Thermodynamics and Micellar Behavior of Surface-Active Drugs in Aqueous Solution of Electrolytes: Surface Tension Study

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Introduction
The action of the amphiphilic drug molecules in the biological system can be subjected to various characteristics molecular properties of such molecules.The various aspects like molecular mass, partition coefficient, permeability, hydrogen bonding, aggregation mechanism, interacting nature etc. are known to be very important in order to characterize a drug.Interestingly, these all are related to the adsorption applicability of the drug in the system [1].Among these characteristics the most widely explored is the micelles formation or self-aggregation of the drug molecules that may be crucial for understanding the pharmacokinetics and pharmacodynamics of drug.The amphiphilic drugs molecules after a certain minimum concentration called critical micelle concentration (CMC) get selfassembled to different shapes viz.spherical, rod-like, lamellar, hexagonal etc.The micelles concentration is very important from various view point including solubilization and emulsification of various water insoluble compounds [2].Micelles due to its peculiar structure (hydrophobic interior and hydrophilic outside core facing water) is of prime importance in the delivery of drugs (specifically water insoluble) and absorption applications [3].Micellization process (formation of micelles) can be studied with the help of different physical properties of solution like conductivity, viscosity, surface tension, etc. [4][5][6][7][8][9][10][11].
A review on the research demonstrates that the study of the surface tension with CMC is an innovative idea for the characteristics of the drug molecules, in the solution.It is a wellknown fact that the drugs carrying diverse characteristic groups may have different aggregation mechanism, interaction behavior, solubilizing nature in the solution or in biosystem.Fascinatingly, the aggregation phenomena of the drug may also be involved in the drug efficiency, that correlates with the applied concentration of the drug in the system [2].
However not only the concentration of amphiphilic drugs but some other factors like presence of additives, such as electrolyte, carbohydrates, amino acids, etc. in the biological system also involve dominantly in the drug mechanism action [12].The study of the critical micelle concentration (CMC), in terms of electrostatic interactions, temperature, charged pseudo-phase separation and further conductometric and spectroscopic techniques can give a batter approach for the formulation as well as idea of adeptness of the efficient drug or bioactive surfactants system [13].
In this regard various research groups have conducted different experiments comprising of drugs with different additives.The strengthening of micellization process and interactions of bio-surfactants (bile salts) as well as conventional surfactants with the drug content has been advocated by different researcher [5,13].The amelioration in micellization in the presence of electrolyte (NaCl) offer the formation the ease of micellization causing decreases in CMC [2].Such results are very encouraging in order to decide the solubility/bioavailability and hence pharmaceutical formulation of the drugs.Keeping these facts in mind in present communication we have analyzed drugs-electrolytes system through surface tension studies.The study has been completed by considering streptomycin sulphate and diphenhydramine in the presence of alakali metal chlorides viz.NaCl, KCl and RbCl (0.01 mol.kg -1 ), and alkaline earth metal chlorides viz.MgCl2, CaCl2 and SrCl2.These electrolytes may influence the micellization of surface active drugs, by binding with the polar head groups of the amphiphiles (drugs) (scheme 1).Various other studies of drugs with NaCl (electrolyte) has also been conducted so far [14,15], but to best of our knowledge the study of hydrochloride in the presence of electrolytes analyzed through surface tension technique has not been reported earlier.
Scheme 1: Micellization of drugs in the presence of electrolytes.
Streptomycin sulphate (SMS), first-generation antibiotic drug intended to be used against a wide range of gram-negative and gram-positive bacteria.Effective in diseases like tuberculosis, endocarditis, brucellosis, burkholderia infection, plague, tularemia and rat bite fever.Chemically, streptomycin is a triacidic base and contains aldose sugar.The two guanidine groups are attached to the streptidine have strong basic nature.The amino sugars are attached to streptidine through glycosidic linkages.The methyl amino group attached to N-methyl-L-glucosamine exhibits weakly basic nature.Therefore, the streptomycin has three basic structural units viz.streptidine (a diguanidino compound), streptose (an aldose sugar) and streptoscamine (N-methyl-L-glucosamine unit) (Scheme 2).
Diphenhydramine hydrochloride (DPH) (Scheme 2) is antihistaminic or anti-allergic drug that may act as antiallergic, sedative and antiemetic agent that belongs to H1-receptor antagonist class (first generation) [16] Chemically, it is 2-(diphenylmethoxy)-N,Ndimethylethylamine hydrochloride and is amphiphilc in nature i.e. possess hydrophobic as well as hydrophilic that make them surface active in nature.
On the other hand, electrolytes are among the crucial constituent present in the living organism.They play imperative role in order to maintain electrical across cell and help in smooth functioning of various physiological phenomena including nerve signal transmission, muscle contraction/relaxation etc.The main objective for the study is to develop the indulgent about the amphiphilic drug salt interaction with the micellar behavior and understanding of intermolecular interactions existing in the drug-electrolyte aqueous system.

Methods
For the Surface tension (γ) study, streptomycin sulphate (0.03 to 0.12 mmol•kg -1 ) and diphenhydramine hydrochloride (0.03 to 0.12 mol•kg -1 ) were prepared aqueous solution of NaCl, KCl and RbCl (0.00 and 0.01mol•kg -1 ), and MgCl2, CaCl2 and SrCl2 (0.00 and 0.002 mol•kg -1 ), at temperatures (298.15-313.15)K by weighing using a balance (Shimadzu) with a precision of 0.0001 g.The surface tension has been determined by using drop number method with the help of Man Singh survismeter, (calibration No. 06070582/1.01/C-0395,NPL, Govt. of India) [17][18][19][20][21] having number of drops for pure water equals to 144 at 298.15 K resulting into surface tension value for water to be o  = 72.14mN•m -1 [22].The average deviation for two-three measurements of a same sample of the solution did not exceed by single drop.The calibration of the survisemeter has been carried out before use at 298.15 K by using solvents, DMSO and 1,4-dioxane having γ values 43.33 and 33.41 mN•m -1 respectively, which were quite close to the values those reported in literature [17,18,20].The reproducibility for the surface tension measurements comes out to be in the range ±0.10 mNm -1 .The density utilized for the calculation of surface tension measured using Density and Sound Analyzer (DSA-5000) procured from Anton Paar, Austria has been reported in our previous study [23].The constant temperature of the solutions under examination has been maintained by circulating thermostated water whose temperature has been maintained by using a high precision water bath fitted with temperature controlled device supplied by Narang Scientific Works (NSW) Pvt. Ltd., New Delhi that keep the temperature constant within the error limit ± 0.01 K.

Surface Tension Studies
Surface tension explore the various properties at the liquid interface and provide better understanding about the bulk and surface behavior of the solution [24].This also helps in understanding the various type of possible interactions pertinent on the liquid surface [25].
The surface tension values for SMS and DPH at different concentration in pure water and 0.01mol•kg -1 aqueous solutions of NaCl, KCl and RbCl and 0.002 mol•kg -1 MgCl2, CaCl2, SrCl2 calculated by below given relation [3]: where, o  , and  are surface tension, o n , n are number of drops, and o  ,  are the densities of pure solvent and solution, respectively.
The calculated surface tension values have been tabulated in Table S1 and S2 provided in the Supporting Information (SI).).These plots show the variation of surface tension with three different experimental parameters as discussed in the following paragraphs.

Effect of the Drug Concentration
The close examination of the obtained results, plotted in Figs. 1 and 2 clearly witnesses the decrease in the surface tension of the solution on increasing the concentration of the both the drugs upto certain concentration, and after then become approximately constant on subsequent addition of drug.The observed trend correlates the affinity of the drug molecules accumulating itself on the solvent surface or adsorbed on the liquid-gas interface.So there is the arrangement of amphiphilic drug in such a manner that the hydrophilic part of the drug project towards solvent (water), while the hydrophobic part towards the air [5,15].
Further, it is interesting to note that the molecules of drug at lower concentration binds to the electrolytic ions through electrostatic forces of attraction, due to which the hydrophobic part of the drug molecules insert into the solvent system and leads to the disruption of structured water molecules i.e. structure breaking due to which drug molecules migrate from bulk to the surface causing in decrease in surface [15,26].The surface tension values decrease sharply with the concentration of drugs upto the formation of Gibbs monolayer [27].Though, after certain concentration of drugs (i.e.CMC), the  values become almost constant or show a little decrease which can be explained due to the supremacy of hydrophobic-hydrophobic interactions resulting into aggregation of drug monomers in the bulk i.e. micelle formation takes place.The aggregation of drug molecules restrict the movement of amphiphilic molecules to the surface from bulk of the solution causing very small change in  values [20].

Effect of the Electrolytes
The acquisitive effect of various interactive forces such as van der Waals forces, hydrogen bonding, electrostatic interaction, solvation and de-solvation of adsorbate and adsorbent are responsible for adsorption process [28].Addition of electrolytes gives a decrease in  values of against both the drugs and follow the order: pure water > NaCl > KCl > RbCl (for alkali metal chlorides) and for alkaline earth metals, pure water > MgCl2 > CaCl2 > SrCl2.Further, comparative analysis shows lower  values for alkali metal salt as compared to alkaline earthmetals that can be attributed to the larger size of alkali metals.The variation can be easily grasped from Table 1 against the examined electrolytic systems for both drug molecules.The obtained surface tension curves are in good agreement with the literature [18,22,29] and almost similar to studies performed with a different procedural approach [30,31].The critical micelle concentration (CMC) values calculated from the plots of  versus log[SMS/DPH] through the break points in curves [32].The obtained CMC values have been tabulated in Table 2 for both the drugs.
The obtained CMC values by the surface tension studies are found to be similar to other studies or literature performed with other methods.The CMC values for the investigated drugs, SMS and DPH have been found to decrease with the addition of electrolytes.The decrease in the CMC in presence of electrolyte follow the same order as discussed above for surface tension.The decrease in the CMC value may be advocated by considering the fact that electrolyte breaks the structure of solvent molecules by binding with the head groups of amphiphillic head groups causing in lowering of hydration layer around ionic head group (Scheme 1).This leads to decrease in the drug hydrophilicity which results in the aggregation and decrease in the CMC value [33].

Effect of Temperature
Temperature plays a vital role on the surface tension as well as on the CMC values.The values of CMC at different temperature have been tabulated in the Table 2.The obtained results clearly show the increase in the CMC values on raising the temperature for all the examined electrolytes.The obtained results can be easily understood by the concept of dehydrations which can be: (i) hydrophobic and (ii) hydrophilic in nature.In case of amphiphilic monomers both types of hydrations seemed to play their role [34], The increase in the micellization is the results of the hydrophobic dehydration and vice-versa.Therefore, both dehydration factors will decide whether the CMC increases or decreases.However, the effect of both these dehydrations further seems to be decreased on increasing the temperature [35].In the present micellar system only hydrophobic dehydration seems to be operative as the temperature leads to increase in CMC, such increase has also been observed earlier [36].
The CMC values varies linearly with temperature (Table 2) as the hydrophobic hydration varies inversely with temperature.Hence high temperature disfavours the micelle formation.
It may be explained by considering the fact that at higher temperature due to hydrophobic dehydration the thermal motion i.e. kinetic energy of the amphiphilic molecules increases and subsequently CMC value increases.

Interfacial Parameters
In order to apprehend the solution behavior, for their quantitative properties it is very important to understand the solution interfacial parameters.The interfacial properties help in the determination of the surface activity.The examination of the solute particles from bulk to the air-water interface help in understanding the various kind of structural rearrangement/ shifting.The shifting inside the amphiphilic molecules i.e.SMS/DPH drugs at the interface or surface solution along with the effect of electrolytes governs the various kinds of interactions in the drug-electrolyte-water system.Various interfacial parameters like maximum surface excess concentration ( max  ), minimum area per surfactant molecule (Amin) and surface pressure at CMC ( CMC  ) have been calculated from the surface tension data.The max  is maximum value that gives the extent of effectiveness of the adsorption.The max  values of the drugs have been calculated by using Gibbs adsorption equations [37]: where, C log   , is calculated from the slope of the graph plotted between  and log[drug] in pre-micellar region, C is the concentration of amphiphilic drug, and 'n' is the number of particles per molecule of the drug, whose surface concentration varies with change in bulk concentration of the amphiphilic drugs and have the values 2 and 3 for ionic (amphiphilic drugs) and dimeric surfactants, respectively [21,38].
The calculated max  have been summarized in Tables 3. The max  values for SMS and DPH in pure water have been found to be 1.42 and 1.90 mol•cm −2 , respectively, at 298.15 K.
The data correlated that there is irregular trend in the max  values obtained at the different examined temperature.The similar trend has been seen in a previous work with ionic surfactants SDS by Chauhan et al. work [19].However, the presence of the electrolyte decreases the max  values and follows the order pure water > NaCl > KCl > RbCl and pure water > MgCl2 > CaCl2 > SrCl2.This is due to the stronger affinity between drug-electrolyte molecules, correlating the increase in the drug hydrophobicity by the electrolyte [13,39].This suggests that the amphiphilic drug molecules get solubilized into bulk resulting into the lower max  values.The Amin calculated by using the below equation [40].
] .[ where, A N is Avogadro's number The obtained data has been tabulated in Table 3, and the values shows that the min A does not follows the regular trend with rise in temperature, similarly to the max  value.
However, the presence of electrolytes increases the min

A values. The increase in min
A values of the drugs by electrolytes correlated the number of drug molecules at the solution-air interface.The addition of electrolytes lower the total number of drug molecules at the interface and subsequently the values for min A increases [41].Interestingly, the trend obtained for the min A is just the reverse of the max  values which is obvious as both the parameter signifies opposite nature to each other.The max  is the maximum value of adsorption, whereas min A is the minimum area occupied by each surfactant molecule.Further, the surface tension reduction (  ), represents the lowering of the solvent surface tension at CMC and be calculated by using the following equation [42]: where,  0 is the surface tension of pure solvent and   is the surface tension at CMC.The obtained   values have been found to increase with temperature as can be observed from Table 3. Increment in the solution temperature increase the thermal motion due to enhanced kinetic energy and increase in the CMC  values.However, the decrease in  0 values with the addition of electrolytes subsequently decreases the CMC  values [43].These phenomena correlated the various types of electrostatic interactions, stability of drugelectrolyte complex at air-water interface, formation of micelles etc. [44].

Thermodynamics of Micellization and Adsorption
The The o ad G  have been calculated by following relation [45]: where, o m G  is representing the change in standard Gibbs free energy of micellization and has been calculated by using the following relation: However the o tr G  have been evaluated by using the following relation, with the help of max  values [46]: Calculated values of all the above discussed parameters i.e. reveal that energetically, adsorption of the drugs at the air-liquid interface is more favored than the micellization.It concludes that, some amount of work has to be done for transference of amphiphilic drug molecules from the interface to the micelle [47].This further can be explained in terms of degree of freedom, i.e. higher degree of freedom values at the air-liquid interface then in the interior of the micelle.This attribute the comparative smaller hydrophobic and aqueous repulsions between the constituents at interface then bulk [5].
The last parameter i.e.
o tr G  , have given smaller negative values.These values validate our above discussion that for the micelle formation, some amount of work done must be done.Collectively all these parameters, enlighten the surface and bulk (in term of micellization) properties of the SMS and DPH in the presence of various electrolytes.
Therefore, from the above discussion it is clear that the process of micellization and adsorption for both the amphiphilic drugs (SMS/DPH) are administered by numerous thermodynamic aspects.

Conclusions
The thermodynamics i.e. energetic of micellization process of two amphiphillic drugs namely streptomycin sulphate and diphenhydramine hydrochloride in the presence of selected alkali metal and alkaline earth metal chlorides at different temperatures has been analyzed through surface tension studies.The surface tension data have been employed for the determination of critical micelle concentration (CMC) of these drugs in the studied solvent system under different experimental temperatures.The CMC values has been obtained from graphs of  versus log [drug].The hydrophobic nature of both the drugs has been clearly illustrated by  as well as CMC values.Further, the surface tension data have been utilized to determine different surface-active parameters that infer the presence of various interactions between drug and electrolytes molecules in solution.Further various thermodynamic parameters evaluated for the considered system advocate the favoured micellization of both drugs in the presence of electrolytes.

Figs. 1
and 2 show the representative plots of surface tension of drugs (SMS/DPH) in aqueous solution of NaCl and MgCl2 at temperature range of 298.15 to 313.15 K. Similarly, the Figs.S1-S3 represent the surface tension values of drugs (SMS/DPH) in the presence and absence of electrolytes (KCl, RbCl, CaCl2, and SrCl2

- 1 . 7 TFig. 2 .
Fig. 2. Representative plots of  versus log [DPH] in aqueous solution of (a) 0.01 mol.kg -1 thermodynamics properties of micellization and adsorption phenomena during the micellization, for both the amphiphilic drugs (SMS/DPH) with electrolytic solutions have been evaluated in term of thermodynamic parameters.The examined parameters are standard Gibbs free energy change ( o ad G  ), change in standard Gibbs free energy of micellization ( o m G  ), and standard Gibbs free energy change of transfer ( of work to transfer monomeric form of amphiphilic drugs molecules to the micellar phase, while, o tr G  is the measure of shifting of molecular species from bulk to the micellar interior.Collectively, both these parameters are the measure of work required for the process of micellization.Also o ad G  is the measure of standard free energy of transfer of one mole of the amphiphilic drug molecule from the solution to the interface.

have been given in Table 4 .
Obtained values represent that, o ad G  value is negative under all the studied experimental conditions.This indicates the energetic spontaneity of micellization process of drugs.Further, more negative values of u, are u(T) = 0.1 K, u ( o tr G  ) = 3×10 -2 kJ•mol-1, u ( o ad G  ) = 2×10 -2 kJ•mol -1 , and u ( o m G  ) = 4×10 -2 kJ•mol -1 (level of confidence = 0.68)

Table 1 .
List of chemicals used with specification.
a Declared by the supplier

Table 2 .
The CMC values (mol.kg - ) for SMS and DPH in pure water and in aqueous solution of different electrolytes at different temperatures.

Table 3 .
Values of surface-active parameters: max  (mol•cm -2 ), Amin (nm 2 ) and CMC  Additionally, the Amin, i.e. the minimum area occupied by each surfactant molecule at saturated air-liquid interface has been calculated by max  value.Amin values vary with the modification in drug structural arrangement in the presence of additive i.e. the electrolytes.

Table 4 .
Values of thermodynamic parameters: