Thermodynamic insights into molecular interactions of sodium lauryl sulfate (SLS) with caffeine and theophylline in aqueous media at different temperatures

Journal Pre-proof Thermodynamic insights into molecular interactions of sodium lauryl sulfate (SLS) with caffeine and theophylline in aqueous media at different temperatures

Shadma Tasneem, Arshid Nabi, Nazim Hasan, Maqsood Ahmad Malik, Khaled Mohamed Khedher PII:

S0167-7322(19)35698-3

DOI:

https://doi.org/10.1016/j.molliq.2020.112776

Reference:

MOLLIQ 112776

To appear in:

Journal of Molecular Liquids

Received date:

14 October 2019

Revised date:

6 February 2020

Accepted date:

24 February 2020

Please cite this article as: S. Tasneem, A. Nabi, N. Hasan, et al., Thermodynamic insights into molecular interactions of sodium lauryl sulfate (SLS) with caffeine and theophylline in aqueous media at different temperatures, Journal of Molecular Liquids(2018), https://doi.org/10.1016/j.molliq.2020.112776

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© 2018 Published by Elsevier.

Journal Pre-proof Thermodynamic insights into molecular interactions of sodium lauryl sulfate (SLS) with caffeine and theophyllinein in aqueous media at different temperatures Shadma Tasneema, Arshid Nabi*b, Nazim Hasana, Maqsood Ahmad Malikc and Khaled Mohamed Khedherd,e a

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Department of Chemistry, Faculty of Science, Jazan University, P.O. Box 114, Jazan, Saudi Arabia b Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia c Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia d Department of Civil Engineering, College of Engineering, King Khalid University, Abha 61421, Saudi Arabia, eDepartment of Civil Engineering, ISET, Nabeul, DGET, Tunisia

Corresponding author. Tel.: +601116942076. E-mail address: [email protected] (A. Nabi)

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Journal Pre-proof Abstract Molecular interactions of the caffeine and theophylline with anionic surfactant sodium lauryl sulfate (SLS), in aqueous media, has been explored by thermodynamic parameters of micellization obtained by the conductometric method. The standard relations associated with micellization were used for the correlation and interpretation of molecular interactions by utilizing the experimental data. The various micellization parameters such as standard free

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o o o energy Gmic , enthalpy H mic and entropy S mic of micellization were calculated. The increase of

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micellization for SLS in theophylline with an elevated temperature is attributed to the prevailing

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hydrophobic-hydrophobic increased interactions. The resonance and the dispersal charge on the

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electron-rich N atoms in caffeine tend to form an ion-dipolar interaction in the vicinity of the

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polar opposite charge. The thermodynamic parameters indicate that the micellization process in caffeine and theophylline with SLS are spontaneous, enthalpy and entropy controlled.

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Keywords: Molecular interactions, Thermodynamic parameters, CMC, Micellization

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Journal Pre-proof 1. Introduction The drug micellization phenomena play a vital role in understanding the possible molecular interactions of drugs with the surfactant in an aqueous medium [1]. In continuation of our earlier studies of CTAB in aqueous paracetamol media [2], here we extend our study on the intermolecular interactions of sodium lauryl sulfate (SLS) in caffeine and theophylline in an aqueous media at different temperatures. The drug micellization phenomena are very important

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in terms of aggregation and the properties related to the thermodynamics of drugs with

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surfactants, which find significant applications in medicinal and pharmaceutical fields [3]. Drug

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molecules have both polarity regions with surfactant molecules that establish an aggregation by

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hydrophobic and hydrophilic interactions [4]. Besides, drugs having well-known hydrophobic or hydrophilic head groups have better opportunities for hydrophobic and hydrogen-bonding

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interactions with the receptor molecules [5]. In the same way, the hydrophobic and hydrophilic

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regions of surfactants are prerequisites for the surface activity and micellization process [6]. The drug molecules having micellar aggregations are proven for alteration in the behavior of

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biological responses [7]. The reason behind so: micelles possess a large radius with slow

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diffusion in comparison to the monomeric species, also the micellar solution, when compared with the ideal solution, shows lower thermodynamic activity [8]. The research-based on molecules having amphiphilic nature called surfactants has become the topic of interest for many research groups because of their unique physicochemical properties and distinctive properties like self–aggregation and adsorption at the interfaces [9–11]. These amphiphilic molecules have self–aggregation features of micelle formation at critical micelle concentration (CMC) [12–15]. These unique properties make them useful in various fields of science and technology like pharmaceuticals, drug delivery systems, petrochemicals, stabilizers in nanoparticle synthesis,

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Journal Pre-proof anticorrosion and coating industries [16–20]. The surfactant SLS micellization behavior with drug caffeine and its derivative theophylline is explored during this study. Caffeine and theophylline are methylxanthines, where the theophylline differs by having H group at position 7 instead of the methyl group. Caffeine and theophylline with moderate concentrations are found in tea, chocolates, coffee and cola beverages [21,22]. Caffeine as a mild central nervous system (CNS) stimulation has significant application in pharmaceutics [23-25], while theophylline also

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being a CNS stimulant is primarily used for the therapy of respiratory disease such as asthma

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[26]. Theophylline also finds application in diuretics [27]. Caffeine and theophylline are water

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and fat-soluble, can easily pass the blood-brain barrier causing stimulation to brain activities and

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are also absorbed after being ingested into the bloodstreams [28-30]. Moreover, the present study of aggregation of drugs with anionic surfactant SLS is taken into

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consideration because of their considerable applications in different industrial and

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pharmaceutical production. Both caffeine and theophylline possess the ability to prevent the aggregation of the amelogenin proteins in the human body, the mode of action of caffeine is after

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A3 adenosine receptor-mediated response with suppression of amyloid-beta precursor protein,

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while theophylline is cyclic adenosine monophosphate (cAMP) mediated signaling adenocarcinoma and airway epithelia stimulant. The availability of higher drug concentrations may lead to toxicity, so it is important to develop new approaches to drug delivery systems. Based on these facts, we have reported the possible molecular interactions and formation of micelles of SLS with caffeine and theophylline in an aqueous medium at different temperatures (298.15K–318.15K) including the approximate physiological temperature (308.15 K).

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Journal Pre-proof 2. Experimental 2.1. Materials Caffeine (CAS number 58-08-2) with purity ≥ 99.0 % and theophylline (CAS number 58-55-9) with purity ≥ 98.0 % were purchased from Bio Xtra (Sigma-Aldrich) and sodium lauryl sulfate of purity 98.0 % was purchased from Merck and their purities were specified by the supplier. The stock solutions of 0.5 m (mol kg-1) of caffeine and theophylline were prepared in an aqueous

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media and further was used as a solvent in the preparation of 0.002, 0.004, 0.006, 0.008, 0.010,

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0.012, 0.014 and 0.016 m SLS solutions. Using an electronic balance (Shimadzu AY220, Japan)

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of precision ± 0.0001 g the weighing was achieved for all the solutions. The freshly prepared

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concentrations were kept in airtight bottles to avoid evaporation. The chemicals utilized during

2.2. Conductivity measurements

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this study are shown in Fig. 2.1 with their chemical structures.

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The conductivity measurement was operated by using a digital conductivity meter (PC 510 Bench/Conductivity Meter (EUTECH instruments). The instrument was calibrated by standard

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(Potassium Chloride, KCl) solutions with conductivity 1413 µS/cm at 298.15 K. The temperature

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was controlled by a thermostatic water bath with standard error in temperature ± 0.0001 K. The standard procedure was used by immersing the assembly of electrodes with a glass tube in a sample solution. Upon the thermal equilibration, conductivity readings were recorded. At higher temperatures, a suitable arrangement of capping jacket over the sample solution assembly was made to avoid evaporation. The uncertainty associated with the conductivity measurements was estimated as ± 0.5%.

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Journal Pre-proof 2.3. Critical micelle concentration, degree of micelle ionization and thermodynamics of micellization The intersection of the straight lines in a conductivity versus concentration (mol kg-1) plots above and below the slope gives the critical micelle concentration, CMC of the surfactant-drug assembly [31,32]. The equation   S2 / S1 was used for the calculation of the degree of micelle ionization  where S1 and S2 are the slopes in pre- and post-micellar regions.

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We adopted here, the pseudo-phase separation model[33,34]for the calculations of Gibbs free

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o energy of micellization, Gmic , as below:

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o Gmic  (2   ) RT ln X cmc

(1)

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where, X cmc is representing the mole fraction of surfactant in the solution at CMC.

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The utilization of the Gibbs-Helmholtz equation and the temperature dependence of CMC and 

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o were essentially involved in the calculation of the standard enthalpy of micellization, H mic as

per the equation:

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 lnX cmc  o H mic   RT 2 (2   )    T  P

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(2)

The results obtained from the Eqn. (2) are an indirect method of calculation of enthalpy of micellization. In addition to the above-derived micellization parameters, we calculated the o standard entropy of micellization, S mic by using the following Gibbs-Helmholtz equation:

o Smic 

o o H mic  Gmic T

The values of

(3)

lnX cmc were determined by fitting ln X cmc to temperature in a polynomial T

function as: 6

Journal Pre-proof lnX cmc  a  b(T / K )  c(T / K ) 2

(4)

where, a, b and c are the respective polynomial constants and consequently we have:

lnX cmc  b  2c(T / K ) dT

(5)

The above mentioned thermodynamic, Eqs. (1) to (5) were implicated for the estimation of the o thermodynamic parameters such as the standard free energy of micellization Gmic , standard

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o o enthalpy of micellization H mic and the standard entropy of micellization S mic .

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3. Results and discussion

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The CMC of the surfactant-drug additives were determined from the plots of the specific

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conductance (  ) against surfactant concentration. The experimentally obtained values of the two

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tangents of intersection on the plots of specific conductance was taken into consideration as the CMC of the surfactant-drug additives. The tangents in the pre- and post-micellar regions have

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the highest regression values of the CMC. The graphical plot of specific conductivity of SLS in the presence of caffeine (0.5 m)and in the presence of theophylline (0.5 m) is shown in Fig. 3.1.

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The obtained value of CMC for the SLS in caffeine are 0.636 × 10-3mol.kg-1are lower than

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compared to the CMC values of0.650 × 10-3mol.kg-1 for SLS in theophylline at 298.15 K. The values of CMC in both systems increase in magnitude with increase in temperature from 298.15 K to 318.15 K. Predominantly, it was observed that the numerical values of CMC were found lower with drug molecules in standard pure solvent (water) state[35]. Caffeine and theophylline are hydrophilic drugs with hydrophilic and hydrophobic atomic groups. In general, the hydrophobic drugs are believed to decrease the CMC of surfactant in drug-additive systems in comparison to surfactants in water media [36]. The possible molecular interactions of caffeine and theophylline with SLS have predominant hydrophobic interactions together with the ionic

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Journal Pre-proof interactions are the reasons behind the decreased CMC values. In turn, the micellization of the SLS in the presence of caffeine and theophylline occurred thermodynamically at lower concentrations as compared to SLS in pure water. The observed overall CMC values of SLS in caffeine were less in magnitude as compared to SLS in theophylline at different temperatures. The possibility of ion-dipolar interactions between the polar N atoms of caffeine ring attached with a methyl group and with the head groups (SO4-)of SLS is stronger than compared with the

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N atom of theophylline ring attached with H atom in theophylline. The ion-dipolar and

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hydrophobic interactions of SLS in caffeine cause favorable possibilities of molecular

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aggregations into the head groups of the surfactant with lower numerical values of CMC in comparison of relatively dis-favorable micelle formation in SLS in theophylline because of

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predominant hydrophobic interactions of N atoms with head groups (SO4-) of SLS in

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theophyllinewith relatively higher CMC values, as shown in Table 3.1.The fact behind the

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increased numerical values of CMC with increase in temperature are feasible because of the influenced factors such as (i) the increased solubility of the polar drugs with stabilization of

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monomeric surfactant molecules, (ii) the disruptions of surrounding water structures leading to

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dis-favorable conditions for micellization and (iii) the increased thermal motions destroying formed palisade layers with destabilization of micellization process. Table 3.2 summed the various parameters of micellization such as the Gibbs free energy of o o o micellization Gmic enthalpy H mic and entropy of micellization S mic for systems (a.)SLS in

o caffeine and (b.)SLS in theophylline. The free energy of micellization Gmic plays an important

role in predicting the readiness of micelle growth. The observed increased negative values of o Gmic from 298.15 K to 318.15 K in both the systems show spontaneous micelle growth. The

o numerical values of Gibbs free energy of micellization, Gmic is the mutual contributions from

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Journal Pre-proof the  and ln X cmc values. The mutual dependence of the two variables  and ln X cmc (Eqn.1) o illustrates that the increased numerical values of Gmic with rising temperatures have a direct

influence on increased values of the former quantity over the decreased values of the latter and vice versa. The relative dependence of  and ln X cmc at different temperatures are revealed in Fig 3.2.The

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corresponding increase in  value of SLS in caffeine is observed up to 313.15 K and becomes

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less in numerical values at 318.15 K with the opposite effects for ln X cmc signifies the

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o spontaneous micelle growth with increased numerical values for Gmic . However, the  and

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o ln X cmc remains constant with similar negative values of Gmic for temperatures 303.15 K and

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308.15 K respectively while, at the relatively higher temperature 313.15 K and 318.15 K, because of the thermal motion the ion-dipolar interactions between head groups of SLS and the

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terminal N-atoms attached with a methyl group in caffeine increase the chances of molecular o aggregation into head of SLS leading to favorable condition of micellization with lower Gmic

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values. On the other hand, the spontaneous micellization with increase in the values of  with subsequent decrease in ln X cmc values are observed for SLS in theophylline mixture. The opposite effects are observed for SLS in theophylline with an increase in temperature because the molecular motions are increased which in turn increases the chances of hydrophobic and the hydrophilic interactions of SLS with the theophylline and stabilizes the micellization process. As for SLS in theophylline, the ion-dipole interactions are weak in comparison to SLS in caffeine mixture. This can be explained by the dispersion of unit change in a ring molecule with N attached with a methyl group in caffeine than that of theophylline ring molecule with N atom attached with a single H atom. 9

Journal Pre-proof The resonance effect is behind the strong ion-dipolar interaction for SLS in caffeine aqueous mixture in comparison to the hydrophobic predominating interactions of SLS in theophylline. This can be further explained based on the possible resonance structures of both SLS in caffeine and SLS in theophylline aqueous systems with the possible insights of molecular interactions are presented in Fig.3.3.The balanced forces which are involved in micellization in an aqueous o o media are analyzed from the contribution of enthalpy H mic and entropy S mic data. The effects of

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these parameters which are temperature-dependent are explored. Temperature dependence of

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o o enthalpy H mic and entropy S mic of micellization for SLS in caffeine and SLS in theophylline

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o o are presented in Fig.3.4.The values of H mic when compared with the values of Gmic for SLS in

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caffeine and SLS in theophylline in their aqueous solutions are numerically negative and become

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more negative with elevated temperatures as shown in Table 3.3. Besides, the individual values o of H mic of SLS in theophylline are more negative in comparison to SLS in caffeine indicating

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the presence of stronger hydrophobic-hydrophobic interactions between non-polar groups of SLS

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and the hydrophobic moiety of theophylline in aqueous media. Alternatively, it suggests that the

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process of micellization is more exothermic for SLS in theophylline than SLS in caffeine. It is worth realizing that the electrostatic interactions between the head (SO4-) groups of SLS and terminal N atom of caffeine disfavors the micellization process because of the overcrowding of surrounding molecules. However, in comparison to SLS in theophylline, the electrostatic interactions are not dominant and the presence of hydrophobic-hydrophobic interactions leads to o the favorable micellization with more negative H mic values. Moreover, the positive trend in the

o values of H mic instead of being always negative trend maybe because of the prevailing

molecular interactions from the solution and structural contributions of additives present in the

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Journal Pre-proof o mixing components. For instance, the values of H mic were found to be 42.27 and 12.24 kJ mol-1

for anionic surfactant sodium N-dodecanoylsarcosinate at 293.15 and 298.15 K in an aqueous o media [37]. Similarly, in cefadroxyl monohydrate with SLS, the H mic values were positive and

the values decrease with elevated temperature and ultimately at a temperature of 323.2 K becomes zero [38].These values decrease with an increase in drug concentration at different temperatures.

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o The entropy of micellization, S mic values are larger and positive summarized in Table 2, for

both SLS in caffeine and SLS in theophylline. The possibility of micelle growth with the transfer

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of the nonpolar hydrophobic tails towards the center of micelle against the bulk which disrupts

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the structural water molecules surrounding the hydrophobic tail with increasing entropy of the

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o o system results in positive trend in S mic values. The higher values of S mic in SLS + caffeine

system as compared to the SLS + theophylline system are observed represents that the entropy of

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micellization is higher for SLS in caffeine with lower numerical values of CMC than that for

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SLS in theophylline with comparatively higher CMC numerical values. However, in both

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o o systems the values of S mic are positive and further increasing in S mic values are observed with

increase of temperature, indicating the process of micellization is favored by entropy gain which serves as a pre-requisite condition for micelle formation [36].Also, the observed higher values of o S mic in both the systems of SLS in caffeine and SLS in theophylline shows entropy dependent o micellization phenomenon can also be explained from the observed experimental S mic o numerical values. The increasing trend in values of S mic at different temperatures for SLS in

caffeine in comparison to SLS in theophylline suggests the difference in the hydration between the hydrophilic and hydrophobic parts of caffeine and theophylline which takes part in 11

Journal Pre-proof micellization process. The increase in temperature causes increase of the thermal motions with increased solubility of monomeric SLS molecules, in turn, the ion-dipole interactions between the N atoms of aromatic caffeine molecules and head groups (SO4-) of SLS increases with o increased S mic numerical values probably associated with water structures and the aromatic

polar N atom causing spontaneous micelle formation with lower values of CMC as compare to

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o the predominant hydrophobic-hydrophobic interaction of SLS with lower values of S mic at all

temperatures in theophylline of comparatively lower numerical values of CMC than SLS in

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caffeine at temperatures between 298.15 K to 318.15 K respectively.

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4. Conclusions

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The present study is based on the conductometric study of SLS in caffeine and theophylline in

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aqueous media at different temperatures. The various thermodynamic parameters have been inspected as well as evaluated and compared with each other. The CMC values are lower in SLS

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in caffeine than the SLS in theophylline. The ion-dipolar interactions between the polar N atoms

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of caffeine ring attached with a methyl group and with the head groups of SLS are stronger than N atom of theophylline ring attached with H atom in theophylline. The observed increased

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o negative values of Gmic from 298.15 K to 318.15 K in both the systems demonstrate the micelle o formation is thermodynamically spontaneous. The H mic values were more negative at individual

temperatures for SLS in theophylline as compared to SLS in caffeine. The presence of more o negative values of H mic for SLS in theophylline suggests the micellization process occurred

more exothermically for SLS in theophylline than SLS in caffeine. In both the systems, the o o values of S mic are positive and further increase in S mic values with the increase of temperature

indicating the process of micellization of the studied systems is primarily an entropy dominated

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Journal Pre-proof one. In general, the results obtained verify the presence of ion-dipole interactions are prevailing in SLS in the caffeine system and hydrophobic-hydrophobic increased interactions arise in SLS

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in theophylline system.

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[31] A. Gonzalez-Perez, J. Czapkiewicz, J. Del Castillo, J. Rodriguez, Micellar properties of tetradecyltrimethylammonium nitrate in aqueous solutions at various temperatures and in waterbenzyl alcohol mixtures at 25 °C, Colloid and Polymer Science, 282 (2004) 1359-1364. [32] Ž. Medoš, M. Bešter-Rogač, Thermodynamics of the micellization process of carboxylates: A conductivity study, The Journal of Chemical Thermodynamics, 83 (2015) 117-122. [33] R.J. Hunter, Introduction to modern colloid science, Oxford University Press1993. [34] A. Gonzalez-Perez, J. Del Castillo, J. Czapkiewicz, J. Rodriguez, Conductivity, density, and adiabatic compressibility of dodecyldimethylbenzylammonium chloride in aqueous solutions, The Journal of Physical Chemistry B, 105 (2001) 1720-1724. [35] J. Liu, L. Li, SDS-aided immobilization and controlled release of camptothecin from agarose hydrogel, European journal of pharmaceutical sciences, 25 (2005) 237-244. 16

Journal Pre-proof [36] M.J. Rosen, J.T. Kunjappu, Surfactants and interfacial phenomena, John Wiley & Sons2012. [37] G.B. Ray, S. Ghosh, S.P. Moulik, Physicochemical studies on the interfacial and bulk behaviors of sodium N‐dodecanoyl sarcosinate (SDDS), Journal of Surfactants and Detergents, 12 (2009) 131-143. [38] F. Akhtar, M.A. Hoque, M.A. Khan, Interaction of cefadroxyl monohydrate with hexadecyltrimethyl ammonium bromide and sodium dodecyl sulfate, The Journal of Chemical

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Thermodynamics, 40 (2008) 1082-1086.

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Journal Pre-proof Table 3.1 Values of CMC (critical micelle concentration),  (degree of ionization of micelles), lnXcmc(cmc values expressed in mole fraction units) for SLS in caffeine and SLS in theophylline at temperatures 298.15 K to 318.15 K and their standard error (SE). Parameters T/K

298.15 K

303.15 K 308.15 K

313.15 K 318.15 K

SE

CMC (10-3 .mol kg-1) 0.636  0.5089 -8.9936 ln X cmc SLS in 0.5 m aqueous theophylline

0.654 0.5120 -8.9654

0.663 0.5346 -8.9518

0.672 0.5630 -8.9379

0.686 0.5361 -8.9179

3.01E-05 1.45E-02 6.00E-07

CMC (10-3.mol kg-1)  ln X cmc

0.675 0.6348 -8.9389

0.688 0.6093 -8.9205

0.709 0.4825 -8.8905

6.48E-05 8.58E-02 1.30E-06

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0.650 0.6093 -8.9768

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SLS in 0.5 m aqueous caffeine

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0.698 0.4889 -8.9064

Journal Pre-proof Table 3.2 o o Micellization parameters Gmic (standard Gibbs free energy of micellization), H mic (enthalpy o of micellization) and S mic (entropy of micellization) of a. SLS in 0.5 m caffeine and of b. SLS in 0.5 m theophyllineand their correlation coefficients (R).

Parameters a. SLS in 0.5 M caffeine T/K 298.15 K -3 -1 o -33.2 Gmic (10 kJ mol ) o -5.0 (10-3kJ mol-1) H mic

303.15 K -33.6

308.15 K 313.15 K 318.15 K -33.6 -33.4 -34.5 -4.1

-3.7

-3.3

0.9989

o 28.3 (10-3J mol-1 K-1) S mic b. SLS in 0.5 M theophylline

29.0

29.5

29.8

31.3

0.7160

o Gmic (kJ mol-1)

-30.9

-30.8

-31.8

-35.0

-35.7

0.9283

o (kJ mol-1) H mic

-7.1

-6.3

-4.9

-4.4

-2.3

0.9765

o (J mol-1 K-1) S mic

23.9

24.4

26.9

30.6

33.4

0.9751

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-4.6

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R 0.7639

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Fig. 2.1. Chemical structure of a. Caffeine, b. Theophylline and c. Sodium lauryl sulfate

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Fig. 3.1. Variation of conductivity against molality (mol kg-1) for (A) SLS in caffeine and (B)

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SLS in theophylline at different temperatures.

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Fig.3.2. Plots of  vs. temperature for (■) SLS in caffeine (0.5 m) and () SLS in theophylline

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(0.5 m) and the variation of ln X cmc vs. temperature for (■) SLS in caffeine (0.5 m) and () SLS

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in theophylline (0.5 m).

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Fig.3.3. Resonance effects and various possible molecular interactions between SLS in aq. caffeine and theophylline

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o Fig. 3.4. Plots of (A) enthalpy of micellization, H mic of SLS in caffeine (■) and theophylline

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o (), (B) entropy of micellization, S mic of SLS in caffeine (■) and theophylline () respectively

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Journal Pre-proof Author Statement Authorship contributions:

All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including

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participation in the concept, design, analysis, writing, or revision of the manuscript.

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Journal Pre-proof Conflict of Interest

The authors whose names are listed in this manuscript declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in

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this paper.

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Journal Pre-proof Highlights

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Micellization behavior of drug-surfactant system has been estimated. Critical micelle concentration was investigated using conductometric technique. Pseudophase separation model was adopted for calculation of various thermodynamic parameters. Estimation of various possible interactions prevailing in the drug surfactant system has been recognized.

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