Thermodynamic solution properties of pefloxacin

Thermochimica Acta 573 (2013) 18–24

Contents lists available at ScienceDirect

Thermochimica Acta journal homepage: www.elsevier.com/locate/tca

Thermodynamic solution properties of pefloxacin mesylate and its interactions with organized assemblies of anionic surfactant, sodium dodecyl sulphate Muhammad Usman a,c , Muhammad Abid Rashid b , Asim Mansha a , Mohammad Siddiq c,∗ a

Department of Chemistry, Government College University, Faisalabad 38000, Pakistan Department of Chemistry and Biochemistry, University of Agriculture, Faisalabad, Pakistan c Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan b

a r t i c l e

i n f o

Article history: Received 23 April 2013 Received in revised form 20 August 2013 Accepted 20 August 2013 Available online xxx Keywords: Drug Surfactant Surface tension Conductivity Spectroscopy Micelle

a b s t r a c t This manuscript reports the physicochemical behavior of antibiotic amphiphilic drug pefloxacin mesylate (PFM) and its interaction with anionic surfactant, sodium dodecyl sulfate (SDS). The data of surface tension and electrical conductivity are helpful to detect the CMC as well as to calculate surface parameters, i.e. surface pressure, , surface excess concentration, , area per molecule of drug and standard Gibbs free energy of adsorption, Gads and thermodynamic parameters like standard free energy of micellization, Gm , standard enthalpy of micellization, Hm and standard entropy of micellization, Sm . The interaction of this drug with anionic surfactant, sodium dodecyl sulfate (SDS) was studied by electrical conductivity and UV/visible spectroscopy. This enabled us to compute the values of partition coefficient (Kx ), free energy of partition, Gp , binding constant, Kb , free energy of binding, Gb , number of drug molecules per micelle, n, and thermodynamic parameters of drug–surfactant interaction. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Amphiphilic compounds have unique property of micelle formation. The micelles are able to enhance the solubility of insoluble or less soluble compounds due to solubilization. The position of solubilized drug molecules in micelle depends on their polarity: non-polar molecules being solubilized in micellar core and those with intermediate polarity will be distributed in certain intermediate positions [1]. Micelles have structural similarity with lipids because both have hydrophobic interior and hydrophilic surface that is why micelles are able to mimic biomembranes. Micelles are, therefore, very good alternative to study the interactions of drugs with membrane and, thus, can be used as model system to conduct in vitro study of drug–membrane interactions [2–4]. Pefloxacin is a synthetic chemotherapeutic agent belonging to the 3rd generation of quinolones introduced in 1979 and approved in 1985. It is commonly referred to as a member of the fluoroquinolone class of antibacterials and possesses the excellent activity against gram-negative aerobic bacteria such as Escherichia coli and Neisseria gonorrhoeae as well as gram-positive bacteria including Streptococcus pneumoniae and Staphylococcus aureus. It possesses effective activity against Shigella, Salmonella,

Campylobacter, Gonococcal organisms and multi drug resistant Pseudomonas and enterobacter. It is an analog of norfloxacin used to cure uncomplicated gonococcal urethritis in males, bacterial infections in the gastrointestinal system, genito-urinary tract infections, lower respiratory tract, bone and joint infections, renal and abdominal infections as well as infections of skin and soft tissues, gonorrhea. Chemically, PFM is 1-ethyl-6-fluoro-1,4-dihydro-7-(4methyl-1-piperazinyl)-4-oxo-, monomethanesulfonate, dihydrate, pefloxacine monomethanesulfonate dihydrate [5–8]. We, in our previous work, have already studied the solution and thermodynamic behavior of anti-allergic drug certizine HCl [9], anti-depression drug citalopram 2HBr [10] and antibiotic drug dexamethasone sodium phosphate [11]. In addition, we have studied the interaction of chloroquine diphosphate with sodium dodecyl sulphate and human serum albumin [12]. In this paper we have discussed how antibiotic drug pefloxacin mesylate does behave in aqueous solution and how does it interact with anionic surfactant sodium dodecyl sulphate. Fig. 1 represents the structure of PFM. 2. Materials and methods 2.1. Materials and preparation of solution

∗ Corresponding author. Tel.: +92 334 5365467. E-mail addresses: m [email protected], [email protected] (M. Siddiq). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.08.014

The antibiotic drug pefloxacin mesylate (PFM) and anionic surfactant sodium dodecyl sulphate (SDS) were obtained from

M. Usman et al. / Thermochimica Acta 573 (2013) 18–24

19

In Eq. (2), m and ms are concentrations of drug and added electrolyte, if any. The value of x is 2 in water and approaches to 1 in the presence of excess amount of inert electrolyte. The minimum area per molecule at interface (A) can be calculated as = 1/NA , where NA is Avogadro’s number. The value of “A” gives information about the degree of packing and orientation of adsorbed surfactant. Free energy of micellization (G◦ m ) can be calculated using the following equation: G◦ m = (2 − ˇ)RT ln XCMC

Fig. 1. Structure of pefloxacin mesylate.

Sigma–Aldrich and used as received. Water was distilled using water still apparatus (Model IM-100) and then deionized by passing through Elga B114 deionizer to minimize its conductivity. For surface tension and conductometric studies, aqueous solution of PFM was prepared in deionized water ranging from pre micellar to post micellar concentration. For spectroscopic study the stock solution of PFM was prepared in distilled deionized water and diluted further so that maximum absorbance may not exceed 1 and Lambert–Beer law may be obeyed. 2.2. Apparatus and methods Surface tension of aqueous solutions of drug was determined using torsion balance (White Elect. Inst. Co. Ltd., UK) equipped with Platinum ring (4.0 cm circumference) along with water circulator (Irmeco I-1800, Germany) to control temperature with the accuracy of ±0.5 K. The instrument was kept free from vibrations. The torsion balance was checked for zero and calibrated with water. It is well known that critical concentration calculated by surface tension is particularly sensitive to impurities. No minima were evidenced in the plot of surface tension versus concentration which was proof of the absence of surface active impurities [13]. Specific conductivities were measured with Jenway 4310 (Bibby Scientific Ltd., UK). This instrument has auto ranging from 0.01 ␮S to 199.9 mS, conductivity control with accuracy of ±0.5% ± 2 digits and temperature control accuracy of ±0.5 K. The electrode used has cell constant of 0.98 cm−1 and was coated with platinum black in order to avoid the polarization effect. The conductivities were measured at temperature range of 293–323 K with the increment of 10 K. The temperature was controlled using water circulator (IRMECO I-2400 GmbH, Germany). The electrode was calibrated using KCl over the appropriate concentration range. All absorption spectra of the sample in UV–visible range were measured on a computer interfaced Perkin Elmer (USA) Double beam lambda 20 UV–visible spectrophotometer equipped with a water jacketed cell compartment to control the temperature. All measurements were taken at 298 K with the accuracy of ±0.5 K. The cells used were square cuvettes of quartz; 1.0 cm thick and slit width used is 1.0 nm.

(3)

In Eq. (3), ˇ is the degree of dissociation, R is the gas constant having value 8.314 J mol−1 K−1 , T is the absolute temperature, XCMC is CMC in term of mole fraction. ˇ can be calculated from the ratio of the slopes of post-micellar and pre-micellar regions of conductivity–concentration plot using following equation: ˇ=

S2 S1

(4)

In Eq. (4), S1 and S2 represent the slopes of the straight lines in the pre micellar and post micellar regions, respectively. The standard free energy of adsorption, G◦ ads for pure surfactant solutions as well as mixed system can be calculated using following equation: G◦ ads = G◦ m −

CMC m

(5)

In Eq. (5), CMC is the surface pressure at critical concentration and is given as CMC = 0 − CMC

(6)

Here 0 is the surface tension of pure solvent and CMC is that of drug solution at CMC. Surface pressure, CMC , is the difference between surface tension of pure solvent and surface tension of drug solution at CMC. 2.4. Calculation of thermodynamic parameters The value of free energy of micellization is calculated from Eq. (3) while that of enthalpy of micellization can be obtained from equation given below [9,10,13].

H ◦ m = −2.3(2 − ˇ)RT 2

∂(log XCMC ) ∂T

(7) P

Here factor ∂(logXCMC )/∂T was obtained from the slope of straight line plotted between log (XCMC ) and T. The entropy of micellization can be calculated by the following equation: S ◦ m =

H ◦ m − G◦ m T

(8)

2.5. Calculation of partition and binding parameters 2.3. Calculation of surface parameters For surface-active solutes, the surface excess concentration, can be considered to be actual surface concentration without significant error. It is actually amount of drug adsorbed at air–water interface. For ionic surfactant, can be determined by the application of Gibbs adsorption equation [9,10,13]. 1 =− 2.303RTx

d

d log m

(1) T

In Eq. (1), R is the gas constant; T is the temperature in Kelvin. The variable x is introduced to allow for the simultaneous adsorption of cations and anions. x =1+

m m + ms

(2)

Partitioning of drug molecules between aqueous and micellar media is governed by partition law. Partition coefficient is determined by differential absorbance method reported by Kawamura et al. [14]. 1 1 1 = + A Kc A∞ (Cd + Csmo ) A∞

(9)

In Eq. (9), Cd is the concentration of drug in mol dm−3 , Csmo represents Cs -CMCo , in the same units. Here, CMCo is CMC of SDS in water and Cs is total surfactant concentration in mol dm−3 . A is differential absorbance and A∞ represents its value at infinity. Kc is partition constant having value in dm3 /mol. The dimensionless quantity partition coefficient, Kx is obtained as Kx = Kc nw , where nw is the number of moles of water per dm3 .

20


The approximate number of drug molecules incorporated per micelle is given as n=

Cm M

70

65

(10)

In Eq. (10), M is micelle concentration and Cm is concentration of solubilized drug. They are calculated from Eqs. (11) and (12), respectively.

Cm =

-1

mN.m )

M=

60

Cs − CMC N

(11)

Ao − A ε − εm

50

(12)

CMC=0.092

In above equations, Cs is total surfactant concentration, N is mean aggregation number of SDS having value 65, Ao is absorbance of drug in the absence of surfactant, A is absorbance at CMC, ε and εm are calculated from Beer–Lambert law [14]. The value of standard free energy change of transfer of additive from aqueous to micellar phase is calculated as Gp = −RT ln Kx

(13)

R is general gas constant and T is the absolute temperature. Following equation provides quantitative approach to calculate binding constant. Cs Cd Cs 1 = + A εl Kb εl

(14)

Here Cd is concentration of drug, Cs is that of surfactant, A is differential absorbance, ε is difference of absorption coefficient; l is path length while Kb stands for binding constant [14–16]. 3. Results and discussion 3.1. Surface tension A plot showing variation in surface tension of PFM verses molal concentration (m) in water at 303 K (Fig. 2) shows that surface tension remains constant after CMC indicating that full Gibbs monolayer is formed at air/solution interface. The value of CMC obtained from surface tension (0.093 mol kg−1 ) and conductivity (0.11 mol kg−1 ) are in close agreement with each other. The slight difference between two CMC values is due to the fact that CMC does not represent a sharp value rather a range of concentration and its value depends on technique being used to find it. The value of standard Gibbs free energy of adsorption (Gads ) at 303 K is −28.98 kJ mol−1 while that of free energy of micellization (Gm ) is −19.72 kJ mol−1 . The value of Gads is more negative than that of Gm because adsorption is more spontaneous and occurs prior to micellization [1–12,14–23]. Table 1 shows the adsorption parameters at air/solution interface. 3.2. Electrical conductivity Fig. 3(a) shows the plots of electrical conductivity as a function of molality for aqueous solution of PFM at different temperatures. The critical concentration was determined by the intersection of two straight lines of conductivity–concentration plot in premicellar and post micellar region as shown in Fig. 3(b). Various parameters calculated from electrical conductivity are given in Table 2.

55

45

40 0.00

0.05

0.10

0.15

0.20

0.25

0.30

-1

m/(mol.kg ) Fig. 2. The plot of surface tension as a function of molality for aqueous solution of PFM at 303 K.

The process of micellization and the value of critical micelle concentration depend on temperature in a complicated way. The increase in temperature causes dehydration of hydrophilic as well as hydrophobic parts. Dehydration of hydrophobic parts favors micellization because it causes increase in entropy while dehydration of hydrophilic groups disfavors micellization because it causes repulsion between charged hydrophilic groups. The relative magnitude of both factors will decide whether CMC will increase or decrease with temperature. In case of PFM, CMC decreases with temperature because degree of hydrophobic dehydration is predominant over hydrophilic dehydration. Addition of amphiphiles to water increases free energy of system because its hydrophobic part has no affinity for water. The natural desire of system to minimize free energy can be fulfilled by micellization and surface adsorption. It is the reason that adsorption and micellization are spontaneous process having negative value of G. At high temperature, the value of Gm becomes more negative because micellization becomes more spontaneous. Both S◦ m and H◦ m have positive values because the process of micellization is entropy driven [25]. The H◦ m is the sum of change in enthalpies arising from hydrophobic interactions, electrostatic interactions, hydration of polar head groups and counter ion binding to micelles. A positive value of H◦ m occurs when hydration of water molecules around hydrophilic heads groups become less important than destruction of water structure around hydrophobic groups of monomers [11,19,25]. Structure of PFM is not flexible so its micellization is only entropy driven process. It, thus, undergoes endothermic micellization. The positive values of S◦ m are due to transfer of hydrophobic chains of drugs from aqueous environment to micelle core [25–31]. According to flickering cluster model of water structure, water consists of structured region in which water molecules are highly ordered as well as region of free, unbound molecules. Thus there is continuous reorientation of water molecules due to nonstop destruction and reconstruction of ordered region. When we add amphiphiles to water, the hydrogen bonding between water

Table 1 Critical micelle concentration, CMC, free energy of adsorption, Gads , free energy of micellization, Gm , surface excess concentration, , and area per molecule, A, for aqueous solution of PFM at T = 303 K. CMC (mol kg−1 )

× 106 (mol m−2 )

A (nm2 )

Gads (kJ mol−1 )

Gm (kJ mol−1 )

0.092 ± 0.001

3.3 ± 0.18

0.52 ± 0.03

−28.98 ± 0.72

−19.72 ± 0.034


(a)

21

(b)

7500

7500

6000

6000

k/( S.cm )

-1

-1

k /( S.cm )

CMC=0.12

4500

4500

3000

3000

0.04

0.08

0.12

0.16

0.04

0.20

0.08

0.12

0.16

0.20

-1

-1

m/(mol.kg )

m/(mol.kg )

Fig. 3. (a) The plots of electrical conductivity (␮S) versus molality, m, (mol kg−1 ), for aqueous solution of PFM at 293 K (o), 303 K ( ), 313 K (×) and 323 K (䊉). (b) Typical plot of electrical conductivity versus molality for aqueous solution of PFM at 303 K.

Table 2 Critical micelle concentrations, CMC, enthalpy of micellization, Hm , free energy of micellization, Gm , entropy of micellization, Sm , degree of counterion binding, ˛, and degree of ionization, ˇ, for aqueous solution of PFM at different temperatures. T (K)

CMC (mol kg−1 )

293 303 313 323

0.120 0.114 0.102 0.089

± ± ± ±

Hm (kJ mol−1 )

0.001 0.001 0.002 0.002

9.29 9.42 9.98 10.2

± ± ± ±

0.29 0.3 0.5 0.31

Gm (kJ mol−1 )

Sm (J K−1 mol−1 )

−19.12 ± 0.21 −19.3 ± 0.18 −19.65 ± 0.21 −20.07 ± 0.21

96.91 94.59 94.38 93.57

molecules is disrupted as no hydrogen bonding is possible between hydrophobic ends of amphiphile and water molecules. As there is no attraction between hydrophobic ends of amphiphile and water molecules, water molecules around hydrophobic parts are more ordered than in pure water due to stronger hydrogen bonding. This phenomenon is called hydrophobic hydration which causes decrease in entropy of system. On micelle formation hydrophobic parts of amphiphiles are expelled from water and enter into core of micelle thus causing increase in entropy. Hydrophobic hydration causes vibrations of hydrophobic chains to be restricted in solution. The more ordered structure of water molecules around hydrophobic chains and restriction on vibrations of hydrophobic groups lead to decrease in entropy of system. The removal of hydrophobic groups from aqueous environment is entropically favorable leading to disruption of highly organized water structure and removal of mobility constraints on hydrocarbon chain [1–4]. The decrease in S◦ m values with temperature is

± ± ± ±

˛

0.89 0.53 0.34 0.51

0.26 0.22 0.20 0.17

ˇ ± ± ± ±

0.01 0.015 0.038 0.015

0.74 0.78 0.80 0.83

± ± ± ±

0.01 0.015 0.038 0.015

due to decrease in the degree of hydration of hydrophobic parts at high temperature.

3.3. UV/visible spectroscopy 3.3.1. Simple UV/visible absorption spectra In order to study the interaction between PFM and SDS, spectroscopic and conductometric measurements were carried out. The concentration of drug was kept constant at 1.65 × 10−5 M while that of surfactant was varied from 0.006 to 0.015 M. The absorption maxima of PFM were shifted toward higher wavelength in the presence of SDS. This change is due to interaction between PFM and SDS. The spectrum of pure PFM in deionized water shows characteristic peak at 276 nm. Fig. 4(a) portrays the absorbance spectra of drug/surfactant/water ternary system. There is a significant shift in the characteristic peak of drug on addition of surfactant.

(a)

(b)

1.0

0.10

0.8

A

A

0.08

0.6

0.06 0.4

0.04 0.2 250

260

270

280

(nm)

290

300

282

285

288

291

294

(nm)

Fig. 4. (a) Simple UV/visible absorption spectra of PFM in the presence of different SDS concentration (mol dm−3 ): ( ) 0, (×) 0.006, (N) 0.0088, and (o) 0.0094. (b) Differential absorbance of aqueous solution of PFM in the presence of different SDS concentration (mol dm−3 ): (o) 0.0088, (N) 0.0094, ( ) 0.01, and (×) 0.0107.

22

M. Usman et al. / Thermochimica Acta 573 (2013) 18–24 1.06

0.112

(a)

(b)

1.04

0.110

CMC=0.00882 0.108

1.00

A

A

1.02

0.106

0.98 0.104

0.96 0.102

0.94 0.006

0.008

0.010

0.012

0.014

0.016

0.008

0.010

-3

0.012

0.014

0.016

-3

Cs/(mol.dm )

Cs/(mol.dm )

Fig. 5. (a) Plot of absorbance as a function of SDS concentration for PFM. (b) Plot of change in differential absorbance of PFM with SDS concentration.

Absorbance of PFM-SDS system increases rapidly till CMC while in post micellar region value of absorbance increases very slowly showing a saturation plateau for the reason that maximum incorporation of drug molecules has taken place in micelles as evident in Fig. 5(a). The CMC of pure SDS is 0.0082 mol dm−3 ; PFM increases its CMC to 0.0088 mol dm−3 . This increase in CMC is due to structure breaking effect of PFM which causes destruction of water structure and causes less increase in entropy thus making micellization entropically less favorable. In addition, there may be chances of hydrogen bonding between hydrophilic parts of drug and water, so orientation of drug molecules is more likely in outer portion of micelle close to micelle water interface. Such fashion of adsorption increases work of micellization by producing less increase in entropy thus making micellization less convenient and increasing the CMC [32–34]. PFM is mainly hydrophobic in nature having cationic hydrophilic group. It is, therefore, expected to have strong interaction with anionic surfactant. It appears that structural environment of drug molecule changes with SDS concentration till CMC of SDS, however, at post micellar concentration drug molecules do not experience any change in environment [34]. Interaction between drug and surfactant occurs in the following manner. Initially drug cations and surfactant anions join each other to form drug–surfactant complex (DS).

calculation of various parameters indicating partitioning of PFM in SDS micellar media. Polar molecules of PFM are solubilized by the virtue of electrostatic interaction between positively charged polar part of drug and negatively charged surfactant. The hydrophilic part of PFM remains attached to surface, whereas their hydrophobic moieties extend partially into hydrophobic core of micelle [14,35]. Fig. 6 helps to calculate partition coefficient using Kawamura equation. Table 4 shows values of parameters calculated from Kawamura equation. The negative value of Gp indicates spontaneous nature of partitioning while the value of Kx is measure of ease with which partition occurs. High value of partition coefficient displays the higher concentration of drug in micelle than in surrounding water. 3.4. The conductometric study of partition of PFM in SDS micelle A plot showing variation of conductance of aqueous solution of SDS in the presence of PFM in temperature range of 293–323 K is shown in Fig. 7(a) while typical plot at 293 K is represented in Fig. 7(b) showing that CMC values are determined from the intersection of premicellar and post micellar region of conductivity–concentration plot. The thermodynamic parameters calculated for SDS-PFM system are given in Table 5.

D+ + S − → DS

9.8

With the passage of time these complexes undergo self aggregation (as given below) and result in increase in the absorbance in premicellar region. 9.6

3.3.2. Differential UV/visible absorption spectra Fig. 4(b) displays differential absorption spectra of PFM in the presence of SDS and Fig. 5(b) shows that the differential absorption increases with increase in surfactant concentration implying stronger interaction between drug molecules and SDS micelles. It indicates that more amount of drug is preferentially being taken into micelle [14,32]. The data of differential absorbance help to calculate value of partition coefficient, Kx , by using Kawamura equation. Table 3 shows

A

When CMC is approached, drug molecules are incorporated in micelles and there is little increase in absorbance after CMC [35]. The increase in CMC value of SDS in the presence of PFM is also confirmed by conductivity measurement.

-1

n(DS) → (DS)n

9.4

9.2 0

400

800 mo

-1

1200 3

1600

-1

(Cs +Ca) /(dm .mol ) Fig. 6. Relationship between 1/A and (1/(Cs + Cs mo ) for the calculation of Kx for PFM/SDS system.


23

Table 3 Concentration of surfactant, Cs , differential absorbance, A, for calculation of parameters indicating partitioning of PFM in SDS micellar media. Cs × 103 (mol dm−3 )

A × 102

Cs mo × 104 (mol dm−3 )

(Cs mo + Ca ) × 104 (mol dm−3 )

08.82 09.37 10.00 10.71 11.54 12.50 13.64 15.00

10.22 10.46 10.56 10.64 10.62 10.7 10.77 10.80

06.20 11.70 18.00 25.10 33.40 43.00 54.40 68.00

06.37 11.87 18.20 25.30 33.60 43.20 54.60 68.20

Table 4 Partition coefficient, Kx , free energy of partition, Gp , binding constant, Kb , free energy of binding, Gb , and number of drug molecules per micelle, n, for PFM/SDS system. Kx × 10−5

Gp (kJ mol−1 )

Kb

Gb (kJ mol−1 )

n

17.22 ± 0.01

−35.58 ± 0.02

50 ± 0.001

−9.70 ± 0.002

1.30 ± 0.01

(a)

800

(b)

700

750

CMC = 0.00879

-1

k/( S.cm )

-1

k/( S.cm )

650

700

650

600

600

550

550

500 0.007

0.008

0.009

0.010

0.011

500 0.007

0.012

0.008

0.009

0.010

0.011

0.012

-3

-3

Cs/(mol.dm )

Cs/(mol.dm )

Fig. 7. (a) Plots of variation in conductivity of SDS as a function of concentration in the presence of PFM at 293 K (o), 303 K ( ), 313 K (×) and 323 K (䊉). (b) Typical plot of variation in conductivity of SDS as a function of concentration in the presence of PFM at 303 K.

Table 5 Critical micelle concentrations, CMC, enthalpy of micellization, Hm , free energy of micellization, Gm , entropy of micellization, Sm , degree of counterion binding, ˛, and degree of ionization, ˇ, for aqueous solution of PFM/SDS system at different temperatures. T (K)

CMC (mol kg−1 )

293 303 313 323

8.60 8.79 8.95 9.00

± ± ± ±

0.001 0.001 0.001 0.001

Hm × 103 (kJ mol−1 ) −1.62 −1.75 −1.93 −2.11

± ± ± ±

0.01 0.01 0.01 0.01

The stability of micellization can be judged by having knowledge about thermodynamic parameters being calculated from CMC, i.e. Gm , Hm , and Sm . The physical behavior of micelle has been visualized as the construction of model membrane to imitate biological system. This experimental model is being used to study the effects of solubilization on micellization of surfactant and thermodynamics of system. The major driving force for micellization is hydrophobic interaction and the negative value of Gm . The value of Gm becomes more negative with temperature, because temperature tends to drive equilibrium toward hydrophobic bonding and, hence, micellization is favored. The negative value of Hm and positive value of Sm are due to flexible structure of SDS which causes it to undergo micellization easily. Therefore micellization of SDS in the presence of PFM is enthalpy as well as entropy driven and exothermic process. The positive value of Sm can be attributed to the fact that solubilization of drug causes distraction of more structured water molecules around hydrophobic parts of drug molecules as the latter move from bulk phase to non-aqueous micelle interior. The large positive value of Sm is clear indicative of the fact that

Gm (kJ mol−1 )

Sm (J K−1 mol−1 )

−29.92 −31.30 −33.41 −33.75

96.63 97.57 100.57 98.28

± ± ± ±

0.01 0.01 0.01 0.01

± ± ± ±

0.01 0.01 0.01 0.01

˛

ˇ

0.40 0.42 0.47 0.44

0.60 0.58 0.53 0.56

the system becomes more haphazard after micellization and governing force of micellization is hydrophobic interaction between surfactant monomers resulting in breakdown of structured water surrounding hydrophobic tails. The increase in CMC of SDS-PFM system with temperature is due to greater degree of hydrophilic dehydration than that of hydrophobic dehydration which causes repulsions between polar groups of amphiphile. This phenomenon opposes micellization thus increasing the CMC [9–25,35–40]. 4. Conclusion The data obtained from surface tension and conductivity enabled us to conclude that adsorption of PFM molecules at the air/water interface and its micellization are spontaneous processes, however, adsorption is more spontaneous than micellization. The spontaneous behavior of micellization increases with temperature. Spectroscopic study indicates the strong interaction between PFM molecules and SDS micelles. The drug molecules are spontaneously partitioned from aqueous media to micellar media. The partition

24


coefficient not only provides an idea about mechanism of solubilization but also helps to understand how drug is partitioned through biological membranes within living body. Acknowledgments Financial support from Higher Education Commission, Pakistan (HEC) under Indigenous PhD Scholarship Program is highly appreciated. References [1] C.O.R. Yagui, A.P. Junior, L.C. Tavares, Micellar solubilization of drugs, J. Pharm. Pharm. Sci. 8 (2005) 147–163. [2] D. Attwood, A.T. Florence, Surfactant Systems, Chapman and Hall, London, 1985. [3] A.M. Khan, S.S. Shah, Determination of critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS) and the effect of low concentration of pyrene on its CMC using ORIGIN software, J. Chem. Soc. Pak. 30 (2008) 186–191. [4] P. Taboada, M. Ruso, M. Garcia, V. Mosquera, Surface properties of some amphiphilic antidepressant drugs, Colloids Surf. A 179 (2001) 125–128. [5] K. Basavaiah, H.C. Prameela, B.C. Somashekar, Spectrophotometric determination of pefloxacin mesylate in pharmaceuticals, Acta Pharm. 57 (2007) 221–230. [6] J.E. Burkhardt, J.N. Walterspiel, U.B. Schaad, Quinolone arthropathy in animals versus children, Clin. Infect. Dis. 25 (1997) 1196–1204. [7] X. Chevalier, E. Albengres, M.C. Voisin, J.P. Tillement, B.L.B. Piet, A case of destructive polyarthropathy in a 17-years old youth following pefloxacin treatment, Drug Saf. 7 (1992) 310–314. [8] L.G. Yamamoto, Chapter VI.5. Antibiotics. Case Based Pediatrics For Medical Students and Residents, Department of Pediatrics, University of Hawaii, 2004, ISBN 1-4184-4728-5. [9] M. Usman, A. Khan, M. Siddiq, Thermodynamic and solution properties of amphiphilic anti allergic drug Cetirizine HCl, J. Chem. Soc. Pak. 31 (2009) 221–227. [10] M. Usman, A. Khan, M. Siddiq, Thermodynamic properties of amphiphilic antidepressant drug Citalopram HBr, J. Chem. Soc. Pak. 32 (2010) 1–6. [11] A. Shah, A.M. Khan, M. Usman, R. Qureshi, M. Siddiq, S.S. Shah, Thermodynamic characterization of dexamethasone sodium phosphate and its complex with DNA as studied by conductometric and spectroscopic techniques, J. Chilean Chem. Soc. 54 (2009) 134–137. [12] M. Usman, M. Siddiq, Surface and micellar properties of chloroquine diphosphate and its interactions with surfactants and human serum albumin, J. Chem. Thermodyn. 58 (2013) 359–366. [13] M.A. Cheema, P. Taboada, S. Barbosa, E. Castro, M. Siddiq, V. Mosquera, A thermodynamic study of the amphiphilic phenothiazine drug thioridazine hydrochloride in water/ethanol solvent, Chem. Phys. 328 (2006) 243–250. [14] H. Kawamura, M. Manabe, Y. Miyamoto, Y. Fujita, S. Tokunaga, Partition coefficient of homologous (-phenyl alkanols between water and sodium dodecyl sulphate micelles, J. Phys. Chem. 93 (1989) 5536–5540. [15] S.K. Mehta, S. Chaudhary, K.K. Bhasin, R. Kumar, M. Aratono, Conductometric and spectroscopic studies of sodium dodecyl sulphate in aqueous media in the presence of organic chalcogen, Colloids Surf. A. 304 (2007) 88–95. [16] S.K. Mehta, K.K. Bhasin, A. Kumar, S. Dham, Micellar behaviour of dodecyldimethylethyl ammonium bromide and dodecyltrimethyl ammonium bromide in aqueous media in the presence of diclofenac sodium, Colloids Surf. A 278 (2007) 17–25. [17] D.F. Evans, H. Wennerstrom, The Colloidal Domain: Where Physics, Chemistry, Biology and Technology Meet, VCH Publishers, USA, 1994. [18] M.J. Rosen, Surfactants and Interfacial Phenomenon, Wiley-Interscience Publication, New York, 1973.

[19] M.A. Cheema, P. Taboada, S. Barbosa, M. Siddiq, V. Mosquera, Effect of molecular structure on the hydration of structurally related antidepressant drugs, Mol. Phys. 104 (2006) 3203–3212. [20] M.A. Cheema, P. Taboada, S. Barbosa, E. Castro, M. Siddiq, V. Mosquera, Thermodynamic study of warfarin sodium salt: surface tension, conductivity and density measurements, J. Chem. Eng. Data 52 (2007) 2315–2318. [21] M.A. Cheema, P. Taboada, S. Barbosa, E. Castro, M. Siddiq, V. Mosquera, Surface bulk properties of two amphiphilic phenothiazine drugs in different aqueous media, J. Chem. Eng. Data 53 (2008) 368–373. [22] M.A. Cheema, P. Taboada, S. Barbosa, E. Castro, M. Siddiq, V. Mosquera, A comparative study of the thermodynamic properties at the air–water interface and in the bulk of structurally related phenothiazine drugs aqueous solutions, J. Chem. Thermodyn. 40 (2008) 298–308. [23] P. Taboada, D. Attwood, M.J. Ruso, M. Garcia, F. Sarmiento, V. Mosquera, Selfassociation of the penicillin sodium nafcillin in aqueous solution, Langmuir 16 (2000) 3175–3181. [24] L.M. Varela, C. Rega, M.J. Saurez-Filloy, J.M. Ruso, G. Prieto, D. Attwood, F. Sarmiento, V. Mosquera, Self-association of Penicillin V in aqueous solution, Langmuir 15 (1999) 6285–6290. [25] F. Akhtar, M.A. Hoque, M.A. Khan, Interaction of cefadroxyl monohydrate with hexadecyltrimethyl ammonium bromide and sodium dodecyl sulfate, J. Chem. Thermodyn. 40 (2008) 1082–1086. [26] D. Attwood, The mode of association of amphiphilic drugs in aqueous solution, Adv. Colloids Interface Sci. 55 (1995) 271–303. [27] P.C. Hiemenz, R. Rajagopalan, Principles of Colloids and Surface Chemistry, 3rd ed., CRC publisher, 1997. [28] R.J. William, J.N. Phillips, K.J. Mysels, The energetics of micelle formation, Trans. Faraday Soc. 51 (1955) 561–569. [29] C.V. Kumar, E.H. Asuncion, DNA binding studies and site selective fluorescence sensitization of an anthryl probe, J. Am. Chem. Soc. 115 (1993) 8547–8553. [30] T. Furias, L.C. Menorval, J. Zajac, A. Rivera, Solubilization of drugs by cationic surfactants micelles: conductivity and 1 H NMR experiments, Colloids Surf. A. 345 (2009) 51–57. [31] J. Luczak, C. Jungnickel, M. Joskowska, J. Thoming, J. Hupka, Thermodynamics of micellization of imidazolium ionic liquids in aqueous solutions, J. Colloid Interface Sci. 336 (2009) 111–116. [32] A. Mishra, P.K. Behera, R.K. Behera, B.K. Mishra, G.B. Behera, Interaction of Nalkyl styryl pyridinium dyes with TX-100 in aqueous medium: role of alkyl chain during solubilization, J. Photochem. Photobiol. A: Chem. 116 (1998) 79–84. [33] M.J. Shick, F.M. Fowkes, Foam stabilizing additives for synthetic detergents. Interaction of additives and detergents in mixed micelles, J. Phys. Chem. 61 (1957) 1062–1068. [34] M. Sarkar, S. Poddar, Studies of the interaction of surfactant with cationic dye by absorption spectroscopy, J. Colloid Interface Sci. 221 (2000) 181–185. [35] A.M. Khan, S.S. Shah, A UV–visible study of partitioning of Pyrene in an anionic surfactant sodium dodecyl sulphate, J. Dispers. Sci. Technol. 29 (2008) 1401–1407. [36] S.S. Shah, G.M. Laghari, K. Naeem, A spectroscopic study of hemicynine dyes in anionic micellar solutions, Thin Solid Films 346 (1999) 145–149. [37] B. Naseem, PhD Thesis, Department of Chemistry, Quaid-i-Azam University Islamabad, Pakistan 2004. [38] S.S. Shah, G.M. Laghari, K. Naeem, S.W.H. Shah, Partition coefficient of amphiphilic hemicyanine dyes between the aqueous and micellar phase of sodium dodecyl sulphate by differential absorbance spectroscopy, Colloids Surf. A 143 (1998) 111–115. [39] S.S. Shah, G.M. Laghari, K. Naeem, S.W.H. Shah, Differential absorbance measurements of amphiphilic hemicyanine dyes, solubilization study in anionic surfactant, Colloids Surf. A 168 (2000) 77–85. [40] S.P. Moulik, M.E. Haque, P.K. Jana, A.R. Das, Micellar properties of cationic surfactants in pure and mixed states, J. Phys. Chem. 100 (1996) 701–708.