a Malvern particle size analyzer 1000HS/3000HS at a fixed scattering angle of 90°. The transmittance (%) at 600 nm of the polymer dispersions diluted to 100 times in water was measured on a Varian spectrophotometer (model Cary Eclipse) from Australia. Transmission electron microscopy (TEM) images were taken

3. RESULTS AND DISCUSSION 3.1. Micelle Templates of LET and LEP for the Synthesis of Nanoparticles of PS and PS-co-PBA. The synthesis of polymer nanoparticles employing micelle templates is largely dependent on the physicochemical characteristics of emulsions of monomers in water. For this study, we have chosen the stable emulsions of monomers in water, formulated using LET and LEP, with maximum weight fraction (ϕ) of monomers at 0.22 and oil/surfactant weight ratio (R) at 4.29 for producing polymer nanoparticles. We mainly focus on the emulsions of styrene, and the mixture of styrene and butyl acrylate at mole ratio of 0.8:0.2. It is to be mentioned here that the emulsions of styrene or styrene-butyl acrylate monomers mixture from LET appeared more transluscent than those from B


Langmuir

Article

The most important observation is the remarkable stability of polymer nanoparticles dispersions in water for minimum of six months. The periodic evaluation of visual appearance and measurements of T and particle size distributions served useful in assessing the storage stability. The surfactant varying between LET and LEP and the monomer, individual or mixtures both seem to influence the particle size of the polymer nanoparticles dispersions under consideration in this study. In respect of both LET and LEP, replacement of 0.2 m styrene with BA, i.e., the polymer, S8B2 contributed to reduction in particle size by about 6.2 ± 2.1. It infers that LEP produced nanoparticles of PS and PS8B2 with particle size larger by about 17.3 ± 2.1 nm. The surface area to volume ratio parameter is useful in understanding the efficiency of nanoparticles, the higher ratio arising from larger surface area has a direct correlation to efficiency of coating of micelles of surfactants. It is significant to note that this ratio in PS and PS 8 B 2 nanoparticles from LET is about 1.8 times that of similar polymers from LEP. This could be explained only on the basis of difference in the emulsification characteristics between LET and LEP for the oils, styrene, and BA. We believe that the localization site of monomer in the micelles is directed by the conformation in the micelle assemblies, wherein the phenol and phenyl groups are packed in the shell and core of LET and LEP micelles. TEM studies on the nanoparticles dispersions threw important information on size and morphology. TEM images of the latex from LET are shown in Figure 1. PS in presence of

LEP. This was also confirmed from higher % transmittance (T) of about 80%, as against 25% with emulsions from LEP. It is to be noted that the formation of emulsion was more spontaneous with LEP in contrast to LET, which required additional mechanical stress in form of vigorous stirring. Photopolymerisation reactions were performed on the emulsions, taking the reaction to almost 98% completion. The polymers isolated from the dispersions by extensive washing with methanol were characterized. The 1H NMR spectra of the polymers confirmed the purity of the isolated polymers to be >96%. The representative 1H NMR spectrum of the copolymer, S8B2 in presence of CDCl3 is presented in Figure S1. In respect of the copolymers, the composition computed from the integral values of the well separated methyl protons of BA at δ: 0.87 ppm and the aromatic protons from styrene, δ 6.45−7.22 ppm was almost in close agreement with the feed ratio of monomers. The molecular weight of the polymers was measured from gel permeation chromatography (GPC) the details of which are previously reported.31 The results on molecular weight and polydispersity index are presented in Table 1. Table 1. Molecular Weight Distribution and Polydispersity Index (PDI) of Polymer Nanoparticles from GPC molecular weight

PDI

monomersa

LET × 105

LEP × 105

LET

LEP

S1B0 S8B2

3.82 7.99

19.81 14.40

1.69 1.93

1.53 1.39

a

The numbers in the subscript of monomers indicate the mole fraction of the monomers, S stands for styrene and B, butyl acrylate.

All polymers showed very high molecular weight with narrow polydispersitry index of about 1.64, the features characteristic of polymerization reaction under controlled conditions. The important inference is that the molecular weight of the polymers produced from LET was lower than those from LEP. This might arise due to difference in polymerization reaction kinetics. The polymers were characterized for the thermal behavior employing DSC measurements. The DSC curves of the copolymers from LET and LEP latex are shown in Figure S2. The Tg was measured as 20.06 °C for the copolymer from LET latex and 17.20 °C, LEP latex, as against the reported values for the related homopolymers, PS, 100 °C and polybutyl acrylate (PBA), at −54 °C.36 This confirms the significant modifications in thermal properties in the copolymers with respect to homopolymers. The results on particle size distribution and the computed parameters of the polymer nanoaparticles, produced in the presence of LET and LEP surfactants are presented in Table 2. It is to be mentioned here that the standard deviation in the particle size measurements was in range of 0.12−0.18.

Figure 1. Transmission electron microscopy (TEM) images of nanoparticles of (a) polystyrene and (b) poly(styrene-co-butyl acrylate) (PS8B2), produced in presence of LET, scale bar: 200 nm.

LET showed spheres of almost uniform size distribution at 24.1 nm, the value in agreement with particle size measured from dynamic light scattering measurements (DLS). PS nanoparticles from LEP showed almost similar spheres of uniform size that is larger by 80.5% (image not shown here) as observed from DLS. The most remarkable inference is the packing of PS-co-BA nanoparticles spheres in regular array with features of porous structures. This feature was characteristic of nanoparticles produced using LET, and not observed with LEP. It infers that with respect to LET, replacement of 20 mol % styrene with BA plays a significant role on the characteristics of polymer nanoparticles in water. From the detailed studies of Kaler et al. and Chern37,38 it can be understood that the particle nucleation mechanism in the initial stage plays a key role in determining the molecular weight distribution and polydispersity index. The occurrence of polymerization inside the monomer swollen micelle assemblies of LET and LEP surfactants in these experiments favors micelle nucleation mechanism and thereby,

Table 2. Characteristics of the Polymer Nanoparticles Dispersions in Water, Temp. 30 °C particle size (r), nm

total surface area ×1022 nm2

surface area/ volume, nm‑1

copolymera

LET

LEP

LET

LEP

LET

LEP

S1B0 S8B2

24.1 20.0

43.5 35.2

3.73 4.50

2.07 2.56

0.12 0.15

0.07 0.09

a

Numbers in the subscript of the copolymer denotes mole fraction of the monomer. C


Langmuir

Article

Figure 2. Gibbs adsorption isotherm plots of surface tension (γ) vs log C of (A) LET and (B) LEP in (a) the absence and (b and c) presence of S1B0 and S8B2 polymers in water.

reached almost constant value at and above a concentration, which is considered as critical micelle concentration (cmc). The results from the surface tension curves on cmc and surface tension at cmc (γcmc) are presented in Table 3. The surface area

polymerization under controlled conditions. It is established that both LET and LEP provide conditions of controlled polymerization reaction similar to microemulsion, at a remarkably low ratio of about 0.25 LET or LEP/monomer with high effective polymer concentration of about 22%. LET and LEP micelle templates play a predominant role in overcoming some of the shortcomings associated with microemulsion polymerization reaction.39−44 This is attributed to the chemical architecture of LET and LEP. In the phase compositions of the emulsions under study, the LET and LEP solutions consist of micellar assemblies, the number of micelles, calculated from aggregation number, more with LET, almost twice, in comparison to LEP. The micelle assemblies in the first place ensure stability controlling the Ostwald ripening process. Furthermore, the difference in number of micelle assemblies, (higher with respect to LET) accounts for the efficient emulsification property of LET. 3.2. Understanding Coating of Micelles of LET and LEP with Polymer Nanoparticles from Surface Tension Measurements. Surface tension measurements offer one of the simplest methods to draw information on the modifications of interfacial adsorption characteristics of surfactants which might arise due to various reasons, including coating with polymer chain. The dependence of organization of polymer from the latex at the interface and adsorption of surfactants on the polymer latex on the physico chemical characteristics of surfactants and the polymer latex, are known. It is to be mentioned here that it is useful to carry out investigations on the latex without additional treatments. In this study, surface tension measurements were performed on polymer nanoparticle dispersions in water (latex) after dilution with water so as to cover a wide range of concentration varying from about 1 × 10−6 to 1 × 10−2 M of LET and LEP in the latex. This means that in all solutions the weight ratio of monomer or monomer mixtures to LET or LEP is about 4.00, the same as that employed in polymerization reaction. This works out to be a molar ratio of roughly 14.1 ± 0.3. The plots of surface tension vs concentration of surfactants, LET and LEP, in the absence and presence of polymer nanoparticles dispersions in water are presented in Figure 2. In presence of polymer nanoparticles, surface tension of LET and LEP decreased progressively with concentration and then

Table 3. Modifications in the Surface Characteristics of (a) LET and (b) LEP in Presence of PS (S1B0) and PS-co-PBA (S8B2) Nanoparticle Dispersions in Water surfactants in water and latex

cmc (M)

LET S1B0 S8B2

3.78 × 10−5 6.45 × 10−4 5.01 × 10−4

LEP S1B0 S8B2

1.26 × 10−4 1.07 × 10−4 1.15 × 10−4

γ cmc (mN/m)

a (Å2)

ratio of increase in a

28.22 38.17 34.92

18.31 50.65 59.81

1.77 2.27

29.37 32.10 33.00

27.66 29.14 33.54

0.05 0.21

(a)

(b)

(a)/molecule of LET or LEP at the air/solution interface in presence of different polymer nanoparticles, was calculated using Gibbs adsorption isotherm equation eq 1. The results are presented in Table 3.45 γ = −nRT Γ ln C = −2.303nRT Γ log C ,

a = 1023/N Γ (1)

where γ refers to surface tension in mN/m, R is the gas constant = 8.31 J mol−1 K−1, T is the temperature in Kelvin, Γ is the surface excess concentration in mol/1000 m2, C is the concentration in mol/L, and a is the area per molecule at the interface in Å2/molecule. From surface tension curves, it is observed that the effect of polymer nanoparticles on the interfacial adsorption characteristics of LET and LEP is very different. The comparative analysis of changes in surface tension at and above cmc of LET or LEP in aqueous solutions threw important information. The surface tension of LET in presence of polymer nanoparticles dispersions was higher than in water, in contrast to LEP, wherein, the changes are almost negligible. For example at a concentration of 4 × 10−5 M, LET in aqueous solution showed γ of 28 mN/m as against 60 mN/m in presence of PS latex. On D


Langmuir

Article

the contrary, with respect to LEP, at 1.50 × 10−4 M, the difference between surface tension of LEP in aqueous solution and PS latex is very small, ∼3 mN/m. It is significant to note that cmc of LEP in aqueous solution and polymer nanoparticles dispersions, remains almost invariant at 1.23 ± 0.16 × 10−4 M as against LET wherein, cmc increased almost by an order. The γcmc of LET in presence of PS nanoparticles is higher by about 35% in comparison to that in aqueous solution in contrast to very small change by about 5% in LEP. It is interesting to observe that a of LET enhances by nearly 1.8 times in presence of PS (S1B0), as against an almost negligible change by about 0.05 times in LEP. A similar trend of substantial changes in interfacial adsorption characteristics were observed for LET as against LEP in presence of S8B2 copolymer nanoparticles. However, the magnitude of modifications in the interfacial adsorption characteristics of LET is larger in presence of PS particles (PS1B0 latex) in comparison to the PS-co-PBA nanoparticles (PS8B2 latex). The important observations, viz. enhancement in surface tension, cmc, and γcmc of LET all substantiate reduction in surface activity of LET or, in other words, loss of LET from the interface in presence of latex. It is known that PS or PS-co-PBA are feebly surface active. Therefore, organization of polymer chain at the interface effecting considerable reduction in surface tension is almost ruled out. The trend of reduction in surface tension of LET in presence of polymer nanoparticles dispersions certainly indicates the organization of LET at the interface, although with different packing density. The decrease in packing density of LET as supported from enhancement in minimum surface area/molecule in presence of polymer nanoparaticle dispersion is explained as the consequence of packing of LET and polymer chain at the air/solution interface. In the solution, the packing of polymer chain alongwith headgroup of LET at the micelle/water interface is envisaged. The presence of polymer chain inside the micelles of LET is not however ruled out. It is to be mentioned here that the polymer nanoparticles were produced in presence of micelles of LET. We propose that the solubilization site of monomer at different regions of micelle assemblies varying between shell and core, as defined in a core shell micelle model, directs coating of micelles with the polymer chain. We believe that the solubilization of the monomer, styrene in the shell region of LET micelles, underlies packing of LET and the polymer chain or in other words, coating of LET micelles at the interface. This is the most distinguishing feature observed with LET surfactant. On the contrary, the difference in interfacial adsorption characteristics of LEP in water in absence and presence of polymer nanoaprticles is almost negligible. We attribute this to solubilization of monomer predominantly inside the core of LEP micelles. A speculative model of organization of LET and LEP in presence of PS latex is presented in Figure 3. In order to establish the difference in the coverage of LET and LEP micelles with the polymer, TGA of solid LET and LEP were compared with that in the dried latex. The TGA curves are presented in Figure 4. The results of thermal degradation characteristics of the latex from Figure 4. are presented in Table 4. in comparison with LET/LEP and the PS. The differences in degradation % (Δdegradation) at different temperatures throw important information. LET showed Δdegradation as high as about 6%, in contrast to LEP, wherein the change was 0.5%, at 300 °C. This means that LEP in the PS latex showed thermal degradation similar to neat LEP, in contrast to LET. The

Figure 3. Speculative model of organization of (A) LET or LEP in water, (B) LET in presence of PS latex, and (C) LEP in presence of PS latex.

coverage of LET with polymer is expected to account for the difference in degradation characteristics. 3.3. Polymer Nanoparticles for Encapsulation of Stearyl Alcohol. The selected polymer dispersions that contain poly(styrene) (S1B0) and poly(styrene-co-BA), (S8B2) produced in the presence of LET and LEP surfactants were evaluated for the encapsulation of the model solid lipid, stearyl alcohol (SA). For reference, the polymer prepared in presence of SDS was employed. The encapsulation experiments were performed at different weight ratios, varying RSA (ratio of weight of SA/surfactant (LET or LEP) as 0.18, 0.35, and 0.70. With respect to LEP latex, encapsulation of SA was almost negligible as observed from the visual separation of SA, at room temperature after a duration of about 15 min at all RSA, irrespective of the nature of the polymer. This was also confirmed from the gravimetry. Quite interestingly, the polymer dispersions from LET showed excellent up take of SA. This is inferred from the preliminary visual assessment of the dispersion, wherein, no separation of SA was observed for more than a month. A comparative account of efficiency of encapsulation of SA is presented in Table 5. It is to be noted that the latex from LET encapsulated maximum of about 30% SA, on the polymer weight basis, which is no doubt remarkable. The encapsulation of SA was confirmed from DSC performed on the polymer dispersion in presence of SA at RSA of 0.70. The representative DSC trace of the PS-co-PBA latex (S8B2) from LET and LEP with and without SA in temperature of 30−60 °C is shown in Figure 5. The DSC trace of solid stearyl alcohol showed an endotherm at 55 °C. (not shown here). The phase transition observed at 47 °C (47.33 is the midpoint of the endotherm) in the SA encapsulated latex as against PS8B2 latex evidence the presence of SA in the former. On the contrary, with respect to LEP, the individual and encapsulated latex showed almost similar DSC trace, confirming the absence of SA. The decrease in melting temperature observed with respect to SA encapsulated in LETS8B2 latex might arise due to reasons of loss of crystalline phase or formation of new phase of encapsulated SA due to interaction with LET and or polymer nanoparticles. The formation of new phase structures and modulation of crylstalline or amorphous phase in stearyl alcohol through interaction with surfactant like Briji or the mixture of surfactants is reported.31 The endotherm due to SA was not observed with LEP-PS8B2 latex and this confirms absence of SA, in line with the visual observation. It is to be mentioned here, that the dispersions from SDS did not encapsulate SA and showed spontaneous separation of SA. The encapsulation is clearly seen in the TEM image (Figure 6), wherein, uniform distribution of SA in the polymer nanoparticle could be observed, in contrast to LEP polymer nanoparticles, wherein, clear phase separation of the SA was E


Langmuir

Article

Figure 4. TGA traces of (A) LET S1B0 and (B) LEP S1B0, polymers prepared by directly drying the latex, 30−800 °C, 5 °C/min, sample size, 5 mg.

Table 4. ΔDegradation of the Dried Latex and Pure Surfactants ΔDegradation = Experimental Degradation − Theoretical Degradation 100 °C sample

exptl

LET LET PS LEP LEP PS

0.63 1.79 1.44 0.80

200 °C

Δdegradation (%)

Δdegradation (%)

exptl 9.17 6.01 35.09 4.41

1.67 0.53

4.28 0.67

RSA = weight of stearyl alcohol/surfactant 0.18

0.35

0.70

LET S1B0 LET S8B2 LEP S1B0 LEP S8B2

milky emulsion milky emulsion separation of solid separation of solid

milky emulsion milky emulsion separation of solid separation of solid

separation of solid milky emulsion separation of solid separation of solid

exptl 73.72 19.50 100.00 19.42

Δdegradation (%) 5.63 0.50

observed. The SEM and OM images also supported this observation (Figures 7 and 8). A model on the SA encapsulated polymer nanoparticle is presented (Figure 9). 3.4. Surface Characteristics of Latex and Effect of Encapsulated SA Nanoparticles. The contact angle of water and hexadecane measured on the latex film was used to calculate the surface energy of the polymer surfaces. The polar and dispersion components of the surface energy of the polymer were calculated using Young Dupre equation (eq 2).

Table 5. Appearance of Latex in Presence of Stearyl Alcohol sample

300 °C

Figure 5. DSC traces of (A) (a) LET S8B2 and (b) LET S8B2 with stearyl alcohol and (B) (a) LEP S8B2 and (b) LEP S8B2 with stearyl alcohol, scanning rate 1 °C/min, sample size 5 mg. F


Langmuir

Article

to mention here, that the contact angle of water showed marked changes on different polymer films, in stark contrast to that of hexadecane that was around 5 ± 2°. This implies that the changes in total surface energy (γS) mainly arise from changes in the polar component of surface energy (γSp). It infers from the Table 6, γSd of all latex films from PS (S1B0) or PS-co-PBA (S2B8) produced using different surfactants, LET and LEP is ∼25 mJ/m2. The γSp of all latex films, from LET is lower than those observed with LEP. In LET S1B0, γSp is about one-third of that of LEP S1B0. This eventually contributes to enhancement of total surface energy, γS in LEP S1B0 by about 1.4 times that of LET S1B0. In LET latex, γSp increases with introduction of BA component, as observed in LET S8B2, wherein, γSp increases by about 3 times and eventually contributes to enhancement in γS = 45.27 mJ/m2, which is about 1.42 times that in LET PS1B0. Similar trend of enhancement in γS and γSp with introduction of PBA was observed with latex from LEP. In the first place, this trend suggests increase in the hydrophilic property of the PS with introduction of PBA. This probably arises due to predominant orientation of the ester group of PBA on the surface of the film. Between LET and LEP, the difference in γSp and γS suggests that the organization of the polymer film at the interface is very different. We believe that this might arise due to difference in the pattern of organization of the polymer film as determined by the site of localization in the LET and LEP films. The changes in the hydrophilic character of the latex films affect the work adhesion (WSL) at the solid/water interface and this could be also recognized from the related eqn. that involves cosine of the contact angle of liquid under consideration, in this case water. In PS latex, WSL increases by 1.32 times, i.e., from 83 to 110 mJ/m2, on replacing LET with LEP. Quite interestingly, in the LET latex, replacement of PS by 20 mol % PBA effects enhancement in WSL almost to a same extent by 1.30 times (Table 6). These results suggest that the nature of surfactant determines organization pattern at the interface and play a predominant role on the important parameters of γSp and γS, correlated directly to hydrophilicity of the surface. The results suggest that for the PS, the hydrophilicity and WSL can be manipulated by varying the surfactant between LET and LEP. The tuning of hydrophilicity of the surface is especially demanded in important biological applications and surface coatings in the context of improving wetting and adhesion characteristics.47 The effect of encapsulation of stearyl alcohol on the surface energy characteristics of polymers was evaluated. A simple comparison of the γSp, γS, and WSL values calculated from measured value of contact angle of water and hexadecane threw important information. In respect of PS from LET, encapsulation of stearyl alcohol resulted in significant enhancement in γSp, γS, and WSL values by 2.85, 1.40, and 1.35 times. On the contrary, with respect to PS from LEP, the changes were almost negligible (Table 6). This suggests in the first place, absence of SA in PS dispersion from LEP in accordance with our observations from preliminary experiments. In the copolymer, LET S8B2, a similar trend of enhancement of γSp, γS, and WSL was observed, as against from LEP. The encapsulation of SA generates the most hydrophilic surface as observed with S8B2 particles from LET. By this, it infers that LET performs as a novel surfactant to generate polymer surfaces with hydrophilic characteristics with a control over the Tg or thermal characteristics of the polymer. This is attributed to typical chemical architecture and conformation of LET, which determines coverage and organization of the polymer film.

Figure 6. Transmission electron microscopy (TEM) images of films from polymeric latex of poly(styrene-co-butyl acrylate) (S8B2) encapsulated with stearyl alcohol using (a) LET and (b) LEP, scale bar: (a) 500 nm and (b) 1 μm.

Figure 7. Scanning electron microscopy (SEM) images of films of poly(styrene-co-butyl acrylate), PS8B2 after encapsulation (a) LET PS 8 B 2 encapsulated with stearyl alcohol and (b) LEP S 8 B 2 encapsulated with stearyl alcohol, scale bar: (a) 1 μm and (b) 2 μm.

Figure 8. Optical microscopy images of films from polymeric latex of poly(styrene-co-butyl acrylate), PS8B2 after encapsualtion (a) LET PS 8 B 2 encapsulated with stearyl alcohol and (b) LEP S 8 B 2 encapsulated with stearyl alcohol, scale bar: 10 μm.

The comparative results are presented in Table 6. The work of adhesion at the solid/liquid interface was also calculated from the contact angle of water using Young Dupre equation.46 γL(1 + cos θ ) = 2(γSd1/2γLd1/2) + 2(γSp1/2γLp1/2)

(2)

where γL is the surface tension of liquid, θ is the contact angle, γSd1/2 is the dispersion component of the solid, γLd1/2 is the dispersion component of the liquid, γSp1/2 is the polar component of the solid, and γLp1/2 is the polar component of the liquid. The surface energy, and in particular the polar and dispersion components serve useful guidelines to assess the nature of the surface that has valuable applications in the surface related properties of wetting, spreading and adhesion.47 It is important G


Langmuir

Article

Figure 9. Speculative model illustrating encapsulation of stearyl alcohol in polymer nanoparticle coated LET micelles.

Table 6. Contact Angle of Water, γSd, γSp, γS, and WSL of Different Polymers in (a) the Absence and (b) Presence of Stearyl Alcohol, Temp. 25 °C

at R of 4.29 of benzene/LET or LEP are presented in Figures 10 and 11.

(a) Without Stearyl Alcohol copolymer LET S1B0 LET S8B2 LEP S1B0 LEP S8B2 SDS S1B0 SDS S8B2

contact angle (deg) 81.96 58.32 62.40 48.28 48.03 37.69 (b) Stearyl

γSd, mJ/m2

γSp, mJ/m2

25.00 6.71 24.77 20.50 25.00 19.81 24.88 27.21 24.15 34.54 25.00 34.13 Alcohol Loaded Latex

γS, mJ/m2

WSL, mJ/m2

31.71 45.27 44.81 52.09 58.69 59.13

82.98 111.03 110.12 121.18 130.14 130.41

copolymer

contact angle (deg)

γSd, mJ/m2

γSp, mJ/m2

γS, mJ/m2

WSL, mJ/m2

LET S1B0 LET S8B2 LEP S1B0 LEP S8B2 SDS S1B0 SDS S8B2

58.39 40.04 60.50 48.55 45.90 39.61

24.85 23.92 24.87 25.00 24.04 23.53

20.41 33.45 32.46 27.03 27.84 33.99

45.26 57.37 57.33 52.03 51.88 57.52

110.95 128.56 128.18 121.04 121.28 128.89

Figure 10. 1H NMR spectra in D2O of 200 mM LET: in (a) the absence and (b) presence of 21.90% benzene.

3.5. Factors Attributed to Encapsulation of SA in the Polymer Nanoparticles and Evolution of a Speculative Model. The polymer nanoparticles from PS (S1B0) or PS-coPBA (S8B2) from LET exhibit remarkable efficiency of encapsulation of SA in stark contrast to those from LEP. The important difference in the solubilization site of the monomers in the micelle assemblies of LET and LEP basically seem to direct the coating of the micelle assemblies with the polymer nanoparticle. In order to understand the difference in the solubilization site of styrene at different regions of the micelles of LET and LEP, 1H NMR measurements were performed on the emulsion of benzene, a model compound of the oil soluble monomer, in 200 mM LET or LEP solutions in D2O, where in the ratio of weight % of benzene to surfactant was 4.29. This composition is same as that was employed in the polymerization reaction. 1H NMR spectra of LET and LEP at a concentration of 200 mM in absence and presence of benzene

Aromatic proton of benzene ring at δ = 6.88 ppm in LET and δ = 7.35 ppm in LEP, merge with those of benzene, in the mixed solutions. The line features of the alkyl chain protons at δ of 0.80 and 1.171 ppm in LET showed almost no change in sharp contrast to that in LEP, wherein the alkyl chain protons showed large upfield shift by 0.12 ppm. The changes in line features of the emulsions must be arising due to ring effect. Considering the packing of the dodecyl (alkyl) chain of LET and LEP micelles in the core, the changes in line features of alkyl chain protons in LEP demonstrate solubilization of benzene near the core as against shell in LET, which showed negligible change in the alkyl chain protons packed in the core region of LET micelles. It is presumed that this difference in the site of solubilization of benzene basically arises due to the difference in the conformations of LET and LEP in the micelles as reported in our recent studies.30 In view of the structural H


Langmuir

Article

after encapsulation. This might arise due to interaction of SA with LET and or polymer nanoparticles. TEM micrographs further evidence encapsulation of SA in LET micelles as against LEP wherein, distinct macrophase separation could be observed. The H bonding interaction between the phenolic group of LET and OH of SA is expected to underlie the encapsulation of SA in the latex from LET. This study opens up novel approaches in the design of potential bio compatible nanocarriers for the encpasualtion of solid lipids that are employed in drug delivery formulations.

■

ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectrum of the copolymer S8B2 in CDCl3 and DSC traces of S8B2 polymer nanoparticles from (a) LET and (b) LEP. This material is available free of charge via the Internet at http://pubs.acs.org.

■

Figure 11. 1H NMR spectra in D2O of 200 mM LEP: in (a) the absence and (b) presence of 21.90% benzene.

AUTHOR INFORMATION

Corresponding Author

similarity with respect to the aromatic ring which provides a scope for π bond interaction, it is reasonable to expect localization of styrene in the core of LEP and shell of LET micelles. The difference in solubilization site of styrene in the micelles, accounts for the formation of polymer nanoaparticles in the shell and the core of LET and LEP micelles, respectively, and, by this, effects significant coating of LET micelles as against LEP. It is significant to mention here, that in dispersions of SA in neat LET and LEP micelles, SA separated spontaneously. This suggests an almost negligible encapsulation of SA in the micelles. By this it is proved that polymer coated LET micelles acts as a suitable matrix for the encapsulation of SA, the strength of which must be arising through H bonding interaction between phenolic group in LET and OH of SA. This study establishes a new and promising approach for the encapsulation of solid lipid using polymer coated micelle assemblies of the amino acid surfactants and is significant in development of high performing eco friendly drug delivery systems.

*E-mail: [email protected]. Tel: +91- 44- 2443 7106. Fax: +91-44-2491 1589. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

The authors are thankful to CSIR, India for funding through Twelfth Five Year Plan - Supra institutional project, STRAIT, grant and Dr. Deepa Khushalani, Assistant Professor, TIFR for the TEM measurements. S.A. thanks CSIR for a CSIR SRF fellowship.

(1) Simovic, S.; Barnes, T. J.; Tan, A.; Prestidge, C. A. Assembling nanoparticle coatings to improve the drug delivery performance of lipid based colloids. Nanoscale 2012, 4 (4), 1220−1230. (2) Volodkin, D.; Arntz, Y.; Schaaf, P.; Moehwald, H.; Voegel, J.-C.; Ball, V. Composite multilayered biocompatible polyelectrolyte films with intact liposomes: stability and temperature triggered dye release. Soft Matter 2008, 4 (1), 122−130. (3) Ding, J.; Chen, J.; Li, D.; Xiao, C.; Zhang, J.; He, C.; Zhuang, X.; Chen, X. Biocompatible reduction-responsive polypeptide micelles as nanocarriers for enhanced chemotherapy efficacy in vitro. J. Mater. Chem. B 2013, 1 (1), 69−81. (4) Meng, H.; Liong, M.; Xia, T.; Li, Z.; Ji, Z.; Zink, J. I.; Nel, A. E. Engineered Design of Mesoporous Silica Nanoparticles to Deliver Doxorubicin and P-Glycoprotein siRNA to Overcome Drug Resistance in a Cancer Cell Line. ACS Nano 2010, 4 (8), 4539−4550. (5) Tang, Y.; Lei, T.; Manchanda, R.; Nagesetti, A.; FernandezFernandez, A.; Srinivasan, S.; McGoron, A. Simultaneous Delivery of Chemotherapeutic and Thermal-Optical Agents to Cancer Cells by a Polymeric (PLGA) Nanocarrier: An In Vitro Study. Pharm. Res. 2010, 27 (10), 2242−2253. (6) Heidel, J.; Davis, M. Clinical Developments in Nanotechnology for Cancer Therapy. Pharm. Res. 2011, 28 (2), 187−199. (7) Landsiedel, R.; Kapp, M. D.; Schulz, M.; Wiench, K.; Oesch, F. Genotoxicity investigations on nanomaterials: Methods, preparation and characterization of test material, potential artifacts and limitationsMany questions, some answers. Mutat.Res.-Rev. Mutat. Res. 2009, 681 (2−3), 241−258. (8) Gwinn, M. R. Nanomaterials: Potential Ecological Uses and Effects. In Encyclopedia of Environmental Health; Jerome, O. N., Ed.; Elsevier: Burlington, 2011; pp 1−11.

4. CONCLUSIONS Polymer nanoparticles coated micelle assemblies of lauryl ester of tyrosine (LET) act as a potential nanocarrier for stearyl alcohol, a model solid lipid, as against the lauryl ester of phenylalanine (LEP). The polymerization of styrene and styrene-co-butyl acrylate in micelle templates of LET and LEP in aqueous medium produced polymer nanopartices of size of 24 to 44 nm as determined by the surfactant and the oil soluble monomers. Higher surface tension numbers of the latex of PS and PS-co-PBA from LET in comparison to LET in aqueous medium is attributed to the coverage of LET with polymer. In stark contrast to this, small variations were observed with respect to LEP, and this suggests an absence of the polymer on the surface of LEP. This significant finding is explained on the basis of the difference in the solubilization site of monomers that varies between shell and core of LET and LEP micelles. Polymer coated LET micelles encapsulated about 30% stearyl alcohol as against almost negligible encapsulation in latex from LEP or the simple micelles of LET and LEP. The encapsulated SA showed a melting endoderm at 47 °C as against 55 °C for pure SA, suggesting that the phase of SA has been modified I


Langmuir

Article

Chemical Architecture in Aqueous Aggregation Properties. J. Phys. Chem. B 2009, 113 (42), 13959−13970. (31) Vijay, R.; Angayarkanny, S.; Baskar, G.; Mandal, A. B. High performance controlled reactors from micellar assemblies of aromatic amino acid amphiphiles for nanoparticle synthesis. J. Colloid Interface Sci. 2012, 381 (1), 100−106. (32) Howard, M. D.; Lu, X.; Rinehart, J. J.; Jay, M.; Dziubla, T. D. Physicochemical Characterization of Nanotemplate Engineered Solid Lipid Nanoparticles. Langmuir 2010, 27 (5), 1964−1971. (33) Mailänder, V.; Landfester, K. Interaction of Nanoparticles with Cells. Biomacromolecules 2009, 10 (9), 2379−2400. (34) Vijay, R.; Angayarkanny, S.; Baskar, G. Amphiphilic dodecyl ester derivatives from aromatic amino acids: Significance of chemical architecture in interfacial adsorption characteristics. Colloids Surf. A 2008, 317 (1−3), 643−649. (35) Suzuki, M.; Sano, M.; Kimura, M.; Hanabusa, K.; Shirai, H. Mediated effects of l-tyrosine esters on quenching of ruthenium(II) complex-containing polymers with C12V2+. Eur. Polym. J. 1999, 35 (6), 1079−1085. (36) Cavaillé, J. Y.; Vassoille, R.; Thollet, G.; Rios, L.; Pichot, C. Structural morphology of poly(styrene)-poly(butyl acrylate) polymerpolymer composites studied by dynamic mechanical measurements. Colloid Polym. Sci. 1991, 269 (3), 248−258. (37) O’Donnell, J.; Kaler, E. W. Microstructure, Kinetics, and Transport in Oil-in-Water Microemulsion Polymerizations. Macromol. Rapid Commun. 2007, 28 (14), 1445−1454. (38) Lin, S.-Y.; Chern, C.-S.; Hsu, T.-J.; Hsu, C.-T.; Capek, I. Emulsion polymerization of styrene: double emulsion effect. Polymer 2001, 42 (4), 1481−1491. (39) Guo, J. S.; El-Aasser, M. S.; Vanderhoff, J. W. Microemulsion polymerization of styrene. J. Polym. Sci. Part A 1989, 27 (2), 691−710. (40) Capek, I.; Potisk, P. Microemulsion and emulsion polymerization of butyl acrylateI. Effect of the initiator type and temperature. Eur. Polym. J. 1995, 31 (12), 1269−1277. (41) Capek, I. Microemulsion polymerization of styrene in the presence of anionic emulsifier. Adv. Colloid Interface Sci. 1999, 82 (1− 3), 253−273. (42) Hentze, H.-P.; Kaler, E. W. Polymerization of and within selforganized media. Curr. Opin. Colloid Interface Sci. 2003, 8 (2), 164− 178. (43) Chow, P. Y.; Gan, L. M., Microemulsion polymerizations and reactions. In Polymer Particles; Okubo, M., Ed.; Springer-Verlag: Berlin, 2005; Vol. 175, pp 257−298. (44) Ming, W.; Zhao, J.; Lu, X.; Wang, C.; Fu, S. Novel Characteristics of Polystyrene Microspheres Prepared by Microemulsion Polymerization. Macromolecules 1996, 29 (24), 7678−7682. (45) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley and Sons: New York, 1978. (46) Cherry, B. W. Polymer Surfaces; Cambridge University Press: New York, 1981. (47) Jin, X.; Yang, S.; Li, Z.; Liu, K. S.; Jiang, L. Bio-inspired special wetting surfaces via self-assembly. Sci. China-Chem. 2012, 55 (11), 2327−2333.

(9) Peng, Y. X.; Zheng, Z. H.; Ding, X. B.; Zhang, W. C.; Ye, Z. H. Nanometer polymer latex dispersion and its application in water-based coating. Prog. Org. Coat. 2003, 48 (2−4), 161−163. (10) Barner-Kowollik, C.; Heuts, J. P. A.; Davis, T. P. Free-radical copolymerization of styrene and itaconic acid studied by H-1 NMR kinetic experiments. J. Polym. Sci. Part A 2001, 39 (5), 656−664. (11) Kim, J.-K.; Howard, M. D.; Dziubla, T. D.; Rinehart, J. J.; Jay, M.; Lu, X. Uniformity of Drug Payload and Its Effect on Stability of Solid Lipid Nanoparticles Containing an Ester Prodrug. ACS Nano 2011, 5 (1), 209−216. (12) Nicolas, J.; Mura, S.; Brambilla, D.; Mackiewicz, N.; Couvreur, P. Design, functionalization strategies and biomedical applications of targeted biodegradable/biocompatible polymer-based nanocarriers for drug delivery. Chem. Soc. Rev. 2013, 42 (3), 1147−1235. (13) Cardenas, M.; Schillen, K.; Nylander, T.; Jansson, J.; Lindman, B. DNA Compaction by cationic surfactant in solution and at polystyrene particle solution interfaces: a dynamic light scattering study. Phys. Chem. Chem. Phys. 2004, 6 (7), 1603−1607. (14) Antonietti, M.; Lohmann, S.; Van Niel, C. Polymerization in microemulsion. 2. Surface control and functionalization of microparticles. Macromolecules 1992, 25 (3), 1139−1143. (15) Antonietti, M.; Bremser, W.; Mueschenborn, D.; Rosenauer, C.; Schupp, B.; Schmidt, M. Synthesis and size control of polystyrene latices via polymerization in microemulsion. Macromolecules 1991, 24 (25), 6636−6643. (16) Atik, S. S.; Thomas, J. K. Polymerized microemulsions. J. Am. Chem. Soc. 1981, 103 (14), 4279−4280. (17) Fu, X.-a.; Qutubuddin, S. Polymerization of Styrene with a Polymerizable Cationic Surfactant in Three-Component Microemulsions. Langmuir 2002, 18 (13), 5058−5063. (18) Perez-Luna, V. H.; Puig, J. E.; Castano, V. M.; Rodriguez, B. E.; Murthy, A. K.; Kaler, E. W. Styrene polymerization in threecomponent cationic microemulsions. Langmuir 1990, 6 (6), 1040− 1044. (19) Ferrick, M. R.; Murtagh, J.; Thomas, J. K. Synthesis and characterization of polystyrene latex particles. Macromolecules 1989, 22 (4), 1515−1517. (20) Oh, S.-G.; Holmberg, K.; Ninham, B. W. Effect of Hydrocarbon Chain Length on Yield of Lipase Catalyzed Triglyceride Synthesis in Microemulsion. J. Colloid Interface Sci. 1996, 181 (1), 341−343. (21) Antonietti, M.; Basten, R.; Groehn, F. Polymerization in Microemulsions of Natural Surfactants and Protein Functionalization of the Particles. Langmuir 1994, 10 (8), 2498−2500. (22) Ryan, L. D.; Kaler, E. W. Effect of Alkyl Sulfates on the Phase Behavior and Microstructure of Alkyl Polyglucoside Microemulsions. J. Phys. Chem. B 1998, 102 (39), 7549−7556. (23) Ryan, L. D.; Kaler, E. W. Microstructure Properties of Alkyl Polyglucoside Microemulsions. Langmuir 1998, 15 (1), 92−101. (24) Ryan, L. D.; Schubert, K.-V.; Kaler, E. W. Phase Behavior of Microemulsions Made with n-Alkyl Monoglucosides and n-Alkyl Polyglycol Ethers. Langmuir 1997, 13 (6), 1510−1518. (25) Häger, M.; Currie, F.; Holmberg, K. A nucleophilic substitution reaction in microemulsions based on either an alcohol ethoxylate or a sugar surfactant. Colloids Surf., A 2004, 250 (1−3), 163−170. (26) von Corswant, C.; Engström, S.; Söderman, O. Microemulsions Based on Soybean Phosphatidylcholine and Triglycerides. Phase Behavior and Microstructure. Langmuir 1997, 13 (19), 5061−5070. (27) Glatter, O.; Orthaber, D.; Stradner, A.; Scherf, G.; Fanun, M.; Garti, N.; Clément, V.; Leser, M. E. Sugar-Ester Nonionic Microemulsion: Structural Characterization. J. Colloid Interface Sci. 2001, 241 (1), 215−225. (28) Folmer, B. M.; Svensson, M.; Holmberg, K.; Brown, W. The Physicochemical Behavior of Phytosterol Ethoxylates. J. Colloid Interface Sci. 1999, 213 (1), 112−120. (29) Pinazo, A.; Pons, R.; Pérez, L.; Infante, M. R. Amino Acids as Raw Material for Biocompatible Surfactants. Ind. Eng. Chem. Res. 2011, 50 (9), 4805−4817. (30) Vijay, R.; Singh, J.; Baskar, G.; Ranganathan, R. Amphiphilic Lauryl Ester Derivatives from Aromatic Amino Acids: Significance of J


Nanocarriers of Solid Lipid from Micelles of Amino Acids Surfactants

Recommend Documents