Acid-base equilibrium of neutral red in benzene-in-water microemulsions

Abstract

Acid-base equilibrium of neutral red (NR) has been studied  spectrophotometrically at 25°C in two o/w microernulsions (ME's) containing benzene/sodium dodecyl sulphate/aqueous buffer/ 1-propanol

(ME1) and benzene/sodium dodecyl sulphate/aqueous buffer/1-butanol

(ME2). NR resides at the interfacial region of the MEs. A modified Henderson-Haselbach (HH) equation is found to be applicable and only one protonation of NR takes place in the <i style="mso-bidi-font-style: normal">p</i>H range of 1.2 to 11 .2. NR appears completely in the protonated form at <i style="mso-bidi-font-style:normal">p</i>H ≤ 4.5, and in the basic form at <i style="mso-bidi-font-style:normal">p</i>H ≥<span style="font-size:14.0pt;font-family:HiddenHorzOCR;mso-bidi-font-family:HiddenHorzOCR"> 9.5. The values of <i>p</i><span style="font-size: 14.0pt;font-family:Arial;mso-bidi-font-style:italic">K_a<i>, </i>for NR in MEl and ME2 are found to be 6.88 and 7.05, respectively. These <i>p</i><span style="font-size:14.0pt;font-family:Arial;mso-bidi-font-style:italic">K_a values were factorized into contributions due to (1) effective dieiectric constant of the interfacial region, (2) interaction between protonated NR and negatively charged droplet, and (3) surface potential of the droplet. The surface potentials of the propanol and

butanol containing droplets have been estimated to be approximately

<span style="font-size:14.0pt;font-family:" times="" new="" roman";mso-fareast-font-family:="" "times="" roman";mso-ansi-language:en-us;mso-fareast-language:en-us;="" mso-bidi-language:ar-sa"="">equal to -75 <span style="font-size:14.0pt; font-family:Arial;mso-fareast-font-family:" times="" new="" roman";mso-ansi-language:="" en-us;mso-fareast-language:en-us;mso-bidi-language:ar-sa"="">± <span style="font-size:14.0pt;font-family:" times="" new="" roman";mso-fareast-font-family:="" "times="" roman";mso-ansi-language:en-us;mso-fareast-language:en-us;="" mso-bidi-language:ar-sa"="">10 and -85 <span style="font-size:14.0pt; font-family:Arial;mso-fareast-font-family:" times="" new="" roman";mso-ansi-language:="" en-us;mso-fareast-language:en-us;mso-bidi-language:ar-sa"="">± <span style="font-size:14.0pt;font-family:" times="" new="" roman";mso-fareast-font-family:="" "times="" roman";mso-ansi-language:en-us;mso-fareast-language:en-us;="" mso-bidi-language:ar-sa"="">10 mV, respectively.</span></span></span></span></span></span></span></span>