Study On Self-Assemble Behavior Of L-Amino Acid Derivatives In Organic Solvent

Organogel is a form of solid or semisolid gel.Having lost its mobility under the short range forces of the solvent molecules,the gel takes shape of a three-dimensional(3D)networks formed by low molecular-weight organogelators(LMWGs)via non-covalent interactions(including hydrogen bonding,Van der Waals’ force,hydrophobic interactions,π-π stacking,etc.)in solvent.The gel’s 3D network structure is formed by LMOGs through self-assembling and rearranging in organic solvent.Due to its unique characteristics,Organogel has attracted much attention since it was found for its future potential application.The experiments synthesized a series of derivatives based L-proline,L-serine,L-tryptophan and L-tyrosine.The amidogen in these amino acid molecules was modified by carboxybenzyl or t-butyloxycarboryl,then,an acyl group with long alkyl chain was introduced to the molecule in order to examine the self-assembling behavior of these gelators in normal organic solvent and the microtopography of the xerogel.Furthermore,the paper studied the influence of molecular structure of the gelator on the gel and discussed the solvent effect in the gelation process.The details of the research process are recorded as follows:(1)The experiment synthesized 14 kinds of derivatives with Cbz or Boc and long chain acyl of L-amino acid and characterized the structures of these derivatives by FT-IR,1H-NMR and LC-MS.It also studied the gelation properties in normal organic solvent formed by these derivatives.The result showed that all the critical gelator concentrations(CGC)of these gelators in organic solvent were less than 3.0wt%,which indicated that these derivatives were classified as good gelators,especially serine and tryptophan derivatives could be called superior gelators as their CGC in some solvent was less than 1.Owt%.It showed that design and synthesis of L-amino acid based gelator could be achieved through chemic modification.The test of gel-to-sol phase transition temperature(Tgel-+sol)revealed the Tgel-sol would increase correspondingly with the rise of the gelator concentration level,which proved the heat stability of the gel was impacted by the gelator concentration level in organic solvent.The higher the gelator concentration level is,the more stable the gel is.It also calculated the gel-to-sol phase transition enthalpy(△Hg)of some representational gel using the van’t Hoff equation.(2)The SEM result showed that different molecule structure gelator formed different microtopography of the gel in the same solvent,and the same result occurred when the same molecule structure gelator in different solvent.The gelator molecule was rearranged and self-assembled into fibrous or sheet structure under the force of hydrogen bonding,hydrophobic interactions,and π-π stacking,etc.For the sheet structure,the width of these sheets was about 10-20μm and the thickness was around hundreds of nanometer,while for the fibrous structure,the diameter of the fiber was about 50-100nm and the length was dozens or even hundreds of micrometer.These aggregations evolved further concentrated into 3D structure with many small holes and gaps,then the solvent was entrapped into these holes and gaps which eventually formed the gel under the effect of short range force.(3)The XRD patterns of the gelators and their xerogel showed that the arrangement and stack form of the gelators had changed,except the derivatives of tryptophan,which meant the molecule of the gelators had been rearranged,or define it as being self-assembled,in organic solvent.The FT-IR spectroscopy showed the main driving force to self-assembling was the hydrogen bonding between gelator molecules,while the secondary role went to the hydrophobic interactions and π-π stacking.The stack form of representational gelator simulated by Hyperchem 8.0 software almost coincided with the results of XRD and FT-IR.Thus,it could be deduced that the gelaors molecule were arranged head to head to yield sheet structure,and then,the sheets stacked further to form 3D networks structure.(4)After thorough study on the relationship of gelation behavior to solvent parameter,the result clearly indicated:the gelation number(GN)of solvent entrapped by proline derivatives did not changed noticeably with the increase of the Hildebrand solubility parameter(δ),the GN of the solvent declined the derivatives of serine and tryptophan,and the GN increased the derivatives of tyrosine.The type of the gelators and the solvent was relevant to the relationship of Hildebrand solubility parameter(δ)to Tgel-sol.The CGC of the derivatives based proline and serine was small in solvent with small solvatochromic ET(30)parameters,meanwhile,there were two different areas of ET(30)parameters for the CGC of the derivatives based of tryptophan and tyrosine.There is no conspicuous obvious rules of the gelation status to ET(30)parameters.It is relatively easier to form gel when the acidity parameter a=0.There was a sphere in Hansen space where gel could be produced when the Hansen parameter(δd、δp、δh)of solvent in it.Teas plot indicated that the solvent which could be geled by L-amino acid gelator was mostly located at the lower right corner of Teas plot intensively.