Volumetric Studies of Sodium Dodecyl Sulphate in Aqueous and Aqueous Amino Acid Solutions

Introduction
Volumetric studies of  SDS in aqueous  and  in aqueous amino acid  provides great help to determine the nature of  interactions and properties of this  solution. And it is very important because of great applications of SDS in Pharmaceuticals and Biotechnological prosses. SDS represents a potentially effective topical microbicide, which can also inhibit and possibly prevent infection by various enveloped and non-enveloped viruses such as the Herpes simplex viruses, HIV, and the Semliki Forest virus.

In this paper, We reports the  changes in the physical properties due to the mixing of  water, sodium dodecyl  sulphate & amino acid  are thought of arising from hydrophobic interactions among  solute and solvent molecules.    Here has been used ternary process and the results of volumetric studies on the binary aqueous solutions of sodium dodecyl sulphate(SDS) and ternary SDS-amino acid-water systems gives vast  information  about  volumetric properties. Here we used two amino acids (glycine & dl-alanine) and sodium dodecyl sulphate(SDS). Surfactant molecules (e.g. CTAB, SDS, TritonX-100, etc.) self-aggregate into super molecular structures when dissolved in water or oil. The simplest aggregate of these surfactant molecules is called a micelle; and the  dispersion of  the aggregates in water or oil  is referred to as a micellar solution.  A typical micelle has size of ~ 50 Å and is made of about 100 surfactant molecules.  The surfactant molecule consists of two parts, namely, a polar hydrophilic head group and an apolar hydrophobic tail (hydrocarbon chain). The concentration above which micelle formation becomes appreciable is termed the critical micelle concentration (c.m.c).

Micelle formation is as typical hydrophobic process in water. In aqueous medium, surfactant molecules with their long hydrophobic tails undergo hydrophobic hydration.  As the surfactant concentration increases, the association of surfactant molecules occurs by hydrophobic interaction and this result in the removal of the non- polar   portion of the molecule from the external aqueous environment to form the interior of   the micelle while the hydrophilic groups are exposed to the aqueous environment

Therefore, the present studies are to determined apparent molar volume, apparent molar expansion and critical micelle concentration (c.m.c) at low region concentration in binary and ternary system. A review of literature shows that only a few authors have attempted to determine of these properties in ternary system at low region concentration

Experimental

The  Surfactant  used  in  this  study  was  sodium  dodecyl  sulphate(SDS).  The c.m.c value of SDS in water reported in the literature value closely around 0.009m at 25⁰C. The values of mean aggregation number of SDS vary from 58 to 64. Glycine  (purity.  Mass fraction   ≥0.99), DL-alanine (purity. Mass fraction ≥0.99) procured from Fluka Chemical Company, Switzerland were used without further purification. Supplied distilled water was redistilled and deionized by passing through two ion exchange columns.  The deionized water was distilled again in alkaline KMnO4 medium and used for preparation of solution. Conductivity of this deionized water was found to  be  5×10^-6 S.cm^-1. An electric balance with an accuracy of   ± 0.0001 g   was used   for weighting. The densitometer (DSA-5000, Anton Paar, Austria) was used for the measurements of density and ultrasonic sound velocity. The solutions were prepared by weight immediately before the measurement. The ternary solutions were prepared by mixing appropriate mass of the components. The amount of each component was later converted into their mole fraction. Precautions were taken to prevent the introduction of moisture into the experimental examples. Each time, the solution was prepared immediately before the density measurement.

Results  and  Discussion

Apparent molar volume

The apparent molar volume,ø_v , was calculated from the measured density of the solution using the relation

Where M₂,  m, ρ and ρ₀   respectively the molar mass of solute, molality of solution, density of solution and density of solvent. Density data of solution and solvent are given in Table 1and the calculated  values at different temperatures are given in Table 2 as a function of molality of the solutions. The plots of the  against  are presented in Figures 1-3.

For surfactant solutions physical properties such as, osmotic pressure, turbidity, electrical conductance and surface tension were used to determine the critical micelle concentration (c.m.c). In our investigation we tried to observe the variation of apparent molar volume of SDS as we focused our study in the low concentration region. The – plots show sudden change in  value at a particular molality after which  show linear change. The c.m.c of SDS in aqueous and aqueous amino acid solutions were determined by extrapolation method. The c.m.c values of SDS in aqueous and aqueous amino acid solutions are presented in Table 4

 

by least square method. The limiting apparent molar volumes, , of SDS are presented in Table 3.

Table 4 reveals that our c.m.c  values of SDS in water are somewhat larger than the literature values (0.009). An increase in c.m.c values in glycine and alanine solution was observed which demonstrates that glycine and alanine behave as water structure breakers. It has been reported that addition of structure breakers causes an increase of cmc [1].

The viscometric study of Devine and Lowe [2] reports that glycine is water structure breaker whereas, alanine is water structure maker. The effect of concentration of glycine and alanine and that of temperature is not regular,  the observed trend is that, c.m.c increases with the increase in amino acid concentration. c.m.c’s of SDS would be expected to increase with increasing temperature [1]. No such distinct behaviour is observed from the study. This may be due to the uncertainties associated with the measurement in very low concentration region. The uncertainty in measurement is greater than the expected variation of cmc values. Further attempts of cmc determination through  measurement will establish this method.

It is evident from Figures 1-3 that values of SDS increase with the increase in solute concentration. The concentration dependence of in aqueous and mixed aqueous solutions has been explained in terms of the solute-solute interaction. The usual interpretation is that the solute species interact through the destructive overlap of their hydration spheres [3]. For apolar species, the positive volume contribution to , originated from the hydrophobic hydration starts decreasing as the solute concentration is increased. The overlap of two hydrophobic hydration cospheres relaxes some water molecules from the salvation sphere to the bulk giving rise to a negative change in volume. For hydrophobic ionic species the volume of water molecules is smaller in the salvation shell due to (a) the effect of electrostriction [4] and (b) a decrease in the hydrogen-bonded network of water molecules in the salvation sphere than in the bulk (the so called structure-breaking effect). Further, if the ions are oppositely charged, this type of interaction also causes   an attraction since the orientation of water molecules by the cations leads to favorable anion-water dipole interactions (and vise versa).  In this way even the electrostricted water molecules may be shared, resulting in a positive volume change.   The structure-breaking influence of ionic species on the hydrophobic hydration sphere of apolar groups gives a negative volume effect

Anionic surfactant sodium dodecyl sulphate has a long hydrophobic chain and charge centre. The concentration dependence of of SDS can be interpreted in terms of the interactions involving the hydrophobic part as well as the charged centres. The values of SDS increase with the increase in molality i.e, S_v value positive. The positive sign of S_v indicates that interactions involving charged moieties dominate over the apolar group-apolar group and apolar group- charged centre interactions.

The  values, obtained by fitting -data by least squares method are tabulated in Table 3.  The values of SDS in aqueous amino acid solutions are greater than these in water and increase with the concentration at all the studied temperatures. The variation of with molality of amino acids can be rationalized in terms of the cosphere overlap model [5].  According to this model, the overlap of cosphere of ion (or a polar group ) with that of another ion (or a polar group) gives positive change in volume, while that of an ion (or a polar group) with that of a hydrophobic group and a hydrophilic group with another hydrophobic group results in a negative change in volume. As in the limit of infinite dilution the solute-solute interaction is minimal, the mutual overlap of the cospheres of zwitterionic amino acid with those of hydrophobic and  hydrophilic group of SDS  produces negative and positive changes in volume, respectively. The greater values in amino acid solution demonstrate strong ion-ion interactions in the ternary system. The relaxation of the electrostricted water molecules, due to strong localized ion-zwitterion interactions, from the cosphers of both the amino acid and SDS to the bulk causes an increase in volume , this increase in  volume outweighs the volume decrease due to the overlap of cospheres of hydrophobic parts of SDS and amino acids. Overlap of cosphere of amino acids gives positive change in volume.

As the concentration of amino acid increases the zwitterions-SDS anion and also the amino acid interactions increase giving rise to an increased value. This in, and all probability account for the increase in  value with the increase in amino acid concentration.

The  of  SDS  in ternary solution can alternatively be thought of as arising from four constituents [6], as

=V_vw +  V_f + V_h + V_{s                                   (3)}

where, V_vw and V_f are van der Waalls volume [7] and volume of empty spaces   present  therein [8], respectively. The V_h and V_s  represent  the contributions due to the hydrophobic and hydrophilic hydration. The V_vw   and  V_f are assumed to be the same in aqueous amino acid as in water [9]. The variation of is, Therefore, due to          the changes in  (V_h + V_s) resulting from SDS-amino acid, amino acid-amino acids   and amino acid water interactions; the contribution from water-water interactions is  assumed to be negligible.

At  the same amino acid concentration, the  increases with the increase in temperature. This may be due to the resultant of the following effects;

Due to the increased thermal energy at higher temperature, the relaxation to the bulk of the electrostricted water  molecules from the ionic interaction regions results in a positive volume change.

An increase in temperature renders the amino acid- amino acid interaction relatively weaker giving rise to a small negative volume changed.

A decrease in amino acid-water interactions causes an increased volume change..

 

Table 1: Density (ρ) and ( ρ0) of SDS in water and aqueous amino acid solutions as a function of molality of SDS at different temperature.   Click here to View table

 

Table  2: Apparent molar volume (Фv/cm-3.mol-1) of  SDS  in aqueous and aqueous aminoacid solutions as a function of molality and different temperature. Click here to View table

 

Table 3: Limiting  apparent molar , of SDS in aqueous  and  aqueous  amino acid solutions as a function of molality and temperature. N= concentration number Click here to View table

 

Table 4: Critical micelle concentration (c.m.c) of  SDS in aqueous  and  aqueous  amino acid solutions as a function of molality and different temperature. Click here to View table

 

Figure 1: Limiting apparent molar volume of SDS in water , water+glycine(1.3), dl-alanine (1.216) as a function of molality of  SDS at  different  temperature; ♦-15оC,  ■-20 оC, ▲-25 оC,  ◊-30 оC,   □-35 оC,  ∆-40 оC, ●-45 оC, ○-50 оC. Click here to View figure

 

Apparent  molar expansion  and  expansibility

Apparent molar expansion, Φ_E  was computed from measured density data using the relation

where, α₀ is the expansibility of water taken from G. S. Kell  [10],  α is the expansibility  of solution calculated using the equation

 

The parameters a₁ and  a₂  were obtained by the regression  of  (ρ – ρ₀) – T data  according to equation

Table  5:  α0 is the expansibility of water taken from G. S. Kell  [10],   Click here to View table

 

Table  6: The parameters a1 and  a2  Click here to View table

 

Table 7: Expansibility ( α ) and apparent molar expansion (ΦE) of  SDS in water as a function of molality and different temperature. Click here to View table

 

Figure 2: Expansibility ( α ) of  SDS  in water, water+glycine(1.3), water+dl-alanine(1.216) as a function of molality of  SDS at different  temperature;  ♦-15оC,   ■-20 оC,▲-25 оC,  ◊-30 оC,  □-35 оC, ∆-40 оC,  ●-45 оC,  ○-50 оC. Click here to View figure

 

Figure 3: Apparent molar expansion (ΦE) of  SDS in water, water+glycine(1.3), water+dl-alanine(1.216) as a function of molality of SDS at different  temperature; ♦-15оC,  ■-20 оC,  ▲-25 оC,  ◊-30 оC,  □-3оC,   ∆-40 оC, ●-45 оC, ○-50 оC. Click here to View figure

 

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